Anders Nylund

Oxidation of tungsten and tungsten carbide in dry and humid atmospheres

Abstract Oxidation experiments were performed on pure tungsten and hot-pressed tungsten carbide. The chemical state and thickness of the oxide products were determined by ESCA. The oxidation of W and WC in dry atmosphere was performed in oxygen at temperatures ranging from 20 to 500 °C. The oxide formed is WO3. The thickness of the oxide layer increases slowly up to 200 °C, after which the oxide growth is rapid. The oxidation behaviour of W and WC in humid atmospheres was studied at room temperature in air at relative humidities of 60 and 95%. It was found that the thickness of the oxide layer increases with increased humidity. No formation of hydroxide was observed. Exposing W to water for one week results in a thick layer of WO2, WO3 and hydroxide. In the case of WC no oxide at all was visible after exposure to water. Furthermore, WC is resistant to further oxidation after exposure.

Surface reactions during pickling of an aluminium-magnesium-silicon alloy in phosphoric acid

Abstract The surface products formed during pickling of a modified AA 6000 alloy (0.70Mg1.52Si) in 0.1 M phosphoric acid were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Cold-rolled and heat-treated samples were compared. Pure Si, Mg2Si and AlFeSi particles are present in the material after the heat treatment. Pickling gives a chemical dissolution of the surface oxide and an anodic dissolution of the Mg2Si particles. The degree of dissolution increases with the pickling time. On the cathodic AlFeSi particles, aluminium phosphate is deposited. After prolonged pickling, micropits are visible on the surface. It is suggested that this is due to the dissolution of small Mg2Si particles precipitated during the annealing process.

Influence of Mg and Si on the Oxidation of Aluminum

Test materials of pure Al, Al alloyed with 10,50, and 190 ppm Mg, and Al alloyed with 10 ppm Mg + 500ppm Si were oxidized at temperatures ranging from 250 to620°C. The composition and thickness of the oxide film were determined by electronspectroscopy for chemical analysis (ESCA) and Augerspectroscopy. Below the crystallization temperature(400°C) of Al2O3,Mg2+ is enriched in the interior portion of the oxide. The enrichment of Mg2+gives a somewhat thinner oxide compared with the oxideformed on pure Al. Above 400°C, MgO is formed as aseparate phase on the surface of theAl2O3. The Si-containing material showed Si4+ enrichment inthe surface oxide. Mg2+ species were notdetectable. Silicon is also strongly enriched in themetal phase just below the metal-oxideinterface.