Abstract
Incidents of intergranular stress corrosion cracking (IGSCC) have been readily observed in boiling water reactors (BWRs) for many years. The electrochemical corrosion potential (ECP) is a major indicator for IGSCC susceptibility of stainless steel components in 288 °C pure water. In the early 1980s, the technology of hydrogen water chemistry (HWC) was developed to mitigate the IGSCC problems in BWRs. However, HWC has side effects of exposing operators to elevated level of radiation and high hydrogen cost. To reduce these side effects, a novel technique of zirconium oxide (ZrO2) coating that eventually requires no hydrogen addition has been proposed.In this study, electrochemical polarization analyses and ECP response monitoring were conducted to investigate the impact on ZrO2 treated specimens in simulated BWR environments. Prior to the electrochemical tests all specimens were thermally sensitized and pre-oxidized in high temperature water containing either 300 ppb O2 or 50 ppb H2. Afterwards, the specimens were treated with 100 nm ZrO2 by hydrothermal deposition at 90 °C and 150 °C for one week. The morphologies of the specimens were examined by scanning electron microscopy and energy dispersive X-ray spectroscopy. The hematite (α-Fe2O3) and magnetite (Fe3O4) structures of oxides were identified by Raman spectrum.The objective of this study was to evaluate the effectiveness of ZrO2 treatment on specimens with different oxide structures. Differences between the sizes and the packing densities of the oxide particles produced from different water chemistry condition could be clearly observed on the SEM images. Based on observation, the coating density of ZrO2 on the oxides with a hematite structure was greater than on those with a magnetite structure. From the electrochemical potentiodynamic polarization results, we observed lower electrochemical corrosion potentials, corrosion current densities, and exchange current densities on the treated specimens than on the untreated ones in high temperature water with either dissolved oxygen or dissolved hydrogen only. Furthermore, the ECPs and surface morphologies of the ZrO2 treated specimens showed almost no changes after ten days of immersion in 288 °C water. The overall results indicated that the ZrO2 treatment could effectively reduce the corrosion rate of Type 304 stainless steel in simulated BWR environments.