How to pickling and passivation stainless steel
Principle of pickling and passivation
The corrosion resistance of stainless steel is mainly due to the addition of high content of Cr element (such as 316L w (CR) = 16.00 ~ 18.00) in the steel. Cr element is easy to oxidize and can rapidly form a dense Cr2O3 oxide film on the surface of the steel, which makes the electrode potential and corrosion resistance of the steel increase abruptly in the oxidation medium, The corrosion resistance of stainless steel mainly depends on the very thin (about 1nm) dense passivation film covered on the surface, which is isolated from the corrosive medium and is the basic barrier for stainless steel protection. If the passivation film is incomplete or defective, the stainless steel will still be corroded.
The surface oil, scratch, rust, impurities, low melting point metal pollutants, paint, welding slag, spatter, etc. brought by the lifting, assembly, welding, weld inspection (such as dye flaw detection, pressure test) and processing of stainless steel chemical tanker’s liner plate, equipment and accessories affect the surface quality of stainless steel and damage the surface passive film, It reduces the surface corrosion resistance and is easy to cause pitting corrosion, intergranular corrosion and even stress corrosion cracking together with the corrosive medium in the later transported chemicals.
In the operation of stainless steel chemical ship, seawater is usually used to wash the cabin. The seawater is rich in chloride ions, which has a great corrosive effect on the passive film. The acid pickling and passivation are indispensable for the poor working conditions.
The passive film of stainless steel has dynamic characteristics, which should not be regarded as the complete stop of corrosion, but the formation of diffusion protective layer. Generally, the passive film tends to be destroyed in the presence of reducing agent (such as chloride ion), but can be protected and repaired in the presence of oxidant (such as air).
Stainless steel will form oxide film when placed in the air, but the protection of this film is not perfect. The average thickness of stainless steel surface is 10 μ The chemical activity of the acid solution makes the dissolution rate of the defect part higher than that of other parts on the surface. Therefore, pickling can make the whole surface tend to be uniform and balanced. More importantly, through pickling passivation, iron and iron oxides are dissolved prior to chromium and chromium oxides, and the chromium poor layer is removed, so that the surface of stainless steel is rich in chromium, The potential of this kind of chromium rich passivation film can reach + 1.0V (SCE), which is close to the potential of noble metal, and the stability of corrosion resistance is improved.
Ways of pickling and passivation
According to the different operation modes, there are six methods for pickling and passivation of stainless steel, including dipping method, paste method, brushing method, spraying method, circulation method and electrochemical method. Among them, dipping method, paste method and spraying method are more suitable for pickling and passivation of stainless steel chemical ship and equipment.
1. Impregnation method:
Stainless steel pipelines, elbows, small pieces and so on are most suitable for this method, and the treatment effect is the best. Because the treated parts can be fully immersed in pickling passivation solution, the surface reaction is complete, and the passivation film is dense and uniform. Chuandong shipyard made use of our pickling workshop to pickling “Ninghua 417” stainless steel pipeline in batches and achieved good results. This method is suitable for continuous batch operation, but new solution should be added continuously with the decrease of reaction concentration. Its disadvantage is that it is limited by the shape and capacity of acid tank, so it is not suitable for large capacity equipment and pipelines with too long and too wide shape; If it is not used for a long time, the effect will be reduced due to solution volatilization and other reasons, so special site, acid pool and heating equipment are needed.
2. Paste method:
Stainless steel pickling passivation paste has been widely used in China, and a series of products are supplied. The main components are 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 ship weld treatment, welding discoloration, deck top Corner dead angle, escalator back and liquid tank large area of painting passivation are applicable.
The advantages of paste method are no special equipment and site, no heating equipment, flexible on-site operation, one-time pickling and passivation, strong independence; The passivation paste has a long shelf life and can be used for one time with a new passivation paste for each coating treatment. The reaction stops immediately after the passivation of the surface layer. It is not easy to be corroded and is not limited by the subsequent washing time. The weak links such as welds can also be passivated. The disadvantages are poor operation environment, high labor intensity, high cost and poor treatment effect on stainless steel pipeline inner wall.
3. Spray method:
It is suitable for pickling and passivation of single product with fixed site and closed environment or equipment with simple internal structure, such as spray pickling process of plate production line, and pickling of inner wall of liquid cargo tank for stainless steel chemical tanker. Chuandong shipyard is preparing to use spray method for pickling passivation of the cargo tank of Sinochem 3450t stainless steel chemical ship “Yingchun”. It has the advantages of fast continuous operation speed, simple operation mode, little influence on workers’ corrosion, and pipeline can be pickled again in the process of liquid transfer. The utilization rate of the solution is high. This method has many restrictions, such as:
- 1. The scaffold in the tank shall be removed completely, the tank and piping shall be cleaned, and there shall be no residue or impurity in the pipeline.
- 2. All deep well pumps, valves, tank washing machines and other liquid cargo systems of the whole ship shall be commissioned for standby.
- 3. If the pickling solution stays in the tank for a long time, it will react and cause excessive corrosion of stainless steel. Therefore, continuous operation is required. The shipyard must highly cooperate and prepare a large amount of cleaning water at any time. In case of power failure, water cut-off and shutdown, serious consequences will be caused.
- 4. The discharge of waste acid and waste water must be in a larger container or temporarily replaced by a tank, but it should be neutralized and discharged as soon as possible to avoid corrosion of the tank.
- 5. With the increase of reaction time and impurities in the solution, the effective components of the pickling solution gradually decrease. It is necessary to detect the concentration of the solution at any time and replenish the new solution in time.
- 6. The residence time of spray liquid in deck, four walls, dead angle and back of escalator is short, and the pickling effect is slightly worse than that of bottom plate.
Pretreatment of pickling and passivation: if there is grease and other dirt on the surface of stainless steel, it will affect the quality of pickling and passivation. Generally, the inner wall of liquid tank should be cleaned with alkaline detergent.
Control of chloride ion concentration in pickling solution / paste and cleaning water: if the chloride ion content exceeds the standard, the passivation film of stainless steel will be damaged. Some stainless steel pickling solutions / paste use hydrochloric acid, perchloric acid and other corrosive media containing chloride ions as the main agent or auxiliary to remove the surface oxide layer, which is unfavorable to prevent corrosion and should be controlled in the inspection, As specified in the ship standard “stainless steel pickling passivation paste” CB / t3595-94, the chloride ion content of stainless steel pickling passivation paste should be controlled in the range of 25ppm ~ 100ppm. In addition, for the cleaning water, the chloride ion content should be controlled below 25ppm, and the requirement can be achieved by adding the chloride ion inhibitor sodium nitrate into the tap water during the construction.
Neutralization and environmental protection discharge of waste liquid: the treatment of acid pickling and passivation waste liquid should meet the national environmental protection discharge requirements, such as adding lime milk or calcium chloride to treat fluorine-containing waste liquid, and adding ferrous sulfate to reduce chromium containing waste liquid.
Quality inspection of pickling and passivation
The quality inspection of stainless steel pickling and passivation effect can refer to the ship standard “stainless steel pickling and passivation paste” CB / t3595-94, international standard “stainless steel electrolytic polishing and surface passivation film inspection formed by grinding and pickling and passivation” iso15730, etc. generally, it can be divided into appearance inspection, chemical test, and chemical test mainly includes artificial seawater coupon corrosion test, corrosion test, corrosion test, corrosion test and so on Copper sulfate titration test and potassium ferricyanide titration test (blue point test) were used to test the pickling and passivation effect of “Ninghua 417” wheel.
Appearance inspection: stainless steel pickling passivation surface should be uniform silver white, smooth and beautiful, no obvious corrosion traces, no oxidation color in weld and heat affected zone, no uneven color spots.
Residual liquid test: use phenolphthalein test paper to check the cleaning degree of residual liquid on the surface of stainless steel. If the pH value is neutral, it is qualified.
Blue point test: the basic principle of the blue point test method is that if the surface passivation film is incomplete or polluted by iron ions, there will be free iron ions, and the potassium ferricyanide solution will react with iron ions to form blue precipitation. The reaction formula is as follows:
Use a 100ml beaker to dissolve 10g potassium ferricyanide in 50ml distilled water, add 30ml concentrated nitric acid after dissolution, and then transfer it into a 1000ml volumetric flask, dilute it to the scale with distilled water, which is the blue spot test solution. The storage period is one week. Test method: put the blue dot test liquid drop on the test paper attached to the stainless steel surface, 30 seconds later, the test paper shows the blue dot, it is unqualified. It should be noted that the chemical test will destroy the passive film of stainless steel. Generally, the test plate and the product can be pickled and passivated at the same time, and then the above test can be carried out on the test plate.
Source: Network Arrangement – China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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