Corrosion Mechanisms of Mild Steel in the Presence of Formic Acid and Acetic Acid

The presence of organic acids such as formic acid, acetic acid and propionic acid in oil field brines has been identified as a significant contributor to corrosion of mild steel. Extensive research indicates that corrosion rates of steel are significantly higher in weak acid environments, such as aqueous CO₂ or acetic acid, as opposed to fully dissociated aqueous strong acids at the same pH. A general observation is that the increase in corrosion rate is due to increase in cathodic current, which is due to the partial dissociation of the weak acid. Most corrosion research with respect to aqueous organic acid environments has focused on acetic acid as it is a prevalent organic acid found in oil fields; it is also a good representative for higher molecular weight carboxylic acids with similar acid dissociation constants (K_a) values that may be present, e.g., propionic acid. However, the difference in acidity of formic acid as compared to acetic acid emphasizes the need to establish a mechanistic understanding of the role of operational parameters such as pH, temperature, and/or concentration of undissociated acid concentrations on corrosion behavior. A conventional three electrode glass cell equipped with a rotating disc electrode was used to conduct electrochemical measurements (potentiodynamic sweeps, electrochemical impedance spectroscopy, and linear polarization resistance) on an API 5L X65 steel working electrode in a 1 wt.% NaCl electrolyte maintained at constant pH and temperature. It was confirmed that both formic acid and acetic acid have a similar effect on the cathodic reaction rate, wherein their contribution to the corrosion process is through chemical dissociation, which induces the buffering effect. However, while acetic acid has a slight inhibitive effect on the anodic reaction rate, a similar effect was not observed in the presence of formic acid. The effects of concentration of undissociated acid, temperature, pH, rotational speed, and presence of CO₂ in the environment on corrosion of mild steel were established.