Corrosion failure analysis of pure nickel thermocouple sheath used in BaTiO₃ hydrothermal synthesis reactor

This paper presents corrosion failure analysis on a commercial pure nickel thermocouple sheath used in BaTiO₃ hydrothermal synthesis reactor. Detailed investigations of the corrosion products have been performed using stereomicroscope, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was found that the corrosion failure of the sheath was mainly due to general corrosion and crevice corrosion of pure nickel, and the main corrosion products were Ni(OH)₂. Many cracks were observed inside the corrosion products and along the interfaces between the corrosion products and metal matrix. BaTiO₃ particles were deposited as parallel white bands along the above cracks inside the corrosion products. Chloride ions also entered the cracks or crevices, resulting in the acidification inside the crevice and aggravated the attack of nickel matrix. Some recommendations to prevent/minimize future similar corrosion failures were proposed.