A Mechanism for the Anodic Dissolution of Magnesium

Abstract

Magnesium anodes were electrolyzed in 3% NaC1 solution in a simple electrolysis cell. Mg in the anolyte, Mg in the corrosion product, total hydrogen evolved, and weight Ioss were determined. It was found that the quantities soluble Mg, insoluble Mg, Rig calculated from the current passed were in approximate one to one relationship with each other and all of these quantities are roughly half of the total weight loss. Hydrogen evolved was always slightly less than the soluble and insoluble Mg. These facts are explained and integrated into existing knowledge of the behavior of Mg anodes by postulating that Mg dissolves in the solution investigated mainly as a univalent ion followed by reaction of this univalent ion with water.The problem of hydrogen evolution at a magnesium anode during electrolysis is an old one with literature references dating back to the work of Beetz (1) and Elsoesser (2). Recently, the problem has been reconsidered by Petty, Davidson, and Kleinberg (3), who measured the apparent valence of Mg anodes in a variety of aqueous electrolytes and found that their results could be explained by postulating the presence of both uniand divalent Mg during electrolysis at a Mg anode. Hydrogen evolution was explained as being a reduction product of the reaction between univalent Mg ions and water. Other substances, such as KMnO4, Cl0~, introduced into the anolyte compartment of their apparatus were also reduced, and in a recent paper by Kleinberg and co-workers (4), where measurements of apparent valence in organic liquids are described, reductions of such test liquids that were electron acceptors were obtained.The existence of univalent Mg has been postulated at various times, and in the earlier work (1, 2, 5) the continued evolution of hydrogen after current shut-off, and the ability of the grayish-black corrosion product that had accumulated on the Mg to evolve hydrogen from water were ascribed to the existence of a suboxide in the corrosion product. However, all efforts to isolate and identify such a compound failed. More recently, Faivre and Michel (6) have shown by x-ray analysis that the grayish product formed on oxidation of Mg in humid air is Mg(OH)~ with Mg atoms inserted in the lattice. Brouchere (7) and Huber (8) claim that x-ray and electron diffraction of corrosion films on Mg indicate that the fihns are mainly MgO with Mg atoms in the lattice.Insofar as none of the work to date has given a clear picture of the mechanism of solution of l~ig at an anode, the work described was undertaken with the purpose of attempting to obtain the necessary information to accomplish this.EXP~I~MI~I~TAL Mg of 99.92% purity with an iron content of 0.0001-0.0005% was used as the anode material. Anodes were of varying lengths and were 16.5 mm in diameter, except for a short portion that was machined slightly smaller to 539 fit inside the untapered portion of a 19/38 ground joint. Before use, the anodes were cleaned by immersion in 5% HN0s and then inserted in the apparatus shown in Fig. 1.All experiments were cond...

An understanding of the electrodeposition process of Al–Mg alloys using an organometallic-based electrolyte

“…The formation of a film has also been observed by other research groups during anodic dissolution of Mg into aqueous electrolytes. In the case of the aqueous electrolytes the black film was identified as MgO [27]. The development of the magnesium hydride film in this study suggests the formation of hydrogen at the anode.…”

The Effect of Halides on the Capacity and Resistance of the Magnesium Electrode in Aqueous Solutions

“…The reproducibility of Rr,~ determinations using Eq. [6] is about -----50%. Potential-current polarization curves of Mg in buffered and unbuffered solutions with halide addi- tions are given in Fig.…”

“…Equation [6] is valid when the time constants for decay across the film and solution double layers are equal or when one of the two decay rates is small compared to the other. Resistance data for the solutions studied are given in Table I.…”

Die Erzeugung von Membranpotentialen mit Hilfe schwerlöslicher Salze

“…und weiterhin muR an der Membranenoberflache (x = 0) die Kontinuitatsbedingung Unter Verwendung der Versuchsergebnisse liefie sich aus der Gleichung (7) die Unbekannte h bestimmen. Dieses geschieht jedoch einfacher durch Verwendung der Gleichung (5) oder (6) und der durch das Experiment zur Zeit t ermittelten Spannung v(t) (8) Aus dieser Gleichung laBt sich fur t -+ 00 auch c, aus V,n,, ermitteln.…”