Chromate conversion coating
In an aqueous salt solution, the oxide film adsorbs the salt and undergoes a chemical reaction, forming basic aluminum chromate [Al(OH)CrO4] and aluminum dichromate [Al(OH)Cr2O7]. These reaction products fill the pores of the film, thereby sealing them and imparting a slight yellow tint to the anodized coating, which in turn enhances its corrosion resistance.
Notes when closed:
The higher the potassium content in the sealing solution, the better the corrosion resistance of the sealed oxide film.
Aluminum workpieces that have undergone anodizing must be thoroughly rinsed before the sealing process to prevent acidic residues on the surface from entering the sealing bath. Additionally, care should be taken to avoid contact between the workpiece and the tank; otherwise, the oxide film may be damaged.
Impurities in the sealing solution should be strictly controlled. When the SO4²⁻ concentration exceeds 0.2 g/L, the oxide film may become lighter in color or turn whitish; this can be mitigated by adding an appropriate amount of calcium chromate (CaCrO₄) and removing it through filtration. At an SO4²⁻ level of 0.02 g/L, the oxide film may also appear whitish and exhibit reduced corrosion resistance; in such cases, potassium aluminum sulfate [K₂Al₂(SO₄)₄·24H₂O] can be added at a concentration of 0.1–0.5 g/L to adjust the situation. Furthermore, when the chloride ion (Cl⁻) concentration exceeds 1.5 g/L, it can cause corrosion of the workpiece’s oxide film, necessitating dilution or replacement of the sealing solution.
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