Study on the black electrolytic coloring of anodized aluminum in cupric sulfate

Research in obtaining the darkest shade of black color on the aluminum surface is challenging but worthwhile. The process starts with forming an anodic film on aluminum, then obtaining the darkest shade of black color using an electrolytic method. The coloring impact factors such as electrolyte concentration, bath temperature, voltage, and time duration were all investigated in the research. The conditions to obtain the darkest shade of black coating with approximately 0.03 of minimum visible light reflectivity and 17.6 of minimum L-value include a mixed solution of 35 g/L copper sulfate, 10 g/L sulfuric acid and 2 g/L sodium sulfate, at a temperature of 40 °C with a voltage of 30 V and a duration of 50 min. The research has found that the bath temperature is the most important coloring factor. The film has a stronger anti-corrosion ability when colored at a low bath temperature.