The microstructures of unsealed anodic coatings of aluminum which had been oxidized by electrolysis in sulfuric acid bath and treated for coloring with aq. solution of various inorganic salts, were studied by means of an X-ray diffractometer and an electron microdiffractometer of high resolving power.
The experiments were conducted by the same processes an in the previous report. Aluminum had previously been coated with porous films by the electrolysis in sulfuric acid bath.
The results of the measurements of crystal structures of coatings under several treatments for coloring were as follows:
1) Peeled off films of unsealed oxide coating consisted of tetragonal lattices of γ-Al
2O
3, which had been slightly deformed from cubic system.
2) The oxide layer was changed to opaque grey film by the cathodic electrolysis in aq. solution of KMnO
4-H
2SO
4 and the formation of α-Al
2O
3 crystals was observed.
3) When the coating was immersed in aq. solutions of Co(Ac)
2 and KMnO
4, alternately, it was changed to brown colored, in which the precipitation of β-Co was observed.
4) When the coating was immersed in aq. solution of (NH
4)
2CrO
2, it showed light yellow color, and in some parts, it was observed to have slightly turned to crystallized structure from the ordinary amorphous state.
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