This paper presents corrosion results obtained on coated specimens of P91 and Alloy 800H. The coatings were deposited using an unbalanced magnetron sputtering technique and were based on AISI 310SS containing controlled additions of Al and Y or Ni-20%Cr with varying amounts of Al. The maximum coating thickness was 10 mum and the grain size was closely controlled. Laboratory testing was carried out at 600degreesC in a forming gas (95%N-2-5%H-2) Mixture containing 1%H2S or 1%H2S + 1%HCl. As a result of the application of these fine-grained coatings, significantly lower corrosion rates were observed accompanied by reduced metal losses.