Corrosion
Even stainless steels are subject to corrosion. However, in the case of stainless steels, corrosion may not produce the visible rust that is observed on the surface of common steels. Therefore, the effects of corrosion on stainless steels can be sudden and devastating. Corrosion of stainless steels occurs in many ways.
Microcorrosion
Microcorrosion, or pinhole corrosion or pitting corrosion (English, pitting) occurs when stainless steel is exposed to an environment lacking oxygen or in an environment where other ions compete with oxygen as an oxidizing agent. Thus, for example, when stainless steel is exposed to chloride solutions, the protective layer of Cr2O3 is destroyed by Cl– anions, resulting in the formation of microscopic pits on the surface of the steel. The pits can develop into cracks which, under relatively low stress, grow rapidly with destructive results.
Microcorrosion is also often observed in cavities or welds of stainless steel components. In this case, we are talking about cavity corrosion, cavitary corrosion, or corrosion of separating surfaces (English, crevice corrosion). Crevice corrosion can be severe even at relatively low temperatures.
Pericrystalline corrosion
Pericrystalline corrosion of stainless steel as observed under a metallographic microscope.
Pericrystalline corrosion (English, intergranular corrosion) occurs when stainless steel is heated and carbides of chromium ((Fe,Cr)7C3, etc.) form around the alloy crystals. These carbides replace the chromium oxide, thus depriving the steel of its protection. Pericrystalline corrosion depends on the alloy's carbon content. Stainless steels with 0.06% C undergo intergranular corrosion within 2 minutes at 700°C; in contrast, stainless steels with 0.02% C do not undergo intergranular corrosion. Pericrystalline corrosion is also observed after welding stainless steels due to local overheating of the alloy.
Corrosion with mechanical stress
Corrosion caused by mechanical stress, dynamic corrosion, or work corrosion (English, stress corrosion cracking) is a complex phenomenon observed when stainless steel is subjected to mechanical stress in a corrosive environment, such as e.g. in chlorinated solutions. Cold-rolled steels are more susceptible to corrosion under mechanical stress due to residual stresses. With annealing, the residual stresses disappear and the steel regains its corrosion resistance. Corrosion under mechanical stress is associated with the creation (or simply the presence) of structural defects in the crystal lattice of the alloy. These defects extend to the surface of the alloy, resulting in local damage to the protective Cr2O3 oxide, the creation of cracks, and the ultimate failure of the alloy.
AISI-SAE 304 and AISI-SAE 316 austenitic stainless steels fail easily due to corrosion under mechanical stress in solutions containing minimal mg/L Cl– when the temperature exceeds 50°C. Austenitic stainless steels with a high content of molybdenum (> 6%) or nickel, ferritic and duplex stainless steels exhibit better resistance to corrosion under mechanical stress.
Electrochemical corrosion
Electrochemical or galvanic corrosion (English, galvanic corrosion) occurs when two different metals come into contact with each other. This creates a local galvanic cell, resulting in corrosion of the more electropositive metal. If, for example, a few specks of common steel on the surface of stainless steel, they will begin to oxidize due to electrochemical corrosion, and then the corrosion may spread to the surface of the stainless steel. In this case, contact corrosion is also mentioned (English, contact corrosion). Contact corrosion is prevented by properly cleaning the alloy with nitric or hydrofluoric acid.
