Technical Standards For Chromium Phosphating On Aluminum And Aluminum Alloy Surfaces

- Aug 19, 2019-

The process of forming a green amorphous chromic acid or phosphate conversion film on the surface of aluminum and aluminum alloy is called chromium phosphating, and is carried out in an acidic solution containing a composition of phosphoric acid, hexavalent chromium and basic fluoride. The International Organization for Standardization has established standards for aluminum and aluminum alloy chromic acid and phosphate conversion coatings.


1 test part


1.1 Test conditions


Material industry pure aluminum L6; temperature 20 ° C; time 10 minutes


Process degreasing, water washing → pickling → water washing → chromium phosphating → water washing → natural drying (or <70 ° C drying =.


Check the strength of the standard reaction (bubbles), the color depth of the film, the adhesion, and the quality of the film.


1.2 film formation factors


1.2.1 Formula composition


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


(2) When H3PO 425ml/L and Cr(VI) 2g/L are fixed, the composite accelerator A can form a film normally at 1~9g/L; A9?15g/L forms a powdery film; when A>15g/L, it is incompetent The film and the substrate are corroded.


(3) Fixing H3PO 425ml/L: When the accelerator A9g/L, the content of Cr(VI) is 1~5g/L, the color of the green film is gradually lighter, and the adhesion of the film is excellent, indicating that Cr(VI) participates in the film formation process. 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 based on trial and error is:


H3PO415 ~ 25ml / L; Cr (VI) 1.5 ~ 2g / L; composite accelerator 6 ~ 9g / L.


1.2.2 Effect of temperature


When the temperature is raised, the reaction is accelerated, and an excellent green film can be formed at 8 to 45 ° C. After >45 ° C, the temperature rises drastically, and the film color becomes shallow and loose, and the film color continues to be increased to corrode the substrate without film. Tests have shown that chromium phosphating can be processed at room temperature.


1.2.3 Impact of time


A rainbow color film was formed at t≤5min, and a light green film was formed at t>5min 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. It has the same film forming law for industrial pure aluminum, rust-proof aluminum, hard aluminum and cast aluminum alloy (silicon-aluminum alloy). Glossy color and roughness have only a slight effect on the gloss and color of the film.


2 film properties


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.