The Role of the Chemical Composition of the Medium in the Protection of Metals from Corrosion

Abstract

The role of pH is important because it characterizes the activity of the dissociation products of the most widely occurring solvent of all corrosive media, water, and consequently, to a first approximation, its chemical composition. A knowledge of the potential, E, of the metal and of the pH of the environment may often be sufficient for assessing the aggressiveness of the environment. Thus, by changing the pH with the value of E held constant it is possible to suppress corrosion by means of either passivation or cathodic protection. This can be seen from Pourbaix diagrams which show the pH-E dependence of the electrochemical reactions that cause corrosion. As a rule, the kinetics of these reactions depends on pH since H₃O⁺ and OH⁻ either participate directly in them or are their products. According to Antropov⁽¹⁰⁸⁾ the rate of corrosion of a metal, K,in acid is associated with pH by the equation:

EquationSource % MathType!MTEF!2!1!+- % feaagCart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 % qaciGGSbGaai4BaiaacEgacaWGlbGaeyypa0JaamyyaiabgkHiTiaa % dkgacaWGWbGaamisaaaa!3F36! $$\log K = a - bpH $$

(2.1)

in which b is a constant dependent on the nature of the metal. Analysis of the values of b has shown that for transition metals, such as iron, it is possible to inhibit corrosion by the screening that results from both adsorption of an additive as well as from changes in the bonding energy of hydrogen with the metal Inhibitors of the cationic type are even capable of affecting the nature of the controlling stage of the cathodic reaction. For non-transition metals, the choice of inhibitors is restricted because many anions accelerate the discharge of H₃O⁺. Grigor’ev and Ekilik⁽³³⁾ used Eq. (2.1) to explain the reduction in the role of the chemical structure of a series of organic inhibitors with decrease in pH of hydrochloric acid solutions. However, for the corrosion of metals in neutral media, Eq. (2.1) is, as a rule, not applicable because of the diffusion limitations of the cathodic reaction, among other reasons.