Determination of Surface Oxide Compositions on Alloy 600 Using Nuclear Analytical Techniques^⋆(1)

The surface composition of oxides formed on Alloy 600 in aqueous solutions similar to those in the primary side of pressurized water reactor (PWR) heat exchangers has been measured as a function of potential using Rutherford backscattering spectroscopy (RBS) and proton inelastic scattering. Electropolished samples of Alloy 600 were exposed at several potentials to a solution of 0.18 M H₃BO₃ (2000 ppm B) with 0.21 mM LiOH (1.5 ppm Li) at 300 C for 450 h. The potentials relative to an internal hydrogen electrode ranged from 0 to 750 mV. RBS analysis showed little or no oxide formation on samples exposed at 0 mV. Above 0 mV the oxide layers formed increased with potential. The RBS also showed significant amounts of aluminum and silicon in the oxide, and the sum of the aluminum and silicon content appeared to maintain a fixed surface concentration independent of the oxide thickness. Boron and lithium concentrations were analyzed with proton inelastic scattering. No lithium above the minimum detectable limit of 0.05 μg/cm² was detected in any sample. The boron concentration was found to follow the thickness of the oxide. The silicon and aluminum concentrations, as determined by RBS and proton inelastic scattering, also followed the thickness of the oxide and maintained the same ratio as in the bulk alloy, indicating that this was the source of these two elements.