Abstract
Recent developments in understanding the mechanism of mild steel corrosion in the presence of carboxylic acids, carbon dioxide, and hydrogen sulfide has challenged the conventional views to corrosion in anoxic environments. Conventionally, the high corrosivity of such environments was associated with the direct reduction of these weak acids. Within the last few years, experimental and theoretical investigations of the electrochemical behavior of these corrosive environments suggest that the buffering effect arising from the dissociation of weak acids at the vicinity of metal surface is the main cause for the observed high corrosivity. These findings suggest that neither carboxylic acids, carbon dioxide, nor hydrogen sulfide are inherently corrosive, they merely exacerbate an existing corrosion process. In this study, the buffering effect is viewed as an inherent property of any weak acid, and it is shown to account for all characteristic behaviors observed in cathodic currents in the cases considered. In order to further elucidate this general property, a comprehensive mathematical model was developed and used to discuss the expected behavior of a hypothetical weak acid depending on the kinetic and thermodynamic properties of its dissociation reaction. The mechanistic findings in the present study is reformed into a generic mechanistic view of corrosion in weak acid solutions. That is presented as a simple and generic categorization of weak acids based on their pKa values to serve as a basis to assess the detrimental effect of any weak acid on mild steel corrosion in acidic solutions.
