High-temperature Oxidation Resistance And Mechanism Of Al-Y Sol Coating OnAISI304 Steel Surface

Austenitic stainless steel has excellent corrosion and high-temperature oxidation resistance and is widely used in the petrochemical,marine,and nuclear industries.When the austenitic stainless steel workpiece is in service at higher temperature or for a long time,it often fails due to serious oxidation of the surface,and even leads to serious accidents.In this paper,AISI304 austenitic stainless steel was selected as the research object,and the effects of polyvinyl alcohol（PVA）modified Al sol and Al-Y sol coating on the high-temperature oxidation resistance of austenitic stainless steel were studied by cyclic oxidation and isothermal oxidation experiments.The oxidation layer was characterized by SEM,EDS,XRD,and TEM.The oxidation resistance mechanism of sol coating at high temperatures was revealed.When pure Al sol is coated on the surface of the sample,there are cracks between the coating and the substrate,and the film-base bonding is poor.When the additional amount of PVA is 2～6wt.%,the bonding ability of the sol coating and substrate is improved,and the thickness of the coating is increased.When the amount of PVA was8wt%,cracks appeared in the coating.After cyclic oxidation at 800℃,the oxidation weight of uncoated AISI304 steel is the largest,the oxide layer is loose and porous,and serious internal oxidation occurs.The oxide phase consists of limonite structure Fe₂O₃ and spinel structure M₃O₄（M indicates Fe and Cr）,and the oxidation depth can reach 50μm.The oxidation weight gain of the samples of PVA-Al Sol coating is significantly reduced,the oxide layer is continuous and dense,and the thickness of the oxide layer is significantly reduced,which inhibits internal oxidation.The oxide phase consists of corundum structure M₂O₃（M indicates Cr and Al）and spinel structure M₃O₄.When the additional amount of PVA is 4wt%,the coating has no peeling,less oxidation weight gain,and better high-temperature oxidation resistance,which is the best amount of PVA.The mechanism of resistance to high-temperature oxidation of PVA-Al sol is as follows:PVA addition improves the adhesion between coating and substrate,PVA-Al sol coating inhibits the formation of Fe₂O₃,and induces the formation of a double layer,dense and continuous high-temperature resistant oxidation layer:The outer layer is spinel structure M₃O₄ phase and the inner layer is corundum structure M₂O₃ phase,which effectively inhibits the internal diffusion of O and the outward transmission of Fe in the matrix,significantly improving the high-temperature oxidation resistance of AISI304 steel.After cyclic oxidation at 900℃,the oxidation spalling of uncoated AISI304 steel is serious,and the oxidation layer loses its protective effect on the substrate.The outer layer is loose porous limonite structure M₂O₃,and the inner layer is spinel structure M₃O₄.Serious internal oxidation occurs,resulting in the formation of Ni Cr₂O₄ spinel with larger particles,which reduces the bonding between the oxide layer and the substrate and leads to the peeling of the oxide layer.After the coating of Al-Y sol,the oxidation layer is continuous and dense,the oxidation phase is Y₃Al₅O₁₂,and the corundum structure is M₂O₃ and M₃O₄.The high-temperature oxidation resistance mechanism of Al-Y sol coating on AISI304 steel surface is as follows:the combination of Y and Al generates the Y₃Al₅O₁₂ phase with better thermal stability,and at the same time,the grain of oxide is refined,the density of the coating is increased,the bonding between the coating and the substrate is improved,and a dense and continuous three-layer high-temperature oxidation resistant layer is obtained:The outermost layer Y₃Al₅O₁₂,the middle layer spinel structure M₃O₄ and the inner layer corundum structure M₂O₃;The dense oxide layer effectively retards the inward diffusion of O and the outward transmission of Fe,inhibits the generation of Fe₂O₃,reduces the degree of internal oxidation and significantly improves the high-temperature oxidation resistance of AISI304 steel.