1 Introduction In the stainless steel "target=_blank> stainless steel processing and production company, there are often stainless steel "target=_blank> stainless steel equipment, stainless steel" target=_blank> stainless steel equipment due to contact with corrosive media, will cause obvious equipment surface Corrosion marks and uneven color marks, so the treatment of stainless steel "target=_blank> stainless steel equipment surface is particularly critical, stainless steel "target=_blank> stainless steel equipment surface passivation treatment is an important link. Equipment surface passivation film formation is not Perfect, pollution caused by contact with iron ions, rust phenomenon will occur during use, resulting in changes in operating medium index, etc. Below is the principle and actual operation of pickling passivation treatment on the surface of austenitic stainless steel "target=_blank> stainless steel equipment" The conventional process talks about some of the ideas for the reference of the relevant personnel.
2 Overview Austenitic stainless steel "target=_blank> stainless steel has good corrosion resistance, and also has good hot and cold processing properties, so it is widely used in the manufacture of various pressure vessels with anti-corrosion requirements, austenitic stainless steel " target=_blank>The passivation film on the stainless steel surface has a great influence on its corrosion resistance. Austenitic stainless steel "target=_blank> stainless steel passivation film is mainly obtained by pickling and passivating the surface.
3 Principle of pickling passivation Passivation: After the metal is treated by oxidizing medium, the corrosion rate of the metal is significantly lower than that before the original treatment. The passivation mechanism can be mainly explained by the theory of thin film, that is, the passivation is considered to be due to the action of the metal and the oxidizing medium. When it acts, it forms a very thin, dense, good covering property on the metal surface, and can be attached to the solid. Passivation film on the metal surface. This film exists as a separate phase, usually
The austenitic stainless steel "target=_blank> stainless steel is treated with an oxidizing medium to form a passivation film that satisfies the above requirements, but the passivation film acts on the activation of Cl-, Br-, F- and other halogen ions. Underneath, it is extremely vulnerable. This is the austenitic stainless steel "target=_blank> stainless steel pressure vessel that has been acid-passed and passivated. If the water stain cannot be removed after the hydrostatic test, the water should be controlled. - One of the reasons why the content does not exceed 25 ppm. In addition, not any metal oxide film can be regarded as a passivation film. For example, an oxide film formed by carbon steel after high temperature oxidation cannot be used as a passivation film because it cannot meet the requirement of being firmly attached to a metal surface.
For austenitic stainless steel "target=_blank> stainless steel is generally treated with a highly oxidizing solution containing nitric acid as the main agent. To ensure the effect of passivation treatment, the passivated surface is pickled before passivation. The entire process is called pickling passivation, referred to as pickling passivation.
4 Pickling solution, passivation solution and pickling paste formula Pickling solution: 20% nitric acid + 5% hydrofluoric acid + 75% water
Pickling passivation solution (two in one): 20% nitric acid + 10% hydrofluoric acid + 70% water
Pickling passivation paste (two in one) formula: 20 ml of hydrochloric acid, 100 ml of water, 30 ml of nitric acid, and 150 g of bentonite are stirred into a paste. Technical Engineer: Zhou Liangqing Mail: [email protected]
5 The conventional process of pickling and passivation treatment ensures the quality of pickling passivation. The pickling passivation first needs to be immersed in pickling passivation solution. It is not convenient to use liquid soaking. The method of pickling the passivation paste, but it is not appropriate to use the method of pickling the passivation solution.
When soaking with pickling passivation solution, the soaking solution should be tested and tested regularly.
5.1 Pretreatment
5.1.1 Remove spatter, flux, dust, etc. from the weld and the surface of the base metal.
5.1.2 Remove oil stains. If necessary, wash with alkali or washing liquid. After washing, rinse the surface with clean water.
5.2 Pickling (pool washing) and rinsing
The pickling temperature and time should be strictly controlled during pickling. The temperature is low for a long time and the high temperature for a short time. Prevent the corrosion of the substrate caused by the pickling effect or excessive pickling, and if necessary, add an etchant to the pickling solution.
The carbon steel parts on the pickled products can be disassembled and removed. If they are not removable, apply protective paint or rubber mud. After pickling, it must be thoroughly rinsed with clean water so that no residual liquid remains on the surface to facilitate the subsequent passivation treatment.
5.3 Passivation (pool washing) and rinsing The passivation time is strictly controlled to form a good passivation film. After passivation, it should be thoroughly rinsed without leaving residual liquid.
5.4 Pickling passivation: pickling passivation (two-in-one) solution (pool washing) and rinsing using pickling passivation (two-in-one) solution can be acid-passivated and passivated once, all other can be referred to the above.
5.5 pickling passivation (two in one) paste (painting) and rinse
5.5.1 Apply the paste evenly to the surface that needs to be pickled and passivated (generally the weld is first applied). The scale of the oxide scale may be slightly thicker.
5.5.2 Keep enough time
5.5.3 Wipe off rust and oxides with cotton or cotton before the paste is dry.
5.5.4 Thoroughly rinse the surface until it is uniform silvery white.
5.6 Post-processing
After pickling and passivation, certain protective measures shall be taken on the passivated surface to protect the passivation film from damage. The passivated surface shall not be exposed to hard objects (including stainless steel "target=_blank> stainless steel wire and wire brush". Welding and grinding are prohibited. .
6 Quality inspection of pickling passivation 6.1 Appearance inspection: The surface of the pickling passivation shall be uniform silver white, and there shall be no obvious signs of corrosion. The surface of the weld and heat affected zone shall not have oxidized color, and there shall be no uneven spots of color. .
6.2 Residual liquid test: Check the rinsing degree of surface mountain residual liquid with phenolphthalein test paper. PH value is neutral.
6.3 Blue point test: The quality of pickling passivation treatment can also be tested by blue dot test method if necessary. Use 1 gram of iron ** potassium K3 [Fe (CN6)] plus 3 ml (65% ~ 85%) of HN03 nitrate and 100 ml of water to prepare a solution (suitable for use). Then, after dipping the solution with a filter paper, attach it to the surface to be tested or directly apply the solution to the surface to be tested, and observe the blue dot condition within 30 seconds, and the blue dot is unqualified. It should be noted that the test is carried out after the acid-washed passivation surface is substantially dry. The test liquid should also be rinsed clean after the test.
The basic principle of the blue dot test is: if the surface passivation film is imperfect or there is iron ion contamination, there will be free iron ions, then the following reaction can occur:
Fe2++K[Fe(CN6)]=KFe[Fe(CN6)]↓Dark blue+2K
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