Stainless steels under an obligation of their corrosion resistance to the shaping of the so-called passive film. This consists of a layer of hydrated chromium oxide (Cr203), which is extremely adherent and resistant to chemical attack. If the passive film is damaged by grinding or scratching, a healing process occurs almost instantly. Yet, under few conditions, stainless steels are high strung to highly localized forms of invasion in relatively mild environments, rendering them underserved for further Help. In this effect, the main types of corrosion attack are intergranular corrosion, pitting corrosion and stress corrosion. However, it should be stressed that these kinds of attack are well researched and thoroughly documented and therefore it is now rare for them to lead to premature or unexpected failure in stainless steel components.
Given favorable temperature conditions, solute atoms can segregate to the grain boundaries, causing enrichment in a particular element or the precipitation of metal compounds. Under highly oxidizing conditions, these effects can cause the grain boundaries of stainless steels to become very reactive, leading to the highly localized form of attack known as intergranular corrosion. Both austenitic and ferritic stainless steel is susceptible to intergranular corrosion and, in either case, the problem is caused by the segregation of carbon to the grain boundaries and the formation of the chromium-rich, M23C6 carbides. The concentration of chromium in these carbides is very much higher than that in the surrounding matrix and, at one time, it was postulated that this resulted in galvanic corrosion between the noble carbides and the more reactive matrix. However, currently, the most widely accepted theory for intergranular corrosion in stainless steels is that involving chromium depletion. Thus, in forming chromium-rich carbides at the grain boundaries, chromium is drawn out of solid solution, and in areas adjacent to the boundaries, the chromium content becomes severely depleted compared to the bulk chromium concentration of the steel. Such areas are then said to have become sensitized in that they no longer contain sufficient chromium to withstand corrosive attack. Corrosion can then proceed along the grain boundaries and a micrograph illustrating this form of attack in austenitic stainless steel.
As its name suggests, pitting is a highly localized form of corrosion which, in its initial form, results in the formation of shallow holes or pits in the surface of the component. However, the pits can propagate at a fast rate, resulting in pin-holing or complete perforation in the wall of the component. Therefore pitting can be completely destructive in terms of further useful life when only a very small amount of metal has been attacked by corrosion. In stainless steels, pitting corrosion generally takes place in the presence of chloride ions and it is widely held that the initiation stage is associated with an attack on non-metallic inclusions. However, other microstructural features may also play a part.
Stress corrosion cracking (SCC) is a form of failure induced by the conjoint action of tensile stresses and particular types of corrosive environments. The stresses can be either applied or residual and cracking takes place in a direction normal to the tensile stresses, often at stress levels below the yield strength of the material. A micrograph illustrating SCC in austenitic stainless steel is shown in Figure 4.18. Cracking can take place in either a transgranular or intergranular manner and can proceed to the point where the remaining material can no longer support the applied stress and fracture then takes place.