Photoelectrochemical investigation of radiation-enhanced shadow corrosion phenomenon

The photoelectrochemical investigation was performed by measuring the photoelectrochemical behavior of various alloys such as Zircaloy 2, 304 stainless steel (SS) and Alloy X-750 in 0.01M Na2SO4 at 25°C or in high purity water at 300°C under intense ultraviolet (UV) illumination. UV was selected because its photon energy (∼5 eV) is similar to the energy gap of the electron-hole pairs in zirconium oxide. The data show that the photoexcitation of ZrO2 caused the corrosion potential to shift in the anodic direction and produced anodic photocurrents under the oxidizing water chemistry condition when Zircaloy 2 electrode was galvanically coupled with dissimilar electrodes, such as Alloy X-750, 304 SS, or Pt, causing accelerated corrosion of Zircaloy. It is thus postulated that the photoelectrochemical enhancement of surface reaction kinetics at the ZrO2 surface may be responsible for the radiation-enhanced corrosion on Zircaloy (i.e., shadow corrosion). In addition, it is revealed that no significant change in galvanic current between Zircaloy 2 and other dissimilar electrodes was measured in high temperature water containing only hydrogen that resulted in similar ECPs. These data thus clearly provide an explanation for why the radiation-enhanced corrosion of Zircaloy only occurs in BWRs but not PWRs. Based on galvanic corrosion, impedance, and transmission electron microscopy analysis, it is proposed that the defect structure of equiaxed grain layers may be responsible for the photoelectrochemical response of ZrO2 in high temperature water. The mechanism of shadow corrosion is still highly debated but it appears to be similar to a process of galvanic corrosion in connection with sufficiently conducting Zr oxide structure. The galvanic corrosion data suggests that a Zr coating on the fuel assembly spacer (Alloy X-750 or 304 SS) may mitigate the shadow corrosion in BWRs. Copyright © 2010 by ASTM International.