AN AES AND XPS STUDY OF THE INITIAL OXIDATION OF POLYCRYSTALLINE MAGNESIUM WITH WATER-VAPOR AT ROOM-TEMPERATURE

The initial oxide formation on polycrystalline magnesium surfaces at r oom temperature has been quantitatively followed from the earliest sta ge using Auger electron spectroscopy (AES) and X-ray photoelectron spe ctroscopy (XPS). Clean surfaces were prepared by sputtering with a min imum dose of argon ions to avoid the creation of heavily disordered cr ystal planes. Crystallographic orientations of various grain faces wer e determined using the electron back scattering diffraction technique in order to allow AES measurements to be made on grains of known orien tation. By using calibrated doses of deuterated water vapour, three st ages of early oxide growth were recognised: (1) a chemisorption stage during doses up to approximately 0.7 langmuir (L) (1 L = 1.3 x 10(-4) Pa.s); (2) a rapid oxide nucleation and island growth stage which is c omplete by about 5 L at an average island height of four monolayers an d (3) a slow, diffusion-controlled growth stage after coalescence of t he islands. Modelling results suggest that the rate of initial oxygen uptake is faster on grain faces that have more open-packed, high index orientations, particularly in the second (island nucleation and growt h) stage. In addition, the slow growth stage has been adequately descr ibed by a logarithmic type growth law for exposures up to 1 X 10(6) L, suggesting a limiting oxide thickness in water vapour of approximatel y 11 monolayers of MgO. The slow growth process was found to be contro lled by the movement of metal cations through the oxide film from the metal/oxide interface to the oxide/gas interface. Finally, the role of hydrogen in the oxide film was studied using XPS and nuclear reaction analysis (NRA). The results indicated that hydrogen was present in th e film only in small relative amounts, likely as hydroxyl groups trapp ed at the metal/oxide interface. Detailed XPS spectra showed two disti nct types of oxygen: one of these is assigned to be oxygen in a normal MgO lattice position; the other is ascribed to oxygen in a ''defectiv e'' chemical environment.