This research shows that acetone degreasing of the AA2024-T3, as per ASTM E1078-97 cleaning protocol, has a photochemical effect on constituent copper-rich intermetallic particles. When an acetone-degreased alloy was exposed to a mist of sodium chloride solution under ambient light, the formation of acetic acid together with a layer of chloride solution resulted in severe pitting. Pitting was inhibited under conditions of darkness. The slow reaction of surface-absorbed acetone with water leading to the formation of acetic acid appeared to be prevented. There was also evidence for the redeposition of dissolved copper onto the alloy matrix under conditions that induced pitting.

Acetone in the presence of water vapor, ambient light, and AA2024-T3 decomposes to acetic acid. This reaction occurs on copper bearing intermetallic particles. A multi-step mechanism has been proposed to explain the observed results. The decomposition of acetone into acetic acid in the presence of aqueous chlorides forms an acid-chloride solution that causes pitting. Copper in the intermetallic particles has been proposed to play two roles in the corrosion process; it photochemically promotes the formation of acetic acid and a self-consuming reagent forming copper chloride. The acetic acid reacts with aluminum eventually resulting in the formation of an insulating layer of aluminum acetate. Free electrons from the alloy tunnel across the thin layer of oxy-hydroxides to cathodic sites on the surface of the alloy and combine with Cu(I) to form metallic copper, which is deposited on the alloy surface, as illustrated in Fig. 1. Hence, the exposure of AA2024-T3 to acetone in the presence of ambient light and water vapor with chlorides leads to pitting corrosion with aluminum acetate, metallic copper and copper chlorides being the primary corrosion products.