Project Material: SELF CATHODIC PROTECTION POTENTIAL OF DRIED PEPPER SEED ON MILE STEEL CORROSION IN ACIDIC MEDIUM

CHAPTER ONE

INTRODUCTION

1.1       Background of the Study

Mild steel is the most commonly use steel; it is used in the industries as well in the different everyday object we use. Even the pans and spoons of the kitchen are sometimes made of mild steel. It is found in most of the chemical industries due to its low cost and easy availability for fabrication or various reactions vessels, tanks, pipes, etc. it is one of the most important metal used in different field of industries, e.g. Automobile, engineering, submarine, etc.  However, this metal is severely affected by the environmental pollutants such ass chlorate, sulphate, nitrate, phosphate, etc (Revie & winton, 2000). In order to minimize this problem, the use of inhibitor is one of the best methods to protect this metal against corrosion. This corrosion of mild steel and its inhibition in acid and other environments have more attention from numerous previous investigators (Umoren, Edouk & oguezie, 2008; Siddiqi & chaubey, 2008; singh & adeyemi, 1987). The corrosion material is one of the main problems facing industrial processes, generating huge financial losses. Metallic industrial structures are exposed to conditions that facilitate corrosive processes. For example, acidic solutions, which are wildly used in acid pickling, industrial acid cleaning and oil refinery equipment cleaning, promotion the acceleration of metallic corrosion, affecting the performance and durability of the treated equipment (Obot & Obi-Egbi, 2010) the use of organic inhibitors to prevent corrosion is a promising alternative solution. These inhibitors are usually absorbed on the metal surface by the formation of coordinate covalent bond (chemical adsorption)or the electrostatic interaction between the metal and inhibitor (physical adsorption) (Ahamad. Prasad& Quraishi, 2010; Noor, 2008). This adsorption produces a uniform film on the metal surface, which reduces or prevents contact with the corrosive medium (Auci, 2008). Because organic inhibitors act by adsorption on the metal surface, the efficiency of these compounds depends strongly on their ability to form complexes with the metals (Shukla & Quraishi, 2009). Both Pelectrons and the polar group containing sulfur, oxygen and/or nitrogen are fundamental characteristics of this type of inhibitor ( Zapta-Loria and pech-Canul, 2014; Yadav-Kumar & Gope,2014; Yadav et al., 2015). The polar functional groups are usually considered the chelation center for chemical adsorption (de souza & spinelli, 2009).

Corrosion is defined as the deterioration of materials by chemical process. The most important by far is the electrochemical corrosion of metals in which the oxidation process     M     M ⁺ +e                                                               is facilitated by the presence of a suitable electron acceptor, sometimes, referred to in corrosion science as a depolarizer.

 In science, corrosion can be viewed as a spontaneous return of metals to their ores, the huge quantities of energy that were consumed in mining, refining and manufacturing metals into useful objects is dissipated by a variety of different routes.

(sir, Humphrey Dary, 2006).

1.2.      Corrosion cell and Reaction

Corrosion cells are a condition on a metal surface in which a flow of electric current occurs between the metal surface and an electrolyte with which it is in contact sufficient to cause the metal to degrade.

Some examples of anodic and cathodic reactions that occur simultaneously on a metal surface in a corrosion cell are as follows;

A typical anodic oxidation that produces dissolved ionic product, for

Fe → Fe²⁺ + 2e^--------------------------------------------- (1)

Examples of cathodic reduction involved in corrosion processes are:

O₂ + 2H₂O + 4e^-  4OH^- --------------------------------- (2)

O₂ + 4H⁺ + 4e^-  2H₂O------------------------------------ (3)

2H⁺ + 2e^- H²--------------------------------------------- (4)

The cathodic reaction represented by eqn.(2) exemplifies corrosion in natural environments where corrosion occurs  at nearly neutral pH values. Eqn. (3) and (4) represents corrosion processes taking place in the acidic environments encountered in industrial processes or for the confined volumes (pits, crevices) where the pH can reach acidic values because of hydrolysis reactions such as:

Fe²⁺ + 2H₂O  Fe(OH)₂ +2H⁺----------------------------(5)

This reaction produces H⁺ ions, the concentration of which can, under certain conditions become large if the H⁺ ion cannot readily move out from a confined volume. The overall corrosion reaction is, of course, the sum of the cathodic and anodic partial reactions. For example, for a reaction producing dissolved ions (sum of reactions (1) & (4)):

Fe + 2H⁺ Fe²⁺ + H₂ ------------------------------------ (6)

Or for a reaction producing dissolved ions (sum of reactions (1) & (2)):

2Fe + O₂ + 2H₂O 2Fe(OH)₂ -------------------------- (7)

1.3       Electrochemical corrosion of iron

           Corrosion often begins at a location (1) in Fig. 1.2 where the metal is under stress (at a bend of weld) or is isolated from the air (where two pieces of metal are joined or under a loosely-adhering paint film). The metal ions dissolve in the moisture film and the electrons migrate to another location (2) where they are taken by a depolarizer. Oxygen is the most common depolarizer, the resulting hydroxide ions react with the Fe²⁺ to form the mixture of hydrous iron oxides known as rust.

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