Corrosion Behavior Of Er₂O₃ Coating And Stainless Steel Substrate In Liaquid Lithium Study By Laser-induced Breakdown Spectroscopy

As a main tritium breeder,liquid lithium(or Li/Pb alloy)was applicated in blanket structure of fusion device.However,the highly corroison of liquid lithium under the high temperature,liquid lithium would short the life time of metal material in blanket structure.Hence,it is useful for researching the corrosion behavior of material and enhancement corrosion resistance in the field of fusion engineer.In our work,Laser-Induced Breakdown Spectroscopy(LIBS)was applied to characterizeelemental concentration distribution in the surface layer of Er₂O₃ coating and austenite stainless steel(type 316)substrate,which were corroded by liquid lithium.Base on the result,the corrosion behavior and mechanism of liquid lithium were deduced.The relationship of intensity of emission lines of corrosion medium lithium as a function of the number of ablation of laser pulse was obtained by LIBS in-depth profile analysis,and the penetration depth of Li was estimated in the surface layer of corroded sample.Owing to the depth of Li permeation is consistent with the thickness of corrosion layer which was calculated by weight loss,it could be employed as the criterion of liquid lithium corrosion resistance of material.The trends of characterization spectral line intensity of transition elements(such as Fe,Cr,Ni and Mn)versus the pulse number were also collected,yet that could not reflect the real variation and distribution of the element content directly.It was attributed the interference factors which induced intensity fluctuation of LIBS spectral signal corresponding to the surface layer of corroded specimen and concealed the information of real elemental distributions.As the inducements of LIBS signal fluctuation,the effect of surface roughness,microstructure and compositional variation of the corroded layer were analysis respectively.To further research on corrosion mechanism,it is necessary to carry out semi-quantitatively analysis for elemental distribution.The conditions of LIBS measurement were optimized,such as(1)selecting reasonable parameter settings;(2)implementation local thermodynamic equalization(LTE)analysis for laser-induced plasma and evaluation of matrix effect during the LIBS testing.The LTE conditions were satisfied during the testing,however,the influence of matrix effect could not be completely neglected.The internal standard method was applied to data treatment of LIBS spectra for reducing the effect of signal fluctuation and gaining component distribution in corroded layer.When it was confirmed effective of LIBS depth profile analysis on content characterization of corroded specimens by EDX testing results,the follow-up work of corrosion study were carried out by LIBS,such as:(1)The specimens of 316 SS with different corrosion times were characterized through depth profile analysis,and found out the elemental distributions have varied follow by the time.(2)Argon atmosphere was used for spectral signal enhancement,that would be realized the compositional characterization of low content or hard to be excited elements(Si,Mo,C)via LIBS measurement.(3)The corrosion resistance of welding joint and base metal were compared by the penetration depths of lithium which as the main criteria of corrosion degree.As a pre-work,the LIBS characterization of corrosion substrate laid the foundation for thestudy of resistance corrosion of Er₂O₃ coatings,while the major research works for coating included three parts,such as fabrication of coating samples,liquid lithium corrosion and components analysis for corroded coating by LIBS.Two processes of direct current(DC)magnetron sputtering were respectively carried out for deposition of Er₂O₃coating:(1)pre-sputtering a metallic Erbium layer on the substrate,and then oxidization it by heating in the air;(2)reactive sputtering.The Er₂O₃coatings with different topographic,structure and grain size were fabricated though variation the conditions of deposition,and the relationships between the conditions and the state of coating were discussed.The coating specimens were treated by liquid lithium corrosion,and then compared the surface morphology,crystal structure before and after corrosion,characterized the corrosion produces and deduced the process of the produces creating.LIBS depth profile analysis under argon atmosphere was utilized to characterize the elemental distributions and concentration variation trends from the boundary of Er₂O₃coating surface to the interface between the coating and the substrate.It is very sensitive of LIBS testing for variation of material on composition,structure and state of testing zone,while there is concentration gradient in the coating samples,therefore,it will make the LIBS component distribution characterization for the coating sample more complex than the corroded 316SS substrate specimens.As the standard specimens,pure Erbium,pure Er₂O₃and 316 SS were employed for optimization conditions of LIBS measurement.The material of oxide ceramic coating and metal substrate are different,hence the influence of matrix effect could not be neglected,and the internal standard method was still applied on spectral data treatment.The different internal standards were investigated,and found Ar emission lines as internal standard performed well on distinguishing the interface between the coating and the substrate.The elemental concentration distributions which were acquired by data treatment shown the diffusion trend of Ni from the substrate to the coating,and that made the distribution of Ni was different from the other transition elements of the substrate,such as Fe and Cr.From the results observation,the major mechanism of the coating failure during the liquid lithium corrosion is the peel-off of the Er₂O₃ layer from the substrate.Hence,enhancing the bond between the coating and the substrate became the priority for all involved problems for coating modifying.The experiment results confirmed that adding transition layer of Erbium,alternately deposition Erbium layer and Erbium oxide layer are effectively means to obtain thicker coating with better binding ability.