Active Dissolution Region Research Articles

Mechanism of pyrite oxidation in copper(II)-ethylenediamine-thiosulphate gold leaching system

Although the Cu2+–ethylenediamine–S2O32− system is a promising alternative to cyanide-based gold leaching systems because of its low cost, nontoxicity, and environmental friendliness, its performance is strongly affected by the presence of pyrite. Therefore, a deep understanding of the mechanism of pyrite oxidation in this system is of high practical importance. Herein, this mechanism was probed to demonstrate the occurrence of rapid reactions on the pyrite surface at high S2O32− concentrations and reveal that the dissolution of bulk pyrite was most favoured at a certain optimal S2O32− concentration, i.e., S2O32− ions strongly affected the surface properties of pyrite. Anodic processes occurring in active, passive, and transpassive dissolution regions were studied by linear sweep voltammetry, with additional insights into the properties of pyrite electrodes and the related electrochemical reactions obtained by electrochemical impedance spectroscopy and equivalent circuit fitting. With increasing potential at range of 0.20 to 0.80 V, the reactions on the pyrite surface accelerated, and its roughness significantly increased. The products of pyrite surface oxidation were identified by Raman spectroscopy and X-ray photoelectron spectroscopy, and a corresponding mechanism was proposed. Thus, this work provides a theoretical basis for the regulation and control of gold leaching reagents in the Cu2+–ethylenediamine–S2O32− system.

New Insights into the Oxidation Mechanism of Pyrite in Copper-Containing Sulfuric Acid: An Electrochemical, AFM, Raman Spectroscopy and XPS Investigation

The oxidation of pyrite in copper-containing sulfuric acid has been the subject of intense research yet has no comprehensive research on electrochemistry and surface species. In this study, pyrite’s electrochemical oxidation mechanism was systematically elucidated in copper-containing sulfuric acid by using electrochemical techniques. Anodic processes occurring in active, passive, and transpassive dissolution regions were studied by linear sweep voltammetry. The corresponding equivalent electrochemical circuits were fitted for these three regions with the potential of ranging from 0.40 V to 0.80 V vs SCE, and the results of fittings were used to clarify the characteristics of electrodes and electrochemical reactions in copper-containing sulfuric acid. With increasing potential, the reactions on the pyrite surface accelerated, and its roughness significantly increased. In addition, the interfacial transfer of electrons is dominated by surface charge transfer. The products of pyrite surface oxidation were identified by Raman spectroscopy and X-ray photoelectron spectroscopy. Based on the results of the electrochemical methods and surface analysis techniques, the mechanism of pyrite oxidation can be clarified and the specific course of the reaction can be described in detail. These results provided further insight into a better understanding of the pyrite oxidation mechanism in the presence of copper ions.

Effects of the α/γ-Phase Ratio on the Corrosion Behavior of Cast Duplex Stainless Steel

The effects of the α/γ-phase ratio on pitting corrosion initiation and growth in cast duplex stainless steel were studied, including the preferential dissolution of the two phases inside the pits, using pitting potential measurement and potentiostatic polarization measurement with a high concentration of chloride ions and a low pH. The initiation of pitting was not dependent on the α-phase ratio. The γ phase preferentially dissolves when a high potential in the active dissolution region is applied, and the α phase preferentially dissolves when a low potential is applied. In addition, with an increase in α-phase ratio, the potential range where the α phase preferentially dissolves enlarged toward the higher potential side. The growth rate of stable pitting increased with the α-phase ratio. Dissolution of the α phase increased with an increase in the α-phase ratio. This phenomenon is presumably caused by the decreased amount of Cr in the α phase, resulting from the increased α-phase ratio, as well as by Cr depletion around Cr nitrides.

Electrochemical and spectroscopic analysis of enargite (Cu3AsS4) dissolution mechanism in sulfuric acid solution

Enargite (Cu3AsS4) is a valuable but arsenic bearing copper resource, which is associated with common copper sulfides. The dissolution mechanism of enargite electrodes in 0.1 mol/L sulfuric acid, at room temperature was investigated by electrochemical and spectroscopic analysis methods. Cyclic voltammetry (CV), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky methods were applied to systematically investigate the electrochemical dissolution of enargite. Spectroscopic analysis methods including X-ray photoelectron spectroscopy (XPS) with cold stage, Raman spectroscopy and Synchrotron X-ray absorption near edge structure (XANES) spectroscopy were used to analyze the product layers on the electrode surface after electrochemical dissolution. The enargite electrode demonstrated an apparent passive potential region from 450 to 750 mV (Ag/AgCl. sat). Results of XPS and Raman suggested an As-sulfide surface lay of As(1-y-z)S4 was formed on the electrode surface, which accounted for the decrease in current density in the passive potential region. A trans-passive region with a small potential window between 750 mV and 900 mV was observed which might be related to the partial dissolution of the passive surface film. It is demonstrated that the trans-passive dissolution of enargite might be linked to the oxidation of sulfur, from the sulfide or polysulfide in the passive film to elemental sulfur. Moreover, copper was found to be more preferentially dissolved than arsenic in the passive and trans-passive regions. At the active dissolution region above 900 mV, the previously formed passive film got completely dissolved accompanying with the oxidation of enargite surface to produce elemental sulfur, which has been confirmed by Raman spectroscopy and sulfur K-edge XANES spectroscopy.

Некоторые особенности анодного поведения Co2Si-электрода в растворе серной кислоты

The effect of anodic film formation conditions on the electrochemical behavior of Co2Si electrode in 0.5 M H2SO4 has been studied. It was shown that the rate of passive dissolution of Co2Si is only moderately greater than that of CoSi2 provided the Co2Si electrode passes through the active dissolution region very quickly. From the current decay curves at E = const, it was deduced that in the middle part of the passive region the oxide film on Co2Si grows according to near logarithmic law. Impedance spectra obtained in measurements at different initial electrode potentials were analyzed.

Effects of Mn on the electrochemical corrosion and passivation behavior of CoFeNiMnCr high-entropy alloy system in H2SO4 solution

The present work investigates the influence of Mn on the electrochemical corrosion and passivation behaviors of CoFeNiMnCr high-entropy alloys in a 0.1 M H2SO4 solution. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results showed that the addition of Mn caused a marked difference in the active dissolution regions and active-passive transition regions. It was deduced from EIS and the potential decay results that the alloyed Mn suppressed the passivation processes and increased the chemical dissolution rate of the passive films. By contrast, the alloyed Mn did not affect the protective properties of the passive films of the alloys. Surface analysis indicated that Mn addition had little influence on the composition and thickness of the passive films. Thus the passivation current densities and transpassive potentials of the CoFeNiMnCr high-entropy alloy remained almost unchanged with the addition of Mn.

First-Principles Investigation of the Effect of Interstitial Carbon on Corrosion Resistance of Martensitic Medium-Carbon Steel

Martensitic carbon steels with fine grains are used for many industrial applications. Interstitial carbon provides high strength to martensitic structure. Although it has been reported that the interstitial carbon has the beneficial effect on the pitting corrosion resistance of steels,1,2 the role of interstitial carbon is still unclear. In this research, first-principles calculations were applied to the clarification of the effect of interstitial carbon on the anodic dissolution of martensite. The base specimen was AISI 1045 carbon steel, and the chemical composition of this steel (mass%) was 0.44%C, 0.20%Si, 0.85%Mn, 0.008%P, 0.002%S, <0.01%Ni, <0.01%Cr, <0.01%Mo, <0.01%Cu, <0.001%Ti, <0.002%Nb, 0.035%Al, 0.003%N, and 0.002%O. The specimens were heat-treated at 1123 K for 10 h in air and then, quenched in water. Since the surface of the specimens was decarburized during this heat-treatment, the low carbon martensitic structure was formed in the surface region of the specimens, and high carbon martensitic structure remained in the core area. Potentiodynamic polarization of the low and high carbon martensitic specimens was conducted using boric-borate buffer solution with 1 mM NaCl at pH 8.0. In addition to the electrochemical measurement, the first-principles calculations based on density functional theory (DFT) were carried out. For the calculation, the carbon concentration was varied from 0.40 at.% to 5.88 at.%. In the potentiodynamic polarization curves of the low and high carbon martensitic specimens in 1mM NaCl containing boric-borate buffer at pH 8.0, the region of active dissolution appeared on both specimens at about -0.5 V. The dissolution current density for high carbon martensitic specimen was lower than that of the low carbon martensitic one, suggesting that the interstitial carbon suppressed the active dissolution. Next, the effect of interstitial carbon atoms on the electronic structure of the martensitic structure was analyzed. It was clarified that the electronic density of states (DOS) of iron atoms at and near the Fermi level decreased by increasing carbon concentration. This result indicates that the interstitial carbon atom makes weak atomic bonding with the nearest iron atom, and stabilizes the electronic structure of martensite. This change in the energy state of iron atom was, therefore, attributed to the high corrosion resistance of martensitic carbon steels. References; M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018).

Effect of HCO3− on electrochemical kinetics of carbon steel corrosion in CO2-saturated brines

The effect of HCO3−(aq) on CO2 corrosion of carbon steel was investigated in deaerated 3.5 wt% NaCl solutions at 30 °C from pH 3.96 to 7.15. In the CO2-saturated solutions, the pH was adjusted with different HCO3−(aq) concentrations, c[HCO3−(aq)]. The corrosion rate decreased by a factor of 2 as the pH and c[HCO3−(aq)] increased. The cathodic current density during polarization increased at higher pH with higher c[HCO3−(aq)], indicating that HCO3−(aq) acted as an additional hydrogen source for the hydrogen evolution reaction. As the pH increased, the active dissolution regions displayed similar anodic Tafel slopes and suggested a modified Bockris mechanism for the Fe oxidation reaction. The exchange current densities for the half reactions were calculated to study kinetics of the anodic and cathodic half reactions independently. The anodic exchange current density (j0,a) increased by one order of magnitude in the presence of CO2, indicating the involvement of CO2(aq) in the Fe oxidation reaction. As the pH and c[HCO3−(aq)] increased, the cathodic exchange current density (j0,c) decreased by a factor of 50 because the increase in j0,c[HCO3−(aq)] was not high enough to compensate the decline from the other hydrogen sources, especially j0,c[H+(aq)]; and j0,a decreased by a factor of 2.4 because HCO3−(aq) may have competed with CO2(aq) for the surface coverage and the increase in j0,a[HCO3−(aq)] could not compensate the decrease in j0,a[CO2(aq)]. It suggests that the reaction rate constant of HCO3−(aq) was smaller than CO2(aq) for the anodic half reaction and was smaller than H+(aq) for the cathodic half reaction. The XPS results verified that the corrosion products transitioned from iron carbonate to hydroxide as the pH increased while iron carbonate remained the major product. As the pH increased with HCO3−(aq), a second time constant was observed at lower frequencies of the electrochemical impedance spectroscopy (EIS) results.

The Effect of Interstitial Carbon on Pitting Corrosion Resistance of Martensitic Carbon Steels

Ultra-high strength steels with excellent ductility are required to reduce the weight of automobiles and other transportation vehicles. While martensite including high amount of interstitial carbon is favored for strengthening steels, the ductility is relatively low. To optimize the balance between ductility and strength, tempering is conducted. In the case of carbon steels, the precipitation of carbides occurs, and the amount of interstitial carbon decreases during tempering. Although tempering provides the balance between the strength and the ductility, it was reported that the pitting corrosion resistance tends to decrease by tempering.1 To achieve a balance between pitting corrosion resistance and mechanical properties, it is important to determine the quantitative effect of tempering on the pitting corrosion resistance. The specimens were AISI 1045 carbon steel (0.47%C, 0.19%Si, 0.85%Mn, 0.015%P, 0.015%S, 0.02%Ni, 0.18%Cr, 0.01%Mo, 0.01%Cu, <0.001%Ti, <0.002%Nb, 0.024%Al, 0.005%N, 0.002%O). The specimens were heat-treated at 1123 K for 3.6 ks and then quenched in water to form a full martensitic structure. To obtain a tempered martensitic structure, the as-quenched specimens were heated to 873 K and kept at this temperature for 0.1, 1, 10, 15, and 20 h in an Ar atmosphere. The metallographic inspection was performed to ascertain the characteristics of the microstructures of the as-quenched and tempered specimens by using FE-SEM. In tempered martensitic specimens, many small white dots were observed. This is evidence of the carbides formed by tempering. The sizes of the carbides increased with tempering time, meaning that the interstitial carbon decreased. The micro-scale anodic polarization measurements of the as-quenched and tempered specimens were conducted in a boric-borate buffer solution with 500 mM NaCl (pH 8.0) under naturally aerated conditions at 298 K. A small area without non-metallic inclusions was selected for the working electrode. The polarization measurements started at -0.1 V to eliminate the influence of active dissolution on pitting corrosion initiation. For 20 h tempered specimen, a stable pit was initiated immediately after immersion. The pitting potential of 20 h tempered specimen was thought to be less than -0.1 V. In the case of 15 h tempered specimen, the sharp increase in current density was observed at 0.05 V due to the pit initiation. In contrast, no pit was initiated on the as-quenched specimen and the specimens tempered for less than or equal to 10 h. It was found that the pitting corrosion resistance of the martensite tempered for less than or equal to 10 h was higher than that of the martensite tempered for more than or equal to 15 h. Pitting corrosion resistance of martensite decreased with tempering time. To clarify the role of the interstitial carbon on the pitting corrosion resistance, the macro-scale anodic polarization curves of as-quenched, 10 h tempered, and 20 h tempered specimens in a boric-borate buffer solution with 1 mM NaCl (pH 8.0) were measured. The polarization started at -1.0 V after a cathodic treatment (-1.2 V, 600s). For the macro-scale polarization, the region of active dissolution appeared on all specimens in the potential region from -0.7 to -0.6 V. The dissolution current density for 20 h tempered specimen was certainly higher than that of other two specimens. It is suggested that the higher pitting corrosion resistance of the as-quenched and short-time tempered specimens was due to the low dissolution rate of those specimens. It is proposed that the inhibition of the active dissolution rate of these specimens is because of the existence of interstitial carbon, and interstitial carbon improves the pitting corrosion resistance. Short-time tempering is one of the answers for striking an optimal balance between pitting corrosion resistance. References; 1. M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).

Electrochemical corrosion characterization of nickel aluminides in acid rain

Nickel aluminides have been extensively studied in recent decades to replace superalloys, in some components of aircraft turbines, because they have excellent corrosion resistance. Many industrial cities have the problem of air pollution, which has forced the study of the degradation of these alloys in the presence of acid rain. The aim of this work is to study the electrochemical corrosion behavior of nickel aluminides in a medium of simulated acid rain. Potentiodynamic Polarization, Linear polarization resistance curves, Rp, Nyquist data and Bode curves obtained by electrochemical impedance spectroscopy were used to study the corrosion behavior of two intermetallic compounds. The polarization curves show that both intermetallic Ni3Al and NiAl have very similar corrosion potential, showing a slightly nobler behavior the NiAl intermetallic. The intermetallic Ni3Al has an active-passive behavior where the anode branch presents a general dissolution of the alloy, indicating that it is under cathodic control. NiAl intermetallic shows an active dissolution region, followed by a passive behavior. At longer immersion times, Ni3Al intermetallic has a higher polarization resistance, which means a lower corrosion rate. Keywords: Nickel Aluminides, Electrochemical impedance spectroscoy, Acid Rain, Nanoindentation.

Corrosion behaviour of electrodeposited nanocrystalline nickel-iron (NiFe) alloys in dilute H2SO4

Electrodeposited nanocrystalline and conventional polycrystalline NiFe alloys of similar iron content were investigated to understand the effect of grain size on the corrosion behaviour of these materials. Potentiodynamic polarization testing in 1N H2SO4 revealed similar active-passive-transpassive behaviour with negligible differences in the Tafel and transpassive regions regardless of grain size. It is shown that differences in the region of active dissolution are the result of strong contributions of sulfur impurities in the nanocrystalline electrodeposits rather than being due to grain size alone.

Corrosion behavior of hafnium in anhydrous isopropanol and acetonitrile solutions containing bromide ions

Abstract The corrosion behaviors of hafnium in Et 4 NBr isopropanol and acetonitrile(ACN) solutions were investigated using electrochemical measurements, ICP-AES and SEM techniques. Results revealed that the open circuit potential gets more positive due to the increased passivity of the surface oxide film with increasing immersion time until it reaches a steady state value. The potentiodynamic anodic polarization curves did not exhibit an active dissolution region near corrosion potential due to the presence of an oxide film on the electrode surface, which was followed by pitting corrosion. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allowed the pitting potential ( ϕ pit ) and the repassivation potential ( ϕ p ) to be determined. ϕ pit increased with increasing potential scanning rate but decreased with increasing temperature, Br − concentration and ACN concentration. The impedance spectra showed that the resistances of the solution and charge transfer decreased with the increase of ACN concentration.

Mechanism of Electrochemical Dissolution of Nickel Grown by Carbonyl Method

A laboratory-grown nickel sample was used to investigate the galvanostatic dissolution under industrial electroplating conditions. The sample contained multiple layers corresponding to changes in the Ni(CO)4 decomposition temperature ranging from 200 to 280°C. Cyclic voltammetry curves were used to determine the active dissolution, passive and transpassive potentials regions of the sample. Chronopotentiometric curves recorded during galvanostatic dissolution displayed oscillations indicating that the dissolution mechanism involved the breakdown of the passive layer and repassivation of the surface. Scanning electron microscopy (SEM) and white light interference microscopy (WLIM) were used to observe changes in morphology and roughness of the surface. The SEM images revealed the formation of pits with lacy covers at the surface during initial stages of the dissolution. At increased dissolution times, a transition from the lacy covered pits to open pits was observed. The images showed that preferential dissolution occurs on Ni layers grown at higher Ni(CO)4 decomposition temperatures. At longer dissolution times, the preferential dissolution also takes place at the boundaries between nickel laminae, which corresponds to the decomposition of Ni(CO)4 at different temperatures.

Protection of chromium-nickel steel in hydrochloric acid solution by a substituted triazole

The corrosion of chromium-nickel steel 12Kh18N10T in 2.0 M HCl (t = 20–100C) was studied using mass loss, potentiometric, and polarization methods. Efficient steel protection under these conditions can be achieved by inhibitors such as triazole derivative IFKhAN-92, alone or in combination with hexamethylenetetramine in a molar component ratio of 1 : 4. The proposed mixed inhibitor often protects steel 12Kh18N10T in HCl solutions in a broad temperature range, 20–100C, more efficiently than IFKhAN-92 alone at the same concentrations. The triazole in question and its blend with urotropine ensure metal protection in acid solutions for at least 8 h. Since for IFKhAN-92 alone and its blend with urotropine the corrosion inhibition coefficients grow with an increase in temperature, at least to t = 100°C, they should be attributed to so-called “high-temperature inhibitors of acid corrosion”. The presence of inhibitors of interest in the solution considerably increases the fraction of diffusion control in the corrosion process, as indicated by the significant decrease in effective activation energy in the presence of these compounds. Steel corrosion occurs in the active dissolution region both in background and inhibited acid solutions. The corrosion potential of steel is more positive in solutions containing inhibitors because they predominantly slow down the anodic reaction. Addition of IFKhAN-92 or its mixture with urotropine to an HCl solution hinders considerably the electrode reactions on steel in the entire temperature range studied. In this case, the cathodic and anodic current densities decrease with time, while the inhibition coefficients of electrode reactions either increase or remain nearly constant. The corrosion potentials decrease with time in the presence of the additives in question at t = 20–60C, which suggests that hindrance of the cathodic reaction is predominantly improved due to adsorption of inhibitors. It has been shown that the efficient inhibition of stainless steel corrosion in HCl solution by addition of IFKhAN92 or IFKhAN-92 + urotropine results from strong hindrance of both electrode reaction on the metal by these inhibitors, which becomes stronger in time and remains in effect as the temperature increases.

Protection of nickel-chromium steel in sulfuric acid solution by a substituted triazole

The corrosion of 12Kh18N10T nickel-chromium steel in 2.0 М H2SO4 (t = 20–100°C) was studied using mass loss, potentiometric, and polarization methods. It was shown that the corrosion resistance of this material in H2SO4 solutions is low, particularly at temperatures close to 100°C. Efficient steel protection in sulfuric acid solutions can be achieved using mixtures of IFKhAN-92 (a triazole derivative) with KI (molar ratio of the components 1: 1), KBr ( 1: 9), or KCNS ( 4: 1) as corrosion inhibitors. Of the IFKhAN-92 formulations studied, the highest corrosion inhibition coefficients are provided by the equimolar mixture with KI, despite an antagonism in the action of its components. Decreasing the concentration of this formulation to 0.005–0.025 mM (depending on temperature) does not considerably decrease the protective effect. Mixed inhibitors based on the substituted triazole protect the metal in the acid solution for at least 8 h. The presence of the inhibitors being studied in the solution considerably increases the contribution of diffusion control to the corrosion process, as indicated by a significant decrease in the activation energy from 60 1 to 35 2 kJ/mol in the presence of these compounds. Steel corrosion occurs in the active dissolution region both in background and inhibited acid solutions. A correlation was noted between the free corrosion potential of steel (Ecor) in an acid solution containing an inhibitor and the protective effect of the latter (): the higher the  value of the inhibitor, the more positive the potential of steel. The corrosion inhibition of stainless 12Kh18N10T steel in H2SO4 solutions by formulations based on IFKhAN-92 results from strong hindrance of electrode reactions on the metal by these compounds in a broad temperature range (up to 100С). The strongest inhibition of electrode reactions on stainless steel is provided by the IFKhAN-9 2+ KI mixture. As a result, it has the maximum efficiency in suppression of metal corrosion. A considerable effect on the characteristics of electrode reactions on stainless steel in background and inhibited acid solutions results from selective dissolution of metal surface components.

Impedance spectra of anodic dissolution of iron in distillery vinasse

Electrocoagulation technique has a great application potential to treat distillery vinasse, a liquid effluent generated from alcohol distilleries. The anodic dissolution of carbon steel in this effluent in the potential range of -750mV to -100mV was investigated by potentiodynamic polarization; likewise, the electrochemical impedance spectroscopy spectra were registered over anodic potentials of -560mV, -500mV and -400mV (overpotentials of 20mV, 80mV and 180mV) corresponding to the active dissolution region in order to propose a mechanism for describing the behavior of the carbon steel electrodissolution. Were explored the effects of the flow regime (Reynolds number of 5,500 and 11,500) and the temperature (60°C and 80°C) over the behavior of the spectra. At low current densities the impedance behavior can be satisfactorily described by the combination of the catalytic and non-catalytic mechanism of iron dissolution. It was also found that the dissolution must be carry out at current densities closely to 1mAcm−2 to prevent the possible formation of iron oxide layers on the anode. The anodic dissolution was favored by increasing the flow regime and temperature as well.

The energetic conditions determining the active dissolution of carbon steel during electrocoagulation in sulfate media

This study aims to investigate the active dissolution of carbon steel under rotating conditions, necessary for continuous dosing of metal cation in electrocoagulation. A reaction mechanism is proposed for its dissolution in 0.1, 0.3 and 0.5M Na2SO4, using electrochemical and chemical techniques (Raman, SEM). A continuous dissolution region was obtained from the potentiodynamic study, as a result of the competition between the rate of electrochemical reactions and the rate at which energetic conditions are imposed on the electrode. On the other hand, the dissolution of carbon steel underwent active, transition and passivation regions in potentiostatic and galvanostatic techniques, which respectively shifted to higher current densities and more positive potentials as the electrolyte concentration (Na2SO4) was increased. In general, the increase of Na2SO4 concentration promotes the iron dissolution and tightens the active region. The results revealed that the Na2SO4 concentration, the Fe(II)/Fe(III) interface concentration, and the time of perturbation influence the occurrence and evolution of the Green Rust Sulphate (NaFe6IIFe3IIIOH18SO42) intermediary, a precursor which hinders electrocoagulation through passive film formation. Scanning Electronic Microscopy and Raman spectroscopy showed that in the active dissolution region, the films grown potentiostatically are porous and contain α-FeOOH, δ-FeOOH, and Fe3O4 as the dominant species. In the transition region, these films become less porous and include α-FeOOH, Fe3O4, α-Fe2O3, γ-Fe2O3, while γ-FeOOH was identified in the passive region.

Influence of electrosynthesis conditions and Al2O3 nanoparticles on corrosion protection effect of polypyrrole films

Purpose – The purpose of this paper was to investigate the anti-corrosion behavior of polypyrrole (PPy) films in different states and presence of alumina nanoparticles synthesized by galvanostatic electropolymerization on stainless steel (SS) electrodes in an artificial seawater solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Design/methodology/approach – The electrochemical measurements were used to examine the effects of PPy and its nanocomposite on the corrosion behavior of SS type 316L in artificial seawater. A standard electrochemical cell with three electrodes was used for the measurements. The electrochemical response of the coated electrodes in the doped and the undoped state was compared with that of a bare electrode. Corrosion rate information was obtained by the Tafel extrapolation method, where the intersection point of a cathodic and an anodic polarization curve provides both the corrosion potential and the corrosion current. EIS measurements confirmed the potentiodynamic and open circuit potential (OCP) results. The microstructure of the obtained films was investigated by scanning electron microscopy. Findings – The results showed that the coated polymer films shifted the electrode potential toward more positive potentials, but this shift did not lead to passivation. However, a notable synergy was observed between PPy undoped film, oxygen reduction and iron dissolution. The potential of the SS remained in the active dissolution region, and it was not possible to produce a passive oxide layer in this region. PPy separates the metal dissolution process from the oxygen reduction process. This would prevent the local pH increase at the metal surface and subsequent delamination. The polarization curves, EOCP and impedance measurements showed that PPy undoped/Al2O3 layers show promise as good candidates for the corrosion protection of reactive metals. Originality/value – This paper presents that electrodes coated with undoped PPy synthesized in the presence of dodecyl sulfate anions and Al2O3 nanoparticles offered a noticeable enhancement of protection against corrosion processes.

Effect of pH, temperature, and H2O2 on the electrochemical oxidation behavior of iron in perchlorate solutions

The effects of pH, temperature, and H2O2 on the electrochemical oxidation behavior of iron were determined in perchlorate solutions. Iron exhibited four distinct oxidation behaviors in perchlorate solutions that had a pH of 3.0. First, the active dissolution potential with two current waves was observed in the range of −0.7 to 0 V. Second, the range of stable passivation was observed between 0 and 0.3 V. Third, unstable passivation was observed in the range of 0.3 to 1.2 V. Finally, pitting corrosion was observed at a potential above 1.2 V. The increase in pH stabilized the passivation of iron, whereas the increase in temperature had a negative influence on both the passivation and pitting behaviors of iron. The presence of H2O2 at concentrations less than 1.45 mM had an adverse effect on the formation of the passive oxide layer. However, H2O2 at concentrations greater than 1.45 mM exerted a beneficial influence on the formation of stable iron oxide in the active dissolution region. In addition, H2O2 mitigated the pitting corrosion of iron regardless of the H2O2 concentration.

In situ investigation of crevice corrosion on UNS S32101 duplex stainless steel in sodium chloride solution

Abstract The crevice corrosion of UNS S32101 in neutral 0.1 M NaCl solutions at room temperature was investigated directly by a facile method. Experimental results showed that both delayed and immediate crevice corrosion can be initiated. Morphology study indicated that the heaviest corrosion attack happened just below the passive/active boundary on the crevice wall. The relocation of the active dissolution regions during crevice corrosion was observed and explained by established theory. The mechanisms of the delayed and immediate types of crevice corrosion on UNS S32101 duplex stainless steel were discussed.