EXPERIENCE OF USING ZINC TO PREVENT CORROSION OF PRE-STRESSED CONCRETE CYLINDER PIPES IN HIGH RESISTIVITY SOILS

摘要

Zinc sacrificial anodes are not generally recommended to be used for the protection of steel in high resistivity soils (>1500 ?.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 re-enforcement 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 re-enforcing 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 Nemours advantages over magnesium and aluminum in sacrificial cathodic protection systems, can then be used in high resistivity soils to protect steel in underground pre-stressed concrete cylinder pipes (PCCP). This paper presents and discusses a one and half year long performance of a retrofit zinc sacrificial anode CP system.