Cathodic Protection of Iron

Abstract

IT has been frequently suggested that the cathodic protection of iron in chloride solutions is due to the depression of its potential to or below the open-circuit potential of the local anodes1,2. However, in neutral solutions containing dissolved oxygen the open-circuit potential of the local anodes cannot be measured directly without assuming the previous statement to be true. Nor can it be calculated from the Nernst equation, since the ferrous ion activity in equilibrium with hydrated ferric oxide is unknown. Since the formation of hydrated ferric oxide may be represented by an equation such as 4 Fe++ + 8 OH′ + O2 = 4 FeO.OH + 2H2O it is implicit that the open-circuit potential of the local anodes should become more electronegative with increasing pH.