Anodic Polarization Curves Research Articles

CORROSÃO LOCALIZADA DO ALUMÍNIO EM MEIOS AERADOS E EM MEIOS COM BAIXO TEOR OXIGÊNIO: ESTUDO E COMPARAÇÃO POR MEIO DE CURVAS DE POLARIZAÇÃO

LOCALIZED CORROSION OF ALUMINUM IN AERATED ENVIRONMENTS AND IN ENVIRONMENTS WITH LOW OXYGEN CONTENT: STUDY AND COMPARISON THROUGH POLARIZATION CURVES. In this study, the influence of deaeration on the electrochemical behavior of aluminum was studied by polarization techniques. The advantages of deaeration for the evaluation of localized corrosion on aluminum are discussed. The influence of the corrosive environment with low oxygen contents on the electrochemical response of aluminum was evaluated by anodic and cathodic potentiodynamic polarization curves. The polarization tests were carried out in chloride solutions followed by optical and scanning electron microscopy characterization. It was observed that deaeration of the solution significantly influences the electrochemical behavior of aluminum, as predicted by corrosion kinetics theories. For the cathodic curves, a decrease in the limiting current density for the oxygen reduction reaction (ORR) was observed for low oxygen solutions, while the “apparent” initiation of the hydrogen evolution reaction (HER) was shifted to higher potentials. In the anodic curves, a common passive behavior was not observed in aerated conditions, however, at the corrosion potential, localized pitting was observed indicating that the material was already above the breakdown potential (Ebr). In the low oxygen content solution, however, a passive region with a breakdown potential, was observed on the anodic curves.

EIS Study on Dissolution Behavior of Al Alloy Covered with Adhesive

Adhesive bonding technology has an attractive attention in the bonding of the various components of fuel cell vehicle for weight reduction. Because the adhesive degradation may occur due to the occurrence of the corrosion of bonded materials caused by penetration of electrolyte into the adhesive, the development of the corrosion evaluation method for metal materials bonded by adhesive is required. An electrochemical impedance spectroscopy (EIS) is useful method to examine the corrosion of metal materials since the impedance spectrum over a wide frequency range can provide detailed information involving the electrochemical properties [1-3]. This method was applied to the evaluation for the water absorption into the adhesive [4-5]. In the present study, the EIS was used to investigate the dissolution behavior of Al alloys covered with epoxy resin. Al alloy electrode was fabricated by the following procedure. The Al alloy and copper wire were electrically connected and embedded in UV-cured resin. In this case, an electrodeposition coating was applied to the electrode surface excluding the test surface used for the measurement in order to prevent crevice corrosion. The test surface area was 2.25 cm2. The epoxy resin was selected as the adhesive in the present study. When covering the electrode surface with epoxy resin, the insulating waterproof tape of arbitrary thickness was applied surrounding the electrode surface, and the epoxy resin was poured into it and cured. Electrochemical measurements were carried out by a three-electrode system. The working electrode was the Al alloy electrode or the Al alloy electrode covered with epoxy resin of arbitrary thickness. A platinum wire and a saturated KCl silver/silver chloride electrode were used as the counter electrode and as the reference electrode, respectively. The electrolyte solution was 0.1 M NaCl or 0.1 M NaCl + 0.01 M NaOH. In the polarization curve measurements, the potential scan rate was set at 1 mV / s. The electrochemical impedance was measured with a frequency range of 100 kHz to 10 mHz and a logarithmic sweep of five points per decade. The video recording of electrode surface was conducted during the electrochemical measurements. The anodic polarization curve and the electrochemical impedance of Al alloys covered with epoxy resin were measured in the test solution. In the anodic polarization curve, an increase of current density was observed at just after starting the measurement. The passive region then appeared owing to the polarization to the noble potential, and the current density was drastically increased at arbitrary potential, implying the occurrence of localized dissolution. When the impedance of the Al alloy covered with epoxy resin was measured during the immersion, the capacitive loops were observed on the Nyquist diagram. The time constants observed in the impedance spectra were different at the early stage and later stage in the immersion. These results implied the occurrence of localized dissolution or the oxide film formation during immersion.

Anodic Dissolution of Al–Fe and Al-Fe-Si Alloys in Emimcl–AlCl3 Electrolyte

Al casting alloys, which contain large amounts of Fe, Si, and Cu, are used for automotive engine components. Recycling used Al casting alloys further leads to effective use of resources and reduction of environmental load.We have reported that various Al alloys can be purified and upgraded by electrorefining using ionic liquids.1,2) In this study, we investigated the anodic dissolution and cathodic deposition behavior of Al–Fe binary alloys and Al–Fe–Si ternary alloys with controlled intermetallic compound.Pure Fe (99.99%Fe), pure Al (99.999%), Al–1.5%Fe, and Al–1.5%Fe–3.5%Si casting alloys were used as anode samples. Ionic liquid preparation and all electrochemical measurements were performed in an Ar-filled glove box. The ionic liquid electrolyte was prepared by mixing 1-ethyl-3-methylimidazolium chloride (EmImCl) and AlCl3 with a molar ratio of 1 : 2. In the electrochemical measurements, Al–1.5%Fe and Al–1.5%Fe–3.5%Si alloys were used for the working electrode, Pt and Cu plates for the counter electrode, and Al wire for the reference electrode. Anodic polarization measurements and constant potential electrolysis were performed at an experimental temperature of 323 K. Anodic polarization curves were measured from the immersion potential to 1.5 V vs Al/Al(III) at a scanning rate of 1 mV s− 1. Constant potential electrolysis was performed at potentials of 0.4, 0.7, 0.9, and 1.4 V at a charge density of 100 C cm− 2. After constant potential electrolysis, samples were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy dispersive spectroscopy (EDS).Intermetallic compounds were identified as Al6Fe in the Al–1.5%Fe alloy and β-AlFeSi in the Al–1.5%Fe–3.5%Si alloy by XRD measurements. In the anodic polarization curves of these alloys, oxidation waves were observed at 0.9 V and 1.4 V in the Al–1.5%Fe–3.5%Si alloy, which were higher than the dissolution potential of pure Fe. SEM–EDS analysis of the Al–1.5%Fe–3.5%Si alloy after electrolysis showed a decrease in Al concentration and an increase in Fe and Si concentrations in β-AlFeSi at 0.9 V. At 1.4 V, Al and Fe concentrations decreased and Si concentration increased.These results indicate that anodic dissolution of Fe in β-AlFeSi in EmImCl–AlCl3 ionic liquid occurs at 1.4 V, which is more noble than the dissolution potential of 0.5 V for pure Fe, and that the addition of Si to the Al–Fe alloy suppressed the anodic dissolution of Fe.From the above, in the Al–Fe–Si alloy containing β-AlFeSi formed by adding Si in the Al–Fe alloy, the dissolution of Fe into the electrolyte is suppressed, and the purity of Al at the cathode is improved. Acknowledgement Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO). We would like to express our gratitude to all parties concerned. Reference 1) J. Nunomura et al., J. Electrochem. Soc., 169, 082518 (2022).2) J. Nunomura et al., Electrochim. Acta, 460, 142601 (2023).

Influence of Phosphorus on Initial and Long-Term Atmospheric Corrosion Behavior of Steel Materials

Phosphorus is contained as an impurity in the manufacturing process of steel materials, and it is known to be difficult to remove it completely, and it is also thought to influence the corrosion resistance of steel materials. However, the effect of phosphorus on corrosion resistance has not been clarified, because the steel materials used in previous research reports also contain copper and chromium. The aim of the present study was to systematically examine the influence of phosphorus on the corrosion behavior from early to long-term.Fe-P binary alloys containing 0.5, 1.0 and 1.5 mass% P were used as the samples. The sample grinded with SiC and diamond paste were prepared for electrochemical measurements. Anodic polarization measurements were performed in 25℃ of boric acid-borate buffer solution (pH 8.0) containing 10 mM NaCl in a non-deaerated environment. Platinum was used as the counter electrode and an Ag/AgCl electrode (3.33 M KCl) as the reference electrode. Before the polarization measurements, the immersion potential was measured for 10 min. and then cathodically treated at -1.0 V for 10 min. The sweep rate was 20 mV/min. In addition, the surface was observed by SEM and EDS after polarization at 0.2 V for 80 s to investigate the starting point of localized corrosion of the Fe-1.5 mass% P.EDS analysis was performed on all sample surfaces before electrochemical measurement. The distribution of phosphorus concentration was not observed on Fe-0.5 mass% P, whereas horizontal banded macro-segregation of phosphorus was observed on Fe-1.0 mass% P and Fe-1.5 mass% P. Furthermore, as a result of EDS analysis with a higher magnification, intergranular segregation of phosphorus was observed only on Fe-1.5 mass% P.In the anodic polarization curves of all samples, a peak was observed in the potential range of about -0.66 to -0.39 V, which could be attributed to the active dissolution of the sample. No increase of the current density was observed in the lower potential range than about 0.05 V, suggesting that the sample surface was passivated in this potential range. On the other hand, the current value increased significantly in the potential range higher than 0.05 V, due to the occurrence of localized corrosion. The current density of Fe-1.5 mass% P increased at lower potentials than for the other samples.Polarization measurement of Fe-1.5 mass% P was conducted at a constant potential to investigate the starting point of localized corrosion. A rapid increase in anode current was observed after approximately 10 seconds. SEM observations and EDS analysis were conducted on the sample surface after the test. A number of localized corrosion of about 5 µm in size was observed on the sample surface after the test. EDS analysis showed that higher concentrations of Si and Mn were detected in the corroded areas. In addition, localized corrosion with an elongated shape was also observed on the Fe-1.5 mass% P surface. Since no alloying elements (Mn and Si) derived from inclusions were detected in the corroded areas, this localized corrosion is considered to be due to intergranular segregation of P.The exposure test of all samples was conducted at Miyakojima Island, Japan for 5 years. The corrosion loss of Fe-1.5 mass% P was the smallest compared to the other samples, suggesting that the higher the P content, the lower the corrosion loss. EPMA analysis was conducted for the cross-section of the rust layer of samples after long-term exposure. P was partly concentrated in the rust layer of Fe-0.5 mass% P, but Cl penetrated into the rust layer and reached the steel substrate. On the other hand, P was fully enriched in the rust layer of Fe-1.5 mass% P and Cl was present only on the surface of the rust layer. The protective function of the rust layer due to phosphorus enrichment may have contributed to the long-term improvement of corrosion resistance.

Localized Corrosion Behavior of Type 304 Stainless Steel Electrolytically Charged with Hydrogen

The authors have been studying the relationship between hydrogen-induced crack initiation and propagation mechanisms in type 304 stainless steels in chloride solution environments. As part of this study, plastic strain was applied to hydrogen-charged austenitic stainless steels to investigate crack initiation and propagation behavior. Interestingly, the induction time of localized corrosion and crack initiation almost disappeared in the case of hydrogen charging. Furthermore, it was observed that localized corrosion initiation also occurred quickly in the case of hydrogen-charged, however not plastically strained, specimens. Such localized corrosion behavior was attributed to the decrease in corrosion resistance of the passive film due to hydrogen charging. However, a detailed evaluation of the corrosion resistance of hydrogen-charged stainless steel has not been conducted. In this study, the effect of hydrogen charging on localized corrosion behavior was investigated by measuring anodic polarization curves of type 304 stainless steel electrochemically charged with hydrogen. The specimen used in the experiments was type 304 stainless steel. Hydrogen charging was performed by cathodic polarization at constant current in 1 M NaCl solution. The anodic polarization curves of hydrogen-charged 304 stainless steel were measured by a three-electrode method, using a Pt wire as the counter electrode and an Ag/AgCl electrode (saturated KCl) as the working electrode. Anodic polarization behavior of type 304 stainless steel after electrolytic hydrogen charging was the same as that of the specimens without hydrogen charging, and current changes in passivity and localized corrosion were observed, however pitting potential became lower due to cathodic polarization. The localized corrosion behavior due to hydrogen charging is discussed in this study.

Oxygen Evolution Behavior of Ni-Containing Alloy Electrodes in NaOH–KOH Hydrate Melt

Hydrogen is expected to be the next-generation energy carrier with zero CO2 emissions. Alkaline water electrolysis (AWE) is one of the methods for hydrogen production; however, improving its energy efficiency is a challenge. We are currently developing a highly efficient water electrolysis using hydroxide hydrate melts at 150–200°C [1,2]. Recently, we reported that the overpotential for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at a Ni electrode in a NaOH–KOH hydrate melt at 200°C under atmospheric pressure was significantly reduced compared to a 30 wt% KOH aqueous solution at 80°C, which is a typical electrolyte for conventional commercial AWE [2]. In addition, the OER overpotential was found to be larger than the HER overpotential at the Ni electrode in the NaOH–KOH hydrate melt [2]. In the present study, we investigated the OER behavior of Ni-containing alloys in the NaOH–KOH hydrate melt.Linear sweep voltammetry was performed in the NaOH–KOH hydrate melt (NaOH:KOH:H2O = 9:61:30 mol%) at 150 and 200°C using a three-electrode cell. Several types of commercially available Ni-containing alloy plates with a flag-shaped and 3 mm diameter were used as the working electrode. A Ni rod was used as the counter electrode. A palladium hydride (Pd–H) electrode was used as the reference electrode and the potential was calibrated with respect to the reversible hydrogen electrode (RHE). All measured potentials were IR compensated. All measurements were performed in stationary electrolytes under Ar gas flow at atmospheric pressure.Figure 1 shows anodic polarization curves at Ni–Fe and Ni–Cu alloy electrodes in the NaOH–KOH hydrate melt at 150°C. The polarization of Ni–Fe and Ni–Cu alloy electrodes is lower than that of the Ni electrode. In this paper, we will also present the results for other Ni-containing alloys.A part of this study was supported by Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP) from Japan Science and Technology Agency (JST) Grant Number JPMJTR23TB.[1] K. Kawaguchi, K. Goto, A. Konno, and T. Nohira, J. Electrochem. Soc., 170, 084507 (2023).[2] K. Goto, K. Kawaguchi, and T. Nohira, Electrochemistry, 92, 043021 (2024). Figure 1

Anodic Dissolution Behavior of Si Electrode in HF Solution Containing H2O2

Silicon is expected to be as anode materials for lithium-ion batteries because the battery performance can be improved by increasing surface area1). Metal-assisted chemical etching (MACE) 2) is a low-cost method to form the nano-silicon structures on Si electrode, which is called silicon nanowires (SiNWs). Though this method enables the fabrication of SiNWs to increase the surface area on Si electrode, the mechanisms on MACE have not been clarified yet. This study aimed to investigate the anodic dissolution behavior of Si electrode to understand the effect of electrolyte concentrations on MACE by electrochemical methods in the HF solution containing H2O2.Electrochemical measurements were carried out by a three-electrode system. An electrochemical cell was made of a polypropylene (PP) to use HF as the electrolyte. The counter electrode was a platinum wire, and the reference electrode was a KCl-saturated Ag/AgCl electrode (SSE) fabricated by the polycarbonate that can be used in the HF solution. The Si, Ag, and Si with deposited Ag particles were used as working electrodes. The Si electrode was made from n-type silicon wafers with a (100) surface. The electrode of Si with deposited Ag particles was prepared using an electroless Ag plating. A mixture of HF and H2O2 was used as the electrolyte solution. To investigate MACE behavior, the measurements of the electrode potential, polarization curve, and electrochemical impedance were carried out at each electrode. The electrode potential of Si with deposited Ag particles was measured for arbitrary time in the electrolyte solution. The potentiostatic polarization of Si electrode was performed to investigate anodic dissolution behavior. The electrochemical impedance measurements were carried out with an AC potential amplitude of 10 mV at arbitrary DC potential. The frequency range was 100 kHz to 0.01 Hz at five frequencies per decade.In the electrolyte solution of HF containing H2O2, the electrode potential of the Si with deposited Ag particles shifted to the less noble potential at the initial stage during MACE regardless of the concentrations of electrolyte solution. The cathodic polarization curve of Ag indicated that the cathodic reactions on Ag electrode were affected by the H2O2 concentrations. The anodic polarization curve of Si electrode demonstrated that the dissolution and film formation may occur on Si electrode, which was depended on the HF concentrations. Based on these results, the electrochemical impedance measurement was performed on the Si electrode. The dissolution and oxide film formation as the anodic reactions and the reduction reactions as the cathodic reactions during MACE were discussed to determine the suitable conditions for fabrication of SiNWs on the Si electrode. References Haibin Li, Shinya Kato, Yosuke Ishii, Yasuyoshi Kurokawa and Tetsuo Soga, Nano Ex., 3, 045010 (2023).Ming-Liang Zhang et al, J. Phys. Chem. C, 112, 4444 (2008).

Effect of ammoniacal thiosulfate solution composition on the gold dissolution rate: An electrochemical study

The electrochemical behavior of ammoniacal thiosulfate solutions (S2O32-- NH3-Cu2+ -EDTA) has been studied for varying electrolyte compositions in the gold electro-dissolution. A gold rotating disk electrode (RDE) was employed to measure anodic and cathodic polarization curves in alkaline thiosulfate solutions. Potentiodynamic polarization showed how thiosulfate, ammonia, and copper concentration influence the cathodic and anodic behavior of the electrolyte. Likewise, the influence of oxygen on the electrochemical behavior system was evidenced from the analysis of polarization curves, and using Koutecky – Levich equation (slope of 59.60 mV/decade), it was possible to determine the peroxide pathway for oxygen reduction reaction (ORR) with two-electron transference. Additionally, gold dissolution and thiosulfate degradation were evaluated using the gold foil leaching tests. It was found solutions with a 0.2 mol l-1 thiosulfate concentration favored the dissolution of gold and by maintaining an adequate ratio between thiosulfate/ammonium (1:3) or thiosulfate/copper (4:1) enable the attainment high gold dissolution and lower degradation of thiosulfate.

Poly(o-anisidine) and poly(o-anisidine-co-aniline) polymers synthesized on the ZnFeNi alloy coated steel surface

Abstract ZnFeNi alloy was synthesized on the carbon steel surface in a sulfate bath using the galvanostatic method at a constant current of 1.5 mA for 300 seconds. Poly(o-anisidine) homopolymer and poly(o-anisidine-co-aniline) copolymer were synthesized on the ZnFeNi coated electrode surface. Poly(o-anisidine) homopolymer was synthesized in 0.05 M o-anisidine+0.2 M sodium oxalate medium, and poly(o-anisidine-co-aniline) copolymer was synthesized in 0.05 M o-anisidine+0.05 M aniline+0.2 M sodium oxalate medium. Electrochemical synthesis was carried out by cyclic voltammetry technique. The synthesized materials were characterized by electrochemical impedance spectroscopy, linear sweep voltammetry, open circuit potential-time and anodic polarization curves. Open circuit potential-time curves showed that polymer coatings had higher open circuit potential. By linear sweep voltammetry measurements, it was determined that ZnFeNi alloys were present at the base of the polymer layers after polymer synthesis. It was understood from the anodic polarization curves that the polymer coated electrodes had lower current values ​​than the uncoated ZnFeNi coated electrode, and the poly(o-anisidine) coated electrode had lower current values ​​than the poly(o-anisidine-co-aniline) coated electrode. From electrochemical impedance spectroscopy measurements, it was determined that the polarization resistance of polymer-coated electrodes was higher than the polymer-free electrode during long periods of waiting in 3.5% corrosive solution. Among the polymer-coated electrodes, it was understood that the homopolymer poly(o-anisidine) showed better corrosion performance than the poly(o-anisidine-co-aniline) copolymer.

Anticorrosive Properties of Different ZnFeNi Alloys Synthesized on Carbon Steel Surface

ZnFeNi alloys were synthesized galvanostatically in a sulfate bath at constant current values of 1, 1.5, and 2 mA on the carbon steel surface. The synthesized alloys were characterized by scanning electron microscopy, energy-dispersive X-ray analysis, and linear sweep voltammetry methods. The corrosion performances of the alloys were examined by electrochemical impedance spectroscopy, anodic polarization curves, and open-circuit potential-time techniques. Based on the anodic polarization curves, it was determined that the alloy with the lowest corrosion current was the ZnFeNi-1.5 alloy synthesized at a constant current value of 1.5 mA. The electrochemical impedance spectroscopy results showed that the alloy with the highest polarization resistance during long exposure to a corrosive environment was ZnFeNi-1.5. By examining the anticorrosive properties, it was understood that the alloy that showed the best protective properties against the corrosion of carbon steel was ZnFeNi-1.5.

Investigation of the Surface Characteristics of GCr15 in Electrochemical Machining.

Bearing steel (GCr15) is widely used in key parts of mechanical transmission for its excellent mechanical properties. Electrochemical machining (ECM) is a potential method for machining GCr15, as the machining process is the electrochemical dissolution of GCr15 regardless of its high hardness (>50 HRC). In ECM, NaNO3 solution is a popular electrolyte, as it has the ability to help in the nonlinear dissolution of many metallic alloy materials, making it useful for precision machining. However, due to high carbon content of GCr15, the electrochemical dissolution of GCr15 is unique, and there is always a black layer with high roughness on the machined surface, reducing the surface quality. In order to improve the electrochemical machining of GCr15 with a high surface quality, the surface characteristics of GCr15 in ECM were investigated. The anodic polarisation curve in the NaNO3 electrolyte was measured and electrochemical dissolution experiments were conducted with different current densities. SEM, XRD, and XPS were employed to analyse the surface morphology and composition formed on the machined surface at different current densities. The initial results showed that there were two parts (black part and bright part) formed on the machined surface when a short circuit occurred, and the test results suggested that the black part contained a mass of Fe3O4 while the bright part was composed of mainly Fe and Fe3C. Further investigation uncovered that a black flocculent layer (Fe3O4) always formed in a low current density (32 A/cm2) with high roughness. With the current density increased, the amount of black flocculent layer was reduced, and Fe3C particles appeared on the machined surface. When the current density reached 81 A/cm2, the entire flocculent oxide layer was removed, only some spherical Fe3C particles were inserted on the machined surface, and the roughness was reduced from Ra7.743 μm to Ra1.783 μm. In addition, due to exposed Fe3C particles on the machined surface, the corrosion resistance of the machined surface was significantly improved. Finally, circular arc grooves of high quality were well manufactured with current density of 81 A/cm2 in NaNO3 electrolyte.

Inhibitory Effect of Fennel Fruit Essential Oil and Its Main Component Anethole of Corrosion on Steel Plates in 1 M HCL

Corrosion worldwide causes large losses of metal, which is why various ways are being sought to slow it down. The aim of this study was to investigate the inhibitory effect of fennel fruit essential oil (Foeniculum vulgare Mill.) and its main component anethole. The inhibitory effect of three different concentrations of the fennel essential oil and anethole (1.0 mL/L, 1.5 mL/L, and 2.5 mL/L) in a solution of 1 M HCl at 298 K for 6 h on a sheet of low-carbon steel was investigated. The inhibitory effect was established using gravimetric methods evaluating weight loss, corrosion rate, and inhibition efficiency, as well as electrochemical methods. In gravimetric studies, the inhibition effect of the inhibitors fennel essential oil and isolate anethole at a concentration of 2.0 mL/L was 70.85% and 45.86%, respectively. The anodic polarization curve data at 298 K demonstrate that the anethole and fennel essential oil adsorption on the metal surface creates a barrier that hinders hydrogen ions’ access and prevents them from being reduced on the steel surface’s cathode sites. Fennel essential oil acting as a mixed type-inhibitor can replace synthetic organic substances and could become an alternative to be used as environmental corrosion inhibitors.

Development of the passive film during passivation in passivating electrolytes containing sodium molybdate

The impact of sodium molybdate on passive films formed on lean duplex stainless steel (STS 329 FLD) was investigated. Elemental depth distribution was examined using glow discharge optical emission spectroscopy, while the analysis covered passive film capacitance, molybdenum's chemical state, and corrosion resistance. The addition of sodium molybdate to the passivation electrolyte resulted in a reduction of iron content in the iron-rich layer (from 48 % to 38 %), an increase in chromium content in the enhanced chromium layer (from 33 % to 42 %), and the presence of molybdenum on the film surface. Electrical impedance spectroscopy measurements indicated a significant increase in the capacitance of the passive film. Furthermore, molybdenum was identified as molybdate. Electrochemical anodic polarization curves revealed that the corrosion potential rose with the addition of sodium molybdate to the nitric acid electrolyte. This improvement in corrosion resistance was attributed to molybdate adsorption on the outermost part of the passive film, preventing chloride adsorption or penetration. Additionally, the corrosion current density decreased due to the presence of the chromium-rich layer.

PbO2 /graphite and graphene/carbon fiber as an electrochemical cell for oxidation of organic contaminants in refinery wastewater by electrofenton process; electrodes preparation, characterization and performance

The electro-Fenton oxidation process was used to treat organic pollutants in industrial wastewater as it is one of the most efficient advanced oxidation processes. The novel cell in this process consists of a prepared PbO2 electrode by electrodeposition on graphite substrate and carbon fiber modified with graphene as a cathode. X-ray diffraction, fluorescence, analysis system, atomic force microscopy, and scan electron microscopy were used to characterize the prepared anode and cathode. XRD patterns clearly show the characteristic reflection of the mixture of  - and β phases of PbO2 on graphite and carbon fiber, and AFM results for cathode and anode present that PbO2 on graphite substrate and graphene on carbon fiber surface are on a nanoscale. Contact angle measurement was determined for the carbon fiber cathode before and after modification. The anodic polarization curve showed a higher anodic current when utilizing the PbO2 anode than the graphite anode. Phenol in simulated wastewater was removed by electro-Fenton oxidation at 8 mA/cm2 current density, 0.4 mM of ferrous ion concentration at 35 °C up to 6 h of electrolysis. Chemical oxygen demand for the treated solution was removed by 94.02 % using the cell consisting of modified anode and cathode compared with 81.23% using modified anode and unmodified cathode and 79.87 % when using unmodified anode and modified cathode.

The anodic dissolution kinetics of Mg alloys in water based on ab initio molecular dynamics simulations

AbstractThe corrosion susceptibility of magnesium (Mg) alloys presents a significant challenge for their broad application. Although there have been extensive experimental and theoretical investigations, the corrosion mechanisms of Mg alloys are still unclear, especially the anodic dissolution process. Here, a thorough theoretical investigation based on ab initio molecular dynamics and metadynamics simulations has been conducted to clarify the underlying corrosion mechanism of Mg anode and propose effective strategies for enhancing corrosion resistance. Through comprehensive analyses of interfacial structures and equilibrium potentials for Mg(0001)/H2O interface models with different water thicknesses, the Mg(0001)/72 H2O model is identified to be reasonable with −2.17 V vs. standard hydrogen electrode equilibrium potential. In addition, utilizing metadynamics, the free energy barrier for Mg dissolution is calculated to be 0.835 eV, enabling the theoretical determination of anodic polarization curves for pure Mg that aligns well with experimental data. Based on the Mg(0001)/72 H2O model, we further explore the effects of various alloying elements on anodic corrosion resistance, among which Al and Mn alloying elements are found to enhance corrosion resistance of Mg. This study provides valuable atomic‐scale insights into the corrosion mechanism of magnesium alloys, offering theoretical guidance for developing novel corrosion‐resistant Mg alloys.

Tailoring Spinel with Perovskite: Sr Substitution in the Perovskite Phase of CoFe2O4–LaCoO3 Nanocomposite for Oxygen Evolution and Methanol Oxidation in Alkaline Medium

CoFe2O4–La1−xSrxCoO3 (x = 0.2, 0.6, 0.8) composites have been synthesized using a two‐step, low‐temperature wet chemical approach that combines coprecipitation and sol–gel techniques. The composite materials have been physicochemically analyzed using Fourier‐transform infrared spectroscopy, X‐ray diffraction, and scanning electron microscope. The electrocatalytic properties of the composite materials have been assessed in terms of their performance in oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) in alkaline medium. Cyclic voltammetry has been used for analyzing the redox behavior of the materials in 1 M KOH solution. The electrocatalytic activity of the materials has been assessed using anodic polarization curves on Ni substrate and the CoFe2O4–La0.2Sr0.8CoO3 film electrode demonstrates excellent activity, with current densities of 11.1 mA cm−2 for OER and 47.4 mA cm−2 for MOR at 650 mV. Also, the CoFe2O4–La0.2Sr0.8CoO3 film electrode has the maximum specific activity of 1.1 × 103 mA cm−2 g−1 at 650 mV toward OER. The electrodes’ stability has been assessed through chronoamperometric experiments and additional insight into the enhancement of electrocatalytic activity gained through the electrochemical surface area estimations using electrochemical impedance spectroscopy. From chronoamperometric experiment, it has been observed that the CoFe2O4–La0.2Sr0.8CoO3 film electrode attains stability within 114 s with a current density of 108.7 mA cm−2. Furthermore, the thermodynamic parameters, including the standard enthalpy of activation (ΔH°#), standard entropy of activation (ΔS°#), and standard electrochemical energy of activation (), have been estimated by anodic polarization curve recorded in KOH as well as KOH with CH3OH solutions at various temperatures.

Influence of compound layer on the corrosion resistance of low alloy steel in a 3.5% NaCl solution

Samples of AISI 4140 low alloy steel were nitrided at different conditions by using plasma nitriding. The optical microscopy observation and x-ray diffraction analysis showed that a 3 ∼ 12 μm thick compound layer composed of ε-Fe2–3N and γ′-Fe4N was formed on the surface after nitriding treatments. The corrosion behavior was evaluated by measuring the anodic polarization curves in a 3.5% NaCl solution along with the observation of corroded surfaces and cross-sectional morphologies using optical and scanning electron microscope. The results indicated that the corrosion resistance of low alloy steel was significantly enhanced through the formation of compound layer. The thickness of compound layer was one important factor in determining its corrosion resistance.

Electrochemical behavior of aluminum in 1-ethyl-3-methylimidazolium chloroaluminate ionic liquids

The mechanism of electrochemical oxidation and reduction of aluminum in 1-ethyl-3-methylimidazolium-based (EMImCl) chloroaluminate ionic liquids is studied across the concentration range of N = 1.1–2.0 (N the molar ratio of AlCl3 to EMImCl) at temperatures between 303 K and 418 К. Anodic stationary polarization curves (SPCs) indicate that the process of anodic dissolution of aluminum is complicated by passivation. The current density in the passive region and the passivation potential decrease with a higher concentration of Al2Cl7– ion in the melt. To elucidate the processes taking place at the aluminum electrode, the relationship between the concentration of the ions in the vicinity of the electrode and the potential applied in the cathodic and anodic region was analyzed by in-situ Raman spectroscopy for the concentrations of N = 1.1 and 2.0. The resulting dependences of relative peak intensity vs. potential correlate with the cathodic and anodic polarization curves. In the anodic region the content of Al2Cl7– grows with the potential, while the concentration of AlCl4– ion decreases. In the cathodic region the opposite process is observed: the concentration of Al2Cl7– decreases and that of AlCl4–increases with growing potential. It is shown that a precipitate of AlCl3 forms on the electrode surface in the region of passive aluminum dissolution, which is responsible for the electrode passivation. Based on the results obtained, we propose a model for the process of aluminum oxidation/reduction preceded and followed by a chemical reaction of aluminum chloride formation for the cathodic and anodic process, respectively. Temperature dependences of diffusion coefficients are studied by chronopotentiometry. The results indicate that diffusion coefficients are not affected by the concentration across the temperature range studied. The activation energy for diffusion was found to be 13.65 kJ∙mol−1. Impedance spectroscopy was used to investigate the concentration and temperature dependences of exchange current density. The exchange currents increase with increasing aluminum chloride content in the melt. The transfer coefficient and electrochemical reaction rate constant were found to be α = 0.17 ± 0.04, ks = (1.7 ± 0.4)∙10−6 cm∙s−1, respectively. The activation energy for the electrochemical reaction rate constant does not depend on the concentration and was found to be 33.5 kJ∙mol−1.

CORROSION EVALUATION OF LPBF-MANUFACTURED DUPLEX STAINLESS STEEL

In this study, the effect of as built and heat-treated additively manufactured 2507 super duplex stainless steel (SDSS) on electrochemical corrosion performance was examined. The corrosion was studied by Tafel polarization and electrochemical impedance spectroscopy (EIS) methods in a 3.5% NaCl solution. For this purpose, the as-built and heat-treated printed samples (solution annealed and stress-relieved) were examined by LOM/SEM and X-ray diffraction to evaluate the phase changes of steel at different processing stages. The correlation between corrosion resistance, structure, and heat treatment was assessed. As a result of the very fast cooling rate of the LPBF process, SDSS reveals ferrite as the major phase in the printed samples. To enhance the corrosion resistance of LPBF-produced duplex stainless steel, it's crucial to balance its two-phase structure and minimize residual stresses. The ferrite grains are elongated in the build direction, with some austenite precipitation along the grain boundaries or as Widmanstatten laths. The stress-relieved and as-printed SDSS exhibits reduced corrosion characteristics by around 20% compared to the solution-annealed SDSS, according to anodic polarization curves. Based on EIS results, the solution-annealed SDSS revealed an almost double increase in corrosion resistance (based on charge transfer resistance values) compared to the as-printed and stress-relieved conditions.

Unveiling the potent corrosion-inhibiting power of Ammophila arenaria aqueous extract for mild steel in acidic environments: An integrated experimental and computational study

Plant extracts have emerged as potent inhibiting compounds for metallic corrosion, offering numerous advantages including simple accessibility, low toxicity, eco-friendliness, biocompatibility, effectiveness, and renewability. In this study, the potential of Ammophila arenaria extract as a green corrosion inhibitor for mild steel in 1.0 M HCl over the concentration range of 25–700 ppm was evaluated, using various electrochemical analyses such as Open circuit potential (OCP), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Results revealed a notable reduction in the corrosion rate of steel, particularly at 700 ppm, where Ammophila arenaria extract demonstrated its highest inhibition efficiency, reaching 83.67% for polarization and 84.32% for EIS. The cathodic and anodic polarization curves indicated that the aqueous extract of Ammophila arenaria functions as a mixed-type inhibitor. The thermodynamic analysis revealed a negative Gibbs free energy value (∆Gads°), suggesting spontaneous physisorption as the predominant adsorption mechanism. To delve into the inhibition mechanism at the molecular level, the surface morphology of mild steel exposed to 1.0 M HCl solutions, both in the absence and presence of Ammophila arenaria aqueous extract, was scrutinized through Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Theoretical techniques, including Quantum Calculation Theory, Non-Covalent Interaction (NCI), and Atom In Molecule (QTAIM), were concurrently employed, providing valuable insights into molecular-level interactions. Remarkably, the computational studies complemented and corroborated the experimental data. These comprehensive results significantly contribute to the understanding of Ammophila arenaria extract's corrosion inhibiting properties, endorsing its potential as a sustainable and environmentally friendly corrosion inhibitor suitable for diverse industrial applications. The synergistic integration of experimental and computational approaches establishes a robust foundation for future endeavors in the field of green corrosion inhibitors.