An investigation of premature, catastrophic failures of Chromel-Alumel thermocouples in petroleum refinery service disclosed that failures consisted of intergranular fractures of the Alumel wires caused by sulfidic corrosion. This type of failure was observed in the laboratory to occur in the 649-816 C (1200-1500 F) temperature range and in the presence of sulfides. Sulfur was detected by electron microprobe x-ray analysis in grain boundaries and cracks in failed Alumel wires. Intergranular sulfidic attack also was observed in progress by means of hot-stage metallography.
A mechanism for the observed type of attack is proposed: At temperatures in the 649 C (1200 F) range, sulfur diffuses into grain boundaries of Alumel and forms the nickel sulfide, Ni3S2±x and the eutectic, Ni-Ni3S2±x. Liquefaction of the eutectic above 645 C (1193 F) produces grain separations and wire failure. Above 810 C (1490 F), however, Ni3S2±x is unstable, hence, in this range Alumel corrosion is not limited to grain boundaries only and catastrophic failures do not occur.
The proposed corrosion mechanism may be applicable to other corrosion problems involving nickel and high-nickel alloys.
