PERFORMANCE EVALUATION OF CONDIMENTS AS …

PERFORMANCE EVALUATION OF CONDIMENTS AS ENVIRONMENTALLY

FRIENDLY CORROSION INHIBITORS FOR AMINE–BASED

CARBON DIOXIDE ABSORPTION PROCESS

A Thesis

Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

For the Degree of

Master of Applied Science

In Process Systems Engineering

University of Regina

By

Sockalingam Sekkappan

Regina, Saskatchewan

February, 2018

Copyright 2018: Sockalingam Sekkappan

UNIVERSITY OF REGINA

FACULTY OF GRADUATE STUDIES AND RESEARCH

SUPERVISORY AND EXAMINING COMMITTEE

Sockalingam Sekkappan, candidate for the degree of Master of Applied Science in Process Systems Engineering, has presented a thesis titled, Performance Evaluation of Condiments as Environmentally Friendly Corrosion Inhibitors for Amine-Based Carbon Dioxide Absorption Process, in an oral examination held on January 12, 2018. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: Dr. Daoyong Yang, Petroleum Systems Engineering

Supervisor: Dr. Amornvadee Veawab, Process Systems Engineering

Committee Member: Dr. Stephanie Young, Environmental Systems Engineering

Committee Member: Dr. Adisorn Aroonwilas, Process Systems Engineering

Chair of Defense: Dr. Doug Durst, Faculty of Social Work

ii

ABSTRACT

Corrosion of process equipment and piping in the amine-based carbon dioxide

(CO2) absorption process causes considerable expenditures for maintenance and repair.

The addition of effective corrosion inhibitors to the amine solutions is a common practice

for corrosion mitigation. Despite their inhibition effectiveness, those corrosion inhibitors

used in the amine-based CO2 absorption process are not environmentally friendly and

require costly waste handling and disposal. To reduce such cost and prepare for more

stringent environmental regulations for chemical uses and disposal, this work

investigated the feasibility of using condiments as environmentally friendly corrosion

inhibitors in the amine-based CO2 absorption process. In this study, five condiments

including powders of garlic, mustard, horseradish, onion and turmeric were selected and

evaluated for their corrosion inhibition performance on carbon steel (CS1018) in the

environment of 5.0 kmol/m3 aqueous solutions of monoethanolamine (MEA) saturated

with dissolved CO2. The evaluation was carried out in corrosion experiments that

employed cyclic and poteniodynamic polarization and electrochemical impedance

spectroscopy for corrosion measurement and analysis. Results show that the powders of

garlic, mustard, horseradish, onion showed great promise for corrosion reduction in both

MEA-CO2 and MEA-CO2-oxygen (O2) solutions. They performed well with inhibition

efficiencies in the range of 80- 95% even at elevated temperatures and the presence of

process contaminants (i.e., chloride and oxalate) which were found to slightly affect the

inhibition performance. These four condiments were proved to be mixed-type (anodic

and cathodic) corrosion inhibitors that protected the metal surface by undergoing

endothermic physical and chemical adsorption with the Langmuir adsorption isotherm.

iii

Sulfur, nitrogen, phosphorus functional groups were the primary contributors to the

inhibition effectiveness. Unlike these four condiments, the powder of turmeric was not

promising as it yielded lower inhibition efficiency and induced pitting corrosion for most

tested conditions.

iv

ACKNOWLEDGEMENTS

I would really like to thank Dr. Amornvadee (Amy) Veawab for providing this

research opportunity .Dr.Amy has always given me the freedom throughout my entire

course and provided a positive environment to try new things as solutions when there

were obstacles in my research. Also, I would like to thank Dr. Adisorn Aroonwillas for

providing lot of support through his suggestions and feedbacks to set up my experiments.

I also gratefully acknowledge the financial support through scholarship and teaching

assistantship from the Faculty of Engineering and Applied Science and Faculty of

Graduate Studies and Research at the University of Regina. I express my earnest

gratefulness for financial support provided by Natural Sciences and Engineering

Research Council (NSERC).

I take this opportunity to express my heartfelt gratitude to my father and brother

for understanding the situation and helping me to overcome the anxiety hurdles and

stresses. With a special mention I would like to thank Mohanned Alammeen and Prathap

IVS for their technical advice and mentorship for completing this project. Also, I take this

opportunity to express my special thanks to Rajesh Murugesan for his innovative ideas

during my research which ignited my curiosity in this project. Finally I would like to

thank my best friends for being my inspiration during my entire research work at difficult

times. So, thanks to Sanjog, Ameer, Ranga, Prakashpathi, Venky, Vaikunth, Vinith

Manoj, Gina and Robyn for their motivation. Thanks are just a word but I am glad to

have met everyone mentioned here and now being a part of my world.

v

TABLE OF CONTENTS

ABSTRACT ii

ACKNOWLEDGEMENTS iv

LIST OF TABLES ix

LIST OF FIGURES xi

NOMENCLATURE xvii

CHAPTER 1 INTRODUCTION 2

1.1 Carbon Dioxide (CO2) Absorption Process 2

1.2 Corrosion Problems and Control 2

1.2.1 Plant practices 3

1.2.2 Corrosion resistant materials 5

1.2.3 Chemical treatment 6

1.3 Corrosion Inhibitor History and Current Status 6

1.3.1 Environmental regulations 8

1.3.2 Eco-friendly corrosion inhibitors 9

1.4 Research Motivation 10

1.5 Research Objectives and Scope 11

CHAPTER 2 FUNDAMENTALS AND LITERATURE REVIEW 13

2.1 Corrosion of Metals 13

vi

2.2 Corrosion Mechanisms in Amine Based CO2 Absorption Process 14

2.3 Factors Affecting Corrosion 15

2.3.1 Amine Type and Concentration 16

2.3.2 CO2 loading 16

2.3.3 Oxygen 17

2.3.4 Operating temperature 18

2.3.5 Heat stable salts (HSS) 18

2.4 Corrosion Inhibitors Classification 18

2.5 Green Corrosion Inhibitors 19

2.6 Criteria for Classification of Inhibitors 21

2.6.1 Open circuit potential (OCP) 21

2.6.2 Tafel slopes 23

2.7 Adsorption 24

2.7.1 Physical adsorption 24

2.7.2 Chemical adsorption 24

2.8 Adsorption Isotherm 25

2.8.1 Langmuir isotherm 25

2.8.2 Temkin isotherm 26

2.8.3 Frumkin isotherm 26

2.9 Standard Free Energy of Adsorption 27

2.10 Arrhenius Plots 29

2.11 Thermodynamic Properties 31

vii

2.12 Electrochemical Impedance Analysis 32

2.13 Theoretical Quantum Chemical Methods 35

CHAPTER 3 EXPERIMENTS 40

3.1 Experimental Setup 40

3.2 Materials 42

3.2.1 Electrodes 42

3.2.2 Chemicals 42

3.3 Experimental Procedure 44

3.4 Data Analysis 47

3.4.1 Tafel extrapolation method 47

3.4.2 Pitting tendency 49

3.4.3 EIS analysis 50

CHAPTER 4 RESULTS AND DISCUSSIONS 52

4.1 Uninhibited System 52

4.1.1 Effect of O2 Concentration in Feed gas 52

4.1.2 Effect of temperature 58

4.1.3 Effect of process contaminants 63

4.2 Inhibited Systems 69

4.2.1 Garlic 69

4.2.2 Mustard 88

4.2.3 Horseradish 105

viii

4.2.4 Onion 124

4.2.5 Turmeric 142

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 148

5.1 Conclusions 148

5.2 Recommendations 150

REFERENCES 151

APPENDIX 168

ix

LIST OF TABLES

Table 1.1 Summary of plant experience on corrosion in CO2 gas absorption

process using alkanolamines 04

Table 1.2 Summary of corrosion inhibitor used in CO2 capture process using

alkanolamines 07

Table 2.1 Green corrosion inhibitors for protecting steel in various corrosive

environments 22

Table 3.1 Summary of the chemicals used 46

Table 4.1 Summary of the parameters and experimental test conditions 53

Table 4.2 Summary of experimental and electrochemical parameters for

uninhibited systems 57

Table 4.3 Summary of experimental and electrochemical parameters for garlic

inhibited systems 71

Table 4.4 Summary of Quantum chemical analysis of garlic 86

Table 4.5 Summary of experimental and electrochemical parameters for mustard


Table 4.6 Summary of quantum chemical analysis of mustard 104

Table 4.7 Summary of experimental and electrochemical parameters for

horseradish inhibited systems 111

Table 4.9 Summary of experimental and electrochemical parameters for onion


Table 4.10 Summary of quantum chemical analysis of onion 140

x

Table 4.11 Summary of experimental and electrochemical parameters for Turmeric


Table 5.1 Summary of Corrosion inhibitors performance 149

Table A.1 Uninhibited MEA solutions under the influence of temperature 168

Table A.2 Garlic inhibited MEA solutions for various inhibitor concentrations 168

Table A.3 Mustard inhibited MEA solutions for inhibitor concentrations 169

Table A.4 Horseradish inhibited MEA solutions for inhibitor concentrations 169

Table A.5 Onion inhibited MEA solutions for Inhibitor concentrations 170

xi

LIST OF FIGURES

Figure 1-1 Schematic Diagram for CO2 gas absorption process 1

Figure 2-1 Adsorption isotherm models: (a) Langmuir isotherm,

(b) Temkin isotherm, and (c) Frumkin Isotherm 28

Figure 2-2 Arrhenius Plots (a) Type I (b) Type II 30

Figure 2-3 (a) Nyquist Plot (b) Bode-Phase Plot and equivalent electrical circuit 33

Figure 3-1 Schematics of experimental setup for electrochemical corrosion testing 41

Figure 3-2 Dimensions and chemical composition of (CS 1018) 43

Figure 3-3 Chittick apparatus CO2 loading and MEA conc. measurement 45

Figure 3-4 Tafel extrapolation methods 48

Figure 3-5 Pitting tendency from poteniodynamic polarization curves

(a) Pitting (b) No Pitting 51

Figure 4-1 Polarization corrosion behavior of uninhibited MEA solutions

in the presence of oxygen 55

Figure 4-2 Corrosion behavior comparison of uninhibited MEA solutions in

the presence and absence of oxygen 56

Figure 4-3 Corrosion behavior comparison of uninhibited MEA solutions under

the influence of temperature 59

Figure 4-4 Corrosion behavior comparison of uninhibited MEA solutions under

influence of temperature (a) Conductivity, (b) Nyquist Plot,and (c) Rp 60

Figure 4-5 Photos (before and after experiment) comparison of uninhibited

MEA solutions under the influence of temperature at (a) 80°C (b) 40°C 61

xii

Figure 4-6 Corrosion behavior of uninhibited MEA solutions under the influence

of temperature (a) Bode-phase plot (b) Equivalent electrical circuit 62

Figure 4-7 Arrhenius Plots for uninhibited MEA solutions under the influence

of temperature (a) Type I (b) Type II 64

Figure 4-8 Corrosion behavior comparison of uninhibited MEA solutions under the

influence of process contaminants (a) Tafel plot (b) Corrosion rate 66

Figure 4-9 Comparison of uninhibited MEA solution under the influence of

process contaminants (a) Nyquist Plot (b) Rp 67

Figure 4-10 Photos of uninhibited MEA solution under the influence of process

contaminants (a) Chloride, (b) Oxalate,(c) Thiosulfate,and (d) Formate 68

Figure 4-11 Comparison of garlic inhibited MEA solutions in the presence

and absence of oxygen 70

Figure 4-12 Comparison of garlic inhibited MEA solutions for Inhibitor

concentrations (a) Polarization behavior (b) Inhibition efficiency 73

Figure 4-13 Comparison of garlic inhibited MEA solutions for inhibitor

concentrations (a) Nyquist plot (b) Rp 74

Figure 4-14 Corrosion behavior of garlic inhibited MEA solutions for Inhibitor

concentrations (a) Bode phase plot (b) Equivalent electrical circuit 75

Figure 4-15 Corrosion behavior of garlic inhibited MEA solutions for inhibitor

concentrations (a) tafel slope (b) Langmuir adsorption isotherm 77

Figure 4-16 Corrosion behavior of garlic inhibited MEA solutions under the

influence of temperature at (a) 40°C, (b) 60°C,and(c) 80°C 78

xiii

Figure 4-17 Comparison of garlic inhibited MEA solutions under the

influence of temperature 79

Figure 4-18 Arrhenius Plots for garlic inhibited MEA solutions under the

influence of temperature (a) Type I (b) Type II 81

Figure 4-19 Comparison of garlic inhibited MEA solutions under

the influence of process contaminants 82

Figure 4-20 Comparison of garlic inhibited MEA solutions under the

influence of process contaminants (a) Nyquist Plot (b) Rp 84

Figure 4-21 Quantum Chemistry structures for Allicin and Diallyl Sulfide 87

Figure 4-22 Comparison of mustard inhibited MEA solutions in the presence and

absence of oxygen (a and b) Polarization behavior,

(c) Open circuit potential, and (d) Inhibition efficiency 89

Figure 4-23 Comparison of mustard inhibited MEA solutions for Inhibitor


Figure 4-24 Comparison of mustard inhibited MEA solutions for Inhibitor


Figure 4-25 Corrosion behavior of mustard inhibited MEA solutions for inhibitor


Figure 4-26 Corrosion behavior of garlic inhibited MEA solutions for inhibitor

concentrations (a) Langmuir adsorption isotherm (b) tafel slope 95

Figure 4-27 Corrosion behavior of mustard inhibited MEA solutions under the

influence of temperature Tafel plot at (a) 40°C, (b) 60°C,and (c) 80°C 97

xiv

Figure 4-28 Comparison of mustard inhibited MEA solutions under the influence

of temperature (a) Inhibition efficiency, (b) Nyquist plot, and (c) Rp 98

Figure 4-29 Arrhenius plots for mustard inhibited MEA solutions

(a) Type I (b) Type II 99


of process contaminants (a) Tafel plot (b) corrosion rate 101


of process contaminants (a) Nyquist Plot (b) Rp 103

Figure 4-32 Quantum Chemistry structures for Allyl isothiocyanate

Benzyl isothiocyanate and Sinigrin (a, b, and c) Optimized molecular

structures (d, e, and f) HOMO (g, h, and i) LUMO 106

Figure 4-33 Comparison of horseradish inhibited MEA solutions in the

presence and absence of oxygen 107

Figure 4-34 Comparison of horseradish inhibited MEA solutions for inhibitor


Figure 4-35 Comparison of horseradish inhibited MEA solutions for inhibitor


Figure 4-36 Corrosion behavior of horseradish inhibited MEA solutions for

Inhibitor concentrations (a) Bode phase plot (b) electrical circuit 113

Figure 4-37 Corrosion behavior of horseradish inhibited MEA solutions for

Inhibitor concentrations (a) tafel slope (b) Langmuir isotherm 114

Figure 4-38 Corrosion behavior of horseradish inhibited MEA solutions under

the influence of temperature at (a) 40°C, (b) 60°C, and (c) 80°C 115

xv

Figure 4-39 Comparison of horseradish inhibited MEA solutions under

the influence of temperature 116

Figure 4-40 Arrhenius Plots for horseradish inhibited MEA solutions



the influence of process contaminants 120


the influence of process contaminants (a) Nyquist Plot (b) Rp 121

Figure 4-43 Quantum Chemistry structures for Peroxidase, Phenethyl

isothiocyanate and Theaflavin (a, b, and c) Optimized molecular

structures (d, e, and f) HOMO (g, h, and i) LUMO 123

Figure 4-44 Corrosion behavior of onion inhibited MEA solutions under the

influence of oxygen (a and b) Polarization behavior (c) Open circuit 125

Figure 4-45 Comparison of onion inhibited MEA solutions for inhibitor


Figure 4-46 Comparison of onion inhibited MEA solutions for Inhibitor


Figure 4-47 Corrosion behavior of onion inhibited MEA solutions for inhibitor


Figure 4-48 Corrosion behavior of onion inhibited MEA solutions (a) tafel slope

comparison (b) Langmuir adsorption isotherm 131

Figure 4-49 Corrosion behavior of onion inhibited MEA solutions under

the influence of temperature at (a) 40°C , (b) 60°C, and (c) 80° C 132

xvi

Figure 4-50 Comparison of onion inhibited MEA solutions under the influence

of temperature 134

Figure 4-51 Arrhenius Plots for onion inhibited MEA solutions under


Figure 4-52 Comparison of onion inhibited MEA solutions under the influence of

process contaminants (a) Tafel plot (b) corrosion rate 137

Figure 4-53 Comparison of onion inhibited MEA solutions under the influence

of process contaminants 139

Figure 4-54 Quantum Chemistry for Dipropyl disulphide and quercetin 141

Figure 4-55 Comparison of turmeric inhibited MEA solutions in the presence and

absence of oxygen 144

Figure 4-56 Comparison of turmeric inhibited MEA solutions for

inhibitor concentrations 145

Figure 4-57 Comparison of turmeric inhibited MEA solutions for inhibitor


xvii

NOMENCLATURE

ASTM American Society for Testing and Materials

C Capacitance (farad)

CCS Carbon capture and storage

Cdl Double layer capacitance (μF/cm2)

CE Counter electrode

CPE Constant phase element

CP Cyclic Polarization

CR Corrosion rate (mmpy)

CS Carbon steel

oC Degree Celsius

D Density (g/cm3)

DFT Density Functional Theory

DC Direct current

E Electrode potential (V)

Eo Standard electrode potential (V)

Eb Breakdown potential or pitting potential (V)

Ecorr Corrosion potential (V)

EHOMO Highest occupied molecular orbital energy (eV)

ELUMO Lowest unoccupied molecular orbital energy (eV)

EIS Electrochemical Impedance Spectroscopy

EPA Environmental Protection Agency

xviii

Epp Primary passivation potential (V)

Erev Equilibrium potential (or Reversible potential) (V)

Erp Re-passivation potential (V)

EW Equivalent weight (g/equivalent)

ΔE Energy gap (eV)

f Frequency (Hz)

F Faraday’s constant (96,500 coulombs per mole)

HSAB Hard and soft acid and base

HSS Heat stable salts

ΔG Free energy change

ΔH Enthalpy change

ia Anodic current density (A/cm2)

ic Cathodic current density (A/cm2)

icorr Corrosion current density (A/cm2)

icrit Critical current density (A/cm2)

iL Limiting current density (A/cm2)

io Equilibrium exchange current density (A/cm2)

ipass Passivation current density (A/cm2)

I Ionization potential (eV)

LC50 Lethal concentration

mmpy millimeter per year

MEA Monoethanolamine

n Number of electrons per atom of the species involved in the reaction

xix

n Hardness (eV)

ΔN Fraction of electrons transferred

OCP Open circuit potential

PC Post combustion

PARCOM Paris Commission

R Gas constant (JK-1mol-1)

RE Reference electrode

RP Polarization resistance (ohm cm2)

RS Solution resistance (ohm cm2)

ΔS Change in entropy

T Absolute temperature (oC)

W Warburg impedance (ohm cm2)

WE working electrode

wt. % Weight percent

vol. % Volume percent

Z Impedance (ohm cm2)

Z' Real impedance (ohm cm2)

Z" Imaginary impedance (ohm cm2)

Greek Letters

βa Anodic Tafel slope (mV/decade of current density)

βc Cathodic Tafel slope (mV/decade of current density)

Ƞa Activation polarization (V)

xx

Ƞc Concentration polarization (V)

θ Phase angle (degree)

μ Dipole moment (Debye)

χ Electronegativity (eV)

ω Angular frequency

2

CHAPTER 1 INTRODUCTION

1.1 Carbon Dioxide (CO2) Absorption Process

The CO2 absorption process is commonly used for removing CO2 from gas

streams for either natural gas purification or flue gas treatment purposes. The process is

operated using amine-based solvents that have the capability to react preferentially with

CO2 [Kohl and Nielsen, 1997]. It is a regenerative process with temperature-dependent

reversible chemical reactions. This process consists of two sequential steps, namely CO2

absorption and solvent regeneration (or CO2 stripping). As illustrated in Figure 1.1, the

gas stream containing CO2 enters the bottom of absorber while the stream of lean amine

solution enters the top of the absorber. The CO2 in the gas stream is absorbed into the

lean amine solution and the gas stream leaves the absorber top with little CO2 contents.

As a result of the CO2 absorption, the lean amine solution becomes rich amine that is

loaded with CO2. The rich amine solution is then preheated through a heat exchanger and

fed to the regenerator where the CO2 is stripped from the rich amine solution by means of

heat. After being regenerated, the rich amine solution becomes the lean amine solution,

leaves the regenerator and is sent back to the absorber for the CO2 absorption cycle.

The amine-based absorption process is widely used in oil and gas industries as

natural gas sweetening plants for gas purification operations. Due to the effect of climate

change, carbon capture and storage (CCS) technologies are gaining momentum

worldwide to control greenhouse gas emissions. Power plants and cement industries are

the major contributors of CO2 emitted into the atmosphere.

1

Figure 1-1 Schematic diagram for CO2 absorption process

2

There are various technologies under development, but the post combustion carbon

(PCC) capture process using amine-based absorption has the potential to become state of

art technology for CCS as it can be integrated into existing power plants and cement

industries [Jang et al., 2016]. The main reason for PCC to be preferred is because of its

efficiency, feasibility and previous operational experiences from a similar process in gas

purification operations. However, there are some differences between these applications

in terms of partial pressure of CO2 and the presence of oxygen (O2) in the flue gas source.

The feed gas streams in gas purification have higher partial pressure of CO2 (in the range

of 100 bar) and contain very little or no O2 whereas those in post-combustion flue gas

treatment have lower CO2 partial pressure (in the range of 0.5 bar) and contain

considerable amounts of O2 [Kittel et al., 2014].

1.2 Corrosion Problems and Control

As per second law of thermodynamics, corrosion is an inevitable and spontaneous

process resulting in metal thickness reduction and, in some cases the formation of pits.

The uncontrolled corrosion failures lead to loss of functionality of equipment or pipeline

crack. Apart from affecting the growth of industries, the consequence of corrosion failure

could even become catastrophes resulting in serious irreparable damage to the

environment and human community through accidents causing injuries and even death to

people. This fact is alarming because in Europe every one out of five major refinery

accidents occurred due to corrosion failure [Groysman, 2016].

3

1.2.1 Plant practices

In the CO2 absorption process, corrosion is one of the major operational

difficulties that directly affect plant economy by causing equipment failure and

unplanned downtime, and indirectly affect the integrity of process by catalyzing

degradation of amine solutions in the presence of O2 and CO2 [Gouedard et al., 2012].

Corrosion failures were observed and well documented for the CO2 absorption process

used in natural gas sweetening process. Approximately 10-30% of maintenance budget

was accounted for corrosion [Garcia-Arriaga et al., 2010]. Types of corrosion found in

the gas plants were general corrosion, stress corrosion cracking, pitting corrosion, and

hydrogen embrittlement [Gui et al., 2008]. The factors affecting corrosion are

temperature, CO2 loading, and solution contaminants (such as amine degradation

products) [Pearson et al., 2013]. Examples of plant corrosion experiences reported for the

amine-based CO2 absorption process are provided in Table: 1.1. It is apparent from Table

1.1 that the integrity of equipment was threatened by corrosion. The process components

including absorbers, regenerators, and heat exchangers were prone to different forms of

corrosion on carbon steel which was a common material of construction for process

equipment. To control corrosion at acceptable levels, process parameters were commonly

adjusted to reduce the corrosiveness of amine solutions [Strazisar et al., 2003]. For

example, the concentration of monoethanolamine (MEA) solutions was kept at 3.0

kmol/m3. In addition, the corrosion was also controlled by using alternative corrosion

resistant materials and chemical treatment.

4

Table 1.1 Summary of plant experience on corrosion in CO2 gas absorption process using alkanolamine

Plant Name Location Material of

Construction Type of Plant Applications Corrosive areas Reference

Tarong Australia Carbon Steel Pilot Plant PCC Absorber section [Cousins et al., 2013]

-- USA Carbon Steel Petroleum

Refinery GP

Mechanical failure

due to cracking at

absorber, regenerator

and heat exchanger.

[McHenry et al.,

1987]

LNG Indonesia Carbon Steel

Natural gas

liquefaction

company,

CO2 removal

unit

Cracking at Amine

Regenerator and

Absorber Columns

due to erosion

corrosion.

[Safruddin et al.,

2000]

NEA USA Carbon Steel

CO2 recovery

Plant from gas

turbine flue gas

Food and

beverage

industry

Both absorber and

stripper [DeHart et al., 1999]

CO2 capture China

Carbon Steel

Pilot Plant PCC

Bottom of the

absorber and rich-

liquid outlet of heat

exchanger

[Gao et al., 2012]

Castor and

CESAR Denmark Carbon Steel Pilot Plant PCC

Liquid outlet from the

stripper and Pitting

corrosion was

observed at the CO2

outlet from the

stripper.

[De Vroey et al.,

2013]

ITC Canada Carbon Steel Pilot Plant PCC Inlet of the stripper [Kittel et al., 2012]

-- USA Carbon Steel Gas Treatment GP

Corrosion products

found as solid

contaminants in heat

exchanger and

regenerator.

[Dingman et al., 1966]

5

1.2.2 Corrosion resistant materials

The material selection for plant equipment plays a key role in construction cost

which is a major capital investment in process industries. The criterion for material

selection is based on compatibility with the operating environment, corrosion resistance

of the material in that environment, cost of the material, and ease of fabrication. For the

CO2 absorption process, alternative materials for construction that could be used instead

of carbon steel based on the above material selection criteria are stainless steel, coated

carbon steel (nickel coated or zinc coated), alloys (Monel 400 or Inconel 625) and non-

metallic lined materials (HDPE or FRP, etc.) [Schweitzer, 1996]. However, the use of

these materials can lead to the followings shortcomings. First, the cost of corrosion

resistant materials is higher than carbon steel. For example, stainless steel costs about

four times the cost of carbon steel [Sedriks, 1996]. Second is the fabrication issue. Metal

(alumina) coated carbon steel is found to resist corrosion in the CO2 gas absorption

process, but the fabrication process of such material is complicated and could affect

mechanical strength of process equipment [Sun et al., 2011(b)]. Third is the compatibility

with temperature. The performance of nonmetallic materials and corrosion resistant

alloys could be degraded at elevated temperatures and no nondestructive methods are

available for performance monitoring [Smallwood, 2006]. Due to these shortcomings,

carbon steel remains the common material of construction for process equipment and

piping. The capital cost saving is possible when carbon steel corrosion rate can be

controlled [Campbell et al., 2017].

6

1.2.3 Chemical treatment

The chemical treatment for corrosion control is through the use of corrosion

inhibitors, the chemical substances that reduce or minimize corrosion when added in a

small quantity to the environment [Riggs, 1973]. The corrosion inhibitors are commonly

selected based on their compatibility with operating environment and type of corrosion

involved. For the CO2 absorption process, the impact of corrosion inhibitors on amine

degradation also needs to be considered [Voice et al, 2014]. The corrosion inhibitors are

the preferred corrosion management method because they are inexpensive compared to

the use of corrosion resistant materials and versatile can be applied directly to the existing

system. This fact is supported by the maintenance reports elsewhere [Cavallaro, 2016].

1.3 Corrosion Inhibitor History and Current Status

Table 1.2 provides the summary of corrosion inhibitors used for carbon steel in

the amine-based CO2 absorption processes. Generally, the corrosion inhibitors function

by means of adsorption of the inhibitor onto the metal surface, or the formation of a

stable layer on the metal surface. The inhibitors can be classified into inorganic, organic,

and the combination of both. The inorganic inhibitor acts as a strong oxidizing agent that

converts the oxidation state of iron to the trivalent state (i.e., ferric oxide layer) [Nielsen

et al., 1995], and reacts electrochemically with the metal surface and forms a stable

passive protecting layer. However, the disposal cost of these inorganic inhibitors is high

due to their toxicity.

7

Table 1.2 Summary of Corrosion Inhibitor used in CO2 capture process using amines

Inhibitor Details Type of

Inhibitor

Process

Applications

Application

Scale

References

Cupric oxide and Zinc

sulfate mixture with

Bronze pieces

Inorganic Industrial gas

Processing Industrial [Trevino et al., 1987]

Formaldehyde thio urea

,Nickel sulfate and amino

ethyl Piperazine mixture

Combination

(Organic and

Inorganic)

Gas

conditioning

in Refinery

Industrial [Hensen et al., 1986]

Carboxylic compounds,

Amine compounds and

Sulfoxide compounds

Organic

Gas

Treatment in

Industries

Lab [Chang et al., 2005]

Polythia ether

compounds Organic

Acid gas

Treatment Industrial

[Veawab et al.,

2000]

Copper carbonate Inorganic Gas

Treatment Lab [Raj et al., 2007]

Ionic liquids

Combination

(Organic and

Inorganic)

Natural Gas

Sweetening Lab

[Hasib-ur-Rahman et

al., 2013]

Sodium metavandate Inorganic Gas

Treatment Industrial

[Williams et al.,

1968]

Proprietary inhibitors Inorganic PCC Lab [Goff et al., 2006]

Antimony-vanadium Inorganic Gas

Treatment Industrial [Mago et al., 1974]

Pyridinium salt ,

thioamide/thiocyanate

mixture with Cobalt salt

Combination

(Organic and

Inorganic)

Sour Gas

Conditioning Industrial [Clouse et al., 1978]

Vanadium and Organic

nitro compounds

Combination

(Organic and

Inorganic)

Acid Gas

Removal Industrial

[McCullough et al.,

1985]

Vanadium and Cobalt Inorganic Sour Gas

conditioning Industrial [Nieh et al., 1983]

Vanadium and amine

Combination

( Organic and

Inorganic)

Sour Gas

Conditioning Industrial [Nieh et al., 1983]

Sulfapyridine , Sulfolane Organic PCC Lab [Srinivasan et al.,

2012]

Carbohydrazide Organic PCC Lab [Fytianos et al.,

2016]

8

Unlike the inorganic inhibitors, the organic corrosion inhibitor forms a thin layer as a

result of physical and/or chemical adsorption onto the metal surface. This thin film layer

acts a barrier between metal and corrosive solution [Kosseim et al., 1984]. In most cases,

the organic inhibitor yields lower corrosion inhibition efficiencies compared to the

inorganic inhibitor.

1.3.1 Environmental regulations

Due to the increasing impact of process effluents over the environment, stringent

environment policies have been developed by governments in all parts of the world for

ecological awareness. The environmental regulations are being enforced by the

continuous monitoring of the process effluent disposed by industry. The North Sea

(United Kingdom, Norway, Denmark, and Netherlands) and the North American

countries have their own sets of environment regulations under various policies. They use

three criteria for considering any chemicals to be environmentally friendly chemicals [Taj

et al., 2006]. These are as follows: 1) toxicity in terms of LC50 (Lethal Concentration) or

EC50 (Effective Concentration) must be greater than 10 mg/L, 2) biodegradation, in 28

days, must be greater than 60% in the North Sea countries, and 3) bioaccumulation in

terms of Log P o/w ( Partition coefficient) must be less than 3.

In UK, the policy does not accept chemicals with carcinogenic and mutagenic

characteristics. Among the above criteria, toxicity plays a key role in the selection of

corrosion inhibitors as it relates to the cost of waste disposal. As the alkanolamine has

been registered under the European Chemicals Agency (ECHA) the production of these

chemicals are expected to increase tenfold by 2050 due to its importance in post-

9

combustion carbon capture [Lag et al., 1984]. As such, environmental standards and

regulations for chemicals use would be strictly monitored.

In the amine-based CO2 absorption process, use of inorganic corrosion inhibitors,

such as vanadium (V) and copper (Cu) leads to the formation of complex compounds

containing dissolved metal ions and inorganic inhibitors, resulting in hazardous reclaimer

waste due to the presence of heavy metal [Thitakamol et al., 2007 ; Léonard et al., 2014].

In addition to the inorganic inhibitors, organic-based inhibitors may not be used due to

their probable connection with toxicity. For instances, despite its inhibition performance,

hydrazine is not used because of carcinogenetic characteristics [Fytianos et al., 2016].

The long aliphatic chains in organic molecules promote corrosion inhibition, but their

toxicity hinders their use in practice [Singh et al., 1996]. Gas emissions from absorbers

also makes things more complicated as the toxicity of used chemicals defines the

maximum permissible concentration limit [Gjernes et al., 2013]. As such, there is a

serious need to find alternative chemicals to control corrosion while complying with the

environment regulations.

1.3.2 Eco-friendly corrosion inhibitors

Global ecological awareness and stringent environment policies of governments

on toxic chemicals have generated a need to replace the toxic chemicals with

environmentally friendly chemicals. Such need has led to a new rapidly growing research

area known as the Green Chemistry or Sustainable Chemistry. Its aim is to reduce or

eliminate the generation of hazardous wastes and replace toxic chemicals with

environmentally friendly ones. As part of this, a new avenue has been created where

10

corrosion inhibitors are derived from the green chemistry. According to the PARCOM

(Paris Commission), the corrosion inhibitor which is non-toxic, readily biodegradable,

and has no bioaccumulation could be termed as the green corrosion inhibitors [Taj et al.,

2006].

1.4 Research Motivation

From plant experience as observed in Table: 1.1, corrosion is one of the most

operational problems in the CO2 absorption process. The corrosion affects not only plant

economy, but also has potential to become one of the key risk factors to initiate

catastrophic industrial accidents. Since the fatal damage to human society by risking

human lives could not be tolerated, the corrosion prevention and control strategies are

considered to be of great importance. Owing to the fact that carbon steel is the preferred

material of construction for the process equipment, the corrosion control using corrosion

inhibitors is an appealing choice. The application of corrosion inhibitors is economical

due to low cost of inhibitors and can be easily integrated to the existing process.

With due diligence and keeping in mind that in the coming years, the amine-based

CO2 absorption process would become major capital investment all around the globe

because of its extensive usage in various applications. The stringent environmental laws

and regulations have been implemented and monitored in most countries. This has put the

use of corrosion inhibitors in the CO2 absorption process in a difficult situation. The

industry is required to implement safer and less hazardous chemical practices to prevent

the generation of toxic wastes. Most corrosion inhibitors that have been used in the CO2

absorption process are not environmentally friendly and could cause unsafe operations.

For instance, sodium metavandate, the conventional corrosion inhibitor, is a heavy metal

11

and highly toxic. Carbohydrazide, the recent corrosion inhibitor tested at the laboratory,

has its own disadvantage as it could explode on heating and proved to be highly toxic.

This has led to serious speculations about the successful possible implementation of the

CO2 absorption process using corrosion inhibitors. Thus, corrosion management using

environmentally friendly corrosion inhibitors is necessary at this moment of time to

tackle corrosion problems and also satisfy the environment regulatory needs. Although

there are intensive database for organic and inorganic corrosion inhibitors used in this

process, there is a still gap of knowledge of eco-friendly corrosion inhibitors to solve

corrosion issues.

1.5 Research Objectives and Scope

The objective of this work was to screen and evaluate effective eco-friendly

corrosion inhibitors for the amine-based CO2 absorption process. Based on the green

chemistry literature [Zaferani et al., 2013], a wide range of plant extracts and food-based

products was reported to perform well as the corrosion inhibitors in various

environments. As such, it is expected that such plant extracts and food-based products

would also be effective in the CO2 absorption process. Thus, five condiments including

powders of garlic, onion, mustard, turmeric, and horseradish were chosen as the tested

inhibitors in this work due to their availability, cost and ease of production in large

volumes.

To evaluate the performance of five condiments, a series of electrochemical

corrosion experiments were carried out in 5.0 kmol/m3 monoethanolamine (MEA)

solutions saturated with CO2 under various test conditions that simulate the operating

12

conditions of the CO2 absorption process. MEA was used to represent the amine solution

and carbon steel 1018 was used to represent the material of equipment and piping in the

CO2 absorption process. Parametric effects, including the effects of dissolved O2,

inhibitor concentration, temperature and process contaminants, on inhibition performance

of the tested inhibitors were examined. In addition, quantum chemical analysis was

performed to gain an understanding of corrosion inhibition mechanism.

13

CHAPTER 2 FUNDAMENTALS AND LITERATURE REVIEW

2.1 Corrosion of Metals

Corrosion is an electrochemical reaction through which a material deteriorates

due to its interaction with the environment. Metal could be considered as electrodes while

the ionically conducting liquid is the electrolyte for the reaction. The two electrochemical

reactions related with the electrode are anodic and cathodic reaction, respectively. They

may involve either oxidation or reduction reactions which lead to the formation of

charged species, i.e., ions. Due to these reactions, the metal deteriorates, leading to the

formation of corrosion products which may be soluble or solid. Oxidation reaction taking

place at anode is known as anodic reaction, i.e., loss of electrons from the metal state

resulting in an increase in valence. As a result, electron is released into the electrolyte. At

cathode, reduction reaction takes place, i.e., accepting electrons from metal resulting in a

decrease in valence is known as cathodic reaction. These reactions occur as evolution

reactions as water is normally the electrolyte involved.

Anodic reaction: Metal → Metal2+ + 2e- (2.1)

Cathodic reaction: 2e- + 2H+ → H2 (2.2)

Corrosion could be prevented when the primary corrosion causing agent or

reactions related with that could be either averted or minimized. Also, it could be

prevented through adsorption of chemical species over the metal surface. This chemical

species is transported from electrolyte to metal mainly by diffusion. Thus, anodic

dissolution reaction occurring at the metal surface could be prevented due to adsorption

of these metal ions [Shaw et al., 2003].

14

2.2 Corrosion Mechanisms in Amine Based CO2 Absorption Process

It is necessary to understand the corrosion mechanisms related with the process

for preventing it. There is various information available in literature related with

corrosion mechanism associated with CO2 capture process using alkanolamine [Kladkaew

et al., 2009; Ali et al., 2012; Hasib-ur-Rahman et al., 2013; Zheng et al., 2014].According

to the literature, when metal is exposed to the CO2 loaded amine, following three

reactions occurred namely anodic, cathodic and corrosion production formation. Even

though different amines are involved and mechanisms were proposed in the presence or

absence of O2, the anodic reaction i.e., metal dissolution remained the same. Metal

dissolution reactions are oxidation reaction where metal loses electron and becomes a

charged ion. Normally, iron is used as metal for the CO2 capture process; it deteriorates

or is oxidized as indicated below to metal ions. Cathodic reactions are generally reduction

reactions which mainly include hydronium and bicarbonate ion reduction reactions. Also,

water is reduced into hydroxyl ions according to [Veawab et al., 2002]. According to

[Duan et al., 2003] carbonic acid present could be reduced into bicarbonate ions when

tertiary amines are used. According to [Kossiem et al., 1984] as a part of corrosion

mechanisms, protonated amine is reduced. Also, in the presence of oxygen, according to

[Brennecke et al., 2001] dissolved oxygen is reduced into hydroxyl ions. Corrosion

product formed in the absence of oxygen was iron hydroxide. In the presence of oxygen,

iron hydroxide is unstable and reacts further with oxygen and water molecules present to

form ferric salt or rust. Iron carbonate was also formed due to the reaction between metal

and carbonate ions [Kladkaew et al., 2009; Emori et al., 2017]. According to [Nielsen,

1995], it was initially thought that acid gases were responsible while some evidences

15

pointed towards bicarbonate or carbonate ions. It was very unclear and difficult to find

out the cathodic reactions responsible for corrosion process. However, there are reports

related such as type of amine, amine concentration and other factors affecting corrosion

in CO2 capture process.

Dissolution of iron/anodic reaction: Fe ↔Fe2++2e- (2.3)

Hydronium ion reduction: 2H3O++2e- ↔2H2O+H2 (2.4)

Bicarbonate ion reduction: 2HCO3-+2e- ↔ 2CO3

2-+H2 (2.5)

Undissociated water reduction: 2H2O + 2e- ↔2OH-+H2 (2.6)

Carbonic acid reduction: 2H2CO3 + 2e- ↔2HCO3-+H2 (2.7)

Protonated amine reduction: RNH2+ 2e- ↔RNH+ H2 (2.8)

Dissolved O2 reduction: O2+2H2O ↔ 4 OH- (2.9)

Iron hydroxide formation: Fe2++2OH- ↔Fe (OH)2 (2.10)

Iron Oxide formation: 2Fe (OH) 2+H2O + ½ O2 ↔ 2Fe (OH)3 (2.11)

Iron carbonate formation: Fe2++CO3

2- ↔ FeCO3 (2.12)

where H2O, H3O+, OH-, HCO3

-, CO32-, RH3, RNH2, RNHCOOH, Fe,H2CO3,Fe(OH)2,

Fe(OH)3, and Fe2CO3 are representing water, hydronium ion, hydroxyl ion, bicarbonate

ion, carbonate ion, amines, carbamic acid, Iron, carbonic acid, iron hydroxide, iron oxide,

and iron carbonate, respectively.

2.3 Factors Affecting Corrosion

Corrosion in the amine-based CO2 capture process is influenced by type of amine,

amine concentration, CO2 loading of amine solution, O2 in feed gas, temperature of the

systems and heat stable salts (HSS).

16

2.3.1 Amine Type and Concentration

Four type of amines namely primary (e.g., MEA), secondary (e.g.,

Diethanolamine, DEA), tertiary (e.g., Methyl Diethanolamine, MDEA) and sterically

hindered amines (2-amino-2-methyl-1-propanol (AMP)) have been used in CO2

absorption process. According to (Gunasekaran et al., 2012), it was found that corrosivity

of amines increased in the following order: tertiary amine < secondary amine< sterically

hindered amines < primary amine. However, there was no strong evidence to identify the

reason behind this. It was also found that in absence of acid gas all amines were non

corrosive [Dupart et al., 1993]. Since amines do not influence directly on corrosion, the

type of amines chosen for the CO2 capture process was based on other process

requirements.

Amine concentration affects corrosion as the increasing MEA concentration leads

to a rise in corrosion rate of carbon steel. This was explained that a large amount of CO2

was absorbed due to high MEA concentration which in turn resulted a large amount of

reducible ions as a result oxidation - a reduction reaction was enhanced. This indicates

that amine concentration resulted in an increased acid content into the solution .As such

high amine concentration should be avoided. Typical amine concentrations are kept as

18-20 wt., % MEA, 30 wt., % DEA and up to 50 wt., % for MDEA respectively [Nouri et

al., 2007 ; Kladkaew et al., 2009].

2.3.2 CO2 loading

Amount of CO2 absorbed into a known quantity of solvent is termed as CO2

loading. It is considered as one of the important factors related with corrosion in CO2

17

capture process as it mainly impacts the cathodic reduction reactions. An increase in CO2

loading leads to increases in amounts of carbonate and bicarbonate ion, making the

solution acidic and corrosive. There was a tenfold increase in corrosion current density

for a rise of CO2 gas loading from 0 to 0.5 mol CO2/ mol amine [Zhao et al., 2011 ;

Kittel, 2014]

2.3.3 Oxygen

The presence of O2 plays a key role on corrosion as the corrosion rate increases in

the presence of O2 due to its role in degradation products formation [Pearson et al., 2013].

In a MDEA/CO2 system with the presence of O2, it was found that corrosion was

accelerated in the presence of heat stable salts [Duan et al., 2013]. For a MEA/CO2

system, similar results were found in the presence of O2 but with absence of heat stable

salts. The corrosive nature was explained based on the fact that the dissolved O2

enhanced oxygen reduction reaction which in turn led to oxidation of iron [Nouri et al.,

2007]. This was further justified when it was found less corrosive for MEA in the

absence of O2 [Zheng et al., 2015]. However, several other studies indicated that

temperature plays a key role when influence of O2 on corrosion was considered.

According to [Sun et al., 2011 (a)] ,there was no big difference in corrosion rate when it

was compared between 40°C and 80°C irrespective of the presence of O2. It was further

proved by [Kittel et al., 2014 (a)] that the influence of O2 on corrosion rate at 80°C for

MEA system was insignificant and it was also reported that an increase in O2

concentration in solution could create passive condition to prevent the metal from

corrosion.

18

2.3.4 Operating temperature

Most electrochemical reactions are thermally activated, on this account, corrosion

rate increases with temperature. This was further justified from the report of [Ali et al.,

2011 (a)], that a higher anodic current density was observed when the temperature was

increased from 40 to 80°C due to the shift of corrosion potential towards active

directions, and anodic metal dissolution.

2.3.5 Heat stable salts (HSS)

Heat stable salts (HSS) such as formate, sulfate, oxalate and chloride are formed

when amines reacts with acids stronger than CO2 .HSS are thermally irreversible and

cannot be regenerated and also increase corrosion rate of the process by increasing

conductivity and lowering pH of the amine solution [Nielsen et al., 1995].According to

[Nouri et al., 2007], the heat stable salt content in amine solution should be limited to 1-2

wt., % to maintain free amine concentration.

2.4 Corrosion Inhibitors Classification

Corrosion inhibitors could be classified based on their mode of blocking the

corrosion reactions or mechanisms or chemistry. Generally, in applications perspective,

they are classified based on their mode of blocking corrosion reactions into three

categories known as anodic, cathodic, and mixed corrosion inhibitors. Anodic corrosion

inhibitors affect anodic reactions to prevent corrosion are known as passivation

inhibitors. Common anodic inhibitors contain ions such as chromate, nitrite and

19

orthophosphate. In CO2 absorption process using alkanolamine, sodium metavandate an

anodic corrosion inhibitor is used to shift the potential and promote passivation resulting

in formation of passive layer over the anodic metal surface and retardation of metal

dissolution reaction. Cathodic corrosion inhibitors act as a barrier in preventing cathodic

corrosion reactions to take place. They achieve this by shifting pH of the solution towards

alkaline forming precipitates, thus reducing sites available for cathodic reaction. In

addition, the cathodic inhibitors can block the diffusion of ions between anodic and

cathodic sites and forming layers over the metal surface. Examples of cathodic corrosion

inhibitors are selenides, arsenic and polyphosphates [Anbarasi et al., 2013]. Majority of

corrosion inhibitors falls under the mixed category as they neither affect anodic or

cathodic corrosion reactions alone, rather affect them both. The mode of inhibition

mechanism for this type of inhibitors is through adsorption at metal solution interface

either through stable bond (chemisorption) or by simply blocking the reaction sites (i.e.,

Physisorption). Because of this nature, they are also known as adsorption inhibitors while

their efficiency is purely based on metal surface coverage [Osokogwu et al., 2012]. This

classification could be done with the help of data obtained from electrochemical

techniques such as open circuit potential (OCP) and observing the shift in Tafel slopes

and current density.

2.5 Green Corrosion Inhibitors

Molasses and vegetable oils used as corrosion inhibitors for acid pickling is the

first patent in corrosion inhibitors category [Putilova et al., 1960] dating back to 1960s.

Then toxic inorganic inhibitors were used for corrosion prevention because of their high

20

efficiency. Due to toxicity and other environment regulations forced the shift towards

developing alternate inorganic corrosion inhibitors. This led to the development of

organic corrosion inhibitors for various corrosion-prone processes. However, they were

not able to meet the requirements for a green ecofriendly demand. Thus, the focus has

been shifted now towards green corrosion inhibitors. According to PARCOM (Paris

Commission), the corrosion inhibitor which is readily biodegradable with no

bioaccumulation and also nontoxic could be termed as green corrosion inhibitors [Frenier

et al., 2000]. Green corrosion inhibitors normally perform the inhibition mechanism by

being adsorbed over the metal surface through either physical or chemical adsorption and

then affect the corrosion reactions .Also, the electric resistance of the solution is

increased by this inhibition mechanism protecting the metal surface from corrosion.

Table 2.1 provides information about the green corrosion inhibitors used for protecting

steel in various corrosive environment. The table mainly focuses on protecting steel in

acidic conditions because it has been reported in various corrosion studies that acidic pH

indicates the most corrosive condition. Even though, in other corrosion environments,

various natural products have been tested and performed well as green corrosion

inhibitors, those kinds of reports have been found lacking for CO2 absorption process.

Developing a green corrosion inhibitor is really a challenging task and requires

the knowledge obtained from the mechanisms of standard corrosion inhibitors used in

that process. Also, factors like availability and cost play an important role in the selection

of green corrosion inhibitor. The following information about the corrosion products and

corrosion inhibitors used in CO2 absorption process environment using alkanolamine

would be really useful. FeCO3 and FeS are the corrosion products formed due to

21

corrosion. It was found that FeS formed over the metal surface acted as a protected film

when carbon steel was immersed in DEA solution [Garcia-Arriaga et al., 2010]. FeS was

found effective and performed well when amines were changed and carbon steel was

immersed in MDEA solutions [Emori et al., 2017]. On the other hand, FeCO3 was found

ineffective to form a protective film, while FeS performed very well and protected the

metal from further corrosion [Nouri et al., 2007]. Because of superior electron donating

ability than nitrogen or oxygen, sulfur containing compounds were found to be effective

corrosion inhibitors. [Hackerman et al., 1954; Khaled et al., 2003]. Also, in natural gas

solutions, sulfur may inhibit corrosion [Emori et al., 2007], while metal thiocyanate was

found as effective corrosion inhibitors in alkanolamine plants [Rooney et al., 2000]. 2-

mercaptobenzimidazole was used as corrosion inhibitor and found effective up to 80° C

for carbon steel in 5 M MEA solutions CO2 absorption process. [Zheng et al.,

2015].Based on this information from the inorganic corrosion inhibitors, it was found that

corrosion inhibition depends largely on electron donating ability of atoms present in the

compounds. Therefore, it could be assumed that sulfur containing natural compounds

could be used as green corrosion inhibitors.

2.6 Criteria for Classification of Inhibitors

2.6.1 Open circuit potential (OCP)

When an inhibitor is added to a corrosive solution, generally there would be a

shift in corrosion potential compared to that of blank solution without inhibitors. If this

displacement on comparison with blank solution is more than 85 mV, then it could be

22

Table 2.1 Green corrosion inhibitors for protecting steel in various corrosive environments

Inhibitor Corrosive Environment Adsorption Type References

Aloe Vera

Multiphase environment with

CO2 gas, sand and brine

solution

- [Ige et al., 2012]

Garlic Peel Extract 1 M HCl Chemisorption [Pereira et al., 2012]

Garcinia Kola Seed 2 M HCl and 1 M H2SO4 Physical adsorption [Oguzie et al., 2007]

Musa sapientum peels H2SO4 Physical adsorption [Africa et al., 2008]

henna (law Sonia) 1 M HCl Chemisorption [Ostovari et al., 2009]

Khillah (Ammi visnaga) seeds 2 M HCl Chemisorption [El-Etre et al., 2006]

Natural Mimosa tannin H2SO4 Chemisorption [Martinez et al., 2002]

Zenthoxylum alatum plant

extract 1 M HCl Chemisorption [Chauhan et al., 2007]

Tryptamine 0.5 M H2SO4 Chemisorption [Moretti et al., 2004]

Olive Leaves 2 M HCl Physical adsorption [El-Etre et al., 2006]

Berberine 1 M H2SO4 Chemisorption [Li et al., 2005]

G.Kola 1 M HCl Physical adsorption [Oguzie et al., 2007]

Alizarin yellow GG 2 M H2SO4 Physical adsorption [Ebenso et al., 2008]

J. Gendarussa extract 1 M HCl Physical adsorption [Satapathy et al., 2009]

Fenugreek Leaves H2SO4 Chemisorption [Noor et al., 2007]

P.Amarus 2 M HCl Chemisorption [Okafor et al., 2008]

Pennyroyal mint 1 M HCl Physical adsorption [Bouyanzer et al., 2006]

Methylene Blue (MB) 2 M HCl Physical adsorption [Oguzie et al., 2007]

Caffeic Acid 0.1 M H2SO4 Chemisorption [de Souza et al., 2009]

23

called as anodic or cathodic inhibitor based on their shift direction., i.e., the difference is

in negative sign and it could be termed as anodic inhibitors, whereas vice versa for

cathodic inhibitors [Riggs et al., 1973 ; Ferreira, 2004]. If the difference is in potential

displacement is less than 25 mV, then those inhibitors could be termed as “modest”

anodic/cathodic corrosion inhibitors [Zheng et al., 2015].

2.6.2 Tafel slopes

On comparing Tafel slopes (βa and βc) of an inhibited solution with that of blank

solution, the inhibitors could be classified either as anodic, cathodic or mixed inhibitor.

When there is no change in Tafel slopes after the addition of inhibitor on comparison

with blank solution, the inhibition mechanism could be due to geometric blocking effect,

i.e., an inhibitor simply blocks the reaction sites on the metal surface, thus decreasing the

available reaction area and it neither affects cathodic or anodic corrosion reaction [Shukla

et al., 2009]. If there is a shift in anodic Tafel slope (βa),while cathodic Tafel slope (βc)

remains same of inhibited solution on comparison with blank solution , the inhibitor

could be termed as anodic and could be assumed affecting the anodic metal dissolution

reaction. For cathodic corrosion inhibitor the criteria are reverse, i.e., cathodic Tafel slope

(βc) varies for inhibited solution on comparison with blank solution whereas anodic Tafel

slope (βa) remains constant. For the mixed inhibitors, shifts on both tafel slopes (βa and

βc) are observed indicating the corrosion inhibition affects both side of corrosion

reactions. [De Souza et al., 2009]

24

2.7 Adsorption

Adsorption is a surface phenomenon where molecules adsorb onto the surface

either through physical or chemical attraction. Performance of corrosion inhibitors is

attributed by adsorption performance of an inhibitor onto the metal surface or interaction

between the inhibitor and the surface and it is classified into two types: namely physical

and chemical adsorption [Jain et al., 1976]

2.7.1 Physical adsorption

The adsorbed molecules attach onto the surface through weak physical attraction

forces known as Van der Waal’s making adsorption of molecules reversible and it

requires less enthalpy to break the bond and retain the interface to its initial state. This

adsorption process increases with a rise in pressure or concentration of adsorbed

molecule while decreases with a rise in temperature. This adsorption occurs in multilayer

and also requires less activation energy for establishing equilibrium. No byproducts are

formed as a result of this type of adsorption [Jain et al., 1976; Atkins et al., 2011].

2.7.2 Chemical adsorption

Chemical reaction occurs between the adsorbed molecules and the metal surface

leading to formation of covalent bands. As a result, surface products are formed and

make the adsorption irreversible. This kind of adsorption requires high activation energy

and time to attain the equilibrium. The adsorption process decreases with an increase in

pressure or concentration of adsorbed molecule while increases with a rise in

25

temperature. This adsorption occurs in monolayer but the forces of attraction are strong.

Chemisorption is said to be an exothermic process [Jain et al., 1976 ; Atkins et al., 2011].

2.8 Adsorption Isotherm

Adsorption is a surface phenomenon. In adsorption, some materials (adsorbate)

from a concentrated source such as bulk vapor or liquid phase gets attached onto the

surface of a solid surface (adsorbent). When a plot is made to understand the amount of

adsorbed as a function of the partial pressure or a concentration at a given temperature is

defined as adsorption isotherm [Adamson et al., 1990]. As the interaction between

inhibitor molecule and the metal surface determines degree of inhibition .Adsorption

isotherms could be used as it relates the amount of inhibitor (or surface coverage)

adsorbed over the metal surface with the inhibitor concentration. Adsorption isotherms

are helpful to understand better about the corrosion mechanism, adsorption equilibrium

constant and surface coverage [Desimone et al., 2011; Manimegalai et al., 2015; Yilmaz

et al., 2016]. The surface coverage (Θ) for corrosion inhibitors [Zhang et al., 2015] is

given by the following equation:

100

(2.13)

where ŋ is inhibition efficiency. Various isotherm models such as Langmuir, Temkin and

Frumkin adsorption isotherms are available for fitting the data related with corrosion

inhibitors.

2.8.1 Langmuir isotherm

This assumes that there is no interaction between adsorbed molecules, and the

metal surface is uniform where all the adsorption occurs through the same mechanism.

26

This kind of isotherm mainly emphasizes chemisorption with some exceptions [Paul et

al., 2012; Kıcır et al., 2016] Langmuir adsorption isotherm is expressed below:

1cc

k

(2.14)

where, c is inhibitor concentration and k is the adsorption equilibrium constant.

2.8.2 Temkin isotherm

This assumes that there is a molecular interaction between adsorbed molecules and

metal surface, resulting in a protecting layer which is non uniform. According to this

isotherm, adsorption heat of all those molecules in the layer decreases with an increase in

coverage. This is valid when Θ is between 0.2 - 0.8 [Paul et al., 2012; Nwabanne et al.,

2012; Nnanna et al., 2013] Temkin adsorption isotherm takes the following form:

2.303log 2.303log

2 2

k c

a a

(2.15)

where, a is the attractive parameter.

2.8.3 Frumkin isotherm

It is based on the assumption that the metal surface is heterogeneous and also takes

lateral interaction between adsorbed inhibitor and metal into account. It also considers

multi molecular layer adsorption and has advantages over other isotherms when

explaining about equilibrium [Sharma et al., 2010; Paul et al., 2012; Al-Mhyawi et al.,

2014]. It is given by the following equation:

log 2.303log 21

c k

(2.16)

27

where, α is lateral interaction term.

The adsorption isotherm models are plotted in Figure 2.1 to represent the above

discussed isotherm models and indicate how the parameters can be obtained from the

plots. For example, an equilibrium adsorption constant (k) can be obtained. The k value is

used for determining the type of adsorption, i.e., physical or chemical adsorption.

According to literature [Desimone et al., 2011; Yilmaz et al., 2016 ], when k value

decreases with a rise in temperature, the interaction between inhibitor molecules and the

metal surface is not flexible and its force is strong due to structural formation over the

metal surface.

2.9 Standard Free Energy of Adsorption

Standard free energy of adsorption (ΔG°ads) could be calculated from k using the

following equation,

ln(1000 )adsG Rt k (2.17)

where R is the Universal gas constant (KJ mol-1 K-1), t is the temperature (K), 1000:

molar concentration of water (gL-1). It should be noted that, to have a correct ΔG°ads it is

necessary to use same concentration unit for both water molecules in the above

expression with that of inhibitor concentration. Generally, the common mistake

committed would be using 55.5 mol/L as concentration of water, while the inhibitor

concentration expressed in different units such as mass/volume or volume/volume %

[Noor et al., 2009; Mourya et al., 2014]. Based on the following fact about ΔG°ads , type

of adsorption could be found out as the interaction between metal surface and inhibitor

molecules are electrostatic and known as physical adsorption when ΔG°ads values is -20

KJ mol-1 or more positive. While ΔG°ads is -40 KJ mol-1 or more negative, it could be said

28

(a)

(b)

(c)

Figure 2-1 Adsorption isotherm models: (a) Langmuir isotherm, (b) Temkin isotherm, and

(c) Frumkin Isotherm

c (g/L)

c /

Θ

log c (g/L)

Θ

Θ

log1

c

29

that adsorption between metal surface and inhibitor is due to electron transfer and

formation of covalent bonds. On the other hand, when ΔG°ads values are found between -

20 KJ mol-1 and -40 KJ mol-1, it could be said that adsorption is of mixed type (both

chemisorption and physisorption occurs) [Atkins et al., 2010; Kıcır et al., 2016].

2.10 Arrhenius Plots

Temperature plays a key role in the performance of corrosion inhibitor which

indicates the Arrhenius type of dependence. For example, with an increase in

temperature, there was an exponential increase in corrosion rate in acid solutions due to

the decrease in hydrogen evolution over potential [Popova et al., 2003]. Apparent

activation energy (Ea) could be calculated using the following Arrhenius equation and

Tafel extrapolation method,

log log2.303

acorr

Ei A

Rt (2.18)

where icorr is corrosion current density (A/cm2), Ea is activation energy (KJ/mol), A is

Arrhenius pre-exponential constant. Arrhenius Plots (Type I) were made by plotting

Log icorr against (1

𝑇) (Lebrini, 2011) to produce a straight line as shown in Figure 2.2.

Inhibitors could be further classified into three groups based on its relation with

Ea and inhibition efficiency. They are as follows: (1) With an increase in temperature,

inhibition efficiency decreases and for those inhibitors Ea of inhibited solution was found

to be greater than that of uninhibited solution; (2) No change in inhibition efficiency

irrespective of the change in temperature and for those cases Ea remains same for

inhibited and uninhibited solutions; and (3) Those inhibitors whose inhibition efficiency

30

(a)

(b)

Figure 2-2 Arrhenius Plots: (a) Type I (b) Type II

1/ T (K-1

)

log i

corr

(A/c

m2)

1/ T (K-1

)

log i

corr

/T (

A /

cm2.K

)

31

increases with increase in temperature, Ea of inhibited solution is smaller than that of

uninhibited solution [Radovici et al., 1965].

From Ea values, lot of information related with adsorption type could be inferred

as it indicates the energy barrier related with corrosion process. If Ea value in the

presence of inhibitor was low on comparison with that of the uninhibited solution, it

indicates chemisorption as the energy barrier of corrosion process decreased in the

presence of inhibitor. If Ea in the presence of inhibitor is higher than Ea of the uninhibited

solution then it indicates physisorption as it could be assumed that physical barrier is

formed by formation of adsorptive film of electrostatic nature over the metal surface

reducing the corrosion rate [de Souza et al.,2009 ; Mourya et al.,2014].

2.11 Thermodynamic Properties

The Arrhenius Equation could be expressed as follows [Lebrini et al., 2011]:

exp expa a

corrRt S H

iNh R Rt

(2.19)

where N is Avogadro’s number, h is Planck’s constant, ΔSa is entropy of activation, ΔHa

is enthalpy of activation. When logcorri

t

was plotted against 1

t

straight lines as

Arrhenius Plot (Type: II) are obtained as shown in Figure 2.2. Showing the intercept as

log2.303

aR S

Nh R

and slope as

aH

R

. From this, thermodynamic properties such

as ΔSa and ΔHa could be found.

When ΔHa is negative, inhibitor adsorption is an exothermic process signifying

either physisorption or chemisorption. While ΔHa is positive it reflects inhibitor

32

adsorption is an endothermic process. Endothermic process is generally chemisorption

for inhibition phenomenon [Singh et al., 2010; Lebrini et al., 2011]. For electrolytic

solutions, Ea should be ideally equal to ΔHa for chemical reactions [Mourya et al.,

2014].If ΔSa of inhibited solution is higher than that of uninhibited solution, then it could

be due to the increase in disorder as a result of shift in adsorption from reactants present

in inhibitors to form adsorption complex. On the other hand, if ΔSa of inhibited solution is

lower than that of uninhibited solution, it is due to the ordering of adsorbed molecules in

the presence of inhibitor [Mourya, 2014 & Lebrini, 2011].

2.12 Electrochemical Impedance Analysis

Electrochemical impedance technique provides mechanistic information of a

corrosion using the frequency-dependent response relationship of corrosion process. This

is useful for evaluating corrosion inhibitor performance through impedance diagrams

such as Nyquist plots and Bode-phase angle plots. From Figure 2.3(a), metal and solution

interface properties such as resistance and capacitance are revealed based on size and

shape of curves obtained.

When a depressed semicircle is observed in the Nyquist plot, indicating surface

heterogeneity due to metal surface roughness, resulting in frequency dispersion

attributing to one of the characteristics of solid electrode [Lebrini et al., 2011 & Zhang et

al., 2015] and a non – ideal electrochemical behavior for the metal solution interface.

When the Nyquist plot of an inhibited solution is compared with that of an uninhibited

solution, if there was a change in shape of the semicircle, it could be said that the

mechanism of corrosion process is altered due to the presence of corrosion inhibitor.

33

(a)

(b)

Figure 2-3 : (a) Nyquist plot (b) Bode-phase angle plot along with its typical equivalent

electrical circuit.

Zreal

(ohms)

Zim

ag (

oh

ms)

Frequency

low high

R1 R

2

Frequency (Hz)

Phas

e an

gle

of

Z (

deg

ree)

CPE

34

If the shape of semicircle remains the same but its size changes, it indicates no change in

corrosion mechanism with change in magnitude of anodic and cathodic reaction rate

[Singh et al., 2012; Zhang et al., 2015].

Bode-phase angle plots as shown in Figure 2.3(b) the influence of inhibitor

concentration can be examined. For example, when a single narrow peak was observed in

the Bode-phase plot, it indicates that the corrosion process has a single time constant

while the increase in height of the peak could be attributed due to the presence of

inhibitor molecules and capacitive nature of the metal solution interface [Mourya et al.,

2014]. From the Nyquist plots, charge transfer resistance (Rp) could be obtained from the

difference in impedance values at lower and higher frequencies [Torres et al., 2011].

The double layer capacitance (Cdl) can be calculated using the following relation

[Ferreira, 2016].

max

1

2dl

p

Cf R

(2.20)

where fmax is the frequency at which imaginary component (Y axis) is maximum for a

Nyquist plot. The relationship between Rp and Cdl plays a key role when influence of

inhibitor concentration is investigated. The fact that there was a reverse dependence

observed between Rp and Cdl results in decreasing Cdl, and it indicates the adsorption of

corrosion inhibitors over the metal surface forming a protective film. When Cdl decreases

with increase in inhibitor concentration, it could be due to an increase in thickness of

protective film by reducing the local dielectric constant of the electrical double layer at

metal-solution interface [de Souza et al., 2009; Zheng et al., 2015].

Generally, a corrosion process can be represented as electrical circuit. As the

double layer at the metal-solution interface does not act as a capacitor or resistor, it is

35

replaced by an imaginary element known as the constant phase element (CPE) which can

provide the electrical circuit with an accurate fit and represent parameters related to mass

transfer and energy barrier related with the corrosion process. However, the CPE is

complicated in terms of physical interpretation with various parameters associated with it.

A typical fit for the results obtained from Bode-phase plots is shown in Figure 2.3(b).

The (ZCPE) impedance related with CPE is given by following relation as follows,

ZCPE = Y0-1(jω)-n (2.21)

where Y0 is proportionality coefficient, ω is angular frequency (rad s-1), j is imaginary

number (j2 is -1), and n is exponent used to measure surface inhomogenitiy and related to

the phase shift. The CPE turns into different electrical circuits based on its value of n.

The CPE becomes capacitor, inductor, resistor and Warburg impedance when n is 1,-1, 0,

and 0.5 respectively, (Lebrini et al., 2011; Yadav et al., 2012).

2.13 Theoretical Quantum Chemical Methods

Generally, traditional experimental methods such as weight loss and

electrochemical techniques such as poteniodynamic polarization and electrochemical

impedance are used to evaluate the performance of corrosion inhibitors. Due to time

constraints and cost effectiveness, computer simulation using quantum corrosion

electrochemistry is used as an advanced tool. Quantum chemical methods have been used

by researchers since 1990s to investigate corrosion inhibitors based on their structures.

These methods relate inhibition efficiency with molecular structure and other related

parameters of corrosion inhibitor. For performing these quantum calculations, various

semi-empirical methods have been developed such as PM6 (parameterized model number

36

6), AM1 (Austin model 1), and MNDO (modified neglect of diatomic overlap). Among

these methods, due to the advantage of its accuracy and providing information on

complex molecules at very cheap cost, density functional theory (DFT) has garnered

attention and used as quantum theoretical method in most of the cases [Becke et al.,

1993] is based on Hohenberg–Kohn theorem with electron density as the fundamental

parameter instead of a single electron wave function for expressing the chemical

quantities of the system [Lee, 1988 & Hohenberg, 1964]. The electron density could

reduce the complexity and simplify the many-bodied Schrodinger equation. The DFT is

helpful to simulate even complex molecules to obtain information related with properties,

structure, reactivity, and dynamics for better understanding of reaction mechanisms

[Lesar et al., 2009]. Corrosion inhibitors are adsorbed over metal surface; the DFT can be

used to explain the inhibitor mechanism through analysis of interaction of inhibitor and

the metal surface [Masoud et al., 2010]. Inhibition efficiency is correlated and

rationalized with chemical indices through DFT using the frontier orbital theory [Parr et

al., 1989].

Corrosion inhibition through adsorption over the metal surface is due to the

exchange of electrons from inhibitor molecules through interactions to fill the vacant

spots in the metal surface atoms [Khalil et al., 2003]; this can be also explained in terms

of reactivity through frontier orbital concepts. The selectivity of the chemical reactions

and the ease of interactions are governed by the Lowest Unoccupied Molecular Orbital

Energy (ELUMO), Energy gap (ΔE), Highest Occupied Molecular Orbital Energy (EHOMO),

dipole moment and electron transfer factor (ΔN) [Fukui et al., 1982].

37

EHOMO indicates the electron donating tendency of inhibitor molecules (i.e., donor)

to metal surface (i.e., acceptor) which has a low energy or an empty molecular orbital.

For high inhibition efficiency, EHOMO values should be high to promote adsorption of

inhibitor molecules over the metal surface [Khaled et al., 2008]. The ionization potential

(I) of the molecule is directly related to EHOMO [Gece et al., 2008] as follows:

I = - EHOMO (2.22)

ELUMO denotes the ability of acceptor (metal) to accept electrons from the donor

(i.e., inhibitor) molecules in their vacant orbitals. The ELUMO values should be low for

high inhibition efficiencies and it is directly related to the electron affinity (A) as follows,

A= - ELUMO (2.23)

Electro negativity (χ) for any system is constant and based on mulliken electro

negativity it [Pearson et al., 1988] is given by the following relation.

( )

2

I A

(2.24)

whereas absolute hardness (ŋh) is a variable and has different values. It can be expressed

as:

( )

2h

I A

(2.25)

As the electron transfer is driven due to the difference in electro-negativity of

inhibitor and metal surface molecules, hardness denotes the resistance to this transfer. As

such hard molecules are characterized with low polarizability and small atomic radius.

While, global softness (σ) is simply the inverse of hardness as shown below:

1

h

(2.26)

38

Soft molecules are characterized as high polarizability and low electro-negativity

[Sastri et al., 2001]. Since there is a transfer of electrons due to the interaction of

corrosion inhibitor over the metal surface, the number of electrons transferred (ΔN) could

be calculated using the following relation,

2( )

m i

m i

N

(2.27)

where χm and χi are electro negativity of metal and corrosion inhibitor respectively and

ŋm and ŋi is absolute hardness of metal and corrosion inhibitor, respectively.

In terms of orbital theory, this electron transfer could be described through energy

gap (ΔE) of the molecule based on the fact that, due to electron transfer in a lower

chemical potential, there is a decrease in energy levels. Reactivity of the corrosion

inhibitor molecule is obtained from this ΔE when the adsorption of the inhibitor molecule

over the metal surface occurs [Obi-Egbedi et al., 2011].

In terms of EHOMO and ELUMO, ΔE is given by the following relation,

ΔE = ELUMO - EHOMO (2.28)

Reactivity is inversely proportional to ΔE as less energy is required for the electron

transfer from inhibitor molecules to the vacant spots in metal surface orbital. Apart from

this, hardness (ŋ) and softness (χ) could be explained in terms of ΔE: Large ΔE results in

hard molecules due to large EHOMO and ELUMO gap with high stability. While soft

molecule has low ΔE due to small EHOMO and ELUMO gap with easier polarizability.

Inhibition efficiency is high when the reactivity of inhibitor molecules is high.i.e. ΔE

should be low. This was proved during the evaluation of Xanthione inhibitors based on

39

their inhibition efficiency performance [Obi-Egbedi et al., 2001] using DFT method and

these concepts.

Polarity of a molecule can be used to relate to inhibition performance using the

dipole moment (µ) which arises due to the non-uniform distribution of polarity charges

on atoms present in molecules. It was found that adsorption strength of corrosion

inhibitor molecules over the metal surface increases for high values of µ and that, if there

is positive values for µ,it denotes the adsorption happening between metal and inhibitor

molecules due to physisorption [Obot et al., 2015].

40

CHAPTER 3 EXPERIMENTS

This chapter provides information on materials used, experimental setup and

procedures, and data analysis. The corrosion experiments were based on electrochemical

experiments, which include Tafel extrapolation, poteniodynamic polarization and

electrochemical impedance spectroscopy. The data analysis was used for determining

corrosion rates.

3.1 Experimental Setup

As illustrated in Figure 3.1, an electrochemical corrosion system consisted of a

corrosion cell (microcell) , temperature controlled water bath, a condenser, a series of gas

supply, a poteniostat, pH and conductivity meters , and data acquisition system. The

corrosion cell was a 100 ml jacketed microcell (Model 636-Ring disk electrode (RDE)

assembly, Princeton Applied Research, USA) fitted with three electrodes, i.e., a

cylindrical working electrode, graphite as counter electrode, and a silver/silver chloride

(Ag/AgCl) as reference electrode, respectively. The water bath was equipped with a

heater-circulator for maintaining the temperature of the tested solution within ± 1.0°C of

the desired temperature. The condenser, a water-cooled double piped heat exchanger was

connected to the microcell to prevent excessive evaporation of solution to maintain

solution concentration. The gas supply set was composed of O2, CO2, and N2 gas

cylinders to provide a desired gas composition to the corrosion cell through gas regulator

and flow meters. The computer-controlled poteniostat was the PARSTAT 4000+ from

the Princeton Applied Research, USA. It was connected to the corrosion cell for the

41

Figure 3-1 Schematic diagram of experimental setup for electrochemical corrosion testing

42

electrochemical measurements. The potential and currents were recorded and analyzed

using the Versa Studio software (Princeton Applied Research, USA). A pH meter

(Oakton pH510 series) and a conductivity meter (YSI 3200 conductivity instrument)

were used for measuring pH and conductivity of the test solutions, respectively.

3.2 Materials

3.2.1 Electrodes

Carbon steel 1018 (CS 1018) was chosen as the working electrode (specimen) as

it is a common construction material for process equipment and piping in gas treating

plants. All specimens were cylindrical in shape with dimensions of height, outside

diameter and center hole diameter of 0.80, 1.20, and 0.60 cm, respectively as shown in

Figure 3.2 also indicating the chemical composition of metal specimen (CS 1018) used.

Prior to each test, each specimen was cleaned and surface prepared by polishing it with

600 grit silicon carbide paper using deionized water and then degreased with methanol

and dried with hot air in accordance with ASTM G1-03 (2003) standard.

3.2.2 Chemicals

Four types of chemicals were used in this work such as test solution, chemicals for

titration, specimen preparation chemicals and corrosion inhibitors. The test solution was

monoethanolamine (MEA) chosen because it has been used as the benchmark solvent in

CO2 absorption process due to its reactivity and low cost. The MEA concentration was

5.0 kmol/m3 or 30 wt. % which represents MEA solution’s strength in industries [Kohl et

al., 1997; Luis et al., 2016; Niegodajew et al., 2016].This aqueous solution was prepared

43

Element Composition (wt. %)

C 0.1840

Mn 0.7500

P 0.0110

S 0.0140

Si 0.1700

Pb 0.0040

Sn 0.0150

Cu 0.1000

Ni 0.0800

Cr 0.1000

Mo 0.0230

N 0.0039

V 0.0040

B 0.0002

Nb 0.0010

Ca 0.0017

Ti 0.0010

1.20 cm

0.60 cm

0.80

cm

Figure 3-2 Dimensions and chemical composition of metal specimen used (CS 1018)

44

from MEA and deionized water, and then purged with CO2 gas to achieve a desired CO2

loading or saturation. To this CO2 loaded MEA solution, a measured weight of tested

corrosion inhibitor was added. Inhibitor concentrations (250-10000 ppm) were varied

based on test conditions. The MEA concentration was determined by titrating it against

hydrochloric acid (1 M HCl) with methyl orange as an indicator. The CO2 loading of

MEA solutions was determined using a chittick apparatus [Horwitz et al., 1975] shown in

Figure 3.3. This apparatus is operated based on the fact that for every drop of HCl added

to the solution a corresponding volume of CO2 was liberated. Using the volume of

liberated CO2 loading, the value of CO2 loading in the solution can be calculated. Table

3.1 presents the summary of the chemicals and main chemical compounds found in the

condiments which are used for the experiments. The chemicals used for titration were

hydrochloric acid and the methyl orange while the chemical for specimen preparation

was methanol. The rest of chemicals were used as corrosion inhibitor.

3.3 Experimental Procedure

The corrosion cell was assembled with three electrodes (working electrode

(specimen), a counter electrode, and a reference electrode) and fitted with a condenser.

The prepared MEA solution is charged into the corrosion cell and purged with CO2 to

control the CO2 loading of the solution. The temperature of solution was raised gradually

to the prespecified value by circulating hot water through the outer jacket of the corrosion

cell. Then, the corrosion cell was connected electrically to the poteniostat. Using the

Versa Software, open circuit potentials (OCP) were recorded.

45

Figure 3-3 : Chittick apparatus - Carbon dioxide loading and MEA concentration

measurement

46

Table 3.1 Summary of the chemicals used

Chemical Formula Supplier

Monoethanolamine C2H7NO Sigma Aldrich

Hydrochloric acid HCl Sigma Aldrich

Methyl orange C14H14N3NaO3.S Sigma Aldrich

Methanol CH4O Fischer-Scientific

Sulfa pyridine C11H11N3O2S Sigma Aldrich

Sodium Metavandate NaVO3 Sigma Aldrich

Turmeric powder - Curcumin C21H20O6 McCormick Canada

Garlic powder

Allicin

Diallyl Sulfide

C6H10OS2

C6H10S

McCormick Canada

Onion powder

Dipropyl disulfide

Quercetin

C6H14S2

C15H10O7

Club House

Mustard powder

Benzyl isothiocyanate

Sinigrin

C8H7NS

C10H16KNO9S2

Club House

Horse radish powder

Allyl isothiocyanate

Peroxidase

C4H5NS

C14H16N2O2

S&B Selected

Spices

47

as a function of time against the reference electrode until a steady state value was reached

at the corrosion potential. After that, electrochemical impedance spectroscopy (EIS)

studies were performed at the open circuit potential using AC signals of 10 mV amplitude

for the frequency ranging from 0.01 Hz to 10 kHz.

Open circuit potentials were then recorded to check the stability of the system

before DC poteniodynamic cyclic polarization studies, where a scan rate of 0.166 mV/s

was initiated. When the scan is completed, all the experimental data and polarization

curves were recorded and stored. Finally, the pH and temperature were measured

followed by taking samples for determining MEA concentration and CO2 loading at the

end of the experiment after breaking the seal of corrosion cell.

3.4 Data Analysis

3.4.1 Tafel extrapolation method

From the data of poteniodynamic polarization experiment, a graph is plotted with

log i (log current density) against the potential. Using this method as illustrated in Figure

3.4, the Tafel extrapolation method was employed to determine the corrosion current

density (icorr) through which the corrosion rate could be calculated as shown below:

CR 0.13 ( . )corri E W

d (3.1)

where CR is Corrosion rate, miles per year (mpy), icorr is corrosion current density,

(µA/cm2), E.W is equivalent weight of corroding species,(g),and d is density of corroding

species,(g/cm3),Inhibition efficiency (IE) could be calculated as follows:

.uninhibited inhibited

uninhibited

CR CRI E

CR

(3.2)

48

Figure 3-4: Tafel extrapolation methods

E

(V

vs

Ag/A

gC

l)

Ecorr

β

a

(Anodic Tafel slope)

βc

(Cathodic Tafel slope)

Log icorr

(A/cm2)

49

Where CRuninhibited is corrosion rate when the system is without inhibitors And CRinhibited is

corrosion rate of the system in the presence of inhibitors. However, due to the application

of large over potential, this method is damaging to the corroding metal. There is no

ASTM standard in selecting the potential range when taking corrosion potential (Ecorr)

into account for the poteniodynamic polarization curves [Baboian et al., 2005].

3.4.2 Pitting tendency

When the poteniodynamic cyclic polarization scanned in forward and reverse

direction, the pitting tendency under the given tested conditions could be predicted. It is

based on the following concept [Gunasekaran et al., 2012]:

Direction of the reverse scan lies to Hysteresis Pitting tendency

Left of the forward scan Negative No

Right of the forward scan Positive Yes

This could be better explained with the help of Figure 3.5. Various information

related with electrochemical kinetic parameters such as primary passivation potential

(Epp), passivation current density (ipass ) and breakdown potential(Ebd) is obtained .Apart

from this, using Versa software parameters such as corrosion rate, icorr ,Ecorr and Tafel

slopes could be obtained.

50

3.4.3 EIS analysis

Using the data from EIS technique plots such as Nyquist plots was made. From

this plot, important parameters such as Rp (charge transfer resistance) were obtained from

the semicircles related with the Nyquist plots. Size and shape of this semicircle provide

information about the corrosion.

From Rp values, inhibition efficiency (I.E) was calculated using the formula

below:

( ) ( )

( )

.p i p blank

p i

R RI E

R

(3.3)

Where Rp(i) is the charge transfer resistance in the presence of inhibitor, and

Rp(blank): charge transfer resistance in the absence of inhibitor. Apart from this, double

layer capacitance (Cdl) could be calculated using the relation as shown in Equation (2.20)

51

(a)

(a)

(b)

Figure 3-5: Pitting tendency from poteniodynamic polarization curves

(a) Pitting (b) No Pitting

(+)

(-)

Potential

Log icorr

Active

Passive

Transpassive

(+)

(-)

Potential

Log icorr

Active

Passive

Transpassive

52

CHAPTER 4 RESULTS AND DISCUSSIONS

This chapter presents results of the electrochemical corrosion experiments that

were carried out to evaluate the inhibition performance of five condiments including

powders of garlic, mustard, horseradish, anion, and turmeric. The evaluation began with

obtaining a set of baseline corrosion data of the carbon steel (CS1018) immersed in the

uninhibited monoethanolamine (MEA) solution (i.e., containing no corrosion inhibitor)

saturated with carbon dioxide (CO2). These data were reported in respect of the effects of

process parameters, including the presence of oxygen (O2) in feed gas, solution

temperature, and the presence of process contaminants. Once the baseline uninhibited

corrosion data were in place, the inhibition performance of the tested condiments was

evaluated under a range of test conditions that enabled the study of parametric effects on

inhibition performance as shown in Table 4.1. The obtained corrosion data were

subsequently analyzed to gain an understanding of inhibition behavior and mechanism of

all tested condiments.

4.1 Uninhibited System

4.1.1 Effect of O2 Concentration in Feed Gas

The O2 concentration in feed gas is an important process parameter that may

contribute to corrosion of carbon steel in the amine-based CO2 absorption process.

Contradicting findings on the effect of O2 on corrosion were reported in literature. No

findings confirm whether the O2 promotes or inhibits corrosion [Pearson et al., 2013].

53

Table 4.1 Summary of the parameters and experimental test conditions

Parameters Test condition

Tested material Carbon steel (CS 1018)

Amine type Monoethanolamine (MEA)

MEA concentration (kmol/m3) 5.0 ± 0.1

CO2 loading Saturated (up to 0.55 mol CO2/mol of MEA)

O2 in feed gas (%) 0, 15

Inhibitor concentration (ppm)

200, 250, 500, 1000, 1500, 2000, 4000, 6000, and

10000

Temperature (°C) 40, 60, 80

Process contaminants Chloride and oxalate

54

Therefore, in this work, corrosion of carbon steel (CS 1018) was studied in the 5.0

kmol/m3 MEA solutions saturated with CO2 at 80°C was experimentally tried at two

different conditions, i.e. 0 and 15 vol.% O2 in feed gas (referred to as the absence and

presence of O2, respectively). The cyclic polarization curves for the MEA-CO2 solutions

with and without O2 show that the carbon steel in both conditions was in the active state

(as illustrated in Figure 4.1 as an example). With the increase in potential, the carbon

steel progressed from active to passive and eventually transpassive state. The negative

hysteresis was developed, indicating that the carbon steel had no pitting tendency.

Figure 4.2 (a) shows that the open circuit potential (OCP) of the MEA solution

exposed to O2 was lower and reached more quickly than that of the solution without O2.

The poteniodynamic polarization curves in Figure 4.2(b) indicate that, with the presence

of O2, there was an increase in icorr along with a shift in cathodic polarization side. This is

also well supported by a decrease in CO2 loading and an increase in conductivity of the

solution (Table 4.2). From these observations, it could be inferred that there was more

iron dissolution in anodic side and a small change in cathodic reactions favoring more

corrosion of the metal specimen. This is further justified when the corrosion rate of the

metal specimen was compared at both conditions as shown in Figure 4.2(c) indicating in

the presence of oxygen, corrosion process was enhanced.

55

Figure 4-1: Polarization corrosion behavior of uninhibited MEA solution in the presence

of oxygen (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading)

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

56

Figure 4-2: Corrosion behavior comparison of uninhibited MEA solution in the presence

and absence of oxygen (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading)

(a) Open circuit potential, (b) Tafel plot, and(c) Corrosion rate comparison

(Original in color)

(a) (b)

(c)

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

Vvs

Ag /

AgC

l)

log icorr (A/cm2)

Presence of oxygen

Absence of oxygen

0

1

2

3

4

Absence of oxygen Presence of oxygen

Co

rro

sio

n r

ate

(mm

py)

-0.8

-0.6

-0.4

-0.2

-10 40 90 140

E (

V v

s A

g /

AgC

l )

Time(s)

Absence of oxygen

Presence of oxygen

57

Table 4.2

Table 4.2 Summary of experimental and electrochemical parameters for uninhibited systems

Experimental Condition

5 kmol/m3

satd.CO2 loading

pH Conductivity

mS/cm

Ecorr,

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

Rp

Ωcm2

Pitting

tendency

80°C,absence of oxygen 8.11

±0.02

75.04

±0.07

-697.05

±2.45

317.45

±0.03

144.69

±2.15

87.24

±4.80

3.75

±0.03 - No

80°C,presence of oxygen

15 vol.% O2 8.16

±0.04

82.09

±0.04

-748.31

±1.85

360.41

±0.04

110.62

±1.85

73.19

±5.20

4.26

±0.01 - No

presence of 15 vol.% O2, 40°C 7.77 53.33 -738.48 138.21 135.04 219.35 1.63 239.44 No

presence of 15 vol.% O2, 60°C 7.98 64.56 -740.44 142.14 92.07 111.20 1.68 138.43 No

presence of 15 vol.% O2, 80°C 8.16

±0.04

82.09

±0.04

-748.31

±1.85

360.41

±0.04

110.62

±1.85

73.19

±5.20

4.26

±0.01 60.25 No

80°C, presence of 15 vol.% O2,

Chloride 8.03 87.31 -737.48 235.00 100.22 77.46 2.78 84.64 No


Oxalate 8.05 73.87 -763.20 212.25 57.09 28.44 2.51 44.56 No


Formate 8.08 84.10 -756.08 199.11 70.86 51.59 2.35 59.75 No


Thiosulfate 8.08 74.70 -681.90 17.50 124.63 67.20 0.21 294.08 No

58

4.1.2 Effect of temperature

The corrosion behavior of carbon steel (CS 1018) immersed in the 5.0 kmol/m3

MEA solutions saturated with CO2 was studied at various temperatures, i.e., 40, 60, and

80°C. Results show that as the temperature increased, there was a continuous rise in the

conductivity of the solution. This indicates that anodic metal dissolution was rapidly

increased with the increase in temperature. The pH of the solution became more alkaline.

There was a slight anodic shift observed in Ecorr with the increase in temperature as

shown in the Figure 4.3(a). From Figure 4.3(b), the corrosion rate for the uninhibited

solution increased with temperature. This is also further justified as icorr was increased

with temperature while βc decreased with the increase in temperature. The decrease in βc

was due to the oxidizer reduction reaction at the cathodic side which promotes corrosion.

However, there was no pitting tendency observed under any circumstances.

When the Nyquist plots were compared, it was observed that size of the

semicircle decreases with the increase in temperature as shown in Figure 4.4(b). This is

well supported from the trend observed for Rp comparisons in Figure 4.4(c). This

indicates that resistance for the corrosion reaction was decreased with temperature. In

addition, the temperature also plays a key role in degradation of amine solution. This was

evidenced by the changes in solution color (Figure 4.5). Bode phase plots in Figure 4.6

(a) was found in good agreement when the data were fitted to the electrical circuit (R

(QR) (Q(R (LR)))) as shown in Figure 4.6(b). The parameter values shown in Appendix

indicate the inverse relationship between the resistors present in the circuit and the

temperature.

59

Figure 4-3: Corrosion behavior comparison of uninhibited MEA solution under the influence of

temperature (5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15 vol. % O2)

(a) Tafel plot (b) Corrosion rate (Original in color)

(a)

(b)

-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l )

log icorr(A/cm2)

80 °C

60 °C

40 °C

0

1

2

3

4

5

40 60 80

Co

rro

sio

n r

ate

(mm

py)

Temperature (°C)

60

(a) (b)

(c)


temperature (5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15vol. %O2)

(a) Conductivity (b) Nyquist plot, and (c) Rp (Original in color)

0

20

40

60

80

40 60 80

Co

nd

uct

ivit

y

(mS

/cm

)

Temperature(°C)

0

50

100

150

200

250

300

40 60 80

RP

(ohm

s)

Temperature (°C)

0

50

100

150

200

250

300

0 100 200 300

Zim

(ohm

s)

Zre (ohms)

80°C

60°C

40°C

0

2

4

6

8

0 20 40

Zim

(oh

ms)

Zre (ohms)

61

Before After

Before (a) After

(b)

Figure 4-5: Photos (before and after experiment) comparison of uninhibited MEA

solution under the influence of temperature (5.0 kmol/m3 MEA, 40-80°C, saturated CO2

loading, presence of 15vol.% O2) (a) 80°C (b) 40°C (Original in color)

62

(a)

(b)

Figure 4-6: Corrosion behavior of uninhibited MEA solution under the influence of temperature

(5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15vol. % O2) (a) Bode-phase

plot comparison (b) Equivalent electrical circuit (Original in color)

0

10

20

30

40

50

60

0.01 1 100 10000

Phas

e an

gle

(d

egre

e)

log frequency (Hz)

40°C

60°C

80°C

63

There are two peaks where one of the peaks is more visible with the rise in

temperature indicating two constant phase elements in the corrosion mechanism. The

Arrhenius plots in Figure 4.7 were developed to obtain thermodynamic properties such as

Ea, ΔHa° and ΔSa

° using two different plots as mentioned previously in Chapter 2. The

positive value of ΔHa° indicates that the metal dissolution reaction is endothermic, and

other data are useful in assessing the corrosion inhibition performance. As the corrosion

rate for this environment is dependent on temperature, the performance of inhibitor needs

to be checked under these temperatures for its recommendation in industrial use.

4.1.3 Effect of process contaminants

The effect of process contaminants including formate, chloride, oxalate, and

thiosulfate, was examined using carbon steel (CS1018) in the 5.0 kmol/m3 MEA-CO2

solution with 15 vol. % O2 at 80°C. The pH of the system indicates the effect of process

contaminants in the uninhibited system. It is apparent that an addition of process

contaminants to the system lowers the pH towards acidic. The pH of the solution

containing no process contaminants is greater than formate, thiosulfate, oxalate, and

chloride. On this basis, it could be said that chloride makes the system more acidic when

compared with no process contaminant containing solutions, making it more favorable

for corrosion reaction. In general, the conductivity of the system changes accordingly

with pH, which is not the case here. The order in which conductivity was decreased is as

follows: Chloride > Formate > No process contaminants > Thiosulfate > Oxalate.

Chloride with more conductivity indicates the condition favorable for anodic metal

dissolution.

64

(a)

(b)

Figure 4-7: Arrhenius Plots for uninhibited MEA solution under the influence of temperature

(5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15 vol. % O2)

(a) Type I (b) Type II

R² = 0.7422

0.00

0.50

1.00

1.50

2.00

2.50

3.00

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

(µA

/cm

2)

1/T x103 (K-1)

Ea = 21.57 KJ/mol

R² = 0.688

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

/T (

µA

/cm

2.K

)

1/T x103 (K-1)

ΔH0a = 18.82 KJ/mol

ΔS0a = - 161.23 J/mol.K

65

From the poteniodynamic polarization curves in the Figure 4.8(a), it was apparent

that Ecorr shifted towards anodic for chloride and thiosulfate, but other way around for

oxalate and formate when compared with no process contaminant system. This was

further explained by βa and βc. For chloride and thiosulfate, βa was greater than the no

process contaminant condition while βc was less than that. This proves that corrosion

reaction equilibrium was shifted due to the influence of process contaminants on the

anodic metal dissolution reaction. Both βa and βc were lower than those in the MEA

solutions containing oxalate and formate. However, there was no significant impact in the

corrosion rate of the system as shown in Figure 4.8(b). The corrosion rates of carbon

steel in MEA-CO2-O2 solution are as follows: no process contaminants > chloride >

oxalate > formate > thiosulfate. In the case of thiosulfate, the corrosion rate was low

similar to a behavior of an inhibitor due to the formation of a protective film over the

metal surface by the sulfur atoms present in the thiosulfate molecule. However, there was

no pitting tendency observed in any of the cases. The Nyquist plots in Figure 4.9(a) show

that there was no change in shape of the semicircle for all the cases except thiosulfate.

The Rp in Figure 4.9(b) decreases in the following order: thiosulfate > chloride > no

process contaminant > formate > oxalate. This suggests that thiosulfate acts as an

inhibitor forming a black protective layer over the metal surface as shown in Figure 4.10.

This was also reported by [Suresh et al., 2012] but not found effective as an inhibitor

when tested for long term exposure. From Figure 4.10, it was also evident that the color

formation in amine was due to the presence of formate as a process contaminant in amine

solution. Based on the high Rp and corrosion rate, oxalate and chloride were chosen to be

tested process contaminants along with the inhibitors in the following sections.

66

(a)

(b)


process contaminants (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, presence of 15vol. % O2)

(a) Tafel plot and (b) Corrosion rate (Original in color)

-0.95

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

-0.60

1.00E-07 1.00E-05 1.00E-03 1.00E-01

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Chloride

Oxalate

Formate

Thiosulfate

0

1

2

3

4

5

Co

rro

sio

n r

ate

(mm

py)

67

(a)

(b)

Figure 4-9: Comparison of uninhibited MEA solution under the influence of process

contaminants (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, presence of 15 vol. % O2)

(a) Nyquist plot (b) Rp (Original in color)

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Zim

(ohm

s)

Zre (ohms)

Uninhibited

Chloride

Oxalate

Formate

Thiosulfate

0

50

100

150

200

250

300

RP

(ohm

s)

68

(a) (b)

(c) (d)

Figure 4-10: Photos of uninhibited MEA solution at the end of experiment under the influence of

process contaminants (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, presence of 15 vol.% O2)

(a) Chloride, (b) Oxalate, (c) Thiosulfate, and (d) Formate (Original in color)

69

4.2 Inhibited Systems

4.2.1 Garlic

4.2.1.1 Effect of O2

The poteniodynamic polarization curves in the Figures 4.11(a and b) indicate that

the presence of O2, increases icorr and Tafel slopes along with a shift in cathodic

polarization side. The pH and conductivity are also found to increase in the presence of

oxygen. The open circuit potentials were compared as shown in Figure 4.11(c), a shift

towards anodic side was observed similar to that of uninhibited systems. The inhibition

efficiencies were compared at both conditions as shown in Figure 4.11(d) and there was

no great change observed in the performance of inhibitor due to the presence of oxygen.

As such, the powder of garlic performs well and the presence of O2 does not have any

impact on its inhibitor performance.

4.2.1.2 Effect of inhibitor concentration

The inhibitor concentration is an important factor because pitting corrosion often

occurs when an insufficient quantity of inhibitors is provided in the solution. In this work,

the garlic concentration was varied from 200 to 10,000 ppm. The test condition was

carbon steel (CS 1018) immersed in the 5.0 kmol/m3 MEA solutions saturated with CO2

and containing 15% O2 at 80°C. Results in Table 4.3 show there was no change in pH of

the solution when the inhibitor concentration was varied. The conductivity of solution

however decreased with increasing inhibitor concentration.

70

(a) (b)

(c) (d)

Figure 4-11: Comparison of garlic inhibited MEA solution in the presence and absence of

oxygen (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, 2000 ppm inhibitor concentration)

(Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Garlic

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Garlic

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

0 50 100 150 200

E (

V v

s A

g /

AgC

l)

Time(s)

Absence of Oxygen

Presence of Oxygen

0

10

20

30

40

50

60

70

80

90

100

Absence of

oxygen

Presence of

oxygen

Inhib

itio

n e

ffic

iency

(%

)

71

Table 4.3 Summary of experimental and electrochemical parameters for garlic inhibited systems


5 kmol/m3,satd.CO2 loading pH

σ

mS/cm

Ecorr

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

CP

I.E

(%)

Pitting

tendency

Rp

Ωcm2

EIS

I.E

(%)

80°C,absence of oxygen,

inhibitor conc: 2000 ppm 8.07

±0.02

76.63

±0.08

-712.60

±2.65

13.27

±1.89

46.70

±2.10

119.78

±4.10

0.16

±0.08 95.83

Yes - -

80°C,presence of 15 vol.% O2 inhibitor conc: 2000 ppm

8.14

±0.04

78.03

±0.07

-759.50

±3.25

25.74

±1.15

52.96

±4.20

123.20

±3.60

0.30

±0.04 92.85

No - -

80°C,

15 vol.% O2

250 8.18 78.29 -734.94 26.13 39.46 122.18 0.31 92.75 No 112.55 46.47

500 8.23 78.88 -734.88 23.02 35.14 127.61 0.27 93.61 No 231.73 74.00

1000 8.16 80.65 -748.09 24.95 48.37 128.51 0.29 93.07 No 263.92 77.17

2000 8.14

±0.04

78.03

±0.07

-759.50

±3.25

25.74

±1.15

52.96

±4.20

123.20

±3.60

0.30

±0.04 91.85 No 399.47 84.91

4000 8.23 78.51 -754.45 37.13 67.93 158.17 0.44 89.69 No 224.67 73.18

10000 8.18 77.67 -760.75 27.52 54.20 127.12 0.32 92.35 No 337.02 82.12

15 vol.% O2 Inhibitor conc.:

2000 ppm

40°C 7.76 52.52 -714.69 4.69 41.29 169.19 0.05 96.61 No 1536.33 84.42

60°C 7.94 67.24 -755.96 10.21 57.13 140.58 0.12 92.82 No 860.22 83.91

80°C 8.14

±0.04

78.03

±0.07

-759.50

±3.25

25.74

±1.15

52.96

±4.20

123.20

±3.60

0.30

±0.04

8.14

±0.04 No 399.47 84.92

80°C,

15 vol.% O2

Inhibitor conc.:

2000 ppm

Chloride 8.03 92.27 -722.38 24.08 58.74 123.03 0.28 89.75 No 246.07 65.60

Oxalate 8.14 81.45 -726.85 47.09 64.86 155.79 0.55 77.82 No 265.97 83.24

72

From the poteniodynamic polarization curves in Figure 4.12(a), the inhibitor

retarded both the anodic and cathodic reactions, resulting in low icorr. This is evident in

Figure 4.12(b) when the inhibition efficiency based on cyclic polarization technique was

in the range of 90%. There was no pitting tendency observed for all inhibitor

concentrations and there was no linearity relationship between the performance and

inhibitor concentration. When the Nyquist plots in Figure 4.13(a) were compared

between the uninhibited and inhibited MEA solutions, it was observed there was a change

in the shape of semicircle with increases in diameter of those semicircles. This suggests

that the corrosion mechanism is affected when garlic is used as inhibitor. This is made

further clear when Rp were compared based on inhibitor concentration as shown in the

Figure 4.13(b). The bode phase plots indicate that there is a one large and one small

phase angle peaks visible for most of the garlic inhibited solutions as shown in the Figure

4.14(a). This shows that there are two time constants phase elements. When the EIS data

were analyzed, they were found fitting for the electrical circuit (R(QR)(Q(R(LR))))

shown in Figure 4.14(b). It was also found that one of the resistor values in that circuit

increased on comparison with the uninhibited solution whenever garlic was used as the

inhibitor. This indicates garlic provides resistance to the corrosion reaction when used.

73

(a)

(b)

Figure 4-12: Comparison of garlic inhibited MEA solutions for Inhibitor concentrations (250

ppm to 10000 ppm) (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, 15 vol. % O2)

(a) Polarization behavior (b) Inhibition efficiency (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited250 ppm500 ppm1000 ppm2000 ppm4000 ppm10000 ppm

0

10

20

30

40

50

60

70

80

90

100

250 500 1000 2000 4000 10000

Inhib

itio

n e

ffic

iency

(%

)


74

(a)

(b)

Figure 4-13: Comparison of garlic inhibited MEA solutions for inhibitor concentrations (250 to

10000 ppm) (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, 15 vol. % O2)


0

50

100

150

200

250

300

350

400

450

500

RP

(ohm

s)


0

50

100

150

200

250

300

350

400

450

500

0 100 200 300 400 500

Zim

(ohm

s)

Zre (ohms)

uninhibited250 ppm500 ppm1000 ppm2000 ppm4000 ppm

0

2

4

6

8

0 10 20 30

Zim

(ohm

s)

Zre (ohms)

75

(a)

(b)

Figure 4-14: Corrosion behavior of garlic inhibited MEA solutions for inhibitor concentrations

(250 ppm to 10000 ppm) (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, 15 vol. % O2)

(a) Bode phase plot comparison (b) Equivalent electrical circuit (Original in color)

0

10

20

30

40

50

60

70

0.01 0.1 1 10 100 1000 10000

Phas

e an

gle

(d

egre

e)

log frequency (Hz)

250 ppm

Uninhbited

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

76

When the Tafel slopes were compared as shown in Figure 4.15(a), to understand

the type of corrosion inhibition mechanisms, it was found that, with increasing garlic

concentration, βa decreased, while the βc performed the other way around, indicating

garlic as a mixed type corrosion inhibitor. To understand the adsorption mechanism,

electrochemical data were fitted for different isotherms and it was found that garlic

followed Langmuir adsorption isotherm as shown in Figure 4.15(b).The standard free

energy of adsorption (ΔG°ads) was found to be in -32.87 KJ/mol indicating mixed

adsorption involving both physisorption and chemisorption between inhibitor molecule

and the metal surface.

4.2.1.3 Effect of temperature

The Poteniodynamic polarization curves were compared for the garlic inhibited

and uninhibited condition for various temperatures as shown in Figure 4.16(a). It was

apparent that there was a shift in Ecorr from anodic to cathodic side with the increase in

temperature. Apart from this as observed in the uninhibited solution, the same trend was

found for pH and conductivity with the rise in temperature. There was no pitting

tendency observed for inhibitor at any of the temperature range. The icorr increased with

temperature, reflecting a higher corrosion rate. On the other hand, there was an inverse

linear relationship observed between βc and the temperature. Inhibitor efficiency obtained

from cyclic polarization (over the range of 90%) decreased with the increase in

temperature as shown in Figure 4.16(b).

77

(a)

(b)



(a) Tafel slope comparison (b) Langmuir adsorption isotherm

0

20

40

60

80

100

120

140

160

0 2000 4000 6000 8000 10000

β(m

V/d

ecad

e)


βa Garlic

βc Garlic

R² = 0.9998

0

2

4

6

8

10

12

0 2 4 6 8 10 12

c/Θ

Inhibitor Concentration (g/L)

ΔG°ads = - 32.87 KJ/mol

78

(a) (b)

(c)

Figure 4-16: Corrosion behavior of garlic inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15vol.% O2) Tafel plot

comparison (a) 40°C, (b) 60°C,and (c) 80°C (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Garlic

-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Garlic

-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

uninhibited

Garlic

79

(a) (b)

(c)

Figure 4-17: Comparison of garlic inhibited MEA solutions under the influence of temperature

(5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15 vol. % O2, 40-80°C)

(a) Inhibition efficiency, (b) Nyquist plot, and (c) Rp

0

10

20

30

40

50

60

70

80

90

100

40 60 80

Inhib

itio

n e

ffic

iency

(%

)

Temperature ( ° C )

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

Zim

(ohm

s)

Zre (ohms)

80°C

60°C

40°C

0

200

400

600

800

1000

1200

1400

1600

40 60 80

RP

(ohm

s)

Temperature (°C)

Garlic

Uninhibited

80

The Nyquist plots for garlic inhibited solutions were obtained for the temperature

effect as shown in Figure 4.17(a). Results show that, similar to that uninhibited solution

at 40°C, the garlic inhibited solution yields a larger semicircle whereas the one obtained

for 80°C was smaller. This was clearly evident when Rp of garlic inhibited solution were

compared as shown in Figure 4.17(b). Since there was no change in shape of the

semicircle, it could be said that temperature did not affect the performance of the

inhibitor. Inhibitor efficiency based on EIS was more or less same with inhibitor

efficiency based on cyclic polarization. The Arrhenius plots were made as shown in

Figure 4.18 to obtain thermodynamic properties. Based on Ea values obtained, it could be

inferred that there was an increase in energy barrier for the corrosion process when

compared with that of uninhibited solution and positive value of ΔHa° indicates that

dissolution reaction is endothermic while dissolution of the metal is difficult and slow in

the presence of the inhibitor. The higher value of ΔSa indicates that it could be due to the

increase in disorder as a result of shift in adsorption from reactants present in inhibitors to

form adsorption complex.

4.2.1.4 Effect of process contaminants

The garlic inhibited solution was tested with process contaminants, i.e., chloride

and oxalate, to understand their impact on the performance of the inhibitor. When the

poteniodynamic polarization curves are compared as shown in Figure 4.19(a), Ecorr for

garlic inhibited solution in the presence of both oxalate and chloride shifted towards

anodic compared to no process contaminant condition. When examined further, Tafel

81

(a)

(b)

Figure 4-18: Arrhenius Plots for garlic inhibited MEA solutions under the influence of temperature

(5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15 vol. % O2)


R² = 0.7422

R² = 0.9851

0.00

0.50

1.00

1.50

2.00

2.50

3.00

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

(µA

/cm

2)

1/T x103 (K-1)

Uninhibited

Garlic

Ea = 41.98 KJ/mol

R² = 0.688

R² = 0.9833-2.00

-1.50

-1.00

-0.50

0.00

0.50

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

/T (

µA

/cm

2.K

)

1/T x103 (K-1)

Uninhibted

Garlic


ΔS0a = - 123.44 J/mol.K

82

(a)

(b)

Figure 4-19: Comparison of garlic inhibited MEA solutions under the influence of process contaminants

(5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, presence of 15 vol. % O2)

(a) Tafel plot (b) Corrosion rate

-1.05

-0.95

-0.85

-0.75

-0.65

-0.55

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

No process contaminants

Chloride

Oxalate

0

1

2

3

4

5

No Process contaminants Chloride Oxalate

Co

rro

sio

n r

ate

(mm

py)

Uninhibited

Garlic

83

slopes for both cases were little different. In the presence of oxalate, βa and βc were larger

for garlic inhibited solution than the no process contaminant and uninhibited oxalate

condition. Whereas, in the presence of chloride, garlic inhibited solution’s βa was larger

than βa of no process contaminant condition, while βc was less than its counterpart. This

situation was reverse when compared for uninhibited chloride solution, i.e., βa of garlic

inhibited solution was less than uninhibited chloride condition’s βa. However, there was

no pitting observed in any of the cases. Oxalate had a little impact on the inhibition

performance of garlic as compared with chloride. The impact was not as significant as the

corrosion rate of garlic inhibited solution with the presence of oxalate, was more than the

no process contaminant condition and whereas less than the uninhibited oxalate

condition. This is shown in Figure 4.19(b). The increase in corrosion rate could be due to

the anodic metal dissolution as even though pH of the solution increased towards

alkaline, it was overcome by the iron metal dissolution as a result there was an increase in

conductivity of the solution with Ecorr shift towards anodic with more βa on comparison

with no process contaminant condition. It shows oxalate has some influence on the

inhibition performance of garlic. However, in the presence of chloride, the garlic

inhibited solution had less corrosion rate compared for both condition. For chloride, there

was no change observed for its pH. EIS (Nyquist Plots) was compared as shown in the

Figure 4.20(a). The size of the semicircle for the garlic inhibited solution in presence of

both chloride and oxalate was less than that of no process contaminant condition.

However, there was no change in shape of the semicircle observed indicating there was

no change in inhibition mechanism due to the presence of these process contaminants.

84

(a)

(b)

Figure 4-20: Comparison of garlic inhibited MEA solutions under the influence of process contaminants

(5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, presence of 15 vol. % O2) (a) Nyquist plot (b) Rp

(Original in color)

0

25

50

75

100

125

150

0 25 50 75 100 125 150

Zim

(ohm

s)

Zre (ohms)

No process contaminantsChlorideOxalate

0

50

100

150

200

250

300

350

400

450


RP

(ohm

s)

Uninhibited

Garlic

85

This was further supported by comparing Rp as shown in Figure 4.20(b).It was observed

that Rp of garlic inhibited solution was less when compared with no process contaminant

condition, but Rp was more than their corresponding uninhibited solution with

contaminants.

4.2.1.4 Quantum chemical analysis

Quantum studies of corrosion inhibitors based on their chemical structure is

useful in assessing their performance and understands the underlying inhibition

mechanism. In this case, condiments are used as corrosion inhibitors. Since they do not

have a specific chemical structure, it could be better assumed to do quantum studies on

the main constituents of chemicals present in it. According to literature [Lanzotti, 2006],

the garlic powder mainly consists of Allicin and Diallyl sulfide. The quantum analyses of

these compounds have been obtained and parameters related with them were obtained as

shown in Table 4.4, it could be understood that EHOMO values on comparison with MEA

were higher, indicating the tendency to donate electrons to metal atom also ELUMO values

of those compounds was less on comparison with MEA indicating higher corrosion

inhibition efficiency. Energy gap (ΔE) was lower when compared with MEA indicates

the binding ability of the inhibitor over the metal surface. A higher electronegativity (χ)

and lower hardness (γ) of these compounds on comparison with MEA are the

characteristics of good inhibitors. For this compounds, fraction of electrons transferred

(ΔN) was less than 3.6 and this shows they could donate electrons to the metal surface for

forming an adsorbed layer to prevent corrosion. The optimized structures of these

compounds are shown in Figures 4.21(a and b), respectively. It is clearly visible this

86

structure has sulfur atoms with different functional groups. From Figures 4.21(c and d)

electron densities in HOMO level are more surrounding the sulfur atoms and same is

observed for LUMO levels from Figures 4.21(e and f) indicating the presence of sulfur

atom is responsible for the inhibition quality of garlic.

Table 4.4 Summary of quantum chemical analysis of chemicals related with garlic

Properties MEA Allicin Diallyl sulfide

EHOMO -6.77 -6.56 -6.17

ELUMO -0.31 -1.71 -0.68

Energy gap (ΔE) 6.46 4.85 5.48

Dipole moment 1.04 2.54 1.69

I 6.77 6.56 6.17

A 0.31 1.71 0.68

Electro-negativity (χ) 3.54 4.14 3.43

Hardness (γ) 3.23 2.42 2.74

(χ)Metal (Fe) 7.00 7.00 7.00

Absolute hardness of Fe

(Hardness (γ)) 0.00 0.00 0.00

Fraction of electrons

transferred (ΔN) 0.54 0.58 0.65

87

Allicin Diallyl sulfide

(a) (b)

(c) (d)

(e) (f)

Figure 4-21: Quantum Chemistry structures for Allicin and Diallyl Sulfide

(a and b) Optimized molecular structures, (c and d) HOMO, and (e and f) LUMO (Original in color)

88

4.2.2 Mustard


When compared with the presence of oxygen, there is an increase in icorr which

ultimately resulted in increase in corrosion rate with a slight decrease in anodic Tafel

slope with an increase in cathodic Tafel slope. This is clearly shown in Figures 4.22 (a

and b) . There is an increase in pH of inhibited solution due to the presence of oxygen.

When the open circuit potentials were compared as shown in the Figure 4.22(c), there

was a shift towards anodic side observed in the presence of oxygen similar to the trend

observed for uninhibited systems. The presence of oxygen does not affect the

performance of the inhibitor as it is obvious from Figure 4.22(d).


The inhibitor concentration of mustard was varied between 250 ppm to 10000

ppm. When the experimental conditions were monitored to examine the effect of

inhibitor concentration with performance, there were some interesting facts found. With

the increase in inhibitor concentration the pH of the solution increased, i.e., it turns to be

more alkaline in nature and the same trend was observed for the conductivity of the

solution. This type of conditions was more favorable for changing the nature of the

corrosive environment making the inhibitor more effective irrespective of the inhibitor

concentration. When electrochemical corrosion results were observed through

comparison of poteniodynamic polarization curves as shown in the following Figure

4.23(a). It could be observed there was a shift of Ecorr more towards cathodic side for all

89

(a) (b)

(c) (d)

Figure 4-22: Comparison of mustard inhibited MEA solutions in the presence and absence of

oxygen (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading,2000 ppm inhibitor concentration)

Polarization behavior in (a) absence, and (b) presence of oxygen, (c) Open circuit potential,

(d) Inhibition efficiency (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

UninhibitedMustard

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Mustard

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0 50 100 150

E (

V v

s A

g /

AgC

l)

Time (s)

Absence of oxygen

Presence of oxygen

0

10

20

30

40

50

60

70

80

90

100

Absence of

oxygen

Presence of

oxygen

Inhib

itio

n e

ffic

iency

(%

)

90

(a)

(b)

Figure 4-23: Comparison of mustard inhibited MEA solutions for Inhibitor concentrations



-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

250 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

0

10

20

30

40

50

60

70

80

90

100

250 500 1000 2000 4000 10000

Inhib

itio

n e

ffic

iency

(%

)


91

Table 4.5 Summary of experimental and electrochemical parameters for mustard inhibited systems


5 kmol/m3,satd.CO2

loading

pH σ

mS/cm

Ecorr

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

CP

I.E

(%)

Pitting

tendency

Rp

Ωcm2

EIS

I.E

(%)

80°C,absence of oxygen,

inhibitor conc: 2000 ppm 8.10

±0.03

79.43

±0.09

-727.80

±2.15

22.64

±0.04

57.67

±1.75

125.81

±2.05

0.27

±0.02 92.87 Yes - -

80°C, 15 vol.% O2

inhibitor conc:2000 ppm 8.15

±0.04

75.76

±0.08

-751.28

±3.75

27.21

±0.07

53.74

±2.15

124.84

±3.15

0.32

±0.03 92.45 No - -

80°C,

15 vol.% O2

250 8.36 77.07 -778.57 24.06 56.55 117.80 0.28 93.38 Yes 319.35 81.13

500 8.29 75.97 -776.01 26.06 50.05 127.21 0.31 92.77 Yes 336.97 82.12

1000 8.31 77.52 -773.79 28.73 58.69 127.46 0.34 92.03 No 436.82 86.21

2000 8.15

±0.04

75.76

±0.08

-751.28

±3.75

27.21

±0.07

53.74

±2.15

124.84

±3.15

0.32

±0.03 92.56 No 339.79 82.27

4000 8.52 72.19 -800.39 25.36 61.08 105.14 0.30 92.96 No 386.56 84.41

10000 8.39 71.50 -788.57 43.54 79.43 116.01 0.51 87.92 No 433.94 86.12

15 vol.% O2 Inhibitor

conc:2000 ppm

40°C 7.81 47.21 -737.19 3.76 58.77 128.73 0.04 97.00 No 1973.88 87.86

60°C 7.89 63.52 -738.30 10.26 37.41 126.23 0.12 93.00 No 354.78 60.98

80°C 8.15

±0.04

75.76

±0.08

-751.28

±3.75

27.21

±0.07

53.74

±2.15

124.84

±3.15

0.32

±0.03 92.56 No 339.80 82.27

80°C,

15 vol.% O2 Inhibitor

conc:2000 ppm

Chloride 8.13 70.20 -714.58 31.42 59.82 137.27 0.37 86.63 No 326.92 74.11

Oxalate 8.15 68.85 -774.64 33.49 59.22 134.95 0.39 84.23 No 269.33 83.45

92

Inhibitor concentrations except 2000 ppm. In that case, the shift was towards anodic side

rather than the other cases. βa obtained for all the cases on comparison with blank

solution was low, while βc was reverse with vice versa. Pitting tendency was only

observed up to the inhibitor concentration range of 500 ppm. When the inhibitor

performance was compared together based on ƞ as shown in the Figure 4.23(b), the

efficiency range of mustard as inhibitor was in the range of 90 % except the case where

the inhibitor concentration was 10000 ppm. When EIS Nyquist plots of mustard based on

their inhibitor concentration were compared together as shown in the Figure 4.24(a)

below. It could be observed that there was a change in size and shape of the semicircle on

comparison with uninhibited solution but with no linearity in relation with inhibitor

concentration. This is evident when Rp of different inhibitor concentration comparisons is

made as shown in Figure 4.24(b). From the inhibitor concentration range above 500 ppm,

the usage could be suggested. Also, the trend observed in the ƞ (obtained from cyclic

polarization technique) was comparable with EIS (ƞ) as well. Bode phase plots

comparisons are shown in Figure 4.25(a).There is only one peak loop indicating

irrespective of inhibitor concentration. This indicates only one constant phase element

associated with inhibitor mechanism. EIS data was found fitted for electrical circuit (LR

(QR(C))) as shown in Figure 4.25(b).This indicated there was a rise in resistance for

inhibited solutions on comparison with uninhibited solution which was also in agreement

with the corresponding obtained Rp values. But, there was no linearity relationship found

between inhibitor concentrations. Mustard followed Langmuir adsorption isotherm as

shown in Figure 4.26(a). Standard free energy of adsorption (ΔG°ads) obtained was

93

(a)

(b)

Figure 4-24: Comparison of mustard inhibited MEA solutions for Inhibitor concentrations



0

25

50

75

100

125

150

175

200

0 25 50 75 100 125 150 175 200

Zim

(ohm

s)

Zre (ohms)

Uninhibited

250 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

0

50

100

150

200

250

300

350

400

450

500

RP

(ohm

s)


94

(a)

(b)

Figure 4-25: Corrosion behavior of mustard inhibited MEA solutions for inhibitor concentrations


(a) Bode phase plot comparison (b) Equivalent electrical circuit (Original in color)

0

10

20

30

40

50

60

70

80

0.01 0.1 1 10 100 1000 10000

Phas

e an

gle

(d

egre

e)

log frequency (Hz)

250 ppm

Uninhbited

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

95

(a)

(b)



(a) Langmuir adsorption isotherm (b) Tafel slope comparison (Original in color)

R² = 0.9999

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12

c/Θ


ΔG°ads = -27.71 kJ/mol

0

20

40

60

80

100

120

140

160

0 2000 4000 6000 8000 10000

β(m

V/d

ecad

e)


βa Mustard

βc Mustard

96

-27.71 KJ/mol, indicating the inhibitor involved mixed type of adsorption.


The poteniodynamic polarization behavior of mustard inhibited solution was

compared with their corresponding uninhibited conditions at various temperatures as

shown in Figure 4.27. Ecorr was found slightly shifting from cathodic to anodic side with

rise in temperature of the solution. As observed in previous cases, the same trend was

observed for pH but there was no linearity in rise of temperature. The conductivity of

mustard inhibited solution on comparison with baseline at the corresponding temperature

was less. There was no pitting tendency observed when the cyclic polarization

performance was analyzed. icorr increased with temperature which was evident in the

corrosion rate as well. But there was no big change in βa while βc remained constant

irrespective of the rise in temperature. Inhibitor efficiency based on cyclic polarization

were compared as shown in Figure 4.28(a) remained in 90% range but decreased with

the increase in temperature. Figure 4.28(b) represents EIS (Nyquist plots) obtained for

mustard at various temperatures as the size of semicircle got reduced with increase in

temperature and this reciprocated as EIS efficiency too decreased with rise in

temperature. This is in a good agreement when Rp was compared as shown in Figure

4.28(c). But the shape of inhibited condition remained same, indicating temperature does

not affect the corrosion inhibition mechanism or its performance. Arrhenius plots were

made as shown in Figure 4.29. The higher values of Ea of the inhibited solution indicate

physical barrier is formed by formation of adsorptive film of electrostatic nature

97

(a) (b)

(c)

Figure 4-27: Corrosion behavior of mustard inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15 vol.% O2)

Tafel plot comparison: (a) 40°C, (b) 60°C, and (c) 80°C (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Mustard

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Mustard

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Mustard

98

Figure 4-28: Corrosion behavior comparison of mustard inhibited MEA solutions under the

influence of temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15vol. % O2,

40-80°C): (a) Inhibition efficiency, (b) Nyquist plot, and (c) Rp (Original in color)

(a) (b)

(c)

0

10

20

30

40

50

60

70

80

90

100

40 60 80

Inhib

itio

n e

ffic

iency

(%

)

Temperature (°C)

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600 700 800

Zim

(ohm

s)

Zre (ohms)

80°C

60°C

40°C

0

500

1000

1500

2000

40 60 80

RP

(ohm

s)

Temperature (°C)

Mustard

uninhibted

99

(a)

(b)

Figure 4-29: Arrhenius Plots for mustard inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, 40-80°C, saturated CO2 loading, presence of 15vol. % O2)


R² = 0.7422

R² = 0.9995

0.00

0.50

1.00

1.50

2.00

2.50

3.00

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

(µA

/cm

2)

1/T x103(K-1)

Uninhibited

Mustard

Ea = 45.11 KJ/mol

R² = 0.688

R² = 0.9995

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

/T (

µA

/cm

2.K

)

1/T x103(K-1)

Uninhibted

Mustard


ΔS0a = - 114.87 J/mol.K

100

over the metal surface reducing the corrosion rate. ΔHa is positive and it reflects inhibitor

adsorption is endothermic process.

4.2.2.4 Effect of process contaminant

The pH and conductivity of the solution could be used to understand the

relationship between the parametric effects as variables and the testing conditions.

Mustard inhibited solution was tested with presence of oxalate and chloride separately to

study their effect on its inhibition performance. When compared with no process

contaminant condition, the pH measured for mustard inhibited solution in the presence of

no process contaminants was less, while it was more on comparing it with the uninhibited

state with process contaminant conditions, i.e., uninhibited state with chloride as process

contaminant. However, the conductivity of the mustard inhibited solution in the presence

of process contaminants was less, indicating that anodic metal dissolution was controlled

by inhibition mechanism of mustard to some extent. When the Poteniodynamic

polarization behavior was compared as shown in Figure 4.30(a). It was observed there

was a shift in Ecorr towards cathodic in the presence of oxalate while it was towards

anodic in the presence of chloride. Also, corrosion rate was compared as shown in the

Figure 4.30(b). It was observed that corrosion rate of mustard inhibited solution in the

presence of these process contaminants was more than no process contaminant condition

but less than their corresponding uninhibited conditions. Apart from this, both the Tafel

slopes for mustard inhibited condition in the presence of process contaminants were more

than their corresponding conditions without process contaminants. It was observed that

with the presence of oxalate,there was a rise in corrosion rate as there was even decrease

101

(a)

(b)

Figure 4-30: Comparison of mustard inhibited MEA solutions under the influence of process



-1.05

-0.95

-0.85

-0.75

-0.65

-0.55

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

No Process Contaminants

Chloride

Oxalate

0

1

2

3

4

5

No Process Contaminant Chloride Oxalate

Co

rro

sio

n r

ate

(mm

py)

Uninhibited

Mustard

102

in conductivity of the solution the corrosion rate was more which might be due to

cathodic side reactions which is well supported by the fact that shift in Ecorr was towards

cathodic with more βc on comparison with no process contaminant condition.There was

no pitting tendency observed for any of the cases.EIS (Nyquist Plots) comparison was

shown in the Figure 4.31(a). The size of the semicircle decreased in the presence of

oxalate and chloride but there was no change in shape of it. Thus, it is clear that

inhibition mechanism remains the same irrespective of the presence of process

contaminants. Also, Rp for mustard inhibited solution in the presence of process

contaminants was compared as shown in the Figure 4.31(b). It is observed that Rp was

less for mustard inhibited solution than that without process contaminant condition,

indicating the resistance of inhibited solution got reduced in the presence of process

contaminants leading to increase in corrosion rate.

4.2.2.4 Quantum Chemical Analysis

The main constituents of chemicals present in mustard are namely Allyl

isothiocyanate, Sinigrin and Benzyl isothiocyanate [Bhattacharya, 2010; Herzallah et al.,

2012]. Parameters obtained for these chemicals through quantum chemical analysis are

shown in Table.4.6. It could be understood that ELUMO values of those compounds were

less on comparison with MEA indicates the tendency to accept electrons while EHOMO

values are less in comparison with MEA, indicating that corrosion inhibition is due to the

vacant innermost orbital present in the inhibitor for accepting electrons. Energy gap (ΔE)

values are lower in comparison with MEA indicating the corrosion inhibition is due to

103

(a)

(b)

Figure 4-31: Comparison of mustard inhibited MEA solutions under the influence of process


(a) Nyquist Plot (b) Rp (Original in color)

0

25

50

75

100

125

150

0 25 50 75 100 125 150

Zim

(ohm

s)

Zre (ohms)


0

50

100

150

200

250

300

350

400

No Process Contaminants Chloride Oxalate

RP

(ohm

s)

Uninhibited

Mustard

104

Table 4.6 Summary of Quantum Chemical Analysis of chemicals related with mustard

Properties MEA Allyl

isothiocyanate

Benzyl

isothiocyanate Sinigrin

EHOMO -6.77 -6.84 -6.76 -7.14

ELUMO -0.31 -0.92 -1.11 -1.32

Energy gap (ΔE) 6.46 5.92 5.64 5.82

Dipole moment 1.04 3.57 3.68 5.66

I 6.77 6.84 6.76 7.14

A 0.31 0.92 1.11 1.32

Electro-negativity (χ) 3.54 3.88 3.93 4.23

Hardness (γ) 3.23 2.96 2.82 2.91

(χ)Metal (Fe) 7.00 7.00 7.00 7.00


(Hardness (γ)) 0.00 0.00 0.00 0.00


transferred (ΔN) 0.54 0.58 0.54 0.47

105

chemical adsorption (Salarvand, 2017).Electro-negativity (χ) values were high on

comparison with MEA, signifying that chemical potential required for corrosion

inhibition is high and hardness (γ), a representation for higher polarizability and better

inhibition, is low [Yilmaz, 2016].Fraction of electrons transferred (ΔN) was less than 3.6,

indicating the formation of adsorption layer to inhibit corrosion through electrons getting

donated from the inhibitors to the metal surface (Salarvand,2017). The optimized

structures of this compounds are shown in Figures 4.32 (a, b, and c) respectively. It is

clearly visible this structure has sulfur and nitrogen atoms with different functional

groups. From Figures 4.32 (c, d, and e) ,electron densities in LUMO level are more

surrounding the sulfur and nitrogen atoms and same is observed for HOMO levels from

Figure 4.32 (f, g, and h) indicating the presence of sulfur and nitrogen atom is

responsible for the inhibition quality of mustard.

4.2.3 Horseradish


When the inhibited solution was compared in the presence of O2, there was not a

big shift in Tafel slopes as shown in Figures 4.33(a and b) and this was also evident in

the inhibition efficiency comparison as shown in Figure 4.33(d). But, for uninhibited

systems, there was a shift in Open circuit potential towards anodic side as shown in

Figures 4.33(c). Thus, it is evident that the inhibitor performance was not affected due to

the presence of oxygen.

106

Allyl isothiocyanate Benzyl isothiocyanate Sinigrin

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 4-32: Quantum Chemistry structures for Allyl isothiocyanate, Benzyl isothiocyanate and

Sinigrin (a, b, and c) Optimized molecular structures (d, e, and f) HOMO (g, h, and i) LUMO

(Original in color)

107

(a) (b)

(c) (d)

Figure 4-33: Comparison of horseradish inhibited MEA solutions in the presence and absence of


Polarization behavior in (a) absence, and (b) presence of oxygen (c) open circuit potential


-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l )

log icorr (A/cm2)

Uninhibited

Horseradish

-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Horseradish

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

0 100 200 300

E (

V v

s A

g /

AgC

l)

Time(s)

Absence of oxygen

Presence of oxygen

0

10

20

30

40

50

60

70

80

90

100

Absence of

oxygen

Presence of

oxygen

Inhib

itio

n e

ffic

iency

(%

)

108


To examine the effect of inhibitor concentration with the inhibitor performance,

various concentration ranges were tried from 250 ppm to 10000 ppm. When the

experimental conditions were monitored based on the concentration range, pH of testing

solution tended to be more alkaline with usage of inhibitors along with the decrease in

conductivity of the solution. There was also a shift in Ecorr towards cathodic side observed

when the Poteniodynamic polarization behavior was compared as shown in Figure

4.34(a). Irrespective of the inhibitor concentration, βa was low for all the inhibitor

concentrations when compared with uninhibited solution whereas the values of βc were

high irrespective of inhibitor concentrations. However, there was pitting tendency

observed above the concentration level of 250 ppm. The ƞ (based on cyclic polarization

curves) remained same in the range of 90% irrespective of inhibitor concentrations as

shown in the Figure 4.34(b). EIS Nyquist plots of horseradish inhibitor concentrations

were compared as shown in the Figure 4.35(a) below. There was a change in shape; size

of the semicircles obtained for the inhibited solution irrespective of their concentration

range, indicating that the inhibitor performs well .It was observed from the comparisons

shown in Figure 4.35(b) that Rp was more than that of the uninhibited solution indicating

that the horseradish inhibits corrosion reaction by increasing the resistance of the

solution. The experimental data related with the horseradish inhibited systems is shown

in Table 4.7 along with electrochemical data obtained. Bode phase plots as shown in

Figure 4.36 (a) indicates that there is only one constant phase element as all the inhibited

solution had a single peak on comparison with that of uninhibited solution.

109

(a)

(b)

Figure 4-34: Comparison of horseradish inhibited MEA solutions for inhibitor concentrations



-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

250 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

0

10

20

30

40

50

60

70

80

90

100

250 500 1000 2000 4000 10000

Inhib

itio

n e

ffic

iency

(%

)


110

(a)

(b)

Figure 4-35: Comparison of horseradish inhibited MEA solutions for inhibitor concentrations



0

25

50

75

100

125

150

175

200

0 25 50 75 100 125 150 175 200

Zim

(ohm

s)

Zre (ohms)

Uninhibited

250 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

0

50

100

150

200

250

300

350

400

450

500

RP

(ohm

s)

Inhibitor concentration(ppm)

111

Table 4.7 Summary of experimental and electrochemical parameters for horseradish inhibited systems


5 kmol/m3,satd.CO2

loading

pH

σ

mS/c

m

Ecorr

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

CP

I.E

(%)

Pitting

tendency

Rp

Ωcm2

EIS

I.E

(%)

80°C,

absence of oxygen, inhibitor conc: 2000 ppm

8.05

±0.02

79.46

±0.11

-734.11

±2.18

19.88

±0.02

53.09

±1.45

125.63

±1.85

0.23

±0.03 93.74 Yes - -

80°C, 15 vol.% O2

Inhibitor conc.: 2000 ppm 8.15

±0.03

75.76

±0.07

-751.28

±4.25

27.21

±0.04

53.74

±1.85

124.85

±2.75

0.32

±0.02 92.45 No - -

80°C,

15 vol.% O2

250 8.41 77.25 -787.39 29.61 58.33 118.43 0.35 91.78 Yes 470.09 87.18

500 8.27 75.86 -758.01 24.76 52.79 127.45 0.29 93.13 No 269.61 77.65

1000 8.28 77.28 -752.35 25.53 55.45 125.14 0.31 92.92 No 347.96 82.69

2000 8.15

±0.03

75.76

±0.07

-751.28

±4.25

27.21

±0.04

53.74

±1.85

124.85

±2.75

0.32

±0.02 92.45 No 343.16 82.44

4000 8.38 77.21 -776.13 21.55 57.36 109.48 0.25 94.02 No 308.34 80.46

10000 8.28 75.35 -766.23 34.23 65.68 117.20 0.40 90.50 No 371.78 83.79

15 vol.% O2, Inhibitor conc:

2000 ppm

40°C 7.79 56.00 -730.89 7.17 46.63 140.05 0.08 94.80 No 227.17 -

60°C 7.93

±0.04

52.63

±0.09

-743.11

±3.15

38.84

±0.02

52.34

±2.96

153.21

±3.15

0.16

±0.01 90.73 No 485.82 71.5

80°C 8.15

±0.03

75.76

±0.07

-751.28

±4.25

27.21

±0.04

53.74

±1.85

124.85

±2.75

0.32

±0.02 92.45 No 342.22 82.39

80°C,

15 vol.% O2, Inhibitor conc:

2000 ppm

Chloride 8.08 71.30 -708.66 34.41 56.17 152.35 0.41 85.36 No 294.89 71.30

Oxalate 8.26 66.68 -787.36 33.31 65.45 131.38 0.39 84.31 No 254.82 82.51

112

The electrical circuit R(QR) as shown in the Figure 4.36(b),was found fitting for

the data obtained and the values are shown in Appendix, it indicates that there is a rise in

resistance of the solution whenever the inhibitor is used and this in good accordance with

the trend observed for Rp data. When Tafel slopes were compared with uninhibited

condition as shown in Figure 4.37(a), shows that horseradish acts as a mixed type

corrosion inhibitor affecting both sides of the corrosion reactions. To understand the

adsorption mechanism involved electrochemical data when fitted for adsorption isotherm

it followed Langmuir isotherm as shown in Figure 4.37(b) with standard free energy of

adsorption (ΔG°ads) as -29.59 KJ/mol, showing that inhibitor undergoes both

physisorption and chemisorption interaction with the metal surface.


Poteniodynamic polarization behavior of Horseradish inhibited solutions were

compared with their corresponding uninhibited solutions at various temperatures as

shown in Figure 4.38. As observed, Ecorr shifted from anodic to cathodic with the rise in

temperature. As observed in previous cases, there was an increase in pH and shifting it

towards alkaline nature. There was no pitting tendency observed while the corrosion rate

increased with rise in temperature. There was no linearity observed in icorr whereas βa

increased with increase in temperature. But βa of horseradish inhibited solution was less

on comparison with corresponding baseline at respective temperatures. However, the

inhibition efficiency remained in the range of 90% for all temperature range as shown in

Figure 4.39(a).EIS Nyquist plots for the Horseradish inhibited solutions at various

temperatures were compared as shown in Figure 4.39(b). There was a change in size of

113

(a)

(b)

Figure 4-36: Corrosion behavior of horseradish inhibited MEA solutions for inhibitor

concentrations (250 ppm to 10000 ppm) (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading,

15 vol. % O2): (a) Bode phase plot comparison (b) Equivalent electrical circuit (Original in color)

0

10

20

30

40

50

60

70

0.01 0.1 1 10 100 1000 10000

Phas

e an

gle

(d

egre

e)

log frequency (Hz)

250 ppm

Uninhbited

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

114

(a)

(b)

Figure 4-37: Corrosion behavior of horseradish inhibited MEA solutions for inhibitor


15 vol. % O2) (a) Tafel slope comparison (b) Langmuir adsorption isotherm (Original in color)

0

20

40

60

80

100

120

140

160

0 2000 4000 6000 8000 10000

β(m

V/d

ecad

e)


βa Horseradish

βc Horseradish

R² = 0.9994

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12

c/Θ


ΔG°ads = -29.59 KJ/mol

115

(a) (b)

(c)

Figure 4-38: Corrosion behavior of horseradish inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15vol.% O2)Tafel plot

comparison: (a) 40°C, (b) 60°C,and (c) 80°C (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

Uninhibited

Horseradish

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

Uninhibited

Horseradish

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

uninhibited

Horseradish

116

(a) (b)

(c)

Figure 4-39: Comparison of horseradish inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15 vol. % O2, 40-80°C)

(a) Inhibition efficiency, (b) Nyquist plot, and (c) Rp (Original in color)

0

10

20

30

40

50

60

70

80

90

100

40 60 80

Inhib

itio

n e

ffic

iency

(%)

Temperature (°C)

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

Zim

(ohm

s)

Zre (ohms)

80°C

60°C

40°C

0

50

100

150

200

250

300

350

400

450

500

40 60 80

RP

(ohm

s)

Temperature (°C)

Horseradish

uninhibted

117

the semicircle with rise in temperature but there was linearity in that as 60°C was found

more optimum for the performance of the horseradish which was further justified when

their Rp was compared as shown in Figure 4.39(c). Arrhenius plots were made as shown

in Figure 4.40 and thermodynamic parameters were obtained. Higher Ea values indicate

that there was rise in energy barrier due to corrosion inhibition process. Since dissolution

of the metal is slow in the presence of inhibitors positive value of ΔHa° was obtained

indicating corrosion inhibition process as endothermic.


Horseradish inhibited solution was tested in the presence of process contaminants

seperately to study their effect on the performance.In the presence of oxalate,pH of the

inhibited solution was more on comparision with no process contaminant condition while

in the presence of chloride it was less.But,on comparision with uninhibited conditions

with process contaminants, pH of the horseradish inhibited solution was more for both

cases, and thus proving that inhibitior clearly increases the pH of the solution. When

conductivity of the inhibited solution was taken into consideration,it was less when

compared with both cases.This is also a clear indication that anodic metal dissolution is

clearly controlled due to the influence of inhibitor as shown in Figure 4.41(a). It could be

observed that process contaminants influence in the performance of this inhibitor. Ecorr

with the presence of oxalate shifted towards cathodic ,whereas in the presence of

chloride shifted towards anodic on comparision with no process contaminant

condition.This shift is clearly supported when Tafel slopes were considered as βa in the

118

(a)

(b)

Figure 4-40: Arrhenius Plots for horseradish inhibited MEA solutions under the influence of



R² = 0.7422

R² = 0.604

0.00

0.50

1.00

1.50

2.00

2.50

3.00

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

(µA

/cm

2)

1/T x103(K-1)

Uninhibited

Horseradish

Ea = 31.88 KJ/mol

R² = 0.688

R² = 0.5591

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

/T (

µA

/cm

2.K

)

1/T x103(K-1)

Uninhibted

Horseradish


ΔS0a = -149.14 J/mol.K

119

presence of oxalate was more than that without process contaminant condition with vice

versa for βc in the presence of chloride. The corrosion rates are compared as shown in

Figure 4.41(b),in the presence of oxalate and chloride corrosion rates were larger than

that of no process contaminant condition but less than their corresponding uninhibited

conditions.There was no pitting tendency reported for any of the cases and there was a

decrease in inhibition effieicincy in the presence of this process contaminants.EIS

Nyquist plots were compared asshown in the Figure 4.42(a). It was clearly visible that, in

the presence of process contaminants size of the semicircle got decreased without any

change in the shape indicating the effect of process contaminant in the performance of the

inhibitior. Horseradish inhibited systems performed well as seen from their increased Rp

when compared correspondingly with their uninhibited conditions as shown in the Figure

4.42(b).

4.2.3.4 Quantum Chemical Analysis

According to literature (Wu et al., 2009; Chen et al., 2012), Allyl isothiocyanate,

Peroxidase, Phenethyl isothiocyanate and Theaflavin were the main chemicals present in

horseradish. The quantum analyses of these compounds have been obtained and

parameters related with them were obtained as shown in Table 4.8. It could be understood

that EHOMO values are less in comparison with MEA, while ELUMO values of those

compounds were also less on comparison with MEA. This indicates the tendency to

accept electrons could be the reason behind corrosion inhibition. Low energy gap (ΔE)

values on comparison with MEA indicating the corrosion inhibition is due to chemical

adsorption (Salarvand, 2017). Electro negativity (χ) values were high on comparison with

MEA, signifying that chemical potential required for corrosion inhibition is high and

120

(a)

(b)

Figure 4-41: Comparison of horseradish inhibited MEA solutions under the influence of process



-1.05

-0.95

-0.85

-0.75

-0.65

-0.55

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

No Process contaminants

Chloride

Oxalate

0

1

2

3

4

5


Co

rro

sio

n r

ate

(mm

py)

Uninhibited

Onion

121

(a)

(b)

Figure 4-42: Comparison of horseradish inhibited MEA solutions under the influence of process



0

25

50

75

100

125

150

0 25 50 75 100 125 150

Zim

(ohm

s)

Zre (ohms)


0

50

100

150

200

250

300

350

400


RP

(ohm

s)

Uninhibited

Onion

122

Hardness (γ) a representation for higher polarizability and better inhibition is low (Yilmaz

et al., 2016).Also, for this compounds, fraction of electrons transferred (ΔN) was also less

than 3.6 and this shows that they could donate electrons to the metal surface for forming

an adsorbed layer to prevent corrosion. The optimized structures of this compounds are

shown in Figures 4.43(a, b. and c) respectively. It is clearly visible this structure has

sulfur and nitrogen, phosphorous atoms with different functional groups. From Figures

4.43(c, d, and e) electron densities in LUMO level are more surrounding the sulfur and

nitrogen atoms and same is observed for HOMO levels on comparison with phosphorous

atoms from Figures 4.43 (f, g, and h) indicating the presence of sulfur and nitrogen atom

or phosphorous are combination of all them is responsible for the inhibition quality of

horseradish. This analysis supports and proves horseradish could be used a corrosion

inhibitor.

Table 4.8 Summary of quantum chemical analysis of chemicals related with horseradish

Properties MEA Allyl

isothiocyanate Peroxidase

Phenethyl

isothiocyanate Theaflavin

EHOMO -6.77 -6.84 -7.22 -6.75 -5.39

ELUMO -0.31 -0.92 -0.80 -0.82 -2.56

Energy gap (ΔE) 6.46 5.92 6.41 5.93 2.82

Dipole Moment 1.04 3.57 2.46 3.54 8.60

I 6.77 6.84 7.22 6.75 5.39

A 0.31 0.92 0.80 0.82 2.56

Electro-negativity (χ) 3.54 3.88 4.01 3.78 3.98

Hardness (γ) 3.23 2.96 3.20 2.96 1.41

(χ)Metal (Fe) 7.00 7.00 7.00 7.00 7.00

Absolute hardness of

Fe (Hardness (γ)) 0.00 0.00 0.00 0.00 0.00


transferred (ΔN) 0.54 0.58 0.46 0.54 1.06

123

Peroxidase Phenethyl isothiocyanate Theaflavin

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 4-43: Quantum chemistry structures for Peroxidase, Phenethyl isothiocyanate and

Theaflavin (a, b, and c) Optimized molecular structures (d, e, and f) HOMO (g, h, and i) LUMO

(Original in color)

124

4.2.4 Onion


The inhibited solution in the presence of O2 had a shift from cathodic side to

anodic side on comparison with corresponding blank solutions as shown in Figure

4.44(a). There is a decrease in conductivity and in both Tafel slopes. Also, there is a

sharp increase in inhibition efficiency under the influence of O2 and also there is a shift

towards to anodic side when open circuit potentials were compared in the presence of

oxygen as shown in Figure 4.44(d).The inhibitor performance got enhanced with the

presence of oxygen. Based on the relation between conductivity of solution and the I corr

which is indirectly related with corrosion rate of metal specimen. It could be found that

whenever there is a decrease in conductivity of the inhibited solution due to the influence

of oxygen there was a less corrosion rate. This could be further explained by the fact that

conductivity of the solution is mainly due to the presence of dissociated Fe ions (metal

ions) as a result of anodic corrosion reaction, i.e., metal dissolution. So, it could be

assessed that under the influence of oxygen, onion forms a passive layer over the metal

surface which prevents the metal dissolution to a certain extent.


It was observed that the conductivity of inhibited solution decreased but not so

drastically. Also, βa of inhibited solution was low when compared with blank solution

while the trend of βc was vice versa. icorr of inhibited solution was low too for all

inhibition concentrations as shown in the Figure 4.45(a),while the corrosion rate

decreased linearly with increase in concentration of inhibitor. ƞ increased after 1000 ppm

125

(a) (b)

(c) (d)

Figure 4-44: Corrosion behavior comparisons of onion inhibited MEA solutions in the presence

and absence of oxygen (5.0 kmol/m3 MEA, 80°C, saturated CO2 loading, 2000 ppm inhibitor

concentration) Polarization behavior in (a) absence (b) presence of oxygen, and (c) Open circuit

(Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l )

log icorr (A/cm2)

Uninhibted

Onion-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibted

Onion

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

0 200 400 600

E (

V v

s A

g /

AgC

l)

Time(s)

Absence of oxygen

Presence of oxygen

0

10

20

30

40

50

60

70

80

90

100

Absence of

oxygen

Presence of

oxygen

Inhib

itio

n e

ffic

iency

(%

)

126

(a)

(b)

Figure 4-45: Corrosion behavior comparison of onion inhibited MEA solutions for inhibitor


15 vol. % O2): (a) Polarization behavior (b) Inhibition efficiency comparison (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

250 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

1500 ppm

6000 ppm

0

10

20

30

40

50

60

70

80

90

100

200 500 1000 1500 2000 4000 6000 10000

Inhib

itio

n e

ffic

iency

(%

)


127

Experimental

Condition

5 kmol/m3,satd.CO2

loading

pH σ

mS/cm

Ecorr

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

CP

I.E

(%)

Pitting

tendency

Rp

Ωcm2

EIS

I.E

(%)

80°C,absence of

oxygen, inhibitor conc.: 2000 ppm

8.09

±0.04

79.40

±0.08

-706.72

±2.85

31.81

±0.03

228.25

±2.15

456.60

±3.25

0.38

±0.02 89.99 Yes 8.09 79.40

80°C, 15 vol.% O2 inhibitor conc.: 2000 ppm

8.10

±0.02

74.69

±0.07

-738.31

±2.15

21.46

±0.02

46.85

±1.15

124.62

±1.25

0.25

±0.03 94.04 No 8.10 74.69

80°C, 15 vol.% O2

200 8.15 79.59 -728.14 147.00 59.04 101.24 1.74 59.21 Yes 60.49 0.40

500 8.18 78.48 -746.42 41.20 47.96 123.17 0.48 88.57 Yes 55.72 -8.12

1000 8.13 77.32 -753.14 33.65 50.75 126.79 0.40 90.66 No 123.91 51.37

1500 8.12 79.03 -743.84 33.71 49.65 123.19 0.40 90.65 No 134.80 55.30

2000 8.10

±0.02

74.69

±0.07

-738.31

±2.15

21.46

±0.02

46.85

±1.15

124.62

±1.25

0.25

±0.03 91.49 No 177.78 66.11

4000 8.13 79.39 -749.69 26.60 52.15 131.33 0.31 92.62 No 285.00 78.86

6000 8.14 76.62 -755.70 27.56 53.12 125.46 0.32 92.35 No 229.34 73.73

10000 8.13 77.45 -758.86 31.12 55.86 128.75 0.37 91.36 No 369.55 83.69

15 vol.% O2

Inhibitor

conc.:

2000 ppm

40°C 7.79 51.07 -734.78 18.97 53.27 108.32 0.22 86.27 No 224.65 -

60°C 8.15 53.69 -747.70 29.68 47.30 109.97 0.35 79.12 No 107.39 -

80°C 8.10

±0.02

74.69

±0.07

-738.31

±2.15

21.46

±0.02

46.85

±1.15

124.62

±1.25

0.25

±0.03 91.49 No 177.78 66.11

80°C, 15 vol.%

O2,

Inhibitor

conc.:

2000 ppm

Chloride 8.12 85.43 -717.22 31.31 53.30 134.36 0.37 86.68 No 123.89 31.68

Oxalate 8.16 80.92 -778.72 23.52 50.31 134.94 0.28 88.92 No 244.20 81.75

Table 4.9 Summary of experimental and electrochemical parameters for onion inhibited systems

128

of inhibitor concentration as it could be seen from Figure 4.45(b),whereas the pitting

tendency was observed up to the concentration level of 500 ppm. Nyquist plots from EIS

was compared for various concentration range of onion as shown in Figure 4.43(a), it was

observed that there was change in size and shape of semicircle on comparison with

uninhibited solutions. Also, from Figure 4.46(b), it was evident that Rp increased after

1000 ppm of inhibitor concentration and this performance is in accordance with ƞ trend

observed from cyclic polarization technique. Bode phase plots when compared as shown

in Figure 4.47(a), two peaks were observed for inhibited solution, indicating two

constant phase elements could be present in inhibition mechanism. Data were fitted for

the electrical circuit (R(QR)(Q(R (LR)))) mentioned in Figure 4.47(b).Also, the values of

those parameters were mentioned in Appendix. It was well understood from these values

that resistance of the solution increased when the inhibitor concentration was over 1000

ppm and this looks in total agreement with Rp values reported above. Onion as a

corrosion inhibitor followed Langmuir adsorption isotherm as shown in Figure 4.48(a),

and standard free energy of adsorption (ΔG°ads) obtained was -30.57 KJ/mol, indicating it

as a mixed type inhibitor. When the Tafel slopes were compared as shown in Figure 4.48

(b), it could be seen that both anodic and cathodic Tafel slopes varied in comparison with

uninhibited condition with a rise in inhibitor concentration, indicating the indicator

follows a mixed type of adsorption.


Cyclic polarization behaviors were compared with their uninhibited solution as

shown above in Figure 4.49(a-c). It was observed there was no big shift in Ecorr with the

129

(a)

(b)

Figure 4-46: Comparison of onion inhibited MEA solutions for inhibitor concentrations



0

50

100

150

200

250

300

350

400

RP

(ohm

s)

Inhibitor concentration( ppm)

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 200

Zim

(ohm

s)

Zre (ohms)

Uninhibited

200 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

1500 ppm

6000 ppm

0

2

4

6

8

0 10 20 30

Zim

(ohm

s)

Zre (ohms)

130

(a)

(b)

Figure 4-47: Corrosion behavior of onion inhibited MEA solutions for inhibitor concentrations


(a) Bode phase angle plot comparison (b) Equivalent electrical circuit (Original in color)

0

10

20

30

40

50

60

70

0.01 0.1 1 10 100 1000 10000

Phas

e an

gle

(d

egre

e)

log frequency (Hz)

200 ppm

Uninhbited

500 ppm

1000 ppm

4000 ppm

10000 ppm

1500 ppm

2000 ppm

6000 ppm

131

(a)

(b)

Figure 4-48: Corrosion behavior of onion inhibited MEA solutions for inhibitor concentrations


(a) Tafel slope comparison (b) Langmuir adsorption isotherm

0

2

4

6

8

10

12

0 2 4 6 8 10 12

C/Θ

Inhibitor concentration (g/L)

ΔG°ads = -30.57 KJ/mol

0

20

40

60

80

100

120

140

160

0 2000 4000 6000 8000 10000

β(m

V/d

ecad

e)


βa Onion

βc Onion

132

(a) (b)

(c)

Figure 4-49: Corrosion behavior of onion inhibited MEA solutions under the influence of

temperature (5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15 vol.% O2) Tafel plot

comparison (a) 40°C, (b) 60°C, and (c) 80° C (Original in color)

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

Uninhibted

Onion

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

Uninhibted

Onion

-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

uninhibted

Onion

133

increase in temperature. There was a change in pH from temperature 40 to 60°C, but there

was a linear increase in conductivity of the solution. The conductivity of inhibited

solution was less than their uninhibited counterparts at corresponding temperatures.

There was no pitting tendency observed while the icorr increased with rise in temperature.

Similar trend was observed for βc and βa remained almost constant with the increase in

temperature. There was no linearity observed for inhibitor efficiency based on cyclic

poteniodynamic behaviors as shown in Figure 4.50(a).EIS Nyquist plots for the onion

inhibited solution for various temperatures as shown in Figure 4.50(b) resembled there

was change in size of the semicircle with no linearity, while 60°C was not found suitable

for the performance of inhibitor whereas the inhibitor performed well for 80°C. This was

further justified when Rp was compared for the onion inhibited solution at various

temperatures as shown in Figure 4.50(c). Arrhenius plots were made as shown in Figure

4.51. Ea values of inhibited solution were low than those of the uninhibited state due to

the lowering of energy barrier for the corrosion process so that chemical adsorption could

be favored. Also, the values of Ea and ΔHa° ideally should be equal for a chemical

reaction in electrolytic solutions. Similarly, it was observed here as well with almost a

constant and small difference between the two values in all the cases. The positive value

of ΔHa° indicates that dissolution reaction is endothermic while dissolution of the metal is

difficult and slow in the presence of all the inhibitors. The superior performance of onion

could be further justified when ΔSa° was considered as it is lower than uninhibited state

due to the ordering of adsorbed molecules in the presence of inhibitor

134

(a) (b)

(c)

Figure 4-50: Comparison of onion inhibited MEA solutions under the influence of temperature

(5.0 kmol/m3 MEA, saturated CO2 loading, presence of 15 vol. % O2, 40-80°C)

(a) Inhibition efficiency, (b) Nyquist plot, and (c) Rp (Original in color)

0

10

20

30

40

50

60

70

80

90

100

40 60 80

Inhib

itio

n e

ffic

iency

(%)

Temperature (°C)

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350 400

Zim

(ohm

s)

Zre (ohms)

80°C

60°C

40°C

0

50

100

150

200

250

300

40 60 80

RP

(ohm

s)

Temperature (°C)

Onion

uninhibted

135

(a)

(b)

Figure 4-51: Arrhenius Plots for onion inhibited MEA solutions under the influence of



R² = 0.7422

R² = 0.828

0.00

0.50

1.00

1.50

2.00

2.50

3.00

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

(µA

/cm

2)

1/T x103(K-1)

Uninhibited

Onion

Ea = 11.22 KJ/mol

R² = 0.688

R² = 0.7287

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

2.80 2.90 3.00 3.10 3.20 3.30

log i

corr

/T (

µA

/cm

2.K

)

1/T x103(K-1)

Uninhibted

OnionΔH0a = 8.46 KJ/mol

ΔS0a = - 209.09 J/mol.K

136


Onion inhibited solution was tested in the presence of oxalate and chloride

separately to understand their influence in the performance of inhibitors.When pH of the

onion inhibted solution was taken into consideration,it was found that the solution tended

to be more alkaline in nature in the presence of oxalate while in the presence of chloride

it almost remained same on comparision with that of no process contamination

condition.When compared with their blank conditions, pH was more, indicating that

inhibitor still has influence in the presence of inhibitor.Conductivity of the solution was

compared with no process contaminant condition and found that it almost remained same

in the presence of oxalate whereas it is increased in the presence of

chloride.Poteniodynamic polarization behaviours were compared as shown in Figure

4.52(a). There was a shift in Ecorr with the presence of process contaminants. In the

presence of Oxalate,shift was towards cathodic while it was towards anodic with the

presence of chloride.This could be further supported when their Tafel slopes were

considered.Compared with the no process contaminant condition, βc was more in the

presence of oxalate and in the presence of chloride βc was more. This clearly explains the

reason behind the shift in Ecorr.The corrosion rate was compared as shown in Figure

4.52(b). It was found that in the presence of oxalte corrosion rate was less on comparison

for both cases,i.e., no process contaminants and corresponding blank condition.In the

presence of chloride the corrosion rate was only less when compared with uninhibited

condition whereas it was larger when compared with no process contaminant case.

137

(a)

(b)

Figure 4-52: Comparison of onion inhibited MEA solutions under the influence of process



-1.05

-0.95

-0.85

-0.75

-0.65

-0.55

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V V

s A

g/A

gC

l)

log icorr (A/cm2)

No Process Contaminants

Chloride

Oxalate

0

1

2

3

4

5


Co

rro

sio

n r

ate

(mm

py)

Uninhibited

Onion

138

There was no pitting tendency observed in any of the cases.EIS Nyquist plots

were compared as shown in the Figure 4.53(a).In the presence of oxalate,the size of the

semicircle obtained was big while in the presence of chloride,it got decreased without any

change in shape of the semicircle.Rp was compared as shown in Figure 4.53(b). It was

found that, in the presence of chloride,it was decreased on comparing it with no process

contaminant conditions and increased when compared with its corresponding uninhibited

condition. This was well supported when Rp of onion inhibted solution was taken into

consideration.

4.2.4.5 Quantum chemical analysis

According to literature [Corzo-Martínez et al., 2007], dipropyl disulphide and

quercetin were the main chemicals present in onion. The quantum analyses of these

compounds have been obtained and parameters related with them were obtained as shown

in Table: 4.10.EHOMO values on comparison with MEA were higher, indicating the

tendency to donate electrons to metal atom also ELUMO values of those compounds was

less on comparison with MEA Indicating higher corrosion inhibition efficiency due to the

tendency to accept electrons from the metal surface. Energy gap (ΔE) values are lower on

comparison with MEA indicating the corrosion inhibition is due to chemical adsorption

(Salarvand,2017).Electro-negativity(χ) values were high on comparison with MEA

signifies that chemical potential required for corrosion inhibition is high and hardness (γ)

a representation for higher polarizability and better inhibition is low (Yilmaz et al.,2016).

Fraction of electrons transferred (ΔN) was also less than 3.6 and this shows they could

donate electrons to the metal surface for forming an adsorbed layer to prevent corrosion

139

(a)

(b)

Figure 4-53: Comparison of onion inhibited MEA solutions under the influence of process



0

25

50

75

100

125

150

0 25 50 75 100 125 150

Zim

(o

hm

s)

Zre (ohms)


0

50

100

150

200

250

300


RP

(ohm

s)

Uninhibited

Onion

140

Table 4.10 Summary of Quantum Chemical Analysis of chemicals related with onion

[Salarvand, 2017].The optimized structures of this compounds are shown in Figures 4.54

(a and b), respectively. It is clearly visible that this structure has sulfur and oxygen atoms

with different functional groups. From Figures 4.54(c and d) electron densities in HOMO

level are more surrounding the both the atoms and same is observed for LUMO levels

from Figures 4.54(e and f) indicating the presence of sulfur and oxygen atom could be

responsible for the inhibition quality of onion. This analysis proves onion could be used a

corrosion inhibitor.

Properties MEA Dipropyl

disulphide Quercetin

EHOMO -6.77 -6.44 -6.05

ELUMO -0.31 -0.76 -2.08

Energy gap (ΔE) 6.46 5.68 3.96

Dipole Moment 1.04 2.38 3.72

I 6.77 6.44 6.05

A 0.31 0.76 2.08

Electro-negativity (χ) 3.54 3.60 4.07

Hardness (γ) 3.23 2.84 1.98

(χ)Metal (Fe) 7.00 7.00 7.00


(Hardness (γ)) 0.00 0.00 0.00


transferred (ΔN) 0.54 0.59 0.73

141

Dipropyl disulphide Quercetin

(a) (b)

(c) (d)

(e) (f)

Figure 4-54: Quantum Chemistry structures for Dipropyl disulphide and Quercetin

(a, b) Optimized molecular structures (c, d) HOMO (e, f) LUMO (Original in color)

142

4.2.5 Turmeric


There was a slight shift from cathodic to anodic when the poteniodynamic

polarization performances of inhibited solutions were compared with their corresponding

blank solutions. With the presence of oxygen there was a decrease in icorr and Tafel

slopes which resulted in less corrosion rate. This was evident from the increase in

inhibition efficiency under the influence of oxygen as shown in Figure 4.55(c) and as

observed for the uninhibited systems, open circuit potential is deviated towards anodic

side with the presence of oxygen. It could be found that, with the presence of oxygen,

inhibitor performance of turmeric was enhanced due to the formation of passive layer

over the metal surface preventing the metal dissolution.


Turmeric was varied with inhibitor concentration ranges from 250-10000 ppm .It

was observed that conductivity of the solution decreased while pH of the solution

tended towards alkalinity on comparison with the uninhibited solutions irrespective of

the inhibitor concentrations. However as shown in Figure 4.56(a). There was no decrease

in icorr with an occasional rise at 1000 ppm for βa and βc at 1000 ppm inhibitor range. This

is evident from Figure 4.56(b) as inhibition efficiency improved after 1000 ppm of

inhibitor concentration. When EIS (Nyquist Plots) were compared as shown in Figure

4.57(a) ,there was no change in shape of the inhibited solution on comparison with

uninhibited solution but there was increase in diameter of the semicircle which looks in

good agreement when Rp values were compared for different inhibitor concentration as

143

shown in Figure 4.57 (b), it was observed from Table 4.11 that for most of inhibition

concentration range the inhibitor had the pitting tendency and also the inhibition

efficiency was not consistent as it could be seen from its performances. So, this inhibitor

was not investigated for further trials and studies.

144

(a) (b)

(c) (d)

Figure 4-55: Comparison of turmeric inhibited MEA solutions in the presence and absence of


Polarization behavior in (a) absence (b) presence of oxygen, and (c) Open circuit potential,


-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

Turmeric

-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

1.00E-08 1.00E-05 1.00E-02

E (

V v

s A

g /

AgC

l)

log icorr(A / cm2)

Uninhibited

Turmeric

-0.74

-0.73

-0.73

-0.72

-0.72

-0.71

-0.71

-0.70

0 200 400 600

E (

V v

s A

g /

AgC

l)

Time(s)

Absence of oxygen

Presence of oxygen

0

10

20

30

40

50

60

70

80

90

100

Absence of oxygen Presence of

oxygen

Inhib

itio

n e

ffic

iency

(%

)

145

(a)

(b)

-1.00

-0.90

-0.80

-0.70

-0.60

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

E (

V v

s A

g /

AgC

l)

log icorr (A/cm2)

Uninhibited

200 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

-60

-40

-20

0

20

40

60

80

100

200 500 1000 2000 4000 10000

Inhib

itio

n e

ffic

iency

(%

)


Figure 4-56 : Comparison of turmeric inhibited MEA solutions for inhibitor concentrations



146

Figure 4-57: Comparison of turmeric inhibited MEA solutions for inhibitor concentrations



(a)

(b)

0

25

50

75

100

125

150

175

200

0 25 50 75 100 125 150 175

Zim

(ohm

s)

Zre (ohms)

Uninhibited

200 ppm

500 ppm

1000 ppm

2000 ppm

4000 ppm

10000 ppm

0

10

20

30

40

50

60

70

80

90

100

RP

(ohm

s)


147

Table 4.11 Summary of experimental and electrochemical parameters for Turmeric inhibited systems

Experimental

Condition

5 kmol/m3,satd.CO2

loading

pH σ

mS/cm

Ecorr

mV

icorr

µA/cm2

βa

mV/decade

βc

mV/decade

CR

mmpy

CP

I.E (%)

Pitting

tendency

Rp

Ωcm2

EIS

I.E

(%)

80°C,absence of oxygen, inhibitor conc.: 2000 ppm

8.17

±0.02

79.76

±3.20

-709.13

±2.25

41.54

±0.04

49.60

±2.25

74.26

±3.48

0.49

±0.02 86.91 No - -

80°C, 15 vol.% O2 inhibitor conc.: 2000 ppm

8.17±

0.05

77.62

±4.70

-743.56

±3.40 29.49

±0.01

43.94

±3.75

106.14

±1.85

0.35

±0.04 91.82 Yes - -

80°C, 15

vol.%

O2

200 8.23

±0.03 78.44

±3.98 -732.31 314.26 94.91 68.01 3.71 12.81 Yes 60.38

22.65

500 8.20

±0.01 78.57

±2.70 -723.95 169.93 56.53 43.79 2.09 52.85 Yes 59.75

-

1000 8.17±

0.05

77.62

±4.70

-743.56

±3.40

29.49

±0.01

43.94

±3.75

106.14

±1.85

0.35

±0.04 91.82 Yes

83.32

27.69

2000 8.17

±0.04 77.62

±4.70 -743.56

±3.40

29.49 43.94 106.14 0.35 91.82 Yes 66.88

9.91

4000 8.23

±0.05 77.40

±3.86 -743.35 43.84 49.09 96.06 0.52 87.84 Yes 79.61

24.32

10000 8.18±0.

03 79.05

±2.89 -735.14 29.17 45.97 132.86 0.34 91.91 Yes 94.99

36.57

148

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The inhibition performance of five condiment including powders of garlic,

mustard, onion, horseradish, and turmeric were evaluated under a wide range of

operational conditions of the amine-based CO2 absorption process. The following are the

main findings:

Table 5.1 indicates the compatibility of condiments when used as corrosion

inhibitors along with their effective concentration range. The powders of garlic, mustard,

onion, and horseradish show great promise as the environmentally friendly corrosion

inhibitors for the amine-based CO2 absorption process. They are mixed-type corrosion

inhibitors that protect the metal surface by retarding both rate of anodic reaction (metal

dissolution) and rate of cathodic reaction (reduction of corroding agents). They undergo

both physical and chemical adsorption onto the metal surface. The adsorption is

endothermic and Langmuir-type that forms a monolayer of the inhibitor onto the metal

surface. The sulfur-functional group in garlic powder, sulfur- and nitrogen-functional

groups in mustard powder, sulfur- and oxygen-functional groups in onion, and sulfur-,

nitrogen- and phosphorous-functional groups in horseradish play a key role in

chemisorption onto the metal surface.

Garlic powder performs well with inhibition efficiencies of up to 96% for the

concentration range of 250-10,000 ppm. Its performance is slightly affected by solution

temperature, the presences of O2 in feed gas, and the presence of process contaminants

(chloride and oxalate) in the amine solution.

149

Table 5.1 Summary of Corrosion inhibitors performance

Inhibitors

Effective

Concentration

ppm

Pitting

Compatibility

Temperature Oxalate Chloride

Garlic 250-10,000 No Yes Yes Yes

Mustard 500-10,000 No Yes Yes Yes

Horseradish 250-10,000 No Yes Yes Yes

Turmeric -- Yes

Onion 500-10,000 No Yes Yes Yes

150

Mustard powder can yield up to 97% inhibition efficiency depending on operating

conditions. Its performance is not affected by the presence of O2, but can be reduced at

elevated temperatures and in the presence of chloride and oxalate. Pitting corrosion may

be induced when insufficient mustard powder (less than 500 ppm) is applied.

Onion powder yields satisfactory inhibition performance with up to 94%

efficiency. Its performance is improved in the presence of O2, but reduced at elevated

temperature and in the presence of chloride and oxalate. Pitting corrosion is observed

when 500 ppm (and less) of onion powder is used.

Horseradish is an effective inhibitor with up to 94% efficiency for the

concentration range of 250-10,000 ppm. Its performance is not affected by the presence

of O2, but slightly decreases with elevated temperature.

Turmeric powder is not recommended as the corrosion inhibitor in the amine-

based CO2 absorption. It induces pitting corrosion in all ranges of test conditions.

5.2 Recommendations

Corrosion inhibitors investigated in this work seems to have a promising future.

In order to use it at industrial level lot of additional experimental work is required. So,

following recommendations are made:

To evaluate the inhibitor performance under the influence of other parametric

effects such as solution velocity and other process contaminants (bicine and acetate).Flow

loop and autoclave experiments to support the electrochemical experimental results.

To examine the effectiveness of these inhibitors, they could be checked using

weight loss experiments conducted which are using solution samples collected from

industries to simulate the actual plant environment.

151

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APPENDIX

Table A.0.1 Uninhibited MEA solutions under the influence of temperature

Table A.0.2 Garlic inhibited MEA solutions for various inhibitor concentrations

Equivalent Electrical circuit for Uninhibited Systems : R(QR)(Q(R(LR)))

Temperature, °C R Q-Yo Q-Yn R Q-Yo Q-Yn R L R

80 1.21E+00 1.86E+00 7.88E-01 2.30E+00 2.69E-03 7.70E-01 1.84E+01 4.60E-01 1.46E+00

60 1.63E+00 2.28E-02 7.39E-01 3.00E+01 1.99E-03 7.76E-01 1.73E+01 1.24E+01 1.68E+01

40 2.12E+00 1.38E+00 1.00E+00 5.19E+07 1.74E-03 7.76E-01 8.25E+01 1.97E-01 1.68E+09

Equivalent Electrical circuit for Garlic inhibited Systems : R(QR)(Q(R(LR)))

Inhibitor Conc, ppm R Q-Yo Q-Yn R Q-Yo Q-Yn R L R

250 1.49E+00 5.22E-01 1.00E+00 2.79E+08 9.74E-04 8.24E-01 1.73E-02 1.23E+03 5.41E+01

500 1.67E+00 2.12E-01 1.00E+00 6.21E+09 8.75E-04 8.58E-01 6.67E-02 2.77E+02 1.34E+02

1000 1.42E+00 1.32E-03 8.89E-01 1.11E+02 1.12E-03 9.28E-01 2.70E+01 6.01E+01 1.94E+01

2000 1.46E+00 1.21E-03 8.94E-01 3.11E+01 1.60E-03 9.20E-01 1.09E+01 3.49E+04 1.00E-02

4000 7.85E+00 2.67E-01 1.00E+00 2.05E+11 1.28E-03 7.77E-01 1.96E-02 2.48E+03 1.11E+02

10000 1.47E+00 1.73E-03 9.75E-01 8.30E+01 8.87E-04 8.70E-01 4.51E+01 4.27E-02 5.63E+09

169

Equivalent Electrical circuit for Mustard inhibited Systems LR(QR(C))

Inhibitor Conc, ppm L R Q-Yo Q-Yn R C

250 2.44E-03 1.40E+00 1.26E-03 7.04E-01 1.38E+02 1.80E-04

500 1.00E-20 1.45E+00 9.00E-04 7.09E-01 1.34E+02 2.21E-01

1000 5.98E-07 2.13E+00 1.40E-03 8.00E-01 1.58E+02 2.40E-04

2000 1.36E-05 1.94E+00 1.09E-03 7.27E-01 1.32E+02 2.08E-04

4000 6.39E-04 1.38E+00 1.92E-03 8.49E-01 1.44E+02 6.61E-12

10000 1.95E-20 1.50E+00 6.78E-03 9.07E-01 1.06E+02 1.29E-20

Table A.0.3 Mustard inhibited MEA solutions for Inhibitor concentrations

Equivalent Electrical circuit for Horseradish inhibited Systems: R(QR)

Inhibitor Conc, ppm R Q-Yo Q-Yn R

250 2.54E+00 1.63E-03 8.00E-01 1.68E+02

500 1.33E+00 8.88E-04 8.27E-01 1.24E+02

1000 1.31E+02 1.03E-03 8.36E-01 1.42E+02

2000 2.28E+01 2.36E-03 7.72E-01 1.33E+02

4000 1.41E+00 2.22E-03 8.20E-01 1.25E+02

10000 1.44E+01 6.05E-03 9.07E-01 1.20E+02

Table A.0.4 Horseradish inhibited MEA solutions for Inhibitor concentrations

170

Equivalent Electrical circuit for Onion inhibited Systems : R(QR)(Q(R(LR)))

Inhibitor

Conc, ppm R Q-Yo Q-Yn R Q-Yo Q-Yn R L R

250 1.55E+00 3.06E-03 7.45E-01 2.09E+01 9.23E+00 8.46E-01 1.00E+16 5.36E+09 1.00E-02

500 1.87E+00 2.83E-03 8.09E-01 2.07E+01 4.30E-03 8.29E-01 8.71E-01 4.92E-01 1.74E+00

1000 1.52E+00 1.65E-03 8.43E-01 4.90E+01 2.44E-03 8.05E-01 9.22E-01 1.54E+01 9.07E+00

1500 1.56E+00 1.20E-03 7.73E-01 6.08E+01 5.66E-03 9.82E-01 1.43E-02 2.66E+01 1.88E+01

2000 1.57E+00 1.35E-03 9.20E-01 8.93E+01 1.77E-03 8.41E-01 7.10E+00 5.93E+01 2.04E+01

4000 1.54E+00 1.34E+03 8.83E-01 1.12E+02 2.11E-03 8.65E-01 4.60E-02 5.20E+02 1.71E+01

6000 1.54E+00 1.74E-03 9.93E-01 7.75E+01 1.39E-03 8.63E-01 1.71E+01 3.92E+01 1.15E+01

10000 1.56E+00 1.86E-03 8.71E-01 1.18E+02 1.50E-03 9.00E-01 1.35E+01 4.56E+01 1.28E+01

Table A.0.5 Onion inhibited MEA solutions for Inhibitor concentrations

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