Electrode Reaction Rates on Straining Aluminum‐Magnesium Wires in Chloride and Sulfate Solutions

Abstract

Electrode reaction rates on aluminum‐7 weight per cent (w/o) magnesium wires exposed to chloride and sulfate solutions have been studied as a function of tensile strain rate, electrode potential, solution pH, degree of aeration, and anion concentration. An increase in rate of both anodic and cathodic reactions was observed on application of plastic strain; the increase depends on the strain rate, the anion concentration, and the degree of anodic or cathodic polarization in the chloride or sulfate solution. At electrode potentials remote from the chloride pitting potential, the difference in electrode‐reaction rates in chloride or sulfate solutions is relatively minor; as the potential approaches the pitting potential, however, the anodic rate in chloride solution becomes significantly greater than that in sulfate solution. The anodic reaction is primarily dissolution and the cathodic is hydrogen‐ion reduction.There is a relationship between current density on the straining electrode and the rate of passivation found on cessation of plastic deformation. The shape of the current/time transients suggests that the change in reaction rate upon straining is controlled by the rate of oxide rupture (caused by slip‐step emergence) and by the subsequent passivation rate of the new bared metal.Stress‐corrosion tests, conducted under potentiostatic control in sulfate and chloride solutions, indicate that the propagation rate of stress‐corrosion cracks in isolated metal has the same dependence on pH, anion type and concentration, and degree of aeration, as does the anodic current density of a straining electrode. This suggests that the crack‐penetration rate depends on the reaction rate at the crack advancing edge, which is controlled by the rate of oxide rupture and by the subsequent rate of passivation.