MAGNESIUM SURFACE SEGREGATION AND ITS EFFECT ON THE OXIDATION RATE OF THE (111)SURFACE OF AL-1.45AT-PERCENT-MG

The temperature dependence of the surface segregation of magnesium onto the (111) surface of Al-1.45at%Mg and its subsequent effect on the oxygen adsorption rate have been studied using optical second harmonic (SH) generation. The SH intensity increases with Mg surface segregation up to 495 K but decreases at higher temperatures due to Mg evaporation. A. bulk diffusion model for Mg segregation was found to be inconsistent with the data. Rather, the segregation rate was found to obey a simple one-dimensional nucleation and growth kinetic law with an activation energy of 7.2 +/- 0.6 kcal/mol and a free energy for segregation of 4.9 +/- 0.3 kcal/mol. The surface modified by the Mg segregation exhibits a sharp increase in the initial oxygen adsorption rate. This is consistent with a multi-site adsorption mechanism. At high annealing temperatures where the Mg surface concentration decreases due to evaporation, the oxygen adsorption rate also decreases. The initial sticking coefficient of the Mg-rich sites increases with Mg surface concentration; its maximum value is found to be of the order of unity, more than 10 times than that of the clean Al(111) surface.