Initial atmospheric corrosion of copper induced by carboxylic acids

A procedure based on infrared reflection absorption spectroscopy integrated with a quartz crystal microbalance has been developed and used for comparison of initial atmospheric corrosion of copper induced by formic, acetic, and propionic acid. The absolute mass of individual constituents of corrosion products formed, mainly cuprous oxide or cuprite, copper carboxylate, and water or hydroxyl groups, could be obtained under in situ conditions during exposure in 120 ppb of carboxylic acid concentration, 95% relative humidity, 20 degrees C, and up to 96 h of exposure. The quantitative data exhibit consistency between all acids and with postanalysis performed by coulometric reduction. Two spatially separated main pathways have been identified: a proton-induced dissolution of cuprous ions followed by the formation of copper(I) oxide, and a carboxylate-induced dissolution followed by the formation of copper(II) carboxylate. The first pathway is initially very fast but levels off, grows more uniformly over the surface, and dominates in acetic acid. The second pathway exhibits a more constant growth rate and localized growth, and dominates in formic acid. Propionic acid exhibits low rates for both pathways. The difference between the carboxylic acids with respect to both total corrosion rate and carboxylate-induced dissolution can be attributed to their dissociation constants and deposition velocity.