Cathodic protection afforded by an intermittent current applied to reinforced concrete

Abstract

In this work, intermittent cathodic protection was applied to chloride contaminated reinforced concrete exposed to simulated tidal zone conditions. It was observed that an integrated protection current of just 6 mA/m 2 induced the passivation of steel exhibiting an initial corrosion rate of 60 mA/m 2 in conditions characterised by weakly polarised cathodic reaction kinetics. This provides the first direct laboratory evidence that protection may be achieved with a cathodic current that is small compared to the corrosion rate. In this case the protective effects of a negative potential shift may be ignored; it is the changes in the environment at the cathode that induce passivation and provide the basis for cathodic protection. Such changes occur slowly and do not give the instantaneous protection offered by a large negative potential shift. A large potential shift combined with a reducing environment may be generated in saturated conditions characterised by strongly polarised cathodic reaction kinetics. Both passivating and reducing environments at the steel will persist following current interruption. The dominance of these persistent effects suggests that the integrated value of the current required for protection will be insensitive to current variations. Positive trends in open circuit potentials or sustained negative potentials are indicative of passivating and reducing conditions respectively, although an intermittent current complicates non-destructive performance assessment.