High Temperature Oxidation of Thin Chromel-Alumel Thermocouples

Abstract

Oxidation of thin chromel-alumel thermoelements 0.3 mm in diameter, heated at the temperature of 800 – 900 °C in medium of air and calcium hydro-silicate is considered in this work. The oxidation rate of these thermoelements, determined by the method of resistance measurement, is very close to the parabolic oxidation rate law. The oxidation rate magnitude of alumel is about ten times larger than one of chromel. By measuring variation of resistance of chromel-alumel thermocouples during its usage it is possible to monitor the oxidation degree as 90 % of the resistance change measured occurs because of alumel oxidation process. Microstructure investigations of the oxide films revealed that alumel oxidation occurs uniformly on the whole surface simultaneously with formation of the three-layered oxide film with even thickness, while oxidation of chromel occurs by developing separate focal points which later join forming the oxide film of non-homogeneous thickness.