CHAPTER I

INTRODUCTION

1.1 Problem Statement

Mild steel is an industrially significant structural material and most commonly used

metals for various purposes including reactors, boilers, drums, petrochemicals process

devices and heat exchangers. The pervasive use of mild steel for commercial

processes is accredited to its unique properties such as strength, low cost and easy

availability for fabrication means. However, mild steel is susceptible to corrosion due

to its thermodynamic instability especially in aggressive media (Pavithra et al., 2010).

1.2 Background of Study

Corrosion in general terminology is the destructive attack of metal. Scientifically,

corrosion is an electrochemical process that involves slow, steady, and irreversible

deterioration or degradation of metal, in both chemical and physical properties

(Roberge, 2000). This phenomenon arise broadly in various industrial fields and

usually related to operating problems and equipment maintenance, leading to recurrent

partial and even total process shutdowns and finally ensue in severe economic losses.

Furthermore, corrosion not only results in huge economic losses but also possesses

social impact and these implications engage with the safety and health of people either

working in the particular industries or living nearby.

The study of corrosion and their inhibition properties is an active field of research.

Corrosion inhibitor is customary and frequently used among all other numerous

anticorrosion measures, which acts as one of the most economical and effective

methods to militate against corrosion. Basically, inhibitors are chemical compounds

which are added in small quantities to reduce the corrosion rate (Lopez et al., 2005).

The purpose of adding inhibitors in low concentrations to corrosive media is to delay

the reaction between metal and the corrosive species in the medium. Inhibitors slow

the corrosion processes by: first, reducing the anodic and cathodic reaction speed;

second, by reducing the movement or diffusion of ions to/and from the metallic

surface; and third, increasing the electrical resistance of the metal surface (Emregu

Orhan Atokol, 2006).

Corrosion can be mitigated by the action of inhibitor which prevent the

adsorption of Cl- ions and/or by the formation of more resistant oxide film on the

metal surface (Nataraja et al., 2011). The adsorption ability of inhibitors onto the

metal surface depends on several factors including the nature and surface charge of

metal, chemical composition of electrolytes, and also molecular structure and

electronic characteristics of inhibitor molecules (Yuce & Kardas, 2012). In addition,

the adsorption of these compounds are also influenced by the electronic structure of

inhibiting molecules, steric effect, aromaticity and electron density at donor site,

presence of a functional group such as -CHO, -N=N, R-OH, etc, molecular area and

molecular weight of the inhibitor molecule (Singh & Quraishi, 2010; Ostovari et al.,

2009; Quraishi & Shukla, 2009).

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Accordingly, the most significant and efficient inhibitors studied are heterocyclic

organic compounds containing mostly nitrogen, oxygen, and sulphur atoms in their

structures which are found to have higher basicity and electron density. These

compounds have shown great effectiveness for inhibiting aqueous corrosion due to

film formation on the metal surface (Loto et al., 2012). Most organic inhibitors adsorb

on the metal surface by displacing water molecules and thus, forming a compact

barrier. Several researchers, such as Soltani et al (2015), Verma et al (2014), and Uday

et al (2013) have proved that the organic molecules inhibit corrosion by adsorption, in

which polar groups acting as adsorption centers thus forming a barrier between the

metal and the environment.

Equally important, the availability on non-bonded (lone pair) and p-electrons

in inhibitor molecules also facilitate the electron transfer from the inhibitor to the

metal (Amitha & Bharathi, 2012). The efficiency of the inhibitor depends on the

stability of the chelate formed (Scendo, 2008), so it mainly depends on the type and

the nature of the substituents present in the inhibitor molecule (Li et al., 1999).

Furthermore, through the light of the tendency to form a stronger coordination bond,

inhibition efficiency should increase in the sequence o£ oxygen < nitrogen < sulphur

(Khadom et al., 2010).

The choice of organic inhibitors is based on two considerations: first, they

could be synthesized conventionally from relatively cheap raw materials; and second,

they contain the electron clouds or the electronegative atoms (Hossein et al., 2007).

For these reasons, among various compounds, thiourea and thiourea derivatives are

acknowledged to be the effective corrosion inhibitor besides having another important

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theoretical and practical application (Abdel-Gaber et al., 2010). Moreover, this

compounds which are possesses both nitrogen and sulphur atoms simultaneously,

always exhibits higher inhibition performance than other inhibitor that possesses

single nitrogen or sulphur atom (Li et al., 2012).

1.3 Scope of Study

Thiourea and its derivatives has been long studied in considerable detail for the use

against corrosion of a wide range of metals in various corrosive environments (Yadav

et al., 2014; Al-Sabagh et al., 2012; Abdel-Rahim et al., 2006). This present research

was carried out in continuation of developing high effective and efficient corrosion

inhibitors. Eight derivatives of benzoyl thiourea (T1-T8) were synthesized and then

characterized via elemental analyzer, Fourier Transform Infrared (FTIR) and 11-1 and

13C Nuclear Magnetic Resonance (NMR) spectroscopies to positively identify them.

Further, three of the eight compounds which are T6, T7 and T8 were selected and

explored a systematic study to investigate their performance in anticorrosion activity.

The corrosion inhibitive action of synthesized thiourea derivatives of mild steel in 1.0

M hydrochloric acid has been investigated at room temperature using weight loss,

electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization

measurements.

1.4 Objectives of Study

This particular work is carried out with the following objectives:

i. to synthesize new series of benzoyl thiourea derivatives

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ii. to characterize the synthesized compound

iii. to investigate the inhibition effect of the synthesized compounds on corrosion

of mild steel in 1.0 M HCI.