Abstract
Micro-arc oxidation (MAO) is an anodizing technique applied to valve metals. With MAO, a crystalline aluminum oxide layer can be grown on aluminum alloys. This paper examines MAO as a surface treatment for metals for vacuum use. A 17.6 μm-thick MAO layer was grown on an A5052 aluminum alloy surface employing 50 Hz AC and a maximum of 450 V. This oxide layer contained α-alumina and γ-alumina crystalline aluminum oxide, and actually consisted of two distinct layers: first, a dense layer about 500 nm thick directly on the substrate, and, second, a 17 μm-thick layer on the dense layer which included numerous voids, each about 10 μm in size. Neither layer had the fine pores - 20-30 nm-structures - exhibited by alumite film. The outgassing rate of a MAO-treated A5052 sample after 10 hours of evacuation was 1.0×10-5 Pa•m3•s-1•m-2, i.e., 1/30 that of alumite with sulfuric acid. The most dominant gas species was water. The MAO sample's corrosion resistance to hydrochloric acid was the same as that of an alumite sample.
