| With the rapid development of nuclear power, the equipment manufacturing in localization has become the key factor for the independent innovation. Nuclear power equipment should have the characteristics of high reliability and long service life. Therefore, the quality assurance requirements for cleanness of nuclear power equipment during manufacturing are proposed. Low melting point metal contamination is one of the surface contaminations during manufacturing. Though the contact of low melting point metal is strongly prohibited by ASME and RCC-M international standards; the phenomena associated with the influence of low melting point metal on the material as well as the interactions between them can be considered as the issues requiring an in-depth study. Stainless steel is a widely used construction material in nuclear power plant. Passivation is an important characterization for stainless steel. However, the interaction of low melting point metal with stainless steel has not been studied till now. Therefore, the interaction of low melting point metal with stainless steel was investigated in the study. The effect of passivation on the interaction was analyzed and the interaction mechanism was revealed. The results could provide a theoretical basis and technical supports to the cleanness manufacturing for stainless steel equipment in nuclear power plant.In the study, taking low melting point metals Sn and Zn as research object, the interaction of low melting point metal with stainless steel after passivation free, exposure in air and passivation in nitric acid was investigated. The microstructure evolution during the interaction was analyzed, and then the assessment method for the effect of passivation on the interaction was proposed. Combined with the chromaticity inspection of passive film on stainless steel, the interaction mechanism was revealed. The effect of low melting point metal on corrosion property at room temperature and corrosion property in high temperature water of stainless steel was discussed. The main conclusions are as follows:(1) The interaction of low melting point metal with stainless steel was identified. Liquid Sn reacted with stainless steel by the formation of interfacial (Fe, Cr)Sn2layer with plated-type and block-type morphology, while there was no metallurgical intermetallics layer formation for the contact of solid Sn with stainless steel at220℃for54h. Liquid Zn and solid Zn reacted with stainless steel by the formation of interfacial columnar8phase (Fe, Cr)Zn10with tiny granule ζ phase (Fe, Cr)Zn13intermetallics.(2) The effect of passivation on the interaction between low melting point metals and stainless steel was clarified. For the interaction of low melting point metal with passivation304stainless steel, the native oxide film grown in air resulted in the non-uniform growth of Fe-Sn intermetallics layer. The chemical passive film grown in nitric acid prevented the formation of Fe-Sn layer. Passivation did not effectively hinder the metallurgical adherence of Zn with stainless steel. For the interaction of Sn with stainless steels, liquid Sn was prone to metallurgically interact with420stainless steel by the formation of (Fe, Cr)Sn2intermetallics. Passivation decreased the growth rate of the intermetallics but can not prevent the intermetallics formation. Fe-Sn intermetallics grown on passivation316stainless steel were non-uniform, and the thickness and coverage ratio of the intermetallics decreased with the increasing exposure time in air. The metallurgical interaction of2205stainless steel with liquid Sn was prevented by passivation in air and chemical passivation.(3) The relationship for pheanthroline colouration measurement—integrity of passive film grown on stainless steel—adherence degree of low melting point metal was revealed. The pheanthroline colouration measurement was employed to detect the stainless steel surface. Pheanthroline colouration value decreased with the increasing exposure time in air, but the pheanthroline solution did not change colour when it contacted with chemical passivation stainless steel. The integrity of passive film could be evaluated by the pheanthroline colouration measurement. Compared to the native film grown on stainless steel exposure air, the relative integrity passivation film grew on chemical passivation stainless steel. It is different for the adherence degree of Fe-Sn and Fe-Zn intermetallics on various passivation stainless steel. The coverage ratio factor was proposed to characterize the effect of passivation on the interaction between low melting point metal and stainless steel. The relationship of coverage ratio factor with pheanthroline colouration value was built. There was a linear relationship for the coverage ratio factor with the colouration value. The larger the colouration value of pheanthroline was, the lower the integrity of passive film was. The inhibition of passive film to the interaction between low melting point metal and stainless steel was decreased, so the coverage ratio of intermetallics increased. The integrity of passive film was an important factor for the interaction between low melting point metals with stainless steels. (4) The effect of Fe-Sn and Fe-Zn intermetallics adherence on corrosion property at room temperature and corrosion property in high temperature water of stainless steel was investigated. For corrosion property at room temperature, Fe-Sn intermetallics adherence accelerated the pitting formation on stainless steel, and the pittings increased with the increasing amount of Fe-Sn intermetallics. For corrosion property in high temperature water, Fe-Sn and Fe-Zn intermetallics adherence changed the oxide scale from the irregularly shaped (Fe, Cr)2O3and faced (Fe, Cr)3O4particles to SnO2, ZnO and the faced substrate oxides. By increasing the oxide time, the corrosion resistance of stainless steel decreased because the SnO2and ZnO particles gradually fell off. |
PDF Full Text Request