Pickling passivation
The corrosion rate is significantly lower than that without any treatment after the metal acting with an oxidizing medium, which is called passivation of the metal.
 
The passivation mechanism can be mainly explained by the thin-film theory, that is, passivation is a very thin, dense, and well-covered passivation film generated by the metal interacting with an oxidizing medium on the metal surface.
 
This layer of film exists independently; usually, it is a compound of oxygen and metal, which plays the role of completely separating the metal from the corrosive medium, and prevents the metal from being in direct contact with the corrosive medium, so that the metal can be protected from corrosion.
 
After the austenitic stainless steel reacts with an oxidizing medium, a passivation film that meets the above requirements can be formed on the surface. However, it must be noted that the passivation film is easily damaged under the action of activating halogen ions such as Cl, Br and F. This is one of the reasons that the Cl ion content in the water must be controlled within 25ppm if the water stains cannot be dried after the hydrostatic test for the austenitic stainless steel pressure vessel after pickling and passivation treatment.
 
For austenitic stainless steel valves, a solution with strong oxidizing nitric acid as the main agent is generally used for passivation treatment. To ensure the effect of passivation treatment, the passivated surface is subjected to pickling treatment before passivation. The whole process is called pickling passivation.
 
Processes
To ensure the quality of pickling passivation, the method of immersion should be first considered. When it is inconvenient to use liquid immersion, brushing pickling passivation paste should be considered, but don’t brush pickling passivation solution.
 
The conventional processes of pickling passivation are as follows:
1. Pretreatment
The oil and dirt on the surface of the workpiece should be degreased and removed. If necessary, alkali cleaning can be used. After alkali cleaning, the surface should be rinsed with clean water. The spatter, welding slag and undercut on the surface of the welded joint should be ground. There should not be scratches and damages on the surface of the workpiece. The welding scars and damage should be ground to meet the relevant requirements.
 
2. Pickling passivation
(1) Pickling passivation solution in a pool
Pickling in a pool and rinse
According to the ratio of pickling solution, the temperature and time of pickling should be strictly controlled to prevent the corrosion of the substrate caused by insufficient or excessive pickling. If necessary, a corrosion inhibitor can be added to the pickling solution. Disassemble the carbon steel parts on the pickled products that can be disassembled, and those that cannot be disassembled need to be painted with protective paint or wrapped. After pickling, rinse the carbon steel part thoroughly with clean water, so that no residue remains on the surface, which is convenient for subsequent passivation treatment.
 
Passivation in a a pool and rinse
According to the characteristics and temperature of the passivation solution, the passivation time should be strictly controlled to form a good passivation film. After passivation, rinse the part thoroughly.
 
(2) Two-in-one pickling passivation in a pool and rinsing methods
The difference from the above-mentioned method is that the two-in-one pickling passivation solution is complete once, and the other procedures are the same.
 
(3) Two-in-one pickling passivation paste and rinsing methods
The steps of using two-in-one pickling passivation paste are as follows:
Apply the paste evenly on the surface to be pickled and passivated. Usually, apply the paste on the welding seam first, and the thicker oxide scale can be applied with more paste.
 
Keep the paste according to the requirements of the instructions of the paste. Wipe the rust and oxides with cotton yarn before the paste is dry. A stainless steel wire brush can be used if necessary. Rinse the surface thoroughly until a uniform silvery white is obtained.
 
Post-treatment
After pickling passivation, certain protective measures should be taken on the passivation surface to prevent the destruction of the passivation film. The passivated surface shall not be in contact with hard objects, and welding and grinding are prohibited.
 
Quality inspection
The quality inspection after acid passivation treatment can generally be considered from the following aspects:
(1) After pickling, no obvious corrosion marks are allowed on the surface of the workpiece, and stainless steel particles are not allowed.
(2) No oxide skin and uneven spots are allowed on the surface. Oxidation color is not allowed on the surface of welded joints, heat-affected zone and hot-worked parts.
(3) After pickling and passivation, the water in and on the surface of the workpiece should be removed immediately. If it cannot be removed, the chloride ion content of the rinse water should be controlled to be no more than 25ppm.
(4) Residual liquid is not allowed on the surface after pickling. PH test paper can be dipped in water and attached to the surface of the inspected workpiece. The workpiece whose pH value is 6.5 to 7.5 is qualified.
(5) After passivation, there is a dull and silver-gray film on the surface.
(6) Testing by the blue dot
The quality of the pickling passivation treatment can be tested by the blue dot test. This method is generally used for the test piece, and the passivation quality of the test piece should be consistent with the workpiece. If dropping the solution on the workpiece must be performed, the inspected part must be treated by the original pickling passivation until the specified requirements are met.
 
Use 1 gram of potassium ferricyanide K3Fe(CN)6, 3 milliliters of nitric acid HNO3 (65% to 85%) and 100 milliliters of water to prepare a solution. The solution should not be used for the second time. Then, after immersing the solution with filter paper, attach it to the surface to be tested or drop the solution directly onto the surface to be tested; observe the surface within 30 seconds. If there is no blue precipitate, the workpiece is qualified.
 
It should be noted that the test can only be carried out after the surface treated by pickling passivation is dry, and the test solution should also be rinsed after the test.