The surface adjustment before phosphating after degreasing is an important step in the formation of phosphating film crystals. It changes the microscopic state of the metal surface and promotes the formation of fine, uniform and dense phosphating film during phosphating. Especially in the case of metal surface cleaned by pickling or high temperature alkali, and immersion low temperature low zinc film phosphating, surface adjustment is particularly required.
The main components of the surface conditioning solution are titanium salt (titanium colloid) and sodium phosphate, a slightly alkaline colloidal solution. Due to the high surface energy of the colloidal particles, the metal surface has a strong adsorption effect. When a large number of crystal nuclei are formed on the surface to be treated, a uniform phosphate crystal is rapidly formed around the crystal nucleus at the initial stage of phosphating, which limits the large The growth of the crystal promotes the refinement and densification of the phosphating film, and improves the film forming property, shortens the phosphating time, reduces the film thickness, and also eliminates the influence of the difference in the state of the metal surface on the phosphating quality.
The main points of control and management of the surface adjustment process are as follows.
(1) Effective titanium concentration: l0ppm or more (that is, 1mg of titanium in lkg aqueous solution is 1ppm), pH 8.5~9.5. When the Ti (titanium) concentration is lower than 10 ppm, the phosphating film is thickened; the pH value is lowered, the surface adjustment effect is lowered, and an iron phosphate film (blue) is formed, and the corrosion resistance is lowered.
(2) In order to prevent the main component titanium colloid from abruptly agglomerating in the treatment liquid, the phosphating film becomes larger and the film becomes thicker after being adhered to the steel sheet by phosphating. The water used to prepare the surface treatment liquid should be pure, and its conductivity should be less than 200 MS/cm. Deionized water should be used in the case of poor water quality of tap water (which is not suitable for chlorine).
(3) At room temperature, the maximum temperature does not exceed 35 ° C, to control the temperature of the workpiece being processed. Processing time is 30~60s.
(4) In order to ensure the stability of the treatment liquid, fresh water and additional surface preparation must be supplied frequently. The amount of surface conditioner used on the production line is not proportional to the number of parts processed. It should be added regularly, and the weekly treatment fluid should be thoroughly updated (should be managed according to the instructions of the material supplier). The treatment liquid should be kept in a good agitation state to avoid precipitation.
The focus of surface adjustment process management is that the workpiece temperature should be as low as possible and undergo strict pH management.
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