Studies On Molten Carbonate Salt Corrosion Of SS316L And FeAl Coating In Concentrated Solar Power Plants

With the increasing depletion of traditional energy,people keep exploring new energy for sustainable development.Concentrated solar power(CSP)technology is a clean and sustainable energy with broad development prospects.High temperature thermal energy storage(TES)system is a very critical module in CSP system,and molten salts are often used as TES and heat transfer media to improve the thermo-electric conversion efficiency.At present,the molten ternary eutectic LiNaK carbonate salt is one of the most potential TES and heat transfer media.However,there are severe corrosion problems of structural materials caused by molten carbonate salt at high temperatures.And the preparation of corrosion-resistant coatings on the surfaces of structural materials is an effective method for both performance and equipment cost.In this work,316L stainless steel,which is widely used as a structural material in industry,was selected as the research object.A series of immersion experiments were carried out on SS316L at 600℃ and 700℃ to investigate the corrosion behavior and potential corrosion mechanism of the samples in molten carbonate salt.The results demonstrated that the corrosion behaviors of SS316L in molten LiNaK carbonate salt at two temperatures were similar,and both followed a linear law,and the corrosion rate increased with the temperature.In the initial stage of corrosion,the alloying elements in SS316L and O2-ions in the molten salt underwent selective oxidation to form Fe3O4,Fe2O3 and Cr2O3,and then these oxides underwent lithiation reaction with highly active Li2O in the melt With the prolongation of corrosion time,the surface corrosion products gradually formed a double-layer structure,and the thickness of the corrosion layer gradually increased.When the thickness of the corrosion layer increased to a certain value,the difference in thermal expansion coefficient between the inner and outer corrosion layers caused the large area spalling of outer corrosion layer,resulting in further corrosion of the inner corrosion layer exposed to the molten salt.A new double-layer structure was gradually formed on the sample surface,and the next round of corrosion cycle started.The FeAl coating was prepared on the surface of SS316L by cold spraying combined with thermal diffusion technology.The pure Al coating was firstly prepared by cold spraying,and then the sample was heat-treated at 800℃,900℃and 1000℃ for different durations.A diffusion layer with a multi-layer structure was formed on the sample surface,and the thickness of the diffusion layer increased with the heat treatment temperature and the holding time.During the thermal diffusion treatment,Fe and Al firstly formed FeAl3 phase.With the prolongation of heat treatment time,FeAl3 phase gradually transformed into Fe-Al intermetallic compounds with low Al content,and finally formed a single FeAl phase.The formation of different Fe-Al intermetallic compounds was analyzed from the perspective of thermodynamics,and the calculated results are in good agreement with the experimental results.The sample formed a dense,crack-free FeAl coating after heat treatment at 1000℃ for 2.5 h.The microhardness of the FeAl coating is as high as 661 HV0.3,which is 5 times that of the substrate.The wear rate of the coating sample is 1.1×10-5 mm3/m,which is only one thirty-sixth of that of the substrate.Long-term corrosion tests were carried out on the prepared coated-FeAl samples in molten LiNaK carbonate salt at 600℃ and 700℃,respectively.The results showed that the weight gains of the corroded samples at both temperatures were very low,being 0.095 mg/cm2 after corrosion at 600℃ for 1000 h and 0.22 mg/cm2 after corrosion at 700℃ for 1000 h,respectively,which were much lower than those of the uncoated SS316L samples under the same conditions.This can be attributed to the formation of a continuous and dense LiAlO2 passivation film on the surface of the coating,which effectively prevented the continuous intrusion of molten salt into the coating.The present study found that the LiAlO2 passivation film transformed from a phase to y phase in the corrosion process,but the phase transformation hardly affected the corrosion resistance of FeAl coating under the experimental conditions.The Al content in the FeAl layer was still as high as 45.7 at.%after corrosion at 700℃ for 1 000 h.The effect of chloride ions on the corrosion behavior of SS316L and FeAl coating in molten carbonate salt was studied by adding 1 wt.%NaCl to molten carbonate salt to simulate the actual service environment in industry.The results showed that the chloride ions in the molten salt can be oxidized and formed chlorine,which penetrated the oxide layers and reacted with the matrix to form highly volatile metal chlorides.These metal chlorides further diffused to the surface of the oxide layers to form metal oxides and the chloride ions were released again,thereby forming a closed-loop process of Cl autocatalysis.The corrosion kinetic curve of SS316L in molten LiNaK-Cl carbonate salt was steeper than that in molten LiNaK carbonate salt,indicating that the addition of Cl accelerated the corrosion rate of SS316L.Obvious holes and transverse cracks appeared in the inner oxide layer of SS316L under the action of chlorides,and wrinkling,cracking and peeling appeared on the sample surface layer,which finally led to the loss of the protective barrier of the substrate and accelerated the corrosion process.The FeAl coating prepared by cold spray combined with thermal diffusion still exhibited excellent corrosion resistance in molten LiNaK-Cl carbonate salt due to the faster formation of continuous and dense protective LiAlO2 passivation film on the coating surface under the catalysis of chloride,which effectively blocked the further corrosion attack by the molten salt.