Behavior of interference-colored oxide films on tungsten in a carbon monoxide atmosphere

A reducing atmosphere not only protects a metal from gaseous corrosion but can also remove the oxide film from a metal surface, a necessary process prior to application of protective coatings. The process by which carbon monoxide reduces thermally produced interference-colored oxides on tungsten was studied. Tungsten samples with dimensions of 20 x 10 x 0.3 mm, having a base metal content up to 99.95%, were ground, polished, degreased with alcohol, and oxidized at a controlled temperature of 550/sup 0/C to a standard blue color. Heating was done in an open transparent, quartz tube. Tarnishing was followed visually after the initial color changes. Electron diffraction patterns showed the oxide films to consist of WO/sub 3/. The oxidized tungsten samples were placed in a cold section of the quartz tube which was then hermetically sealed and saturated with carbon monoxide for 2 h. The reaction zone of the tube was next heated to the required temperature and the samples were quickly introduced into the heated zone by means of a special rod. The reducing atmosphere above 575/sup 0/C reverses the sequence of film interference colors produced during oxidation: azure blue ..-->.. blue ..-->.. violet ..-->.. brown ..-->.. yellow, attesting to amore » decreasing oxide layer thickness. Since the process temperature can evaporate WO/sub 3/, it is logical to presume that the film thinning observed is the overall result of reduction and sublimation, the possibility of which was verified in an argon atmosphere.« less