Effect of moisture on the spatial uniformity of cathodic protection of steel in reinforced concrete

A numerical model for the cathodic protection (CP) of steel in reinforced concrete was developed. Parameters were set to represent a three-dimensional section of a bridge beam exposed to the atmosphere and coated with a thermally sprayed zinc anode. Both the diffusion of oxygen and the conduction of charge within the concrete were considered explicitly through a two-dimensional finite element model. The diffusivity and conductivity were represented as functions of concrete moisture content. Electrochemical reactions considered at the rebar-concrete interface included the reduction of oxygen, the oxidation of iron, and the evolution of hydrogen in a constant-potential CP circuit. Reaction-kinetic parameters for actively corroding steel (not passivated steel) were used. Reactions at the zinc-concrete interface were not considered explicitly. The effectiveness of protection was found to vary significantly with both concrete moisture content and position on the rebar. For spatially uniform pore saturation, the drier the concrete, the greater the corrosion current and the greater the nonuniformity. Protection was significantly more effective at the "front" of the rebar (closest to the zinc anode) than at the "back" (closest to the center of the beam). The corrosion current was greater under drying conditions than under wetting conditions. The numerical model was applied toward interpretation of the "100-mV polarization decay criterion" that is often used to assess the effectiveness of CP. It was found that the polarization decay predicted from relaxation of oxygen concentration gradients was comparable in magnitude to that observed experimentally, but depended on location on the rebar.