Abstract
Zinc sacrificial anodes are not generally recommended to be used for the protection of steel in high resistivity soils (>1500 Ω.cm) as the driving potential of the zinc will not be sufficient to provide the required current needed to drive the structure to soil potential to the protection criterion of -850 mV w.r.t. Cu/CuSO4. This criterion guarantees that the structure potential is shifted from the active region to the immunity region of the potential-pH diagram, where the corrosion process is to be completely halted. Generally, more than 300 mV of negative shift is required. Steel in concrete however, is in the passive region due to the alkaline environment provided by the concrete surrounding which allows the steel surface to develop a protective iron oxide film. Ingress of some species such as chloride ions leads to the destruction of this protective film and pitting of the steel reinforcement initiates. Potential of the steel in concrete is relatively less negative (generally <-200mV) especially if the film damaging species have not reached the steel/concrete interface. The steel in this environment stays intact so long as the environment is not altered. Generally, one tenth of the CP current is sufficient to protect new reinforcing concrete structures where the passivity can be maintained. This means that immunity which requires a protection current of ten folds will not be required in this case. Zinc known for its numerous advantages over magnesium and aluminum in sacrificial cathodic protection systems, i.e. zinc anodes have relatively longer design life more efficient when delivering low current outputs, can then be used in high resistivity soils to protect steel in underground pre-stressed concrete cylinder pipes (PCCP). The high resistivity of the surrounding soil can initially have negative influence on the protective current density however, this detrimental effect does not last for long. This paper discusses some practical findings supporting this hypothesis.
