Transpassive Potential Research Articles

Anodic behavior of cobalt silicides in potassium hydroxide solutions

Anodic behavior of a CoSi2 electrode in 0.1–4 M KOH solutions and a Co2Si electrode in 1 M KOH was studied in a range from the corrosion potential (Ecor) to the oxygen evolution potentials with the use of polarization and impedance measurements. A number of results indicate enrichment of electrode surface layers in cobalt. At potentials from Ecor to Etp transpassivation potential, impedance spectra correspond to the passive state of the silicides. At potentials from Etp to the onset potential of oxygen evolution, impedance spectra can be described taking into account the inclusion of hydroxide ions in the passivating film. Equivalent electric circuits are proposed for different electrode potential ranges.

Passive and transpassive behaviour of Alloy 31 in a heavy brine LiBr solution

The passive and transpassive behaviour of Alloy 31, a highly alloyed austenitic stainless steel (UNS N08031), has been investigated in a LiBr heavy brine solution (400g/l) at 25°C using potentiostatic polarisation combined with electrochemical impedance spectroscopy and Mott–Schottky analysis. The passive film formed on Alloy 31 has been found to be p-type and/or n-type in electronic character, depending on the film formation potential. The thickness of the film formed at potentials within the passive region increases linearly with applied potential. The film formed at transpassive potentials is thinner and more conductive than the film formed within the passive region. These observations are consistent with the predictions of the Point Defect Model for passive and transpassive films on metals and alloys.

Effect of initial transpassive treatment on properties of passive film formed on Fe–Mn–Al–Cr alloy

The present study explores an electrochemical surface modification process to improve the corrosion resistance of Fe–24Mn–4Al–5Cr alloy. The effects of transpassive aging potential and time on the corrosion resistance, composition and stability of the passive film formed on an Fe–24Mn–4Al–5Cr alloy in 1M Na2SO4 solution were studied using combined electrochemical measurements, Auger electron spectroscopic and X-ray photoelectron spectroscopic analysis. The passivation of Fe–24Mn–4Al–5Cr alloy in 1M Na2SO4 solution was performed under the optimal parameters of critical potential of 1100 mV and for a passivation time of 30 min. The corrosion resistance in 1M Na2SO4 solution of Fe–24Mn–4Al–5Cr alloy modified by this treatment is superior to that of Fe–13Cr–0·1C stainless steel. Auger electron spectroscopic and X-ray photoelectron spectroscopic depth profiles of the passive film formed on the alloy using this treatment show Al2O3 and Cr2O3 enrichment together with Fe and Mn depletion in the passive film. This compositional change provides the increase in stability and in corrosion resistance of the alloy.

Corrosion Characteristics of Alloy 800 in Aqueous Solutions up to 200°C

The corrosion behavior of Alloy 800 in sulfate solutions containing hydrogen peroxide in the temperature range of 100–200°C was studied. Electrochemical techniques were used to study both the cathodic and anodic processes. The reduction of hydrogen peroxide showed an increase in the steady state current density and the H2O2 diffusion coefficient by increasing the temperature from 100 to 200°C. The electrochemical corrosion of Alloy 800 showed a decrease in corrosion potential and a decrease in transpassive potential in accordance with calculations for CrO4−2/Cr2O3 and HCrO4−/Cr2O3 couples. The increase in corrosion current density was compared with a predictive model based on both reaction kinetics and water property changes. The findings were compared with weight loss measurements and surface characterization studies.

Systematic study of the corrosion properties of selected high-resistance alloys in natural seawater

Electrochemical measurements were conducted to evaluate localized corrosion on UNS S31603, UNS S31803, UNS S32750, UNS S31254 and UNS N08825 in natural seawater. Critical pitting and crevice temperatures were assessed using a potentiostatic technique and critical potentials for pitting and crevice corrosion initiation and repassivation were identified using potentiodynamic polarization at temperatures from 5 to 40°C. Passivity breakdown always occurred through pitting and crevice growth above a transition temperature. Below this temperature, pitting corrosion was not observed on any of the alloys regardless of the applied potential, but initiation of crevice corrosion occurred after the alloys reached a transpassive potential.

Passivation of a CoCrMo PVD Alloy with Biomedical Composition under Simulated Physiological Conditions Studied by EQCM and XPS

Kinetics of passive film growth on a CoCrMo biomedical alloy have been studied using the Electrochemical Quartz Crystal Microbalance technique (EQCM) in phosphate buffer solution at room temperature and 37°C. CoCrMo layers were deposited on the quartz crystals by physical vapor deposition (PVD) reaching a dense and compact deposition film with fine-grain structure. EQCM measurements were performed under potentiodynamic and potentiostatic conditions (at applied passive and transpassive potentials). Furthermore, ex-situ X-ray Photoelectron Spectroscopy (XPS) analysis of the each tested sample was performed at the end of the electrochemical test. The use of EQCM allows distinguishing between electrochemical oxidation, passive and transpassive dissolution and passive film growth. In the passive domain the passive film thickness stabilizes within 200 to 400 s after an initial fast growth. The increase in current at the onset of the transpassive domain does not affect the passive dissolution rate. Only at higher potential dissolution rate increases due to the dissolution of Cr(VI), Co(III) and Mo(VI) species. The observed constant mass loss rate at transpassive potentials indicates that the passive film at these potentials is cracked or porous. Increasing temperature accelerates the mass loss through the oxide/electrolyte interface enhancing the passive and transpassive dissolution and increasing the thickness of the oxide film.

Electrochemistry of Actinides and Selected Fission Products in the Head End of Spent Nuclear Fuel Reprocessing

The data on the uranium metal corrosion rate in the solutions of nitric acid (0,1 – 4M) and effect of complex forming agents on uranium corrosion properties are presented. The increase of HNO3 concentration caused the shift of corrosion potential from 38mV to 446mV and the increase of the corrosion rate from 0,02 to 0,62mg.cm-2h-1. Transpassivation potential of U metal was found weakly effected by HNO3 concentration varying from 448 to 470mV/Ag/AgCl. The addition of HCOOH to the electrolytes containing less than 3M HNO3 found to shift the values of corrosion potentials about 500mV towards negative direction reducing the passivation of U metal. The data on the kinetics of oxidative dissolution of PuO2 using Ag(II) and Am(VI,V) as mediators and the effect of the mediator generation techniques are discussed. The electrochemical properties of UC in the solutions 2 – 4M HNO3, results of the quantitative determination of “oxidizable carbon” in dissolver solutions are presented. The results of corrosion and dissolution studies of Tc metal and Tc - Ru alloys containing from 19 to 70 at.% Ru in 0.5 0– 6M HNO3 indicate the formation of passive films of Tc(IV) – Ru(III,IV) hydroxides at the electrode surface in the solutions containing less than 2M HNO3 at the potentials less than 650mV/Ag/AgCl. The increase of HNO3 concentration to values exceeding 3M and the shift of the electrode potential towards positive direction causes the transition of the Tc and Tc-Ru alloys to transpassive state. The values of transpassivation potentials increase with the increasing with HNO3 concentration. Quantitative dissolution of Tc metal without application of oxidation potential becomes possible in the electrolytes, containing more than 4M HNO3. The rate of Tc – Ru alloys dissolution is noticed to slow down with the increase of Ru content in the alloy.

Microstructure and Electrochemical Behavior of Pd–Cu–Ni–P Bulk Metallic Glass and Its Crystallized Alloys

The microstructure and electrochemical behavior of the Pd42.5Cu30Ni7.5P20 as-quenched bulk metallic glass and its crystallized alloys after heat-treatment at 623 and 723K were investigated. The results revealed that the Pd42.5Cu30Ni7.5P20 bulk metallic glass and its crystallized alloys were spontaneously passivated in 0.144M NaCl and Hanks’ solution with a low passive current density and a wide passive region. The transpassive potential of the fully crystallized alloy heat-treated at a higher temperature was lower than those of the as-quenched bulk metallic glass and partially crystallized alloy. The difference in the microstructure of the as-quenched bulk metallic glass, the partially crystallized alloy and the fully crystallized alloy was responsible for their different transpassive potentials. The wider passive region of the alloys in Hanks’ solution is attributed to the presence of HCO3 ¹ , HPO4 2¹ ,S O4 2¹ and H2PO4 ¹ ions in the solution, which act as corrosive inhibitors.

The experiment research of corrosion behaviour about Ni-based alloys in simulant solution containing H 2S/CO 2

Aimed at the problem of tubing corrosion in environment that containing hydrogen sulfide (H 2S), carbon dioxide (CO 2), and chlorides (Cl −), the corrosion behaviour of two nickel based alloys (UNS 06985 and UNS 08825) in 15 wt%NaCl solution containing H 2S/CO 2 in high temperature and high pressure environment was investigated. The pitting corrosion behaviour of Ni-based alloys was studied in FeCl 3·6H 2O solution by means of polarisation curve and immersion tests. The scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) was applied to analyse the microstructure and corrosion performance of the samples. The results showed that the pitting-resistant of nickel alloy UNS 06985 was superior to UNS 08825. With the rising of experimental temperature, the corrosion increased and some slight pitting attacks appeared on the surface of UNS 08825. The test temperature was the crucial factor that influenced not only the compactness and the growing rate of corrosion product scale, but also the corrosion rate of the alloys. Elemental sulfur is a strong oxidant, the presence of S0 leads to a serious localized corrosion. XRD showed that the corrosion films formed on nickel base alloys consisted of NiS, CrO 3, and the oxides of Ni and Fe. The polarisation curves showed a different corrosion behaviour of two alloys, anodic curve of UNS 06985 has a wider passivation area, and there has higher transpassive potential.

Theoretical investigation of the evolution of the passive state on Alloy 22 in acidified, saturated brine under open circuit conditions

It is observed in many systems that as a metal or alloy is exposed to an aqueous solution over a long period of time, the corrosion potential drifts in the positive direction and the corrosion rate decreases. This implies the possibility that the potential may become sufficiently positive that it will exceed the transpassive potential and result in depassivation of the surface and hence to massive corrosion. On the other hand, if depassivation does not occur, the corrosion rate is predicted to continuously decrease with time, such that experiments carried out over short periods of time may seriously over-estimate the true corrosion rate over extended periods. This is an important issue in the selection of materials for the containment of high-level nuclear waste (HLNW), because of the excessively long performance horizon (up to 10 6 years), and in interpreting corrosion data from archeological artifacts. The theory developed in this work provides a physico-electrochemical explanation of the ennoblement phenomenon that is in good agreement with experiment and, in principle, allows one to optimize the model on short-term data and then predict the corrosion behavior over the very long term.

Transpassive Dissolution of Copper and Rapid Formation of Brilliant Colored Copper Oxide Films

This paper describes an electrochemical technique for growing adhesive copper oxide films on copper with attractive colors ranging from gold-brown to pearl with intermediate colors from red violet to gold green. The technique consists of anodically dissolving copper at transpassive potentials in hot sodium hydroxide, and then depositing brilliant color films of Cu2O onto the surface of copper after the anodic potential has been turned off. The color of the copper oxide film depends on the temperature, the anodic potential, the time t1 of polarization, and the time t2, which is the time of immersion after potential has been turned off. (At potentials lower than the transpassive region, that is, in the passive region, the oxide film is a dark gray or dark green matte with a blue powder hydroxide on the surface and it is not attractive.) The brilliant colored films were characterized using glancing angle x-ray diffraction, and the film was found to be primarily Cu2O. Cyclic voltammetry, chronopotentiometry, scanning electron microscopy, and x-ray photoelectron spectroscopy were also used to characterize these films. These techniques show that the different colors were due to differences in porosity and thickness rather than changes in composition.

Transpassive Electrochemistry of Chalcopyrite Microparticles

Most of the electrochemistry of chalcopyrite reported in the literature has been done using either massive samples of the mineral or carbon paste electrodes. Despite its importance, reports of the electrochemical behavior of this mineral carried on individual microparticles are scarce. In this work the microelectrochemistry of chalcopyrite is studied in sulfuric acid solutions, focusing the study on its behavior at transpassive potentials. It is shown that microparticles with sizes typically found in chalcopyrite concentrates (20 to 100 μm) can be leached to completion in less than 4 minutes at room conditions under the influence of 1.2 V (SCE). According to EDX analysis, atomic absorption spectroscopy and ICP, around 88% of the sulfur in the mineral is transformed into a compact layer of amorphous plastic sulfur, while the rest is transformed to sulphate. The plastic sulfur gradually equilibrates to more stable S8-rings orthorhombic phase. This nearly instantaneous dissolution of chalcopyrite opens the window to the development of new technologies.

Electrochemical and surface investigation of zirconium based metallic glass Zr 59Ti 3Cu 20Al 10Ni 8 alloy in nitric acid and sodium chloride media

To understand the corrosion behavior of zirconium based bulk metallic glass (BMG); electrochemical, and surface investigations were carried out on Zr 59Ti 3Cu 20Al 10Ni 8 alloy in HNO 3 acid, and NaCl medium. Electrochemical studies were carried out in 1 N, 6 N and 11.5 N HNO 3 by recording open circuit potential–time, potentiodynamic polarization, and impedance characteristics. Atomic force microscope was used for e x situ surface morphology study of the specimens after potentiodynamic polarization in 1 N, 6 N, and 11.5 N HNO 3, and in situ surface morphology study by free immersion of the specimens in both 0.5 M, and 1 M NaCl solution. The results revealed attaining of steady state noble open circuit potential, wider passive region, and decrease in transpassive potential ( E transpass) with increase in HNO 3 concentration. Surface morphology in free immersion, and after polarization showed, deep pit propagation in both 0.5 M, and 1 M NaCl solution, and aggressive surface dissolution in HNO 3 medium.

New material for orthopedic implants: Electrochemical study of nickel free P558 stainless steel in minimum essential medium

Nickel, a component of stainless steels (SS) applied in orthopedic implants may cause allergic processes in human tissues. P558 nickel free SS was studied to verify its viability as a substitute for stainless steel containing nickel. Its performance is compared to ISO 5832-9 and F138 most used nowadays grades in implants fabrications, in minimum essential medium, MEM, at 37 °C. Potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and “in vitro” cytotoxicity were used as techniques. From the electrochemical point of view P558 SS is comparable to ISO 5832-9 SS in MEM. It remains passivated until the transpassivation potential, above which generalized corrosion occurs. F138 presents pitting corrosion at 370 mV/SCE. The cytotoxicity results showed that P558, ISO 5832-9 and F138 do not present cytotoxic character. Therefore, these results suggest that P558 SS can be applied in orthopedic implants.

Passivity and Pitting Corrosion of X80 Pipeline Steel in Carbonate/Bicarbonate Solution Studied by Electrochemical Measurements

This work investigated the effects of chloride ions and hydrogen-charging on the passivity and pitting corrosion behavior of X80 pipeline steel in a bicarbonate-carbonate solution by electrochemical and photo-electrochemical techniques. It was found that a stable passivity can be established on the steel in the absence and presence of chloride ions. The hydrogen-charging does not alter the transpassive potential, but increases the passive current density. When chloride ions are contained in the solution, pitting corrosion will be initiated. The pitting potential is independent of the hydrogen-charging. Hydrogen-charging would enhance the anodic dissolution and electrochemical activity of the steel, but does not affect the pitting potential, which indicates that the charged hydrogen is not involved in the pitting initiation. However, hydrogen may accelerate the pit growth. Photo illumination could enhance the activity of the steel electrode, resulting in an increase of photo-induced anodic current density.

Estudo eletroquímico comparativo do aço ISO 5832-9 em diferentes meios de interesse biológico

Foi estudado o comportamento eletroquímico a 37°C do aço inoxidável ISO 5832-9, em meios de NaCl 0,9 %, de Ringer Lactato e meio mínimo de Eagle (MEM), por voltametria linear e análises da superfície por microscopia eletrônica de varredura (MEV) e por espectroscopia por dispersão de energia (EDS). Foram feitos ensaios mecânicos e testes de toxicidade. O aço ISO 5832-9 se encontra passivado no potencial de corrosão e não apresenta corrosão por pite nos três meios estudados em toda faixa de potencial investigada, desde o potencial de corrosão até 50 mV acima do potencial de transpassivação. Em meio de MEM, no entanto, as análises por MEV e EDS mostraram que o referido aço, nesse valor mais elevado de potencial, apresentou um comportamento diferente, com perda das inclusões de óxido de manganês. Os potenciais de corrosão, Ecorr (potencial de circuito aberto estacionário) bem como os valores de densidade de corrente de passivação, variaram na seguinte ordem: Ecorr, RL < Ecorr, NaCl < Ecorr, MEM. e jMEM << jRL ≅ jNaCl. No ensaio de citotoxicidade, o aço foi caracterizado como não-tóxico.

Corrosion performance of TiO2 coated type 304L stainless steel in nitric acid medium

The corrosion resistance of sputter deposited TiO 2 on austenitic type 304L stainless steel has been evaluated in the oxidizing medium of 1 M and 8 M nitric acid by monitoring corrosion potential, polarization behaviour and electrochemical impedance. Glancing incidence X-ray diffraction study of the TiO 2 coated surface revealed anatase phase and surface morphological study using an atomic force microscope showed evenly spaced particles with nano pores in the matrix. The particle size measurement showed that particles are within the range of 20–50 nm and measured root mean square roughness of coated surface was around 10 nm. The open circuit potential-time results in 1 M and 8 M nitric acid showed an increase in corrosion potential in both the test solutions for TiO 2 coated specimens. Electrochemical impedance analysis revealed an increase in magnitude of total impedance as well as phase angle for TiO 2 coated specimens in both the concentrations as compared to uncoated specimens. Potentiodynamic polarization study of coated specimens revealed overall resistance to corrosion by increase in corrosion potential, decrease in passive current density and increase in transpassive potential. Morphological examination of the polarized specimens using an atomic force microscope revealed resistance to surface dissolution in 1 M nitric acid and uniform surface dissolution at higher concentrations of nitric acid (8 M).

Corrosion Properties of Ag-Au-Cu-Pd System Alloys Containing Indium

In this study, the corrosion resistance of Ag-Au-Cu-Pd system alloys consisting of 5 or 10 mass% indium was evaluated. Levels of element release and tarnish were determined and electrochemical measurements performed. Results were compared with those for commercial silver-palladium-gold alloy. In terms of electrochemical behavior, the transpassive potential of these experimental alloys was 168-248mV. Experimental alloys with 25 mass% Au showed similar corrosion resistance to control gold-silver-palladium alloy. Amount of released elements was 14-130microg/cm(2) at 7 days, which is in the allowable range for dental alloys. Addition of indium to Ag-Au-Cu-10mass%Pd system alloys was effective in increasing resistance to tarnish and alloys containing 10 mass% of indium showed a minimal decrease in L(*) values after immersion. These findings indicate that 25Au-37.5Ag-15Cu-10Pd-2Zn-10In-0.5Ir alloy is applicable in dental practice.

Effects of hydrogen on the anodic behavior of Alloy 690 at 60 °C

Polarization and electrochemical impedance spectroscopy (EIS) measurements, Mott–Schottky (M–S) analysis and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of hydrogen on the anodic behavior of a one-dimensionally (1D) 25% cold worked (CW) Alloy 690 thermal treated (TT) in a boric acid and sodium sulphate solution at 60 °C. The pre-hydrogen-charged specimen exhibited a higher anodic current than that of the uncharged specimen below the transpassive potential. The charged hydrogen can be trapped in the metal. Electrochemical impedance spectroscopy (EIS) showed that the resistance capacitance loop of the hydrogen-charged specimen was significantly smaller than that of the uncharged specimen. Mott–Schottky analyses indicated that the passive film formed on Alloy 690 at −0.2 V SCE was an n-type semiconductor, with a p–n hetero-junction at 0.2 V SCE. Charged hydrogen increased the carrier density and the thickness of the passive film both at −0.2 V SCE and 0.2 V SCE. The Ni/Cr ratio in the surface film decreased after hydrogen charging, indicating that charged hydrogen could enhance the oxide film growth by increasing the OH − (O 2−) concentrations through its reaction with vacancies.

Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying

Amorphous coatings with a composition of Fe 48Cr 15Mo 14C 15B 6Y 2 were prepared by means of high velocity oxygen fuel (HVOF) thermal spraying under different conditions. Microstructural studies show that the coatings present dense layered structure and low porosity with a fraction of nanocrystals precipitated. The porosity and amorphous fraction of the coatings decrease as the kerosene and oxygen flow increase within the parameter range examined. Corrosion behavior of the amorphous coatings was investigated by electrochemical measurement. The results show that the coatings are spontaneously passivated with wide passive region and low passive current density in 3.5% NaCl, 1 N HCl and 1 N H 2SO 4 solutions, and exhibit an excellent ability to resist localized corrosion. However, the corrosion resistance of the coatings decreases in 1 N NaOH solution with lower transpassive potential and passive region. In addition, the optimal spraying parameter improves the corrosion resistance of the amorphous coatings obviously due to the proper proportion of porosity and amorphous fraction.