Development In Corrosion Resistance Of Molybdenum Alloys By Nitriding

For pure Mo nitrided at 1123K to 1373K in NH3 gas, the microstructural observations of molybdenum nitride layer were carried out through an optical and a transmission electron microscope and X-ray diffraction analysis. Corrosion behavior of the nitrided specimens was also examined. A surface nitriding layer consisted mainly of two Mo nitrides: an outer one of gamma-Mo2N and an inner one of beta-Mo2N. A great number of (011)[0 (1) over bar1] type microtwins were observed in the beta-Mo2N layer whereas the microstructure of the gamma-Mo2N layer is almost free from lattice defects. The nitrided specimens exhibited excellent corrosion resistance even in 75wt% suffinic acid solution at the boiling temperature of 460.8K compared with unnitrided specimens. The selective corrosion of molybdenum matrix occurred at the tip of cracks in the molybdenum nitride layer. The molybdenum nitride itself exhibited perfect corrosion resistance up to in boiling 85 wt% sulfuric acid.