1. Chemical corrosion
(1) Surface contamination: The oil, dust, acid, alkali, and salt attached to the surface of the workpiece are transformed into corrosive media under certain conditions, and chemically react with certain components in the stainless steel parts, resulting in chemical corrosion and rust. Common sense of stainless steel surface treatment
(2) Surface scratches: All kinds of scratches damage the passivation film, which reduces the protective ability of stainless steel, and is easy to react with chemical media, causing chemical corrosion and rust.
(3) Cleaning: After pickling and passivation, the remaining liquid will remain and the stainless steel will be directly corroded (chemical corrosion) if the cleaning is not clean.
2. Electrochemical corrosion
(1) Carbon steel pollution: scratches and corrosive media caused by contact with carbon steel parts form galvanic cells and cause electrochemical corrosion.
(2) Cutting: The adhesion of rust-prone substances such as cutting slag, splash and corrosive medium forms a galvanic cell and produces electrochemical corrosion.
(3) Baking: The composition and metallographic structure of the flame heating zone change unevenly, and form a galvanic cell with the corrosive medium, which causes electrochemical corrosion.
(4) Welding: physical defects (undercuts, pores, cracks, lack of fusion, insufficient penetration, etc.) and chemical defects (large grains, chromium depletion at grain boundaries, segregation, etc.) in the welding area and corrosive media are caused by the formation of galvanic cells Electrochemical corrosion.
(5) Material: The chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (loose, blisters, cracks, etc.) of the stainless steel material are conducive to forming galvanic cells with corrosive media and causing electrochemical corrosion.
(6) Passivation: poor pickling and passivation results in uneven or thin passivation film on the surface of stainless steel, which is prone to electrochemical corrosion.
(7) Cleaning: The remaining pickling passivation residue and the chemical corrosion products of stainless steel will form electrochemical corrosion with stainless steel parts.
Concentration of force is easy to cause stress corrosion. In short, due to its special metallographic structure and surface passivation film, stainless steel is generally difficult to chemically react with the medium and be corroded, but it is not impossible to be corroded under any conditions. . In the presence of corrosive media and incentives (such as scratches, splashes, cutting slag, etc.), stainless steel can also be corroded by slow chemical and electrochemical reactions with corrosive media, and the corrosion rate is quite fast under certain conditions. Rust phenomena, especially pitting and crevice corrosion. The corrosion mechanism of stainless steel parts is mainly electrochemical corrosion. Therefore, all effective measures should be taken during the processing of stainless steel products to try to avoid rust conditions and inducements. In fact, many rust conditions and inducements (such as scratches, splashes, cutting slag, etc.) also have significant adverse effects on the appearance of the product, and they should and must be overcome.