Concrete Impressed Current Cathodic Protection in a Black Rust Environment

Cathodic protection has been a widely used technique to control corrosion of steel in reinforced concrete for several decades. One of the key parameters in the design of a CP system is the design current density, which is commonly taken as 20 mA/m2 of steel reinforcement area. While this is  based on the upper limit of typical current density required to stop corrosion normally encountered in reinforced concrete structures, it should be noted that this may not be adequate for all scenarios. 

An external wall of a seawater swimming pool was suffering from significant reinforcement corrosion due to chloride attack. During the substantial repairs, 16 mm diameter bars were found to be totally corroded away or with substantial section loss, with black rust formation prevalent throughout the damaged areas. A titanium ribbon based Impressed Current CP system was designed and installed, based on a current density of 20 mA/m2. 

Initial performance assessment of the system showed that even with running the system at current densities significantly greater than the 20 mA/m2 design current density, the reinforcement at most references still failed to meet criteria as per Australian Standard AS2832.5. An actual current density of up to 35 mA/m2 was required in this case. 

For extremely aggressive and corrosive environments, higher design current densities than the usual 20 mA/m2 value may be required.