Chromium Diffusion Research Articles

A New Generation of Protective Coating Materials Based on CuMn1.7Fe0.3O4-Me2O3 (Me=Y, Gd) Ceramic Bilayers for Solid Oxide Cells Interconnect

In SOCs (solid oxide cells) technology, coating of interconnects is one of the main processes that determine the long-term operation of the stack. Currently, the main material used for interconnects are steels with high chromium content (e.g. Crofer 22 APU). They are known for their high resistance towards high-temperature corrosion due to the formation of passivation layer composed of chromium oxide. This oxide is characterized by relatively high conductivity compared to other single metal oxides, but at the same time, chromium evaporation causes cathode poisoning, what causes a significant decrease in the cell efficiency. Coating the interconnects reduces the thickness of chromium oxide layer and also protects the cathode. Materials used as protective coatings must be characterized by an appropriate coefficient of thermal expansion (suitable for steel, sealant and cathode) and high electrical conductivity, because target value of the area specific resistance cannot exceed 0.1 Ω cm2. Currently, the state of the art coating material is MCO (MnCo2O4), which is characterized by high conductivity and very good protection against chromium diffusion. Due to the fact that cobalt is a critical element for many areas, it was immediately necessary to find cobalt-free alternatives.This work will present the research on the spinel material CuMn1.7Fe0.3O4 in combination with a single metal oxide – Y2O3 or Gd2O3. These combinations were made due to high affinity of the spinel material for chromium, and thin base layers of single metal oxides were intended to reduce the diffusion of chromium into the spinel layer. The layers were deposited using two techniques – electrolytic and electrophoretic deposition. The prepared samples with the layers fabricated in three configurations were subjected to oxidation tests at a temperature of 700 °C for 4000 hours. The reaction kinetics was determined by measuring weight gain and determining the Kp coefficient and instantaneous Kp. Electrical resistance measurements were also performed at various stages of oxidation to demonstrate the changes occurring during the oxidation. SEM-EDS measurements allowed for the assessment of the quality of the layer, the distribution of the elements, the thickness of chromium oxide layer and the efficiency for suppression chromium diffusion. Confocal Raman imaging, X-ray adsorption spectroscopy and X-ray diffraction measurements allowed to demonstrate potential changes occurring in the material. The results indicated very good corrosion protection properties with good chromium diffusion blocking properties. A very low value of the area specific resistance was obtained, which after 4000 hours of oxidation ranged from 8 to 20 mΩ cm2, depending on the type of a sample. Acknowledgement This research has been supported by National Science Centre (NCN) Harmonia 9 project number UMO-2017/26/M/ST8/00438: ‘Quest for novel materials for solid oxide cell interconnect coatings’.

Effect of bias voltage on high-temperature oxidation resistance and electrical properties of Mn–Co coating with metal interconnects for solid oxide fuel cell

Abstract This study investigated the effects of bias voltage on the microstructure, high-temperature oxidation resistance, electrical conductivity, element diffusion, and barrier of chromium poisoning cathode of Mn–Co coating on the surface of SOFCs metal interconnect SUS441 ferritic stainless steel. A series of Mn–Co coatings were prepared by magnetron sputtering technique at different bias voltages (−10, −50, −90 V) and oxidized at high temperatures for 175 h at 850 °C in an air environment. The results showed that the surface of each coating before oxidation exhibited a cauliflower-like morphology, with the crystallinity of the coating increasing with higher bias voltage. After high-temperature oxidation, especially the Mn–Co coatings prepared at −90 V bias, a dense and stable MnCo2O4 spinel structure was formed, which is crucial in inhibiting the growth of the Cr2O3 oxide layer. In addition, the coating also exhibits excellent electrical conductivity (Ea = 0.35 eV), good high-temperature oxidation resistance (1.182 mg cm−2), and a stronger ability to prevent the diffusion of Cr elements.

A Study of the Features of Coating Deposition on a Carbide Substrate Using Preliminary Etching with Glow-Discharge Plasma

The properties of coatings obtained using two surface preparation methods were compared: heating and etching by ion bombardment with plasma generation by arc evaporators and heating and etching by a glow discharge. A Ti-TiN-(Ti,Cr,Al)N coating was deposited. The use of a glow discharge provides better resistance of the coating to destruction during the scratch test and wear resistance of metal-cutting tools when turning steel. As the cutting speed increases, the advantage in wear resistance of the coating deposited using a glow discharge increases. During the process of heating and etching by ion bombardment with metal ions, a nanolayer rich in cobalt and tooling elements (iron, molybdenum) is formed in the area of the interface of the coating and the carbide substrate. When heated and etched by a glow discharge, such a layer does not form. When using both methods, there is identical diffusion of tungsten into the coating and diffusion of chromium and possibly titanium into the substrate. Thus, the glow-discharge heating and etching method can be effectively used in the process of PVD coating deposition.

Development of the ENIGMA fuel performance code for LWR applications with chromium-coated cladding

Zirconium-alloy cladding with a chromium coating is the most advanced of the near-term concepts amongst Accident Tolerant Fuel (ATF) materials for Light Water Reactor (LWR) applications. The ENIGMA fuel performance code has been updated to model the thermo-mechanical behaviour of such cladding in both normal and off-normal operating conditions. The focus was on accurately simulating the behaviour in Loss Of Coolant Accident (LOCA) conditions to evaluate the increase in coping time during design-basis accidents. New low-temperature and high-temperature models were incorporated for cladding oxidation and hydriding and cladding creep which take into account the impact of the chromium coating on the overall cladding behaviour. Furthermore, the consumption of the chromium coating due to high-temperature diffusion of chromium into the cladding base alloy’s β-Zr phase is simulated. The new models have been validated using measurements on chromium-coated cladding from irradiated rods, high-temperature annealing experiments and semi-integral LOCA tests. The validation showed good agreement between ENIGMA’s predictions and the experimental data; thereby demonstrating the applicability of the new models for simulating the performance of LWR fuel rods with chromium-coated cladding in both normal operation and accident conditions.

Diffusion nickel-cobalt coatings for protection of solid oxide electrolysis cells’ current collectors made of Crofer 22 APU steel

The evolution of the microstructure and composition of Ni-Co coatings for the protection of current collectors made of stainless chromium steel Crofer 22 APU from oxidation in the operating mode of the anode chamber of solid oxide electrolysis cells (SOEСs) has been studied. It is shown that due to the diffusion of the components of the steel and the coating, as well as the redox reactions occurring under the coating in the operating mode of the SOEС, the diffusion of chromium to the surface of the current collector is blocked. During operation in the air atmosphere of the anode chamber, the composition of the protective coating changes from metallic Ni-Co to a mixture of highly conductive oxides (Fe,Ni,Co)3O4 and (Ni,Co)O, which leads to a change in the type of time dependence of the specific surface resistance of the junction current collector-anode. At the same time, the obtained values of ~17 mOhm cm2 during tests of 7000 hours are quite low and these coatings can be used to protect SOEС stack current collectors made of stainless chromium steel from oxidation.

Unusual intermediate layer precipitation in low-temperature salt bath nitrocarburized 316L austenitic stainless steel

This study focuses on investigating the precipitation behavior of the expanded austenite during low-temperature salt bath nitrocarburizing by adjusting the treatment temperature and time. The results indicate that the nitrocarburizing layer consists of a N-rich expanded austenite layer near the surface, and a C-rich expanded austenite layer closer to the substrate. Notably, all precipitates occur within the N-rich expanded austenite. When the temperature exceeds 430 °C, Cr3C2 and M2-3N emerge in the near-surface region. Furthermore, at 470 °C, with the prolongation of time, the M2-3N near the surface gradually decreases and even disappears, as the increase in carbon content within the N-rich expanded austenite enhances its stability. Specifically, at 430 °C, due to the high chemical potential of carbon and the nitrogen-induced lattice expansion, M5C2 forms near the interface between the N-rich layer and the C-rich layer. However, when the temperature rises to 470 °C, a significant amount of thermodynamically more stable M7C3 and M23C6 precipitates with the assistance of chromium diffusion. These findings may provide new insights for the process design and performance optimization of nitrocarburizing.

The oxide films formed under the low oxygen pressure in a particular system containing competing chromium and aluminium

AbstractIn this paper, pre‐oxidation treatment was carried out at 950 °C under oxygen pressure of 10−16 atm for nickel‐10 % chromium‐x % aluminium‐1 % silicon‐0.5 % yttrium(x=0 wt.–%, 1 wt.–%, 3 wt.–%, 5 wt.–%) alloys. Morphology and structure of oxide layers on the alloys after pre‐oxidation were analyzed by scanning electron microscopy and electron discharge spectroscopy. The results showed that a chromium oxide film was formed on the surface of the alloy without aluminium addition, while the oxide layer on the surface of the alloy with aluminium addition gradually transformed into M2O3 (M=aluminium and chromium) oxide film. The oxidation behavior was analyzed by means of chemical thermodynamics, and it was seen that because the interaction coefficient,εijof aluminium and chromium is a positive value, the increase of aluminium addition will strengthen the interaction and increase the diffusion coefficient,Diof aluminium and chromium. The diffusion of chromium and aluminium in each other therefore promotes their interaction, which is beneficial to the formation of the outer oxide film. Thus, with the increase of aluminium addition, the thickness of the outer oxide film gradually increased, and the aluminium content in M2O3 (M=aluminium and chromium) increased significantly, and with 5 % aluminium added, the aluminium oxide of the outer oxide film became the main body.

Mitigating fuel cladding chemical interactions by enhancing chromium diffusion barrier performance using pulse reverse electroplating

Fuel cladding chemical interaction (FCCI) is a critical issue affecting the safe usage of metallic nuclear fuel in sodium-cooled fast reactors (SFRs). Although a chromium (Cr) coating diffusion barrier has been developed to effectively mitigate FCCI, Cr coatings formed by direct current (DC) and pulse current (PC) electroplating generate hydrogen-induced micro-cracks that reduce diffusion barrier performance. In this study, electroplating with a pulse reverse (PR) current and optimized anodic time was utilized to minimize the effects of hydrogen. Consequently, this approach enabled the formation of a crack-free and highly stable microstructure in the electroplated Cr coating. A diffusion couple test (FCCI test) was conducted at 923 K for 25 h to evaluate that the diffusion barrier performance of the Cr coating formed by DC, PC and PR electroplating. The test results demonstrate the superior diffusion barrier performance of the Cr coating formed by PR electroplating through a systematic analysis comparing the microstructures of Cr coatings formed by different waveforms.

Investigation of the Relationship between Molten Fluoride Salt Structure and Fluoroacidity Using Chromium As a Probe Redox Ion

Molten fluoride salts have been studied for decades due to their applications in the production of aluminum as well as in the operation of advanced nuclear reactors. Advanced fission and fusion nuclear reactor designs may include molten fluoride salts as coolants, fuel carriers (fission), or fuel breeding blankets (fusion). Relevant to their nuclear applications are the thermophysical and thermochemical properties of the salt. These include viscosity and thermal conductivity, as well as density, vapor pressure, and heat capacity. Underlying these properties are the interactions of the ions constituting the liquid, characterized by their coordination number, correlation times, and the possible formation of polymeric networks in the melt. The composition and structure of a melt, that is, what ions are present and how they interact, dictates the macroscopic properties relevant to reactor operation. Fluoroacidity, or the activity of dissociated fluoride anions, is one metric that may be used to describe the relationship between salt chemistry and structure. Overall, understanding the relationship between molten fluoride salt chemistry and structure enables deeper understanding and, thus, prediction of property changes that may take place throughout a reactor’s lifetime.In this poster, we present cyclic and square wave voltammetry studies of chromium speciation and diffusivity in various molten fluoride salts, connecting results with current understanding of the structure of the melts. Chromium is a thermodynamically favored corrosion product in molten salt systems, and in this study, it serves as a probe ion in different fluoride melts. Chromium trifluoride is added to various fluoride solvents and the diffusion coefficients of its various oxidation states are calculated and interpreted in light of salt structure. Experimental apparatus design and know-how for high-temperature molten salt electrochemistry are also highlighted.

Microstructures and mechanical behaviour of bimetallic structures of tungsten alloy (90WNiFe) and nickel alloy (In625) fabricated by wire-arc directed energy deposition

ABSTRACT This study reported the fabrication of bimetallic structures (BS) from tungsten alloy (90WNiFe) and nickel alloy (In625) via wire-arc directed energy deposition (DED)-based additive manufacturing (AM). Its microstructures and mechanical properties were investigated for three different heat input conditions: low (180A), medium (200A), and high (220A). The highest average ultimate tensile strength of 618 MPa was achieved in the low heat input condition, with an average elongation of 49%. Although the ultimate tensile strength of the produced BS is lower than that of 90WNiFe and In625 individually, its yield strength closely resembles that of In625 processed by wire-arc DED. Tensile samples from each heat input condition experienced different fracture locations and exhibited distinct fracture morphologies. It can be concluded that the bonding strength is attributed to the diffusion of chromium, nickel, molybdenum, and niobium from In625 into the γ-(Ni, Fe, W) binding matrix of the 90WNiFe substrate.

CuMn1.7Fe0.3O4 – RE2O3 (RE = Y, Gd) bilayers as protective interconnect coatings for solid oxide cells

Efficient replacement of materials based on critical elements such as cobalt is one of the greatest challenges facing the field of solid oxide cells. New generation materials, free of cobalt show potential to replace conventional materials. However, these materials are characterised by poor ability to block chromium diffusion. This article described the study of CuMn1.7Fe0.3O4 (CMFO) spinel combined with single metal oxide (Y2O3 or Gd2O3) thin films as protective coatings for steel interconnects. CMFO was examined using XRD and TPR. Coated steel samples were oxidised in an air atmosphere at 700 °C for 4000 h. The coatings and oxide scale microstructures and cross-sections were examined by CRI, XRD, and SEM-EDX. The electrical properties of the steel-coating system were evaluated using Area Specific Resistance measurements. Based on the results obtained, it can be concluded that the use of thin layers of rare earth oxides allowed for better blocking of chromium diffusion.

High corrosion resistance of nanograined and nanotwinned Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion

This study examines the corrosion resistance of ultrafine-grained Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion (HPT), focusing on stored energy, diffusion, and the high-entropy effect. Microstructural analysis revealed a single-phase FCC structure with numerous defects and reduced crystallite size following the HPT process. Samples with higher HPT turns formed high-angle grain boundaries, equiaxed nanograins, and numerous nanotwins, contributing to a notable increase in Vickers microhardness from 159 Hv for the initial sample to 550 Hv after 5 turns of HPT. Corrosion behavior is found to be influenced by stored energy and chromium diffusion. Low HPT turns increased the corrosion rate due to sluggish diffusion of chromium and high stored energy in the form of dislocations. Conversely, corrosion resistance improved with increased HPT turns due to fast diffusion through non-equilibrium grain boundaries and the formation of nanotwins with lower stored energy. These findings suggest that a relatively low entropy plays a critical role in the chromium segregation from the solid solution for forming a passive film and enhancing corrosion resistance.

Experimental investigation of electrochemical and mechanical properties of stainless steel 309L at different built orientation by cold metal transfer assisted wire arc additive manufacturing

In this article, a multi layered wall of stainless steel 309 L (ER309L) material was WAAM printed over a substrate, at optimized parameters using gas metal arc welding as heat input source assisted by cold metal transfer machine, aiming to evaluate mechanical and electrochemical properties at different built orientations i.e., 0°, 30°, 45°, 60°, and 90°. Wall samples were cut-out by wire cut electric discharge machine to compare their properties. Microstructural properties and phases were confirmed by Scanning Electron Microscopy, Energy-Dispersive Spectrometry (EDS) and X-ray diffractometry (XRD), as they change on melting and solidification along the building direction. Columnar dendrite and vertical ferritic grains were observed through an optical microscope. SEM images suggests the presence of Widmanstatten austenite laths forming δ/γ duplex phase. Element distribution by EDS was further verified by Schaeffler’s diagram. XRD pattern confirms various elements and their phase compositions. The tensile strength and yield strength of samples decreased with increase in orientation angle, which is attributed to decrease in the inter-layer length which restricts the slipping of grains and reestablishment of the residual forces., tableau software was used to theoretically predict values for 45° sample as it broke outside the gauge length. electrochemical properties and corrosion behaviour were studied by electrochemical impedance spectroscopy (EIS) and Potentiodynamic polarization (PDP) where in highest Rct=170 Ω and lowest corrosion rate, 0.56 mils per year (MPY) was observed for 45° is attributed to diffusion of chromium to the metal-solution interface forming a passive film. The corrosion rate range 0.48–2.7 MPY, advocates SS309L have a uniform passive film. The properties studied certifies that WAAM printed stainless steel component can be exploited for critical applications.

Evaluation of Sensitization Behaviors on the Heat-Affected Zone of Austenitic Stainless Steel Weld by Thermal Cycles of Actual Multi-pass Welding

The sensitization behavior of the welding heat affected zone (HAZ) in austenitic stainless steels (SSs) was investigated through simulated thermal cycles emulating actual multi-pass welding processes using the Gleeble simulator. The tests were performed with austenitic SSs, considering carbon contents, heat input, and distance from the fusion line to determine the thermal cycle conditions of the HAZ. Higher carbon content led to increased sensitization (degree of sensitization, DOS) values, while the influence of the thermal cycle in the final weld pass was that even though it was rapidly heated to over 1000 °C and cooled at a rapid rate, the DOS value decreased due to partial carbide dissolution and chromium diffusion. Therefore, effective management of the final thermal cycle in the HAZ contributes to improved intergranular stress corrosion cracking resistance. Even with prolonged exposure of the HAZ to the sensitization region, the discovery that corrosion resistance improves when the final heating cycle reaches 1000 °C underscores the importance of HAZ heat cycle management and provides valuable insights for materials engineering and industrial applications.Graphical abstract

Influence of ageing treatment time on microstructural evolution, sensitization and self-healing of Inconel 600

Samples of Inconel 600 were thermally aged at 739 °C from 1 up to 300 h followed by water quenching. The changes in microstructure were characterized and its effect on sensitization was evaluated by subjecting the heat-treated samples to double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The evolution of the microstructure was analysed in terms of grain size, carbide formation and chromium diffusion by optical and scanning electron microscopy. The results showed that the formation of chromium carbides is rapid at this temperature, increasing the density of carbides at the grain boundaries in the early stages of the ageing treatment. The formation of Cr7C3 and Cr23C6 along the grain boundaries and within the austenitic matrix modified the corrosion behaviour as seen in the measurements of the degree of sensitization (DOS). A decrement in DOS was seen with ageing time up to 200 h, due to the redistribution of chromium, induced by the equilibrium between chromium at grain boundaries and within the matrix. Electron backscattering diffraction suggested that self-healing of the alloy may be due to the mechanism of near-boundary gradient zones, where the increased number of dislocations leads to precipitation of carbides by promoting diffusion and thereby decreasing the Cr-depleted zones.

Stress-corrosion cracking sensitization by hydrogen upon oxidation of nickel-base alloys by water – An experiment-guided first-principles study

This work describes a viable hydrogen-induced sensitisation process toward stress corrosion cracking in chromia-forming nickel base alloys owing to oxidation by water. A mechanistic chemical understanding of the stress corrosion based on repeated cracking and re-healing of the oxide scale emerges from experiment-guided first-principles calculations. Piggybacking processes during repeated oxide scale cracking-healing cycles are understood to cause sensitisation towards the formation and growth of macroscopic cracks. Under light-water reactor conditions, the healed oxide scale comprises an outer region composed of chromium depleted nickel ferrite, and non-protective chromia, nickel oxide, and nickel hydroxide. An inner nickel iron chromite layer provides the passivating barrier that controls the oxidation rate at a steady state. Early during the re-healing of a crack in the oxide scale, water is conveyed to the alloy/scale interface by hydration/dehydration of composite nickel oxy-hydroxide Ni(OH)2∙NiO inclusions in the scale. At later stages, Ni(OH)2∙NiO serves as oxidant of preferentially chromium, while hydrogen is released as H2(g) into the primary water or becomes picked up by the alloy, possibly while assisting in the reduction of NiO to form nickel metal particles. Oxidation by water equivalents at the confining alloy/oxide scale interface favours hydrogen pick-up in the alloy that becomes increasingly detrimental owing to enrichment and inward diffusion of hydrogen along alloy grain boundaries. The pinning of alloy vacancies by hydrogen causes mitigation of outward diffusion of chromium, which in turn hampers crack re-healing while promoting alloy embrittlement.

Influence of the Composition of Saturating Mixture on the Depth of Diffusion of Chromium and the Coating Structure During Chromovanadizing

Influence of the Composition of Saturating Mixture on the Depth of Diffusion of Chromium and the Coating Structure During Chromovanadizing

Chromium diffusion coatings for improving the oxidation behavior of refractory metals at intermediate temperatures

Due to their very high melting points and high mechanical strength at elevated temperatures, refractory metals are promising structural materials for high temperature applications. However, their application is strongly restricted by their insufficient oxidation resistance. In this fundamental study, four different refractory metals (Mo, Nb, Ta, and W) were chromized by pack cementation to investigate the influence of chromium diffusion coatings on their oxidation resistance at intermediate temperatures. Thermodynamic calculations were used to find the most suitable conditions for the pack cementations. The formation of a homogeneous single-phase bcc Cr layer with a thickness below 10 μm on the sample surface was observed on all four refractory metals. In the case of Nb and Ta, intermetallic Cr2X-Laves phases (X = Nb, Ta) formed underneath the bcc Cr layer while an interdiffusion zone formed on Mo. The concentration profiles of the deposited Cr layers were used to determine the diffusion coefficients graphically via Boltzmann-Matano analysis. The Cr-coated samples were analyzed via thermogravimetric analysis at 700 °C and 900 °C for up to 100 h in dry synthetic air. Optical and electron microscopy, EPMA, EBSD and XRD were used to investigate the chromium diffusion layers and the oxide scales.

The incipient oxidation behaviors of Haynes 282 at high temperatures

The purpose of this study was to investigate the oxidation behavior and the development of residual stress in the nickel-based superalloy Haynes 282 at temperatures ranging from 800 to 950 °C. Detailed examinations of the oxide scale surfaces were conducted using Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB). The residual stresses in the Cr2O3 scales were measured using X-ray diffraction (XRD). A layered chromia scale was formed as a result of the oxidation process, and the kinetics of this process were found to be parabolic, controlled by Chromium diffusion. An external Cr2O3 layer was observed in cross-sections, along with Kirkendall voids at the alloy-oxide interface. All residual stresses of chromia observed were tensile intrinsically.

On oxidation film structure of SIMP steel exposed in stagnant oxygen-saturated LBE at 600 °C

To understand the impact of multilayered oxide films on the liquid-metal resistance, SIMP steels with three different Si levels were investigated. A novel composite oxidation film, namely amorphous SiO2 packed by Cr2O3, was formed along the prior high-angle grain boundaries in internal oxidation zone to impede the inward diffusion of oxygen. Evolved into the inner layer, the amorphous SiO2 films stayed there, while Cr2O3 transformed into FeCr2O4. Moreover, outward diffusion of chromium facilitated the transformation from lower sublayer to upper sublayer in inner layer. This multilayer plays a critical role in improving the oxidation resistance of SIMP steel.