Corrosion notes

When metal atoms are in a chemical environment that allows or causes them to give up electrons, they become positively charged ions that take part in chemical reactions, provided an electrical circuit can be completed. The net effect is that the metal component corrodes away where the electrons are given up and the useful cross-sectional area is reduced. This effect can be concentrated locally to form a pit or, sometimes, a crack if a high level of tensile stress is acting, or it can extend across a wide area to produce general wastage. The load carrying capacity is therefore reduced and an eventual failure may occur simply because a load in the upper part of the normal spectrum exceeds the residual strength of the component.
Localised corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.

Some metals display an inherently greater corrosion resistance than others, but even those most resistant to normal atmospheric conditions are vulnerable to some reagent. Stainless steels, for example, are almost never attacked under oxidizing conditions because they have an inbuilt chromium content (at least 12 per cent), which forms an electrically insulating and self-healing protective oxide film preventing most corrosive agents being able to set up an electrical circuit. If the oxide film is mechanically damaged it immediately reforms and keeps the reactive agent away from the metal. However, if the environmental conditions are reducing or exclude the oxygen necessary to re-form the chromium oxide film, stainless steels will corrode away almost as fast as non-stainless varieties.

Stress corrosion
For certain metallic alloys the effects of stress and corrosion combine to produce the troublesome phenomenon of stress corrosion cracking. It requires all of the following ingredients to occur simultaneously.'
(a) a susceptible microstructure;
(b) an applied or residual tensile stress - residual stresses contain balanced regions of tension and compression; usually resulting from forming operations
(c) a mildly aggressive chemical environment, but seldom one producing any visible corrosion product on the surface of the component.