Chromium Phosphating Technology of Aluminum and Aluminum Alloys

The process of forming a green amorphous chromic acid and 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 alkali 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 minutes

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) When composite accelerator A9g/L, Cr(VI) 2g/L, H3PO4 can form excellent adhesion film in the range of 5-40ml/L, and H3PO4≥40ml/L form loose powder film. .

(2) When H3PO4 25ml/L and Cr(VI) 2g/L are fixed, composite accelerator A can form a film normally at 1~9g/L; A 9~15g/L form a powdery film; A> 15g/L , Incompetent membranes and corrosive substrates.

(3) When fixing H3PO4 25ml/L: Accelerator A 9g/L, Cr(VI) content is 1~5g/L, the color of green film gradually becomes lighter, and the film adhesion is excellent, indicating that Cr(VI) participates in film formation The process enters the membrane 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.

1.2.2 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

An iridescent film was formed after t ≤ 5 min. A light green film was formed after t> 5 min and the green color was deepened with time, and a dark green film was formed during 25 ~ 35 min. When the time is extended to several hours, the film layer is rough and loose.

1.2.4 Adaptability of Substrates

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. Fresh chrome phosphate film is easily soluble in 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.