Mechanisms of corrosion protection inferred by Anodic Coatings fall into two classes
Anodic coatings for the protection of iron and steel substrates are almost entirely limited to zinc and aluminium coatings or their alloys. Where coatings anodic to the substrate are applied, the corrosion protection is referred to as cathodic protection or sacrificial protection. The substrate is made to be the cathode and the coating the sacrificial corroding anode. The mechanisms of corrosion protection inferred by these coatings fall into two classes:
- Cathodic or sacrificial protection
- A barrier to the environment
An ordinary sprayed coating of zinc or aluminium although somewhat porous, to a large extent excludes the environment and provides cathodic protection. Where desired the porosity can be sealed with organic sealers, or the coating painted, which can in some cases prolong the life of the protective system by increasing the barrier effect. It is argued that sealing or painting these coatings reduce the cathodic protection effect and reduce the overall effectiveness. Sealing and painting certainly reduces the cathodic protection by decreasing the area of contact of the coating with the environment, but in many cases where the substrate becomes exposed there is more than enough coating exposed to keep the substrate as the cathode. Also, the barrier effect prolongs the life of the coating. It could be argued that this is not applicable to all situations. Depending on choice of coating system and environment the life expectancy can be well in excess of 20 years with no maintenance. This method is generally regarded as providing superior corrosion protection than galvanising, plating and painting without excessive cost penalties.