In this review the relative importance of the various protective effects operating when electrochemical treatment is applied to reinforcing steel in atmospherically exposed concrete is assessed. The available evidence suggests that, on balance, the principal protective effect of a cathodic current applied to steel in atmospherically exposed concrete is to improve the environment at the steel. This promotes the formation of a stable passive film on the steel that polarises the anodic dissolution reaction. While the textbook understanding of cathodic protection suggests it cathodically polarises the steel, its ultimate effect in concrete is to cause anodic polarisation. Indeed, it can be shown that the protective effects of a negative potential shift may be negligible compared to the protective effects of improving the environment at a steel cathode in atmospherically exposed concrete where oxygen access is not restricted. Electrochemical treatment may also modify the solid phases present at the steel surface, which in turn results in a persistent increase in the tolerance to the presence of chloride ions. The effect of generating a protective environment at the steel has been observed in both galvanic and impressed current electrochemical protection systems applied to reinforced concrete.
Various forms of electrochemical treatment are used to arrest or prevent the corrosion of steel in concrete [1,2]. These treatments all deliver a current from an installed anode to the reinforcing steel which forms the cathode of the resulting electrochemical cell. Differences arise from changes in the anode system and from changes in the magnitude and duration of the protection current density.
The protective effects of applying such a current to steel have been previously documented [3]. They include shifting the potential of the steel to more negative values, producing hydroxyl ions at the steel to increase the local pH and removing chloride ions from the steel surface. Effects such as the increase in the pH and reduction in chloride content at the steel constitute an improvement in the local environment that stabilises the passive film on the steel. In these cases, the aggressive local environment is rendered protective.
The aim of this review is to assess the relative importance of the various protective effects operating when electrochemical treatment is applied to reinforcing steel in atmospherically exposed concrete, and the practical implications this might have.
ELECTROCHEMICAL PROTECTION
Electrochemical treatments for steel in concrete may be divided into temporary, intermittent and continuous treatments. Impressed current cathodic protection is the most widely used electrochemical treatment to prevent or arrest the corrosion of reinforcing steel. It is usually applied on a continuous basis and it is good practice to monitor such a system as a common failure results from loss of the power [4]. Conventional wisdom would suggest that protection is achieved in such a system by shifting the steel potential in the negative direction [5].
More recently sacrificial and intermittent cathodic protection systems have been developed for steel in concrete [6,7]. Sacrificial cathodic protection, also known as galvanic protection, is usually based on the use of zinc anodes and both discrete and surface applied zinc anodes have been developed (Figure 1). In these cases the current delivered by the zinc is very dependent on the local environment [8]. Thus, for example, zinc will deliver more current in moist warm conditions. An example of the temperature dependence of the current output of a discrete zinc sacrificial anode is given in Figure 2. Such anodes may be
