The goal of the research reported herein was to accomplish a quantitative mechanistic analysis of iron dissolution in strong acid in a potential range in the proximity of its open circuit potential (OCP), leading to articulation of a revised narrative of BDD† mechanism for iron dissolution; additional mechanistic pathways were postulated in addition to the hypothesized mechanisms of BDD and Heusler. Thirty-eight different pathways were investigated here and theoretical Butler-Volmer equations were written for each. The kinetic consequences of each pathway and the corresponding theoretical values of the main kinetic parameters were determined, and the theoretical outcomes were compared to the experimental observations. It was found that in strong acids (pH ≤ 4) in the potential range of ±50 mV vs. OCP, the mechanism of iron dissolution agrees well with three pathways, and all three were explainable within the same framework of BDD mechanism, where the reaction of OH− with iron produces the adsorbed intermediate FeOHads. One single dissolution pathway which corresponds to the conversion of FeOHads to Fe(II)sol is dominant in the potential range adjacent to the OCP. Near OCP the effect of hydrogen reduction was taken into account using the linearity of the cathodic potentiodynamic branch to approximately extract the pure anodic data points from both anodic and cathodic sweeps.
Reviewing literature related to corrosion research brings to light the importance of understanding the mechanisms involved, and how this is essential to aid in development of mathematical models for corrosion prediction. The current research documents possible mechanisms for the dissolution of pure iron in strong acid in a potential range in the potential range of ±50 mV vs. OCP, providing explanations for corrosion engineers and researchers working with mild steel. Prediction of corrosion rate relies on the precise understanding of the anodic and cathodic processes at the metal surface in the potential range close to the OCP. In the case of iron dissolution, not far from OCP, there are two common mechanisms in strong acids (pH ≤ 4) reported in the literature; namely, the "catalytic mechanism" proposed by Heusler et al.1, and the "consecutive mechanism" postulated by Bockris, et al. .2. which is also known as "BDD mechanism". Heusler’s model is based on the second order dependence on OH- ions and the anodic Tafel slope of 30 mV/decade, while BDD mechanism predicts a first order of dependency on OH- and an anodic Tafel slope of 40 mV/decade. Over a wider range of overpotentials far from the OCP, Keddam, et al., reported that iron dissolution occurs through three different but interrelated dissolution paths in which four adsorbed intermediates are involved in seven elementary steps.3 Bockris’ approach2,4 for elucidation of the mechanism near the OCP was methodical in terms of utilizing the Butler-Volmer equation as a means to reasonably deduce the mechanism since it immediately provides the metrics to prove, or disprove, a particular hypothesis.
