Chromium phosphating technology of aluminum and aluminum alloys

The process of forming a green amorphous chromic acid-phosphorus 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 10min. 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 Formulation 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. Loose powdery 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 can 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 green film color gradually becomes lighter, and the film adhesion is excellent, indicating that Cr(VI) participates in the film formation The process enters the membrane in the form of Cr(III). In order to protect the environment and reduce pollution, Cr(VI) should be selected in a low content. According to trial and error determination of chromium phosphide the best process formula: H3PO4 15 ~ 25ml / L; Cr (VI) 1.5 ~ 2g / L; composite accelerator 6 ~ 9g / L. 1.2.2 Influence of temperature Temperature rise, reaction is accelerated, excellent green film can be formed at 8～45°C, and after 45°C, the temperature rises violently, the film color becomes lighter and looser, and continues to improve without film. Corrosion of the substrate. Tests have shown that chromium phosphide 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. 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 Substrate Chromium Phosphorizing has a wide adaptability to the processing of 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 law of film formation, the gloss color of the part surface and the roughness etc. only have a slight influence on the gloss and color of the film. 2 The properties of the film The phosphatized film is amorphous. When the film is thin, it is light green with rainbow colors. The slightly thick film is olive green. The film is excellent in flexibility and can withstand various deformation processing of parts. The brittleness of the 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.