Fundamentals of corrosion and its prevention

Abstract

Most metals other than the noble metals, do not exist in their native states but exist as compounds of the metals (oxides, sulphides, etc.) indicating that such states are energetically more favourable. It is no wonder therefore that most metals tend to revert back to their more stable compound state through corrosion. Corrosion is thus inevitable for metals and alloys. Corrosion is the most predominant cause of metal failures today, surpassing other failure modes like fatigue, creep, impact, and others. The cost of corrosion is not only the cost of replacement but additional costs as well such as: ➢ Loss of production due to shutdown or failure > High maintenance costs ➢ Compliance with environmental and consumer regulations ➢ Loss of product quality due to contamination from corrosion of the materials ➢ High fuel and energy costs as a result of leakage from corroded pipes > Extra working capital and larger stocks Corrosion is differentiated from chemical attack in that it normally involves two or more electrochemical reactions. Thus while an insulating material can react with a strong chemical, such a degradation would not be considered corrosion as there are no electrochemical reaction involved. Any degradation suffered by a chemical or component resulting from electrochemical reactions involving species of the constituent material is corrosion. Corrosion may involve substantial material loss, but that is not universally true. Corrosion when manifested as pitting can cause very small total loss of material while destroying the component integrity. Similarly, hydrogen embrittlement can occur with no material loss, when hydrogen produced in an electrochemical fashion, enters the material leading to its loss of material properties. The study of corrosion therefore is also a study of material-environment interaction through a set of electrochemical reactions. It naturally requires detailed understanding of the metallurgy of the material including time dependent changes in the metallurgy, stresses present in the component, operational parameters, environment chemistry and reaction thermodynamics and kinetics. A point of view proposed by Professor Staehle, a well known corrosion expert, is that all engineering materials are reactive chemically and that the strength of materials depends totally upon the extent to which environments influence the reactivity and subsequent degradation of these materials. In order to define the strength of an engineering material for a corrosion based design it is essential to define the nature of the environments affecting the material over time. The corrosion event is a culmination of the conjoint action of various factors.