Pickling passivation process of stainless steel pipes

With the rapid development of stainless steel pipe manufacturing, The application of stainless steel is becoming more and more widely.In practical applications, during the manufacturing, processing and use of stainless steel, the surface may be affected by oil stains, scratches, rust, impurities and other pollutants. These pollutants not only damage the surface quality of stainless steel pipes, but may also damage their passivation film and reduce corrosion resistance. In order to maintain the corrosion resistance of stainless steel, pickling passivation has become an indispensable link.

1. The mechanism of pickling passivation

Pickling passivation is to use chemical methods to clean, pickling and passivate the inner and outer surfaces and other locations of stainless steel pipes, and remove all kinds of oil stains, rust, scales, welding spots and other dirt in the welding joints and damaged areas of stainless steel material, and then passivation operations are adopted to protect. The inner and outer surfaces of the stainless steel tube after pickling passivation are uniform silver-white, and can form a uniform and dense passivation film.(approximately 1nm). The passivation film has a continuous structure, a dense and hard texture, usually not damaged by erosion by the external environment. At the same time, due to its low chemical potential energy, it is inert to react to many media. This effectively prevents the corrosion reaction from further corrosion and protects the matrix from being further corroded, has a certain protective effect.

Passivation film can effectively isolate corrosive substances from stainless steel, thereby making stainless steelThe tube has good oxidation resistance and corrosion resistance. The passivation film is mainly a compound formed by the reaction of oxygen and metal., Although it adheres to the metal surface, it exists independently. By preventing contact between metal and corrosive substances, the metal will be substantially stopped dissolution, can greatly improve the corrosion resistance of stainless steel pipes.

2. The process of pickling passivation

The passivation treatment of stainless steel pipes is generally carried out with a highly oxidizing nitric acid solution. To ensure the effect of passivation treatment, Before passivation, the passivation surface must be pickled. The entire process is called pickling passivation treatment.

The general process of pickling passivation is shown in the figure.

1. 2.1 Surface pretreatment

Before pickling and passivation treatment of stainless steel pipes, Stainless steel should beMechanical cleaning of oil and other dirt on the surface of the pipe, and remove the grease. If these tasks are not done well in advance, it will greatly affect the stainless steelPipe-clearing passivation quality, make stainless steelThere are some gaps in the passivation film of the tube. The commonly used methods to remove oil stains on the surface of stainless steel pipes are, Use emulsifiers, steam or organic solvents to treat oil stains.

1. 2.2 Pickling and passivation

According to the technical requirements of stainless steelWhether it is the impregnation method or the paste method, the two-in-one method (commonly known as one-step washing) is generally used. The pickling passivation treatment is related to the concentration, ambient temperature and contact time of the pickling passivation solution. It must be operated strictly in accordance with the technical requirements of the pickling passivation. If the temperature is too low, pickling passivation will lead to incompleteness of the product passivation film, and the treatment should be carried out at an ambient temperature greater than 10℃. Generally speaking, increasing the passivation temperature of pickling will accelerate and improve the cleaning effect, but may also bring about the risk of increasing surface contamination or damage, which is the so-called "over-picking". The final rinsing must be rinsed with deionized water with a chloride ion concentration less than 25 mg/L.

1. 2.3 Post-processing

After pickling and passivation, certain protective measures should be taken for the passivation surface., to prevent damage to the passivation film (about 1 nm thick). The formed passivation film should not contact hard objects, and welding and grinding are prohibited. ASTM/A380 "Cleaning, Rust removal and Passivation of Stainless Steel Products and Equipment" emphasizes that the protection of passivation films runs through manufacturing, transportation, inspection, storage, installation and other steps, and also stipulates that polyvinyl chloride and other packaging materials shall not be used. Polyvinyl chloride materials will corrode stainless steel under the action of sunlight or ultraviolet light.

3. Quality control of pickling passivation

According to relevant quality inspection standards for pickling passivation of stainless steel containers in my country, We can put stainless steelThe quality inspection process of pickling passivation of the tube is divided into three parts. first, People need to do stainless steelTubes are inspected for appearance, that is, observe stainless steelIs the surface of the tube evenly distributed silvery white?, If the color is different or the color distribution is uneven, it means that stainless steelThe pipe's pickling passivation quality is not up to standardOtherwise, it means that the pickling passivation quality is qualified. Secondly, the staff need toTest the residual solution after tube pickling passivation, If the phenolphthalein test strip shows that the pH of the residual solution is neutral, it will be qualified, otherwise it will be unqualified. Third, stainless steel needs to beBlue point inspection for tube, Place the test paper soaked in a mixed solution of potassium ferricyanide and nitric acid on stainless steelThe surface of the tube, If the test paper has no color change after a period of time, it means that the pickling passivation quality is good. If the test paper turns blue, it means that the stainless steelThe pickling passivation quality of the tube is not qualified.

4. Knotlanguage

Stainless steel pipes mainly rely on the passivation film formed by pickling passivation treatment to resist the corrosion of various water media., puts forward high requirements for process flow and quality control in actual operation.Therefore, when performing pickling and passivation treatment of stainless steel pipes, it is necessary to strictly follow the operating procedures to ensure that each step meets the quality standards. At the same time, the quality inspection after pickling passivation treatment cannot be ignored. Only by passing strict inspection can the quality and service life of stainless steel pipes be ensured. In addition, with the continuous advancement of technology, we should also actively explore new pickling passivation technologies and inspection methods to better meet the production and use needs of stainless steel pipes.