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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Zeta-potential and particle size studies of silver sulphide nanoparticles

Silver sulfide (Ag{sub 2}S) nanoparticles (NPs) were prepared successfully for the first time using diethylene glycol (DEG) as a surfactant. X-ray diffraction (XRD) data revealed single phase nature of the compound and energy-dispersive X-ray (EDX) confirmed its nominal composition. Their sizes were 43 nm from XRD, 50 nm from atomic force microscopy (AFM) and 19 nm & 213 nm from dynamic light scattering (DLS); their differences have been discussed. Autotitration study of zeta potential of these NPs in deionized water by DLS at different pH values confirmed an isoelectric point at pH = 5.14 and their very unstable nature in deionized water.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

How do operating conditions affect As(III) removal by iron electrocoagulation?

Iron electrocoagulation (Fe-EC) has been shown to effectively remove arsenic from contaminated groundwater at low cost and has the potential to improve access to safe drinking water for millions of people. Understanding how operating conditions, such as the Fe dosage rate and the O 2 recharge rate, affect arsenic removal at different pH values is crucial to maximize the performance of Fe-EC under economic constraints. In this work, we improved upon an existing computational model to investigate the combined effects of pH, Fe dosage rate, and O 2 recharge rate on arsenic removal in Fe-EC. We showed that the impact of the Fe dosage rate strongly depends on pH and on the O 2 recharge rate, which has important practical implications. We identified the process limiting arsenic removal (As(III) oxidation versus As(V) adsorption) at different pH values, which allowed us to interpret the effect of operating conditions on Fe-EC performance. Finally, we assessed the robustness of the trends predicted by the model, which assumes a constant pH, against lab experiments reproducing more realistic conditions where pH is allowed to drift during treatment as a result of equilibration with atmospheric CO 2 . Furthermore, our results provide a nuanced understanding of how operating conditions impact arsenic removal by Fe-EC and can inform decisions regarding the operation of this technology in a range of groundwaters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

CdSe/ZnS Fluorescent Nanoparticles as Nanoprobes of Local pH in Diagnostics of Oncology

We discuss the fluorescence spectra from a set of points in histological sections of colon tissue with different levels of pathology that were stained with hydrophilic semiconductor CdSe/ZnS nanoparticles that were modified by a unique method at the phase interface. The shifts in the fluorescence spectra that were recorded for stained cells at different pathologies are described using the electrochromism of the nanoparticles. Aqueous solutions of the CdSe/ZnS nanoparticles with different pH values are used as systems that model the acidity of a biological medium. It has been shown that the shifts of the fluorescence bands of the CdSe/ZnS nanoparticles are caused by a change in the local electrical field that is induced by solvated ions near their surface at different pH values of the solutions. The application of the CdSe/ZnS nanoparticles as nanoprobes for the local pH in biological tissue is discussed in the context of this model.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

pH-Dependent Single-Step Rapid Synthesis of CuO Nanoparticles and Their Optical Behavior

This paper reports the pH-dependent growth of copper oxide (CuO) nanoparticles by wet chemical precipitation method using pH value of the samples as the only variable parameter. The phase purity, morphology, optical behavior, and elemental analysis of synthesized nanoparticles are shown to be critically dependent on the pH of the samples. Scanning electron microscope (SEM) results shows that a higher pH results well-defined CuO nanoflakes. X-ray powder diffraction (XRD) results disclosed that the growth of pure CuO with monoclinic structure at higher pH 8, whereas mixed phase was formed at lower pH 7. The average crystallite size of samples prepared at pH 8 to 10 was varying from 23.36 to 25.78 nm. The infrared spectroscopy showed that the O–H stretching peaks become narrow with an increase in the pH value. Optical results revealed the existence of the sharp absorption edges with precise excitonic features and photoluminescence bands both located at visible and near infrared spectral region attributed to the excellent optical behavior and narrow size distribution of particles. The additional near infrared band in photoluminescence spectrum located at 860 nm is attributed the defect-related luminescence. The growth mechanism of CuO nanostructures was discussed in the light of our findings.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Evaluation of experimentally measured and model-calculated pH for rock-brine-CO2 systems under geologic CO2 sequestration conditions

pH is an essential parameter for understanding the geochemical reactions that occur in rock-brine-CO2 systems when CO2 is injected into deep geologic formations for long-term storage. Due to a lack of reliable experimental methods, most laboratory studies conducted under geological CO2 sequestration (GCS) conditions have relied on thermodynamic modeling to estimate pH. The accuracy of these model predictions is typically uncertain. In our previous work, we have developed a method for pH determination by in-situ spectrophotometry. In the present work, we expanded the applicable pH range for this method and measured the pH of several rock-brine-CO2 systems at GCS conditions for five rock samples collected from ongoing GCS demonstration projects. Experimental measurements were compared with pH values calculated using several geochemical modeling approaches. The effect of different thermodynamic databases on the accuracy of model prediction was evaluated. Results indicate that the accuracy of model calculations is rock-dependent. For rocks comprised of carbonate and sandstone, model results generally agreed well with experimentally measured pH; however, for basalt, significant differences were observed. These discrepancies may be due to the models’ failure to fully account for certain reaction occurring between the basalt minerals the CO2-saturated brine solutions.

pH, carbon sequestration↗
DOE OSTI2019

Effect of Solution pH on Layered Double Hydroxide Formation on Electrogalvanized Steel Sheets

The effect of solution pH on layered double hydroxide (LDH) formation on electrogalvanized steel sheets (EG steel) was investigated in this study. EG steel with a Zn coating weight of 64 g/m{sup 2} was used as a substrate and was immersed in a 0.25 M Na{sub 2}Al{sub 2}O{sub 4}-based solution having several pH values to form a Zn-Al-based LDH conversion coating. After rinsing and drying, corrosion resistance was evaluated by EIS. It was found that corrosion resistance increased with increasing pH up to pH 12.6. However, above this pH, corrosion resistance tended to display a sharp decrease. To understand this trend, the conversion coating was characterized by FT-IR, XRD and GDOES. The conversion coating was found to consist mainly of Zn{sub 2}Al(OH){sub 6}(CO{sub 3}){sub 0.5}·xH{sub 2}O crystals (an LDH). The thickness of the conversion coating layer increased with increasing pH up to pH 12.4, but above this pH, its thickness tended to decrease due to the formation of ZnO. The reason for the dependence of corrosion resistance on solution pH is discussed from the viewpoints of the thickness and changes in the microstructure of the conversion coating layer, including its crystal size, density and uniformity, which could vary during the initial formation of ZnO at pH values above 12.6.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Synthesis and characterization of nickel doped cadmium sulfide (CdS:Ni{sup 2+}) nanoparticles

Highlights: ► CdS:Ni{sup 2+} nanostructures are synthesized by wet chemical method for different pH values. ► Monodisperse hexagonal wurtzite phase nanostructured Cd{sub 1−x}Ni{sub x}S and rhombohedral NiS secondary phase are identified from XRD. ► The UV–vis absorption shows strong blue shift in its absorption region that is correlated with pH of synthesizing medium. ► The FT-IR study evidences the Cd{sup 2+} weak interaction. ► The variation in magnetic properties of the CdS:Ni{sup 2+} nanomaterial is evaluated with respect to pH. -- Abstract: Nickel doped cadmium sulfide (CdS:Ni{sup 2+}) nanocrystals were synthesized at three different pH values. Nanocrystalline state of nickel doped cadmium sulfide material was established using X-ray diffraction analysis. Peak broadening and peak shifts in the XRD spectra were caused by the replacement of Cd{sup 2+} with Ni{sup 2+}. Particles size (∼62 nm) and the morphology of the synthesized nanoparticles were established through SEM analysis. The TEM analysis further established the presence of nanostate with particles size of 35 nm. The EDS analysis confirmed the existence of Ni in all the three samples. Blue shift in the absorption region and increase in the bandgap to the larger value (3.72 eV) were attributed to the nano dimensional state and incorporation of dopant into the host lattice. Weak metal sulfur interaction was identified from the FT-IR spectroscopy. The room temperature magnetic studies showed that 10% nickel doped CdS at 10 pH, 11.0 pH and 9.0 pH showed weak ferromagnetic and strong ferromagnetic hysteresis. The variations in the magnetic property of the nanomaterials were caused by the weak interaction of Ni{sup 2+} with the host material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Optimization of biogenic methane production from coal

Given continuously increasing global energy needs, diversified efforts have been made to find and exploit new natural gas resources. These include coalbed methane (CBM), which represents an important global, unconventional source of natural gas. Efforts have been underway for some time to more effectively generate methane in-situ in coal plays by introduction of nutrients and/or microbial consortia. However, much is still to be learned about the limitations and environmental conditions that support microbial growth and are conducive to biogenic methane production from coal. Here we evaluated environmental conditions that led to increased methane production from subbituminous coal by introducing a foreign methanogenic consortium that included Methanobacterium sp. Furthermore, we used a central composite design (CCD) to explore a broad range of operational conditions, examine the effects of the important environmental factors, such as temperature, pH and salt concentration, and query a feasible region of operation to maximize methane production from coal. An anticipated detrimental effect of NaCl concentration on methane production was observed for the consortium assessed. The range of feasible operational conditions comprised initial pH values between 4.2 and 6.8, temperatures between 23 °C and 37 °C, and NaCl concentrations between 3.7 mg/cm 3 and 9.0 mg/cm 3 . Coal biogasification was optimal for this consortium at an initial pH value of 5.5, at 30 °C, and at a NaCl concentration 3.7 mg/cm 3 (i.e., 145,165 ppm, which is 25.6 sft 3 /ton).

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2021

Binding mechanism and binding free energy of amino acids and citrate to hydroxyapatite surfaces as a function of crystallographic facet, pH, and electrolytes

Hydroxyapatite (HAP) is the major mineral phase in bone and teeth. The interaction of individual amino acids and citrate ions with different crystallographic HAP surfaces has remained uncertain for decades, creating a knowledge gap to rationally design interactions with peptides, proteins, and drugs. In this contribution, we quantify the binding mechanisms and binding free energies of the 20 end-capped natural amino acids and citrate ions on the basal (001) and prismatic (010)/(020) planes of hydroxyapatite at pH values of 7 and 5 for the first time at the molecular scale. We utilized over 1500 steered molecular dynamics simulations with highly accurate potentials that reproduce surface and hydration energies of (hkl) hydroxyapatite surfaces at different pH values. Charged residues demonstrate a much higher affinity to HAP than charge-neutral species due to the formation of superficial ion pairs and ease of penetration into layers of water molecules on the mineral surface. Binding free energies range from 0 to -60 kJ/mol and were determined with ~10% uncertainty. The highest affinity was found for citrate, followed by Asp(-) and Glu(-), and followed after a gap by Arg(+), Lys(+), as well as by His(+) at pH 5. The (hkl)-specific area density of calcium ions, the protonation state of phosphate ions, and subsurface directional order of the ions in HAP lead to surface-specific binding patterns. Amino acids without ionic side groups exhibit weak binding, between -3 and 0 kJ/mol, due to difficulties to penetrate the first layer of water molecules on the apatite surfaces. We explain recognition processes that remained elusive in experiments, in prior simulations, discuss agreement with available data, and reconcile conflicting interpretations. In conclusion, the findings can serve as useful input for the design of peptides, proteins, and drug molecules for the modification of bone and teeth-related materials, as well as control of apatite mineralization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes

Highlights: • Lipid in municipal biomass would not inhibited the anaerobic digestion process. • A lipid concentration of 65% of total VS was the inhibition concentration. • The amount of Brevibacterium decreased with the increasing of the lipid contents. • Long chain fatty acids stacked on the methanogenic bacteria and blocked the mass transfer process. - Abstract: The influence of the lipid concentration on the anaerobic co-digestion of municipal biomass waste and waste-activated sludge was assessed by biochemical methane potential (BMP) tests and by bench-scale tests in a mesophilic semi-continuous stirred tank reactor. The effect of increasing the volatile solid (VS) concentration of lipid from 0% to 75% was investigated. BMP tests showed that lipids in municipal biomass waste could enhance the methane production. The results of bench-scale tests showed that a lipids concentration of 65% of total VS was the inhibition concentration. Methane yields increased with increasing lipid concentration when lipid concentrations were below 60%, but when lipid concentration was set as 65% or higher, methane yields decreased sharply. When lipid concentrations were below 60%, the pH values were in the optimum range for the growth of methanogenic bacteria and the ratios of volatile fatty acid (VFA)/alkalinity were in the range of 0.2–0.6. When lipid concentrations exceeded 65%, the pH values were below 5.2, the reactor was acidized and the values of VFA/alkalinity rose to 2.0. The amount of Brevibacterium decreased with increasing lipid content. Long chain fatty acids stacked on the methanogenic bacteria and blocked the mass transfer process, thereby inhibiting anaerobic digestion.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

Monitoring of U(VI) Bioreduction after ARCADIS demonstration at Savannah River Site F-Area - 15699

The F/H Area Seepage Basins located in the center of Savannah River Site (SRS) received approximately 1.8 billion gallons of acidic waste solutions. The acidic nature of the basin waste solutions triggered the mobilization of metals and radionuclides including soluble uranium (VI). SRS F-Area sediments are acidic and composed predominantly of quartz sand with varying amounts of fine-grained minerals and iron-oxides. In 2010, ARCADIS implemented in-situ injections of a carbohydrate substrate to establish anaerobic reactive zones for metal and radionuclide remediation via the Enhanced Anaerobic Reductive Precipitation (EARP) process at the SRS F-Area (Figure 2). The addition of the molasses substrate solution to groundwater produces anaerobic conditions with redox values in the methanogenic or sulfate-reducing range conducive to the reductive precipitation of uranium. Objectives: Determine the mineral composition of SRS fine clay fractions and identify all iron-oxide phases. Establish if ferrous carbonate and ferrous sulfide compounds will be present in the sediment after the bioreduction process has occurred. The presence of these compounds will indicate that anaerobic reductive conditions were established. Determine the types of reactions that might occur in the anaerobic aquifer. Determine if conditions will return to the initial state once samples are returned to aerobic conditions. Microcosm studies were prepared with SRS sediments treated using a basal medium solution augmented with sulfate and molasses as well as a trace metal solution. To create anaerobic conditions necessary for the experiment, a vinyl anaerobic airlock chamber from COY Lab Products was used to ensure that no oxygen reached the samples. Half of the samples were augmented with sulfate in an attempt to create iron sulfide and stabilize reduced uranium. A pH evolution study was conducted to find the type of pH changes that occurred in the samples. Samples were sieved to 180 mm before X-ray Diffraction (XRD) analysis was conducted to identify any mineralogical changes in the samples. The experimental patterns were compared against known XRD patterns for siderite and pyrite. Liquid samples were filtered through 0.45 mm filters and were diluted by a factor of 200 in nitric acid (1%) for ICP-OES analysis to determine the ferrous iron concentration. XRD analysis indicated that the sediments contained quartz, kaolinite, montmorillonite, and goethite. All of the samples have followed a similar trend, with a decline in the pH values. It was noted that samples amended with sulfates had slightly higher pH than sulfate-free samples. The pH drop can be attributed to the fermentation process of molasses and the natural acidity of SRS soil used for the microcosm study. The low pH conditions are believed to have hindered the formation of bicarbonate/carbonate ions used to form the iron precipitates. XRD results confirmed the assumption that with low pH values no iron compounds would precipitate in the samples. The experimental patterns for each sample was compared with the patterns for siderite and pyrite; the results for these analyses were negative. ICP-OES analysis confirmed that there is ferrous iron present in the samples. The highest iron concentration being 13312.8 ppb for Batch 1, Sample 1-2, the lowest were in Batch 2 Samples 2,3, and 4 which may be below the calibration curve. This may also be an indication that the iron has precipitated in these samples. Future work: Conduct ICP analysis on the Batch 2 samples using a lower dilution factor. Determine the types of reactions that might occur in the anaerobic aquifer. Verify the continued sequestration of U(VI) in relation to the possible re-oxidation of minerals from the bioreduction zone. Sulfate concentrations will be examined using an ion analysis technique. Determine pH levels, sulfate concentrations, and iron concentrations after re-oxygenation.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2013

Morphology control of open-framework zinc phosphate Zn{sub 4}(H{sub 3}O)(NH{sub 4}){sub 3}(PO{sub 4}){sub 4} via microwave-assisted technique

Open-framework zinc phosphates were synthesized by microwave-assisted technique, and it was shown that the morphology of as-prepared materials could be easily tailored by changing synthesis temperature, reaction time and pH value. During the synthesis, when the reaction temperature increases from 130 °C to 220 °C, the products transformed from hexagonal prisms to polyhedron along with the disappearance of the hexagonal prisms vertical plane. Simultaneously, both the reaction time and pH value could promote the nucleation and growth of crystal particles. More interestingly, the target products with different morphologies could be obtained by varying the usage of NaOH or NH{sub 3}·H{sub 2}O at 130 °C during the microwave synthesis process. - Graphical abstract: Zinc phosphates with variable morphologies can be obtained by simply tuning the microwave-heating temperatures. Display Omitted - Highlights: • Synthesis of open-framework Zn{sub 4} (H{sub 3}O) (NH{sub 4}){sub 3}(PO{sub 4}){sub 4} compounds employing microwave technique. • Dependence of morphology on the reaction conditions. • Morphology transformation from hexagonal prisms to polyhedron was observed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Fluoride removal performance of glass derived hydroxyapatite

Research highlights: {yields} Novel sodium calcium borate glass derived hydroxyapatite (G-HAP) is prepared. {yields} Micro-G-HAP adsorbs F{sup -} ions in solutions more effectively than commercial nano-HAP. {yields} The adsorption kinetics and isotherms are well fitted by a second order kinetic model and Freundlich isotherm model. -- Abstract: A novel sodium calcium borate glass derived hydroxyapatite (G-HAP) with different ranges of particle size was prepared by immersion sodium calcium borate glass in 0.1 M K{sub 2}HPO{sub 4} solution by the ratio of 50 g L{sup -1} for 7 days. The unique advantage of G-HAP for the adsorption of fluoride ions in solutions was studied. The effects of size and quantity of particles, pH value and adsorption time on adsorption performance were investigated. The maximum adsorption capacity was 17.34 mg g{sup -1} if 5 g L{sup -1}, <100 {mu}m G-HAP was added to a solution with an initial pH value of 6.72 and the adsorption time was 12 h. The results showed that the micro-G-HAP could immobilize F{sup -} in solution more effectively than commercial nano-HAP, which makes potential application of the G-HAP in removing the fluoride ions from wastewater. The adsorption kinetics and isotherms for F{sup -} could be well fitted by a second order kinetic model and Freundlich isotherm model respectively, which could be used to describe the adsorption behavior. The mechanism of G-HAP in immobilizing F{sup -} from aqueous solutions was investigated by the X-ray diffraction (XRD), infrared spectra (IR) and scanning electron microscopy (SEM).

36 MATERIALS SCIENCE↗
NASA NTRS1979

Gas diffusion liquid storage bag and method of use for storing blood

The shelf life of stored whole blood may be doubled by adding a buffer which maintains a desired pH level. However, this buffer causes the generation of CO2 which, if not removed at a controlled rate, causes the pH value of the blood to decrease, which shortens the useful life of the blood. A blood storage bag is described which permits the CO2 to be diffused out at a controlled rate into the atmosphere, thereby maintaining the desired pH value and providing a bag strong enough to permit handling.

Bank, H.↗
DOE OSTI2014

The Solubility of 242 PuO 2 in the Presence of Aqueous Fe(II): The Impact of Precipitate Preparation

The solubility of different forms of precipitated 242 PuO 2 (am) were examined in solutions containing aqueous Fe(II) over a range of pH values. The first series of 242 PuO 2 (am) suspensions were prepared from a 242 Pu(IV) stock that had been treated with thenoyltrifluoroacetone (TTA) to remove the 241 Am originating from the decay of 241 Pu. These 242 PuO 2 (am) suspensions showed much higher solubilities at the same pH value and Fe(II) concentration than previous studies using 239 PuO 2 (am). X ray absorption fine structure (XAFS) spectroscopy of the precipitates showed a substantially reduced Pu-Pu backscatter over that previously observed in 242 PuO 2 (am) precipitates, indicating that the 242 PuO2(am) precipitates purified using TTA lacked the long range order previously found in 239 PuO 2 (am) precipitates. The Pu(IV) stock solution was subsequently repurified using an ion exchange resin and an additional series of 242 PuO 2 (am) precipitates prepared. These suspensions showed higher redox potentials and total aqueous Pu concentrations than the TTA purified stock solution. The higher redox potential and aqueous Pu concentrations were in general agreement with previous studies on 242 PuO 2 (am) precipitates, presumably due to the removal of possible organic compounds originally present in the TTA purified stock. 242 PuO 2 (am) suspensions prepared with both stock solutions showed almost identical solubilities in Fe(II) containing solutions even though the initial aqueous Pu concentrations before the addition of Fe(II) were orders of magnitude different. By examining the solubility of 242 PuO 2 (am) prepared from both stocks in this way we have essentially approached equilibrium from both the undersaturated and oversaturated conditions. The final aqueous Pu concentrations are predictable using a chemical equilibrium model which includes the formation of a nanometer sized Fe(III) reaction product, identified in the 242 PuO 2 (am) suspension both by use of 57 Fe Mössbauer spectroscopy and transmission electron microscopy (TEM) analysis.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
NASA NTRS2011

One-carbon (bio ?) Geochemistry in Subsurface Waters of the Serpentinizing Coast Range Ophiolite

Serpentinization - the aqueous alteration of ultramafic rocks - typically imparts a highly reducing and alkaline character to the reacting fluids. In turn, these can influence the speciation and potential for metabolism of one-carbon compounds in the system. We examined the aqueous geochemistry and assessed the biological potential of one-carbon compounds in the subsurface of the McLaughlin Natural Reserve (Coast Range Ophiolite, California, USA). Fluids from wells sunk at depths of 25-90 meters have pH values ranging from 9.7 to 11.5 and dissolved inorganic carbon (DIC concentrations) generally below 60 micromolar. Methane is present at concentrations up to 1.3 millimolar (approximately one-atmosphere saturation), and hydrogen concentrations are below 15 nanomolar, suggesting active consumption of H2 and production of CH4. However, methane production from CO2 is thermodynamically unfavorable under these conditions. Additionally, the speciation of DIC predominantly into carbonate at these high pH values creates a problem of carbon availability for any organisms that require CO2 (or bicarbonate) for catabolism or anabolism. A potential alternative is carbon monoxide, which is present in these waters at concentrations 2000-fold higher than equilibrium with atmospheric CO. CO is utilized in a variety of metabolisms, including methanogenesis, and bioavailability is not adversely affected by pH-dependent speciation (as for DIC). Methanogenesis from CO under in situ conditions is thermodynamically favorable and would satisfy biological energy requirements with respect to both Gibbs Energy yield and power.

McLaughlin Natural Reserve↗
DOE OSTI2019

Effects of temperature and pH on the electrochemical behaviour of alloy 600 in simulated pressurized water reactor primary water

The effects of temperature and pH on the electrochemical properties of Alloy 600 were studied in oxygenated borate buffer solutions by the measurements of open circuit potential (OCP), anodic potentiostatic polarization, Mott-Schottky analysis (MSA), and electrochemical impedance spectroscopy (EIS). At each temperature and pH, the stabilized anodic passive current density was independent of the formation potential, and the slope of the M - S plot for the passive film formed at +0.3 V OCP (OCP + 0.3 V) is positive, indicating that the passive films formed on Alloy 600 have n-type semiconductor character. With increasing temperature (in each solution), the OCP and the impedance modulus decreased, while the passive current density and the defect density calculated from MSA increased, indicating a significant reduction in corrosion resistance. With the increase in pH value, the passive current density decreased, the absolute value of impedance increased slightly, revealing a slight increase in corrosion resistance. According to the optimization of Mixed Potential Model (MPM) on the experimentally measured EIS data for Alloy 600 at OCP and +0.3 V OCP , a series of fundamental parameters related to the micro-characteristic of passive film were extracted. In the barrier layer of the passive film, the concentration of cation interstitials is much higher than that of anion vacancies, indicating that the former was a dominant defect. With increasing temperature, the linear increase in the defect density and film thickness resulted in a linear increase of the anodic current density. However, with increasing pH, the decrease of defect density and the slight increase of film thickness caused a linear reduction in anodic current density. Elevated temperature has a more significant deterioration effect on the corrosion resistance of Alloy 600 than the increased pH value. Additionally, all the kinetics parameters describing the generation of cation interstitials and oxygen vacancies at the substrate/film were obtained.

36 MATERIALS SCIENCE↗