The specimens of AA-6063 extruded aluminum alloys, Al-Si casting alloys and Al-Mg casting alloys were anodized in 10% sulfuric acid solution and then the anodized specimens were colored electrolytically in nickel sulfate solution with an alternating current.
The effects of the structures of anodic oxide films on alternating current color anodizing of aluminum alloys were discussed and the coloring origin of this process was examined with an electron microscope. Also the reasons why the aluminum castings a considerable amount of silicon and magnesium are not colored by the alternating current color anodizing, were studied.
The results lead to the following conclusions. The color obtained by the alternating current color anodizing was independent of the structure of the oxide layer, such as micropore diameter, cell size, the oxide layer thickness, etc. and was determined by the amount of a deposit in the bottom of micropore. The color became darker with increasing the amount of the deposit. The deposit in the bottom of micropore by electrolysis in aqueous solution of $NiSO_4$, was found to be metallic nickel and its oxide or hydoxide was not detected. The silicon particles contained in Al-Si castings remained unoxidized during anodization. These anodic oxide films of Al-Si castings containing a considerable amount of silicon did not show the rectification characteristics and so the castings were not colored by the process of alternating current color anodizing.
The β phase formed continuously in the matrix of Al-Mg castings were dissolved in the anodizing electrolyte and possibly accellerated by the anodizing itself. Therefore it was difficult to obtain anodic oxide films which have definite rectification characteristics and so the Al-Mg castings were not appropriate to be colored. However, when the Al-Mg castings were solution treated, they could be uniformly anodized and so colored by the alternating current color anodizing.