Passivity Degradation Mechanism Of Steam Generator Alloys Of PWRs In Sulfur-Containing Environment

The corrosion of steam generator in the second circuit crevices between tubes and support palates is one of the problems which affects the safe operation of nuclear power plant.This paper studies the corrosion electrochemical behavior in simulated crack under water chemistry of tubes made in Alloy 800,Alloy 690 and Alloy 600.We systematically studies passivity degradation mechanism of steam generator alloys in sulfur-containing environment PWRs under the influence of temperature,electrode potential and ratio of chloride and thiosulfate.The influence factors of the passivity degradation of alloy800 were studied by polarization curves,electrochemical impedance at different potentials and in-situ morphology of in the near neutral chloride and thiosulfate simulated sulfur-contaning crevice water chemistry.At high concentration ratios,there is a combined effect between these two ions on film degradation.However,at low concentration ratios,this combined effect is vanished,because there are no enough chloride ions to attack the passive film.The passive layer is broken down inevitably and thiosulfate reduce to form low valence sulfur at high anodic potential.The pit growth rate in chloride + thiosulfate solution is very fast due to the accumulation of thiosulfate and the subsequently reduction of thiosulfate within the pits to form sulfide and elemental sulfur.Through the advanced technologies of transmission electron microscopy and X-ray photoelectron spectroscopy,this paper finds that passive film is a multi-layer structure with spinel structure,metal oxide and hydroxide in the outer layer,and mainly metal oxides in the inner layer.The formation of sulphide on passive film surface was influenced by the temperature.Through high resolution transmission electron microscopy and selection diffraction,the dislocation and stacking fault structure can be found at interior of the oxide structure while some different direction twins form simultaneously.Meanwhile,at the interface of the inner layer oxide between outer oxide,there is a twin crystal boundary and a polycrystalline structure.Adjusting the ingredient of iron-nickel-chromium alloy can change the stability of anti-corrosion,the resistant to chloride of iron-base Alloy 800 is better than nickel-base Alloy 690 and Alloy600,nickel-base alloy portends that the higher anti-corrosion properties appear under the depravation of sulfur-containing crevice water chemistry effects.Increase the content of chromium in the alloy can improve the comprehensive corrosion resistance of passivation film.The performance of nickel in sulfur-contaning crevice water chemistry is relatively stable,and iron is the most unstable under the influence of sulfur-containing crevice water chemistry.