The Chemical Nature of Aluminum Corrosion: V . Energy Transfer in Aluminum Dissolution

Abstract

The transfer of energy during the dissolution of aluminum is represented by a potential energy surface. In chloride solution, the high energy aluminum metal ionizes rapidly to the A1 + § ion which also hydrolyzes rapidly. At the start of the reaction, in the microsecond range, there are two species available for reaction, A1 +++ and AI(OH) § The reaction follows two alternative paths; the energy levels of the alternative species, A1C1 +* and AI(OH)C1 + have been measured. These two species react further to form a reasonable stable basic aluminum chloride that is transformed slowly to AI(OH)3 and finally to AI~_O~ 9 H20. The energy levels of these species have been measured. Reaction in sulfate solution differs because the intermediate species, AI(OHh + and AI(OH)SO~ occupy low energy levels, and the basic aluminum sulfate has a lower energy than AI(OH)3. In most practical cases of aluminum corrosion, the reaction proceeds in the reverse direction. The reactants are the oxide-covered aluminum surface and the anion combining to form the basic aluminum salt.The complete undersCanding of a metal dissolution reaction includes a knowledge, not only of the reactants and final products, but also the intermediates, which are often transitory in nature. This includes the rate of formation and decomposition of the intermediates along with the energies of activation or formation of the various species. In the present paper, it is intended to trace the reaction path from aluminum in the atomic stage to the final form, the hydrated aluminum oxide.The reaction of interest is the corrosion of aluminum in aqueous electrolytes, a reaction central to such phenomena as aluminum pitting, stress cracking of aluminum, and localized corrosion. The specific objective of the present paper is to develop the free energy surface for the dissolution of aluminum in chloride and sulfate aqueous solutions. Potential Energy SurfacesIn recent years, some success has been achieved in chemical kinetics in describing chemical reactions with potential energy surfaces (1). However, because of the * Electrochemical Society Active Member.Key words: localized corrosion, aluminum pitting, activation energies, equilibrium constants, limited amount of data available, accurate potential energy diagrams have been developed for only the simplest of gas phase reactions. The classical potential energy diagram would describe a reaction such asThe course of the reaction would be represented as a three-dimension plot with interatomic distances, rA-B and rB-C as coordinates in the x-y plane and potential energy represented by contour lines in the z direction. The reaction proceeds from a valley in which the reactants A-B + C are stable, across an energy barrier, the saddle point of which is occupied by the activated complex, A-B-C, to another valley in which A q-BC are stable. So, the course of the reaction is represented by an extension of the A-B bond to the formation of A-B-C, and then a shortening of the B-C bond.For a multistep process in aqueous soluti...

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