On the oxidation mechanism of pure tungsten in the temperature range 600–800°C

Abstract

Abstract:The oxidation behavior of International Thermonuclear Experimental Reactor (ITER) reference tungsten grade has been evaluated in dry air in the temperature range 600 800 °C. At 600 °C, the scale remained protective while the integrity of W 18 O 49 layer was kept. Rapid increase in mass gain resulted from mas sive cracking at local areas in the W 18 O 49 layer. Then, a coarse non protective columnar WO 2.92 scale was developed which favoured rapid inward oxygen transport into the alloy. At 700 and 800 °C, growth stres ses in the scale were released through local cracking. At this stage, WO 2.92 became progressively trans formed into WO 3 when the oxygen partial pressure increased across the scale thickness.

Influence of processing route and yttria additions on the oxidation behavior of tungsten

“…These results suggest that yttria additions might modify the stress state in the scale, delaying the cracking stage for longer exposure times that in the scale established in W-CP. In the case of W-PM, the outermost thin layer of the scale behave similar to the scale developed in W-0.6Y 2 O 3 , while the innermost thick part of the scale was generated in a similar way to that established on W-CP [8].…”

“…In agreement with mass gain curves, maximum thickness corresponded to W-CP, about 95 lm and minimum to W-0.6Y 2 O 3 , about 18 lm. Thickness of the scale on W-PM was not uniform, presenting thin dense regions of about 20 lm, similar to that observed in W-0.6Y 2 O 3 , and thick regions of about 75 lm whose microstructure resembled rather well that found in W-CP [8]. The oxide scale formed on W-CP consisted of two layers, clearly depicted by a nearly continuous band of cavities between both layers (see Fig.…”

“…The present work reports how tungsten processing by a powder metallurgical route and yttria additions affect the oxidation resistance of tungsten in dry air between 873 and 1073 K. The results are compared to previous oxidation data for ITER-reference tungsten grade [8] and commercial pure tungsten of different sources [6,9,10].…”

“…The oxidation mechanism on pure tungsten has been reviewed by Cifuentes et al [8]. Basically, the oxidation proceeded through a cyclic mechanism involving periodic cracking at local areas when a dense oxide layer attained a critical thickness.…”

“…As reference, the mass gain curve for pure W (W-CP) of a ITER-reference tungsten grade, 99.97% purity, is shown (for more detail see Ref. [8]). Mass gain curves proved that oxide scale formed on these materials conferred protection only at 873 K, being non-protective above this temperature.…”