Influence of Water Quality on the Corrosion and Electrochemical Behavior of Mild Steel in Synthetic Acid Mine Waters

Corrosion rates of mild steel immersed in synthetic acid mine waters (pH 2.5) containing Fe3+ or Fe2+, Cu2+, Mg2+, Ca2+, K+, SO42–, Cl−, and SO32– ions in various combinations were measured by weight loss technique. The corrosion rates were significantly increased by Fe3+ and Cu2+ due to their reduction to Fe2+ and metallic Cu, respectively. An empirical relationship between ferric ion reduction rate and corrosion rate was established. Changes in corrosion potential with an increase in Fe3+ ion concentration gave a slope of 50 mV for dE/dlog [Fe3+]. A similar slope was observed in presence of Cu2+ ions. The cathodic Tafel slopes were in the range -112 to -133 mV/decade in accordance with the well known H2-evolution reaction mechanism of Bockris. The agreement between the observed and calculated limiting current densities confirms that the corrosion process is predominantly under diffusion-controlled cathodic processes. The corrosion rates calculated by the Tafel extrapolation method were 30% lower than those obtained from weight loss measurements. Attempted methods for control of mild steel corrosion by acid mine waters are discussed.