Effect of nickel base superalloy composition on oxidation resistance in SO2 containing, high pO2 environments

Abstract

Modern gas turbine materials are subjected to increasing operating temperatures in service environments, which contain high concentrations of gaseous species originating from fuel impurities, e.g. sulfur. In the present study, the oxidation behavior of two commercially available nickel‐base alloys, Rene 80 and PWA 1483, was investigated for up to 500 h duration in synthetic air and synthetic air containing 2% SO2 at 1050 °C. In order to investigate the mechanisms of SO2 attack, specimens of the two alloys after different oxidation times were characterized using a number of surface analytical techniques such as scanning electron microscopy, XRD, and glow discharge optical emission spectrometry. The corrosion reactions in the presence of SO2 were found to be strongly alloy dependent and could not be explained simply in terms of the contents of the main scale‐forming alloying elements, Cr and Al. Severe internal sulfidation was observed underneath the chromium‐rich oxide scale on Rene 80, leading to breakaway oxidation after longer exposure times. For PWA 1483 sub‐scale formation of a near‐continuous alumina occurred, which resulted in substantial suppression of sulfidation. Consequently, no breakaway oxidation of PWA 1483 was observed up to 500 h exposure at 1050 °C in synthetic air with 2% SO2. The formation of a dense alumina sub‐scale on PWA 1483 is believed to be correlated with the Ta‐addition in this material. It is proposed that Ta can tie up Ti by forming a mixed oxide of the type TiTaO4. In this way, it prevents the incorporation of Ti into the chromia scale, which can lead to enhanced scale growth.