Participation of Aluminum Hydride in the Anodic Dissolution of Aluminum in Alkaline Solutions

Abstract

The mechanism of anodic alkaline dissolution of aluminum was investigated through the analysis of cyclic voltammetry (CV) and potential step experiments. Attention was focused on the role of aluminum hydride (AlH3) as a reaction intermediate, as suggested by the recent detection of AlH3 formation during open-circuit dissolution. Potential step experiments at pH 11.75 revealed that the potential at the metal–surface film interface was close to the Nernst potential of AlH3 oxidation. This finding suggested a reaction mechanism in which an interfacial AlH3 layer is formed continuously by reaction of cathodically formed H with Al, and is then oxidized to the dissolution product, aluminate [Al(OH)4−] ions. However, potential step experiments at pH 11 did not indicate the presence of interfacial AlH3 ; instead, the metal–film interface was close to the equilibrium potential of Al oxidation. Analysis of the CV indicated an abrupt transition in dissolution behavior between the two pH values, from a relatively rapid dissolution controlled by diffusion and film conduction in highly alkaline solutions, to a slow dissolution at a lower pH controlled by a highly resistive surface film. The formation of interfacial AlH3 occurs readily at the higher pH, but is suppressed as the pH approaches neutrality.

Atom Probe Tomography Characterization of Thin Copper Layers on Aluminum Deposited by Galvanic Displacement

“…Such a film if composed of pure alumina would be electrically insulating; thus, direct reduction of Cu 2+ ions by Al could occur only at isolated conductive defects, producing Cu particles rather than the observed thin continuous film. However, the presence of hydride in the surface layer is demonstrated by the present APT results, as well as the electrochemical potential and SIMS measurements discussed above. − In solution, this hydride might be in the form of a continuous AlH 3 layer, or interstitial ions such as H – in the oxide. Upon addition of CuSO 4 to the alkaline bath, either type of hydride could reduce cupric ions directly to copper metal, for example 2 AlH 3 + 8 OH − + 3 Cu + 2 → 3 Cu + 2 Al ( OH ) 4 − + 3 normalH 2 In experiments with no copper addition, upon emersion of the sample, water vapor or oxygen would oxidize the hydride layer to aluminum oxide or hydroxide, in a reaction such as AlH 3 + 3 2 normalO 2 → 1 2 Al 2 normalO 3 + 3 2 normalH 2 normalO Thus, the oxide thickness on the alkaline etched sample would be greater than that on the sample with the copper deposit, as is consistent with the APT results Table .…”

“…The films consisted of small particles overlying copper layers of order nanometers in thickness. In the same pH range, electrochemical measurements demonstrated that the Al potential lies close to the Nernst potential determining AlH 3 stability, suggesting the presence of a hydride-containing surface layer on Al. − The formation of hydride during alkaline dissolution was detected by secondary ion mass spectrometry (SIMS) . In the copper deposition experiments, QCM and open circuit potential transients revealed a distinct initial stage of deposition occurring near the hydride oxidation potential, in which the mass change was equivalent to a deposited Cu layer of thickness 2–3 nm .…”

Developments in electrode materials and electrolytes for aluminium–air batteries

“…The corrosion rate of aluminium alloys in alkaline solutions varies with time [36], which is particularly evident at the opencircuit potential for aluminium alloys when the changes in the rate of hydrogen evolution exactly mirror the changes in the open-circuit electrode potential with time [37]. Upon immersion, the open-circuit potential of aluminium alloys shifts to a very negative value of À2.0 V vs. Hg/HgO due to the presence of aluminium hydrides [38,39], and possibly hydrides of the alloying elements, creating a hydroxide-free surface and a high rate of hydrogen evolution [37]. Once the alloy surface becomes passivated by a hydroxide layer, its potential shifts more positively, which is accompanied by a reduction in the rate of hydrogen evolution.…”

Electrochemical and Metal-Phase Processes Accompanying Hydrogen Absorption in Aluminum During Aqueous Corrosion

“…2) along with observations of timedependent concentrations of metal vacancies detected by PAS and hydride particles found by SIMS and AFM. 2,4 These results indicate chemical pathways to corrosioninduced tensile stress increases and hydride formation, factors that play important roles in SCC mechanisms.…”