THE DEPENDENCE OF THE PITTING POTENTIALS OF ALUMINUM-ALLOYS ON THE SOLID-STATE COHESION OF THE ALLOYING METALS

It is shown that the pitting potentials of aluminum alloys reported by Inturi and Szklarska-Smialowska [Corros Sci. 34, 705 (1993)] can be c ausatively correlated with the metal-metal bond energy, b(M-M), values of the alloying elements: high b(M-M) values raise the pitting potent ials towards nobler values and vice versa. This has been interpreted a s resulting from pitting being nothing but active dissolution of the b are metal sites in the micro-fissures and defects of the oxide coverin g the alloy, in accordance with the pioneering studies of Richardson a nd Wood [Corros. Sci. 10, 313 (1970)] and Galvele et al. [U. R. Evans Conf. on Localized Corrosion, Williamsburg Dec 1971)]. Further, interp retations of pitting phenomenon being controlled by the pH of the surf ace oxide in relation to pH of the electrolyte solution as proposed by Natishan et al. [J. electrochem. Soc. 135, 321 (1988)], cannot be sus tained by a further analysis: such relationships between pitting poten tials and pH(pzc) can arise because of the correlation between b(M-M) and pH(pzc). Quantitatively, the slope of the correlative line between the pitting potentials of aluminum alloys and the b(M-M) values of th e alloying elements has been found to be unity: no theoretical signifi cance can be attributed to this slope at the present time, however.