AN INVESTIGATION INTO THE MECHANISMS OF PROTECTION AFFORDED BY A CATHODIC CURRENT AND THE IMPLICATIONS FOR ADVANCES IN THE FIELD OF CATHODIC PROTECTION

The influence of a cathodic current on the corrosion behaviour of mild steel exposed to both dilute sodium nitrate solutions and chloride contaminated concrete has been examined. In both cases the formation of passive films was promoted by stimulating a cathodic reaction on the metal surface, the cathodic kinetics being under activation control in the near passive region. The sodium nitrate solution was not capable of supporting a passive film on its own and it is suggested that, in this environment, film formation is promoted by the production of inhibitors (hydroxyl ions, free radical intermediates and metal oxide intermediates) resulting from the cathodic reduction reaction. In chloride-contaminated concrete an additional effect is the removal of aggressive anions via the flow of negative ionic current away from the metal surface. These passivating effects of the cathodic current provide persistent protection and may be harnessed to develop intermittent cathodic protection for use in areas such as the tidal zone where an electrolyte is not always present. The improvement in the local environment at the steel surface may facilitate a reduction in the protection current density, a factor which may be used to reduce the risk of overprotection when cathodic protection is applied to prestressing steel. The use of potential depolarisation and current density related criteria may be more appropriate than the achievement of a predetermined absolute potential in cases where the purpose of the cathodic protection system is to induce passivation.