CHEMICAL HETEROGENEITIES IN AA2024: DIOXYGEN REDUCTION AS A PROBE OF REMOVAL OF CU-RICH INCLUSIONS VIA CHEMICAL ETCHING

摘要

For light-weight aluminum alloys, such as AA2024, dioxygen reduction at Cu-rich intermetallic inclusions is an important cathodic reaction that can drive corrosion. To better understand this process, electrochemical measurements were used to examine dioxygen reduction at AA2024 samples using a wall jet flow cell. These experiments were done in a potential regime where anodic processes are unimportant. This allows unambiguous measurement of dioxygen reduction currents at the alloy and determination of the influence of various surface treatments on this current. Dioxygen reduction was first studied at untreated alloy surfaces (i.e. surfaces subjected to polishing under aprotic conditions) and then again after subjecting the surfaces to a chemical etchant solution designed to remove Cu from the intermetallic inclusions at the surface of the alloy. These etching solutions contain both an oxidant (persulfate) and a Cu~(2+) complexing agent (diethylene triamine). A significant decrease in the dioxygen reduction current results from exposure of the alloy samples to the etchant solutions. The influence of etching time on this decrease was also studied.