Stainless steel pickling passivation knowledge

The corrosion resistance of stainless steel is mainly due to the addition of a higher content of Cr element in steel (such as 316L W (Cr) =16.00 ~ 18.00), Cr element is easy to oxidize, and can quickly form a dense Cr2O3 oxide film on the surface of steel, so that the electrode potential of steel and the corrosion resistance in the oxidizing medium are mutated. The corrosion resistance of stainless steel mainly depends on the surface covered by this layer of extremely thin (about 1nm) dense passivation film, this layer of passivation film is isolated from the corrosive medium, is the basic barrier of stainless steel protection, if the passivation film is incomplete or defective destroyed, stainless steel will still be corroded.

Surface oil, scratches, rust, impurities, low melting point metal pollutants, paint, welding slag, spatters, etc. brought by stainless steel chemical tanker in the lifting, assembly, welding, weld inspection (such as color detection, pressure resistance test) and processing process, which affect the surface quality of stainless steel and destroy the passivation film on its surface. The surface corrosion resistance is reduced, and it is easy to interact with the corrosive medium in the chemicals that are transported later, causing pitting corrosion, intergranular corrosion, and even stress corrosion cracking. In GB150-1998 "Steel pressure vessels", it is stipulated that "the surface of the container made of stainless steel with anti-corrosion requirements should be pickled and passivated".

Stainless steel chemical ship also due to the transport of a variety of different chemicals, to prevent the pollution of goods have high requirements, and the surface quality of domestic stainless steel plate is relatively poor, usually should be stainless steel plate, equipment, accessories mechanical, chemical or electrolytic polishing and other finishing treatment after cleaning, pickling passivation, so that stainless steel has stronger corrosion resistance. In the operation of stainless steel chemical tanker, seawater washing process is often used, seawater is rich in chloride ions, which has a greater corrosion effect on the passivation film, and the working condition is harsh, pickling passivation is indispensable. Stainless steel passivation film has dynamic characteristics, should not be regarded as a complete stop of corrosion, but in the formation of a diffuse protective layer, usually in the presence of reducing agents (such as chloride ions) tend to destroy the passivation film, and in the presence of oxidizing agents (such as air) can protect and repair the passivation film. Stainless steel placed in the air will form an oxide film, but the protection of this film is not perfect, through pickling the stainless steel surface has an average thickness of 10μm of a layer of surface is corroded, the chemical activity of the acid makes the dissolution rate of the defect part is higher than other parts on the surface, so pickling can make the entire surface tend to be uniform and balanced, more importantly, through pickling passivation, So that iron and iron oxides than chromium and chromium oxides priority to dissolve, remove the chromium-poor layer, so that the surface of stainless steel rich chromium, under the passivation action of oxidizer surface to produce a complete and stable passivation film, this chromium-rich passivation film potential up to +1.0V(SCE), close to the potential of precious metals, improve the stability of corrosion resistance. According to the different mode of operation, stainless steel pickling passivation treatment mainly includes impregnation method, paste method, brushing method, spray method, circulation method, electrochemical method and other 6 methods, of which, impregnation method, paste method, spray method is more suitable for pickling passivation of stainless steel chemical ships and equipment.

First, impregnation method: stainless steel pipelines, elbows, small parts, etc. are the most suitable for this method, and the treatment effect is the best. Because the treated parts can be fully soaked in the pickling passivation solution, the surface reaction is complete, and the passivation film is compact and uniform. The "Ninghua 417" stainless steel pipeline has been pickled in batch by our pickling workshop in Chuandong Shipyard, and good results have been achieved. This method is suitable for continuous batch operation, but the new solution needs to be added continuously as the reaction concentration of the solution decreases. Its disadvantage is that it is limited by the shape and capacity of the acid tank, and is not suitable for large-capacity equipment and pipelines with too long and too wide a shape; If not used for a long time, the effect will decrease due to volatilization of the solution, and special sites, acid pools and heating equipment are required.

2. Paste method: Stainless steel pickling passivation paste has been widely used in China and has a series of products supply, the main components are composed of nitric acid, hydrofluoric acid, corrosion inhibitor, viscous agent, etc., according to a certain proportion, the ship standard "Stainless steel pickling passivation paste" CB/T3595-94 has specific inspection rules, manual operation, suitable for on-site construction, For stainless steel chemical tanker weld treatment, welding discoloration, deck top, corner dead Angle, escalator back and large area of the tank smear passivation are applicable. The advantages of the paste method are that it does not require special equipment and site, does not require heating equipment, and has flexible on-site operation, complete pickling and passivation at one time, and strong independence; Passivation paste has a long shelf life, each application treatment is used with a new passivation paste, the reaction stops after the end of a layer of passivation on the surface, it is not easy to over-corrosion, is not limited by subsequent washing time, and weak links such as welding seams can also strengthen passivation. The disadvantage is that the worker's operating environment is poor, the labor intensity is high, the cost is high, and the treatment effect of the inner wall of the stainless steel pipeline is slightly poor, which needs to be combined with other methods.

Third, spray method: suitable for site fixed, closed environment of a single product or equipment with simple internal structure pickling passivation, such as the spray pickling process on the plate production line, stainless steel chemical tanker, can be used for liquid cargo tank inner wall pickling. The liquid cargo tank of Sinochem 3450t stainless steel chemical tanker "Yingchun" under construction is ready to be acid-washed and passivated by spraying method in Chuandong Shipyard. Its advantages are fast continuous operation, simple operation mode, small corrosion impact on workers, and the pipetting process can acid wash the pipeline again. The solution utilization rate is higher. This method has more restrictions (public number: pump pipe home), such as: 1, the scaffolding in the tank needs to be removed, the cabin and piping should be cleaned, and there should be no residue, impurities, etc. 2. All deep-well pumps, valves, tank washing machines and other liquid cargo systems of the ship should be adjusted and ready for use. 3. If the acid washing liquid stays in the cabin for too long, it will react to cause excessive corrosion of stainless steel, so it is necessary to continue operation, and the shipyard must be highly cooperative and ready to clean a large amount of water at any time. In case of power failure, water interruption and shutdown, serious consequences will be caused. 4, waste acid, waste water discharge must have a larger container or temporary use of a tank instead, but to neutralize as soon as possible to avoid corrosion of the tank. 5, with the increase of reaction time and the increase of impurities in the solution, the effective composition of the pickling solution is gradually reduced, and the solution concentration must be detected at any time and the new solution should be added in time. 6, the deck, four walls, dead corners, the back of the escalator and other parts of the spray liquid residence time is short, the pickling effect is slightly worse than the bottom plate. Pre-treatment of pickling passivation: If there is dirt such as grease on the surface of stainless steel, it will affect the quality of pickling passivation. Generally, alkaline detergent should be used first to clean the inner wall of the liquid tank. Control of chloride concentration in acid lotion/paste and cleaning water: Excessive chloride ion content will destroy the stainless steel passivation film, some stainless steel pickling liquid/paste using chloric acid, perchloric acid and other chlorine-containing erosion media as the main agent or auxiliary to remove the surface oxidation layer, which is adverse to prevent corrosion, should be controlled in the inspection, such as the ship's standard "Stainless steel pickling Passivation paste" CB/T3595-94, The chloride content of stainless steel pickling passivation paste is controlled in the range of 25ppm ~ 100ppm. In addition, for cleaning water, the chloride ion content should also be controlled to be less than 25PPM, and the chloride ion corrosion inhibitor sodium nitrate can be added to the tap water in the construction. Waste liquid and environmental protection discharge: For the treatment of pickling passivation waste liquid, should meet the national environmental protection discharge requirements, such as fluoride waste liquid can be added to lime milk or calcium chloride treatment, for chromium containing waste liquid, can be added ferrous sulfate reduction treatment. The quality inspection of the passivation effect of stainless steel pickling can refer to the ship's standard "Stainless Steel Pickling Passivation paste" CB/T3595-94, the international standard "Stainless steel electrolytic polishing and polishing and pickling passivation surface passivation film inspection" ISO15730, etc., generally can be divided into: Appearance inspection, chemical test, chemical test and mainly include artificial seawater hanging plate corrosion test, copper sulfate titration test, potassium ferricyanide titration test (blue dot test), etc. The test of the passivation effect of "Ninghua 417" round pickling is carried out in the following three ways: Appearance inspection: Stainless steel pickling passivation surface should be uniform silver white, clean and beautiful, no obvious traces of corrosion, weld and heat affected zone should not have oxidation color, there should be no uneven color spots.

Residual liquid test: Using phenolphthalein test paper to check the cleaning degree of residual liquid on the surface of stainless steel, the PH value is neutral. Blue dot test: The basic principle of the blue dot test method is that if the surface passivation film is incomplete or there is iron ion pollution, there will be free iron ions, and the potassium ferricyanide solution will react to produce blue precipitation when it meets iron ions. The reaction formula is as follows: K++Fe+2+[Fe(CN)6]-3→KFe[Fe(CN)6]↓ Dissolve 10g potassium ferricyanide in 50ml distilled water with 100mL beaker, add 30ml concentrated nitric acid after dissolution, and then transfer to 1000ml volumetric bottle and dilute with distilled water to the scale, that is, blue dot test solution, storage period is one week. Test method: Drop the blue dot test liquid on the test paper attached to the stainless steel surface, 30 seconds later the test paper shows a blue dot is unqualified. It should be noted that the chemical test will destroy the passivation film of stainless steel, which can usually be pickled and passivated with the test plate and the product at the same time, and then the above test is carried out on the test plate.