"Stainless steel is the material that thermowells are made of most frequently. A thermowell made of stainless steel is inexpensive and has excellent heat and corrosion resistance. A high strength stainless steel used for pressurised vessels is chromium/molybdenum steel. Molybdenum increases corrosion resistance when added.
Because of its excellent strength and corrosion resistance, Monel Thermowells are well known. It functions in a broad temperature range and is offered in a number of formats for a range of products. Monel is often used in piping (for pipes). Because of its durability and anti-corrosive qualities, it is ideal for heavy-duty applications like transfer (pipe) pipework used in oil refineries. It is also resistant to both steam and seawater."