Ternary Zr–Nb–Fe(O) system: phase diagram at 853 K and corrosion behaviour in the domain Nb<0.8%

Several zirconium-based alloys in the domain Nb < 0.8 wt%, Fe < 0.5 wt% and Sn < 1 wt% (and oxygen in the range 600-1200 ppm) were elaborated. Several process routes were used to produce sheets and claddings. The microstructure was investigated by TEM, with a particular attention payed to the SPP nature and composition. Corrosion tests in steam in autoclave or in air were performed. This study allowed us to precise the isothermal cut at 853 K of the Zr-rich corner of the Zr-Nb--Fe ternary phase diagram, and particularly the solubility limit of Nb in αZr which appears equal to 0.37 ± 0.05 wt%, which is much lower than the usually assumed value of 0.6 wt%. In the investigated range, the tin content has no significant influence on the phase diagram, but tin degrades the corrosion resistance in autoclave at 773 and 688 K. Last, the steam corrosion behaviour shows a very strong dependence on the exact process for the lowest SPP-VF, and very high weight gain can be obtained. The air oxidation is much less affected by this parameter.