Green pigment coating through micro arc oxidation of aluminum alloy 6061

• Main Authors: Jian-mao Wang, 王建貿
• Other Authors: Dah-Shyang Tsai
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/83782387241654734980

碩士 === 國立臺灣科技大學 === 化學工程系 === 104 === Coloring aluminum alloys with micro arc oxidation (MAO) technique is quite difficult, because, unlike anodizing, the MAO coating does not contain regular pore structure. This study demonstrates that we are capable of achieving uniform coloring via addition of negatively charged pigment nanoparticles in the electrolyte. We select green chromia pigment to color the surface of aluminum alloy 6061 and show the feasibility of MAO coloring. Experimental results indicate the pigment particles, with adsorbed negative charge, drastically raise the oxidation current and the power consumption under the constant-voltage mode. Hence we divide the coloring operation into two steps, first depositing a barrier layer without pigment using the constant-voltage mode, second growing a green layer with pigment under the constant-current mode. Still, the positive voltage of the second step easily runs away if the electrical parameters are not set properly. The phases of chromia oxide and α-alumina are isostructural. Although commercial chromia pigment and in-house chromia powder are highly defective chromium oxide, Cr2O2.4, yet the green layer turns into a much less oxygen deficient crystal with parameters similar to those of hexagonal Cr2O3. Microstructure analysis of commercial powder precursor shows the green coating consists of two layers; the top layer contains approximately 50% chromia and 50% alumina, a dense layer underneath contains ~100% alumina. The total thickness increases with increasing duty ratio, or increasing ratio of positive (I+) divided by negative current (I-). With 10 g dm-3 pigment in the electrolyte for green coating on aluminum alloy 6061 for 40 minute, the optimal parameters are 92 mA cm-2 in current density, 500 Hz in frequency, 20% in duty ratio, and 1.0 in the I+/I- ratio.