Comparative evaluation of corrosion behaviour of type K thin film thermocouple and its bulk counterpart

This paper investigates the corrosion behaviour of type K thermoelements and their thin films, and compares the performance of chromel–alumel thin film thermocouple with its wire counterpart before and after exposure to 5% NaCl medium. Potentiodynamic polarisation tests reveal that chromel and alumel films are more “noble” than their respective wires. Alumel corrodes faster when coupled with chromel in films than as wires. Secondary electron micrographs and electrochemical impedance spectroscopy measurements suggest that chromel shows localised corrosion while alumel undergoes uniform corrosion. Corrosion adversely affects the thermocouple output and introduces an uncertainty in the measurement.

► Anodic vacuum arc deposited chromel and alumel films are more “noble” in 5% NaCl solution than their respective wires.
► Chromel undergoes localised corrosion while alumel shows uniform corrosion.
► Virgin samples of chromel–alumel TFTCs exhibit good thermoelectric response.
► Their thermoelectric outputs remain largely unaffected when shelved under normal atmospheric conditions.
► After 288 h of exposure in salt spray environment, their thermoelectric outputs show noticeable change due to size effects.