Common causes of corrosion in stainless steel
1. Chemical corrosion
1.1 Surface contamination:
The oil stains, dust, acids, alkalis, salts, etc. attached to the surface of the workpiece are transformed into corrosive media under certain conditions, and undergo chemical reactions with certain components in the stainless steel parts, resulting in chemical corrosion and rusting.
1.2 Surface Scratches:
Various scratches can damage the passivation film, reducing the protective ability of stainless steel and easily reacting with chemical media, resulting in chemical corrosion and rusting.
1.3 Cleaning:
Poor cleaning after acid pickling and passivation results in residual liquid retention, which directly corrodes stainless steel parts (chemical corrosion).
1.2 Electrochemical corrosion
1.2.1 Carbon steel pollution:
Scratches and corrosive media caused by contact with carbon steel parts form a primary battery, resulting in electrochemical corrosion.
1.2.2 Cutting:
The attachment of rust prone substances such as slag cutting and splashing to the corrosive medium forms the primary battery, resulting in electrochemical corrosion.
1.2.3 Baking calibration:
The composition and metallographic structure of the flame heating area change and become uneven, forming a primary battery with the corrosive medium and causing electrochemical corrosion
1.2.4 Welding:
The physical defects (undercuts, pores, cracks, lack of fusion, lack of penetration, etc.) and chemical defects (coarse grains, poor chromium at grain boundaries, segregation, etc.) in the welding area form galvanic corrosion with the corrosive medium, resulting in electrochemical corrosion.
1.2.5 Material:
The chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (looseness, sand holes, cracks, etc.) of stainless steel materials are conducive to the formation of primary batteries with corrosive media, resulting in electrochemical corrosion.
1.2.6 Passivation:
Poor pickling and passivation results in uneven or thin passivation film on the surface of stainless steel, which is prone to electrochemical corrosion
1.2.7 Cleaning:
The remaining acid pickling passivation residue and the products of chemical corrosion of stainless steel form electrochemical corrosion with stainless steel parts.
1.3 Stress concentration is prone to stress corrosion.
In short, stainless steel, due to its special metallographic structure and surface passivation film, is generally difficult to undergo chemical reactions with the medium and be corroded, but it cannot be corroded under any conditions. Under the conditions of corrosive media and inducing factors (such as scratches, splashes, slag cutting, etc.), stainless steel can also undergo slow chemical and electrochemical reactions with corrosive media to be corroded. Moreover, under certain conditions, the corrosion rate is quite fast, resulting in corrosion 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 avoid the occurrence of corrosion conditions and incentives as much as possible. In fact, many rust conditions and triggers (such as scratches, splashes, slag cutting, etc.) also have significant adverse effects on the appearance quality of products, and they should and must be overcome.
1.1 Surface contamination:
The oil stains, dust, acids, alkalis, salts, etc. attached to the surface of the workpiece are transformed into corrosive media under certain conditions, and undergo chemical reactions with certain components in the stainless steel parts, resulting in chemical corrosion and rusting.
1.2 Surface Scratches:
Various scratches can damage the passivation film, reducing the protective ability of stainless steel and easily reacting with chemical media, resulting in chemical corrosion and rusting.
1.3 Cleaning:
Poor cleaning after acid pickling and passivation results in residual liquid retention, which directly corrodes stainless steel parts (chemical corrosion).
1.2 Electrochemical corrosion
1.2.1 Carbon steel pollution:
Scratches and corrosive media caused by contact with carbon steel parts form a primary battery, resulting in electrochemical corrosion.
1.2.2 Cutting:
The attachment of rust prone substances such as slag cutting and splashing to the corrosive medium forms the primary battery, resulting in electrochemical corrosion.
1.2.3 Baking calibration:
The composition and metallographic structure of the flame heating area change and become uneven, forming a primary battery with the corrosive medium and causing electrochemical corrosion
1.2.4 Welding:
The physical defects (undercuts, pores, cracks, lack of fusion, lack of penetration, etc.) and chemical defects (coarse grains, poor chromium at grain boundaries, segregation, etc.) in the welding area form galvanic corrosion with the corrosive medium, resulting in electrochemical corrosion.
1.2.5 Material:
The chemical defects (uneven composition, S, P impurities, etc.) and surface physical defects (looseness, sand holes, cracks, etc.) of stainless steel materials are conducive to the formation of primary batteries with corrosive media, resulting in electrochemical corrosion.
1.2.6 Passivation:
Poor pickling and passivation results in uneven or thin passivation film on the surface of stainless steel, which is prone to electrochemical corrosion
1.2.7 Cleaning:
The remaining acid pickling passivation residue and the products of chemical corrosion of stainless steel form electrochemical corrosion with stainless steel parts.
1.3 Stress concentration is prone to stress corrosion.
In short, stainless steel, due to its special metallographic structure and surface passivation film, is generally difficult to undergo chemical reactions with the medium and be corroded, but it cannot be corroded under any conditions. Under the conditions of corrosive media and inducing factors (such as scratches, splashes, slag cutting, etc.), stainless steel can also undergo slow chemical and electrochemical reactions with corrosive media to be corroded. Moreover, under certain conditions, the corrosion rate is quite fast, resulting in corrosion 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 avoid the occurrence of corrosion conditions and incentives as much as possible. In fact, many rust conditions and triggers (such as scratches, splashes, slag cutting, etc.) also have significant adverse effects on the appearance quality of products, and they should and must be overcome.
