Stainless steel is not easy to rust.
The main alloy element of stainless steel is Cr (chromium).
Only when the Cr content reaches a certain value can it have corrosion resistance.
Generally, the Cr content of stainless steel is at least 10.5%.
Illustration of bell coating on stainless steel surface
The corrosion resistance mechanism of stainless steel is the bell coating theory, that is, an extremely thin, solid, fine and stable Cr rich bell coating is formed on its surface to prevent oxygen atoms from further infiltration and oxidation, so as to achieve the ability to prevent corrosion.
Many people think that “stainless steel does not rust”.
In fact, this statement is wrong. Stainless steel will rust under certain conditions.
Related reading: Why Stainless Steel Rust & How to Prevent It From Rusting?
Note: If we can intuitively understand various types of stainless steel corrosion, we can take corresponding measures to reduce losses when facing stainless steel corrosion.
Most of the corrosion damage of stainless steel is local corrosion damage, and the most common are intergranular corrosion (9%), pitting corrosion (23%) and stress corrosion (49%).
In many industrial applications, stainless steel can provide satisfactory corrosion resistance.
According to the use experience, apart from mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is local corrosion (i.e. stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion).
The failure cases caused by these local corrosion almost account for more than half of the failure cases.
In fact, many failure accidents can be avoided through reasonable material selection.
Stress corrosion cracking (SCC):
A general term that refers to the mutual failure of stressed alloys due to the propagation of severe cracks in corrosive environments.
SCC has brittle fracture morphology, but it may also occur in materials with high toughness.
The necessary condition for stress corrosion cracking is the existence of tensile stress (whether residual stress or external stress, or both) and specific corrosion medium.
The formation and expansion of the dimple are roughly perpendicular to the direction of tensile stress.
The stress value that leads to stress corrosion cracking is much smaller than that required for material fracture in the absence of corrosion medium.
Microscopically, cracks passing through grains are called transgranular cracks, while cracks extending along grain boundaries are called intergranular cracks.
When stress corrosion cracking extends to a certain depth (here, the stress on the section of the material under load reaches its fracture stress in air), the material will be broken as normal cracks (in ductile materials, usually through the polymerization of microscopic defects).
Therefore, the section of a part that fails due to stress corrosion cracking will contain the characteristic area of stress corrosion cracking and the “tough rich” area associated with the polymerization of micro defects.
It is a form of local corrosion that causes corrosion.
The intergranular boundaries are the boundaries of disordered and staggered intergranular elements with different crystallographic orientations.
Therefore, they are favorable areas for the segregation of various solute elements or the precipitation of metal compounds (such as carbides and δ phases) in steel.
Therefore, in some corrosive media, it is not surprising that the grain boundary may be corroded first.
This type of corrosion is called intergranular corrosion.
Most metals and alloys may exhibit intergranular corrosion in a specific corrosion medium.
It is a form of local corrosion, which may occur in the gap where the solution stops or in the shielded surface.
Such a gap can be formed at the joint of metal and metal or metal and nonmetal, for example, at the contact with bell studs, bolts, gaskets, valve seats, loose surface sediments and marine organisms
A term used to describe the corrosion phenomenon that occurs on the entire alloy surface in a relatively uniform manner.
In case of overall corrosion, the material will gradually become thinner due to corrosion, and even the material will become invalid due to corrosion.
Stainless steel may show overall corrosion in strong acid and alkali.
The failure problem caused by overall corrosion is not very worrying, because this kind of corrosion can usually be predicted by simple immersion test or consulting corrosion literature.