Refractory metals are a unique class of metals known for their exceptional resistance to extreme heat and wear. The category includes tungsten, molybdenum, niobium, rhenium and tantalum, as well as their alloys. Because of their high melting point, it is necessary to produce refractory metals via powder metallurgy instead of fabrication by casting.
Due to their remarkable properties, refractory metals have gained widespread use in specialized manufacturing processes for products a such as nuclear reaction control rods, lubricants, lighting equipment and tools. Specific applications include:
- Tungsten: rocket nozzles, semiconductor supports, high-capacity cathodes, electronic tube emitters.
- Molybdenum: rocket and missile engine components, electronic furnace heating elements, zinc refining pumps.
- Tantalum: electrolytic capacitors, thermometer wells, vacuum tube filaments
- Niobium: numismatics, optics, electronics.
- Rhenium: jet engines, filaments for mass spectrographs, high-octane gasoline.
The Vulnerability of Refractory Metals to Oxidation
While these metals offer numerous benefits, they do pose certain challenges. One of their biggest downfalls is their susceptibility to surface oxidation when exposed to extremely high temperatures. The resulting corrosion can eat away at the metal and jeopardize the structural integrity of the part or component. The end-user will experience a shorter product lifespan and need to bear the expense of more frequent replacement.
Preventing Oxidation via the Application of Protective Coatings
It is possible to significantly limit the impact of corrosion on refractory metals by applying a coating to the base material. The coating serves as a protective barrier that prevents the formation of surface oxides. The most effective coating choice depends on a variety of factors such as the desired product lifespan, temperature (could range from 1100° F to 4500° F depending on the application) and the atmospheric conditions to which the finished product will be exposed (i.e., indoor vs. outdoor, damp vs. dry).
Also, each refractory metal reacts differently to various coating, which underscores the importance of ensuring compatibility. Examples of matches that have yielded successful outcomes include:
- Molybdenum: Compatible coating materials with molybdenum include silicon, nickel, chromium, precious metals (gold, silver, platinum, etc.), glass and refractory oxides.
- Tantalum: The most suitable metals when coating tantalum-based substrates include beryllide, aluminide, silicon and oxide-based materials. *Tantalum will not electroplate with aqueous solutions.
- Tungsten: Sufficient protection for tungsten substrates typically requires a multi-coat process. Rhodium serves as the base coat followed by nickel, chromium, precious metals (gold, silver, platinum, etc.), chromium, rhenium or silicon.
Contact SPC for All Your Refractory Metal Coating Needs
Coating refractory metals is one of the more challenging metal finishing processes — it requires specialized expertise that few companies in the industry possess. As one of the most reputable metal finishing companies with more than 90 years of experience, SPC can develop and implement a high-precision refractory metal coating technique that will work for your manufacturing applications. Contact us to learn more today.
