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Decarburizing phenomena resulted from the reaction of various gases with carburized thoriated tungsten filaments were studied by the mass spectrometric method.
Analysis of the data when the filament was heated at 2000°K or at room temperature leads the following results ;
(1) Carbon monoxide was easily formed by the reaction between oxygen and tungsten carbide on the surface of the filament.
(2) Methane, carbon monoxide, carbon dioxide and water vapour were formed when hydrogen was introduced.
(3) Forming rate of methane was proportional to the throughput of hydrogen, while those of carbon dioxide, carbon monoxide and water vapour were proportional to the square root of the throughput.
The mechanism of formation of methane is supposed to be a direct reaction of hydrogen with tungsten carbide on the surface. Water vapour seems to be formed by two processes. First, hydrogen molecules dissociate to atoms on the hot filament (H₂-2H). Secondly, the atoms react with oxide (glass surface) to form water vapour. Carbon monoxide and carbon dioxide are probably formed by the surface reaction between tungsten carbide and the water vapour.
From these results, it is considered that the carburized thoriated tungsten filament used in an electron tube is decarburized by oxygen or hydrogen evolved from various electrodes and inner surface of the envelope.

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