In situ X-ray diffraction study of the oxide formed on alloy 600 in borated and lithiated high-temperature water

In situ x-ray diffraction (XRD) measurements of the oxide film formed on Alloy 600 in borated and lithiated high-temperature water were conducted to demonstrate a capability to investigate rapid changes in oxide films during transient water chemistry conditions. In the presence of dissolved hydrogen (DH)= 30 cm³/kg [H₂O] and dissolved oxygen (DO) < 0.06 ppm, only spinel oxides were detected and no significant NiO peak was found even after 1,220 h exposure. By contrast, once the DO was increased to 8 ppm, a NiO peak grew rapidly. Within 7 h, the amount of NiO became comparable to that of spinel oxide. However, when DO was decreased again below 0.3 ppm and DH was increased up to 30 cm³/kg [H₂O], the ratio of NiO to spinel did not change during 10 h. Thus, the rate of dissolution of NiO in DH=30 cm³/kg water seemed to be lower than the growth rate of NiO in high DO conditions. After these in situ XRD measurements, additional ex situ scanning electron microscope and energy dispersive spectrometry observations on cross sections of the oxide film were also conducted to check the validity of the results of the in situ XRD measurements. It was demonstrated that in situ XRD measurement is suitable for investigating time-dependent phenomena during formation of oxide films. The real time investigation of time-dependent phenomena is the benefit of in situ measurement. The details of the measurement method and the results are reported in this paper.