Study On Corrosion Behaviors Of Inconel625 Alloy In Chloride Molten Salts

Chloride molten salts are potential thermal storage mass for solar thermal plant, compared with molten salt nitrate, they can work at a higher temperature, which has positive effects for improving efficiency of the system, however, chloride molten salts are more corrosive than that of nitrate molten salts, the corrosion mechanism of chloride molten salts is unclear and protection technique and means is imperfect. This dissertation, based on the background of solar thermal plants industry, studied the corrosion behaviors of Inconel 625 alloy in chloride molten salts, which focus on the effects of cations and adding of trace rare earth elements on the corrosion behaviors. And preliminary explored the influence of thermal diffusion coatingson reducing chloride molten salt corrosion rate. The results are expected to lay the foundation for development of a new type of the chloride molten salt used to the solar thermal power generation and selection for storage pipeline.The corrosion behavior of Inconel625 alloy in four different chloride molten salt(Na Cl, KCl,Mg Cl2, Ca Cl2) and their mixturesalt at 900℃ was studied, and the influence of cation on corrosion behavior of Inconel625 alloy was analyzed. The results showed that corrosion rate of Inconel625 in alkaline earth metal chloride molten salt( Mg Cl2, Ca Cl2)or the higher alkaline metal chloride salt content mixture moltenwas higher. Cations in chloride molten salt could affect molten salt’s alkalinity through thermal chemical properties(such as vapor pressure and hydroscopicity). High hydroscopicity and vapor pressure of alkaline earth metal chloride molten salt caused high molten salt alkalinity and rapid dissolution rate of oxides on alloy surface during corrosion process.The corrosion behaviors of Inconel625 alloy with thermal diffusion aluminide or chromate coating in six different chloride mixture molten salts were studied the influence of different diffusion layer on corrosion behavior was analyzed. The results indicated that thermal diffusion Cr and thermal diffusion Al coating could decrease corrosion rate of Inconel625 alloy in chloride molten salt, and thermal diffusion Al coating was better. The reason was that content of Al or Cr on alloy surface was increased by thermal diffusion treated. Al2O3 or Cr2O3 film would formed on the surfaces of the alloy during the corrosion process, but the oxide films were instable and un-protective in chloride molten salt so the decreasing of the corrosion rate of the thermal diffusion Cr and thermal diffusion Al coatings on the surface of Inconel625 alloy in chloride molten salt was not significant. Cr2O3 was more easily dissolved in the molten salt than Al2O3, in other words, stability of Cr2O3 was worse than Al2O3, thus the effect of thermal diffusion Al was better.The influence of adding trace rare earth elements on the corrosion behaviros of Inconel625 alloy in chloride molten salt was studied. Corrosion mechanism of chloride molten salt and effect of trace rare earth elements were analyzed by using electrochemical impedance spectroscopy(EIS). It turned out that the corrosion rate of Inconel625 alloy in mixed chloride molten salt at 900℃ decreased through adding Y2O3 in chloride molten salt. Trace rare earth elements changed electrochemical characteristics of Inconel625 alloy, the corrosion potential of the alloy was shifted to positive direction and corrosion pits were decreased, so that samples occurred larger polarization rate and corrosion current was reduced, thereby improving the corrosion resistance of alloy effectively. Only one capacitive arc could be found in Nyquist figure of molten salt electrochemical impedance, which indicated electrochemistry principle were not changed whether add trace rare earth elements in mixed molten salt or not, alloy corrosion was controlled by charge transfer in molten salt.