Oxidation kinetics and oxygen diffusion in low-tin Zircaloy-4 up to 1523 K

This paper deals with the study of oxidation kinetics and the identification of oxygen diffusion coefficients of low-tin Zy-4 alloy at intermediate (973 K ⩽ T ⩽ 1123 K) and high temperatures (T ⩾ 1373 K). Two different cases were considered: dissolution of a pre-existing oxide layer in the temperature range 973 K ⩽ T ⩽ 1123 K and oxidation at T ⩾ 1373 K. The results are the following ones: in the temperature range 973–1123 K, the oxygen diffusion coefficient in αZr phase can be expressed as Dα = 6.798 exp(−217.99 kJ/RT) cm2/s. In the temperature range 1373–1523 K, the oxygen diffusion coefficients in αZr, βZr and ZrO2, were determined using an ‘inverse identification method’ from experimental high temperature oxidation data (i.e., ZrO2, and αZr(O) layer thickness measurements); they can be expressed as follows: Dα = 1.543 exp(−201.55 kJ/ RT) cm2/s, Dβ = 0.0068 exp(−102.62 kJ/ RT) cm2/s and DZrO2=0.115exp(−143.64kJ/RT)cm2/sDZrO2=0.115exp(−143.64kJ/RT)cm2/s. Finally an oxygen diffusion coefficient in αZr in the temperature range 973 K ⩽ T ⩽ 1523 K was determined, by combining the whole set of results: Dα = 4.604exp(−214.44 kJ/RT) cm2/s. In order to check these calculated diffusion coefficients, oxygen concentration profiles were determined by Electron Probe MicroAnalysis (EPMA) in pre-oxidized low-tin Zy4 alloys annealed under vacuum at three different temperatures 973, 1073 and 1123 K for different times, and compared to the calculated profiles. At last, in the framework of this study, it appeared also necessary to reassess the Zr–O binary phase diagram in order to take into account the existence of a composition range in the two zirconia phases, αZrO2αZrO2 and βZrO2βZrO2.