Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

•Strong effect of oxidation temperature around 1000 °C on breakaway oxidation.•No significant effect of small chemical variations around nominal composition of M5Framatome.•Effect of one-side versus two-side oxidation in some cases.•Large hydrogen pickup fraction after the onset of breakaway oxidation.•Microstructural and microchemical specific features associated with breakaway oxidation. The breakaway oxidation of Zircaloy-4 and M5Framatome alloys oxidized in flowing steam around 1000 °C was studied. The influences of oxidation temperature between 950 and 1050 °C, one-side versus two-side oxidation and chemical variations around the nominal composition of M5Framatome were investigated. Breakaway oxidation is sensitive to the oxidation temperature, the material and the oxidation type. A phenomenological mechanism for breakaway oxidation is postulated on the basis of microstructural and microchemical analyses.