Comparison between oxidation of Fe–Cr–Al sputter coatings in air and air–HCl environments at 550°C

Abstract

In biomass fired power plants the superheaters and reheaters are known to be particularly susceptible to chloride induced fireside corrosion. One approach to giving them longer lives is to develop new coatings that are resistant to this type of fireside corrosion damage. This paper reports the initial stages of such an approach using the combinatorial model alloy development method. Physical vapour deposition (PVD) using a two-target magnetron sputtering system (99·95 wt-%Cr and Fe–30 wt-%Al) has been used to obtain a range of coating compositions. The coatings were deposited onto an array of sapphire discs (10 mm diameter; 3 mm thick) placed in front of the targets. This resulted in a group of samples with coatings with a range of different Cr to Fe/Al ratios, which have been characterised using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) and X-ray diffraction (XRD). Two groups of eleven coatings have been exposed at 550°C for up to 150 h in air and air–315 vppm HCl. Weight change data was gathered from these exposures after 50 and 150 h. After each exposure period, the surfaces of the oxidation/corrosion products were characterised using SEM/EDX and XRD. This analytical data has been used to identify the phases formed and the morphology of the scales generated. The best performing coatings from the mass change data were cross-sectioned to characterise the damage.