Application of yttrium coatings on chromium- base alloys by metalliding Final report
Electroplating and vacuum evaporation of yttrium oxide-chromium oxide coating of chromium alloys for nitrogen embrittlement protection
Berkowitz-Mattuck, J.↗
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Inhibition of Chromium(III) Oxidation through Manganese(IV) Oxide Passivation and Iron(II) Abiotic Reduction
Manganese (Mn) oxides are strong oxidants that are ubiquitous in soils and can oxidize redox-active metals, including chromium (Cr). In soil environments, trivalent chromium (Cr(III)) is a benign, immobile micronutrient, whereas the hexavalent Cr(VI) form is present as a highly mobile, toxic chromate oxyanion. Although many studies have characterized the capacity of Mn(III/IV) oxides to oxidize Cr(III) to toxic Cr(VI), the oxidative capacity of Mn oxides in the presence of potentially passivating soil constituents, specifically reduced soluble iron (Fe(II) aq ), remains unresolved. We hypothesized that chemical processes at redox interfaces, such as diffusion-limited environments within soil aggregates, can lead to decreased Cr(VI) production from Mn oxide-driven oxidation due to passivation by Fe(II) aq . A multichamber diffusion-limited reactor was used to simulate transport at soil redox interfaces and investigate the capacity of poorly crystalline and crystalline Mn oxides to oxidize solid Cr(III) minerals to Cr(VI) in the presence of Fe(II) aq . As predicted, Cr(VI) was produced through the Mn oxide-catalyzed oxidation of Cr(III) at a rate controlled by the solubility of Cr(OH) 3 . However, in the presence of Fe(II) aq , the concentration of aqueous Cr(VI) decreased as a function of the Fe(II) aq concentration, where high concentrations of Fe(II) aq completely inhibited Cr(VI) production, likely through both the passivation of the Mn oxide and the direct reduction of Cr(VI) by Fe(II). At both low (14 μM) and high (100 μM) Fe(II) aq concentrations, the iron oxide minerals hematite (Fe 2 O 3 ) and goethite (α-FeOOH) were associated with the Mn oxides, which can cause surface passivation, a likely role that decreases Cr(III) oxidation. Additionally, the Cr(III) oxidation rate decreased with increasing crystallinity of the Mn oxides whether or not Fe(II) was present.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Statistical fracture behavior of doped UO 2 using a ball-on-ring equibiaxial flexure test method
Metal oxide dopants, such as titanium and chromium oxides, have garnered considerable attention for their potential to increase grain size (≥ 30 µm) in UO 2 fuel, purportedly enhancing fission gas retention during reactor operation. Fuel performance is significantly impacted by fuel fracture behavior, so it is important to understand the effects of enhanced grain size and dopant content on UO 2 fuel fracture. UO 2 pellets were doped with 0.1 wt% TiO 2 and 0.3 wt% Cr 2 O 3 to alter density and grain size. Inductively coupled plasma mass spectroscopy measured dopant levels pre- and post-sintering. X-ray diffraction revealed lattice changes and microstrain via Rietveld refinement. Field emission scanning electron microscopy determined grain sizes of approximately 30 µm for TiO 2 doping and 7 µm for Cr 2 O 3 doping. Transverse rupture strength tests were performed on over 30 samples per dataset to obtain characteristic strength and Weibull modulus. Results indicate no statistical difference in fracture strength between 0.1 wt% TiO 2 doped UO 2 and undoped UO 2 , while 0.3 wt% Cr 2 O 3 doped UO 2 exhibited a 20% decrease in fracture strength. Doped UO 2 samples also showed reduced Weibull modulus compared to undoped UO 2 , suggesting increased scatter in fracture strength. This study's findings suggest that titanium and chromium oxide doping in UO 2 , regardless of grain size, induce residual stresses, decreasing fracture strength and increasing variability in fracture behavior.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
High temperature oxidation resistant cermet compositions
Cermet compositions are designed to provide high temperature resistant refractory coatings on stainless steel or molybdenum substrates. A ceramic mixture of chromium oxide and aluminum oxide form a coating of chromium oxide as an oxidation barrier around the metal particles, to provide oxidation resistance for the metal particles.
Phillips, W. M.↗
Interaction of beryllium with 316H stainless steel in molten Li 2 BeF 4 (FLiBe)
The increased demand in renewable energy resources has led to a renewed interest in nuclear reactors including a new generation of molten salt reactors. One of the issues encountered in handling of molten chloride or fluoride salts is the inability of metals to form and retain a protective oxide surface layer, generally chromium oxide. The low chromium content, nickel-based alloy INOR-8, later marketed as Hastelloy N, was specifically developed to contain molten fluorides. This alloy had some deficiencies, but its composition resulted in it incurring far less chromium depletion by the molten fluoride salts than alloys like 316 stainless steel. For the next generation of molten salt reactors, an alloy with pressure vessel code approval and higher temperature capability than Hastelloy N is required. Consequently, several reactor designers have reportedly chosen to build the reactor containment vessel from 316H stainless steel. In order to minimize corrosion of the stainless steel by the molten fluoride salt (2LiF-BeF 2 ), it is proposed to add beryllium metal to the salt to react with impurities and provide a means to lower the oxidation potential thus making the salt less corrosive. Even so, there is a concern whether the beryllium would react with components of the stainless steel. To address this concern, static capsule tests were conducted in which selected amounts of beryllium were added to capsules containing 2LiF-BeF 2 (FLiBe) salt and 316H stainless steel and Hastelloy N samples. Following exposure, the samples were cleaned and examined using a variety of analysis techniques including, optical microscopy, SEM-EDS, XPS, LIBS, XRD and EPMA. Tensile samples were also exposed in capsules, and those were subjected to tensile testing. Notably, It was found that for the level of beryllium additions used in this study, intermetallic compounds were formed which could be detrimental to the long-term performance of the 316H stainless steel.
36 MATERIALS SCIENCE↗
Oxidation behavior of nickel-chromium-aluminum-yttrium - Magnesium oxide and nickel-chromium-aluminum-yttrium - zirconate type of cermets
The 1100 and 1200 C cyclic oxidation resistance of dense Ni-Cr-Al-Y - MgO, Ni-Cr-Al-Y - CaZrO3, Ni-Cr-Al-Y - SrZrO3, Ni-Cr-Al-Y - MgZro3 cermets and a 70 percent dense Ni-Cr-Al-Y developmental material was determined. The cermets contained 60 and 50 volume percent of Ni-Cr-Al-Y which formed a matrix with the oxide particles imbedded in it. The cermets containing MgO were superior to cermets based on zirconates and to the porous Ni-Cr-Al-Y material.
Zaplatynsky, I.↗
First-principles insights into chromium-induced oxide phases in NiO
The high-temperature oxidation of Ni–Cr alloys leads to complex oxide scales comprising Ni(Cr)O solid solutions, NiCr 2 O 4 spinel, and Cr 2 O 3 corundum within the NiO matrix. Understanding the atomic-scale mechanisms of Cr segregation and precipitate formation is crucial for enhancing oxidation resistance. Here, we employ density functional theory calculations to investigate Cr behavior on NiO(100), (110), and (111) surfaces and in the bulk. Our results reveal that isolated Cr atoms preferentially segregate to the surfaces, stabilizing Ni(Cr)O solid solutions via strong Cr–O bonding, whereas Cr pairs and clusters favor subsurface migration and bulk aggregation, promoting nucleation of NiCr 2 O 4 and Cr 2 O 3 phases. These findings elucidate a size-dependent segregation mechanism linking Cr coordination environments to oxide phase evolution. This atomic-scale insight informs strategies to tailor oxide microstructures and enhance the high-temperature oxidation resistance of Ni–Cr alloys.
Chromium Segregation↗
Kinetics of Radiation-induced Cr( II ) and Cr( III ) Redox Chemistry in Molten LiCl-KCl Eutectic
Chromium (Cr) is a frequent constituent of the metal alloys proposed for molten salt nuclear reactor (MSR) applications, and is typically the least noble metal ion present. Consequently, chromium is preferentially corroded into molten salt solutions. Here, the redox poise and redox cycling of chromium ions in the salt can greatly influence its corrosivity towards structural alloys, ultimately impacting the longevity of MSR systems. Radiation-induced chemistry is expected to play a significant role in determining the chromium oxidation state distribution during MSR operations. In the present research, electron pulse radiolysis techniques were employed to characterize the reactivity of Cr(II) and Cr(III) ions with primary radiolysis products in molten lithium chloride–potassium chloride (LiCl–KCl) eutectic over a temperature range of 400–600 °C. Both chromium oxidation states were found to rapidly react with the primary products of molten chloride salt radiolysis, i.e., the solvated electron (e S - ) and the dichlorine radical anion (Cl 2 ˙ - ). For reactions with the e S - , second-order rate coefficients (k) of k = (4.1 ± 0.2) and (6.1 ± 0.3) × 10 10 M -1 s -1 at 400 °C for Cr(II) and Cr(III), respectively, were determined. Temperature-dependent measurements allowed for the derivation of activation parameters for electron capture by Cr(II) and Cr(III). Both chromium ions also react with Cl 2 ˙ - , k = (7.2 ± 0.3) and (1.4 ± 0.1) × 10 9 M -1 s -1 at 400 °C for Cr(II) and Cr(III), respectively.
37 - INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL C↗
Steam oxidation of chromium corrosion barrier coatings for sic-based accident tolerant fuel cladding
SiC-based materials are currently being considered for accident tolerant fuel (ATF) cladding, however, there are concerns regarding hydrothermal corrosion rates and hermeticity. These issues may be addressed with the utilization of a Cr corrosion barrier coating (CBC) but the interaction of Cr with SiC under loss of coolant accident (LOCA) conditions needs to be investigated. In the present work, Cr coatings were deposited onto chemically vapor deposited (CVD) SiC with either high power impulse magnetron sputtering (HiPIMS), cathodic arc (CA), or a combination of both. Annealing and steam thermogravimetric analysis (TGA) was used to study reaction product formation with and without oxidation. Additionally, after 1200 °C annealing, formation of a Cr x C y carbide layer at the ambient interface and a Cr x Si y C z silicide layer at the SiC interface was observed. After 1200 °C TGA exposure, similar carbide and silicide layers were observed but with a Cr 2 O 3 outer reaction layer and a Si and O rich interfacial layer forming between the carbide and silicide layers. Initial mass gain of the coatings during the 1200 °C TGA exposure was parabolic and were similar to reported rates of chromia formation. Mass gain behavior and observed transitions in chromia microstructure indicate that metallic Cr may be consumed within the first ~1 h at 1200 °C and that subsequent chromia formation occurs at the expense of the underlying Cr carbide layer. Overall, the results show no detrimental impact of Cr coatings on the steam oxidation resistance of SiC up to 4 h at 1200 °C.
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
Tribological Behavior of Structural Materials in High Temperature Helium Gas-Cooled Reactor Environments
The High-Temperature Gas-cooled Reactor (HTGR) is a Generation IV concept designed to produce electricity and hydrogen at high efficiencies via high operating temperatures (700 °C or higher). Helium, the primary coolant for HTGRs, contains impurities (e.g., CO, H2O and CH4) that can induce corrosion reactions at high temperatures such as oxidation and (de)carburization, which in turn can affect the tribological behavior of components. Incoloy® 800HT (Ni-Fe-Cr austenitic solid-solution alloy) and Inconel® 617 (Ni-Cr-Co-Mo solid-solution alloy) are two high-temperature superalloys currently selected as candidate structural materials for the HTGR. The objective of this study is to evaluate the high-temperature tribological performance of these two candidate alloys after conditioning them in HTGR environments. Four regimes of corrosion were considered: a non-conditioned regime (Regime I), an oxidizing regime (Regime II), a carburizing and oxidizing regime (Regime III), and a carburizing regime (Regime IV). To simulate oxidation in HTGR, samples were conditioned for 22 days at elevated temperatures in a once-through helium loop with 4 ppmv H2O. Carburization of the samples was achieved by the commercial process, Kolsterising®. In addition, two surface treatments – shot peening and aluminization – were considered as potential routes of improving the alloys’ wear resistance. Surface-treated samples were tested both before and after conditioning in Regime II. Tribological testing of the samples was performed via a pin-on-disk tribometer at elevated temperatures in ambient environment – 650 and 750°C for 800HT; 850 and 900°C for 617 – with applied loads of 1N, 2N and 5N. The wear behavior of the alloys was assessed via wear volume and friction coefficient measurements, supported by morphological, structural and compositional analyses of the wear tracks. Conditioning the samples in Regime II led to the formation of a chromium oxide on both alloys. This protective scale increased the wear resistance compared to that of as-received samples (Regime I) due to the formation of a compacted ‘glaze’ oxide layer during sliding, rendering the wear track nearly undistinguishable from the unworn background and resulting in wear volumes below the detection limits of the measurement technique. The variability of the friction coefficients of the conditioned samples was also considerably reduced compared to that of the as-received samples due to the glaze layer. Additionally, the initial friction coefficients of the samples conditioned in Regime II were reduced by 45% and 54% compared to those of as-received samples for alloys 800HT and 617, respectively. Carburizing the samples (Regime IV) hardened the surfaces of both alloys and promoted the formation of an iron oxide on 800HT during tribological testing, thereby increasing the wear resistance and decreasing the initial friction coefficient by a factor of two compared to those of as-received 800HT. Alloy 617 exhibited an enhanced wear resistance but similar initial friction coefficient compared to those of as-received samples due to the increased surface hardness. The Mn-Cr oxide developed during Regime III conditioning of 800HT did not develop a glaze layer during tribotesting, even at lower loads. Thus, it was not as protective as the oxide produced during conditioning of Regime II samples, leading to lower wear resistance of 800HT in Regime III compared to that measured in the Regime II condition. 800HT benefited from the aluminization surface treatment, particularly before conditioning in Regime II, due to the promotion of a wear-resistant aluminum oxide layer during testing and the increase in surface hardness due to the presence of a FeAl intermetallic phase. The presence of this phase caused an order-of-magnitude reduction in wear volumes compared to those of as-received 800HT. Aluminization of 617 decreased its wear resistance, particularly after conditioning in Regime II, due to significant wear of the aluminum oxide via a ceramic wear mechanism. This aluminum oxide was removed during sliding, causing the wear volumes to increase by a factor of five compared to those of 617 conditioned in Regime II. Shot-peened 800HT exhibited a tribological behavior similar to that of as-received 800HT, despite an enhancement in the rate of chromium oxide formation. Shot-peened 800HT conditioned in Regime II showed lower wear resistance compared to that of 800HT conditioned in Regime II due to the poor adhesion between the oxide and the underlying metal, preventing the formation of the glaze layer during sliding. Shot peening of 617 increased its wear resistance at higher load, reducing worn volumes by 60% compared to that measured with as-received 617. Tribotesting of shot-peened 617 in Regime II revealed that the glaze-oxide layer was formed at every load, resulting in negligible wear, similar to what was observed from testing 617 after conditioning in Regime II. Overall, alloy 617 exhibited tribologically superior behavior compared to that of alloy 800HT, as demonstrated by its lower friction coefficients and its order-of-magnitude lower wear volumes when measurable.
36 MATERIALS SCIENCE↗
Steam reforming of ethylene over manganese-chromium spinel oxides
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99 GENERAL AND MISCELLANEOUS↗
Auger electron spectroscopy study of oxidation of a PdCr alloy used for high-temperature sensors
A Pd-13 wt. percent Cr solid solution is a promising high-temperature strain gage alloy. In bulk form it has a number of properties that are desirable in a resistance strain gage material, such as a linear electrical resistance versus temperature curve to 1000 C and stable electrical resistance in air at 1000 C. However, unprotected fine wire gages fabricated from this alloy perform well only to 600 C. At higher temperatures severe oxidation degrades their electrical performance. In this work Auger electron spectroscopy was used to study the oxidation chemistry of the alloy wires and ribbons. Results indicate that the oxidation is caused by a complex mechanism that is not yet fully understood. As expected, during oxidation, a layer of chromium oxide is formed. This layer, however, forms beneath a layer of metallic palladium. The results of this study have increased the understanding of the oxidation mechanism of Pd-13 wt. percent Cr.
Boyd, Darwin L.↗
Contact Sensor Attachment to Titanium Metal Composites
A Pd-13wt%Cr solid solution is a promising high-temperature strain gage alloy. In bulk form it has a number of properties that are desirable in a resistance strain gage material, such as a linear electrical-resistance-versus-temperature curve to 1000 C and stable electrical resistance in air at 1000 C. However, unprotected fine wire gages fabricated from this alloy perform well only to 600 C. At higher temperatures severe oxidation degrades their electrical performance. In this work Auger electron spectroscopy has been used to study the oxidation chemistry of the alloy wires and ribbons. Results indicate that the oxidation is caused by a complex mechanism that is not yet fully understood. As expected, during oxidation, a layer of chromium oxide is formed. This layer, however, forms beneath a layer of metallic palladium. The results of this study have increased the understanding of the oxidation mechanism of Pd-13wt%Cr.
Vargas-Aburto, Carlos↗
NASA PS400: A New Temperature Solid Lubricant Coating for High Temperature Wear Applications
A new solid lubricant coating, NASA PS400, has been developed for high temperature tribological applications. This plasma sprayed coating is a variant of the patented PS304 coating and has been formulated to provide higher density, smoother surface finish and better dimensional stability than PS304. PS400 is comprised of a nickel-molybdenum binder that provides strength, creep resistance and extreme oxidative and dimensional stability. Chromium oxide, silver and barium-calcium fluoride eutectic are added to the binder to form PS400.Tribological properties were evaluated with a pin-on-disk test rig in sliding contact to 650 C. Coating material samples were exposed to air, argon and vacuum at 760 C followed by cross section microscopic analysis to assess microstructure stability. Oil-Free microturbine engine hot section foil bearing tests were undertaken to assess PS400 s suitability for hot foil gas bearing applications. The preliminary results indicate that PS400 exhibits tribological characteristics comparable to the PS304 coating but with enhanced creep resistance and dimensional stability suitable for demanding, dynamic applications.
DellaCorte, C.↗
Characterizing Radiation-Induced Chromium Redox Chemistry
The redox behavior of Cr(III) and Cr(VI) in nuclear environments were studied by analyzing aqueous irradiated chromium ions using UV-Visible spectroscopy. The formation of different chromium oxidation states can occur through gamma radiolysis of chromium solutions. Characterization of radiation-induced chromium is not well understood, and yet important as chromium can make its way into the primary coolant of reactors because of the corrosion of stainless steel reactor components. Furthermore, Cr(VI) is toxic and not suitable for environmental release, therefore Cr speciation is important. Fricke dosimetry was performed to collect doses rates, specific extinction coefficients were obtained from various analytical methods for the two oxidation states of chromium. Chromium samples--conditioned at various pH's and concentration to understand the effects--were then irradiated using a Coblat-60 gamma irradiator. Three optical spectroscopy analytical methods were used , including direct oxidation state measurements and complexation by either EDTA or DPC. Complexation by EDTA was found to not be a reliable method. Radiolytic oxidation of Cr(III) to Cr(VI), and reduction of Cr(VI) to Cr(III) were clearly seen, except for Cr(III) samples were a pH less than or equal to 2, where no change occurred.
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells
The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr 2 O 3 -forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H 2 O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr 2 O 3 -forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. As a result, long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H 2 O to provide further guidance for SOFC BOP component alloy selection.
08 HYDROGEN↗
Electrochemical cleaning: An in-Situ method to reverse chromium poisoning in solid oxide fuel cell cathodes
Chromium poisoning is one of the primary factors that affect solid oxide fuel cell (SOFC) durability. In this study, an electrochemical cleaning method has been employed to reverse the effects of chromium poisoning. Two cells were fabricated and poisoned under identical conditions by operating them at 800°C in the presence of a Cr source. Both poisoned cells exhibited a significant reduction in maximum power density and an increase in polarization resistance. One of the poisoned cells, designated as the baseline cell, was cooled to room temperature after poisoning. Microstructural observation of the baseline cell showed significant contamination of the cathode active layer by chromium oxide and (Cr,Mn) spinel deposits. The other poisoned cell was electrochemically cleaned for two hours under a mild electrolytic condition. As a result of the electrochemical cleaning process, the chromium oxide deposits were substantially removed leading to a significant reduction of Cr concentration in the cathode, especially at the cathode/electrolyte interface. Furthermore, a substantial fraction of the cell performance loss was also reversed. To the best of our knowledge, this is a first demonstration of reversing the effects of chromium poisoning using a rapid in-situ process.
30 DIRECT ENERGY CONVERSION↗