Coloring the aluminum alloy surface in plasma electrolytic oxidation with the green pigment colloid

Chromium oxide pigment is a promising electrolyte additive to green the aluminum alloy surface with plasma electrolytic oxidation (PEO) because of its opacity, outdoor durability, and hardness. Initial coating efforts reveal that pigment and adsorbed dispersant, with extra negative charges, dramatically raise positive current and cause power fluctuations under the constant-voltage mode. Accordingly, research attention is redirected to a search of proper electrical parameters under constant current with a required amount of pigment and dispersant. The parameters for green coating involve; 92mA cm−2 in positive and negative current densities, 10–20% in duty ratio, 500Hz in frequency, when the electrolyte is formulated with 10g dm−3 pigment and 0.8g dm−3 dispersant. Microstructure analysis shows that the resultant coating comprises two layers of different compositions; the top layer is a mixture of chromia and alumina, a dense layer underneath involves nearly 100% alumina. Phase analysis indicates that the aluminum oxide crystallizes in γ- and α-alumina both. A small amount of α-alumina exists in solid solution with chromium. Although the as-received pigment is oxygen deficient, similar to Cr2O2.4 in crystal structure, the oxidative plasma of PEO converts it into stoichiometric Cr2O3. •Green pigment of chromium oxide is used to color Al alloys in PEO.•Extra charge carriers of pigment and dispersant cause power instability.•Constant current mode is suitable for PEO coloring with pigment. The resultant green coating comprises a two-layer structure.•A layer of chromia and alumina is on top of the dense γ-alumina layer.