Degradation of thermocouple in a temperature programmed sulphidation reactor

•We report sulphidation of thermocouple at high temperature.•Sulphidation indicated that H2S selectively reacts with the specific alloy component.•As a function of temperature H2S reaction with the thermocouple started at about 400 °C.•Sulphidation at lower temperature is surface phenomena while at high it is a bulk.

Sulphidation due to sulphur bearing gases such as H2S with refinery materials is a serious problem in the petroleum industry. Measuring instruments such as thermocouples (TCs) used in the refining industry to measure and control temperatures in critical areas are also susceptible to sulphidation when exposed to the sulphur bearing process media. The paper presents an investigation carried out to understand the failure of a TC in a temperature programmed sulphidation (TPS) reactor. The TC was incorrectly installed in the reactor without the external protective sheath. As a result of incorrect installation, the TC was exposed directly to sulphidizing gas of H2S (5% H2S in H2) at a programmed temperature profile (150–1000 °C). The qualitative H2S consumption with a change in temperature indicated that the H2S reaction with the TC started at about 400 °C, which corresponds to a physical phenomena on the metal surface. The TPS result indicated that the first H2S consumption peak in the range 600–780 °C corresponded to the surface phenomena, mostly due to the H2S chemisorption reaction on the metal surface. However, the bulk phenomena of sulphidation or corrosion require diffusion as a function of further increase in temperature, hence a mass consumption peak in the range of 780–900 °C is recorded. Detailed investigations of corroded TC were carried out using TPS, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The investigation revealed that the failure of the TC was due to catastrophic sulphidation taken place as a result of incorrect installation. The results further suggest that the TPS technique has the potential to study the sulphidation behavior of metals and alloys.