Prevention: Inhibiting Corrosion
An inhibitor is a compound that retards the corrosion rate of the acid on the metal by interfering with the electrochemical process which causes corrosion. This is accomplished by separating the metal from its environment by creating an inert, impervious surface barrier. In order for the barrier to be effective it must be firmly adhering, continuous, densely packed, non-reactive, and self-healing.
Corrosion Barrier: How it Works
The barrier mechanism involves the adsorption of the polar molecules within an organic inhibitor onto the anodic and cathodic sites of the metal surface.
- The cathodic inhibitor molecules migrate to the cathode and the anodic inhibitor molecules attach to the anode. These are adhesive bonds (A) noted in Figure 2.
- Lateral bonds between adjacent polar groups of molecules are also possible (B).
- A third bond (C) occurs between the non-polar hydrocarbon portions of different inhibitor molecules.
When all of these types of bonds are developed, a firmly adherent film is ensured. This film offers a resistance to the diffusion of acid on the metal. If the lateral bonds (B) and Van Der Waals bonds (C) are properly developed, the adhesive bond (A) will seldom be required to keep the film anchored to the surface.
Other considerations for optimum film formation and adherence include the structure of the non-polar groups above the metal, the dimensions of the polar groups, and steric effects.
When an appropriately dense concentration of molecules of inhibitor is maintained in the surrounding media, a barrier film will be self-healing. This concentration will provide an equilibrium between the inhibitor solution and the inhibitor film.
An effective, long-lasting inhibitor must take into account each factor influencing the corrosion process.