Journal of The Electrochemical Society       OPEN ACCESS Reactive Condensation of Cr Vapor on Aluminosilicates Containing Alkaline Oxides To cite this article: T. K. van Leeuwen et al 2024 J. Electrochem. Soc. 171 091501   View the article online for updates and enhancements. You may also like Evaluation of compressive and split tensile strength of slag based aluminosilicate geopolymer reinforced by waste polymeric materials using Taguchi method Amirreza Khezrloo, Morteza Tayebi, Abbas Shafiee et al. - Determination of Brønsted Acid Sites In Porous Aluminosilicate Solid Catalysts Using Volumetric And Potentiometric Titration Method A Purwaningsih, A N Kristanti, D Z Mardho et al. - Chemistry in confined space through the eyes of surface science—2D porous materials J Anibal Boscoboinik - This content was downloaded from IP address 153.90.170.56 on 24/10/2024 at 21:26 https://doi.org/10.1149/1945-7111/ad7061 https://iopscience.iop.org/article/10.1088/2053-1591/abe101 https://iopscience.iop.org/article/10.1088/2053-1591/abe101 https://iopscience.iop.org/article/10.1088/2053-1591/abe101 https://iopscience.iop.org/article/10.1088/2053-1591/abe101 https://iopscience.iop.org/article/10.1088/1755-1315/217/1/012002 https://iopscience.iop.org/article/10.1088/1755-1315/217/1/012002 https://iopscience.iop.org/article/10.1088/1755-1315/217/1/012002 https://iopscience.iop.org/article/10.1088/1755-1315/217/1/012002 https://iopscience.iop.org/article/10.1088/1361-648X/aaf2ce https://iopscience.iop.org/article/10.1088/1361-648X/aaf2ce https://iopscience.iop.org/article/10.1088/1361-648X/aaf2ce https://pagead2.googlesyndication.com/pcs/click?xai=AKAOjstr01CYWvv29_rPER1ICTyeVhGXOGzbT8OfNN9LDwXk-vraFcFPWuWqh-Ysx8cj2jWtqCBgFUwbkOf8m7JYb2BM1-k3Nlb-Vk0NJl0xwuror7uVLRdiIaSVYnOQoYpOY3OwGaokm5qB8fmcGXD1TxROK-gA_6SE9mtWofK93kJB2A87bMAOxfKHPk1OH50igZkGuVLpSUdoD-BFdpXPQ-Xz32rZ-C5Lkxecjf67zHAOPXd58WEVrGL49uipA44xHFEj0ZgiPMPeJfiYVzfzZ7WjuilsZf_xJtXlQr3zhGz36tscLtnLPNnbJO2ZZS6GRQBB-GT1gcLsUXtltHqnYHttZQLj03mE5M1uZHknUbKW&sig=Cg0ArKJSzHz9taO8Iqn_&fbs_aeid=%5Bgw_fbsaeid%5D&adurl=https://www.el-cell.com/products/test-cells/electrochemical-dilatometer/ecd-4-nano/%3Fmtm_campaign%3Diop%2520pdf%2520advert%26mtm_kwd%3Decd-4-nano%26mtm_source%3Dpdf%26mtm_cid%3D2024 Reactive Condensation of Cr Vapor on Aluminosilicates Containing Alkaline Oxides T. K. van Leeuwen,1,z A. Guerrero,1 R. Dowdy,1 B. Satritama,2 M. A. Rhamdhani,2 G. Will,3 and P. Gannon1 1Department of Chemical and Biological Engineering, Montana State University, Bozeman, Montana 59718, United States of America 2Department of Mechanical and Product Design Engineering, Swinburne University of Technology, Melbourne, VIC 3122, Australia 3School of Science, Technology and Engineering, University of the Sunshine Coast, Sippy Downs, QLD 4556, Australia This study is part of a series with the objective of improving fundamental understanding of reactive condensation of Chromium (Cr) vapors, which are generated from Cr containing alloys used in many high-temperature (>500 °C) process environments and can form potentially problematic condensed hexavalent (Cr(VI)) species downstream. This study specifically focuses on the effects of alkaline oxide additives in aluminosilicate fibers on Cr condensation and speciation. Cr vapors were generated by flowing high- temperature (800 °C) air containing 3% water vapor over chromia (Cr2O3) powder, with aluminosilicate fiber samples positioned downstream where the temperature decreases (<500 °C). Total condensed Cr and ratios of oxidation states were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and diphenyl carbazide (DPC) colorimetric/direct UV–vis spectrophotometric analyses. Results indicate presence of hexavalent Cr (Cr(VI)) species condensed on all samples investigated. The ratio of Cr(VI) to total Cr detected was consistently higher on aluminosilicate fiber samples containing alkaline oxide (CaO and MgO) additions. Computational thermodynamic equilibrium modelling corroborated experimental results showing stabilities of Ca and Mg chromate (Cr(VI)) compounds. Comparative results and analyses are presented and discussed to help inform mechanistic understanding and future related research and engineering efforts. © 2024 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/ad7061] Manuscript submitted May 24, 2024; revised manuscript received July 4, 2024. Published September 2, 2024. Chromium (Cr) vapors are generated in high-temperature (>500 °C) systems that employ chromia (Cr2O3)-forming alloys such as stainless steels used in exhaust manifolds, steam turbines, boilers, or solid oxide fuel/electrolysis cell (SOFC/SOEC) systems. Reactive evaporation of Cr is known to occur in oxygenated environments and increases in the presence of water vapor.1,2 The generated Cr oxide and hydroxide vapors proceed downstream and interact with the surfaces of other components in the system. These component surfaces could be ceramic insulating fibers such as those found in insulation blankets used in power and chemical plants or the ceramic sealants or electrodes used in SOFCs. Similar to observed Cr evaporation trends, condensation of these vapors onto ceramic insulating fibers was also found to increase in the presence of water vapor.1 Understanding the interactions between volatilized Cr species and downstream components during operation is critical for improving system performance, environmental health, and safety as some condensed Cr forms carcinogenic hexavalent Cr(VI) species. Reactive evaporation of Cr from chromia-forming alloys, like stainless steels, commonly used in these systems is well- documented as the primary Cr vapor source implicated in Cr poisoning of SOFCs; however, the interactions between volatilized Cr species and surrounding interfaces during complex and dynamic system exposures is poorly understood. The objective of this study is to explore the effects of alkaline oxide additives in aluminosilicate fibers on Cr collection/condensation and speciation onto these ceramic fibers. Reactive Evaporation Reactive evaporation of chromia from the surface of stainless steels occurs at high temperatures (>500 °C) in oxygenated envir- onments with or without water vapor.1 Volatilized Cr from the oxide layer or reactive evaporation interacts with the surrounding system and can form compounds that pose risks to human health, the environment, and degrade performance in electrochemical devices like SOFCs. The evaporation rate increases in the presence of water vapor and leads to the formation of CrO2(OH)2(g). 3–5 Experimentation reveals a partial pressure of CrO2(OH)2(g) that is larger than that of CrO3(g) by several orders of magnitude in the presence of water vapor up to ∼1400 K3–8 and thermodynamic data corroborate these observations that partial pressure of Cr oxyhydroxide (CrO2(OH)2(g)) is greater in the presence of water vapor by several orders of magnitude up to ∼1500 K.9 Differences in vapor pressure between CrO3 and CrO2(OH)2 are attributable to both stability and intermolecular forces. A mechanism for the formation of surface CrO3 species in wet and dry conditions has been proposed by other researchers.10,11 First, oxygen reacts with chromia to form [-O-(CrO2)-O-(CrO2)-] repeating units with a given vapor pressure at a given temperature and this is followed by conversion of CrO3 to CrO2(OH)2 that occurs through hydrolysis reactions with repeat surface units that results in the severing of Cr- O-Cr bonds to form CrO2(OH)2. If CrO3 volatilizes first, then gas phase hydrolysis occurs to form CrO2(OH)2. The difference in vapor pressure of CrO3 and CrO2(OH)2 is attributed to two factors: dissociation reactions of CrO2(OH)2 facilitated by hydrolysis and the thermodynamic stability of CrO2(OH)2 up to 1400 K.2 Reactive condensation.—While reactive evaporation mechan- isms of Cr are well-documented, there are relatively few publica- tions focusing on the chemical and physical processes that occur during downstream reactive condensation.1,2,12–14 Reactive conden- sation of Cr onto various surfaces under different conditions such as refractory ceramic materials like alumina and silica,15,16 and on SOFC components, such as those made from manganese or strontium, is a well-known occurrence.17,18 Cr is also commonly used as a catalyst on ceramic supports, such as silicas, aluminas, and aluminosilicates.19–31 However, unlike reactive condensation of Cr vapor, the impregnation of Cr onto catalyst supports is completed in an aqueous environment. Speciation of Cr deposited on catalyst supports depends on the support material, processing temperature, and Cr loading. Speciation of Cr condensed from its vapors can bezE-mail: travisvanleeuwen@gmail.com Journal of The Electrochemical Society, 2024 171 091501 https://orcid.org/0000-0002-7625-7238 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ https://doi.org/10.1149/1945-7111/ad7061 https://doi.org/10.1149/1945-7111/ad7061 mailto:travisvanleeuwen@gmail.com https://crossmark.crossref.org/dialog/?doi=10.1149/1945-7111/ad7061&domain=pdf&date_stamp=2024-09-02 initially identified by the color of stains appearing on the ceramic substrate materials (e.g., alumina). Green is indicative of trivalent chromium species whereas brown and yellow are indicative of hexavalent chromium species, or mixtures thereof. In one study, the following compounds were linked to specific staining colors: Condensed Cr2(OH)2 or CrO3 (brown staining), CrO4 (yellow staining), and Cr2O3 (green staining).32 Solid oxide fuel cell degradation.—Reactive evaporation of Cr vapors from stainless steel interconnects used in SOFC/SOECs is well documented as the source of Cr poisoning in these systems.33–36 SOFC/SOECs that operate at lower temperatures (600–850 °C) may use metallic interconnects, such as ferritic stainless steel (FSS).37 As discussed earlier in the introduction, chromia-forming stainless steels like FSS may release volatile chromium trioxide (CrO3) and/or chromium oxyhydroxide (CrO2(OH)2)) when exposed to air and/or H2O. These volatile species proceed downstream and deposit on the cathode through chemical and electrochemical deposition, Cr poisoning also occurs at the cathode/electrolyte interface. This interface is referred to as the triple-phase boundary (TPB) and is the point at which the gas phase, electrolyte, and cathode meet. Cr deposition occurs at two-phase boundary points (cathode/gas, electrolyte/gas) as well. These Cr deposits degrade SOFC perfor- mance over time by forming secondary phases, which decrease the efficiency of ion exchange. In addition to Cr poisoning, chemical interactions between glass–ceramic sealants and FSS interconnects have also been observed during operation.38 Chromium vapor interactions with sealing glasses.—Sealing materials in SOFC/SOEC stacks are used to protect against air and gas leaks between interconnects. Commonly used sealing materials are ceramic glasses belonging to the SiO2–Al2O3 system and contain different compositions of oxides including CaO, Na2O, MgO, K2O, B2O3, Y2O3, and BaO.39,40 Another commonly used sealant material is compressed mica paper.41,42 Two types commonly used in SOFC applications are muscovite (potassium aluminum silicate hydroxide fluoride, KAl2(AlSi3O10)(F,OH)2), and phlogopite (potassium mag- nesium aluminum silicate hydroxide, KMg3(AlSi3O10)(OH)2). Two sealing glass materials are classified as G18 and G#36. G18 is a barium alumino-silicate (BCAS)-based glass–ceramic43 and G#36 is a mixture of SrCO3, CaCO3, boric acid, and various oxides.17 Strontium chromate was observed to form from interac- tions between volatile Cr species and SOFC cathode materials such as lanthanum strontium cobalt ferrite (LSCF) and on sealing glasses containing SrO. Researchers placed G#36 pastes on 430 stainless steel substrate surfaces and held them at 800 °C for one to two weeks.17 Yellow staining on the glass was observed first forming around the edges before progressing inward over time. In another study, chromia and G18 powder were mixed and heated to 950 °C in dry air for 24 h. The resultant product was stained yellow and was analyzed via XRD which confirmed the formation of SrCrO4. 44 Researchers have reported similar findings from barium-calcium aluminosilicate-based glass-ceramic G18 and FSS. Reaction couples between G18 and FSS 446 coupons were heated to 850 °C for one hour then lowered to 750 °C for an additional four hours. Barium oxide was observed reacting with chromia and/or volatile Cr species to form a yellow BaCrO4 resulting in the G18 detaching from the FSS 446 coupon. Separation of the two materials was due to differences in the respective coefficients of thermal expansion between BaCrO4 and G18/FSS 446. While BaCrO4 was not identified using a phase identification technique in this study, a separate study identified this phase using XRD.18 Chromium “getters”.—To combat Cr poisoning of SOFC systems, materials that readily absorb and trap volatile Cr species have been developed. These materials, dubbed “chromium getters,” are those that readily react with gaseous Cr to form thermodynami- cally stable surface species.45 Cr getters are inserted within SOFC stacks to attract gaseous Cr and decrease the total amount in the system. A commonly used design involves coating fibers, wools, or honeycomb substrates with SrxNiyOz. 46–49 Researchers have demonstrated novel methods to mitigate cathode degradation using Cr getters manufactured from low-cost materials.47,49 The getters use a cordierite honeycomb substrate coated with alkaline Earth and transition metal oxides. The first getter design was tested for 500 h under SOFC cathode exposure conditions.49 Chemical and structural analyses show gaseous Cr concentrated at the getter with only the end of the getter free of Cr. The second study measured the electrochemical performance of the SOFC system for 100 h at 850 °C with and without the Cr getter.47 Stable electrochemical performance was maintained for the cell tests with getters, whereas the cell performance in the test without Cr getters rapidly decreased after 10 h. Results from tests with SOFCs and getters demonstrated the high efficiency of Cr capture tech- nology for the preservation of cell performance. Of these Cr getters, few if any consider disposal/recycling methods. For example, in some getters the thermodynamically stable Cr compounds formed are in a hexavalent oxidation state such as SrCrO4(s). 46,49 The presence of Cr(VI) compounds on the getter renders it hazardous waste and requires further chemical processing to become recyclable. The authors of one Cr getter study, however, did take this issue into account and developed a catalytic “Cr getter.” Using a Ti/TiO2 surface to capture Cr compounds, captured Cr(VI) compounds are then reduced to Cr(III).50 The reduction of Cr(VI) compounds to Cr(III) allows for the contaminated Cr getter to be recycled as a regular metal at the end of its service life. The studies presented above show the potential for volatile Cr species generated from stainless steel in high-temperature systems to engage in reactive condensation onto ceramic surfaces to form trivalent Cr(III), and hexavalent Cr(VI) species. The influence of oxides on the surface is of interest as Cr(VI) species formation was observed to increase in the presence of Sr and Ba. Cr(VI) species also formed without these elements for example, Cr catalysts on ceramic supports that often lack alkaline Earth metals, still support Cr(VI) formation.31,51–54 Studying the interactions between these materials and Cr vapors has direct implications for the development of improved high-temperature processes and systems. A path towards this goal includes exploring the interactions between volatile Cr and ceramic surfaces such as aluminosilicates with or without alkaline oxide additives. Experimental An experiment, based on a “transpiration experiment” developed by another research group,3 was designed to investigate reactive evaporation of Cr vapors generated from chromia exposed to 800 °C air saturated with ∼3 vol% water vapor, intended to simulate prototypical high-temperature system conditions. The experimental sample fibers are positioned at the furnace exit where temperatures range from ∼100 °C–500 °C. Figure 1 displays a schematic of experimental design,. Precision-controlled furnace systems are used to simulate common service environments. A water evaporator held at room temperature to achieve the desired water vapor content is attached to the inlet air line from a laboratory bench air supply to effectively filter incoming air and maintain water vapor saturation before reaching the furnace. Heat wrap controlled by a variable AC power supply is used to avoid water condensation before delivery to the tube furnace system. A flow meter is positioned near the exit of the furnace and a water-cooled condenser is used to further filter exhaust material. Temperatures are recorded throughout the system during flow conditions to promote uniformity in experiments. A series of experiments were conducted to assess the effects of alkaline oxide additives on Cr reactive condensation. Prototypical ceramic fiber insulation materials used in this study included high- purity quartz and alumina fibers from laboratory supply vendors Wale Apparatus and EA Consumables, respectively, while the two silica-based alkaline Earth silicate (AES) fiber samples with varying alkaline oxide content from Lynn Manufacturing, an industrial Journal of The Electrochemical Society, 2024 171 091501 thermal insulation vendor. Approximately 2 g of chromia powder and 2 g of ceramic insulating fiber material are used in each experiment. This is an arbitrary amount that is used for repeatability across samples. It could also be considered an extreme case of Cr vapor exposure to the HTIW samples as the Cr source would be mixed with other stainless-steel components if observed in the field. The tube furnace center where the chromia powder is placed was calibrated to 850 °C and temperature was recorded at the exit in three distinct locations where the ceramic material is placed using a thermocouple probe. These locations include inside the furnace at exit, at the threshold of the furnace exit, and outside the furnace exit are marked on the 2.54 cm (1 inch) quartz tube. The temperatures at each location are ∼800 °C, ∼500 °C, and ∼160 °C, respectively. Flow is calibrated to a face velocity of ∼2 cm/s for each experiment and the experiment is run for 150 h. The resulting volumetric flow rate is ∼10 c.c./s or ∼600 c.c./min which places the Cr evaporation rate within the non-equilibrium linear evaporation rate regime according to other researchers,55,56 however these Cr evaporation studies were conducted using alloys as the Cr source and not pure chromia. The location of the laboratory in which these experiments were conducted is at an elevation of 1,470 meters (4,820 feet) above sea level where the ambient pressure is approximately 84.9 kPa (637 mmHg). Pre-exposure samples are characterized using a Zeiss SUPRA 55VP field emission scanning electron microscope (FE-SEM) with an energy-dispersive X-ray spectroscopy (EDX) detector. Figures 2 and 3 below display photographs and FE-SEM images of the fiber samples. Table I below presents fiber diameter statistics measured from the FE-SEM image and chemical analysis of each sample gathered from EDX results. Total Cr and Cr(VI) is quantified using a SPECTROBLUE EOP TI inductively coupled plasma optical emis- sion spectrometry (ICP-OES) and direct UV–vis using a ThermoSci Genesys 20 spectrophotometer with a visual colorimetric diphenyl carbazide (DPC) water test kit from Chemetrics.57 ICP-OES can detect metals and several non-metals in liquid samples at very low concentrations from 1–5 ppb and up to 100 ppm. DPC is a visual or spectroscopic colorimetry technique that utilizes a reaction between DPC and Cr(VI) in acidic conditions, creating a red-violet color in direct proportion to the Cr(VI) concentration. Measurements are made against a color comparator and expressed as ppm (mg/L) CrO4. DPC is limited to lower concentrations (0–0.8 ppm) before relative standard deviation increases to 20%–50% whereas direct UV–vis can be used for concentrations above 0.8 ppm and up to 100 ppm with relative standard deviations of 0.5%.58 Absorbance on the spectrophotometer is measured at λ= 540 nm for DPC and λ= 340 nm for direct UV–vis. ICP-OES, DPC, and direct UV–vis under these conditions produce uncertainties no greater than 10%. The uncertainty for ICP-OES measurements under 0.01 ppm, how- ever, increases to 40%. Solid samples, such as the fibers in this study, need to be prepared via acid digestion before being analyzed. 0.7 M nitric acid is used to digest samples on a hot-plate at 90–95 °C for one hour. Resulting solutions are diluted to 0.3 M before being analyzed via ICP-OES or DPC/direct UV–vis. Figures 2 and 3 below display the four fiber samples used in this study pre-exposure. Table I below presents the physical and chemical characteristics of each sample. Pre-exposure samples of AES #1 and AES #2 wools were also acid digested under the same conditions outlined above and then analyzed via ICP-OES to observe changes in leached Cr, Ca, Mg, Na, and K. The results are presented in the Results section as histogram plots in Figs. 10 and 11. Computational thermodynamic equilibrium modelling of the system was conducted using FactSage 8.2 to obtain information on evaporation and condensation behaviors of Cr species at different temperatures, water vapor concentrations, and fiber materials. Calculations in FactSage were completed using Equilib and Phase Diagram modules and FactPS (pure substances) and FToxid (oxide compounds) databases. At lower temperatures (50 °C ⩽ T ⩽ 100 °C), the Aqueous database was also selected. Equilib calculations were carried out in two stages: evaporation of 2 g of Cr2O3(s) with O2(g) and H2O (g) and condensation of the gas products from the evaporation stage on the fibers of interest (quartz, alumina, AES #1, and AES #2). Addition of O2 and H2O were simulated based on the experimental set up (0.6 L/min gas flowrate, 150 h reaction, and atmospheric air mixed with H2O). Ideal gases, pure substances, and Figure 1. Experimental design diagram (top), experimental setup (bottom left), and chromia powder with ceramic insulation in furnace (bottom right) with temperature gradient locations. Journal of The Electrochemical Society, 2024 171 091501 all solutions were selected as products. The parameters used in the calculations include: 1 atm total pressure, 850 °C for evaporation, and 100–800 °C for condensation. It is worth noting here that the total pressure used in calculations does not match the ambient pressure where the experiments were conducted at altitude, but the ratios of gases (O2 and H2O) used in the calculations are the same as those used in the experiment series. Results Results below are photos of samples, observed staining colors on samples, as well as DPC/UV–vis and ICP-OES measurements. Figure 4 below shows the ceramic insulating fibers post-exposure. Following this figure, Table II presents the resulting observed staining colors and Cr(VI)/Total Cr measurements for each ceramic insulating fiber sample. Figure 2. Fibers pre-exposure photographs (top) and SEM images (bottom) from left to right: quartz wool, alumina wool. Figure 3. Fibers pre-exposure photographs (top) and SEM images (bottom) from left to right: manufacturer AES #1, and manufacturer AES #2. Journal of The Electrochemical Society, 2024 171 091501 Next, Fig. 5 below presents total Cr and Cr(VI) measurements obtained from ICP-OES and DPC/UV–vis, respectively, as a histogram plot with error bars (10% uncertainty) for each fiber sample. To ensure repeatability, experiments for each sample have been at least duplicated (some triplicates). However, the metho- dology has evolved, and more refined characterization techniques were used in later iterations. For example, total Cr collected on samples was initially measured using inductively couples plasma mass spectroscopy (ICP-MS), but in later iterations was measured using ICP-OES, and initial measurements of Cr(VI) collected on samples was estimated using a visual color comparator provided with the DPC test kit, but in later iterations was measured directly using DPC/Direct UV–vis with the aid of a spectrophotometer. The exact measurements have changed but the trends/differences in total Cr/Cr(VI) collection agree with respect to changes in water vapor concentration. As a result, the 10% error bars in Fig. 5 may be an underestimate of the total error involved due to fiber packing and positioning in the quartz tube. It can be inferred from Fig. 5 that alkaline content in the fiber samples increases Cr and Cr(VI) formation. A possible explanation for this observation is that alkaline-Cr compound formation reac- tions are more favorable compared to those formation reactions of the quartz or alumina fiber surface chemistries. Assuming that is the case, it is expected that leached alkaline elements between pre- and post-exposure samples will increase. To test this hypothesis, unexposed samples of each ceramic insulating fiber, also weighing Figure 4. Ceramic insulating fibers post-exposure, counterclockwise from top left: quartz, AES #1, AES #2, and alumina wools. Table II. Appearance and Cr content for high temperature insu- lating wool fiber samples. Metric Quartz Alumina AES #1 AES #2 Color Green, yellow, brown Green, yellow, brown Yellow Yellow, brown DPC/UV–vis 0.739 ppm 0.403 ppm 0.873 ppm 1.747 ppm ICP-OES 1.339 ppm 1.976 ppm 2.170 ppm 3.040 ppm Figure 5. Collected Cr/Cr(VI) comparison amongst fiber types with error bars. Table I. Information on the ceramic fibers. Quartz Alumina AES #1 AES #2 Fiber Diameter, μm (Avg, Std Dev) 21.45, 14.30 9.67, 3.63 15.63, 7.03 15.05, 7.08 EDX Chemical Analysis Si (98.2 wt%), Al (96.5 wt%), Si (61.8 wt%), Si (53.3 wt%), (Oxygen Balance) S (1.1 wt%), Si (3.3 wt%) Ca (34.8 wt%), Ca (39.8 wt%), and Na (0.6 wt%) K (1.1 wt%), Mg (5.6 wt%), Al (0.9 wt%), S (0.9 wt%), Mg (0.7 wt%), and Na (0.6 wt%) and Na (0.6 wt%) Journal of The Electrochemical Society, 2024 171 091501 2 g each, were acid digested under the same conditions as described in the experimental design section. The results of this experiment are presented below in histogram plots of ICP-OES elemental composi- tion results for AES #1 and AES #2 fibers. Each histogram plot contains results in ppm for Cr, Ca, Mg, Na, and K for post- and pre- exposure samples with error bars (10% uncertainty). Figure 6 below displays the post- and pre-exposure ICP-OES results for AES #1 as well as the difference between post- and pre-exposure. The greatest difference in detected elemental concentrations between post- and pre-exposure are Cr and Ca at 2.17 ppm and 4.20 ppm, respectively. Na and K increased from pre- to post-exposure by 0.61 ppm and 0.35 ppm, respectively. Finally, Mg decreased from pre- to post- exposure by 0.21 ppm. Next, Fig. 7 below displays the post- and pre-exposure ICP-OES results for AES #2 as well as the difference between post- and pre- exposure. The greatest difference in detected elemental concentra- tions between post- and pre-exposure are Cr and Ca at 3.03 ppm and 3.43 ppm, respectively. Mg increased from pre- to post-exposure by 0.12 ppm. Finally, Na and K decreased from pre- to post-exposure by 0.23 ppm and 0.03 ppm, respectively. FactSage-generated condensation reaction calculations of Cr vapor onto Quartz, Alumina, AES #1, and AES #2 fibers as functions of water vapor concentration were completed to interpet experimental results. Results are presented as calculated total condensed Cr species formed in grams on each ceramic insulating fiber from 100 °C to 800 °C for 3% H2O water vapor content. Condensed Cr species formed for each fiber surface in the greatest quantity are interpreted as being the most stable. For quartz wool, the calculated primary condensed Cr compounds formed are Cr2O3, Cr2(SO4)3, and Cr2(CrO4)3 (Cr(III) chromate), however Cr2(CrO4)3 was calculated to only form at 100 °C. For alumina wool, the calculated primary condensed Cr compounds formed are Cr2O3 (Cr (III)) and Cr2(CrO4)3, however Cr2(CrO4)3 was calculated only to form at 100 °C here as well. For AES #1, the calculated primary condensed Cr compounds formed are CaCrO4 (Cr(VI)) and Ca3Cr2Si3O12 (Cr(III) uvarovite), however Ca3Cr2Si3O12 was calcu- lated to form only at 100 °C. Finally, the calculated primary condensed Cr compounds formed for AES # 2 is CaCrO4 (Cr(VI)). Figure 8 shows the partial pressure of some potential Cr vapor species at 850 °C, with higher lines indicating a higher likelihood of formation during evaporation. CrO2(OH)2 is the dominant com- pound formed, and increasing H2O concentration can increase the amount of Cr vapor compounds. Below 1% H2O addition, these Cr vapor compounds significantly decrease, as the formation of Cr vapor barely occurs without H2O addition, with CrO3 being the predominant possible species. Increasing water vapor concentration not only increases the Cr vapor compounds (Fig. 8), but also increases the amount of condensed Cr based on Fig. 9a. Lower temperatures also resulted in increased condensed Cr compounds as more Cr species are stable at lower temperatures. Condensation of Cr vapor on quartz starts with the formation of Cr2O3 at higher temperatures, then some Cr reacts with S from the fibers to produce Cr2(SO4)3 at below 425 °C. At temperature lower than 170 °C, Cr2O3 formation starts being replaced by CrO2. Further cooling below 130 °C leads to the formation of Cr2(CrO4)3. Aqueous compounds are formed at temperatures below 100 °C as Cr2(CrO4)3 and Cr2(SO4)3 begins to gradually decrease (Figs. 9b–9c). Most Cr compounds formed are HCrO4 − and Cr2O7 2− ions (hexavalent Cr), while some formed Cr compounds become trivalent as Cr3+ and Cr(OH)2+. The behavior of Cr vapor condensation on alumina fiber is comparable to condensation on quartz fibers, except for formation of Cr2(SO4)3 condensate which does not occur due to the lack of S present in the alumina fibers (Fig. 10). At higher temperatures, Cr condenses as Cr2O3 in corundum phase along with alumina and then becomes primarily Cr2O3 solid phase as alumina can react with water vapor to form hydroxides. Condensed compounds at lower temperatures are comparable to quartz fibers with most of the Cr formed as hexavalent in HCrO4 − and Cr2O7 2− ions and some trivalent Cr as Cr3+ and Cr(OH)2+. Condensation on AES #1 and #2 (CaO-containing fibers) have similar results: all condensed Cr formed is hexavalent CaCrO4 at every temperature and some ionic CrO4 2− at temperatures near 50 °C (Fig. 11). These results show that hexavalent Cr is most stable and formed in the existence of calcium oxide in the insulating fiber materials. Phase diagrams are made to understand more about Cr con- densation behavior as a function of temperature and p(O2). Four phase diagrams represent the condensation on the four fibers studied in this research project, which are quartz (Fig. 12), alumina (Fig. 13), AES #1 (Fig. 14), and AES #2 (Fig. 15). All phase diagrams are calculated at a p(H2O) of 0.03 atm and total pressure of 1 atm as the experiment uses 3 vol% of H2O in air in the Cr evaporation studies. Phase diagram calculations for condensed Cr on quartz fibers used Figure 6. Histogram plot of ICP-OES analysis of AES#1 pre- and post- exposure reported in ppm. Figure 7. Histogram plot of ICP-OES analysis of AES#2 pre- and post- exposure reported in ppm. Journal of The Electrochemical Society, 2024 171 091501 inputs for the composition of Cr2O3/(Cr2O3+SiO2+Na2O+S)= 0.5, Na2O/(SiO2+Na2O+S)= 0.004, and S/(SiO2+Na2O+S)= 0.005. These compositions are a resemblance of the quartz fiber composi- tion used in the experiment. At p(O2) of 0.21, condensed Cr compound starts with Cr2O3, then Cr2(SO4)3 starts to form at 779 °C, while CrO2 and Cr(CrO4) starts to form at 133 °C. Cr2(CrO4)3 becomes stable at below 92 °C and then aqueous starts to form below 63 °C. The input on the phase diagram calculation on alumina fibers are Cr2O3/(Cr2O3+Al2O3+SiO2)= 0.5 and SiO2/(Al2O3+SiO2)= 0.0368 based on the alumina fiber composi- tion used in this experiment. Most of the Cr formed is Cr2O3 in the corundum phase or pure solid compound. At temperatures below 150 °C, the Cr starts to form CrO2, Cr(CrO4), Cr2(CrO4)3, and aqueous phase below 65 °C. Thermodynamic equilibrium calculations for Cr species formed on AES #1, presented in Fig. 14, use an input of Cr2O3/ (Cr2O3+SiO2+CaO+K2O+Al2O3+MgO+Na2O)= 0.5, CaO/Z= 0.262, K2O/Z= 0.0072, Al2O3/Z= 0.0094, MgO/Z= 0.0066, and Na2O/Z= 0.004 with Z= SiO2+CaO+K2O+Al2O3+MgO+Na2O. Meanwhile, calculations for condensed Cr species on AES #2, presented in Fig. 15, use an input of Cr2O3/(Cr2O3+SiO2+CaO+ MgO+Na2O+S)= 0.5, CaO/Z= 0.307, MgO/Z= 0.05, Na2O/Z= 0.004, and S/Z= 0.0103 with Z= SiO2+CaO+MgO+Na2O+S. The formation of aqueous phase Cr on AES #1 and #2 fibers occurs at higher temperature compared to quartz and alumina fibers. Its formation starts at below 145 °C for AES #1 and 238 °C for AES #2. Similar with the Equilibrium calculation result, most of the Cr condenses on AES #1 and #2 as CaCrO4, while can also be stable as CrO2, Cr(CrO4), and Cr2(CrO4)3. Discussion The photos, ICP-OES and DPC/UV–vis results, and thermo- dynamic modelling corroborate observed trends of increased loading of total Cr and Cr(VI) in the presence of alkaline oxide additions in the aluminosilicate fibers. Photos of post-exposure samples show that staining becomes more evident on each sample with greater total alkaline oxide content. Total Cr measurements from ICP-OES and Cr(VI) measurements from DPC/Direct UV–vis also increase with increasing alkaline oxide content. The process, as it is understood according to these observations, by which reactive condensation of Cr vapors occurs on ceramic fiber surfaces is as follows: as CrO3 or Cr2(OH)2 gases generated from high-temperature processes cool and fall onto surfaces they encounter eventually forming various stable Cr compounds depending on the surface chemistry on which they condense onto. These Cr compounds, according to the thermody- namic equilibrium calculations and experimental observations, include trivalent Cr compounds (Cr2O3, Cr2(SO4)3, Ca3Cr2Si3O12), hexavalent Cr compounds (CaCrO4), and combinations of hexava- lent and trivalent Cr compounds (Cr2(CrO4)3). Furthermore, ac- cording to experimental observations presented in Figs. 6 and 7, it is possible that Cr compounds formed on alkaline surfaces form chemically bonded alkaline chromates and pull the alkaline compo- nents out of the fiber during acid digestion. The solubility of relevant Cr compounds could also be of importance to explaining these observations. Cr(III) compounds are generally insoluble to slightly soluble in water, whereas Cr(VI) compounds are very soluble. For example, Cr(III) oxide (Cr2O3) is completely insoluble in water at 20 °C while dehydrated chromic acid has a solubility of 1000 g l−1 and calcium chromate has a solubility of 22.3 g l−1.59 Similar to a previous study on the influence of water vapor concentration on reactive condensation of Cr vapors,1 a similar pattern in staining is observed on each fiber sample. Observing the samples from left to right coordinates with the temperature zones of hot to cold, respectively, where the sample was placed in the furnace exit. Each sample starts with unstained sections on the left side where the temperature is >500 °C. This observation agrees with expectations as the condensation point for Cr vapor, the boiling point of chromic acid, is ∼250 °C, and deposition at temperatures above the boiling point is not to be expected. Staining becomes more apparent in the middle and right side as the sample cools down below the boiling point of chromic acid. The computational thermodynamic equilibrium modelling also showed a variation in the species of stable condensed Cr and in the total amount of condensed Cr formed on each fiber. Stable Cr compounds from thermodynamic modelling are ascertained from the calculated total (in grams) of each calculated stable Cr species. The species with the largest calculated totals are determined to be the most stable. The calculated total Cr formed and was greater for AES #1 and AES #2 compared to quartz and alumina across all temperatures and the most stable species were calculated to be Cr(VI). These results are reinforced by observing the staining on each sample. Staining on AES #1 and AES #2 was more yellow and continuous than the stains Figure 8. Partial pressure of chromium vapor compounds at 850 °C and different water vapor concentrations. Journal of The Electrochemical Society, 2024 171 091501 Figure 9. Equilibrium calculation of condensation of Cr vapor compounds from Fig. 8 on quartz fibre at air atmosphere and 1%–10% H2O (a) at 100–800 °C and (b)-(c) at 50–100 °C. Journal of The Electrochemical Society, 2024 171 091501 Figure 10. Equilibrium calculation of condensation of Cr vapor compounds from Fig. 8 on alumina fibre at air atmosphere and 1%-10% H2O (a) at 100–800 °C and (b)-(c) at 50–100 °C. Journal of The Electrochemical Society, 2024 171 091501 observed on quartz or alumina. Cr(III) chromate only formed at 100 °C on quartz and alumina and therefore would fall towards the trailing edge of the ceramic insulating fiber plug in the tube furnace. Hydroxyl populations on the surface are assumed to act as anchoring points for condensed Cr vapor species. De-hydroxylation, the process by which surface hydroxyl groups are effectively removed through bonding with hydrogen to form water, may also occur at elevated temperatures (>500 °C).60 When water vapor is introduced into the atmosphere, rehydroxylation of dehydroxylated surfaces can occur and this rehydroxylation effect is increased with increased water vapor concentration. A partially dehydroxylated oxide surface has hydroxyl groups and coordinatively unsaturated metal cations/oxygen anions. Coordinatively unsaturated cations at the surface can accept free electron pairs of adsorbed molecules. On the dehydroxylated surface, the M+ sites behave like Lewis acids while the O- ions are more basic than the bulk ions in the material. Coordinative unsaturation becomes more extensive if dehydroxyla- tion takes place.61 The strength of these acid sites depends on the charge and size of the cations, both of which vary with the oxidation number of the cation. In hard soft acid base (HSAB) theory, cations of a higher oxidation state are harder acids. The strength of these acid sites depends on the charge and size of the cations which vary with the oxidation state of the cation. Harder cations are smaller and less polarizable, and they will absorb or bind hard bases stronger than soft or polarizable bases. A study on the role of hardness in the adsorption of Cr(VI) onto metal oxide nanoparticles indicated that the adsorption onto an oxide surface is influenced by their chemical hardness.62 The hardness of the cation determines the basicity of surface OH groups which regulates adsorption as well as protonation and deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity and this was found to be the driving force for adsorption of Cr(VI) anions onto the oxides. Magnesium is a harder acid than calcium which is harder than aluminum which is harder than silicon, therefore the bonding between magnesium/oxygen or calcium/oxygen is harder and more stable than that of aluminum/oxygen and silicon/oxygen, leading to a greater affinity for adsorption of Cr anions for the AES wools compared to alumina or quartz wool. In a study of Cr(VI) formation in stainless steel refractory slags, the authors observed Cr(VI) formation increased in the Figure 11. Equilibrium calculation of condensation of Cr vapor compounds from Fig. 8 on AES fibre at air atmosphere and 1%–10% H2O at 50–800 °C. Figure 12. Phase diagram of Cr condensation on quartz in Cr2O3—O2— H2O—SiO2—Na2O—S system in a function of temperature and p(O2). Figure 13. Phase diagram of Cr condensation on alumina in Cr2O3—O2— H2O—Al2O3—SiO2 system in a function of temperature and p(O2). Journal of The Electrochemical Society, 2024 171 091501 presence of Ca.15 The authors of the study also found that “slag basicity,” the ratio between CaO/SiO2, increases Cr(VI) formation. The replacement of Al2O3 by SiO2 in Ca-aluminate slags decreases the amount of uncombined CaO that can react with chromite particles to form Cr(VI). This study also found that a decrease in chromite particle size increases the content of Cr(VI) because a larger surface area is available for the reaction to take place. Another study of stainless-steel slag also found the main Cr compound distributed in the soluble phase as hexavalent CaCr2O4 was formed below 1200 °C.63 Increased boron oxide content was also found to increase Cr(VI) formation. The authors of both studies also observed that cooling rate affects the formation of Cr(VI), however the observations and conclusions drawn from each study do not agree. In the first study, an increase in cooling rate was observed to decrease the formation of Cr(VI) and this was assumed to be the result of limiting the kinetics of the formation. The second study’s results demonstrated that a faster cooling rate prohibited Cr from entering the spinel phase and remained primarily in the silicate matrix. In this state, there was more tendency for Cr to oxidize to Cr (VI) in the subsequent cooling process. The Gibb’s free energy of formation for various condensed phase Cr species is presented below in Table III. The reactions of alkaline chromate/chromite compounds are generally more favorable than those of oxide or hydroxide Cr compounds due to their lower Gibb’s free energy. The observation that alkaline oxides have a higher propensity for forming compounds with Cr vapors is reinforced by HSAB theory, studies of refractory slags, and the relative thermodynamic stability of alkaline chromates. Furthermore, when viewing the histogram plots in Figs. 6 and 7, it is evident that Cr and alkaline oxides, particularly Ca, are linked. A significant amount of Cr and Ca were detected in post-exposure samples when compared to pre-exposure samples. This could be evidence of calcium-chromium compounds forming chemical bonds and essentially removing Ca from the fibers during the acid digestion process. The results from thermodynamic modelling of the system also corroborate this assertation as the primary stable Cr compounds formed for the AES wools are Ca- based. Furthermore, the calculated thermodynamic equilibriums for condensed Cr compounds show increasing total condensed Cr across all temperatures and water vapor contents when comparing quartz to alumina to AES #1 and #2 fibers. The calculated primary condensed Cr species also show an increase in Cr(VI) compounds formed on AES #1 and AES #2 compared to quartz and alumina fibers. This is reinforced by the experimental data in Fig. 5 that displays the same trend of increasing total condensed Cr compounds from quartz and alumina to AES #1 and #2 fibers. Limitations of study and future work recommendations.— Various experimental factors in this study limited this investigation and provided context for future work. For example, the placement and packing of the fibers within the furnace exit could influence Cr condensation. AES #1/#2 fibers are produced by the same manu- facturer and have similar fiber dimensions and orientation; however, the quartz and alumina fibers are oriented differently and have different dimensions compared to one another and the AES fibers. Furthermore, the specific placement and packing of each sample could have an impact on internal pressure of the reaction tube, influencing the fluid mechanics of the gas flow across the fiber surfaces as well. Fluid flow past the fibers is not uniform and Cr vapors will not distribute equally across all fibers. This observation is reinforced by the accumulation of green staining around the sample on the inside surface of the quartz tube which could be related to fluid flow past the fibers but has also been observed in other studies. This occurrence has been addressed by other researchers to optimize Cr vapor-focused experiments.65 The authors of the study identified a method using a sodium carbonate coated thin alumina tube which effectively mitigates interference caused by chemical interactions between Cr vapor species and quartz and alumina furnace tube, allowing for improved assessment of Cr evaporation. Another limitation is the acid digestion used for preparing samples to be analyzed via ICP-OES or DPC/UV–vis spectrophotometry. The efficacy of the digestion process used was quantified by measuring the difference in Cr(VI) detected after 1 h at 90–95 °C hot plate assisted nitric acid digestion and allowing the fibers to continue digesting for 12 h unassisted. Measured Cr(VI) decreased by ∼80%, meaning ∼20% of the Cr remains on the fibers after hot plate assisted acid digestion. Using more advanced digestion methods such as pressurized or microwavable vessels could achieve a more complete digestion of samples. A third limitation is the differences in analytical techniques used. A single characterization technique that could be used for total Cr and Cr(VI) would provide more reliable data for comparison. Currently, the three different methods used limit the inter-sample comparisons or conclusions that can be drawn from the data. Finally, the thermo- dynamic equilibrium calculations and modeling have limitations as they do not consider some kinetic constraints or limitations from the Figure 14. Phase diagram of Cr condensation on alumina in Cr2O3—O2— H2O—SiO2—CaO—K2O—Al2O3—MgO—Na2O system in a function of temperature and p(O2). Numbers on graph: 1 K2MgSi5O12, 2 KAl3Si3O10(OH)2, 3 KAlSi3O8, 4 Na2Mg2Si6O15, 5 Na2Ca3Si6O16, 6 NaAlSi3O8, 7 Quartz, 8 Clinopyroxene, 9 Feldspar, 10 Garnets, 11 Spinel. Figure 15. Phase diagram of Cr condensation on alumina in Cr2O3—O2—H2O—SiO2—CaO—MgO—Na2O—S system in a function of temperature and p(O2). Numbers on graph: 1 CaSO4, 2 Na2SO4, 3 Na2Ca3Si6O16, 4 Quartz, 5 Clinopyroxene, 6 Spinel. Journal of The Electrochemical Society, 2024 171 091501 experiment previously mentioned. The calculations assume a closed system and equilibrium, meaning they have 100% reaction effi- ciency and do not factor in reaction duration, as it treats all reactants have equal potential to react. However, this is not the case in experimentation, as possibly only the surface of the chromia source interacts with the reactant gases. Nevertheless, the FactSage results explain some phenomena observed in the experiment and predict the chromium compounds formed during evaporation and condensation. The overall trends of condensed Cr species observed from thermo- dynamic calculations can be compared to experimental results, even though the amount of condensed Cr is not comparable as FactSage models equilibrium reactions, while the experiment does not reach equilibrium. Total Cr (in all forms) and Cr(VI) collected on the ceramic insulating fiber samples increases in the presence of alkaline oxides in the fibers, specifically calcium content. As evident in Fig. 5, measurements from ICP-OES and from DPC/UV–vis, also presented in Table II, show an increase in total Cr and Cr(VI) with increased alkaline oxide content. While the trends agree, the individual quantified data does not experience the same rate of change. For example, alumina collected more total Cr than quartz wool, but quartz wool collected more Cr(VI). This could be due to the small amounts of S (∼0.5 wt%) and Na (∼0.3 wt%) present in the quartz wool sample. This observation is reinforced by the thermodynamic model- ling of the quartz wool that calculated Cr2(SO4)3 as a stable compound formed in the reaction. However, further investigation into these observations is warranted and in situ Cr/Cr(VI) measurements of the fibers using IR or Raman spectroscopy during testing as a function of time, temperature, and Cr loading could provide more insight into the mobility and evolution of Cr species on different surfaces. High- temperature environments, such as those generated in the experi- mental system, generate significant amounts of blackbody radiation, and limit the methods of in situ characterization of reaction mechan- isms. However, advancements in IR and Raman spectroscopy in the past decade have successfully integrated these characterization techniques into SOFCs operating above 650 °C to correlate electro- chemical performance with chemical processes occurring simulta- neously within the system.66,67 These advanced spectroscopy systems could be applied to the experimental design employed in this study to obtain a more comprehensive picture of the dynamic reactions occurring during complex high-temperature processes. Another area for future work is to continue probing environ- mental influences on reactive evaporation and condensation mechan- isms of Cr vapors. For example, the authors of this manuscript are currently involved in a study that focuses on the influence of alkaline oxides in the Cr source upstream from the aluminosilicate fibers. Studying combinations of upstream and downstream components has the potential to provide additional insights into the complex and dynamic reactions occurring in the high-temperature systems of interest. Conclusions In this study, silica-based aluminosilicate fibers were exposed to Cr vapors produced by reacting chromia powder to high-temperature (800 °C) air with 3% water vapor content. Aluminosilicate fibers from different manufacturers were evaluated under similar condi- tions for comparison. Chemical analyses were then conducted for each sample for quantitative comparisons. Total Cr and Cr(VI), quantified using ICP-OES, were observed to collect on all samples including those without an appreciable amount of alkaline oxide content. The presence of alkaline oxides in the fiber, however, provides an opportunity to capture more Cr(VI). Total Cr collected increased in the presence of increasing alkaline oxide content as well. Computational thermodynamic equilibrium calculations car- ried out in FactSage were used to interpret these observations. The different amounts and speciation of Cr observed on the ceramic substrates also suggests different Cr condensation mechanisms for different ceramic surfaces and conditions, which invites further investigation. Implications for SOFC/SOEC systems include mate- rial selection and system design to consider the impacts of increased alkaline oxide content on corrosion of stainless-steel components within these systems. While this study confirms the preference for Cr vapors to condense on surfaces containing alkaline oxides, the results do not represent the likelihood or quantification of concentrations of Cr(VI) that would be found in actual industrial applications. Furthermore, as all fibers regardless of alkaline oxide content used in these applications capture Cr(VI), workplace regulatory rules governing the exposure to Cr(VI) should guide actions taken by users of stainless steel used at elevated temperatures. Acknowledgments This work was partially funded by the Montana Space Grant Consortium (MTSGC) and the Raymond E. and Erin S. Schultz Emerging Fellowship. Furthermore, the work performed herein was in part at the Montana Nanotechnology Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant# ECCS- 2025391). ORCID T. K. van Leeuwen https://orcid.org/0000-0002-7625-7238 References 1. T. K. van Leeuwen, R. Dowdy, A. Guerrero, and P. Gannon, J. Electrochem. Soc., 171, 011503 (2024). 2. T. K. van Leeuwen, R. Dowdy, A. Guerrero, and P. Gannon, J. Power Sources, 572, 233065 (2023). 3. E. J. Opila, D. L. Myers, N. S. Jacobson, I. M.-B. Nielsen, D. F. 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