Ion microprobe investigation of large decalibrations in inconel-sheathed magnesia-insulated platinum-rhodium/platinum thermocouple assemblies during use at 1200 °C

Abstract

After 24–72 h at 1200 °C, Inconel-600-sheathed MgO-insulated Pt 90Rh 10/Pt compacted thermocouple assemblies exhibited decalibrations of 5–47%. Metallographie and ion microprobe mass analysis showed that reactions had occurred within the thermocouple assemblies at the Pt-MgO and Pt 90Rh 10-MgO interfaces, resulting in the solution of up to 12 at.% Mg in the thermocouple wires. Magnesium concentrations were less in the Pt-Rh wires than in the Pt wires. Aluminum, Ca and Si in concentrations about 10 –1000 times lower than Mg were also found in the wires. The origin of these impurities was the Al 2O 3, CaO and SiO 2 impurities in the MgO powder insulation. Manganese found in the wires was shown to come from the Inconel sheath. It was concluded that Pt/Rh thermocouples insulated with compacted MgO powder and sheathed in Inconel-600 are inherently unstable at temperatures of 1200 °C because oxidation of the Inconel sheath produces a very low O 2 partial pressure inside the sheath that allows Pt and Pt/Rh to reduce MgO at the metal-oxide interface.