Effects of Flow Rate on Dissolution of Monocrystal Alumina and Monocrystal Yttria-Stabilized Zirconia in High-Temperature Pure Water

The effects of flow rate on the dissolution of monocrystal alumina with an impurity of 0.003 wt% or monocrystal yttria-stabilized zirconia (YSZ) with an impurity of 0.08 wt% in high-temperature pure water were studied under the following conditions: temperature = 280 C; inlet electrical conductivity <10 mu S.m(-1); flow rate = 0.002-2.6m.s(-1); and dissolved oxygen concentration <10 mu g.kg(-1). Alumina or YSZ is used as an inorganic insulant material from which the reference electrode is constructed to measure the electrochemical corrosion potential of structural materials used in boiling water reactors (BWRs). The weights per unit area of alumina specimens linearly decreased for 200h. The apparent dissolution rate for a flow rate of 2.6m.s(-1) (-0.80 g.m(-2).h(-1)) was about 10 times as large as that for a flow rate of 0.002 m.s(-1) (-0.078 g-m(-2).h(-1)). Assuming the equation, 1/R-r = 1/k(s) + 1/k(d), where k(d) = alpha . K (R-r, apparent dissolution rate (g.m(-2).h(-1)); k(s), dissolution rate controlled by surface reaction (g-m(-2).h(-1)); k(d), dissolution rate controlled by diffusion of aluminum ion (g.m(-2).h(-1)); K, mass transfer coefficient (m.s(-1)); alpha, constant (g.s.m(-3).h(-1))), dissolution rates controlled by surface reaction k(s) and constant alpha were calculated as -2.6 g.m(-2).h(-1), -633 g.s.m(-3).h(-1), respectively. The dissolution rate was thought to be controlled by the diffusion of aluminum from the surfaces to bulk water under the experimental conditions. The weight change was within the measurement limit for each flow rate for YSZ specimens. The specimens did not flake nor crack. The apparent dissolution rate was calculated as <3.9 x 10(-3) g.m(-2).h(-1) from the measurement limit, which was less than one-thousandth of that for alumina for the flow rate of 2.6 m-s(-1). From these results, YSZ was judged as a more desirable material from the viewpoint of stability under BWR conditions.