What are the forms of stainless steel corrosion?

In many industrial applications, stainless steel materials have very satisfactory corrosion resistance. Judging from actual use experience, in addition to mechanical failure, the corrosion performance of stainless steel materials is mainly localized corrosion. It includes stress corrosion cracking, pitting corrosion, crevice corrosion and general corrosion.

Stress corrosion cracking is a term that refers to the failure of stress-bearing alloys due to crack propagation in corrosive environments. Stress corrosion cracking has a brittle fracture morphology, but it may also occur in tougher materials. The necessary conditions for stress corrosion cracking to occur are the presence of tensile stress and a specific corrosive medium.

Crack formation and propagation are generally perpendicular to the direction of tensile stress. This makes the stress value of stress corrosion cracking much smaller than the stress value required for material fracture when there is no corrosive medium. Cross-sections of stainless steel parts that have failed due to stress corrosion cracking have areas characteristic of stress corrosion cracking and "dimple" areas associated with the aggregation of micro-defects.

To understand what intergranular corrosion is, we must first understand that the grain boundaries are the disordered interlocking boundaries between grains with different crystallographic orientations, which are caused by the segregation of various solute elements in stainless steel or metal compounds (such as carbides and carbides). δ phase) is a favorable area for precipitation. Therefore, in some corrosive media, it is normal for the grain boundaries to be corroded first. This type of corrosion is called intergranular corrosion. Most metals and alloys may have intergranular corrosion in specific corrosive media.

Crevice corrosion is a form of localized corrosion that may occur in crevices where the solution is stagnant or in covered surfaces. Such gaps can develop at metal-to-metal or metal-to-nonmetal joints, such as where they come into contact with rivets, bolts, gaskets, valve seats, loose surface sediments, and marine growth.

Finally, let’s talk about general corrosion, which is a term used to describe the corrosion phenomenon that forms in a relatively uniform manner across the entire alloy surface. Whenever comprehensive corrosion forms, the material will slowly become thinner due to corrosion, and may even cause corrosion failure of the stainless steel material. However, generally only strong acid and strong alkali can cause comprehensive corrosion of stainless steel materials. Therefore, in actual use, comprehensive corrosion rarely occurs, but there is no need to worry too much.