| Abstract: | Critical potentials in corrosion chemisry and their measurement It is shown with the aid of a potential-pH-diagram that the corrosion behaviour of a given system can be evaluated on the basis of the border lines between the different zones of the metal behaviour. Such regions are: thermo-dynamic stability of the metal, active and transpassive corrosion, surface layer passivity (= thermodynamic stability on the surface layer), chemical passivity etc. These border lines are thermodynamic equilibrium lines or correspond to such lines. With the exception of the equilibrium line between stability and corrosion of the metal, the other transition must be interpreted in cinetic rather than in thermodynamic term. The influences of Cr-content, pH, temperature and specimen pretreatment are discussed. Under certain conditions local corrosion may appear within certain potential regions; such corrosion phenomena are e.g. pitting and stress corrosion cracking. The border potentials of these corrosion regions are discussed with particular attention to measurement problems involved. With corrosion protection by cathodic or anodic polarisation the border potentials have the meaning of protection potentials. The latter are, consequently, no equilibrium potentials but border potentials having an analogous significance; they can be measured best with the aid of stationary methods. |
