10
In situ study of dew point corrosion by electrochemical measurement Yange Yang a , Tao Zhang a,b,, Yawei Shao a,b , Guozhe Meng a,b , Fuhui Wang a,b a Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Ministry of Education, Nantong ST 145, Harbin 150001, China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China article info Article history: Received 29 October 2012 Accepted 2 March 2013 Available online 13 March 2013 Keywords: A. Carbon steel B. EIS B. SEM C. Acid corrosion abstract Dew point corrosion (DPC) is an electrochemical process in a dynamic electrolyte layer, which makes it difficult to carry out conventional electrochemical measurements and thus is not conducive to corrosion mechanism study. In situ electrochemical measurements including electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) are realized by a novel DPC simulation set-up and special electrode arrangements in this work. DPC of carbon steel is studied by in situ test method and ex situ test method, respectively. Different corrosion mechanisms are obtained from two methods. Thus, it is significant to study DPC by in situ test method so as to understand DPC essentially. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Dew point corrosion (DPC) occurs when vapours are cooled below the saturation temperature pertinent to the concentration of condensable gases, such as sulfur trioxide (SO 3 ), hydrogen chloride (HCl), nitrogen dioxide (NO 2 ) and water (H 2 O). The con- densed acids (H 2 SO 4 , HCl and HNO 3 ) are very corrosive to steel and caused corrosion damage to plant materials. Corrosion failures frequently occur in the process and power plant because of condensing gases [1,2]. Consequently, it is significant to carry out research on material DPC resistance performance evaluation and mechanism discussion. DPC is an unusual corrosion type on which little literature re- ports. Let us consider the DPC environment: When temperature drops below the dew point, condensable gases begin to condense either as an acid droplet or as a thin layer on metal surface. With more liquid condensing, acid droplets may fall off from metal sur- face due to the effect of gravity and then a new acid layer will form again. The periodic dropping of acid droplets and formation of a new acid layer can be defined as ‘‘a dynamic acid electrolyte layer’’. Therefore, DPC is in fact an electrochemical process in a dynamic acid electrolyte layer on metal surface. At present, two test meth- ods are proposed to study DPC: ex situ electrochemical method and in situ weight loss method. It is quite difficult to carry out elec- trochemical measurements using conventional three-electrode cell under dynamic acid electrolyte layers. Ex situ electrochemical method employs acid simulation solution to replace dynamic acid electrolyte layers so as to realize kinds of electrochemical mea- surements. Advantage of this method is that electrochemical infor- mation concerning corrosion process can be obtained [3]. However, the condensation of gases, the adsorption and drop of acid droplets on metal surface are ignored by this method, which might result in the error of the data. In situ weight loss method is a direct way to study DPC by exposing samples to the actual DPC environment [4,5]. Merit of this method is that it provides the ‘‘in situ’’ corrosive environment, but insufficient electrochemical information is not conducive to DPC mechanism study. Therefore, it is very important to realize the in situ electrochemical measurement in order to bet- ter understand DPC. The nature of the DPC is an electrochemical corrosion in a dy- namic electrolyte layer on metal surface which is similar to atmo- sphere corrosion to some extent. In the early stage of atmosphere corrosion, Nishikata et al. realized EIS measurement in a thin elec- trolyte layer using a two electrode cell system [6–8]. Referencing the idea of atmosphere corrosion, we design two and three identi- cal electrode arrangements in this work for electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) test of DPC, respectively. The objective of this paper is to realize the EIS and EN test in DPC environment simulating by a novel set-up to assess the applicability of electrochemical measurement for DPC study. Hydrochloric acid DPC problem is taken as an example in this work to demonstrate the reliability of the electrochemical measurements. In addition, corrosion behaviour in the DPC envi- ronment and acid simulation solution is compared in order to char- acterize DPC. 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.03.001 Corresponding author at: Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax: +86 451 8251 9190. E-mail address: [email protected] (T. Zhang). Corrosion Science 71 (2013) 62–71 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci

In situ study of dew point corrosion by electrochemical measurement

  • Upload
    fuhui

  • View
    222

  • Download
    2

Embed Size (px)

Citation preview

Page 1: In situ study of dew point corrosion by electrochemical measurement

Corrosion Science 71 (2013) 62–71

Contents lists available at SciVerse ScienceDirect

Corrosion Science

journal homepage: www.elsevier .com/locate /corsc i

In situ study of dew point corrosion by electrochemical measurement

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.03.001

⇑ Corresponding author at: Corrosion and Protection Laboratory, Key Laboratoryof Superlight Materials and Surface Technology, Harbin Engineering University,Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax: +86 4518251 9190.

E-mail address: [email protected] (T. Zhang).

Yange Yang a, Tao Zhang a,b,⇑, Yawei Shao a,b, Guozhe Meng a,b, Fuhui Wang a,b

a Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Ministry of Education, Nantong ST 145,Harbin 150001, Chinab State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 October 2012Accepted 2 March 2013Available online 13 March 2013

Keywords:A. Carbon steelB. EISB. SEMC. Acid corrosion

Dew point corrosion (DPC) is an electrochemical process in a dynamic electrolyte layer, which makes itdifficult to carry out conventional electrochemical measurements and thus is not conducive to corrosionmechanism study. In situ electrochemical measurements including electrochemical impedancespectroscopy (EIS) and electrochemical noise (EN) are realized by a novel DPC simulation set-up andspecial electrode arrangements in this work. DPC of carbon steel is studied by in situ test method andex situ test method, respectively. Different corrosion mechanisms are obtained from two methods. Thus,it is significant to study DPC by in situ test method so as to understand DPC essentially.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Dew point corrosion (DPC) occurs when vapours are cooledbelow the saturation temperature pertinent to the concentrationof condensable gases, such as sulfur trioxide (SO3), hydrogenchloride (HCl), nitrogen dioxide (NO2) and water (H2O). The con-densed acids (H2SO4, HCl and HNO3) are very corrosive to steeland caused corrosion damage to plant materials. Corrosion failuresfrequently occur in the process and power plant because ofcondensing gases [1,2]. Consequently, it is significant to carry outresearch on material DPC resistance performance evaluation andmechanism discussion.

DPC is an unusual corrosion type on which little literature re-ports. Let us consider the DPC environment: When temperaturedrops below the dew point, condensable gases begin to condenseeither as an acid droplet or as a thin layer on metal surface. Withmore liquid condensing, acid droplets may fall off from metal sur-face due to the effect of gravity and then a new acid layer will formagain. The periodic dropping of acid droplets and formation of anew acid layer can be defined as ‘‘a dynamic acid electrolyte layer’’.Therefore, DPC is in fact an electrochemical process in a dynamicacid electrolyte layer on metal surface. At present, two test meth-ods are proposed to study DPC: ex situ electrochemical methodand in situ weight loss method. It is quite difficult to carry out elec-trochemical measurements using conventional three-electrode cell

under dynamic acid electrolyte layers. Ex situ electrochemicalmethod employs acid simulation solution to replace dynamic acidelectrolyte layers so as to realize kinds of electrochemical mea-surements. Advantage of this method is that electrochemical infor-mation concerning corrosion process can be obtained [3]. However,the condensation of gases, the adsorption and drop of acid dropletson metal surface are ignored by this method, which might result inthe error of the data. In situ weight loss method is a direct way tostudy DPC by exposing samples to the actual DPC environment[4,5]. Merit of this method is that it provides the ‘‘in situ’’ corrosiveenvironment, but insufficient electrochemical information is notconducive to DPC mechanism study. Therefore, it is very importantto realize the in situ electrochemical measurement in order to bet-ter understand DPC.

The nature of the DPC is an electrochemical corrosion in a dy-namic electrolyte layer on metal surface which is similar to atmo-sphere corrosion to some extent. In the early stage of atmospherecorrosion, Nishikata et al. realized EIS measurement in a thin elec-trolyte layer using a two electrode cell system [6–8]. Referencingthe idea of atmosphere corrosion, we design two and three identi-cal electrode arrangements in this work for electrochemicalimpedance spectroscopy (EIS) and electrochemical noise (EN) testof DPC, respectively. The objective of this paper is to realize theEIS and EN test in DPC environment simulating by a novel set-upto assess the applicability of electrochemical measurement forDPC study. Hydrochloric acid DPC problem is taken as an examplein this work to demonstrate the reliability of the electrochemicalmeasurements. In addition, corrosion behaviour in the DPC envi-ronment and acid simulation solution is compared in order to char-acterize DPC.

Page 2: In situ study of dew point corrosion by electrochemical measurement

Fig. 1. Microstructure of the investigated carbon steel.

Y. Yang et al. / Corrosion Science 71 (2013) 62–71 63

2. Wavelet analysis: background

As a relative new way to analyze EN data in recent years, wave-let analysis overcomes the problems of conventional Fourier trans-form [9–11]. The wavelet approach consists essentially inrepresenting the time record xn ¼ ð1;2; . . . ;NÞ using a linear com-bination of basis functions wj;k and /j;k [9]:

xðtÞ �X

k

sJ;k/J;kðtÞ þX

k

dJ;kwJ;kðtÞ þX

k

dJ�1;kwJ�1;kðtÞ þ � � �

þX

k

d1;kw1;kðtÞ ð1Þ

where sJ;k;dJ;k; . . . ; d1;k are the so-called wavelet coefficients. The ba-sis for this decomposition is formed from mother wavelet wðtÞ andfather wavelet /ðtÞ by translating in time and dilating in scale [10]:

wj;kðtÞ ¼ 2�j=2wð2�jt � kÞ ¼ 2�j=2wt � 2jk

2j

!ð2Þ

/j;kðtÞ ¼ 2�j=2/ð2�jt � kÞ ¼ 2�j=2/t � 2jk

2j

!j; k 2 Z ð3Þ

where k = 1,2,N/2j and j = 1,2, . . ., J and J is often a small naturalnumber which depends mainly on N and the basis function. 2j actsas the scale factor and 2jk as the translation parameter.

Discrete wavelet transform (DWT) is used to analyze the ENdata, performed by the fast wavelet transform (FWT) algorithmin practice. According to the algorithm, filters of different cutofffrequencies (low-pass filter and high-pass filter) are used for theanalysis of the signal at different scales. The signal is passedthrough a series of high-pass filters to collect the high frequencycomponent of the signal (the detail) and through a series of low-pass filters to retain the low frequency component (the smooth).After filtering, the outputs are down-sampling, which consists ofdeleting one of every two consecutive coefficients of the filteredoutputs. At the end, the signal is decomposed into the detail coef-ficients, d1,d2, . . .,dJ and the smooth coefficients, sJ, containing theinformation about the local fluctuations and the general trend ofthe signal, respectively. Each of the sets of coefficients d1,d2, . . .,dJ

and sJ is called a crystal. The scale range of each crystal can be com-puted roughly from the following equation:

ðCj1;C

j2Þ ¼ ð2

jDt;2j�1DtÞ ð4Þ

where Dt is the sampling interval and j stands for the correspondingcrystal.

Energy distribution plot (EDP) is thought to be a convenientway to represent wavelet transform in more detail. In this repre-sentation, the contribution of each crystal to the overall signal, Ej

named as the relative energy of a crystal, is plotted versus the crys-tal name. To calculate Ej, it is necessary to define the total energy ofthe signal as follows:

E ¼Xn

n¼1

x2n n ¼ 1; . . . ;N ð5Þ

Then the relative energy of a crystal can be calculated asfollows:

Edj ¼

1E

XN=2j

k¼1

d2j;k j ¼ 1; . . . ; J ð6Þ

Esj ¼

XN=2J

k¼1

s2j;k ð7Þ

Since the chosen wavelets are orthogonal, the followingequation is satisfied:

E ¼ Esj þXJ

j¼1

Edj ð8Þ

3. Experimental method

3.1. Material

The experiments were carried out using ordinary carbon steelwhich has a nominal composition (wt.%): 0.14–0.22 C, 0.3–0.65Mn, 0.30 max Si , 0.05 max S, 0.045 max P, and Fe balance. Micro-structure of the carbon steel is shown in Fig. 1, which consists offerrite and pearlite.

3.2. DPC simulation set-up and electrode specimen design

A schematic diagram of the DPC simulation set-up is shown inFig. 2. It mainly consists of an oil bath, an evaporation bottle, acondenser tube and a tail gas absorption bottle. Continuous mixingvapours containing HCl and H2O are produced from the evapora-tion bottle containing hydrochloric acid solution under the heatingof oil bath which is kept at 120 ± 1 �C. When reaching the con-denser tube, HCl and H2O will condense as a mist of acid dropleton the surface of electrode specimen under the effect of circulatingcooling water. A mist of corrosive acid droplet with different tem-perature and pH can be got by changing the concentration ofhydrochloric acid in the evaporation bottle and the position ofthe electrode specimen in the condenser tube. The higher theconcentration of hydrochloric acid in the evaporation bottle, thelower the pH of the acid droplet. The farther the distance betweenelectrode specimen and evaporation bottle, the lower the temper-ature of the acid droplet. Solutions in the evaporation bottle andtail gas absorption bottle are 0.5 mol/L hydrochloric acid and1 mol/L NaOH respectively in this work.

Structure details of the electrode specimens (L = 5 mm, Xw = 1 -mm, Xg = 0.5 mm) used for EIS and EN test are shown in Figs. 3 and4, respectively. Two and three identical electrode arrangementswere separately designed for EIS and EN test of DPC study. Testspecimens were encased in epoxy powder coating at 220 �C withadhesion force more than 80 MPa to avoid crevice corrosion beforebeing embedded in a glass tube using epoxy resin. The copper wire

Page 3: In situ study of dew point corrosion by electrochemical measurement

oil bath

evaporation bottle

water inlet

condenser tube

water outlet

electrode specimen

tail gas

absorption bottle

Fig. 2. A schematic diagram of the DPC simulation set-up.

Xw

Xg

L

(b)

(a)

Resin

Electrode

Copper wire

Glass tube

Powder coating

Fig. 3. Two identical electrode arrangements used for EIS measurement in DPCenvironment: (a) side view, and (b) bottom view.

L

Xw

Xg(b)

(a)

Electrode Powder coating

Resin

Copper wire

Glass tube

Fig. 4. Three identical electrode arrangements used for EN measurement in DPCenvironment: (a) side view, and (b) bottom view.

64 Y. Yang et al. / Corrosion Science 71 (2013) 62–71

in Figs. 3 and 4 is used to connect electrochemical workstation forelectrochemical measurements. The bottom face exposed to thevapours (HCl and H2O) of both EIS and EN electrode specimen isa Ø12 mm rounded area, which is larger than the electrode size,so the acid layer can cover the electrode specimen surfacecompletely.

3.3. Electrochemical measurements

Before EIS and EN test, specimens were wet ground to a 1000-grit finish by silicon carbide paper, washed using deionized waterand cleaned with acetone. For better reproducibility, all theelectrochemical measurements were carried out more than twiceusing electrochemical workstation Zennium. A sinusoidal pertur-bation of 50 mV was applied at the open circuit potential overthe frequency range 100 kHz–0.01 Hz for EIS measurement. EIStest was carried out every two hours with a total of 10 h.

Each set of EN records was 500 s with a data sampling interval0.244 s, a total of 2048 data points. This procedure was repeatedfor 10 h. EN signals were recorded when a single electrolyte drop-let formed on the electrode surface. Even though the droplet wouldfall off during EN measurement period, there was still a completeelectrolyte layer covering on electrode surface and then a newdroplet would form again. Therefore, the EN records of carbon steelin DPC environment were continuous. The direct trend of electro-chemical noise data was removed by using 5-order polynomialdetrending method [12]. EN records were analyzed both by statis-tical analysis and wavelet analysis. As for statistical analysis, awidely used parameter noise resistance (Rn), defined as the ratioof the standard deviations of the potential noise and current noise,was calculated. Regarding for wavelet analysis, FWT was per-formed by MATLAB 7.0 software and nine crystals were obtained(J = 8).

3.4. Weight loss measurement

Specimens were cut into 20 � 10 � 2.5 mm3 for weight lossexperiment. The specimens were firstly wet ground to a 1000-gritfinish by silicon carbide paper, washed using deionized water,dried and weighed by means of an analytical balance (SartoriusCP225D) with a precision of 0.0001 g for the original weight. Andthen the specimens were exposed to the corrosion environment.

Page 4: In situ study of dew point corrosion by electrochemical measurement

Y. Yang et al. / Corrosion Science 71 (2013) 62–71 65

After test, corrosion products of the specimens were removed byultrasonic cleaning in the hydrochloric acid solution with hexa-methylenetetramine as the inhibitor. Finally, the specimens with-out corrosion products were washed with deionized water, driedand weighed again for the final weight. Weight loss of the carbonsteel every 2 h during 10 h test was obtained and each experimentwas performed two times for reproducibility.

3.5. Corrosion morphology observation

In order to gain the direct information of the corrosion process,surface of the corroded carbon steel specimen after different expo-sure to the corrosion environment was examined by scanning elec-tron microscope (SEM). The procedure was as following: Firstly, thespecimen was wet ground to a 2000-grit finish by silicon carbidepaper and polished by diamond spray. Second, the microstructureof the specimen was etched in the solution containing 97 mL alco-hol + 3 mL nitric acid for 3�5 s. Third, the specimen was exposed tothe corrosion environment for 10 min, 30 min, 2 h and 10 h. Finally,corrosion product of the specimen was removed by ultrasoniccleaning in the hydrochloric acid solution with hexamethylenetet-ramine as the inhibitor, dried and prepared for SEM observation.

4. Results and discussion

4.1. Determination of DPC environment parameters

It is necessary to make clear the DPC environment parametersincluding temperature and pH of the acid droplet condensed onelectrode specimen surface prior to electrochemical tests. Fromtop to bottom, temperature and pH distributions in different posi-tion of the condenser tube are schematically shown in Fig. 5 whenthe concentration of hydrochloric acid in the evaporation bottle is0.5 mol/L. Acid droplet with temperature 60 �C and pH 2.0 is cho-sen to be the DPC environment in this work. The measurementprocedure of the parameters is as followed:

Put a thermometer in different positions of the condenser tube,the temperature is determined when an acid droplet forming onthe bottom of the thermometer. Electrode specimen is fixed at

30 °C

50 °C

60 °C

70 °C

40 °C

80 °C

Bottom

90 °C

Top

pH=2.0

pH=1.5

pH=7.0

pH=3.0

Fig. 5. A schematic diagram of the temperature and pH distributions in differentposition of the condenser tube.

the position where temperature of the condensed acid droplet is60 �C in this work.

Considering that H+ concentration is equal to the Cl� concentra-tion in acid droplet in this work, pH value of the acid droplet can becalculated from the potential of the Ag/AgCl electrode which de-pends on Cl� concentration. Ag/AgCl ion selective electrode wasfabricated using high purity silver wire with 1 mm diameter. Thesilver wire was anodized in 0.1 mol/L HCl solution for 7 h at0.15 mA/cm2 and aged in 0.1 mol/L KCl solution for 24 h. Beforeuse, the Ag/AgCl ion selective electrode was calibrated by measur-ing a series of standard solutions with known Cl� at 60 �C and thecalibration curve is presented in Fig. 6. The measurement proce-dure of the pH can be divided into three stages: First, a combinedreference electrode including a Ag/AgCl ion selective electrode anda reference electrode shown in Fig. 7 is constructed by exposeadhesive. Then, put the combined reference electrode in the con-denser tube, the position of which is the same as the electrodespecimen, and the potential of the Ag/AgCl ion selective electrodeis measured when acid droplet forming on the bottom of the com-bined reference electrode. Finally, the H+ concentration (equal tothe Cl� concentration) can be calculated according to the calibra-tion curve in Fig. 6.

4.2. Reliability demonstration of electrochemical measurements forDPC study

EIS test results of carbon steel during 10 h exposure in DPCenvironment are shown in Fig. 8. Generally, a peak and a valleyin the figure ‘‘Phase vs Frequency’’ of Bode plot represents a capac-itive loop and an inductive loop, respectively. It can be seen fromFig. 8 that the EIS plots exhibit a capacitive loop in the high fre-quency range and an inductive loop in the low frequency range.The equivalent circuit in Fig. 9 is used to fit the EIS data, whereRs is the solution resistance; Qf and Rf represents the adsorptioncapacitance and the resistance of corrosion products covered oncarbon steel surface, respectively; Qdl is the double layer capaci-tance and Rt is the charge transfer resistance of the carbon steel;L and RL represents the inductance and resistance which may be re-lated to the adsorption species as Cl�ads and Hþads on the electrodesurface [13,14].

The polarization resistance, Rp, is an important parameter toevaluate corrosion resistance, the reciprocal of which is propor-tional to the corrosion rate [15]. Rp, can be calculated from theequivalent circuit in Fig. 9 as follows:

Rp ¼ Rs þ Rf þRtRL

Rt þ RLð9Þ

10-3 10-2 10-1 100

150

200

250

300

350

400

Ag/A

gCl p

oten

tial/m

VSH

E

Cl-/mol.L-1

Fig. 6. The calibration curve of Ag/AgCl ion selective microelectrode.

Page 5: In situ study of dew point corrosion by electrochemical measurement

SCE (RE)

Ag/AgCl chloride electrode

Fig. 7. A schematic diagram of the combined reference electrode.

10-2 10-1 100 101 102 103 104 105-10

-5

0

5

1040

60

80

100

120

140

-Pha

se a

ngle

Frequency/Hz

|Z|/Ω

.cm

2

0.5h 2h 4h 6h 8h 10h Fitting

Fig. 8. EIS of carbon steel during 10 h exposure in DPC environment.

Fig. 9. The equivalent circuit for carbon steel EIS data fitting in DPC environment.

Fig. 10. EN records of carbon steel during 10 h exposure in DPC environment.

0 2 4 6 8 10

0

2

4

6

8

10

12

0.000

0.003

0.006

0.009

0.012

0.015

0.018

EISC

orro

sion

rate

by

wei

ght l

oss/

g.m

-2.h

-1

Time/h

Corrosion rate by EIS and EN

/Ω-1.cm

-2

EN

Fig. 11. Variation of corrosion rate for carbon steel obtained by EIS (1/Rp), EN (1/Rn)and weight loss test during 10 h exposure in DPC environment.

0 2 4 6 8 10

0

20

40

60

80

100

120

Wei

ght l

oss/

g.m

-2

Time/h

acid simulation solution DPC environment

Fig. 12. Weight loss comparison result in DPC environment and acid simulationsolution.

66 Y. Yang et al. / Corrosion Science 71 (2013) 62–71

EN records of carbon steel during 10 h exposure in DPC environ-ment are shown in Fig. 10. Electrochemical noise resistance (Rn),which is equivalent to polarization resistance Rp [16–18], can beused to calculate the corrosion rate. The ratio 1/Rn is proportionalto the corrosion rate [19].

In order to verify the reliability of the electrochemical measure-ments results (EIS and EN), corrosion rate (v) of carbon steel everytwo hours in DPC environment is also calculated by weight lossmethod according to the equation as follows:

v i ¼DWi � DWi�2

A� 2ði ¼ 2;4;6;8;10Þ ð10Þ

where i stands for the exposure time (h), DW is the weight loss afterexposure (g) and A is the exposure area of the specimen (m2).

Corrosion rates of carbon steel obtained by different methodsincluding EIS (1/Rp), EN (1/Rn) and weight loss in DPC environmentare compared Fig. 11. It can be seen that EIS and EN results agreewith each other very well and moreover variation of corrosionrates by electrochemical methods (EIS and EN) are consistent with

Page 6: In situ study of dew point corrosion by electrochemical measurement

10-2 10-1 100 101 102 103 104 105-10-505

101520

40

60

80

100

-Pha

se a

ngle

Frequency/Hz

|Z|/Ω

.cm

2

0.5h 2h 4h 6h 8h 10h Fitting

Fig. 13. EIS of carbon steel during 10 h immersion in acid simulation solution.

0 2 4 6 8 100.000

0.004

0.008

0.012

0.016

0.020

0.024

0.028

Rn-1

/ Ω-1.c

m-2

Time/h

acid simulation solution DPC environment

Fig. 16. Comparison result of 1/Rn in DPC environment and acid simulationsolution.

Fig. 17. Macroscopic corrosion morphologies of carbon steel after 10 h weight lossin two environments: (a) DPC environment, and (b) acid simulation solution.

Fig. 14. EN records of carbon steel during 10 h immersion in acid simulationsolution.

0 2 4 6 8 10

0.000

0.006

0.012

0.018

0.024

Rp-1

/ Ω-1.c

m-2

Time/h

acid simulation solution DPC environment

Fig. 15. Comparison result of 1/Rp in DPC environment and acid simulationsolution.

Y. Yang et al. / Corrosion Science 71 (2013) 62–71 67

weight loss method, i.e. the in situ electrochemical measurementsresults of DPC are reliable.

4.3. Corrosion behaviour comparison of carbon steel in DPCenvironment and acid simulation solution

Measurements of DPC in a dynamic acid electrolyte layer aredefined as in situ test method and ex situ test method means

employing acid simulation solution to replace the dynamic acidelectrolyte layer. It is necessary to compare the corrosion behav-iour studied by two methods in order to characterize DPC andhighlight the importance of the in situ test method. Therefore,the same measurements including weight loss, EIS and EN are per-formed in hydrochloric acid solution, where the temperature(60 �C) and pH (2.0) are the same as the acid droplet in DPCenvironment.

4.3.1. Weight loss test and electrochemical measurementsFig. 12 represents the weight loss comparison result in two

environments. It is noted that carbon steel shows more weight lossin acid simulation solution than that in DPC environment, whichindicates higher corrosion rate in acid simulation solution. EIS testresult of carbon steel during 10 h immersion in acid simulationsolution is shown in Fig. 13. EIS data is fitted using the equivalentcircuit in Fig. 9 and the meaning of each electrochemical parameteris the same as that in the DPC environment, which has beenexplained in previous section. Meanwhile, the parameter Rp is

Page 7: In situ study of dew point corrosion by electrochemical measurement

Fig. 18. Corrosion morphologies of carbon steel after different exposure time in the DPC environment: (a) 10 min, (b) magnification of (a); (c) 30 min, (d) magnification of (c);(e) 2 h, (f) magnification of (e); and (g) 10 h.

68 Y. Yang et al. / Corrosion Science 71 (2013) 62–71

calculated using Eq. (9) to obtain the corrosion rate of carbon steel.EN records of carbon steel in acid simulation solution are pre-sented in Fig. 14 and noise resistance Rn is calculated. Corrosionrates in two environments calculated from EIS (1/Rp) and EN (1/Rn) are contrasted in Figs. 15 and 16, respectively. Both 1/Rp and1/Rn show that corrosion rate of carbon steel in acid simulationsolution is higher than that in DPC environment, which agrees wellwith the weight loss result.

Macroscopic corrosion morphologies of carbon steel after 10 hweight loss in two environments are shown in Fig. 17. From the

macroscopic, there are obviously more corrosion product coveredon specimen surface after exposure test in DPC environment(Fig. 17a) compared with that in acid simulation solution(Fig. 17b). Differences between corrosion morphologies could berelated to the discrepancy between two corrosion environments.Corrosion products of the specimen in acid simulation solution falloff easily under the effect of hydrogen stirring resulting fromhydrogen evolution reaction of the carbon steel surface. However,hydrogen runs off easily from the electrolyte layer and the hydro-gen stirring effect is weakened when the specimen is exposed to

Page 8: In situ study of dew point corrosion by electrochemical measurement

Fig. 19. Corrosion morphologies of carbon steel after different immersion time in acid simulation solution: (a) 10 min, (b) magnification of (a); (c) 30 min, (d) magnification of(c); (e) 2 h, (f) magnification of (e); and (g) 10 h.

Y. Yang et al. / Corrosion Science 71 (2013) 62–71 69

dynamic acid electrolyte layer, which indicates that more corro-sion products are remained (Fig. 17a). The corrosion products onspecimen surface in DPC environment played a protective role tosome degree compared with that in acid simulation solution. Thatmay be why the corrosion rates in two environments are different.

4.3.2. Corrosion morphologies observationCorrosion morphologies of carbon steel after different exposure

time in the DPC environment are shown in Fig. 18. According to thechange of the corroded specimen surface morphology, the corro-

sion process of carbon steel in the DPC environment can be de-scribed as followed: First, after 10 min exposure to the DPCenvironment, many pits can be observed on the surface of the spec-imen due to the condensation of acid micro-droplet shown inFig. 18a and 18b shows that the pits generate preferentially onthe boundary and interior of ferrite phase (marked ‘‘1’’ inFig. 18b) and boundary between pearlite and ferrite (marked ‘‘2’’in Fig. 18b). Second, with the increase of exposure time, pit sizesincrease obviously (Fig. 18d) and a number of pits begin to coalesce(Fig. 18c). After 2 h exposure, pit sizes increase continuously and

Page 9: In situ study of dew point corrosion by electrochemical measurement

3000 3050 3100 3150 3200 3250

-60

-30

0

30

60

90

120 acid simulation solutionDPC environment

i/μA.

cm-2

Time/s

Fig. 20. Typical current signals of carbon steel in DPC environment and acidsimulation solution.

1

2

3

4

5

6

7

8

Time/h

d

0

0.1094

0.2188

0.3281

0.4375

0.54690.7000

0 2 4 6 8 10

0 2 4 6 8 101

2

3

4

5

6

7

8

Time/h

d

0

0.1094

0.2187

0.3281

0.4375

0.54690.7000

(a)

(b)

Fig. 21. Variation of EDPs corresponding to the current noise of the carbon steel intwo environments: (a) DPC environment, and (b) acid simulation solution.

70 Y. Yang et al. / Corrosion Science 71 (2013) 62–71

the boundaries between pearlite and ferrite become misty (Fig. 18eand f). Third, as exposure time increases to 10 h, most of the pitscoalesce with each other and a few pits exist on specimen surface(Fig. 18g).

Corrosion morphologies of carbon steel after differentimmersion time in acid simulation solution are depicted inFig. 19. It is noted that the corrosion process in acid simulationsolution is completely different from that in DPC environment.

After 10 min immersion in acid simulation solution, the bound-aries between pearlite and ferrite in carbon steel are still clear(Fig. 19a) and the steel shows the characteristic of uniformcorrosion (Fig. 19b). With the increase of immersion time, theboundaries between pearlite and ferrite become misty (Fig. 19c)and the specimen surface begin to be rough (Fig. 19d). After2 h immersion, the specimen surface becomes rougher, whichmay result from the nonuniform corrosion of pearlite and ferrite(Fig. 19e and f). As immersion time increases to 10 h, the speci-men surface is mainly suffering from uniform corrosion(Fig. 19g).

4.3.3. EN analysisTypical current signals of carbon steel in DPC environment and

DPC environment are shown in Fig. 20. It can be seen that the cur-rent transients in two environments are different: First, the num-ber of transient peaks above the baseline is much more in DPCenvironment than that in acid simulation solution. Second, currenttransients in DPC environment are characterized by a quick risefollowed by a quick decay to the base line and the features of cur-rent transients in acid simulation solution are a slow rise followedby a slow decay to the base line. These phenomena may be relatedwith the different corrosion morphologies (Figs. 18 and 19), whichclearly show that carbon steel is mainly suffering from pitting cor-rosion in DPC environment and uniform corrosion in acid simula-tion solution.

In order to explain the different corrosion mechanism in twoenvironment, the evolution features of energy distribution plots(EDPs) corresponding to the current noise of the carbon steel intwo environments with 10 h test are shown in Fig. 21a and b,respectively. The relative energy value of each crystal showingby different colour in EDP stands for the contribution of the crys-tal to the corrosion process. The higher the relative energy valueof the crystal, the greater the contribution to the corrosion pro-cess. Quick step of the corrosion process appears in the high fre-quency domain, while slow step in the low frequency. Accordingto Eq. (4), the time constants of crystals d1–d8 are between0.244 s and 62.5 s, which is associated with the events from highfrequency domain to low frequency domain. If low frequency do-main corrosion events are dominant in the corrosion process, cor-rosion tends to be localized corrosion, such as pitting corrosion.Otherwise, it will be uniform corrosion. The distribution changeof the relative energy of each crystal can reflect different corro-sion process.

In the DPC environment, the corrosion process of carbon steelcan be divided into two stages from Fig. 21a. From the beginningto about 4 h, the energy is mainly accumulated among the crystalsd7–d8 (corresponding to the time constants between 15.63 s and62.5 s), which may correspond to the slow localized corrosionevents, such as pitting (Fig. 18a–f). In the second stage (4 h to10 h), the energy scattered in d3–d8 and the energy percentage ofthe crystals d7–d8 decreases compared with that in the first stage.Therefore, the number of pits on specimen surface decreased obvi-ously (Fig. 18g).

From the beginning to 4 h in acid simulation solution shown inFig. 21b, the energy is mainly accumulated among the crystals d5–d7 (corresponding to the time constants between 3.9 s and 31.25 s).The frequency of the dominant corrosion events d5–d7 in acid sim-ulation solution is higher than d7–d8 in the DPC environment fromthe beginning to 4 h, which indicate that the corrosion morpholo-gies will be more uniform (Figs. 19a–f and 18a–f). From 4 h to 10 hin acid simulation solution, the energy scattered in d1–d8. There-fore, the corrosion morphology shows the characteristic of uniformcorrosion (Fig. 19g).

Page 10: In situ study of dew point corrosion by electrochemical measurement

Y. Yang et al. / Corrosion Science 71 (2013) 62–71 71

5. Conclusions

In situ electrochemical measurements including EIS and EN areapplied to estimate corrosion rates of carbon steel in DPC environ-ment by a novel simulation set-up and particular electrodearrangements in this paper. The electrochemical measurements re-sults are in agreement with the weight loss test, which demon-strates the reliability of the electrochemical measurements.

Corrosion behaviour of carbon steel in DPC environment andacid simulation solution is compared in order to differentiatein situ test method and ex situ test method for studying DPC.The results indicate that carbon steel shows higher corrosion rateand different corrosion behaviour in acid simulation solution com-pared with that in DPC environment. Therefore, it is recommend-able to study DPC by in situ test method so as to understand DPCin nature.

Acknowledgements

The authors wish to acknowledge the financial support of theprogram for the Hundred Talents Program of China, New CenturyExcellent Talents in University of China (NCET-09-0052) and theFundamental Research Funds for the Central Universities(HEUCF201210001).

References

[1] V. Lins, E. Guimaraes, Failure of a heat exchanger generated by an excess of SO2

and H2S in the sulfur recovery unit of a petroleum refinery, J. Loss Prevent Proc.Ind. 20 (2007) 91–97.

[2] W.M.M. Huijbregts, R. Leferink, Latest advances in the understanding of aciddewpoint corrosion: corrosion and stress corrosion cracking in combustion gascondensates, Anti-Corros. Method Mater. 51 (2004) 173–188.

[3] X.Q. Cheng, F.L. Sun, S.J. Lv, X.G. Li, A new steel with good low-temperaturesulfuric acid dew point corrosion resistance, Mater. Corros. DOI: 10.1002/maco.201006046.

[4] I. Matsushima, Low-alloy Corrosion Resistant Steels-a History of Development,Application and Research, Metallurgical Industry Press, Beijing, 2004.

[5] U. Kivisäkk, A test method for dewpoint corrosion of stainless steels in dilutehydrochloric acid, Corros. Sci. 45 (2003) 485–495.

[6] A. Nishikata, Y. Ichihara, T. Tsuru, An application of electrochemical impedancespectroscopy to atmospheric corrosion study, Corros. Sci. 37 (1995) 897–911.

[7] A. Nishikata, Y. Yamashita, H. Isatayama, T. Tsuru, A. Usami, K. Tanabe, H.Mabuchi, An electrochemical impedance study on atmospheric corrosion ofsteels in a cyclic wet-dry condition, Corros. Sci. 37 (1995) 2059–2069.

[8] G.A. El-Mahdy, A. Nishikata, T. Tsuru, AC impedance study on corrosion of55%Al–Zn alloy-coated steel under thin electrolyte layers, Corros. Sci. 42(2000) 1509–1521.

[9] A. Aballe, M. Bethencourt, F.J. Botana, M. Marcos, Wavelet transform-basedanalysis for electrochemical noise, Electrochem. Commun. 1 (1999) 266–270.

[10] B. Zhao, J.H. Li, R.G. Hu, R.G. Du, C.J. Lin, Study on the corrosion behavior ofreinforcing steel in cement mortar by electrochemical noise measurements,Electrochim. Acta 52 (2007) 3976–3984.

[11] C. Cai, Z. Zhang, F.H. Cao, Z.N. Gao, J.Q. Zhang, C.N. Cao, Analysis of pittingcorrosion behavior of pure Al in sodium chloride solution with the wavelettechnique, J. Electroanal. Chem. 578 (2005) 143–150.

[12] U. Bertocci, F. Huet, R. Nogueira, P. Rousseau, Drift removal procedures in theanalysis of electrochemical noise, Corrosion 58 (2002) 337–347.

[13] H.J.W. Lenderink, M.V.D. Linden, J.H.W. De Wit, Corrosion of aluminium inacidic and neutral solutions, Electrochim. Acta 38 (1993) 1989–1992.

[14] M.A. Amin, S.S. Abd El-Rehim, E.E.F. El-Sherbini, R.S. Bayoumi, The inhibition oflow carbon steel corrosion in hydrochloric acid solutions by succinic acid: PartI. Weight loss, polarization, EIS, PZC, EDX and SEM studies, Electrochim. Acta52 (2007) 3588–3600.

[15] M. Stern, A.L. Geary, Electrochemical polarization: I. A theoretical analysis ofthe shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63.

[16] U. Bertocci, C. Gabrielli, F. Huet, M. Keddam, Noise resistance applied tocorrosion measurements: I. Theoretical analysis, J. Electrochem. Soc. 144(1997) 31–37.

[17] F. Mansfeld, H. Xiao, Electrochemical noise analysis of iron exposed to NaClsolutions of different corrosivity, J. Electrochem. Soc. 140 (1993) 2205–2209.

[18] U. Bertocci, C. Gabrielli, F. Huet, M. Keddam, P. Rousseau, Noise resistanceapplied to corrosion measurements: II. Experimental tests, J. Electrochem. Soc.144 (1997) 37–43.

[19] J. Chen, W. Bogaerts, The physical meaning of noise resistance, Corros. Sci. 37(1995) 1839–1842.