Electrochemical Corrosion Principles

CHAPTER-1

INTRODUCTION

1.1 General:-

Corrosion is a natural process. Just like water flows to the lowest level, all natural processes tend toward the lowest possible energy states. It is everywhere, air, water, soil and in every environment. Corrosion is mainly due to chemical or electrochemical reaction of the metal with environment. Marine environments are highly corrosive to the sea water steel structure belonging to the shipping, offshore oil and gas production, power plants and coastal industrial plants .the corrosion problems in these systems have been well studies over many years .The most characteristics feature of seawater is its high salt content, which makes it a good electrolyte and can cause galvanic corrosion and crevice corrosion.

Thermodynamic principles

2. Physical & Chemical principles

3. Metallurgical principles

4. Electrochemical principles

Thermodynamic and electrochemical principles play a major role in determining the corrosion behaviour of materials. Thermodynamic principles can indicate the spontaneous direction of a chemical reaction. They are used to determine whether the corrosion is theoretically possible or not. Thermodynamic principles are applicable to free energy, electrical double layer, Nernst equation for electrode potentials and Pourbaix diagrams [4,5].

Electrochemical principles are extensively used to determine the corrosion behavior of materials. Here, the corrosion reaction can be represented by partial reactions such as metal oxidation and reduction of some reducible species of the environment both occurring simultaneously at equal rates, at the mixed potential [6] of the reaction. Corrosion reaction mainly occurs at the metal-environment interface.

The electrochemical nature of corrosion can be illustrated by the attack of iron in hydrochloric acid. When the iron is dipped in acid a vigorous action occurs as a result hydrogen gas is evolved and iron gets dissolved.

Hence the reaction

The operating potential of an anode materials is its potential when coupled with a structure . since all commercial anode materials are formulated to suffer only slight polarization under most conditions of exposure , the operating potential approximates to the open circuit potential . Indeed, any substantial difference between these two potentials will call into question the suitability of the anode in the particular environment .

The protection potential refers to the potential at which experience shows corrosion of a metal will cease . different materials have different protection potentials (table 1.).

Occasionally a less negative protection potential will specified because some degree of corrosion is permissible . It should be noted that in a mixed metal system the protection potential for the most base metal is adopted