Laboratory experiments established that corrosion of steel reinforcement can be arrested within relatively short periods if a sufficient cathodic current density higher than 20 mA/m2 is applied. After corrosion is arrested, it is only necessary to maintain steel passivity longer-term by cathodic prevention, a process that requires a much-reduced current density. Once the two-stage principle was tested and established in the laboratory, self-contained anodes were developed, each product comprising a battery-operated Impressed Current Cathodic Protection element and a galvanic anode component. A high initial current output substantially above 20 mA/m2 by area of steel is delivered over a period of weeks during which corrosion arrest is achieved. The galvanic anode components then deliver the lower cathodic prevention current long-term. This paper introduces the concept and principles of the technique, illustrates products developed and presents medium-term field studies to demonstrate the success of the Two-Stage CP technique.
There is no doubt that Impressed Current Cathodic Protection (ICCP) of steel reinforced concrete has, over the last decades, become a well-established technique for controlling reinforcement corrosion of structural elements. The expectation of long-life protection has, however, been somewhat reduced as some anode systems fail, monitoring equipment become antiquated, and lack of adequate maintenance makes the systems inoperable with the average service life of any ICCP system falling to 15-20 years.1 Inevitable additional costs are involved in maintaining and prolonging correct operation of the system. It appears that there is a requirement by structure managers and owners for simpler cathodic protection (CP) systems which will involve less maintenance and monitoring requirements.
ISO EN 12696:20222 and NACE SP0290-20173 have clear performance criteria that need to be satisfied to ensure that a cathodic protection system is working. One popular criterion is a depolarization potential of 100 mV over a period of 24 hours. ISO EN 12696:20222 also defines in Figure A3 that a successful CP system either passivates the steel or reduces the corrosion rate of the steel reinforcement. The implication is that achieving 100 mV of polarization does not necessarily mean that corrosion has been arrested but that an acceptable reduction in corrosion can be achieved. Nonetheless, it has been shown that if a CP system had run for periods of around 5 years, and is then turned off, re-initiation of corrosion of the steel is avoided over a significant time period.4 What appears to delay the onset of corrosion are some important secondary effects, primarily, the increase in alkalinity and reduction in chloride concentration at the steel/concrete interface5 which in effect reduces the [Cl−]/[OH−] ratio considerably below the critical ratio for initiation or maintenance of corrosion. Furthermore, re-alkalization of the acidified pits occurs which allows steel repassivation within them.6 Once repassivation of the steel is achieved, it is reasonable to expect maintenance of the passive conditions long-term by the application of cathodic prevention.7
