Corrosion behavior of ferritic and ferritic-martensitic steels in supercritical carbon dioxide

Oxidation tests on a ferritic–martensitic steel were carried out in deaerated supercritical water at 600–700 °C under 25 MPa pressure. The oxidation kinetics followed near-parabolic rate law at 600 °C and obeyed near-cubic rate law at 650–700 °C. The deviations from parabolic behaviour may have been related to the development of growth stresses within the oxide. The oxidation rate did not always increase significantly with increasing temperature. The oxidation rates at 650 and 700 °C were approximately equal. The reason for this may be attributed to the formation of large pores at the interface between oxide scale and substrate at 700 °C. The influence of temperature on the microstructure of oxide scale and oxidation kinetics is discussed.

Impedance spectroscopy has been used to measure the electrical properties of oxide scales formed from oxidation of IN738LC superalloy at high temperature. Electrical resistance and capacitance of the oxide scales were obtained from the simulation of the measured impedance diagrams based on the equivalent circuit model, which represents the features of the oxide scales. For oxidation of IN738LC superalloy, the electrical resistance of oxide scales increased with increasing oxidation time for the specimens exposed to air at 900°C. However, for the specimens oxidised at 1,200°C, the oxide scales showed very low electrical resistance, which indicated that cracking and spallation in oxide scales occurred continuously. By using scanning electron microscopy and X-ray diffraction techniques, the composition and microstructure of the oxide scales were examined. It was found that electrical properties were determined, not only by the microstructure of oxide scales, but also by the composition of the oxide scales. By determining the relationship between electrical properties, microstructure and composition of oxide scales, impedance spectroscopy could be used as a non-destructive technique for monitoring the oxidation of metallic alloys at high temperature.

Effect of gas composition on the oxide scale growth mechanisms in a ferritic steel for solid oxide cell interconnects

Radiation Effects in Ferritic Steels and Advanced Ferritic-Martensitic Steels

To promote the application of resource-saving heat-resistant stainless steels, the study on high temperature oxidation behavior and oxide-scales formation mechanism of X10CrAlSi18 ferritic heat-resistant stainless steels (FHSSs) and 310S austenitic heat-resistant stainless steels (AHSSs) was carried out in air up to 140 h, and the microstructure of oxide scales, oxidation kinetics and thermodynamics theories, and the classical hypothesis of the third-element effect analyzed. The results showed that the cost-effective X10CrAlSi18 FHSSs presented good oxidation resistance at 800 and 900 °C due to the compact multicomponent oxide scales composed of Al2O3, Cr2O3, MnCr2O4 and MnFe2O4 and good adhesion to the matrix. However, the oxidation resistance of the alloy at 1000 °C was deteriorated, which is mainly due to the non-protective Fe2O3. The oxide scales of 310S AHSSs containing Cr2O3, MnCr2O4 and inner SiO2 exhibited good oxidation resistance, while the internal oxidation of silicon weakened the adhesion to the substrate.

We report the study of supercritical flow of carbon dioxide in a vertical tube under non-uniform heat flux. The investigation of supercritical fluid is crucial for advanced generation IV (GIF) nuclear reactors since both supercritical carbon dioxide and supercritical water are planned to be used in those designs. The supercritical carbon dioxide is going to be used in secondary loop of liquid metal fast reactors, the supercritical water will be used as a coolant itself. Thanks to scaling parameters for fluid-to-fluid modelling at supercritical conditions based on inlet conditions approach it is possible to predict response for different supercritical fluids at different scaled conditions. Hence, the results can be used for investigation of other supercritical fluids if it is needed. Since, the heat flux in particular fuel element varies axially it is imperative to investigate the influence of non-uniform flux in this direction. Current literature provides experimental results only when uniform heat flux is applied, whereas non-uniform heat flux can show the thermal response at different stage of fuel cycle. Hence, it is possible to simulate how the flow behaves from the beginning of cycle (BOC) when the shape of the curve is a cosine distributed to the end of cycle (EOC) moment when the peaking factor is skewed to the bottom of the fuel element. The numerical investigation is performed for a 2-D axisymmetric model of a tube with use of CFD code. First, the model is validated for a case with uniform heat flux against results found in the literature. Then, non-uniform heat flux is represented with two parameter equation to describe the variation along axial direction. Sensitivity study related to influence of pressure, temperature and average heat flux in order to better understand the phenomena are conducted. Obtained results provide information about heat transfer coefficient (HTC), heat transfer deterioration (HTD) in supercritical conditions and which parameters can influence it. With better understanding of this phenomena it is possible to prevent or to avoid it, since HTD may cause overheating of the fuel elements inside reactor core and lead to an accident.

Corrosion of austenitic and ferritic-martensitic steels exposed to supercritical carbon dioxide

For last half century the development of creep strength enhanced ferritic steels has been continued and presently ASME grades 91, 92 and 122 extremely stronger than conventional low alloy steels have extensively been used worldwide in high efficient power plants. However the use of these creep strength enhanced 9-12%Cr steels is limited to around 630°C or 650°C at maximum in terms of high temperature strength and oxidation resistance. Consequently the appearance of ferritic steels standing up to higher temperature of around 700°C to substitute of high strength austenitic steels is strongly desired. Under the state, the addition of high nitrogen to ferritic steels is attracting considerable attention because of improving high temperature strength and oxidation resistance of them. This work was done to evaluate the oxidation resistance of high nitrogen steels and to investigate the effect nitrogen and microstructure on oxidation resistance using 9-15%Cr steels with about 0.3% nitrogen manufactured by means of Pressurized Electro- Slag Remelting (PESR) method in comparison with ASME grades 91 and 122. As a result, high nitrogen ferritic steels showed excellent oxidation resistance comparing with nitrogen-free steels and ASME grades 91 and 122. The oxidation resistance of 9%Cr ferritic steels depends on the nitrogen content in the each steel. That is, the weight gain decreases with an increase in nitrogen content. Moreover, the oxide scale of high nitrogen steel contained a high concentration of Cr. It is conjectured that, in high temperature oxidation, nitrogen plays a key role in promoting the formation of the oxide scale which has high concentration of Cr, inhibiting oxidation from proceeding. And also it was found that the oxidation resistance of the high nitrogen steels does not depend greatly on Cr content but on their microstructure. The oxidation resistance of high nitrogen ferritic heat-resistant steels increased as the fraction of martensite structure increased. These results indicate for high nitrogen steels Cr diffusion along grain boundaries is further promoted resulting in the formation of protective oxide scale having high Cr concentration. Furthermore as new findings it was confirmed that the Cr diffusion in substrate of steels to form Cr concentrated oxide scale on the metal surface is accelerated by nitrogen while suppressed by carbon in matrix of steel.

Extraction of iron and calcium from low rank coal by supercritical carbon dioxide with entrainers

Exfoliation of oxide scales from high-temperature heating surfaces of power boilers threatened the safety of supercritical power generating units. According to available space model, the oxidation kinetics of two ferritic-martensitic steels are developed to predict in supercritical water at 400°C, 500°C, and 600°C. The iron diffusion coefficients in magnetite and Fe-Cr spinel are extrapolated from studies of Backhaus and Töpfer. According to Fe-Cr-O ternary phase diagram, oxygen partial pressure at the steel/Fe-Cr spinel oxide interface is determined. The oxygen partial pressure at the magnetite/supercritical water interface meets the equivalent oxygen partial pressure when system equilibrium has been attained. The relative error between calculated values and experimental values is analyzed and the reasons of error are suggested. The research results show that the results of simulation at 600°C are approximately close to experimental results. The iron diffusion coefficient is discontinuous in the duplex scale of two ferritic-martensitic steels. The simulation results of thicknesses of the oxide scale on tubes (T91) of final superheater of a 600 MW supercritical boiler are compared with field measurement data and calculation results by Adrian’s method. The calculated void positions of oxide scales are in good agreement with a cross-sectional SEM image of the oxide layers.

The effect of low dose irradiation on the impact fracture energy and tensile properties of pure iron and two ferritic martensitic steels

Parabolic rate constants, kp, were collected from published reports and calculated from corrosion product data (sample mass gain or corrosion product thickness) and tabulated for 75 alloys exposed to temperatures between ~800 and 2000 K (~500–1700 oC; 900–3000 oF). Data were collected for environments including lab air, ambient and supercritical carbon dioxide, supercritical water, and steam. Materials studied include low- and high-Cr ferritic and austenitic steels, nickel superalloys, and aluminide materials. A combination of Arrhenius analysis, simple linear regression, supervised and unsupervised machine learning methods were used to investigate the relations between composition and oxidation kinetics. The supervised machine learning techniques produced the lowest mean standard errors. The most significant elements controlling oxidation kinetics were Ni, Cr, Al, and Fe, with Mo and Co composition also found to be significant features. The activation energies produced from the machine learning analysis were in the correct distributions for the diffusion constants for the oxide scales expected to dominate in each class.

Iron or chromium based alloys are currently the most preferred materials for SOFC interconnects. The poisoning of the cathode of the membrane electrode assembly (MEA) due to evaporation of chromium species from the metallic interconnect (MIC) and the oxidation of the interconnect surface during stack operation at elevated (800-900 °C) temperatures are regarded to be the major mechanisms affecting the degradation behaviour of the SOFC stack. The oxidation at the cathode side of the interconnect can cause up to one third of the total stack degradation. Despite intensive investigations on corrosion stability of MIC for SOFC application over the past decades, the main attention was focused mainly on the material properties in the cathode side gas conditions (ambient air). The growth behavior and the composition of oxides grown in the SOFC anode gas environment as well as the influence of the oxides on the contact resistance between MIC and anode contacting are still not investigated comprehensively. The MIC materials have to be stable over more than 40,000 hours to satisfy a demand for a continuous increase of operation times of the SOFC stacks. Real time tests over such long time scales are cost intensive. A modification of standard material test procedures, with the aim to accelerate material degradation, is therefore necessary. In order to realize this, three ferritic steels (Crofer 22 APU, Crofer 22 H, AISI 441) and a chromium-based alloy were tested in SOFC anode gas environment in a temperature range between 725 – 875°C. The experiments were carried out under variation of the water vapor content in the gas mixture for different exposure times in order to create the accelerated degradation testing conditions for MIC and to investigate the behavior of these materials caused by the formation of growing chromium oxide based scales. Both gravimetric measurements and FESEM/EDX data, as well as polished microimage sections were analyzed to characterize the oxidation kinetics and the microstructure of the oxide scales and interconnect materials. A clear correlation between increasing temperatures and increasing oxide growth rate constants kp,w can be demonstrated in all materials. This interrelation results in thicker surface oxide scales. A comparison of surface oxide thicknesses after 1000 h oxidation in reducing atmosphere show an increase between 725°C and 875°C with a maximum factor of about 3.8 in ferritic steels and 2.4 for CFY. The comparison of kp,w values at temperatures between 725 and 875°C show acceleration factors of 8,2-11.9 in ferritic steels and 2.3 for CFY. No significant dependence of the oxide growth rate in ferritic steels and CFY by variation of the water content in fuel gas for concentrations within H2/H2O=87/13 vol.% and H2/H2O= 50/50 vol.% was found. The structures of the oxide layers are specific for each material and consist mainly of Cr2O3 (CFY) and Cr2O3/(Cr,Mn)3O4 in ferritic steels with different element distributions and thickness ratios. Beside this, the zone of an inner oxidation of MICs with the Al-, Si- and Ti- rich oxide inclusions can be seen in ferritic samples, whose microstructures differ depending on analyzed materials and temperatures. In Nb containing materials (Crofer H, AISI 441), Silicon precipitations as Laves phases as well as continuous SiO2 scales were observed beneath the formed Cr2O3 top layer. Furthermore, an increase of the internal oxidation zone was identified with increasing temperatures in ferritic steels. In CFY, the internal oxidation zone with Cr2O3 inclusions in the material bulk remains almost constant over 1000 h oxidation in the tested temperature range. Electrical measurements in the dual gas atmosphere reveal also an increase of resistance within 1000 h material exposure under anode gas conditions. Comparison of these results suggests that materials, forming such isolating SiO2 layers, show a higher resistance increase. The results of the oxide scale formation under the anode gas conditions in tested Crofer 22 APU and CFY samples are compared with data derived from the real SOFC stacks. Only minor differences in the oxide thicknesses were observed between the gas inlet and gas outlet (region with higher water content) at the anode side of the stacks. This behavior is similar to the results of our artificially oxidized MIC samples in which we also have not found a clear evidence for an increase in oxide scale growth rate with higher water content in the humidification range tested. Such effects can indicate, that the reactions at phase boundary are dominated by solid-state diffusion and the rate supply of adsorbed oxygen species (from H2O) is not the limiting factor (in the tested H2O concentrations range).

Different phase compositions and microstructures of oxide scales were formed on the surface of SS400 hot rolled alloys by employing various heat treatment processes. Cyclic wet-dry immersion corrosion test, electrochemical impedance spectroscopy were used to investigate the corrosion resistance of strips with scales fabricated by different heat treatment processes. The results reveal that difference in the corrosion resistance of the various scales is due to the difference in the grain size of Fe3O4phase. Furthermore, the difference in the corrosion resistance of different oxide phases, exhibited by various scales, also render the strips to give various corrosion behaviors. It is surmised that the strip with oxide scale, which consist of a small mount of the outer layer Fe2O3phase distributed continuously and a large quantity of the inner layer Fe3O4phase with the fine grain size, and possess nice compactness, continuity, integrity in the morphology structure, has the best corrosion resistance.

The Supercritical Carbon Dioxide Brayton Cycle has drawn much attention due to its high efficiency and stability in the power plants. Evaluation of the corrosion behavior and mechanical properties as an essential part of the power plant is of great importance for structural integrity assessment. In this work, the three types of possible candidate materials were chosen to examine high-temperature thermal stability and corrosion resistance under the high-temperature carbon dioxide environment: i) ferritic-martensitic steels (T92), ii) austenitic steels (304L & 316L), and iii) Ni-based alloys (Alloy 600 and 738LC). The experiments were performed during 1000 hours at temperatures ranging from 500 to 800°C. The corrosion behaviors of the tested materials were evaluated from weight change, composition and thickness of oxide scale, and analyses of surface and cross-section morphologies. The results show that Ni-based alloys and austenitic steels have better corrosion resistance than ferritic-martensitic steels at the temperatures tested. The corrosion resistances of the tested materials were compared with those from other publications.