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Views: 6     Author: Site Editor     Publish Time: 2021-09-23      Origin: Site

Acid pickling passivation of valve

1 pickling passivation principle

The corrosion resistance of stainless steel is mainly because of the addition of a higher content of Cr element in the steel (such as 316L W (Cr) =16.00 ~ 18.00), Cr element is easy to oxidation, can quickly form a dense Cr2O3 oxide film on the surface of the steel, so that the steel electrode potential and corrosion resistance in the oxidation medium mutagenicity improvement, The corrosion resistance of stainless steel mainly depends on the surface cover of this layer of very thin (about 1nm) dense passivation film, this layer of passivation film and corrosion medium isolation, is the basic barrier of stainless steel protection, if the passivation film is incomplete or defective is destroyed, stainless steel will still be corroded.

Stainless steel chemical tanker in the tank plate, the lifting equipment and accessories, assembly, welding, weld inspection (e.g., penetrant inspection, pressure test) and processing process of surface oil pollution, scratch, rust, impurities, low melting point metal pollutants, paint, welding slag, spatter, etc., which affects the surface quality of stainless steel, undermined its surface passivation membrane, The surface corrosion resistance is reduced, and it is easy to interact with the corrosive medium in the chemicals carried later, resulting in pitting corrosion, intergranular corrosion, and even stress corrosion cracking.

In the operation of stainless steel chemical ship, seawater washing process is often used. The seawater is rich in chloride ions, which has a great corrosion effect on passivation film. In bad working conditions, pickling passivation is indispensable.

Stainless steel passivation films have a dynamic character and should not be seen as a complete cessation of corrosion, but rather as a diffuse protective layer that tends to destroy the passivation film in the presence of a reducing agent (e.g., chloride ions) and protects and repair the passivation film in the presence of an oxidant (e.g., air).

Stainless steel is a deposit may form in the air oxidation film, but this kind of membrane protective imperfect, make through pickling of stainless steel surface with an average thickness of 10 microns of a layer of surface corrosion, acid dissolving rate of defective parts of the chemical activity of higher than other parts on the surface, so the pickling can make the whole surface tends to be even balance, and, more importantly, through pickling passivation, Make iron and iron oxide than chromium and chromium oxide preferential dissolution, remove the chromium poor layer, make the stainless steel surface chromium rich, in the oxidizer passivation effect of the surface to produce a complete and stable passivation film, the chromium rich passivation film potential up to +1.0V(SCE), close to the potential of precious metals, improve the stability of corrosion resistance.

2 pickling passivation

According to the different operation modes, stainless steel pickling passivation treatment mainly includes dipping method, paste method, coating method, spraying method, circulation method, electrochemical method and other six methods. Among them, dipping method, paste method, spraying method is more suitable for pickling passivation of stainless steel chemical vessels and equipment.

First, impregnation method:

Stainless steel pipelines, elbows, small parts and so on are most suitable for this method, and the treatment effect is the best. Because the processed parts can be fully immersed in pickling passivation solution, the surface reaction is complete, passivation film formation compact and uniform. The batch pickling of "Ninghua 417" stainless steel pipelines in the pickling workshop of our factory has achieved good results in Chuandong Shipyard. This method is suitable for continuous batch operation, but new solution needs to be added continuously with the decrease of solution concentration. Its disadvantage is limited by the shape and capacity of acid tank, it is not suitable for large-capacity equipment and pipelines with too long and too wide shape. Long-term use will be due to the volatilization of the solution and other reasons, and the effect of the need for a special site, acid pool and heating equipment.

Two, paste method:

Stainless steel pickling passivating paste has been widely used in China and a series of products supply, the main composition is composed of nitric acid, hydrofluoric acid, corrosion inhibitor, thickener and so on according to a certain proportion, the ship "stainless steel pickling passivating paste" CB/T3595-94 has specific inspection rules, manual operation, suitable for site construction, It is applicable to weld treatment of stainless steel chemical carrier, welding discoloration, top of deck, corner dead corner, back of escalator and large area of smear passivation in liquid tank.

The advantages of the paste method are that there is no need for special equipment and site, no need for heating equipment, flexible site operation, pickling passivation completed at a time, strong independence; The passivation paste has a long shelf life, and each smear treatment is used with a new passivation paste for one-time use. After the passivation of a surface layer, the reaction will stop. It is not easy to be over-corroded, and it is not limited by the subsequent washing time. Disadvantages are poor operating environment of workers, high labor intensity, high cost, slightly poor treatment effect of stainless steel pipeline wall, need to combine with other methods.

Three, spray method:

It is suitable for pickling passivation of single product with fixed site and closed environment or equipment with simple internal structure, such as spray pickling process on plate production line, and pickling of inner wall of liquid cargo tank on stainless steel chemical ship. The liquid cargo tank of the Sinochem 3450T stainless steel chemical tanker yingchun under construction is prepared to be passivated by pickling with spray method in Chuanchong Shipyard. Its advantages are fast continuous operation, simple operation mode, small impact on workers corrosion, pipetting process can be pickling the pipeline again. The solution utilization rate is high. This method has many restrictions, such as:

1 . The scaffolding in the liquid tank shall be removed completely, the cabin and the pipe system shall be cleaned, and there shall be no residue and impurities in the pipe. . All deep well pumps, valves, tank washing machines and other liquid cargo systems of the ship shall be adjusted and ready for use. . Acid washing solution staying in the cabin for too long will react and cause excessive corrosion of stainless steel, so continuous operation must be carried out. The shipyard shall be highly cooperative and ready to use a lot of cleaning water at any time. . Waste acid and wastewater discharge shall be stored in a larger container or temporarily replaced by a cabin, but it shall be neutralized and discharged as soon as possible to avoid corrosion of the cabin. . With the increase of reaction time and the increase of impurities in the solution, the effective components of acid washing solution are gradually reduced. The concentration of solution must be detected at any time and the new solution must be added in time. 6, the deck, four walls, dead corner, the back of the escalator and other parts of the spray time is short, pickling effect is slightly worse than the bottom plate.

3 . Special notes

Pretreatment of pickling passivation: if there is grease and other dirt on the surface of stainless steel, it will affect the quality of pickling passivation. Generally, alkaline detergent should be used to clean the inner wall of liquid tank.

Control of chloride ion concentration in acid washing solution/cream and cleaning water: Excessive chlorine ion content will destroy stainless steel passivation film, some stainless steel acid washing solution/paste with hydrochloric acid, perchloric acid and other corrosive media containing chlorine ions as the main agent or additives to remove the oxide layer on the surface, is adverse to prevent corrosion, should be controlled in the inspection, such as the ship "stainless steel acid cleaning passivation paste" CB/T3595-94 provisions, The chlorine ion content of stainless steel pickling passivation cream should be controlled in the range of 25ppm ~ 100ppm. In addition, the chloride ion content in the cleaning water 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.

Neutralization of waste liquid and environmental discharge: the treatment of pickling passivation waste liquid should meet the national environmental discharge requirements. For example, the waste liquid containing fluorine can be treated with lime milk or calcium chloride, and the waste liquid containing chromium can be treated with ferrous sulfate reduction.

4 . Quality inspection of pickling passivation

The quality inspection of 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 polishing and pickling passivation formed by the surface passivation film inspection" ISO15730, generally can be divided into: Appearance inspection, chemical test, chemical test and mainly artificial seawater hanging corrosion test, copper sulfate titration test, potassium ferricyanide titration test (blue spot test), etc., "Ninghua 417" wheel pickling passivation effect is tested in the following three ways:

Appearance inspection: stainless steel pickling passivation surface should be uniform silver white, smooth and beautiful, no obvious corrosion traces, weld and heat affected area should not have oxidation color, no uneven color spots.

Residual liquid test: phenolphthalein test paper to check the stainless steel surface residual liquid flushing degree, PH neutral for qualified.

Blue point test: The basic principle of blue point test is that if the passivation film on the surface is incomplete or there is iron ion pollution, there will be free iron ion. Potassium ferricyanide solution reacts with iron ion to generate blue precipitation, and the reaction formula is as follows:

K++ Fe 2 + + [Fe (CN) 6] - 3 - > KFe [Fe (CN) 6] left

Dissolve 10g potassium ferriccyanide in 50ml distilled water in a 100mL beaker, then add 30ml concentrated nitric acid, and then move it into a 1000ml volumetric flask and dilute it with distilled water to the scale, which is the blue dot test solution. The storage period is one week. Test method: the blue dot test drops on the surface of stainless steel test paper, 30 seconds after the test paper shows blue dot is unqualified. It should be noted that the chemical test will destroy the stainless steel passivation film, usually can be used to test the plate and the product at the same time pickling passivation, and then the test on the test plate.

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