The process of forming a green amorphous chromic acid phosphate conversion film on the surface of aluminum and aluminum alloys is called chromium phosphating and is carried out in an acidic solution containing phosphoric acid, hexavalent chromium, and alkaline fluoride. The International Organization for Standardization has established standards for chromic acid and phosphate conversion coatings for aluminum and aluminum alloys.
1 test section
1.1 Test conditions
Material industry pure aluminum L6; temperature 20°C; time 10 min.
Process oil derusting → washing → pickling → washing → chromium phosphating → washing → natural drying (or <70 °C drying).
Test the standard reaction (bubble) strength, film depth, adhesion, film quality.
1.2 Film Formation Influencing Factors
1.2.1 Formula composition
(1) Composite accelerator A9g/L, Cr(VI) 2
In the case of g/L, H3PO4 can form a film with excellent adhesion in the range of 5-40 ml/L, while H3PO4 ≥ 40 ml/L forms a loose powdery film.
(2) When H3PO4 25ml/L and Cr(VI) 2g/L are fixed, the composite accelerator A can form a film normally at 1~9g/L; A 9?
15g/L formed a powdery film; A> 15g/L, no energy film and corrosive substrate.
(3) Fix H3PO4 25ml/L: Accelerator A
At 9g/L, the content of Cr(VI) is 1～5g/L, the color of green film becomes lighter, and the film adhesion is excellent, indicating that Cr(VI) is involved in the film formation process and enters the film in the form of Cr(III). . To protect the environment and reduce pollution, it is advisable to select a low content of Cr(VI).
The preferred process recipe for chromium phosphide determination based on trial and error is:
H3PO4 15 ~ 25ml / L; Cr (VI) 1.5 ~ 2g / L; composite accelerator 6 ~ 9g / L.
.2.2 Effect of temperature
With the increase of temperature, the reaction speeds up, and an excellent green film can be formed at 8 to 45°C. After a temperature of >45°C, the temperature rises violently and the film color becomes lighter and looser, and the film continues to increase so that the substrate is corroded. Tests have shown that chromium phosphide can be processed at room temperature.
1.2.3 Influence of time on the relationship between film weight and processing time.
t ≤ 5min to form an iridescent film, t>5
Min forms a light green film and prolongs the green color with time, forming a dark green film in 25 to 35 minutes. When the time is extended to several hours, the film layer is rough and loose.
1.2.4 The adaptability of the substrate
Chromium phosphide has a wide adaptability to the processing of aluminum and aluminum alloy materials. For industrial pure aluminum, rust-proof aluminum, hard aluminum and cast aluminum alloy (silicon aluminum alloy), etc., all have the same film-forming rule of the substrate, the surface of the parts Gloss color, roughness, etc. only slightly affect the gloss and color of the film.
2 film properties
Chromium phosphide film is amorphous, the film is light green with a rainbow color when the film is thin, and the slightly thick film is olive green. The film is excellent in flexibility and can withstand various deformation processing of parts. The brittleness of thick film is improved. A fresh chrome phosphate film is easily soluble in a 1:1 nitric acid solution. After drying and storage for a period of time, the corrosion resistance of the film is improved and it is insoluble in 1:1 nitric acid, phosphoric acid and sulfuric acid. Placed in the atmosphere for a year without corrosion, fade, film intact.
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