(Corrosion Division Morris Cohen Graduate Student Award Address) Active Metallic Corrosion in Weak Acid Solutions: A Unified Mechanistic View to Cathodic Reactions
Authors: Kahyarian Aria , Nesic Srdjan 
Published: February 2020

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 (CO2), and hydrogen sulfide (H2S). 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 CO2, H2S 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 (H2CO3), bicarbonate ion (HCO3
-), H2S, 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 H2CO3, and HCO3
- in CO2 corrosion, as well as HAc, and H2S.

With the focus on the cathodic reactions, the mechanism of corrosion in aqueous solutions containing HAc, CO2, and H2S 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 H2CO3, HCO3
-, HAc, and H2S 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 steel with only two electrochemical reactions: hydrogen ion (H+) reduction as the sole cathodic reaction, and iron dissolution as the sole anodic reaction. However, the combined effect of the coupled chemical reactions along with solution pH, and other environmental conditions such as temperature and fluid flow on the surface concentration of H+, leads to a great diversity in the observed behavior of the cathodic polarization curves and corrosion rate trends, as have been observed in the previous studies.

In its simplest form, the chemical dissociation of the weak acid at the vicinity of the metal surface acts as an additional source of H+, hence, results in increased limiting currents. Nevertheless, the results of this study showed that depending on the chemical properties of the involved weak acid and other environmental conditions, the same mechanism may lead to more complex polarization behavior. The reduction of HCO3
- and H2S are examples of such scenario, where the cathodic polarization curves show two “waves”, as demonstrated in Figure 1. Even though the existence of this “double wave” was considered as solid proof of the direct reduction of these weak acid until very recently, the mathematical simulations based on the newly proposed mechanism showed that this behavior is also merely another example of the effect of solution chemistry on the surface concentration of H+.

In light of this cumulative knowledge, a unifying and generic mechanistic view for Active
corrosion in weak acid solutions can now be put forward. Based on this mechanistic view a generic categorization of weak acids, simply based on their pKa values, is developed to represent their extent of influence on the cathodic polarization and corrosion rates. Such a framework can be used to assess the expected behavior of any given species and provides the guidelines for any future research in this field of study.

Figure 1