Pure niobium begins to become significantly hydrogen embrittled at hydrogen concentrations greater than 100 ppm. In the chemical-processing industry (CPI), pure niobium will absorb hydrogen and become embrittled when exposed to hot HCl and hot H2SO4 at conditions illustrated in FIGS. 1 and 2. Where niobium and niobium alloys are used in the CPI to contain hot and concentrated acids, hydrogen embrittlement, rather than a loss of wall thickness due to corrosion, is the predominant failure mechanism.
U.S. Pat. No. 4,784,830 discloses that oxidation resistance of alloys can be improved by a controlled addition and retention of nitrogen. Put another way, it has been discovered that the microstructure of the alloys of the type under consideration, notably grain size, can be controlled or rendered relatively structurally stable over extended periods at elevated temperature through a microalloying addition of nitrogen. In addition, and most advantageously, a special ratio of silicon to titanium should be observed in seeking extended service life.
U.S. Pat. No. 3,592,639 relates to a ternary Ta—W alloy which contains from 1.5 to 3.5 percent of tungsten. Niobium can also be present in the alloy from 0.05 to 0.5 weight percent. Molybdenum is limited to 0.5% maximum (less than 5000 ppm) to promote smaller grain size in the alloy.
U.S. Pat. No. 4,062,679 claims a wrought tantalum product of, substantially pure tantalum containing less than 300 parts per million of columbium, less than 200 parts per million of iron, chromium and nickel combined, less than 50 parts per million of tungsten, less than 10 parts per million of molybdenum, less than 30 parts per million of chromium, and less than 20 parts per million of calcium, the improvement which comprises the inclusion of from about 50 to about 700 parts per million of silicon in the composition of said product whereby said product is improved in resistance to embrittlement when exposed to elevated temperatures in an oxygen-containing environment.