Corrosion mechanism and oxide scale evolution of austenitic stainless steels in supercritical CO2

A simple method has been devised to calculate the mass transfer in liquid metal loops. It is based on the use of mass transfer coefficients which determine the mass flux from the wall into the fluid. These coefficients depend on a dimensionless characteristic thermo-hydraulic number, namely the Sherwood number, which itself depends under forced convection flow conditions on the Reynolds number and the Schmidt number. This is supplemented by the application of the mass conservation law, which allows the calculation of the conditions in the bulk of the fluid. The newly developed kinetic model for mass transfer under forced convection flow conditions has been implemented in the computer code MATLIM. The formation of protective oxide scales on stainless steel components is of prime importance for lead and lead-bismuth loops. As the propensity for mechanical effects like delamination and spalling increases with the thickness of the oxide scale, it is indispensable to have a clear understanding of the physical effects contributing to their growth. Thus, the importance of dissolution effects on oxide scales must be investigated and quantified, especially the dependence on the oxygen content in the liquid metal. This determines also the mass transfer from the hot leg to the cold leg in these loops. At higher temperatures dissolution attack of stainless steel specimens has been observed in some cases for oxygen contents in lead-bismuth alloy normally sufficient for oxide scale formation. Dissolution effects on pre-existing oxide or other protective scales are also of great importance for lead-lithium loops. These loops are operated at very low oxygen contents in order to avoid the formation of Li 2 O. With the loss of protective scales heavy dissolution attack of stainless steel components commences.

High temperature corrosion studies in today’s austenitic steels are of great interest to develop novel corrosion- and creep-resistant austenitic steels for advanced zero-emission power generation plants. These efficient power plants can offer advantageous considerations for the global need for energy and mitigation of climate change. However the aggressive environmental conditions poised challenges to current austenitic steels for long-term applications. To date, long-term application of austenitic steels has been restricted to service temperatures lower than 635 °C while future boilers are targeting temperatures of 700 – 750 °C. Therefore, extensive microstructural evolution studies of Fe-Cr-Ni steels exposed to high temperature conditions will contribute useful information to help retard degradation or enhance corrosion resistance properties while in search of novel compositions in austenitic steels.Protective Cr-rich oxide films formed on austenitic stainless steels play an effective role to lower high temperature corrosion. These oxide scales of low porosity, good adherence, low growth rate, and high mechanical and thermodynamic stability can provide good corrosion resistance properties. Hence, in the formation of a protective Cr-rich oxide scale as a barrier to further corrosion, diffusional transport of Cr within the alloy to the oxide scale is critical.The conditions of the exposure environments of the austenitic steels highly affect the growth and stability of these protective Cr-rich oxide scales. For example, the formation of volatile Cr species can lead to a depletion of Cr in the oxide scales and form Fe-rich oxide films, which inadequately function as a barrier to corrosion. Since the rate of diffusional transport of Cr in the oxide scale is compensated, accelerated localized attacks within the grains occur. In addition, the presence of H2O and CO2 in the exposure environment results in the formation of Fe-rich oxide scales and accelerated oxidation rates of most technical steels.Sulphurous corrosive components (SO2 and sulphate-based salt components) in commercial high temperature applications, for e.g. combustion, accelerate the corrosion rate, resulting in adverse modification of degradation properties in Fe-Cr-Ni steels. This deposit-induced intensified oxidation process is known as hot corrosion. In addition, Na and S exist as fuel impurities in power plants and lead to deposits of Na2SO4. These molten deposits can cause sulphate-deposit induced accelerated oxidation processes, which destroys the reaction-product oxide scales. Propagation modes of hot corrosion determined by these mechanisms are also dependent on the exposure temperatures and durations, and compositions of exposure gas, oxides and deposits. Hence, the influence of different exposure temperatures and durations on the mechanisms of the oxidation behavior of Fe-Cr-Ni steels studied in this work will help predict the behavior of austenitic steels in hot corrosion environments.In this work, the isothermal high temperature oxidation behaviors of a 22 wt.% Cr austenitic stainless steels, Sanicro25 (42Fe22Cr25NiWCuNbN), subjected to various exposure temperatures have been investigated. The exposure environment consists of O2+H2O+CO2+SO2+Na2SO4. The alloys were subjected to respective exposure temperatures of 600, 700 and 750 °C with different exposure durations of 300 h and 1000 h. They were also compared to results observed in Sanicro25 alloys which were subjected to similar environmental conditions at 700 °C but for a shorter exposure duration time of 168 h [1].Detailed microstructural investigations within the individual grains of the formed oxide, the oxide-metal interface and in the vicinity of the oxide-steel interface using XRD, SEM, FIB/SEM, TEM and EDX have been performed. These results yield insights to the microscopic mechanisms governing the oxidation behavior of steels with deposits of Na2SO4when subjected to aggressive high temperature environments related to oxy-fuel combustion.

Fe - 9 to 12%Cr alloys are a material for the thick sections boiler components and steam lines of a power plant. The role Fe - 9 to 12%Cr alloys is becoming more prominent in the development of a new generation of Ultra-Supercritical (USC) Power Plant due to the target operating temperature is reaching 620 °C (893 K), in 100% steam condition as well as pressure in excess of 300 bar (30 × 106 Pa). In such condition, the integrity of Fe - 9 to 12%Cr alloys relies on the oxide scale formed during the time of exposure. However due to the high temperature and water vapor condition, it is a well known fact that, the formation of oxide scale is accelerated thus depleting the structural integrity of the Fe - 9 to 12%Cr alloys over the time. Studies show that not only the formation of protective oxide scale was suppressed but the formation of non-protective oxide scale was accelerated instead. Decades of studies done by various groups around the globe has yet to have consensual on the exact mechanism of this phenomenon. Initial stage oxidation of these alloys plays great roles in hope to understand the formation of oxide scale in water vapor condition at high temperature. This paper reviews previous research works to understand the initial stage oxidation of Fe - 9 to 12%Cr alloys at high temperature in water vapor condition.

Effect of complex municipal solid waste incineration flue gas on the corrosion of various alloys at 550 °C

New insight on the formation of uneven oxide scales on AFA steel in supercritical CO2: Roles of recrystallization degree on the high temperature corrosion resistance

Irradiation accelerated corrosion of alumina-forming austenitic steels in supercritical CO2: The oxide scale formed within an individual grain or affected by grain boundary

Oxide scale on stainless steels and its effect on sticking during hot-rolling

As internal combustion engine (ICE) systems are increasingly exposed to severe thermal and oxidative environments, the oxidation resistance and structural integrity of exhaust valve materials have become critical for maintaining long-term engine reliability and efficiency. This study presents a comparative evaluation of the cyclic oxidation behavior of two candidate valve steels, 1.4718 (ferritic stainless steel) and 1.4871 (austenitic stainless steel), under service-temperature conditions. The specimens were exposed to repeated oxidation at 550 °C, 650 °C and 750 °C for 25 cycles in ambient air. The surface and cross-sectional morphologies of the oxide layers were analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to investigate oxide scale composition, thickness, and growth characteristics. The oxidation behavior of both alloys proceeded in two distinct stages: an initial phase marked by accelerated oxidation, followed by a slower, more stable growth period. The extent of oxidation intensified with increasing temperature. The 1.4718 alloy developed relatively porous but compositionally stable oxide layers consisting primarily of Fe- and Cr-based spinels such as FeCr2O4 and Cr2SiO4. In contrast, the 1.4871 alloy formed a dense, adherent, dual-layered oxide scale composed of an outer Mn2O3-rich layer and an inner Cr2O3-rich layer, attributable to its high Mn and Cr content. The results underscore the critical influence of elemental composition, particularly Cr, Mn and Si, on oxide scale stability and spallation resistance, demonstrating the superior cyclic oxidation resistance of the 1.4871 alloy and its potential suitability for exhaust valve applications in thermally aggressive environments.

High-temperature corrosion of preoxidized Ni-Cr-Ce alloys in chlorine gas (Cl2) at temperatures between 400°C and 700°C was investigated using thermogravimetric analysis and examining the corrosion products by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Corrosion resistance of preoxidized alloys against chlorination was improved by formation of protective nickel oxide-chromium oxide-cerium oxide (NiO-Cr2O3-CeO2) scale. At lower temperature, the protective oxide scale played a role in the chlorination processes. With increasing reaction temperatures, the oxide scale was destroyed after exposure to Cl2 for a period of time. The corrosion rate increased rapidly, and weight descreased linearly with time. In Cl2 atmospheres at high temperature, Cl2 attacked the oxide scale by three mechanisms: mechanical damage by volatile chlorides, scale pitting and spalling by Cl2 dissolving in the scale, and oxides being converted to chlorides.

A new wrought alloy has been developed for use as furnace tubes in ethylene pyrolysis plants. This alloy has an excellent carburization resistance due to uniform formation of protective Al2O3 oxide scale on the metal surface. At temperatures between 1000°C and 1150°C, laboratory corrosion tests have been carried out to evaluate carburization resistance of the developed alloy. Commercial austenitic stainless steels used for furnace tubes were tested for comparison. In a simulated carburizing environment of 15vol%CH4-3%CO2-82%H2 gas mixture, the developed alloy has a three times better carburization resistance than the conventional austenitic stainless steels containing more than 25mass% chromium and high silicon at evaluated temperatures. Another laboratory test has been conducted to clarify carburization and coking resistance under conditions of cyclic carburization and oxidation environments. Carbon ingress and coke deposition of conventional alloys forming chromium oxide scale increased with increasing heat cycles, whereas the developed alloy remained unchanged. Based on these results, carburization and coking behavior of alloys used as ethylene pyrolysis furnace tubes has been explained.

Effects of thermal cycling on the formation of oxide scales were investigated in Fe–Cr alloy interconnects for solid oxide fuel cells. Thermal cycling between 293 and 1073 K with a heating rate of 250 K/h was examined. Relatively large oxide scale grains were formed on the surface. These grains mainly consisted of the spinel-based oxides formed by fast diffusion of Mn along the grain boundaries of the Fe–Cr alloy. Although the oxide scale/alloy interface showed strong contact without exfoliation, some cracks were found at the oxide scale surface. The elemental distribution in the oxide scale was similar to that of samples with the normal annealing (without thermal cycling). The growth of oxide scale was promoted by thermal cycling especially at the grain boundary of the Fe–Cr alloy, which eventually resulted in the formation of a thick oxide scale. An increase of oxide-scale growth rates with the thermal cycling was evaluated taking into account microstructures and elemental distributions.

Exposed to 650°C air, TP304H stainless steel with two different grain size was oxidized at this temperature. At the meantime, comparison of their oxidation was through the oxidation kinetics curves and analysis of the morphology and composition of oxide scale which conducted by SEM and X-ray. The results showed that the oxidation rate of TP304H stainless steel was slowed down by grain refinement and oxide scale of fine-grained TP304H steel was thinner than that of coarse-grained steel. The nucleation and the growth of nuclei of coarse-grained oxide scale were more rapid. In addition, the grain refinement of austenitic stainless steel accelerated the diffusivity of Cr and made for the formation of dense and continuous oxide scale, so that the oxidation of stainless steel can be effectively inhabited.

Oxide scale on stainless steels in hot rolling, which affects the coefficient of friction, thermal conductivity and surface quality, is more complicated than that on carbon steels. In this study, an austenitic stainless steel 301 was selected. The humid air with 12% water vapour content was adopted for the formation of uniform oxide scale at 1,100°C. Oxidation kinetics study was carried out by using the thermogravimetric analyser (TGA). Specimens were isothermally heated for 25 and 35 minutes to obtain different thickness of scales. Hot rolling was performed on a 2-high Hille 100 experimental rolling mill at various reductions with dry and water cooling conditions. Oxide scale thickness and compositions were analysed with SEM and XRD. Surface roughness was measured after rolling. Inverse calculation of coefficient of friction was employed to analyse and discuss the friction condition between rolls and the strip under different rolling parameters.

This paper considers the effect that surface finish has on early oxide growth. Austenitic stainless steel of grades 304 and 316L, with ground (240 grit) and polished (1 µm) surface finishes were subjected to temperatures of 750°C and 800°C for times of up to 24 h. It was found that the oxide morphology was heavily dependent on surface finish and substrate microstructure. The ground surfaces were found to initially produce a homogenous oxide scale, while the polished samples formed a scale made up of many oxide islands. The formation of the oxide islands was due to the fast diffusion of chromium through grain boundaries and prior ferrite regions. These fast diffusion paths provided localised oxidation resistance at the surface which caused breaks in the oxide scale. With the ground surface finish, these fast diffusion paths become negated as the grinding of the surfaces creates an even distribution of fast diffusion paths. Mechanisms of early oxide growth in relation to the substrate microstructure and surface finish have been summarised in a schematic diagram.

This paper considers the effect that surface finish has on early oxide growth. Austenitic stainless steel of grades 304 and 316L, with ground (240 grit) and polished (1 µm) surface finishes were subjected to temperatures of 750°C and 800°C for times of up to 24 h. It was found that the oxide morphology was heavily dependent on surface finish and substrate microstructure. The ground surfaces were found to initially produce a homogenous oxide scale, while the polished samples formed a scale made up of many oxide islands. The formation of the oxide islands was due to the fast diffusion of chromium through grain boundaries and prior ferrite regions. These fast diffusion paths provided localised oxidation resistance at the surface which caused breaks in the oxide scale. With the ground surface finish, these fast diffusion paths become negated as the grinding of the surfaces creates an even distribution of fast diffusion paths. Mechanisms of early oxide growth in relation to the substrate microstructure and surface finish have been summarised in a schematic diagram.