Study On Initial Oxide Phase Diagram Of The Fe-Ni-Al-O System And Preparation Of Alumina Film On 316L Stainless Steel

In recent years,hydrogen energy,nuclear energy and so on are pollution-free,and renewable secondary energy has been widely valued.However,hydrogen and deuterium and tritium required for nuclear fusion have a strong corrosive effect on containers during storage and transportation,so it is necessary to select specific container materials and cover them with hydrogen-resistant coatings to prevent or reduce the permeation of hydrogen to container substrates.There are many kinds of hydrogen-resistant coatings,among which alumina and Fe-Al intermetallic compounds have good hydrogen-resistant effects,while Fe-Ni-based super steel and 316L stainless steel are commonly used materials for hydrogen storage vessels.In this study,the phase equilibrium of Fe-Ni-Al-O quaternary system was calculated and analyzed by thermodynamic software and experiments,which provided theoretical guidance for the preparation of aluminum oxide hydrogen barrier coating by selective oxidation of Fe-Ni-Al alloy.In addition,the composite hydrogen resistant coating of Fe-Al compound and alumina was prepared on 316L stainless steel by hot dip aluminizing,high temperature diffusion and high temperature oxidation.The main contents are as follows:(1)The 750? isothermal cross section correlation system of Fe-Ni-Al-O quaternary system was studied by means of thermodynamic calculation and experimental phase diagram.In this isothermal section,there were mainly two oxides,Al₂O₃ and Fe Al₂O₄.The alloy balance between Al₂O₃ and Ni-Al side,and the alloy balance between Fe Al₂O₄and Fe-rich angle,and the boundary between the two oxides was determined in the phase diagram of Fe-Ni-Al ternary alloy.The following9 regions were confirmed:(1)?+Corundum?(2)?+?+Corundum?(3)?+?+?+Corundum?(4)?+?+Corundum(5)?+Corundum?(6)?+Spinel?(7)?+?+Spinel?(8)?+?+?+Spinel?(9)?+Spinel.Compared with the oxide boundary obtained by thermodynamic calculation,the oxide boundary determined by comparative experiment shifted to Ni-Al side.The solubility of Ni in oxide Fe Al₂O₄ increased with the increase of Ni content in the balance Al₂O₃,while the solubility of Fe in oxide Al₂O₃decreased with the decrease of Fe content.The change of temperature not only changed the phase balance of alloy system,but also changed the balance of oxide phase and matrix combination.With the increase of temperature,the boundary between the Corundum phase and Spinel phase shifted to the Fe-Ni side,which provided an important basis for the subsequent preparation of selective oxidation coatings.(2)Metal and metal oxide powder were used to provide oxygen pressure for the selective oxidation of Fe-Ni-Al alloy.The effects of temperature,powder type and oxidation time on the type and distribution of oxides on the surface of Fe-Ni-Al alloy were studied.The results show that with the increase of oxygen pressure,the growth rate of oxide particles along the grain boundary of Fe-Ni-Al alloy was accelerated at the same time,but the internal oxidation of Al occured when the oxygen pressure was too high.When Fe and Fe O powders were used to provide oxygen pressure and annealed at 750?,the oxide layer on the surface was compact and complete.During the annealing process of Fe-Al-Ni alloy,the oxide layer on the surface of the alloy was formed and became dense gradually with the increase of time.However,too long annealing time would lead to local spalling of the oxide layer.When Fe and Fe O powders were used to provide oxygen pressure and annealed at 750? for about 30 min,the oxide layer on the surface was compact,smooth and no spalling.(3)316L stainless steel was coated hot-dip aluminized by flux method and annealed at high temperature.Morphology of surface and cross-section was observed by SEM and the phases were analyzed by EDS and XPS.The oxidation behavior of samples at high temperature was studied by mass gain method.The results showed that after hot dip aluminizing and diffusion,the surface was mainly composed of?-Fe phase layer,Fe Al phase layer and mixed phase layer of the Fe₂Al₅ and Fe Al₂ phases.After diffusion at 900?,the Fe₂Al₅ phase disappeared and the Fe Al ductile phase and porous Fe Al₂ phase were formed.After oxidation at 800? and 1000?,the Fe-Al compounds in the diffusion layer gradually transformed into the Fe Al phase,and the outside surface was composed of Al₂O₃.High temperature oxidation rate of the samples after hot dip aluminizing and diffusion annealing was significantly lower than that of the samples without hot dip aluminizing.When the sample was diffused at 800? and oxidized at 1000? for 50 h,the surface of the sample was smooth,no spalling,and had good high-temperature oxidation resistance.Excessive high-temperature oxidation time would lead to the spalling of the alumina layer.This work provides basic data for the analysis of the balance between alloy and oxide under low oxygen pressure,and also provides experimental basis for the design and development of hydrogen resistant coating.