Study On High Temperature Chlorine Corrosion Properties Of The Modified Alloys And High-Entropy Alloys Based On 904L Steel

In order to improve the corrosion resistance of alloys or coatings serving in high temperature chlorine-containing environments,increase the operating efficiency of equipment under such conditions,reduce the cost of conventional alloys based on nickel-based high temperature alloys,such as Inconel625,and improve the ratio of cost/performance,in this study,Fe-based super austenitic alloy 904L with high Ni and Cr content was selected as the matrix,and Ti,Al and Nb elements,which showed high corrosion resistance in high temperature chlorine-containing environments,were selected as modified elements.Ti modified 904L alloy,Al modified 904L alloy,Al Fe Ni Mo Nb modified 904L alloy and Al Fe_2.5Ni Mo Nb Cr high-entropy alloy were prepared by vacuum arc melting technology and hot pressing combined with vacuum arc melting technology,respectively.High temperature chlorine corrosion experiments of the obtained alloys were carried out under the atmosphere of N₂-2.6 vol.%CO₂-1.3 vol.%O₂-2700 vppm HCl at 600°C,700°C and 800°C for 55 h,respectively.Through the calculation of thermodynamic and kinetic parameters in the corrosion process of the alloy,the analysis of the surface and cross-sectional morphology and composition of the corrosion products,the high temperature chlorine corrosion characteristics of the alloy were examined and the corrosion mechanism were analyzed.The main conclusions are as follows:（1）Through the thermodynamic calculations,the high temperature chlorine corrosion process,the phase composition and the distribution of corrosion products were theoretically inferred.In the oxidizing chlorine-containing atmosphere,the oxidation reaction of the alloying elements occurred first to obtain the corresponding metal oxides.At the same time,metal oxychlorides were prone to further oxidation,forming metal oxides.It was more difficult for the Cl element to react with the metal oxide directly,but instead passed through the corrosion products and reached the oxide layer/matrix interface,where the chlorination reactions of the alloying elements occurred.The obtained metal chlorides continuously diffused towards the gas/oxide layer interface and were oxidized,while Cl₂was released.Due to the presence of a concentration gradient,Cl₂will return to the oxide layer/matrix interface to begin a new round of corrosion.In the corrosion products,protective layers of Ti O₂,Al₂O₃tended to form close to the matrix,while Fe oxides were easy to form on the surface of the corrosion layer.（2）After the addition of the Ti element,a refined microstructure of 904L was obtained.Ti and Ni elements were enriched in areas with low corrosion resistance in metallographic corrosive fluids（aqua reams）,especially Ni elements.Cr₁₃Fe₃₅Ni₃Ti₇intermetallic compound was formed in the 904L:Ti alloy.At the same time,a small amount of Fe Ni₃,Fe Ti and Ni₃Ti compounds were obtained in the modified alloys.The precipitation of intermetallic compounds at the grain boundaries of 904L:Ti alloys was conducive to inhibiting the diffusion and corrosion of chloride ions along these regions.As the corrosion temperature increased,the mass change of the alloy became smaller and smaller.The mass gain of the 904L:Ti alloy with 10 at.%Ti added was-0.44 mg/cm²after the corrosion at 800°C for 55 h.Due to the high oxidation activity of Ti element and the low oxygen partial pressure required for its chloride oxidation,the 904L:Ti alloy could easily form a stable Ti O₂film close to the matrix during the corrosion process.Finally,the 904L:Ti alloy obtained a high temperature chlorine corrosion resistance much higher than that of the 904L alloy by forming a protective Ti O₂/Cr₂O₃double-layer oxide film during the high temperature chlorine corrosion process.（3）The addition of Al element gradually transformed the main phase of the904L alloy from austenitic?phase to intermetallic compound phase contained Al element.In the test temperature range,the corrosion kinetic curves of 904L:Al_xalloys were basically comply with the parabolic law.When the Al content was 5 at.%,it was not enough to form a continuous Al₂O₃layer,and the corrosion resistance of the alloy was not improved,but because of the high activity of Al,the corrosion rate of the alloy was accelerated.When the Al content was 20 at.%,904L:Al_0.20alloy obtained the highest corrosion resistance,and the higher the corrosion temperature,the more obvious the advantage of its corrosion resistance.After corrosion at 800°C,the mass gain and the corrosion rate constant of the 904L:Al_0.20alloy was 0.0625mg/cm²and 0.014 mg/(cm²·h^1/3),respectively.The effective addition of Al enabled the modified 904L alloy to obtain a much higher corrosion resistance than 904L alloy by forming a protective Al₂O₃/Cr₂O₃double layer of oxide film during high temperature chlorine corrosion process.（4）Different from 904L and Al Fe Ni Mo Nb alloys,the branch crystal structure of 904L:Al Fe Ni Mo Nb high-entropy alloy was obtained and the distribution of elements were more uniform.Similar to the corrosion characteristics of Inconel625alloy,the corrosion layer of 904L:Al Fe Ni Mo Nb high-entropy alloy had a relatively flat oxide/matrix interface,and the entire corrosion process was carried out evenly.The corrosion layer composed of mixed oxides of Al₂O₃and Cr₂O₃had high corrosion resistance,which could effectively protect the alloy matrix from further corrosion of Cl element.Among the test samples corroded at 600°C,700°C and800°C,the 904L:Al Fe Ni Mo Nb high-entropy alloy had a high resistance to high temperature chlorine corrosion and had the most stable corrosion kinetic properties as the corrosion temperature changed.The mass gain of the 904L:Al Fe Ni Mo Nb high-entropy alloy was-0.01136 mg/cm²after corrosion at 800°C for 55 h.（5）A high ratio of cost-effective Al Fe_2.5Ni Mo Nb Cr high-entropy alloy with BCC structure was obtained.The high-entropy properties,as well as the refinement and uniformity of microstructure,were conducive to the high chlorine corrosion resistance of the alloy.The kinetic curve of high temperature chlorine corrosion of the alloy roughly conformed to the parabolic law.The corrosion rate constant of Al Fe_2.5Ni Mo Nb Cr high-entropy alloy was lower than that of Al Fe Ni Mo Nb alloy,and was in the same order of magnitude as that of Inconel625.The mass gain of the Al Fe_2.5Ni Mo Nb Cr high-entropy alloy was 0.59524 mg/cm²after corrosion at 800°C for 55 h.In the corrosion process of Al Fe_2.5Ni Mo Nb Cr high-entropy alloy,the elements in the Cr depletion zone were homogenously dispersed,which promoted the formation of a continuous Al₂O₃oxide layer.The dendritic distribution of Al₂O₃connected to the corrosion layer in the matrix improved the adhesion of the corrosion layer through the"oxide pegging"mechanism.Compared with the Inconel625 alloy that obtained a single Cr₂O₃corrosion layer,the Al Fe_2.5Ni Mo Nb Cr high-entropy alloy with Al₂O₃/Cr₂O₃double corrosion layer was expected to obtain higher corrosion resistance under chlorine containing environment with longer time or higher corrosion temperatures.（6）The modified element Ti and Al,selected by thermodynamic calculations,both played a significant role in improving the high temperature chlorine corrosion performance of the 904L alloy,especially the element Al.The composition and microstructure of the alloy were changed by the formation of intermetallic compounds containing the modified elements and improved the protection of the layer of corrosion product to the substrate through the formation of stable oxides of Ti O₂and Al₂O₃closest to the substrate.High-entropy alloys designed on the basis of the constituent elements of 904L:Al_xalloy had much higher cost/performances ratio than 904L alloy.The improved corrosion resistance of high-entropy alloys was determined by their refined microstructure and the formation of protective Al₂O₃layers.The higher the corrosion temperature,the more obvious the stability advantage in corrosion resistance of the high-entropy alloy in chlorine-containing environments,and was expected to obtain better corrosion resistance than Inconel625 alloy at higher temperatures.The order of corrosion resistance of the modified alloys from highest to lowest was 904L:Al_0.20>904L:Al Fe Ni Mo Nb>Al Fe_2.5Ni Mo Nb Cr>904L:Ti.For the defects（porosity）formed in the corrosion surface of Al-modified 904L alloy,which could be avoided by optimizing the preparation process of the alloy and thus reducing the defects in the base alloy.The corrosion resistance of the high-entropy alloy at higher temperatures was expected.