High-Temperature Oxidation Behavior Of Electroplating Ni,Co,and Mn Coatings On The Stainless Steel Surface

Solid oxide fuel cell is an important type of fuel cell due to its high efficiency,fuel flexibility and wide application.However,the single cell output power of SOFC is not sufficient to meet the requirement,and it needs a connector to connect multiple single cells and form cell stack so as to enhance the output power.With constant development of SOFC,the metal material is possible to be used as a connector.Moreover,the ferrite stainless steel(FSS)is widely applied owing to low price,easy to machine,matching of thermal expansion with the cell components.However,Cr2O3 is developed on the surface of FSS in cathode environment of SOFC,which results in rapid growth of contact resistance.Moreover,the volatilization of Cr results in cathode Cr-poisoning and degradation of the cell performance.At present,the main method solving such problem is to fabricate coating on the FSS connector.To realize the purpose of protecting FSS connector,Ni,Co and Mn coatings were deposited on SUS 430 by electroplating method.For Ni,Co and Mn coated steels,long-term oxidation at 800℃ and short-term oxidation at 700℃ and 900℃ were conducted in air.The appearance and structure of the samples were characterized by XRD,SEM and EDS.For long-term oxidation samples at 800℃,the electrical property of the surface oxide scale was tested.By analyzing the oxidation kinetics,appearance and structure of samples to study on high-temperature oxidation behavior of Ni,Co and Mn coated samples.In addition,the inhibition of Cr volatilization and elelctrical performance were evaluated.The results were as follows:Ni,Co and Mn coatings were prepared on the FSS using electroplating method.The coating was about 6 μm thick,and it was uniform and well bonded to the steel substate.For long-term oxidation at 800℃ in air,the weight gain of coated steels kept stable after one week.The surface oxide scale of Ni coated steel had compact structure and good adhesion.The outer layer of the oxide was NiFe2O4 spinel,the middle layer was NiO and the inner layer was Cr2O3.During oxidation,Ni diffused toward the steel substrate and subsequently formed NiFe phase.The surface oxide scale of Co coated had exfoliated.The outer layer of the oxide was Co3O4 spinel with the trace of Fe element.The inner layer was Cr2O3.The surface oxide scale of Mn coated had bulges.The outer layer of the oxide was Mn2O3.The middle layer was oxide of Fe and Mn.The inner layer was still Cr2O3.The test of long-term oxidation showed that the three coatings had effect on the inhibition of Cr volatilization.Hereinto,the surface oxide scale of Ni coated steel had the best structure stability.For 5 weeks oxidation at 800℃ of Ni,Co and Mn coatings,the scale ASR were 39.312 mΩ·cm2、20.736 mΩ·cm2 and 53.352 mΩ·cm2.They all met the requirement of the connector electrical performance.During the short-term oxidation for 100 hours at 700℃ in air,the weight gain of coated steels kept stable after 10 hours.During the short-term oxidation for 50 hours at 900℃ in air,the weight gain of coated steels kept rising.The surface had different degree of exfoliation and bulges.The anti-oxidation of Ni coatings was relatively better.The surface oxide scale had compact structure and good adhesion.The inner layer was Cr2O3.The outer layers of the oxide were NiFe2O4 spinel and NiO.During the oxidation,both the surface on Co and Mn coatings exfoliate badly.The structure of surface oxide scale had damaged.