Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide layer

•Dye emulsions can be used in coloring aluminum alloys during PEO.•Addition of dye emulsion suppresses the micro arc intensity during PEO.•A large number of synthetic dyes are applicable using this coloring technique.•Addition of dye emulsions flattens the PEO surface.

Coloring a dense oxide layer is accomplished during plasma electrolytic oxidation (PEO) with an electrolyte mixed with dye emulsions. We demonstrate this coating technique using the aluminum disks of 6061 alloy and five synthetic dyes of red, yellow, orange, blue, and black color. The prepared dye emulsions are kinetically stable, containing negatively charged oil droplets of ~ 50 nm in size. To avoid color speckles, a barrier oxide layer must be first grown under the normal PEO conditions with an electrolyte without dye, followed by the growth of colored γ-alumina film with a stepwise increasing voltage of positive polarization in the electrolyte containing dye emulsion. Although the presence of surfactant in emulsion causes a large amount of bubbles that obscures the light of electric sparks, the spark sound can be heard clearly on site. These electric discharges can produce an uneven surface with many volcano craters of micrometer size which is the icon of PEO. Fortunately this typical unevenness can be smoothed out by synthetic dyes with proper voltage adjustment, resulting in a glossy surface.