A Thermodynamic Investigation on Corrosion of Cu-bearing Steel in Aqueous Solutions

Abstract

This paper investigated the corrosion behavior of Cu-bearing steel using thermodynamic methods and presented the results in the graphical information widely known as “Calculated Potential-pH Diagram”. The chemical and electrochemical reactions which potentially occur during the corrosion process of Cu-containing steel and significant thermodynamic data such as standard Gibbs free energy of ions or compounds were first evaluated. Nernst’s equation and Van't Hoff’s equation were then employed to establish two sets of thermodynamically important equations. Later, these equations were used to construct a potential-pH diagram, which can be used as a guideline to investigate the corrosion behavior of Cu-bearing steel. It is found from the obtained diagram that CuFeO₂ can be formed on the steel substrate for almost pH value. Fe₃O₄ appears from the middle to high pH value. The region in which CuFeO₂ and Fe₃O₄ overlap is found, indicating that two compounds potentially coexist in the rust layers covering on the steel substrate. The presence of copper ions in the rust can decrease the rust conductivity and retard oxygen ingress. Thus, this rust can would enhance the corrosion resistance of Cu-containing steel in a slightly acidic to a high alkaline environment.