Corrosion of Alloy 690 process pot by sulfate containing high level radioactive waste at feed stage

Prolonged exposure of Alloy 690 process pot to sulfate containing high level radioactive waste leads to (a) depletion of Cr from the alloy, (b) intergranular attack and (c) building up of Cr2O3–Ni2O3–Fe2O3 mixed oxide surface layer containing Na and Cs sulfate precipitates. Time dependence of material loss from Alloy 690 is found to follow a linear relationship of the type Δw (material loss) = −7.05 + 0.05t. Corrosion rate calculated for 2400 h exposure is 3.66 mpy. Cr and Ni leach rates obtained for the same sample are 1.61 g m−2 d−1 and 2.52 g m−2 d−1, respectively. Ni leach rates followed a linear time dependence relationship of the type dNLNi/dt (leach rate) = −0.09 + 0.027t, whereas Cr leach rates obeyed a non-linear relationship of the type dNLCr/dt (leach rate) = 0.241 + 0.027t − 1.33 × 10−4t1/2.