Why do stainless steel forgings produce rust?

2019-03-28 10:57:28 19

Some forgings have a long time or improper processing will inevitably cause rust, we will try to minimize it as much as possible, so today to explore the common causes of corrosion caused by stainless steel forgings.

1. Chemical corrosion

      (1) Surface contamination: Oil stains, dust and acids, alkalis, salts, etc. attached to the surface of stainless steel forgings are converted into corrosive media under certain conditions, and chemically react with certain components in stainless steel parts to cause chemical corrosion and rust.

      (2) Surface scratching: damage to the passivation film caused by various scratches, which reduces the protection ability of stainless steel, easily reacts with chemical medium, and causes chemical corrosion and rust.

      (3) Cleaning: After pickling and passivation, the cleaning is not clean and the residual liquid remains, directly corroding the stainless steel parts (chemical corrosion).

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 and corrosion of rust-prone substances such as cutting slag and splashing form a primary battery to generate electrochemical corrosion.

     (3) Baking school: The composition of the flame heating zone changes and the metallographic structure is uneven, and the primary battery is formed with the corrosive medium to cause electrochemical corrosion.

     (4) Welding: physical defects (biting, pores, cracks, unfused, incomplete penetration, etc.) and chemical defects (grain coarse, grain boundary chromium deficiency, segregation, etc.) and corrosive medium forming the primary battery Electrochemical corrosion.

     (5) Material: The chemical defects of stainless steel forging materials (uneven components, S, P impurities, etc.) and surface physical defects (loose, blisters, cracks, etc.) are beneficial to the electrochemical cell corrosion caused by the formation of galvanic cells with corrosive media.

     (6) Passivation: The passivation effect of pickling is not good, resulting in uneven or thin passivation film on stainless steel surface, which is easy to form electrochemical corrosion.

     (7) Cleaning: The acid-washed passivation residue remaining and the chemically corroded product of stainless steel form electrochemical corrosion with the stainless steel piece.

3. Stress concentration is prone to stress corrosion.

      In short, due to its special metallographic structure and surface passivation film, stainless steel is generally difficult to be corroded by chemical reaction with the medium under normal conditions, but it cannot be corroded under any conditions. In the presence of corrosive media and incentives (such as scratches, splashes, slag, etc.), stainless steel can also be corroded by slow chemical and electrochemical reactions with corrosive media, and the corrosion rate is relatively fast under certain conditions. Corrosion, especially pitting and crevice corrosion. The corrosion mechanism of stainless steel forgings is mainly electrochemical corrosion.

      Therefore, all effective measures should be taken during the processing of stainless steel products to avoid rust conditions and incentives. In fact, many rust conditions and incentives (such as scratches, splashes, slags, etc.) have a significant adverse effect on the appearance quality of the product and should and must be overcome.