Effect of polishing treatment and thermal oxidation on the total hemispherical thermal emittance of austenitic stainless steel AISI 321

The optimum value of the hemispherical thermal emittance (?_h) of austenitic stainless steel (SS) AISI 321, polished with “AB Polishing Alumina” of particle sizes 0.05, 0.3, and 5.0 μm and abrasives of various grit sizes, is found to be 0.13 ± 0.01 and is significantly lower than the corresponding value of 0.23 ± 0.01 for the unpolished samples. Although the ?_h value at a given firing temperature is constant, a small but systematic upward trend exists when the temperature is raised from 823 to 1293 K. At a firing temperature of 1293 K and for firing times of 3 to 15 min, ?_h increases from 0.15 to 0.22 ± 0.01.     

Oxidation of BrCN in shock waves and formation of NO

The oxidation of BrCN has been studied in shock waves over the temperature range 1680–2550K. The kinetics of the oxidation was monitored by following it emission from NO at the wavelength of 5.34 μm. A computer simulation study was performed to determine the important elementary reactions. It was found from this computer study that the NO formation from BrCN oxidation is mainly governed by 9 elementary reactions, and the main reaction in producing NO is NCO+O=NO+CO.The rate constant of this reaction was determined as k₅=10^13.5±0.4 cm³ mol⁻¹ s⁻¹.The rate constants of Br+BrCN=Br₂+CN,were also determined as k₂=10^15.0±0.2 exp[−(157.4±8.7) kj/RT] cm³ mol⁻¹ s⁻¹,k₆=10^13.8±0.4 exp[−(159 kj/RT] cm³ mol⁻¹ s⁻¹.     

Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water

Oxidation coefficient plays a significant role in sewage sludge treatment in supercritical water, such as supercritical water oxidation (SCWO) and supercritical water partial oxidation (SWPO). In this work, the influences of oxidation coefficient (n) on sewage sludge treatment in supercritical water are investigated systematically and the corresponding reaction mechanisms are discussed objectively. Moreover, corrosion properties of stainless steel 316 considered as reactor material are also explored. The results show that H₂ yield first rises and then decreases with an increase in n. Its maximum value is approximately 190.4 ml/L when sewage sludge is disposed at 450 °C, 25 MPa, n = 0.6 and a residence time of 2.5 min. Under the reaction conditions of 540 °C, 25 MPa, n = 2.0 and 2.5 min, liquid product COD (Chemical Oxygen Demand) removal ratio, TOC (Total Organic Carbon) removal ratio and ammonia removal ratio of liquid product can reach up to 99.95%, 99.8% and 99.7%, respectively. However, if coupling SWPO and SCWO, we can obtain a certain amount of H₂ and CH₄, achieve the above removal ratios even at 450 °C and a lower total n (0.74), and meanwhile liquid products can meet corresponding discharge standards. However, special attention should be paid to reactor material corrosion in the above SWPO. We also confirm that stainless steel 316 undergoes more severe pitting corrosion in the absence of oxygen, compared with general corrosion in the excessive oxygen environment.     

Investigation of corrosion resistance of materials in the presence of sulphuric acid and its decomposition products applied in the thermochemical cycle of hydrogen production

The corrosion resistance of some nickel constructional steels and alloys was investigated in an atmosphere containing SO₃ + SO₂ + O₂ + H₂O, in the temperature range 600–900°C, and in boiling cone, sulphuric acid. The composition of the corrosion products is analyzed. Also the corrosion resistance of stainless nickel-chromium steel, implanted with singly charged ions, Ni⁺, Cr⁺, Al⁺, Pb⁺ and He⁺, was investigated in boiling cone, sulphuric acid.     

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a CrC alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of Cr_xC_y@Cr₂O₃/Al₂O₃ was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.     

Heat transfer correlations for open-cellular porous materials

A general correlation for volumetric heat transfer coefficient between stream of air and open-cellular porous materials was derived utilizing experimental data obtained by several researchers. The derived correlation is written in form of h_v = (A/D_s²⁻ⁿ)uⁿ. Here, h_v denotes the volumetric heat transfer coefficient, A is the constant, n is the velocity exponent, u is the mean fluid velocity and D_s is the equivalent strut diameter of Dul'nev's unit cell for open-cell foam. The parameters, A and n, were determined by a least-square fit of the expression to the above-mentioned experimental data to give A = 13.0 and n = 0.791. Moreover, from the h_v−u correlation thus determined, the following Nusselt vs. Reynolds number heat transfer correlation was proposed: Nu_s = 0.124(Re_sPr)^0.791, where Nu_s represents the Nusselt number defined by h_vD_s²/k_f, Re_s denotes the Reynolds number defined by uD_s/ν_f, Pr is the Prandtl number, k_f is the thermal conductivity of fluid and ν_f is the kinetic viscosity of fluid. It is found that this correlation approximates 78.1% of the available experimental data with an error of less than ± 40%.     

Solar global UV (280–380 nm) radiation and its relationship with solar global radiation measured on the island of Cyprus

We report the first observations of solar global UV (280–380 nm) radiation on the island of Cyprus obtained during an ongoing joint research project between the University of Athens and the Meteorological Service of Cyprus. Hourly global UV (G_uv) and global (G_h) solar irradiances are measured and the relationship between the two radiant fluxes is investigated at Athalassa, Cyprus (35°15′N, 33°40′E, 165 m above MSL). These data are used to determine the temporal variability of the percentage ratio of solar global UV to solar global irradiation (G_uv/G_h) and its dependence on various atmospheric conditions. Analyzing the data set, an inverse correlation between ozone column amount and the precise UV/Global ratio was found, which can be rather attributed to the change of various atmospheric parameters than the ozone column. The analysis of hourly percentage ratio values reveals a definite daily pattern with lower values during sunrise/sunset and higher values around noon. The variation of the percentage ratios (G_uv/G_h) ranges from 3.95±0.29% in September to 2.92±0.42% in August for hourly values, while for daily values, the variation is between 2.85±0.32% in August and 3.68±0.1% in September, with annual mean values of 3.33±0.21% for hourly and 3.19±0.17% for daily data. Finally, the present data reveals a seasonal contribution of the aerosol extinction on the precise (G_uv/G_h) ratio values.     

Experimental investigation and analysis of a new photoelectrochemical reactor for hydrogen production

In this research paper, an experimental investigation of photoactive material titanium dioxide (TiO₂) coated on 180 cm² 316 stainless steel anode is undertaken to study the photoresponse on photoelectrochemical (PEC) hydrogen production. The TiO₂ nanoparticles are first prepared via sol-gel method. A large surface 316 stainless steel anode is coated with TiO₂ nanoparticles by a dip coating apparatus at a withdraw rate of 2.5 mm/s. The nanoparticles are carried on the stainless steel substrate by two-step annealing procedure. The potentiostatic studies confirm the photoactivity of TiO₂ nanoparticles in a photoelectrochemical reactor when exposed to solar ultraviolet (UV) light. The photon to current efficiency measurements carried out on the PEC reactor with TiO₂ coated large surface stainless steel as photoanode demonstrate a significant increase of photoresponse in UV light compared to the uncoated stainless steel prepared under similar conditions. Upon illumination at a power density of 600 W/m², the hydrogen production is observed in TiO₂ coated stainless steel substrate at a measured rate of 51 ml/h while no illumination conditions show a production rate of 42 ml/h. In comparative assessments, the TiO₂ coated substrate shows an increase in photocurrent of 10 mA with an energy efficiency of 1.32% and exergy efficiency of 3.42% at an applied potential of 1.6 V. The present results show a great potential for titanium nanoparticles semiconductor metal oxide in photoelectrochemical hydrogen production application.     

Oxidation of porous stainless steel supports for metal-supported solid oxide electrolysis cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide electrolysis cells (MS-SOEC), is studied under oxygen-side and steam-side conditions. The impact of oxygen content on the oxygen side and steam:hydrogen ratio on the steam side is determined at 700 °C for bare, as-sintered samples. For these conditions, oxidation is more aggressive in the steam-side atmosphere. Oxygen with 3% humidification and steam:hydrogen ratio of 90:10 are selected for further assessment with pre-oxidized, catalyst-coated, and CuMn_1·8O₄-coated samples. The rapid oxidation at 700 °C and breakaway oxidation at 600 °C observed for bare stainless steel in 90:10 steam:hydrogen is mitigated by pre-oxidizing the sample in air before exposure. In oxygen, addition of the catalyst or CuMn_1·8O₄ coatings moderately increases the oxidation rate, due to consumption of Cr via reaction between the coatings and Cr-oxide scale. The results for ex-situ controlled oxidation are similar to oxidation observed after 1000 h operation of a full MS-SOEC. In general, the oxidation behavior at 700 °C is found to be acceptable.     

Electrochemical nitridation of a stainless steel for PEMFC bipolar plates

AISI446 steel has been electrochemically nitrided in 0.1 M HNO₃ + 0.5 M KNO₃ solution at room temperature. XPS analysis revealed surface NH₃ and a deeper nitride layer. The surface layer of the stainless steel modified by electrochemical nitridation was thus composed of a nitrogen-incorporated oxide film. The nitrided steel showed very low interfacial contact resistance (ca. 18 mΩ cm² at 140 N/cm²) and excellent corrosion resistance in simulated PEMFC environments. Electrochemical nitridation provides an economic way to modify the stainless steel’s surface, and is very promising for application to fuel cell bipolar plates.     

Acoustic emission study of deformation behaviour of sensitised 304 stainless steel

Abstract

A systematic study has been undertaken to correlate the changes in acoustic emissions during tensile deformation of sensitised AISI type 304 stainless steel. Samples of a typical 304 stainless steel were sensitised at 700°C for 4, 14 and 24 h after being austenised at 1050°C for 30 min. AE signals were recorded during tensile test by using two sensors with 125 kHz resonant frequency. The results showed significant change in generation of AE during tensile deformation of sensitised AISI 304 stainless steel in compare to solution annealed material. This type of behaviour could be attributed to the microstructural changes in the sensitised specimens especially formation of continuous Cr₂₃C₆ carbides on grain boundaries which lead to increase in shearing by dislocations.     

High temperature rate coefficient measurements of CO + O chemiluminescence

Spectral intensities from the chemiluminescent reaction CO + O → CO₂ + hv have been measured in the range 2600–7000 Å from reacting mixtures of H₂O₂COCO₂argon, shock heated in a shock tube to temperatures of 1300 and 2700K. Intergrals of the photon production rate yield an overall rate coefficient I₀ = 6.8 (±0.6) × 10⁵, exp(− 1960/T) cm³ mole⁻¹ s⁻¹. Extrapolated to 300K, this rate coefficient is in excellent agreement with the room temperature measurements of Pravilov.     

Fabrication of a double-layered Co-Mn-O spinel coating on stainless steel via the double glow plasma alloying process and preoxidation treatment as SOFC interconnect

To improve oxidation resistance, prevent Cr evaporation and maintain appropriate electrical conductivity of AISI 430 stainless steel (430 SS) as the solid oxide fuel cells' (SOFCs) interconnect, a double-layered Co-Mn-O spinel coating is fabricated successfully on 430 SS via a simple double glow plasma alloying process (DGPA) followed by heating in the air (preoxidation treatment). The double-layered Co-Mn-O spinel coating is composed of a thick MnCo₂O₄ spinel outlayer and a thin mutual-diffused (MnCoFe)₃O₄ oxide innerlayer. The isothermal and cyclic oxidation measurements are used to investigate the oxidation resistance, and the ASR test is performed to evaluate the conductivity for the coated and uncoated specimens. The coated specimen has a lower oxidation kinetics rate constant (9.0929 × 10⁻⁴ mg² cm⁻⁴ h⁻¹) than the uncoated one (1.900 × 10⁻³ mg² cm⁻⁴ h⁻¹) and the weight gain of the coated specimen (0.84 mg cm⁻²) is less than that of bare steel (1.29 mg cm⁻²) after 750 h oxidation. Meanwhile, the coated specimen holds a lower area specific resistance (0.029 Ω cm²) compared to the uncoated one (2.28 Ω cm²) after 408 h oxidation. Furthermore, the compact Co-Mn-O spinel coating can effectively impede Cr-volatilization. Additionally, the probable mechanism of the Co-Mn alloy conversion into spinel and the electronic conduction behavior in the spinel are discussed. The effects of mutual-diffused oxide innerlayer on oxidation behavior and conductivity are investigated.     

Sputtered Fe1·5CoNi0.5 coating: An improved protective coating for SOFC interconnect applications

In an attempt to optimize the properties of FeCoNi coating for planar solid oxide fuel cell (SOFC) interconnect application, the coating composition is modified by increasing the ratio of Fe/Ni. An Fe_1·5CoNi_0.5 (Fe:Co:Ni = 1.5:1:0.5, atomic ratio) metallic coating is fabricated on SUS 430 stainless steel by magnetron sputtering, followed by oxidation in air at 800°C. The Fe_1·5CoNi_0.5 coating is thermally converted to (Fe,Co,Ni)₃O₄ and (Fe,Co,Mn,Ni)₃O₄ without (Ni,Co)O particles. After oxidation for 1680 h, no further migration of Cr is detected in the thermally converted coating region. A low oxidation rate of 5.9 × 10^?14 g² cm^?4 s^?1 and area specific resistance of 12.64 mΩ·cm² is obtained for Fe_1·5CoNi_0.5 coated steels.     

Oxidation of porous stainless steel supports for metal-supported solid oxide fuel cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide fuel cells (MS-SOFC), is studied under anodic and cathodic atmospheres. Temperature- and atmosphere-dependence is determined for as-sintered and pre-oxidized stainless steel. Pre-oxidation reduced the long-term oxidation rate. For pre-oxidized samples, the oxidation rate in air exceeds that in humid hydrogen for temperatures above 700 °C. The influence of PrOx, LSCF-SDC, and Ni-SDC coatings is also examined. The coatings do not dramatically impact oxide scale growth. Oxidation in C-free and C-containing anodic atmospheres is similar. Addition of CO₂, CH₄, and CO to humidified hydrogen to simulate ethanol reformate does not significantly impact oxidation behavior. Cr transpiration in humid air is greatly reduced by the PrOx coating, and a PrCrO₃ reaction product is observed throughout the porous structure. A dense and protective chromia-based scale forms on steel samples during oxidation in all conditions. A thin silica enriched oxide layer also forms at the metal-scale interface. In general, the oxidation behavior at 700 °C is found to be acceptable.     

Performance of stainless steel interconnects with (Mn,Co)3O4-Based coating for solid oxide electrolysis

Mixed transition-metal oxide coatings are commonly applied to stainless steel interconnects for solid oxide cell stacks. Such coatings reduce oxidation and Cr evaporation rates, leading to improved degradation rate and stack lifetime. Here, the ChromLok? MCO-based composition (Mn,Co)₃O₄ is applied to Crofer 22 APU stainless steel and evaluated specifically for application in solid oxide electrolyzer stacks operating around 800 °C and utilizing oxygen-ion-conducting solid oxide cells. The MCO coating is found to decrease the stainless steel oxidation rate by about one order of magnitude, and decrease the Cr evaporation rate by fourfold. The coating also dramatically lowers the rate of area-specific resistance increase for stainless steel coupons oxidized for 500 h with constant current applied, from 33 mΩ1cm² kh^?1 for an uncoated coupon to less than 4 mΩ1cm² kh^?1 for coated coupons. The coating is demonstrated on full-scale interconnects for single-cells, where the coating dramatically reduces degradation rate, and for a stack, which displays stable operation for 700 h.     

Ferritic stainless steel interconnects for protonic ceramic electrochemical cell stacks: Oxidation behavior and protective coatings

Protonic ceramic fuel or electrolysis cells (PCFC/PCEC) have shown promising performance at intermediate temperatures. However, these technologies have not yet been demonstrated in a stack, hence the oxidation behavior of the metallic interconnect under relevant operating environments is unknown. In this work, ferritic stainless steels 430 SS, 441 SS, and Crofer 22 APU were investigated for their use as interconnect materials in the PCFC/PCEC stack. The bare metal sheets were exposed to a humidified air environment in the temperature range from 450 °C to 650 °C, to simulate their application in a PCFC cathode or PCEC anode. Breakaway oxidation with rapid weight gain and Fe outward diffusion/oxidation was observed on all the selected stainless steel materials. A protective coating is deemed necessary to prevent the metallic interconnect from oxidizing.To mitigate the observed breakaway oxidation, state-of-the-art protective coatings, Y₂O₃, Ce_0.02Mn_1.49Co_1.49O₄, CuMn_1.8O₄ and Ce/Co, were applied to the stainless steel sheets and their oxidation resistance was investigated. Dual atmosphere testing further validated the effectiveness of the protective coatings in realistic PCFC/PCEC environments, with a hydrogen gradient across the interconnect. Several combinations of metal and coating material were found to be viable for use as the interconnect for PCFC/PCEC stacks.     

Hydrogen generation by hydrolysis of Mg-Mg2Si composite and enhanced kinetics performance from introducing of MgCl2 and Si

This paper reported the performance and mechanism of hydrogen generation via hydrolysis of ball-milled Mg-Mg₂Si composite (5.3 wt % Si-94.7 wt % Mg) in deionized water and in MgCl₂ solution. The results showed that the obtained Mg-Mg₂Si composite presented relatively higher hydrogen generation performance than pure magnesium. Adoption of 0.5 M MgCl₂ solution to replace deionized water sufficiently and vastly enhanced the hydrolysis properties of the Mg-Mg₂Si composite. The composite in 0.5 M MgCl₂ solution generated 445 mL/g hydrogen in 5 min, 688 mL/g hydrogen in 10 min and 889 mL/g hydrogen (conversion rate 99%) in 40 min at 328 K. This remarkable improvement is due to that the addition of Si element in the composite and the introduction of MgCl₂ in solution, as well as the special preparation process of the materials, could decrease the formation of continuous magnesium hydroxide passive layer on the particle surface, directly or indirectly. Moreover, the apparent activation energies for composite hydrolysis in deionized water, in 0.5 and 2.0 M MgCl₂ solution were calculated to be 30.1 ± 0.6, 9.5 ± 0.1 and 3.7 ± 0.2 kJ/mol, respectively. This work demonstrates that the hydrogen generation system based on low-cost and high-performance Mg-Mg₂Si composite is very applicable and promising; and it may open a new avenue for onsite hydrogen supply.     

Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect

Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at operating temperatures 600–800 °C. Chromium (Cr)-forming stainless steel as an interconnect plate possesses a low oxidation resistance at high temperature and electrical conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective strategy is to use a surface coating to improve interconnect performance. This work is to form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic stainless steel. The area specific resistance of samples was measured as a function of time. The area specific resistance of coated sample with 2% of activator content and holding time of 2 h is 90.21 and 108.32 mΩ cm² after 450 h of oxidation in air, respectively. Additionally, the coated sample with 2% of activator content and holding time of 2 h has a weight change of merely 0.299 and 0.231 mg/cm² after 650 h of isothermal oxidation at 800 °C, separately. The results displayed that the formation of CoFe₂O₄ spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions.     

Fast biodiesel production from beef tallow with radio frequency heating

Efficient biodiesel production from beef tallow was achieved with radio frequency (RF) heating. A conversion rate of 96.3 ± 0.5% was obtained with a NaOH concentration of 0.6% (based on tallow), an RF heating for 5 min, and a methanol/tallow molar ratio of 9:1. Response surface methodology was employed to evaluate the influence of NaOH dose, RF heating time, and methanol/tallow ratio. The alkaline concentration showed the largest positive impact on the conversion rate. Similar fast conversion from canola oil to biodiesel was achieved in our previous work, indicating that RF heating, as an accelerating technique for biodiesel production, had a large applying area. Viscosities of biodiesel products from beef tallow and canola oil were measured as 5.23 ± 0.01 and 4.86 ± 0.01 mm² s^?1, respectively, both meeting the specification in ASTM D6751 (1.9–6.0 mm² s^?1).