Stainless Steel Corrosion

Corrosion Doctors Web pages related to iron

Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications. Along with iron and chromium, all stainless steels contain some carbon. It is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.

Stainless steels are mainly used in wet environments. With increasing chromium and molybdenum contents, the steels become increasingly resistant to aggressive solutions. The higher nickel content reduces the risk of SCC. Austenitic steels are more or less resistant to general corrosion, crevice corrosion and pitting, depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion is very important if the steel is to be used in chloride containing environments. Resistance to pitting and crevice corrosion typically increases with increasing contents of chromium, molybdenum and nitrogen.

Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately.

Many pages of the Corrosion Doctors site discuss specific issues related to the corrosion behavior of steels. The following are references to some of these pages:

To find much more on stainless steels and other nickel based alloys consult the plentiful Nickel Development Institute site.