Optimisation of the water chemistry regarding stability of the oxide film on alloy 690 during simulated hot functional testing of a pressurised water reactor

In the present paper, an attempt is made to quantify the effect of water chemistry in simulated HFT conditions on the electrical and electrochemical properties, and thus stability and corrosion resistance, of the passive film formed on a PWR steam generator material, namely, Alloy 690. For the purpose, a quantitative procedure to interpret simultaneously electrochemical impedance and surface analytical results is used to obtain first-hand estimates of the interfacial rate constants of oxidation of individual alloy constituents (Ni, Cr and Fe), the diffusion coefficient of oxygen vacancies, as well as field strength in the growing oxide. The main conclusion that can be drawn from both experimental and calculational results is that increasing LiOH concentration in the simulated HFT water leads to higher dissolution rates, higher metal oxidation and transport rates through the oxide, or in other words, the formed oxide is found to be the more defective, the higher the concentration of Li in the electrolyte is. On the other hand, boron added as H3BO3 was found to have a beneficial effect that counter-acts the accelerating influence of Li. On the basis of our experiments and their interpretation, a HFT water chemistry with low to intermediate Li contents and boron addition seems to produce films that exhibit better electrochemical stability and thus corrosion resistance.