Surface Roughness Evolution to Identify Incubation Time for Hot Corrosion of Nickel-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS at 550 °C

Abstract

In the absence of protective scales, nickel-base superalloys have an extremely limited hot corrosion incubation period before increased rates of attack are experienced. This paper reports on the nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS subjected to 550 °C hot corrosion exposures of durations ranging from 0 to 800 h, during which none of the superalloys developed a fully protective scale. The aim of the research was to identify the incubation period of each superalloy and this was achieved by means of surface roughness evaluations. A metrology exercise was performed on the cross section of test specimens which produced Cartesian data points which were subsequently converted to Ra and Rz data. Statistical analysis of the results suggested the incubation period lasted approximately 400, 500 and 200 h, respectively, for each superalloy. It was concluded that refractory metal phases within the microstructure were associated with the relatively short IN6203DS incubation period. This paper demonstrates that monitoring the changes in surface roughness provides a plausible method to identify the transition from incubation to propagation when studying 550 °C hot corrosion attack.

Mechanical Stress Effects on 550 °C Hot Corrosion Propagation Rates in Precipitation Hardened Ni-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS

“…Similar hot corrosion exposures to those performed by Kistler et al [14] have also been performed on CMSX-4, CM247LC DS and IN6203DS using a 4/1 molar ratio of Na 2 SO 4 :K 2 SO 4 deposits, with a surface loading of 0.5 mg cm −2 applied every 100 h, in a gaseous environment of 300 ppm SO 2 in air at 550 °C [15]. Those exposures indicated that a continuous layer of protective scale had failed to establish itself at the relatively low temperature.…”

“…Cylindrical specimens were used to evaluate the evolution of surface roughness during exposure to the hot corrosion conditions [15]. The data obtained were gathered during a metrology exercise on the polished cross-sections and enabled equations to be derived which provided an estimation of the maximum penetration rate of hot corrosion during the propagation stage of attack.…”

“…One cylindrical specimen for each material was exposed for durations of 0, 100, 200, 300, 400, 500, 600, 700 and 800 h. Subsequent surface roughness evaluations [15] indicated each material had entered the propagation stage of hot corrosion after approximately: 400 h for CMSX-4, 500 h for CM247LC DS and 200 h for IN6203DS.…”

“…That is, does the application of mechanical stress affect the 550 ˚C hot corrosion propagation rate on CMSX-4, CM247LC DS and IN6203DS or not? This was investigated by comparing the respective hot corrosion propagation rates in specimens without the application of mechanical stress [15] against those that were subjected to CF testing using high frequency and high R-ratio parameters.…”