In acid solutions the anodic process of corrosion is the passage of metal ions from the oxide-free metal surface into the solution, and the principal cathodic process is the discharge of hydrogen ions to produce hydrogen gas. In air-saturated acid solutions, cathodic reduction of dissolved oxygen also occurs, but for iron the rate does not become significant compared to the rate of hydrogen ion discharge until the pH exceeds a value of 3. An inhibitor may decrease the rate of the anodic process, the cathodic process or both processes.
The change in the corrosion potential on addition of the inhibitor is often a useful indication of which process is retarded. Displacement of the corrosion potential in the positive direction indicates mainly retardation of the anodic process (anodic control), whereas displacement in the negative direction indicates mainly retardation of the cathodic process (cathodic control). Little change in the corrosion potential suggests that both anodic and cathodic processes are retarded.
The following discussion illustrates the usage of anodic and cathodic inhibitors for acid cleaning of industrial equipment. The combined action of film growth and deposition from solution results in fouling that has to be removed to restore the efficiency of heat exchangers, boilers and steam generators.
Pourbaix or E-pH diagrams indicate that the fouling of iron-based boiler tubes, by Fe3O4 and Fe2O3, can be dissolved in either the acidic or alkaline corrosion regions. In practice, inhibited hydrochloric acid has been repeatedly proven to be the most efficient method to remove fouling. Four equations are basically needed to explain the chemistry involved in fouling removal. Three of those equations represent cathodic processes:
Fe2O3 + 4 Cl- + 6 H+ + 2 e--->2 FeCl2(aq) + 3 H2O ... (A)
Fe3O4 + 6 Cl- + 8 H+ + 2 e--->3 FeCl2(aq) + 4 H2O ... (A')
2 H+ + 2 e- -->H2 ... (A'')
and one anodic process, i.e. the dissolution of tubular material
Fe + 2 Cl- -->FeCl2(aq) + 2 e- ... (C)
These equations indicate that the base iron functions as a reducer to accelerate the dissolution of iron oxides. Since it is difficult to determine the end point for the dissolution of fouling oxides, an inhibitor is generally added for safety purpose. Both anodic and cathodic inhibitor could be added to retard the corrosion of the bare metal after dissolution of the fouling oxides. This is illustrated in two figures showing the action that could be played by either an anodic inhibitor or a cathodic inhibitor. It can be seen that while the anodic inhibitor retards the anodic dissolution of iron at the end point, it concurrently decreases the rate of oxide dissolution permitted by the chemical system.
On the other hand, the cathodic inhibitor retards both the reduction of protons into hydrogen and the dissolution of the base metal while the reduction of the fouling oxides is left unaffected. The E-pH diagrams also indicate that the dissolution of the fouling oxides is also possible in alkaline solutions. But the kinetics of anodic and cathodic reactions in high pH environments are much slower and therefore these reactions less useful.