The Oxidation Kinetics of Some Zirconium Alloys in Flowing Carbon Dioxide at High Temperatures

The oxidation kinetics of three zirconium alloys (Zr—2.2 wt% Hf, Zr—2.5 wt% Nb, and Zr— 3 wt% Nb—1 wt% Sn) have been measured in flowing carbon dioxide in the temperature range from 873 to 1173 K to 120 ks (2000 min). At all oxidation temperatures, Zr—2.5 Nb and Zr—3 Nb—1 Sn showed a transition to rapid linear kinetics after initial parabolic oxidation. The Zr—2.2Hf showed this transition at temperatures in the range from 973 to 1173 K; at 873 K, no transition was observed within the oxidation times reported. The Zr—2.2 Hf showed the smallest weight gains, followed in order by Zr—2.5Nb and Zr—3 Nb—1 Sn. Increased oxidation rates and shorter times-to-rate-transition of Zr—2.2 Nb and Zr—1 Sn as compared with Zr—2.2 Hf can be attributed to the presence of niobium, tin, and hafnium in the alloys. This is considered in terms of the Nomura-Akutsu model, according to which hafnium should delay the rate transition, while niobium and tin lead to shorter times-to-rate-transition. The scale on Zr—2.2 Hf was identified as monoclinic zirconia, while the tetragonal phase, 6ZrO₂·Nb₂O₅, was contained in the monoclinic zirconia scales on both other alloys.