In this paper, oxidation tests on tantalum in steam between 600 and 1100℃ are presented and kinetics data are given. The samples after the test have shown different behaviors: the plates underwent stronger oxidation than the cylinders. This is probably due to the different shapes and thicknesses, since the coupons oxidized strongly at the edges and the cylinders, due to the stresses developed on the convex surface, developed a denser oxide layer. Regarding the oxidation kinetics, three different stages can be recognized:
At 600 and 700 ℃, an initial parabolic stage is followed by two different linear stages. While the first parabolic stage follows the Wagner law, the linear stage is correlated to the start of the breakaway, due to the metastable sub-oxides, which foster the production of cracks. The transition between the first and the second linear regime can be due to the transition between TaOz to TaOy.
At 800 and 900℃ the oxidation follows only the linear law. The scale proves to be dense enough not falling apart, probably due to the formation of a different sub-oxide (TaO). However, it exhibits stratification and longitudinal cracks. This structure supports the hydrogen absorption due to the pump effect. The linear rate constants are lower than at 600 and 700 ℃.
At 1000 and 1100 ℃ the linear oxidation constant is the same order of magnitude than at 600 and 700℃. The oxide scale shows columnar features, and in particular at 100 kPa steam partial pressure high porosity is developed
Overall, the hydrogen uptake is higher than the theoretical value estimated from the equilibrium with the ambient hydrogen partial pressures. This has been explained considering the ‘‘pump effect’’, especially at 800 ℃ due to the characteristic oxide scale.
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