Reaction Rates of Second-Phase Constituents in Aluminium During Etching, Brightening and Oxalic Acid Anodizing Processes

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Abstract
The behaviour has been examined of second-phase constituents in super-purity aluminium during etching in caustic soda, chemical brightening and anodizing in oxalic acid. Specimens containing large second- phase constituents were prepared using a carefully selected casting technique. Surfaces of specimens were metallographically polished and then the specimens were either caustic etched or chemically brightened or anodized. After these treatments the surfaces and cross-sections of specimens were photographed. It was found that the majority of constituents examined reacted to the chemical and electrochemical treatments at a different rate from that of the aluminium matrix. Furthermore, the relative rate of reaction often varied from one type of treatment to another. The intermetallic constituents included Mg2Si, MnAl6, CuAl2, FeAl3, NiAl3, Si, TiAl3, β-AlMg and CrAl7. On the basis of experimental results they were divided into groups reacting slower, at an equal rate and faster than the aluminium matrix respectively. This grouping was carried out separately for caustic etching, chemical brightening and anodizing.

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