Failure mechanisms in Pt-Rhx thermocouple sensors caused by gaseous phosphorous species

Interactions between Pt-Rh sensor alloys and P-containing gas in an environment simulating a carbon feedstock gasification process were investigated. A series of exposure tests revealed materials failure through two distinct P diffusion mechanisms, depending on Rh concentrations; intergranular diffusion in low Rh alloys and intragranular diffusion in high Rh alloys. Upon exposure, P rapidly migrated into the alloys, lowering the melting temperature of the alloys and/or forming intermediate phases with Rh at grain boundaries or within grains. The failure mechanisms of Pt-Rh alloys by P-bearing gas in the conditions studied are proposed.