An investigation of palladium oxidation in the platinum/palladium thermocouple system

The work reported explores the effect of Pd oxidation on the behaviour of Pt/Pd thermocouples within the temperature range 250 degreesC to 1000 degreesC. The results confirm that oxidation of Pd takes place primarily in the temperature range 600 degreesC to 800 degreesC. Uniform (isothermal) heat treatment in the critical temperature range can increase the thermoelectric emf by 1.5 muV (equivalent to 0.15 degreesC) at the zinc freezing point, with the peak change occurring for a heat-treatment temperature of 750 degreesC. When subjected to non-uniform heat treatment, an initially homogeneous thermocouple may develop a thermoelectric inhomogeneity of I muV (equivalent to 0.14 degreesC) at the tin freezing point when subjected to temperatures between 600 degreesC and 800 degreesC. The oxidation of palladium is more easily interpreted from our parallel investigation of a Pd resistor fashioned as a resistive temperature detector (RTD). For the Pd RTD, the peak of the palladium oxide formation occurs at 725 degreesC and oxide does not form at or above 800 degreesC. The changes in thermoelectric emf of the Pt/Pd thermocouple are less than would be expected for a Pt-Rh/Pt thermocouple such as a type S. However, the Pt/Pd thermocouple does not exhibit the stability of the gold/platinum thermocouple due to oxidation of the Pd component and is not recommended in its stead unless the maximum temperature of use exceeds 1000 degreesC.