The chemical interaction between H21 tool steel and molten A380 alloy was investigated at 700 degrees C under dynamic conditions. Cr2O3-coated H21 samples were also investigated under the same condition for comparison. Samples were rotated at 300 rpm. The test time was varied between 1 and 36 hours. The reaction zone was characterised by SEM and EDS analysis. It was found that, from A380 alloy to H21 steel, FeAl3, Fe2Al5 and FeAl2 intermetallic compounds formed in sequence. Major alloying elements from both H21 steel and A380 alloy were present in those intermetallic compounds. FeAl3 compound was porous, while Fe2Al5 and FeAl2 compounds were compact. The thickness of Fe2Al5 layer kept constant in the range of 7-10 mu m during the tests, while the thickness of FeAl3 and FeAl2 compounds increased with increasing test time. Under the dynamic conditions, the high erosion rate of H21 sample in molten A380 alloy may be attributed to dissolution, swift melt agitation and poor protective effect of the reaction zone. It was also found that although Cr2O3-coating could protect H21 steel to some extent from erosion in molten A380 alloy, it can only act as a diffusion barrier because the coating itself was attacked by molten Al-alloy through chemical dissolution.