The static isothermal corrosions of Ti-50A1 and Ti-45A1-8Nb alloys in liquid zinc were studied and their positron lifetime spectrums were measured. The densities of free electrons of the bulk and microdefects in these alloys were calculated by using the positron lifetime parameters. It is found that the density of free electrons is lower in the bulk TiAl alloy than in Ti or Al matrix due to the localization and aggregation of some valence electrons in Ti and Al atoms to form TiAl alloy with a mixture bond of metallic bond and covalent bond. The large open defects occurs in the grain boundary of TiAl alloy and the bonding strength of grain boundary is weak due to the low density of free electrons. Then the densities of free electrons in the bulk and the grain boundary will simultaneously increase due to the great decrease of dissolution rate in liquid zinc as the TiAl alloy is alloyed with Nb