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

The temperature dependence of the surface segregation of magnesium ont o the (111) surface of Al-1.45at%Mg and its subsequent effect on the o xygen adsorption rate have been studied using optical second harmonic (SH) generation. The SH intensity increases with Mg surface segregatio n up to 495 K but decreases at higher temperatures due to Mg evaporati on. A. bulk diffusion model for Mg segregation was found to be inconsi stent with the data. Rather, the segregation rate was found to obey a simple one-dimensional nucleation and growth kinetic law with an activ ation 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 e xhibits a sharp increase in the initial oxygen adsorption rate. This i s consistent with a multi-site adsorption mechanism. At high annealing temperatures where the Mg surface concentration decreases due to evap oration, the oxygen adsorption rate also decreases. The initial sticki ng coefficient of the Mg-rich sites increases with Mg surface concentr ation; its maximum value is found to be of the order of unity, more th an 10 times than that of the clean Al(111) surface.