A green amorphous chromic acid process  phosphate conversion coating on aluminum and aluminum alloy surfaces called chromium phosphate, is composed of an acidic solution containing phosphoric acid, hexavalent chromium and alkaline fluorides performed in . The International Organization for Standardization has established standards for aluminum and aluminum alloy chromic acid-phosphate conversion coatings.
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1 Test part 1.1 Test conditions Material industrial pure aluminum L6; temperature 20 ° C; time 10 min.
Process degreasing, water washing, pickling, water washing, chromium phosphating, water washing, natural drying (or drying at <70 °C).
Test the strength of the standard reaction (bubbles), the color depth of the film layer, the adhesion, and the quality of the film.
1.2 Influencing factors of film formation 1.2.1 Formulation composition (1) When the composite accelerator A9g/L and Cr(VI) 2 g/L, H3PO4 can form a film with excellent adhesion in the range of 5-40 ml/L, H3PO4≥ 40ml/L forms a loose powdery film.
15 g/L forms a powdery film; when A>15 g/L, it is incapable of film and corrodes the matrix. ï¾(2) When H 3 PO 4 25ml/L and Cr(VI) 2g/L are fixed, the composite accelerator A can form a film normally at 1～9g/L; A 9
(3) Fixing H 3 PO 4 25ml/L: Accelerator A 9g/L, Cr(VI) content is 1～5g/L, the color of green film is gradually lighter, and the adhesion of film layer is excellent, indicating Cr(VI) Participate in the film formation process and enter the film layer in the form of Cr(III). In order to protect the environment from pollution, it is advisable to choose a low content of Cr(VI).
The best process recipe for determining chromium phosphating according to trial and error is:
H 3 PO 4 15 to 25 ml/L; Cr (VI) 1.5 to 2 g/L; composite accelerator 6 to 9 g/L.
1.2.2 Influence of temperature The temperature rises and the reaction speeds up. Excellent green film can be formed at 8~45°C. After >45°C, the temperature rises sharply, and the film color becomes shallow and loose, and continues to increase without film. Corrosion of the substrate. Tests have shown that chromium phosphating can be processed at room temperature.
1.2.3 Effect of time The relationship between film weight and processing time is shown in the figure.
A rainbow color film was formed at t≤5min, and a light green film was formed at t>5 min and the green color was deepened with time, and a dark green film was formed at 25 to 35 minutes. When the time is extended to several hours, the film is rough and loose.
1.2.4 Adaptability of the substrate Chromium phosphating has a wide processing adaptability to aluminum and aluminum alloy materials, and has the same matrix for industrial pure aluminum, rust-proof aluminum, hard aluminum and cast aluminum alloy (silicon-aluminum alloy). The film formation rule, the gloss color and roughness of the surface of the part only slightly affect the gloss and color of the film.
2 Properties of the film The chromium phosphate film is amorphous, the film is light green with a rainbow color, and the slightly thick film is olive green. The film is excellent in flexibility and can withstand various deformation processes of parts. The brittleness of the thick film is improved. Fresh chromium phosphate film is soluble in a 1:1 nitric acid solution. After drying and storage for a period of time, the film has improved corrosion resistance and is insoluble in 1:1 nitric acid, phosphoric acid and sulfuric acid. It is left in the atmosphere for one year without corrosion, fading, and the film is intact.