Abstract
The GILDES computer based model was successfully applied to the atmospheric corrosion of zinc exposed to low concentrations of carboxylic acids in humidified air at room temperature. Under these exposure conditions the expected precipitated phases are zinc oxide (ZnO), zinc hydroxide (Zn(OH)2) and several forms of hydrated zinc carboxylate, Zn(CH3CH2COO)2·2H2O, Zn(CH3COO)2·2H2O and Zn(HCOO)2·2H2O. The results were compared to those from laboratory exposures obtained in the same conditions. The model correctly predicts the trend for ZnO and zinc carboxylate formation found in experimental exposures for the three acids tested. According to the simulations, surface protonation, surface acid base reactions, as well as ligand- and proton- induced dissolution reactions play a major role in the initial atmospheric corrosion of zinc. Henry's law constant (KH) is found to be an important parameter but uncertain due to different reported literature values. When KH is increased the formation rate of zinc carboxylate also increases. KH increases in the sequence propionic acid < acetic acid < formic acid, the same order as found for the calculated dissolution rate.
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