As pointed out by Jones (1988, p. 21), the technology of corrosion inhibition is largely an art. An inhibitor is a chemical substance or a mixture that decreasescorrosion when added to an environment (usually in small concentration). A passivator, on the other hand, is an inhibitor which appreciably changes the potential of a metal to a more noble value(cathodic value).
Anodic inhibitors for iron in water are soluble hydroxides, chromates, phosphates, carbonates, and silicates. These substances increase anodic polarization, probably by helping toform (or to repair) a protective film on the metal surface.
Cathodic inhibitors for iron partially immersed in water are magnesium, zinc, and nickel salts. In aqueous medium, as O2 is reduced atcathodic areas, the pH is increased, resulting in the precipitation of Mg(OH)*, Zn(OH)2, or Ni(OH)2 over the cathodic surfaces as a fairly adherent porous deposit. Thus, the reaction is slowed downbecause O2 must diffuse through these deposits in order to reach the cathodic surfaces. In waters containing C02, the calcium salts act similarly by precipitating CaC03 on the cathodic areas as aresult of increasing
An insufficient concentration of an anodic inhibitor in a system that is under cathodic control intensifies the attack on small, localized areas.
This results in pittingand early perforation. The required concentration of anodic inhibitor depends on the concentrations of ions such as chloride or sulfate, which interfere with the formation of passivating films.Other factors that are considered in determining the required concentration of anodic inhibitor include:
1. Agitation of the liquid
2. Composition of the environment
3. Stresses in the metal
4.Composition of the metal
5. Contact with dissimilar metal
Certain inhibitors change the electrochemical potential of a metal to more cathodic or noble value. These inhibitors...