Pickling and Passivation of Stainless Steel
Date:2017-11-06 keywords:pickling, passivation, stainless steel
1. Pickling passivation pretreatment
Stainless steel pickling passivation before any surface dirt, etc., should be mechanically cleaned, degreasing and then degreasing. If the pickling solution and the passivation solution can not remove the grease, the grease on the surface will affect the quality of the pickling passivation. Therefore, degreasing and degreasing can not be omitted. Lye, 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 and sodium chloride and other chloride-containing corrosive media as the main agent or additives to remove the surface oxide layer, with the exception of grease trichlorethylene such as chlorine-containing organic solvents, from the prevention of stress corrosion cracking is not appropriate. In addition, industrial water may be used for the initial rinse water, but the halide content is strictly controlled for the final rinse water. Usually deionized water. Such as petrochemical austenitic stainless steel pressure vessel water pressure test control C1- content of not more than 25mg / L, such as can not meet this requirement, can be added to the water treatment of sodium nitrate to meet the requirements, C1-content exceeded, will be Destroying stainless steel passivation film is the root cause of pitting, crevice corrosion, stress corrosion cracking and the like.
3. Pickling passivation operation of the process control
Nitric acid solution alone is used to remove free iron and other metal contaminants is effective, but to remove the scale, thick corrosion products, tempering film, etc., generally should be used HNO3 + HF solution, for the convenience and safety of operation, fluoride substitute for HF. HNO3 alone solution can be added without corrosion inhibitor, but need to add Lan-826 when HNO3 + HF pickling. Use HNO3 + HF pickling, to prevent corrosion, the concentration should maintain a 5: 1 ratio. The temperature should be below 49 ℃. If it is too high, HF will volatilize.
4. Stainless steel pickling control sensitization conditions
Some stainless steels are sensitized by poor heat treatment or welding, pickling with HNO & HF may cause intergranular corrosion, and cracks caused by intergranular corrosion may condense halides during operation, during cleaning, or during subsequent processing, resulting in Stress corrosion. These sensitized stainless steel generally should not use HNO3 + HF solution descaling or pickling. After welding such as the need for such pickling, ultra-low carbon or stabilized stainless steel should be used.
5. Stainless steel and carbon steel assembly pickling
For stainless steel and carbon steel assemblies (eg, stainless steel tubes in heat exchangers, tube sheets, and carbon steel housings), pickling passivates severe corrosion of carbon steels with HNO3 or HNO3 + HF and proper corrosion inhibition agents such as Lan-826. When the combination of stainless steel and carbon steel in the sensitized state, can not be washed with HNO3 + HF, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor, temperature 93 ℃, time 6h or ammonium EDTA base neutral solution + corrosion inhibitor, temperature: 121 °C, time: 6h, then rinsed with hot water and immersed in 10 mg / L ammonium hydroxide + 100 mg / L hydrazine.
6. Pickling passivation of the post-processing
Stainless steel parts after pickling and water rinse, available 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 alkaline permanganate solution soaked in 71 ~ 82 ℃ 5 ~ 60min to remove acid wash the residue, rinse thoroughly with water and allow to dry. After the stainless steel surface by pickling passivated spot or stain, available fresh passivation solution or higher concentration of nitric acid to eliminate. The final pickling passivation stainless steel equipment or components should pay attention to protection, polyethylene film can be covered or bandaged, to avoid contact with different metals and non-metallic.