1. Do you know how to clean and passivate stainless steel facilities?

1. Pretreatment for pickling passivation

　　 Stainless steel workpieces should be cleaned mechanically before pickling passivation if there is any surface dirt, and then degreasing and degreasing. If the pickling solution and passivation solution cannot remove grease, the presence of grease on the surface will affect the quality of the pickling passivation. Therefore, degreasing and degreasing cannot be omitted, and lye, emulsifier, organic solvent and steam can be used.

　　 2. Control of Cl- in pickling solution and rinse water

Some stainless steel pickling liquids or pickling pastes use hydrochloric acid, perchloric acid, ferric chloride and sodium chloride and other corrosion media containing chloride ions as the main or auxiliary agent to remove the surface oxide layer, and use trichloroethylene for grease removal Such chlorinated organic solvents are not suitable for preventing stress corrosion cracking. In addition, industrial water can be used for the initial washing water, but the final washing water requires strict control of the halide content. Deionized water is usually used. Such as petrochemical austenitic stainless steel pressure vessel for hydrostatic test water, control the C1- content not to exceed 25mg/L, if this requirement cannot be met, sodium nitrate can be added to the water to meet the requirements, and the C1- content will exceed the standard. Destroying the passive film of stainless steel is the source of pitting corrosion, crevice corrosion, and stress corrosion cracking.

　　 3. Process control in pickling passivation operation

Nitric acid solution alone is effective for removing free iron and other metal contaminants, but it is not effective for removing oxide scale, thick corrosion products, tempered films, etc. Generally, HNO3+HF solution should be used. For convenience and safe operation, fluorine can be used. Compound instead of HF. There is no need to add corrosion inhibitor to the HNO3 solution alone, but Lan-826 is required for HNO3+HF acid washing. Use HNO3+HF pickling, in order to prevent corrosion, the concentration should be maintained at a ratio of 5:1. The temperature should be lower than 49℃. If it is too high, HF will volatilize.

For the passivation solution, HNO3 should be controlled between 20%-50%. According to electrochemical tests, the quality of the passivation film treated with HNO3 concentration less than 20% is unstable and easy to produce pitting corrosion, but the HNO3 concentration should not be greater than 50%. To prevent overpassivation. The one-step method is used to process the deoiling and pickling passivation, although the operation is simple and labor-saving, but the pickling passivation solution (paste) contains corrosive HF, so the final protective film quality is not as good as the multi-step method.

　　 During the pickling process, the acid concentration, temperature and contact time are allowed to be adjusted within a certain range. As the use time of pickling solution increases, attention must be paid to changes in acid concentration and metal ion concentration, and attention should be paid to avoid over pickling. Titanium ion concentration should be less than 2%, otherwise it will cause serious pitting corrosion. Generally speaking, increasing the pickling temperature will speed up and improve the cleaning effect, but it may also increase the risk of surface contamination or damage.

　　4. Control of pickling under the sensitized condition of stainless steel

Some stainless steels are sensitized due to poor heat treatment or welding. The use of HNO3+HF pickling may cause intergranular corrosion. The cracks caused by intergranular corrosion can concentrate halide during operation, cleaning, or subsequent processing. And cause stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling with HNO3+HF solution. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used.

　　 5. Pickling of stainless steel and carbon steel assembly

For stainless steel and carbon steel assemblies (such as stainless steel tubes, tube sheets and carbon steel shells in heat exchangers), if HNO3 or HNO3+HF is used for pickling passivation, carbon steel will be severely corroded, and appropriate corrosion inhibitors should be added. Agent such as Lan-826. When the stainless steel and carbon steel assembly is in the sensitized state and cannot be pickled with HNO3+HF, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor can be used, the temperature is 93℃, the time is 6h or the ammonium EDTA Base neutral solution + corrosion inhibitor, temperature: 121°C, time: 6h, then rinse with hot water and immerse in 10mg/L ammonium hydroxide + 100mg/L hydrazine.

　　6. Post-treatment of pickling passivation

After the stainless steel workpiece is pickled and rinsed with water, it can be soaked in an alkaline permanganate solution containing 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 at 71-82℃ for 5-60 minutes to remove the acid Wash the residue, then rinse with water and dry. After pickling passivation, stains or stains appear on the surface of stainless steel, which can be eliminated by scrubbing with fresh passivation solution or higher concentration of nitric acid. The stainless steel equipment or parts that are finally pickled and passivated should be protected and can be covered or wrapped with polyethylene film to avoid contact between foreign metals and non-metals.

　　The treatment of acidic and passivated waste liquids shall comply with the national environmental protection discharge regulations. For example, fluorine-containing wastewater can be treated with lime milk or calcium chloride. The passivation solution does not use dichromate as much as possible. If there is chromium-containing wastewater, ferrous sulfate can be added for reduction treatment.

　　Pickling may cause hydrogen embrittlement of martensitic stainless steel. If necessary, heat treatment can be used to deoxidize (heat to 200°C for a period of time).

　　7. Stainless steel pickling passivation quality inspection

　　Because the chemical inspection will destroy the passivation film of the product, the inspection is usually carried out on the sample board. Examples of methods are as follows:

　　(1) Copper sulfate titration test

　　 Use 8g CuS04+500mL H20+2～3mLH2SO4 solution to drip into the surface of the sample plate, keep it wet, if no copper precipitation occurs within 6min, it is qualified.

　　(2) potassium ferricyanide titration test

　　 Use 2mL HCl+1mL H2SO4+1g K3Fe(CN)6+97mL H20 solution to drop on the surface of the sample plate, and identify the quality of the passivation film by the number of blue spots generated and the length of time.