Pit morphology and depth after low-temperature hot corrosion of a disc alloy

Abstract

Hot corrosion of the low solvus, high refractory (LSHR) disc alloy was studied at 700 °C. The purpose of this study was to determine the conditions which result in a discrete, isolated pit morphology and to examine the influence of SO2 gas additions and various salt concentrations on the depth of those pits. Three salts, pure Na2SO4 and two Na2SO4–MgSO4 compositions, were used. It was found that with a eutectic Na2SO4–MgSO4 salt, there was no significant increase in pit depth between 0 and 30 ppm SO2 when O2 was also present in the gas stream. Gas flow was observed to affect pit formation, but the variation in the position of the corrosion mounds/pits on the sample surface was unexpected. There was limited evidence that pit nucleation was not associated with grain boundaries or grain triple point junctions. An evolution from single, isolated pits, to coalesced pits, to overlapping pits on a single sample was observed. At higher SO2 concentrations, the extent of attack increased, resulting in a uniform type of attack morphology with significant metal loss across the sample surface. It was concluded that hot corrosion attack by pit formation for these conditions is not easily explained or predicted.