1 The influence of different pretreatment processes on the formation of phosphating film is very obvious. For the use of strong alkali degreasing and strong acid etching, the obtained phosphating film is coarse, and the hydrogen evolution process is large during the phosphating process, and the phosphating time is long.
2 After the steel parts enter the phosphating solution, the surface is continuously dissolved by the phosphating solution, so that the phosphate content on the surface of the part rises, and the orthophosphate precipitates and forms a film on the metal surface. If there are many surface active points on the part, the dissolution rate is high after being etched by the free acid in the phosphating solution, causing a large amount of hydrogen evolution, and the acid content of the phosphate on the surface of the part rises and the pH rises. This process causes an increase in the orthophosphate content of the part surface. The nucleation rate is fast, and the formed phosphating film is loose and more than L. The phosphating solution passes through the porous membrane and continues to etch the parts to deposit hydrogen. As a result, the surface of the part was severely etched, the film formation rate was slow, the phosphating time was prolonged, and the formed phosphating film was coarse.
3 The strong acid and alkali pretreatment process has the most obvious adverse effect on the easily phosphatized parts such as high carbon steel and alloy steel. This is because in the phosphating process, not only the chemical dissolution of the metal, but also the corrosion of the surface of the part. Battery reaction. The pretreatment process of strong acid and alkali causes the metal lattice to be completely exposed, which aggravates the two reaction speeds of metal dissolution and deteriorates the conditions of film formation.
Approach:
1 After phosphating parts are pretreated, the surface should remove oil and oxide film, and the crystal lattice should not be exposed too much, that is, the surface activity of the parts should not be too high, which is the requirement of phosphating for pretreatment;
2 Where there are conditions, medium and high carbon steel parts can be degreased by organic solvent and blasted to remove scale. The phosphating film obtained is fine and compact, and the hydrogen evolution is less during phosphating, and the phosphating speed is fast. High membrane corrosion resistance;
3 For phosphating parts before lacquering, a large number of low carbon steel stamping parts are not suitable for degreasing organic solvents, and the phosphating film is required to be thin, crystallized, and rust-proof. Therefore, when selecting the pre-treatment process, for the case of too many parts and no serious rust, the descaling process can be omitted, and only the degreasing process of adding the surfactant to the weakly alkaline is used, and after degreasing, the surface is directly adjusted and phosphatized;
4 For alloy steel and other parts, because the machined surface is free of scale, and only machined oil, use the surface of the wiped parts, without acid etching, direct phosphating;
5 For the rest of the mass-produced parts, most of them use weak alkaline, medium temperature, surfactant-containing degreasing solution. After washing, the acid salt is used to etch the solution to remove the oxide film. The free acid and the salt of the same anion are mixed to form an acid etching solution. Under the same ion effect, the H+ ionization amount is small, but continuous, and a certain amount of H+ content can be maintained. Therefore, the acid etching solution is not acidic. However, it lasts for a long time and is effective for oxide film removal, but does not have large etching on the base metal. The surface of the part thus treated can remove oil and oxide film without causing too much corrosion on the surface of the base metal. That is to say, the activity of the metal surface is not too high after pre-treatment, and after phosphating, a fine and dense phosphating film can be obtained, and the film weight is small, and the corrosion resistance is also required.
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Label: Phosphating film Hydrogen evolution Phosphating solution Pretreatment process Oxidation scale
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