| Exploring tritium permeation barriers with high permeation reduction factor(PRF)by an industrial scale process is crucial for solving tritium self-sufficiency and tritium safety issues in future commercial fusion reactors.The hot-dipping aluminizing(HDA)method is considered to be suitable for the preparation of aluminized coatings on the inner walls of vessels and pipes.Based on this point,combined with oxidation technology,the preparation of Fe-Al/Al2O3 coatings can be achieved.However,recent research shows that the coating prepared by this method generally exhibits a large number of pores and cracks in the Fe-Al alloy layer,as well as a loose and porous oxide layer,which makes it unable to act as an effective tritium barrier.In this paper,China Low Activation Martensitic steel(CLAM)which is one of the fusion structural materials is taken as the substrate,and the growth of HDA coating and the formation of holes and cracks during thermal diffusion or oxidation process were investigated.Through the optimized the oxidation strategy,A dense Fe-Al/Al2O3 as tritium permeation barrier on CLAM steel was prepared under air atmosphere by using FeAl alloy topcoat oxidation strategy.The main results are as follows:1.An aluminized coating was prepared on CLAM steel by HDA followed by subsequent thermal diffusion treatment.By comparing the microstructure of HDA coatings on CLAM steel under different HDA processes,it was found that there no cracks were present in the coating obtained at 720℃ for 10 s,and the thickness of the Fe-Al layer was about 10 μm.Subsequently,after the thermal diffusion treatment,a large number of cracks were observed in the aluminide coating on CLAM steel.By comparing the changes in microstructure and mechanical properties of CLAM steel and Fe-1Cr substrate before and after aluminizing treatment,it was found that the thickness of the newly obtained Fe-Al layers(FeAl2,FeAl,Fe3Al,α-Fe(Al))was 77.4 μm,after aluminizing treatment under 700℃ for 100 hours.While,the thickness of newly obtained Fe-Al layers in Fe-1Cr sample was up to 136.7 μm.This phenomenon indicated that the presence of Cr element in the CLAM steel substrate inhibited the diffusion of Fe and Al elements during the aluminizing process.Meanwhile,as a result of the low solubility of Cr in Fe2Al5,the excess Cr would precipitate in the form of Al8Cr5,which may affect the mechanical properties of coating.After aluminizing treatment at 700℃ for 10 hours,the Vickers hardness of the aluminide coating in CLAM steel was 994.7 HV,which was 66.7 HV higher than that of the aluminide coating in Fe-1Cr sample.Therefore,although the Cr element in the substrate could suppress the formation of pores(Kirkendall effect)caused by diffusion differences between Fe and Al elements during the aluminizing process,it could also hinder the transition from high aluminum content brittle phase to low aluminum content ductile phase,which was not conducive to suppressing the generation of cracks in the aluminide coating.Therefore,it is necessary to further control the growth of Fe2Al5 phase during the aluminizing process.2.In order to suppress the formation of holes and cracks in aluminide coating that prepared by the traditional oxidation strategy,in this study,the Al topcoat of HDA sample was removed by mechanical polishing process.This treatment limits the growth of Fe2Al5 phase during the preparation process of aluminide coatings.Subsequently,the FeAl topcoat samples were obtained by thermal diffusion treatment and then subjected to subsequent oxidation treatment,which is also called as the FeAl alloy topcoat oxidation strategy.By comparing the structure and composition of the coating under the two oxidation strategies through SEM,XPS,TEM,and other technologies,it was observed that the oxide layer prepared by the FeAl alloy topcoat oxidation strategy was denser,and had fewer obvious defects,such as holes and cracks,in the alloy layer.Additionally,the oxide formed on the surface of the FeAl alloy topcoat in air is composed only of γ-Al2O3.XPS analysis was conducted on samples prepared at different oxidation times to investigate the formation of oxide layer on the surface of the FeAl alloy topcoat in air.It was found that although FexOy and CrxOy were generated during the initial stage of oxidation(≤1 h).However,as the oxidation time extended to 4 hours,due to the lower formation energy of Al2O3,the oxide layer finally transformed into an Al2O3 layer.This means that the FeAl alloy layer can be oxidized selectively under formation of an Al2O3 layer in atmosphere.3.This study compared the effects of different preparation processes on the coating performance by testing the deuterium permeability,mechanical properties,and corrosion resistance of the coating samples obtained.The results showed that the coating prepared using the FeAl topcoat oxidation strategy had stronger adhesion to the substrate,remained intact after 100 cycles of thermal shock at room temperature to 550℃,and had a PRF value of 2.39×103 at 550℃,which can meet the requirement for coatings used in breeding blanket modules(PRF>103).Furthermore,the coating prepared by oxidation at 750℃ for 4 hours under the FeAl alloy layer oxidation strategy possessed the best corrosion resistance among the tested samples,with a maximum polarization resistance of 8.06×103 Ω·cm2.Meanwhile,the Al topcoat of HDA sample was selectively removed by electrochemical etching.The optimized preparation method was used to obtain a dense Fe-Al/Al2O3 coating with low defects on the surface of CLAM steel tube samples,and the e PRF value of the obtained coating was 1.48×103 at 650℃,indicating that the coating of tube sample prepared by this method may had good deuterium permeation resistance performance.In summary,A dense Fe-Al/Al2O3 as tritium permeation barrier on a CLAM steel tube sample was prepared under an air atmosphere by an optimized preparation method that consisted of HDA,electrochemical etching and FeAl alloy topcoat oxidation strategy.The coating prepared by this method has excellent PRF value.This study provided a feasible technical route for the engineering preparation of tritium permeation barrier on fusion reactor components and may promote the application of aluminide coatings in other hydrogen isotope-related fields. |
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