Magnesium surface segregation and its effect on the oxidation rate of the (111) surface of A1-1.45at%Mg

Abstract

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.