Heat exchangers in coal gasification plants are exposed to highly reducing and sulphidising process gases. This paper presents high temperature corrosion test results for two candidate heat exchanger materials, P91 and Alloy 800H, coated using an unbalanced magnetron-sputter deposition technique. The coatings of plain and modified type 310SS stainless steel on P91 provided a reasonable improvement of corrosion resistance compared to uncoated P91. The Ni-Cr-Al coatings with up to 5 wt-% Al enhanced the corrosion resistance of the already very stable Alloy 800H base material considerably, by the formation of an Al and O rich intermediate corrosion layer.