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...
The oxide-metal interface of samples of two iron-and nickel-containing aluminum alloys and commercial aluminum was examined by optical microscopy after short exposures to high-temperature pure water. The initial attack proceeds faster on the second phase particles in the aluminum than on the surrounding matrix, but with time a reversal in relative corrosion rates occurs. The time at which this reversal occurs varies with temperature, and after it occurs the aluminum matrix surface recedes past the second phase particles leaving them isolated and not further corroded. The method of preparing the sample for examination is described briefly.Draley et al. (1)(2)(3) have shown that aluminum can be made very much more resistant to corrosion in high-temperature water by alloying with cathodic metals, such as iron and nickel. Krenz and Biefer (4, 5) have reported considerable testing of two such alloys (alloy 10155 and alloy 10157) developed at Atomic Energy of Canada Ltd., and have correlated the enhanced properties of these alloys with presence of second phase particles in the metal and their size and distribution. This requirement for a uniform fine distribution of second phase particles had also been put forward by Draley and Ruther (6), Coriou et al. (7), and Brown et aI. (8). Carlsen (9) has made observations on the behavior of second phase material when segregated in grain boundaries of cast alloys and showed that attack occurred more rapidly in these regions. However, in the previous work no specific information was obtained as to how the second phase particles themselves behaved.The program of investigation at the Naval Research Establishment has concentrated on the initial period of film formation, and films from specimens exposed for periods varying from 1 rain to 7 hr have been examined by electron and optical microscopy. Some of the results of this work, as reported previously (10-12), had revealed the behavior of second phase particles at the metal-oxide interface after 7 hr of exposure and the relation between these particles and topological features of the innermost and outermost surface of the oxide film. More recently, complete sets of exposures were completed on commercial aluminum and two special iron-and nickel-containing alloys over the time range 1 min to 7 hr at 5 different temperatures. Each metal-oxide interface was examined in detail on cross-sectioned samples using specially developed metallographic techniques (13-15). By following the condition of the oxide-metal interface during the initial stages of corrosion, this work has yielded new information on the behavior of second phase particles.
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