Chapter 2: Corrosion Principles: Rates, Electrochemistry & Environment

To accurately measure metallic deterioration for engineering applications, the rate of attack is quantitatively expressed as mils per year, a measurement that represents the depth of penetration and allows engineers to predict the remaining life of a specific component. At its core, corrosion is an electrochemical phenomenon characterized by two or more simultaneous partial reactions: oxidation, or the anodic reaction, which produces electrons as the metal dissolves; and reduction, or the cathodic reaction, which consumes those electrons through processes like hydrogen evolution or oxygen reduction. The speed of these electrochemical reactions is often limited by polarization, which is categorized as either activation polarization, where the rate is governed by the chemical reaction sequence at the metal-interface, or concentration polarization, where the rate is restricted by the physical diffusion of ions from the solution to the metal surface. A critical concept discussed is passivity, where certain metals and alloys, such as stainless steel and titanium, lose their chemical reactivity and form a protective surface barrier that leads to a massive reduction in corrosion rates even in highly oxidizing environments. Various environmental factors—including temperature, velocity of the corrosive medium, and the concentration of the oxidizers—can drastically alter these rates, often leading to exponential increases in damage or, conversely, assisting in the formation of passive films. Metallurgical properties also significantly influence durability, as the specific arrangement of atoms in crystal lattices and the high-energy regions known as grain boundaries serve as primary sites for localized chemical attack. Ultimately, the chapter concludes that corrosion control is an economic challenge, requiring the use of financial tools like net present value and return on investment to ensure that the chosen engineering solution is the most cost-effective over the entire life cycle of the equipment.