Stainless steel pickling passivation operation basic considerations

1. Pickling passivation of pickling passivation processing stainless steel workpiece before the surface dirt, if any, should be through mechanical cleaning, then oil removal. If the acid lotion and passivation solution cannot remove grease, oil surface will affect the quality of the pickling passivation, therefore oil removal cannot omit, lye, emulsifier, organic solvent and steam can be used, etc. 2. Cl - acid wash and rinse water Control of some stainless steel pickling liquor or pickling paste by using hydrochloric acid and perchloric acid, ferric chloride and sodium chloride, chlorine ion erosion medium as the main agent or agents to remove surface oxide layer, in addition to the grease with chlorinated organic solvent such as trichloroethylene, from preventing stress corrosion cracking is not appropriate. In addition, the initial flushing water can be used in industrial water, but the final rinse water requires strict control of halide content. Usually using deionized water. Such as petrochemical austenitic stainless steel pressure vessel for hydrostatic test water, C1 - control Content is not more than 25 mg/L, if cannot meet the requirements, can be added to sodium nitrate in water treatment, make it meet the requirements, C1 - Content that would harm the passivation of stainless steel membrane, is pitting, crevice corrosion, stress corrosion cracking. 3. Pickling passivation operation in the process control nitrate solution separately used to remove free iron and other metal contamination is effective, but to remove oxide, thick corrosion products, tempering membrane is invalid, such as general HNO3 + HF solution should be adopted, in order to convenient and safe in operation and can be substituted for fluoride HF [ 2] 。 Separate HNO3 solution without corrosion inhibitor, but HNO3 + HF acid pickling, need to add a Lan - 826. Using HNO3 + HF acid, to prevent corrosion, the concentration of the ratio of 5:1 shall be maintained. High temperature should be lower than 49 ℃, such as, HF will evaporate. The passivation solution, HNO3 should be controlled in 20% - 50%, according to the electrochemical tests, HNO3 concentration less than 20% treatment of passivation film quality is not stable, easy to produce pitting [ 8] , but also should not be greater than 50% HNO3 concentration, to prevent passivation. Using one-step processing oil removal pickling passivation, although easy to operate, save time, but the pickling passivation solution ( Paste) There will be aggressive in HF, so its ultimate footwork than protective film quality. Allowed within a certain range adjustment in the process of pickling acid concentration, temperature and contact time. As the growth of the acid wash time, must pay attention to the change of the acid concentration and metal ion concentration, attention should be paid to avoid acid, titanium ion concentration should be less than 2%, otherwise it will lead to serious pitting. In general, improve pickling temperature will accelerate and improve the cleaning effect, but may also increase the risk of pollution or damage to the surface. 4. Stainless steel pickling sensitization conditions control [ 2] Some stainless steel due to poor heat treatment or welding cause sensitization, use HNO& HF acid may produce intercrystalline corrosion, caused by the intergranular corrosion cracks at runtime, or cleaning, or in the subsequent processing, can enrichment halide, caused by stress corrosion. These sensitizing descaling stainless steel should not be commonly use HNO3 + HF solution or acid. Such as the pickling must be conducted after welding, ultra-low carbon stainless steel or stabilization should be adopted. 5. Stainless steel and carbon steel pickling of stainless steel and carbon steel composite assembly ( Such as stainless steel pipe, tube plate in heat exchanger with carbon steel shell) , if pickling passivation using HNO3 or the serious corrosion of carbon steel, HNO3 + HF should then add appropriate corrosion inhibitors such as Lan - 826. When the stainless steel and carbon steel assembly in the sensitized status, cannot use HNO3 + HF acid, glycolic acid can be used ( 2%) + formic acid ( 2%) + corrosion inhibitors, the temperature is 93 ℃, time of 6 h or EDTA + ammonium neutral solution corrosion inhibitor, temperature: 121 ℃, time: 6 h, and then rinse with hot water and soaked 10 mg/L ammonium hydroxide + 100 mg/L hydrazine [ 3] 。 6. Pickling passivation of post-processing stainless steel workpiece after pickling and flushing water, available containing 10% ( The mass fraction) NaOH+4％( The mass fraction) KMnO4 alkali 1 raw potassium permanganate solution soak 5 in 71 ~ 82 ℃ ~ 60 min, to remove acid residue, and then rinse thoroughly with water, and dry. Stainless steel surface after pickling passivation spotted or blotch, can use fresh passivation solution or relatively high concentrations of nitric acid scrubbing and eliminated. Final pickling passivation of stainless steel equipment or parts should pay attention to protect, use polyethylene mulch or dress, avoid different metal and nonmetal contact. For acid and passivation liquid waste processing, should comply with the national environmental emissions regulations. Such as the fluoride waste water can add lime or calcium chloride processing. Passivation solution as far as possible need not bichromate, such as wastewater containing chromium, can add ferrous sulfate reduction processing. Pickling may cause martensitic stainless steel hydrogen embrittlement, such as oxygen need to go through heat treatment ( Heated to 200 ℃ heat preservation for a period of time) 。