Corrosion

Introduction:Corrosion is defined as the degradation of material due to chemical or electrochemical reaction with the environment. The destruction or degradation of the material due to physical causes is called erosion, galling or wear. Sometimes both physical and chemical attack on the material occurs at the same time and it is called corrosion-erosion or corrosive wear.Importance of corrosion is for three main reasons: Economics, Safety, and conservation. Studies show that the corrosion damages costs the industrialized nations about 3 to 5 % of the gross national product (GNP).Corrosion was the reason for some catastrophes in the industry as it adversely affects the materials what causes failure. Corrosion Mechanisms:As mentioned corrosion is an electrochemical process. In aqueous media, the corrosion reactions are similar to those that occur in a flashlight cell consisting of a center carbon electrode and a zinc cup electrode separated by an electrolyte consisting essentially of NH4Cl solution. An incandescent light bulb connected to both electrodes glows continuously, with the electrical energy being supplied by chemical reactions at both electrodes. At the carbon electrode (positive pole), chemical reduction occurs, and at the zinc electrode (negative pole) oxidation occurs, with metallic zinc being converted into hydrated zinc ions, Zn 2+nH2O. The greater the flow of electricity through the cell, the greater the amount of zinc that corrodes.

Two basic mechanisms of corrosion:1- Electrolytic corrosion: Electrolytic corrosion is a result of direct current from outside sources entering and then leaving a particular metallic structure by way of the electrolyte. It occurs as a result of the difference potential energy of two metals in electrolyte.

2- Galvanic corrosion: It occurs due to difference in electrolyte.

Carbon content effect on corrosion:

The higher the carbon content of steel, the higher the corrosion rate.

A pure iron corrodes in acids at a much lower rate than an iron or steel high in residual elements, such as carbon, nitrogen, sulfur, and phosphorus.

Page | 2