There are two main factors that cause surface corrosion of precision seamless pipes: chemical corrosion and electrochemical corrosion.
(1) Chemical corrosion
1. Surface contamination: oil, dust, acid, alkali, salt, etc. 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.
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, resulting in 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 rusty substances such as cutting slag, splash and corrosive media form galvanic cells and produce electrochemical corrosion.
3. Baking: The composition and metallographic structure of the flame heating area change unevenly, and form a galvanic cell with the corrosive medium to cause electrochemical corrosion.
4. Welding: physical defects (undercuts, pores, cracks, lack of fusion, incomplete penetration, etc.) and chemical defects (large grains, chromium depletion at grain boundaries, segregation, etc.) in the welding area and corrosive media form galvanic cells to generate electricity Chemical 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 acid pickling passivation residue and the chemical corrosion products of stainless steel will form electrochemical corrosion with stainless steel parts.