The mechanism of zinc wettability defects on high silicon and high aluminium annealed steels has been studied by transmission electron microscopy of specimens prepared from synthetic steels recrystallized on a simulator. Wettabillity defects on high silicon steels are associated with a continuous amorphous silica layer that forms during annealing. On high aluminium steels, wettability defects result from a continuous crystallized alumina layer thicker than 6 to 8 nm.