CORROSION

CORROSION
It is a common observation that certainmetals(except those which are least reactive like Au, Pt, Pd etc ) are slowly eaten up on long exposure to atmosphere. For example, silver gets tarnished, copper develops green coating on its surface, iron rusts and lead looses its lustre. In fact, such metals react with gases or moisture present in the environment to form undesirable compounds. This process in general is referred to as corrosion . Corrosion may, thus be defined as the process of slow conversion of metals into their undesirable compounds(usually oxides) by reaction with moisture and other gases present in the atmosphere.
FACTORS WHICH AFFECT CORROSION
The factors which affect corrosion are :
Reactivity of the metal : The more active metals are prone to corrosion .
Presence of impurities : Presence of impurities helps in setting up a corrosion cell and makes the corrosion to occur rapidly. For example, pure iron does not rust.
Air and moisture : Air and moisture are quite helpful in corrosion. The presence of gases like CO 2 and SO 2 in air makesit still rapid. For example no rusting is caused if iron is kept under vaccum.
Strains in metal also help in corrosion : For example, in iron articles, rusting is more pronounced on the areas having bends, scratches, nicks and cuts.
Presence of electrolytes : The presence of electrolytes also make the corrosion process faster. For example, iron rusts more rapidly in saline water in comparison to pure water.
Let us study the mechanism of corrosion by studying the familiar example of rusting of iron. Chemically, rust is hydrated iron (III) oxide,
Fe 2 O 3 .x H 2 O . It is generally causedby moisture, CO 2 and O 2 of air. Rust isnon-sticking brown coloured material which can be easily removed by scratching. There are a number of theories about the mechanism of rusting. The most widely accepted theory is electrochemical theory.