Pickling and Passivation of Stainless Steel
Date:2020-01-01View:879Tags:Pickling and Passivation of Stainless Steel
1. Pickling and passivation pretreatment
Before pickling and passivation of stainless steel, the surface dirt shall be mechanically cleaned and degreased before degreasing. If the pickling solution and passivation solution cannot remove the grease, the grease on the surface will affect the quality of pickling passivation. Therefore, degreasing and degreasing cannot be omitted. Alkali, emulsifier, organic solvent and steam can be used.
2. Pickling and flushing water CL control
Some stainless steel pickling or pickling by adding hydrochloric acid, perchloric acid, ferric chloride, sodium chloride and other corrosive media containing chlorine as the main reagent or additive to remove the surface oxide layer, except for trichloroethylene lubricating grease (such as containing chlorine) organic solvent, so as to prevent stress corrosion cracking. In addition, industrial water can be used as the initial rinse water, but the halide content of the final rinse water should be strictly controlled. Deionized water is usually used. If the C1 - content in the hydrostatic test of austenitic stainless steel pressure vessel in petrochemical industry is not more than 25mg / L, if this requirement cannot be met, the sodium nitrate can be treated with water to meet the requirement. If the C1 - content exceeds the standard, the passivation film of stainless steel will be damaged, which is the root cause of pitting corrosion, crevice corrosion, stress corrosion cracking, etc.
3. Process control of pickling and passivation operation
It is effective to use nitric acid solution alone to remove free iron and other metal pollutants, but to remove oxide skin, thick corrosion products, tempering film, etc., HNO3 + HF solution should be used generally to facilitate operation and safe use, and fluoride can replace HF. HNO3 solution can be added alone without corrosion inhibitor, but Lan-826 needs to be added in HNO3 + HF pickling. HNO3 + HF pickling shall be used to prevent corrosion, and the concentration shall be kept at the ratio of 5:1. The temperature shall be lower than 49 ℃. If it is too high, HF will evaporate.