(Corrosion Division Morris Cohen Graduate Student Award Address) Active Metallic Corrosion in Weak Acid Solutions: A Unified Mechanistic View to Cathodic Reactions

Metallic corrosion in an aqueous acidic solution is one of the most common corrosion scenarios in industrial applications and energy sector, in particular the oil and gas extraction and transmission facilities. In such systems corrosion is usually caused by the presence of the co-produced aqueous phase, acid gases, and other dissolved acidic compounds. The three major compounds that are conventionally believed to be causing the high corrosivity in such environments are carboxylic acids, carbon dioxide (CO 2 ), and hydrogen sulfide (H 2 S). The presence of these species in the aqueous phase form an acidic, highly corrosive solution. Nevertheless, mild steel remains the inevitable choice of material due to practical and economical limitations of such large scale infrastructures. The use of mild steel, with its low resistance to corrosion, makes the understanding, prediction, detection, and mitigation of corrosion an essential aspect of this industry. The exceptionally higher corrosivity of the aqueous solutions containing CO 2 , H 2 S and carboxylic acids (e.g. acetic acid or HAc in short) as compared to solutions of strong acids (e.g. HCl) with the same pH, has been known for decades. Species like carbonic acid (H 2 CO 3 ), bicarbonate ion (HCO 3 - ), H 2 S, and HAc, as weak acids, are only partially dissociated in the solution. Hence, they are present both in their dissociated and undissociated forms. In the conventional mechanistic view, the higher corrosivity of these solutions have been associated with the additional reduction reactions of the undissociated weak acids (to hydrogen and their corresponding anion) including H 2 CO 3 , and HCO 3 - in CO 2 corrosion, as well as HAc, and H 2 S. With the focus on the cathodic reactions, the mechanism of corrosion in aqueous solutions containing HAc, CO 2 , and H 2 S are revisited in a series of investigations. In all three cases the experimental data and the quantitative evaluations showed that the direct electrochemical reduction of H 2 CO 3 , HCO 3 - , HAc, and H 2 S are not occurring to any appreciable extend, in contrast to the conventional mechanistic understanding. Furthermore, the deficiencies in previous studies both in the experimental approaches and quantification of the results leading to misinterpretation of the data, were discussed. In the mechanistic view developed in these studies, the corrosion of mild steel in all of the abovementioned scenarios is an extension of simple acidic corrosion of st