High temperature metallographic microscope studies of the initial oxidation of tantalum

High temperature metallographic microscope studies of the initial oxidation of tantalum in the temperature range 300°–1300°C at oxygen pressures of 0.1, 0.01 and 0.001 torr have been carried out. The results are presented as a consecutive series of photographs of specimen surface during the oxidation. In all cases an incubation period precedes the first visual observation of oxide formation. At any temperature the duration of this incubation period becomes longer the lower the oxygen pressure. During the incubation period oxygen dissolution in the metal is the predominant part-process. The initial oxide formation, as revealed by the metallographic microscope, never results in a continuous film or scale with an approximately even thickness, but always involves the formation of individual oxide nuclei or crystallites and their subsequent growth. The results are correlated with previous studies of the oxidation of tantalum.