ABSTRACT
Exploratory experiments were conducted seeking to provide empirical information on whether an intrinsic (predominantly interfacial effects of polarization) or an extrinsic (predominantly concentration changes due to polarization) mechanism may be dominant in the beneficial effect of cathodic polarization during cathodic prevention of steel in concrete. Tests used concrete specimens, each with a cast-in steel plate, constantly exposed to a high-chloride environment. The specimens were polarized at 4 different levels ranging from open circuit to -400 mV vs Saturated Calomel Electrode. Shortly after corrosion started specimens were demolished and measurements of pH using a novel procedure, and of chloride ion concentration, were done on the steel concrete interface. The experimental data confirmed the general expectation that concrete reinforcement benefits from cathodic polarization, as the specimens polarized at high cathodic polarization voltage tended to take longer to reach an active corrosion condition. The interfacial pH and chloride ion concentrations measured favored to some extent a dominant intrinsic mechanism interpretation, while the evidence in support of a dominant extrinsic mechanism interpretation at these moderate polarization levels remained elusive.
INTRODUCTION
Corrosion of carbon steel rebar in marine concrete structures remains a persistent problem. While carbon steel in the high pH concrete medium is capable of producing a passive film that naturally prevents corrosion, this film can be easily broken down by the presence of chloride ion1-2 from seawater. Steel corrosion ensues that creates expansive corrosion products that crack the concrete needing costly repairs.
The chloride content on the steel surface that, if exceeded, causes passive film breakdown is called the chloride threshold CT.1,3 Among other control methods, Cathodic Prevention (CPrev) has received increasing attention as a means to retard or even completely prevent corrosion from occurring. CPrev involves applying cathodic polarization while the steel is in its passive condition, which can significantly increase the value of CT4,5 as indicated by a small but increasing body of data. Although it is generally recognized that Cprev can indeed increase the service life of rebar in concrete,4,6 there is relatively little information on the mechanism that is responsible for such effect.
