Mechanism of Pit Initiation on Aluminum Alloy Type 7075.

摘要

The influence of anions on the initiation of pitting and the kinetics of pit growth on aluminum alloy type 7075 has been investigated. The order of reaction (i.e. the number of anions per Al surface reaction site) and the energy of activation for pitting initiation has been measured in F(-), Cl(-), Br(-) and I(-) solutions. The technique involved potentiostating the alloy in the range of anodic passivity, injecting a solution of halide ion, and recording the time, Tau, at which an exponential increase in corrosion current occurs. The slope of the plot of the logarithm of 1/Tau versus the logarithm of the concentration gives, 'n', the number of anions associated with an Al surface reaction site in the primary pitting initiation step. The value of 'n' in 1NH2SO4 varies with the anion, as, for Cl(-), n=8, for Br(-), n=4, and for I(-), n=2. In an electrolyte of pH 5.8 the n value for Cl(-) is 2, for Br(-), 2, and for F(-), 3. The activation energies measured over the temperature range 5C to 60C in 1NH2SO4 were, for Cl(-), Ea = 18 kcal/mole, for Br(-), 26 kcal/mole -1, and for I(-), 3.0 kcal/mole. In an electrolyte of pH 5.8, the Ea is 12 kcal/mole for Cl(-), 10 kcal/mole for Br(-), and 0.46 kcal/mole for F(-). These results are interpreted in terms of the formation of transitory species akin to activated complexes, of specific composition, such as AlBr(-)4. (Author)