As molybdenum moved from rare metal status to a vital component of high temperature manufacturing, its unique combination of toughness, weldability, hardenability and elevated temperature strength became an attractive alternative to more expensive, heavier tungsten products. The only real drawback to using it was its tendency to oxidize rapidly at elevated temperatures. In fact, it was impossible to use it at 1292 deg F (700 deg C) or above without a protective coating.
The problem was solved by applying a plating process based on nickel and chromium to the molybdenum. The alloys developed, referred to as the C-type nickel molybdenum alloys, exhibit excellent resistance to reducing acids such as hydrochloric and sulfuric. They also withstand non-oxidizing acids such as acetic, formic and phosphoric acids.
The corrosion resistance is attributed to the high content of nickel in the alloys which results in very low carbon contents and molybdenum content levels above about 30%. A few examples of the alloys are Alloy 400 (UNS K93600), Invar(r) (UNS K93600), Waspaloy 25(tm) and Ni-Span(r). Other alloys which contain 72-83% nickel are used in electrical applications because they have very low thermal expansion rates at room temperature and cryogenic temperatures. They are used for transformers, inductors and magnetic amplifiers. Nickel is ferromagnetic and therefore can be incorporated in permanent magnets. These materials are produced from laterite and magmatic sulfide ores and from secondary processes such as hydro-metallurgy.
