The effectiveness of titanium additions on mitigating flow-accelerated corrosion under PWR feedwater conditions

Titanium as an additive in CANDU registered primary coolant has been shown to reduce the rate of flow-accelerated corrosion (FAC) of the carbon steel feeder pipes that direct the coolant from the core to the steam generators. The mechanism apparently depends upon the incorporation of titanium (Ti) into the magnetite oxide that normally forms on the steel in high-temperature water. Localized corrosion in steam generator crevices has also been mitigated by additions of Ti, apparently by a similar mechanism. It would seem possible, therefore, that Ti as an additive would mitigate FAC of carbon steel piping in pressurized-water reactor (PWR) feedwater systems. To explore this concept, an experimental program sponsored by the Electric Power Research Institute has utilized a high-temperature water loop to evaluate the effectiveness of adding Ti-based compounds to simulated PWR feedwater. On-line tubular probes measured corrosion directly at system conditions by monitoring the FAC-induced thinning of the tube walls with an electrical resistance technique. In the program, three compounds were tested in the loop, each at a concentration producing several μg . kg-1 equivalent of elemental Ti at the probes. Coolant chemistry conditions were: ammoniated water at pH25C of 9.2, hydrazine at 40 μg . kg-1 (ppb) and temperature of 140 C. For each additive, plots of the probe internal radius against time directly showed the progression of FAC, and comparison with periods before the addition demonstrated any mitigating effect of the Ti. Indications of the effects of coolant velocity on FAC during Ti addition were obtained by varying loop pumping rates. Destructive examination of probes after exposure revealed any change in surface appearance and constitution brought about by the additive. The paper describes the experiments and results and assesses the possibility of using Ti compounds for mitigating FAC in feedwater systems in operating PWRs. (orig.)