Abstract
Mg-V and Mg-Zr alloys with nominal compositions 1, 6, 17.5, 27 wt% V and 2, 8.6 and 10.6 wt% Zr respectively were produced by PVD. All deposits exhibited compositional inhomogeneity, columnar microstructures and a strong basal texture. The solid solubilities of V and Zr in Mg were extended approximately to 17 wt% V and 10 wt% respectively. Grain refinement occurred with increasing solute content. The solid solution break up temperature decreased as the V and Zr content in the alloys increased. Pure V precipitated when the extended solid solubility of was exceeded. Both c and a lattice parameters, as well as the c/a ratio decreased with increasing V content in the Mg-V alloys. The slight increase of the a-lattice parameter and the decrease of the c one led to a decrease of the c/a ratio with increasing Zr additions in the Mg-Zr alloys. The air-formed oxide on the surfaces of the Mg-V alloys consisted predominantly of hydromagnesite at the outermost surface with Mg(OH)2 in excess of MgO underneath. No evidence of V oxide in the surface film was found. Magnesium oxide was also found between the grains of the deposits. The air-formed oxide on the surfaces of the Mg-Zr alloys consisted of ZrO2, MgO and possibly Zr sub-oxide. The presence of the oxides beween the columnar grains gave rise to graded metal/oxide interfaces. The outermost surfaces of the Mg-Zr alloys were similar to the Mg-V ones. Analysis of changes of the Auger parameters of the Mg-V and Mg-Zr alloys was also undertaken in order to investigate the electronic changes that take place upon alloying Mg with V and Zr. Charge transfer between 0.09 and 0.11 electrons/atom from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. Electron transfer between 0.02 and 0.03 electrons/atom from Mg to Zr was also found to occur upon alloying Mg with Zr. The electron transfer has been related to changes in crystal structure. The Mg-V and Mg-Zr alloys were examined after immersion in 3 wt% NaCl solution for 5 and 15 minutes, 9 hours and 7 days. The dramatic increase in the corrosion rate of the Mg-V alloys was attributed to the precipitation of pure V. The unsatisfactory corrosion performance of the Mg-V alloys was attributed to the absence of compositional uniformity through the thickness of the Mg-V deposits and the low thermodynamic stability of the corrosion products in the saline environment. Hydromagnesite at the outermost surface and Mg(OH)2, MgO and V2O4 in the bulk of the corrosion layer were the corrosion products. MgH2 and areas enriched in metallic V within the bulk of the corrosion products were also detected. The low corrosion rates of the Mg-Zr alloys, the lowest ever reported for Mg alloys, were attributed to the nature of the corrosion products and particularly the Zr contribution. The corrosion products were enriched in Zr, and were non-porous and in many cases well adherent. X-ray and electron diffraction suggested the existence of only Mg(OH)2 and MgO in the corrosion products, indirectly implying the participation of zirconium oxide/hydroxide in an amorphous/nanocrystalline state. Surface analysis indicated that a Zr oxide coexisted with Mg(OH)2 and MgO below a magnesium carbonate overlayer and also suggested the existence of Zr hydrous oxide (hydroxide). The repetition of the substrate pattern, as well as the fact that Zr hydroxide was replaced with ZrO2 and Zr sub-oxide as the metal-oxide interface was approached, implied a corrosion mechanism involving inwards diffusion of the anionic species.