Cathodic protection afforded by an intermittent current applied to reinforced concrete

In this work. intermittent cathodic protection was applied to chloride cont aminated reinforced concrete exposed to simulated tidal zone conditions. It was observed that an integrated protection current of just 6 mA/m(2) induc ed the passivation of steel exhibiting an initial corrosion rate of 60 mA/m (2) in conditions characterised by weakly polarised cathodic reaction kinet ics. This provides the first direct laboratory evidence that protection may be achieved with a cathodic current that is small compared to the corrosio n rate. In this case the protective effects of a negative potential shift m ay be ignored: it is the changes in the environment at the cathode that ind uce passivation and provide the basis for cathodic protection. Such changes occur slowly and do not give the instantaneous protection offered by a lar ge negative potential shift. A large potential shift combined with a reduci ng environment may be generated in saturated conditions characterised by st rongly 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 variati ons. Positive trends in open circuit potentials or sustained negative poten tials are indicative of passivating and reducing conditions respectively, a lthough an intertmittent current complicates nun-destructive performance as sessment. (C) 2001 Elsevier Science Ltd. All rights reserved.