Anodic Protection

Anodic protection is a method of corrosion control that was developed more recently than cathodic protection, but it is used less frequently. As its name implies, anodic protection shields the anodic electrode in the system from corrosion rather than the cathodic electrode as in CP. The principle behind anodic protection, however, is not quite analogous to that of CP. Essentially, instead of shifting corrosion potential from the metal to be protected to an anodic material as in CP, anodic protection involves passivation of the metal to be protected. A passive film forms on the surface of the metal with the application of an electrical current. Once this film is formed, it acts to protect the metal from dissolution, and the film itself is nearly insoluble in the environment which it formed. Passivation causes metals to become very non-reactive and consequently very resistant to corrosion. The limitation of this type of corrosion control is that not every metal can be protected this way; only certain metals in specific environments can be anodically protected. These include the metals and solutions shown in Table 59.

Anodic protection requires three electrodes, a potential controller (potentiostat), and a power source. The necessary electrodes are a cathode, a reference electrode and an anode, which is by definition the metal to be protected. The reference electrode monitors the voltage on the anode, and is very important since it is necessary to maintain proper protection and avoid accelerated corrosion. The cathode should be resistant to dissolution; it can be platinum on brass, steel, silicon cast iron, copper, stainless steel, or nickel-plated steel, among others. The potential controller actively controls the potential on the anode.

A notable advantage of anodic protection is that after the passive film has formed, the amount of current required to maintain this protective film is very small. A further advantage is that the applied current is equal to the corrosion rate of the protected metal. This allows the instantaneous corrosion rate to be measured, which is not the case for CP. Moreover, anodic protection is effective in weak and strong corrosive media. Furthermore, the operating conditions for anodic protection systems can be determined accurately by laboratory-scale experiments, whereas, to do so for CP is hardly a scientific procedure. A general comparison of anodic protection and cathodic protection methods is provided in Table 60.