The steam generator heat transfer tube is the heat exchange hub of the primary and secondary loops of the pressurized water reactor (PWR) nuclear power plant. The corrosion resistance of its material affects the service life of the PWR nuclear power plant. In view of this, the work aims to study the corrosion resistance of 690-NiCr coated tube under normal and abnormal conditions, and reveal the effect of temperature and pH on the formation process of oxide film of 690-NiCr coated tube. Ni-Cr coating was prepared by air plasma spraying (APS) technology, and the real working condition of pressurized water reactor (PWR) was simulated by autoclave equipment of water circulation system. The experimental schemes of secondary loop water environment (285 ℃), 320 ℃ deviation from working condition water environment, subcritical water environment (360 ℃) and different pH values (pH=7, pH=9.8, pH=10.6) were designed. The morphology, composition, element content and corrosion mechanism of the oxide film were analyzed by SEM, EDS and XRD.
In the secondary loop water environment, the surface oxide shapes of the coated tubes in three pH water environments were similar, and the surface was covered with granular oxides accompanied by needle-like oxides. In the water environment of 320 °C deviating from the working condition, the coated tubes formed irregular granular outer oxide film and dense inner oxide film under three pH conditions. The outer oxide film was rich in Cr, followed by Ni, and the inner oxide film was the opposite. In the subcritical water environment, the composition of the oxide film formed in the inner and outer layers of the coated tube was less different, and both were rich in Cr and Ni elements. The difference was that the oxygen content was lower when pH was 10.6. The coating tube had three layers of oxide film in the secondary water environment, and the outer oxide film was mainly composed of Cr(OH)3, Ni(OH)2 and NiO. The intermediate layer oxide film was mainly composed of Ni, a small amount of NiO and Fe oxides. The inner oxide film was composed of Cr2O3. As the temperature increased, the oxide film on the surface of the coated tube thickened, the outer oxide particles coarsened, and the Ni and Fe ions incorporated into the inner oxide layer reduced the density of the Cr2O3 film. As a result, the inner oxide layer was damaged and became less protective, leading to an accelerated corrosion rate of the substrate. The surface of the Inconel 690-NiCr coated tube was irregular and uneven, making it difficult to calculate the exact surface area. Therefore, the corrosion rate was evaluated per unit height. As shown in the corrosion tests under different water environments, the samples generally exhibited initial weight loss, primarily due to the detachment of loosely bonded flaky coating materials after immersion for about 100 h. With prolonged immersion, the rates of oxide formation and dissolution gradually approached equilibrium. In pure water (pH=7.0), the corrosion mass change reversed after 300 h, whereas at pH=9.8, the reversal occurred after 600 h and at pH=10.6, weight loss proceeded at a relatively constant rate. After 1 000 h in all environments, the samples showed net weight gain, indicating the formation of a more compact and adherent oxide film. This dense oxide layer inhibited further dissolution while promoting continued deposition of oxides and hydroxides on the coating surface.
Key words
steam generator /
Inconel690-NiCr coated tube /
high temperature caustic solution /
oxide film composition /
corrosion mechanism
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Funding
China Postdoctoral Fund General Project (2024M750723)
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