Figure 1a,1b shows schematically five potentiokinetic polarization curves relating to Armco iron in chloride-free solutions of pH 5, 7, 9, 11, and 13. From these polarization curves, it is easy to deduce Figure 1b which represents, as a function of pH and of electrode potential, the experimental conditions of immunity, general corrosion, and passivity.
The potentials separating the regions of immunity and general corrosion are the equilibrium potentials, C, of the corrosion reaction, below which, this reaction is thermodynamically impossible. The potentials separating the regions of general corrosion and passivity are the passivation potentials, P, above which, a protective film is formed on the metal.
Figure 2a shows schematically five polarization curves in solution of the same pH, 5 to 13, but containing 10−2 gram ion Cl− per liter (355 ppm Cl−). The r is the rupture potential (or pitting potential), above which, the...
