What are Stainless Steels ?

Stainless steels are defined as iron based alloys containing at least 10.5 % chromium and a maximum of 1.2 % carbon. One of the most important properties of stainless steels is their resistance to corrosion. Corrosion resistance in stainless steels is provided by a passive surface film which acts as a barrier between the alloy and the surrounding medium. The passive film is a continuous, non-porous and insoluble film which, if broken, is self-healing under normal conditions. Chromium plays an essential role in the formation and the stabilisation of the passive film. Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element can, by itself, create the properties of stainless steels. Increasing the chromium content from 10.5 % to the 18 % level typical of the austenitic stainless steels, provides increased stability of the passive film. In austenitic stainless steels, the nickel content does not contribute directly to the composition of the passive film, but does promote repassivation, especially in reducing atmospheres. Molybdenum in combination with chromium is very effective in stabilising the passive film in the presence of chlorides. It also considerably improves the resistance to pitting and crevice corrosion in neutral chloride solutions.