ABSTRACT
The fact that different metals require different energy inputs to win them from their ores implies that the energy liberated when they corrode is also different. Intuitively, this suggests that the tendencies to corrode will also vary in a way that is linked to the amount of energy released. By the sign convention of thermodynamics, this means that there is a net outflow of heat energy from the reacting species to the surrounding universe. The two reactions rapidly approach a situation whereby the chemical advantage of anodic dissolution is balanced by the electrical advantage gained in cathodic redeposition process. Thermodynamics tells us that any chemical process will only occur spontaneously if the process results in a decrease in enthalpy. The fact that the Cathodic Protection (CP) of steel can be effectively achieved without applying sufficient cathodic polarisation to depress the potential into the region of immunity has recently prompted a debate among onshore CP practitioners over the mechanism of CP.
