Chapter 9: Modern Corrosion Theory: Thermodynamics & Kinetics

Modern Corrosion Theory: Thermodynamics & Kinetics begins by examining thermodynamics as the science of energy changes, specifically using the concept of free energy to determine the spontaneous direction of chemical reactions. By establishing a standard reference through the hydrogen electrode, the text explains the development of the electromotive force series and the use of redox potentials to predict metallic behavior. The Nernst equation is introduced to calculate electrode potentials when reactants are not at standard concentrations, while Pourbaix diagrams are presented as essential tools for visualizing the stability of metals and their compounds across different pH levels. Transitioning into electrode kinetics, the discussion highlights how corrosion rates are determined by polarization—the displacement of electrode potential from equilibrium. Detailed analysis is provided for activation polarization, which is controlled by reaction sequences like electron transfer, and concentration polarization, which is limited by the diffusion of ions. These concepts culminate in the mixed-potential theory, which posits that the total rate of oxidation must balance the total rate of reduction on any corroding surface. The final sections delve into the complex phenomenon of passivity, explaining how certain materials form ultra-thin protective films that drastically reduce corrosion rates. By looking at models like the point defect theory, the chapter explains the growth and eventual breakdown of these films, providing a comprehensive framework for understanding and controlling metal degradation in engineering applications.