Study On The Surface Modification Of SUS430 Metal Connector And Its Oxidation Behavior In Anode And Cathode Atmosphere

Fe-Cr alloy is the most suitable material for SOFC batteries due to its excellent expansion coefficient,suitable oxidation resistance,good electrical and thermal conductivity,excellent comprehensive mechanical properties,and low cost.However,in the long-term service environment,especially the different oxygen partial pressures on the cathode and anode ends,the commonly used commercial 430 alloy exhibits insufficient oxidation resistance,resulting in thicker,peeling and increased surface resistivity of the oxide film layer on the alloy surface.Other problems have severely restricted the development and application of this alloy material.Therefore,in view of the above problems of Fe based alloy,SUS430 alloy is selected as the research object,and its oxidation resistance and surface specific resistance are optimized by surface modification technology.First,the surface of the alloy was modified by implanting different doses of rare earth Y on the surface.The oxidation kinetics and the change of specific resistance of SUS430 and three kinds of modified samples with different dosages in air and moist atmosphere of 5vol%H₂+95vol%N₂ at 800?were studied.The oxidation mechanism in the two atmospheres and the action mechanism of rare earth Y in the oxidation resistance and specific resistance of the alloy were analyzed.The main factors influencing the surface specific resistance are revealed,which provides a theoretical basis for the further modification of the surface specific resistance.Subsequently,combined with the defects in the modification of rare earth Y and the theoretical basis obtained,the Mn_xCo_3-xO₄ spinel anti-oxidation coating with good conductivity was prepared by sol-gel dip pulling method.By controlling the ratio x of Mn/Co,the influence of different ratio x of Mn/Co on the thermal expansion coefficient and phase structure of Mn_xCo_3-xO₄?x=0.4,0.8,1.0,1.2?was analyzed.The long-period oxidation kinetics and the change law of surface specific resistance of the coating under the atmosphere of cathode and anode were discussed.The conclusions are as follows:?1?Rare earth Y itself has a strong O-affinity.The alloy implanted with rare earth Y initially formed a small amount of rare earth oxide at the grain boundary,and formed a rich Y buffer layer with a thickness of about 50 nm on the surface.However,Y₂O₃ particles in the grains can promote the rapid diffusion of oxides along the grain boundary to the grains,resulting in a uniform chromium rich oxide layer on the surface of the alloy,which can promote the initial oxidation rate of the alloy.At the same time,the wedge-shaped oxide rich in y along the grain boundary forms the pinning effect on the external oxide film,and then under the combined action of the buffer layer and the pinning effect on the grain boundary of the oxide film,the adhesion and anti peeling performance of the oxide film are improved.?2?The results of oxidation kinetics and surface specific resistance analysis show that the water vapor contained in the anode atmosphere and the reducing gas react with the oxide film,resulting in the thicker oxide film and the formation of Mn O and Fe O phase with worse conductivity,which makes the oxidation weight gain and surface specific resistance in the anode atmosphere larger than that in the cathode atmosphere.At the same time,it is found that the conductivity is not greatly improved after adding rare earth Y,because the high resistivity of the oxide itself has a far greater effect on the specific resistance than the thickness of the oxide film.Therefore,if we want to further improve the surface specific resistance of the alloy,we must change the phase composition of the oxide film on the surface of the alloy by a method,or coating a high temperature resistant coating material with good conductivity.?3?Mn_xCo_3-xO₄?x=0.4,0.8,1.0,1.2?spinel coatings with controllable phase composition were successfully synthesized on the surface of SUS430 alloy by sol gel dip method.The coating has the advantages of fine grain size and uniform density.It is also found that with the increase of Mn/Co ratio x,the crystallinity and stability of Mn_xCo_3-xO₄ become better and better.Among them,the linear expansion coefficient(11.98×10^-6 K^-1)of Mn_1.2Co_1.8O₄ coating is very different from that of SUS430(?12×10^-6 K^-1),which ensures the good adhesion between the coating and the substrate.?4?The results of oxidation kinetics and surface specific resistance analysis show that Mn_xCo_3-xO₄ spinel coating can effectively prevent the migration of cations and the inward diffusion of oxygen ions.The oxidation resistance and conductivity of spinel coating on the alloy surface are significantly improved.In particular,Mn_1.2Co_1.8O₄ spinel coating shows the best oxidation resistance,conductivity and cation migration inhibition in both negative and anode atmosphere.