Samples of niobium, tantalum, molybdenum and tungsten metals have been given phosphided cases by heating in a phosphorous atmosphere. The samples were subsequently exposed to air at an elevated temperature. In tests at 800°C, the phosphided samples resisted air oxidation markedly compared to the pure untreated samples. The phosphided cases were only a few microns thick yet oxidation resistance of all these samples extended for several hours. Phosphided samples with 90° edges did not resist oxidation at the edges as well as rounded edge samples.
Witte, James Henry and Wilhelm, Harley A., "Oxidation studies of some refractory metals" (1964). Ames Laboratory Technical Reports. 66.