Effects of temperature and time on the microstructure and phase evolution for different thermal treatments were investigated with respect to the measurement of intermetallic layer thickness, phase identification and microhardness distribution in the aluminized zone of a steel substrate. The intermetallic phases present in the aluminized region after hot dip aluminizing is mainly Fe2Al5. The thickness of the intermetallic layers increases with increasing oxidation temperature and time. In the oxidation treatments of the aluminized steel in air, the initial Fe2Al5 phase remains at the temperature below 950 degrees C in 2-h, and the Fe2Al5 phase is completely transformed into low iron content Fe-Al intermetallics due to oxidation at 950 degrees C for 4 h. However, the Fe2Al5 phase remains in the outer layer of the aluminized samples diffusion-treated in vacuum regardless of diffusion time. The microhardness values of the Al2O3 and the intermetallic Fe2Al5, FeAl2, FeAl and Fe3Al phases are HV1150, HV1010, HV810, HV650 and HV320, respectively. The oxide layer formed on the steel substrate has an extremely fast adherence to the steel substrate and excellent properties of thermal shock resistance, high temperature oxidation resistance and anti-liquid aluminum corrosion. (C) 2007 Elsevier B.V. All rights reserved.