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.
INTRODUCTION
In order to develop a generic view to the corrosion in the presence of weak acids, a brief review of the literature on the mechanism of corrosion in the presence of weak acids such as organic acids, carbonic acid and hydrogen sulfide is necessary. In all cases, the historical developments show a similar trend. The higher corrosion rates of mild steel in the solution of different weak acids as compared to those seen in strong acid solution with the same pH have (without exception) been explained by presuming that the weak acid itself is directly involved in the underlying electrochemical reactions. The argument was that the higher corrosion rates are caused by the “direct reduction” of the weak acid as an additional electrochemical reaction. Such arguments have been used and slightly modified over decades of research in the field, until more inclusive calculations were introduced that allowed for a detailed account of the effect of homogeneous reactions on the electrochemical response of the system. By doing so in recent years, a major shift in opinion has been made in the current mechanistic understanding of these systems. The present attempt on reforming the existing perspective to corrosion in weak acid solutions as one general, and unified topic, stems from this cumulative experience. Following the literature review, a generic mathematical model was developed for the present study. The model was used to demonstrate the extent of the influence of homogeneous chemical reactions on the observed electrochemical behavior of such a corroding system. The results showed that this aspect, which is by definition inherent to all weak acids, is most likely the cause of the higher corrosion rates, not the presumed electrochemical activity of these species.
