Zinc Corrosion in Borated, Buffered Post-LOCA Solutions
This current research provides a broad assessment of recently evaluated zinc corrosion in simulated non-buffered and buffered post-LOCA solution as a function of pH, temperature, velocity, boron concentration and surface area. Corrosion rates obtained can be used for further zinc borate formation analysis or to plan further zinc corrosion studies. Experiments are conducted utilizing a three-electrode electrochemical system in a glass cell. Aqueous solutions used in testing simulate reactor containment building (RCB) pool water under various LOCA scenarios that are representative of US PWR RCB pool water: boric acid only, boric acid with trisodium phosphate, boric acid with sodium hydroxide, and boric acid with sodium tetraborate. The specimens used in this work are machined from 99.6% pure zinc rods that are used as a surrogate for the high purity zinc layer of galvanized steel. Prior to the experiment, the specimens are first polished and then left exposed to the atmosphere for approximately one day at room temperature. Prior to each test, the zinc specimen are sequentially rinsed with deionized water and acetone, and then dried by evaporation in air at room temperature. Prepared samples are then mounted in the glass cell test apparatus and rotated on a spindle at a desired velocity for a specified duration. Instantaneous corrosion rate as a function of time and total zinc mass corroded, which may either be released into solution or incorporated into surface scale, is determined by electrochemical impedance spectroscopy (EIS) analysis. Corrosion rates after 24 hours of immersion are determined from a full potential dynamic scan and from weight loss measurements of descaled post-test samples. The solution zinc concentration resulting from zinc corrosion is determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Preliminary results show that zinc corrosion with a zinc surface area-to-liquid volume ratio of 0.0003 m2/L in a borated, buffered solution (55 deg. C, pH 7.2) exposed to a velocity of 0.5 ft/s is between 4.9 X to 2.3 X less than that obtained by the Kyrk et al study, which examines a lower temperature, lower pH with a 5.5 X larger surface area-to-volume ratio. This initial finding suggests that the corrosion of zinc, which supplies a zinc source term to form zinc borates, may not be insignificant under conditions of interest (borated, buffered post-LOCA solutions). Further experiments are currently underway that evaluate the influence of pH, velocity, temperature, buffer type, boron concentration and surface area-to-volume ratio on zinc corrosion. Preliminary results suggest two of the three buffer types examined as conditions of interest to US PWRs likely produce zinc corrosion within the range observed by Kyrk et al. Current preliminary results indicate that buffer type, temperature, pH and velocity each have an impact on the zinc corrosion rate. Quantification of zinc mass lost to solution and incorporated into scale is currently underway, as is the characterization of velocity and boric acid effects on zinc corrosion.