Cathodic protection (Topic 11052)

When an aluminium structure is cathodically protected, the potential is lowered into the passive area. This can be done by impressed direct current or by sacrificial anodes.

An impressed current system will require careful design and a high degree of reliability in order to avoid so called over protection. This means that the structure is brought to a potential lower than 1400 mV vs. Cu/CuSO4 electrode.

When sacrificial anodes made of zinc or special aluminium alloys are used, the risk of overprotection is eliminated. While too many magnesium anodes would give a potential lower than the limit for over protection. Even if there is no risk for over protection, the amount and placement of sacrificial anodes is a job for experts, in order to get sufficient protection of the whole structure with a minimum amount of anodes.

An obvious advantage when aluminium is protected in this manner is that current drawn from the anodes usually is very low. This is because the area that needs protection is small. Because under most conditions, large areas of the aluminium surface will be protected by the natural oxide film and only areas that would be susceptible to pitting or crevice corrosion would need protection. The necessary current will depend on many parameters like flow rate and the corrosivity and conductivity of the water. An efficient cathodic protection is only possible if the structure is totally immersed in water.

A special kind of cathodic protection is achieved by applying an anodic metallic surface layer to the aluminium alloy. The anodic layer can be an aluminium-zinc alloy, pure aluminium or pure zinc. The anodic metal can be applied by cladding, spraying or hot dipping. An anodic surface layer can give protection even at atmospheric conditions. It is often necessary to heat-treat these layers in order to achieve the best corrosion performance.