First, the classification of stainless steel passivation process:
1. Wet passivation, including chemical and electrochemical methods.
A chemical passivation: divided into nitric acid type (nitric acid, nitric acid dichromate type passivation, nitric acid + hydrofluoric acid type passivation and other chromic acid type passivation), sulfuric acid type passivation, hydrogen peroxide type passivation, alkali Liquid type passivation.
B Electrochemical passivation: DC passivation, carrier DC passivation.
2. Dry passivation has the following two categories: A room temperature passivation B heat treatment passivation
In summary, according to the composition of the stainless steel alloy, the processing type can select a suitable passivation process. However, the passivation effect depends both on the passivation process and on the stainless steel material itself.
Second, the quality control of the passivation film: strict quality inspection of the passivation film of stainless steel, that is, the evaluation of the passivation film, but also the identification of the effect of the use of stainless steel. See the table for the quality inspection method of stainless steel.
Quality inspection of stainless steel passivation film
Experimental name test method description document standard number
Exterior
The visual surface should be a clean metallic color with no rust, spots or hair. Allow slight discoloration, size can not be damaged
Ff-10-01
QQ-P-35B
Water immersion test immersed in water surface without rust MIL-STD-753 method 100
High humidity test in 100 °F, 95%~100% humidity chamber, no rust on the surface of the experiment for 24h, the appearance does not decrease ff-10-01
QQ-P-35B
Salt spray test 5% sodium chloride salt spray, no less than 2h surface without corrosion ASTM-B117
Surface contact
Copper test copper oxide solution for passivation film, surface treated with weak acid, copper contact deposition for 300 type stainless steel (sulphur, niobium steel type 303., 303SC, 307SC), instead of salt spray test MIL-STD-753 - method 102
Ferric chloride
Corrosion test FeCl3 6%, temperature (35 ± 1) ° C, time 24h The experimental results are corrosion rate [g / (m2. h)]:
Passivation after polishing 8~12
GB4334.7-87
Unpolished passivation 30~40
Polished unpassivated 15~20
Passivation film finish
For the integrity test, see the presence or absence of deposited copper HB5292-84 on the surface of HB5292-84 parts.
The boiling water test stainless steel passivation parts boil in boiling water, the longer the rust point occurs, the better
Third, the factors affecting the surface passivation of stainless steel:
There are many factors that can damage the passivation film of stainless steel and convert the passive state of stainless steel into a living state.
1. Use environmental media. The stainless steel passivation film is a thermodynamically suppressed metastable structure. Its protection effectiveness is related to the environmental medium. It should be cleaned regularly during use to remove harmful substances and adhere to the surface for a long time. Especially in environments with chloride ions, avoid long-term adhesion of chloride ions to the surface and concentration in water. If stainless steel is used in food industry appliances, it should be washed after each contact with food to avoid chloride ions and damage the passivation film. For containers and equipment used in dairy products, chloride ions in the dairy product have a destructive effect on the passivation film. For example, if the dairy product is used for a long time, the capacity and equipment will be corroded and perforated. Therefore, it should be cleaned regularly to restore the passivation film.
2. Chloride ion. Chloride ions are extremely harmful to stainless steel. The chloride ion content in the passivation solution should be strictly controlled during the passivation process, and the passivation chemical materials used have a limited amount of chloride ions. The preparation of passivation water and washing water also have strict water quality requirements for chloride ions, so as to ensure that the passivated product does not adhere to chloride ions, so as to avoid future troubles.
3. Surface cleanliness. For stainless steel alloys, the lower the surface roughness, the smoother the surface, the harder the foreign matter adheres, and the lower the local corrosion rate of each part. Therefore, stainless steel should be used as much as possible to finish the surface. In addition, the cleanliness of the stainless steel surface is also important, and the final cleaning after passivation should be carried out carefully because the residual acid promotes the cathodic reaction, causing the film to rupture, thereby activating the stainless steel and greatly reducing the corrosion resistance.
4. The internal factors of stainless steel. The martensite content and chromium-nickel content of stainless steel have a great influence on the passivation properties of stainless steel. The nickel content is low and the passivation performance is low. The passivation film properties of martensitic stainless steel are not as good as those of austenitic stainless steel.
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