Hydrothermal treatments of TiO2 on Type 304 stainless steels for corrosion mitigation in high temperature pure water
By Yeh, Tsung-Kuang; Huang, Yu-Jen; Wang, Mei-Ya & Tsai, Chuen-Horng
Published in Nuclear Engineering and Design
2013
Abstract
Incidents of intergranular stress corrosion cracking (IGSCC) have occurred in boiling water reactors (BWRs) for decades. The electrochemical corrosion potential (ECP) is currently a major indicator for the IGSCC susceptibility of stainless steel (SS) components in BWR environments. This study proposes a novel technique of titanium dioxide (TiO2) treatment to mitigate the IGSCC problems in BWRs that could eventually lead to a lower demand of dissolved hydrogen for hydrogen water chemistry (HWC). Electrochemical polarization analyses and ECP measurements were conducted to investigate the impact of ultraviolet (UV) radiation on the electrochemical behavior of oxygen and TiO2 treated specimens in 288 °C pure water. Prior to the electrochemical tests, all specimens were thermally sensitized and pre-oxidized in high temperature pure water containing 300 ppb dissolved O2. Afterwards, 38 nm TiO2 nanoparticles were deposited on the specimens by hydrothermal deposition at 150 °C for 96 h. The surface morphologies of the specimens were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and laser Raman spectra (LRS). SEM and EDX results showed that the distribution of TiO2 deposited on the oxides with both hematite (α-Fe2O3) and magnetite (Fe3O4) structures were not uniform and continuous. LRS results showed that the TiO2 particles on the treated specimens had an anatase-type structure. In addition, the ECPs of the TiO2 treated specimens with UV radiation were 100 mV lower than those without UV in high temperature water containing various levels of dissolved O2. The results of electrochemical potentiodynamic polarization analysis revealed that the corrosion current densities of the treated specimens and the exchange current densities of the O2 reduction reactions were comparatively lower in the presence of UV radiation. Without UV radiation, however, no significant differences were observed between the TiO2 treated and untreated specimens. These results indicate that the TiO2 treatment in combination with UV radiation would effectively reduce the corrosion rate of Type 304 stainless steels in high temperature oxygenated environments.