Mechanisms of Hydrogen Evolution in Weak Acid Solution Investigated By Using EIS

Abstract

EIS was used in this mechanistic study of the electrochemical reaction mechanisms associated with corrosion of mild steel in the presence of weak acids, such as carbonic acid found in CO2 saturated aqueous solutions. Over the past decade, comprehensive mechanistic studies using DC electrochemical techniques have indicated that direct reduction of carbonic acid is not a significant reaction in CO2 aqueous solutions, contrary to what the conventional wisdom was for a long time. The new results suggest that the only cathodic reaction is hydrogen reduction, and that the main role of carbonic acid is to increase the limiting current by the so-called buffering effect. However, the problem is that the same limiting current is obtained theoretically whether one assumes that the direct reduction of carbonic acid or the buffering effect is true. Therefore, by using DC electrochemical techniques it is impossible to distinguish these two mechanisms in the limiting current region which is always seen in the polarization sweeps. An attempt has been made here to use the EIS technique to identify the role of carbonic acid in accelerating the cathodic reaction. Results indicate that the buffering effect is dominant, as recently proposed.