Abstract

Thermocouples are the sensor of choice for monitoring high temperature process. Sensor failure can be caused by a number of reasons related to severe service environments; including slag attack, abrasive wear, shear, protection tube failure, and vapor attack. Depending on the industrial process being monitored, post-modern analysis of a failed thermocouple assembly to determine causes of failure cannot always be conclusive because process disruption is not always possible. The gasification of coal occurs at temperatures from 1300-1575 °C, with H2 and CO (syngas) production being critical for power or chemical generation. In severe service environments thermocouples are placed in protection assemblies that are in contact with refractory liners and the process. Refractory lining shifts or the nature of the process can cause breakage or failure of the thermocouple protection assembly, allowing the corrosive gaseous environment of a process to contact thermocouple wires. Corrosive gases in gasifier originate from the carbon feedstock or refractories - materials that can contain arsenic, sulfur, phosphorous and other impurities. Current research investigated the effects of phosphorous gas on the Pt-Rh thermocouple degradation. Gaseous phosphorous interactions with a type-B thermocouple conducted non-isothermally by heating to 1500 °C and isothermally at 1012 °C in high temperature resistance furnace. CO and CO2 gases were used to simulate reducing environment similar to those in industrial gasifier and to promote evolution of phosphorus gas. Separately, individual Pt wires with varying Rh contents were tested isothermally in the phosphorous rich reducing environment with different exposure times. The analysis revealed the material degradation was caused by a combination of Pt liquidus lowering and the intermediate phase formation at grain boundaries or gas/solid interfaces. The phosphorous diffusion in the Pt-Rhx alloys depended on Rh contents. In the low Rh (0, 6, 10 wt.%) alloys, intergranular diffusion was dominant, while intragranular diffusion was governing the P transport in the high Rh alloys (30 wt.%) due to higher interactions with Rh. Upon isothermal P exposure, the Rh2P intermediate phase formed in all the wires containing Rh, and the pure Pt wire extensively melted at grain boundaries due to phosphorous enrichment within one minute exposure contributing to material degradation. Mechanisms of ultimate thermocouple failure by phosphorous are proposed.