Why does stainless steel corrode?
As we all know, stainless steel has the ability to resist atmospheric oxidation, that is, will not rust, but also corrode in the medium like acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of stainless steel is conditional, that is, stainless steel in a certain medium is corrosion-resistant, but in another medium may be destroyed. Correspondingly, no one of stainless steel is resistant to corrosion in all environments.
Stainless steel can provide excellent corrosion resistance in various industries, strictly speaking, they show excellent corrosion resistance in most media, but it is exceptional in some media due to low chemical stability and corrosion but. Therefore, stainless steel can not be corrosion-resistant to all media except mechanical failure. The corrosion of stainless steel is mainly manifested as a serious form of corrosion of stainless steel is local corrosion (i.e., stress corrosion cracking, pitting, intergranular corrosion, corrosion fatigue and crevice corrosion). This local corrosion causes almost half of the failure. To understand why stainless steel corrodes, we must first understand the type of corrosion of stainless steel.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking (SCC) is the failure of stainless steel subjected to stress in a corrosive environment due to the expansion of strong grain. SCC has a brittle fracture morphology and can occur in materials with high toughness in the presence of tensile stress (whether residual stress or applied stress or both) and corrosive media. In the micro term, crack through the grain called transgranular crack, and the cracks along the grain boundary expansion graph called the intergranular crack, when the SCC extended to one depth (load stress on the section of materials to achieve its fracture stress) in the air, stainless steel as normal crack (in ductile material, usually through microscopic defect aggregation) and disconnect.
Therefore, the section of a part that has failed due to stress corrosion cracking will contain areas characterized by stress corrosion cracking and “dimples” areas associated with the polymerization that has been slightly defective.
Pitting Corrosion
Pitting corrosion refers to the most non-corrosion or scattered slight local corrosion on the surface of metal materials. The size of the common pitting point is less than 1.00mm, and the depth is often greater than the surface aperture, which may be a shallow pitting pit or perforation.
Intergranular Corrosion
Intergranular corrosion: A disordered dislocation of grains at the boundary between different grains and, therefore, a favorable zone for segregation of solute elements or precipitation of metallic compounds such as carbides and δ phases in steels. Therefore, in some corrosive media, it is common that the grain boundaries may be corroded first, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.
Crevice Corrosion
Crevice corrosion refers to the occurrence of speckled corrosion in the cracks of stainless steel parts, which is a kind of local corrosion. It may occur in the cracks of solution stagnation or in the shielding surface. Such gaps may form at metal-to-metal or metal-to-nonmetal junctions, for example, at rivets, bolts, gaskets, valve seats, and loose surface deposits.
General Corrosion
Uniform corrosion on the surface of stainless steel. Stainless steels may exhibit general corrosion in strong acids and bases. When general corrosion occurs, the stainless steels gradually thin and even fail, which is not much of a concern because such corrosion can usually be predicted by a simple immersion test. It can be said that stainless steel refers to the corrosion resistance of steel in the atmosphere and weak corrosion medium, corrosion rate is less than 0.01mm/ year, that is “completely corrosion resistance”; Stainless steels with corrosion rates less than 0.1mm/ year are considered “corrosion-resistant”.
