The influence of thermal diffusion treatment on the microstructure and corrosion behavior of hot-dipped Al coated Fe-Cr-Si-B cast steel in a 5 wt.% NaCl solution was studied. The results showed that the product obtained from the interfacial reaction between the Fe matrix in Fe-Cr-Si-B cast steel and molten Al was FeAl₃ with a columnar crystal structure. Periodic layered structures at the (Cr, Fe)₂B/Al interface were formed during the subsequent thermal diffusion treatment but not hot dip aluminizing. The corrosion behavior of the residual Al layer predominated that of the hot dip aluminizing and thermal diffusion treatment coatings over a short duration till the Al topcoat was consumed completely. Localized corrosion caused by Cl^- preferentially began from the block-like B-Al-Cr phase/FeAl₃ interface and propagated along periodic layered structures. The Si particles acted as a cathode and promoted the corrosion of the coating. The steel substrate retained its cathodic protection until the Fe-Al intermetallic compounds layer was corroded completely. Additionally, the corrosion in the NaCl solution at an elevated temperature was greatly accelerated.