Stainless steel pickling and passivation

1.Pickling passivation principle

The corrosion resistance of stainless steel is mainly due to the higher content of Cr added to the steel (such as 316L W (Cr) = 16.00 ～ 18.00). Cr is easily oxidized and can form a dense Cr2O3 oxide film on the surface of the steel. The electrode potential of the steel and the corrosion resistance in the oxidizing medium are suddenly improved. The corrosion resistance of stainless steel mainly depends on the extremely thin (about 1 nm) dense passivation film covering the surface. This layer of passivation film Isolation from corrosive media is the basic barrier for stainless steel protection. If the passivation film is incomplete or defective, the stainless steel will still be corroded. The stainless steel chemical tanker lifts, assembles, welds, inspects welds (such as color inspection, pressure test) and the surface oil contamination, scratches, rust, impurities, low Melting point metal contaminants, paints, welding slags, spatters, etc. These materials affect the surface quality of stainless steel, destroy the surface passivation film, reduce the surface corrosion resistance, and are easy to work with the corrosive media in chemicals carried later. Effect, causing pitting corrosion, intergranular corrosion, and even stress corrosion cracking. Stainless steel chemical tankers usually use seawater to wash the tank during operation. The seawater is rich in chloride ions, which has a large corrosive effect on the passivation film, and the working conditions are harsh. Pickling and passivation is indispensable. The stainless steel passivation film has dynamic characteristics and should not be considered as a complete stop of corrosion. Instead, it forms a protective layer of diffusion. Generally, it tends to destroy the passivation film in the presence of reducing agents (such as chloride ions), while oxidants (such as Air) protects and repairs the passivation film in the presence. The stainless steel will form an oxide film when placed in the air, but the protection of this film is not perfect. The surface of the stainless steel has an average thickness of 10 μm. The surface is corroded by acid washing. The chemical activity of the acid solution causes the dissolution rate of the defect site. It is higher than other parts on the surface, so pickling can make the entire surface tend to be uniform and balanced. More importantly, passivation makes the iron and iron oxides preferentially dissolve than chromium and chromium oxides, eliminating the poor The chromium layer makes the stainless steel surface rich in chromium. Under the effect of oxidant passivation, a complete and stable passivation film is produced on the surface. The potential of this chromium-rich passivation film can reach + 1.0V (SCE), which is close to the potential of precious metals, which improves Stability against corrosion.

2.Pickling passivation method

According to different operation methods, stainless steel acid pickling passivation treatment mainly includes 6 methods such as dipping method, paste method, brushing method, spray method, circulation method, electrochemical method, etc. Among them, dipping method, paste method and spray method are compared. Suitable for pickling and passivation of stainless steel chemical tankers and equipment. I. Dipping method: Stainless steel pipelines, elbows, small parts, etc. are most suitable for this method, and the processing effect is the best. Because the treated part can be fully immersed in the pickling passivation solution, the surface reaction is complete, and the passivation film is dense and uniform. Chuandong Shipyard has used our pickling workshop to pickle "Ninghua 417" stainless steel pipes in batches, and achieved good results. This method is suitable for continuous batch operations, but it is necessary to continuously replenish new liquid as the reaction concentration of the solution decreases. The disadvantage is that due to the limitation of the shape and capacity of the acid tank, it is not suitable for large-capacity equipment and pipelines that are too long and wide in shape; long-term use will reduce the effect due to solution volatilization and other reasons. Special sites, acid pools and heating equipment are required. 2. Paste method: The stainless steel pickling passivation paste has been widely used in China and has a series of products. The main ingredients are nitric acid, hydrofluoric acid, corrosion inhibitor, viscous agent, etc. CB / T3595-94 has specific inspection rules, manual operation, suitable for on-site construction, stainless steel chemical ship welding treatment, welding discoloration, top of deck, corner dead corner, back of escalator and large area in tank. Applicable for passivation. The advantages of the paste method are that no special equipment and site are needed, no heating equipment is required, and the site is flexible in operation. The pickling and passivation are completed at once, and the independence is strong; the passivation paste has a long shelf life, and a new passivation paste is used for each application. In general, the reaction stops after the passivation of the surface layer is finished, it is not easy to be over-corroded, and it is not limited by the subsequent flushing time. Weakness such as welds can also strengthen passivation. Disadvantages are poor operating environment for workers, high labor intensity and high cost, and the effect of treating the inner wall of stainless steel pipelines is slightly worse, which needs to be combined with other methods. 3. Spray method: suitable for a single product with a fixed site and a closed environment or equipment with a simple internal structure for pickling and passivation, such as the spray pickling process on a plate production line. For stainless steel chemical tankers, it can be used for acid in the tank wall. wash. Chuandong Shipyard is preparing to pickle and passivate the liquid tank of Sinochem 3450t stainless steel chemical tanker "Yingchun" under construction by spraying. Its advantages are fast continuous operation, simple operation, and little impact on workers' corrosion. The pipetting process can be acid-washed again. Higher solution utilization. This method has many restrictions, such as: 1. The scaffolding in the tank must be completely removed, the cabin and piping must be cleaned, and there must be no residues or impurities in the pipeline. 2. All deep-well pumps, valves, tank washing machines and other liquid cargo systems of the whole ship should be debugged and reserved. 3. The pickling solution stays in the cabin for too long, and the reaction will cause over corrosion of the stainless steel, so it must be operated continuously. The shipyard must be highly cooperative and ready to prepare a large amount of cleaning water at any time. It will cause serious problems in the event of power outage, water outage, and work stoppage. as a result of. 4. The waste acid and waste water discharge must be replaced by a large container or a tank is temporarily opened, but it should be neutralized and discharged as soon as possible to avoid corrosion of the tank. 5. As the reaction time increases and the solution impurities increase, the effective components of the pickling solution gradually decrease. The solution concentration must be detected at any time and new solutions must be added in time. 6. The staying time of the spray liquid on the deck, the four walls, the dead corner, and the back of the escalator is shorter, and the pickling effect is slightly worse than that of the bottom plate.

3. Special considerations

Pre-treatment of pickling and passivation: If there is dirt such as grease on the surface of stainless steel, the quality of pickling and passivation will be affected. Generally, the inner wall of the tank should be cleaned with alkaline detergent. Control of chloride ion concentration in pickling solution / paste and cleaning water: Excessive chloride ion content will destroy the stainless steel passivation film. Some stainless steel pickling solutions / pastes use chloride ion-containing erosion media such as hydrochloric acid and perchloric acid as the main agent or The removal of the surface oxide layer by the additive is not good for preventing corrosion and should be controlled during the inspection. As specified in the ship's standard "Stainless Steel Pickling Passivation Paste" CB / T3595-94, the chloride ion content of stainless steel pickling passivation paste Controlled in the range of 25ppm to 100ppm. In addition, for cleaning water, the chloride ion content should be controlled below 25PPM, and the chloride ion corrosion inhibitor sodium nitrate can be added to the tap water during construction to meet the requirements. Neutral environmental protection discharge of waste liquid: For the treatment of pickling passivated waste liquid, it should meet the national environmental protection discharge requirements. For example, for the fluorine-containing waste liquid, lime milk or calcium chloride can be added to the treatment, and for chromium-containing waste liquid, subsulfate can be added. Iron reduction treatment.

4. Pickling passivation quality inspection

The quality inspection of the stainless steel pickling passivation effect can refer to the ship's standard "Stainless Steel Pickling Passivation Paste" CB / T3595-94, the international standard "Stainless Steel Electrolytic Polishing and Grinding Polishing and Pickling Passivation Surface Inspection" ISO15730, etc., can generally be divided into: appearance inspection, chemical test, chemical test mainly includes artificial seawater chip corrosion test, copper sulfate titration test, potassium ferricyanide titration test (blue dot test), etc. "The inspection of the passivation effect of the round pickling is carried out in the following three ways: Appearance inspection: The surface of the stainless steel pickling passivation should be uniform silver-white, smooth and beautiful, there should be no obvious signs of corrosion, and the weld seam and heat-affected zone must not have oxidation colors. , There must be no uneven spots. Residual liquid inspection: Use phenolphthalein test paper to check the cleanliness of the residual liquid on the surface of stainless steel. A neutral pH is acceptable. Blue dot test: The basic principle of the blue dot test method is that if the surface passivation film is incomplete or contaminated with iron ions, there will be free iron ions. The potassium ferricyanide solution will react to form blue precipitates when it encounters iron ions. The reaction formula is as follows: K ++ Fe + 2 + [Fe (CN) 6] -3 → KFe [Fe (CN) 6] ↓ Use a 100mL beaker to dissolve 10g potassium ferricyanide in 50ml distilled water, add 30ml after dissolution Concentrated nitric acid, then transferred into a 1000ml volumetric flask and diluted with distilled water to the mark, which is the blue dot test solution, the storage period is one week. Test method: The blue dot test liquid drops are placed on the test paper attached to the stainless steel surface. After 30 seconds, a blue dot is displayed on the test paper as a failure. It should be noted that the chemical test will destroy the passivation film of stainless steel. Usually, the test plate and the product can be acid-passivated at the same time, and then the above test is performed on the test plate.

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