The stainless steels are typically used in environments where a good corrosion resistance is required. The alloy composition of different grades varies a lot and so does the corrosion resistance of each grade respectively. The basic reason for the corrosion resistance is the extremely thin chromium oxide film forming on the steel surfaces in an oxydizing environment. This protective oxide film is very adhesive and capable to self-repairing while the environment of service contains free oxygen sufficiently. The chromium content must be 10.5 % at least for any grade to be corrosion resistant.

The stainless steels are not completely immune to different forms of corrosion. These are the most typical forms of corrosion of the stainless steels:

Intergranular corrosion may arise because of the chemical composition of the steel or because of the improper heat treatment leading to chromium containing carbides precipitated on the grainboundaries. The carbides bind chromium and form zones of lower chromium content around the precipitations. The zones of lower chromium are not capable to form the protective passive film any more and thus the grainboundaries are attacked first in the corrosive environment.

A low carbon level (typically < 0.030%) or stabilizing by titanium or niobium is used to increase the resistance to intergranular corrosion. Care must be taken while heat treating or welding the steels containing more than 0.03 % carbon without titanium or niobium alloying. Especially in the critical temperature range of 600 to 800 C during slow cooling, after welding or heattreatment, carbides tend to precipitate on grainboundaries. The extra low carbon grades (or stabilized grades) should be used for welding of heavy sections because of insufficient cooling speed.

Pitting corrosion will typically arise in chlorine containing solutions. Almost any stainless steel grade is prone to pitting corrosion if the chlorine contents and service temperatures are sufficiently high. Molybdenum is the major element improving the resistance to pitting corrosion.

Stress corrosion may typically appear on the welded constructions made of some austenitic grade. The increased chlorine content of solution and the high residual stresses around the weldings cause higher risk for stress corrosion cracks.

Fatigue corrosion is the usual fatigue loading combined to some corrosive environment. Compared to fatigue under pure dry air condition, the fatigue will arise earlier and progress more quickly under corrosive circumstances. Once again, chlorine is the dangerous component of solutions.

Last update December 26th, 2000