Comparison of the combined oxidation and sulphidation behavior of nickel- and cobalt-based alloys at high temperature

The combined oxidation-sulphidation behaviour at high temperature of two nickel-based and two cobalt-based cast alloys is analysed and discussed. These materials are evaluated in view of their possible application for building the components of the high temperature heating furnace up to 1380 °C. Typical service conditions combine extremely high temperatures with the simultaneous presence of sulphur-bearing impurities, due to the combustion of fuels derived from the coke production. The main requirements for this demanding application are simultaneous resistance to high thermal levels and to degradation caused by oxidation and sulphurization. The atmosphere chosen to simulate this harsh environment consists of atmospheric air enriched by 12% of sulphur. The testing procedure has been performed at three temperatures (1080 °C, 1180 °C and 1280 °C) and three times of exposure (24 h, 48 h and 96 h). Macroscopic observations have been coupled to microstructural investigations, performed by SEM-EDS. The analysis is mainly focused on the identification of the microstructural alterations experienced by each alloy and on the extent of internal attack caused by oxygen and sulphur. The combination of the macroscopic experimental data and the information from the microstructural observations alloys have been shown to point out the performances of alloy grade throughout the testing trials in order to define the chemical compositions associated to the moderate weight loss and limited depth of surface attack. The new chemical composition of a Ni-superalloy points out performance of corrosion resistance that are even better than the cobalt based alloys usually selected for such working conditions.