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

Abstract

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 pH 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 Acm -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: A1 + H20 --> AIOHads + I-I+ + e-, the rate being independent of pH and sulfate (0.5M) or chloride (I-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 /at ~ /am exp(--~tn). Initially n ----I, but at longer times changes to 0.5, the changeover occurring earlier with increase in anodic polarization and by the presence of C1-.The generalized theory of stress-corrosion cracking, originally suggested by Mears, Brown, and Dix (1) and expanded upon by others (2-4) has long been accepted for aluminum alloys since it is capable of explaining qualitatively the variation of cracking rate with second-phase morphology and, possibly, with elementary changes in electrochemical conditions at the crack edge. In essence this theory proposes that crack propagation progresses from a nucleating notch (usually formed by intergranular attack associated with discrete secondphase particles) when sufficient stress concentration exists to cause ductile tearing at the notch edge. Accelerated dissolution then occurs preferentially at the resultant film-free regions and continues until the dissolution rate is decreased by oxide formation or the intergranular penetration is halted at a grain boundary of unfavorable orientation. Thereupon the cycle of slow intergranular corrosion followed by rapid growth of the oxide film starts again.Recently, however, there is increasing evidence (5, 6) that an alternative mechanism involving hydrogen embrittlement may be valid in humid and aqueous environments. This evidence is based primarily on the similarity between the potential dependence of stresscorrosion susceptibility and of hydrogen permeation (7,8), the fact that the tensile ductility may be reversibly * Electrochemical Society Active Member. Present address" ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.237.29.138 Downloaded on 2015-03-09 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.237.29.138 Downloaded o...

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

“…A general mechanism for the dissolution of aluminium in the presence of chloride ions would be similar to that reported in the literature [110,111]. …”

Electrochemical and molecular dynamics simulation studies on the corrosion inhibition of aluminum in molar hydrochloric acid using some imidazole derivatives

“…Corrosion of aluminum in aqueous solution, which depends on the concentration of anions in solution, has been reported [24,25]. A general mechanism for the dissolution of aluminum would be similar to that reported by Ford et al [26] as well as by Nguyen and Foley [24,27].…”

Repassivation of 304 Stainless Steel Investigated with a Single Scratch Test

“…The working electrodes were made from 304 (UNS S30400) (1) stainless steel sheet machined into disks 1.5 mm thick and 11.3 mm in diameter. The composition of the stainless steel in weight percent was 18.0 Cr, 0.22 Mo, 8.53 Ni, 1.86 Mn, 0.84 Si, 0.07 C, 0.64 S, 0.42 Cu, and balance Fe.…”

“…[1][2][3] The cell was filled with a solution containing 1 M Na 2 SO 4 , with a measured pH of 6.7. The potential of the working electrode was controlled by a potentiostat.…”

“…The charge that passed through the external circuit during the transient Q was calculated by subtracting the stationary current prior to the scratch (I s ) from the measured current (I u ) and integrating the current transient. [1][2][3] The formula was:…”