Bare Surface Reaction Rates and Their Relation to Environment Controlled Cracking of Aluminum Alloys: I . Bare Surface Reaction Rates on Aluminum‐7 Weight Percent Magnesium in Aqueous Solutions

Data have been obtained for electrode reaction rates on bare aluminum‐7 weight percent magnesium immersed in aqueous solutions. The reactions investigated were metal dissolution, proton reduction, and oxide growth in sulfate and chloride containing solutions in the p H range 1.0–6.5. The bare metal surface was produced by rapidly and lightly scratching a potentiostatically controlled rotating disk electrode. The resulting current transients were measured under various conditions of electrode potential, electrolyte composition, temperature, and electrode rotation rate. Current densities of up to 8 A cm −2 on the scratch were measured in this way. The rates of metal dissolution and proton reduction are increased by many orders of magnitude by removing the high impedance surface oxide. Dissolution of the bare surface occurs with the following rate‐determining step: , the rate being independent of p H and sulfate (0.5M) or chloride (1–4M) content. Proton reduction occurs with the discharge step being rate controlling. The rate of nucleation and growth of the oxide film follows the empirical law . Initially , but at longer times changes to 0.5, the changeover occurring earlier with increase in anodic polarization and by the presence of Cl − .