Anodic Behavior of a Titanium–Aluminum Hybrid Electrode: Formation of Hydroxide-Oxide Compounds

The electrochemical behavior of a titanium–aluminum hybrid electrode in aqueous solutions of electrolytes containing halide ions (F^– and Cl^–) was studied. The effects of current density, solution composition, and ratio of the working surface area of titanium and aluminum on the anodic dissolution rate of a Ti?Al hybrid electrode and its electrochemical characteristics were revealed. The joint anodic dissolution of aluminum and titanium in the aqueous media under study made it possible to obtain precursors of the highly disperse oxide system Al₂O₃–TiO₂. Data of X-ray and electron-microscopic analysis confirmed the results obtained.     

Galvanostatic study of the breakdown of Zn passivity by sulphate anions

The anodic behavior of Zn electrode in Na₂SO₄ solutions was studied by galvanostatic polarization technique. The polarization curves are characterized by a distinct arrest corresponding to ZnO formation, after which the potential rises linearly with time up to a well-defined value, the breakdown potential, at which the potential drops with time, down to more negative direction. This denotes the destruction of the passive film and initiation of pitting corrosion. It was found that, the breakdown potential, the time consumed till the breakdown potential and the rate of potential rise with time depend on the sulphate anions concentration, solution temperature and magnitude of the imposed current density. Addition of increasing concentration of phosphate, molybdate, tungstate or chromate anions causes a shift of the breakdown potential into the noble direction, indicating the inhibitive effect of these anions. The inhibitive effect of these inhibitors decreases in the order: CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > HPO₄²⁻.     

Anodic oxide films on nickel electrode in borate solutions

The effect of different concentrations of sodium borate, current densities, pH and temperature on the anodic oxidation of nickel was studied using galvanostatic polarization technique. The anodic potential-time curves showed four regions: the charging of the anodic double layer, an arrest corresponding to anodic dissolution and/or oxide film formation, linear rise in the potential due to the formation of barrier oxide film and finally deviation from linearity to reach steady-state potential value attributed to oxygen evolution reaction. The duration time of the arrest decreases with increasing current density and concentration of borate anion while the rate of oxide film formation increases. On the other hand, the duration time of the arrest increases with increasing pH and temperature of the solution while the rate of oxide film formation decreases. Increasing the borate concentration and pH of the solution shifted the starting potential of the arrest towards more negative values. Linear relationships were obtained between the starting potential of the arrest and both borate concentration and pH.     

Corrosion properties of Co-based cemented carbides in acidic solutions

The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H₂SO₄ solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H₂SO₄ solution. The specimen in H₂SO₄ solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.     

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

The corrosion behaviors of hafnium in Et₄NBr 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 (?_pit) and the repassivation potential (?_p) to be determined. ?_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.     

Electrochemical behaviour of an inhibitor film formed on copper surface

The potential dependence of adsorption and kinetic properties of inhibitor film formation on copper surface was studied in acidic environment by using surface sensitive techniques: electrochemical quartz crystal microbalance (EQCM) and electrochemical scanning tunneling microscopy (EC-STM). The investigated inhibitor was 5-mercapto-1-phenyl-tetrazole (5-MPhTT) in 1 mmol dm⁻³ H₂SO₄ solution. The QCM investigations were done to study the electrode mass changes in absence and presence of 5-MPhTT at rest potential. The apparent mass and current variation as a function of the electrode potential was registered in order to study the protective film and its breakdown. Results revealed that the anodic current density and the electrode mass loss is twice less in presence of 5-MPhTT. We also present results of an in situ STM study on the surface morphology and anodic dissolution of Cu(1 1 1) electrodes in inhibited/uninhibited electrolytes. The images gave information about the nature of the protective layer. The results were analyzed and discussed.     

Anodic behaviour of tin in maleic acid solution and the effect of some inorganic inhibitors

The anodic behaviour of a tin electrode in maleic acid solutions was investigated by potentiodynamic and chronopotentiometric methods. Measurements were conducted under different experimental conditions. The results demonstrated that the polarization curves exhibit active/passive transition. In active regions, tin dissolves as Sn²⁺ which is subsequently oxidized to Sn⁴⁺ and the dissolution process is controlled partly by diffusion of the solution species. The passivity is due to the presence of thin film of SnO₂ on the anode surface formed by dehydration of precipitated Sn(OH)₄. The active dissolution of tin increases with increasing acid concentration, temperature and scan rate. The potential transients showed that the passivation time decreases with increasing applied current density. The effect of adding increasing concentrations of CrO₄²⁻, MoO₄²⁻ and NO₂⁻ ions on the anodic behaviour of tin in maleic acid was studied. These ions inhibit the active dissolution of tin and promote the attainment of passivity. The extent of these changes depends upon the type and concentration of the inhibitor.     

Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

In an effort to examine the combined effect of HNO₃, NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations promoted active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increased HNO₃ concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO₃ concentrations. Susceptibility to pitting was also greatest at intermediate HNO₃ concentrations: the pit initiation and repassivation potentials decreased with increasing HNO₃ concentration until the HNO₃ concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.     

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H2SO4 solutions

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H₂SO₄ solutions were studied by potentiodynamic polarization curves and electrochemical impedance spectra techniques, as well as the corrosion morphology was characterized. The results indicate that electroless Ni–P coating with about 25 µm is stable in 30 days immersion in NaCl solution. Although it was corroded with prolonged immersion days, the corrosive medium has not penetrated through the coating. During the H₂SO₄ concentration ranging from 5 to 10%, the corrosion current density of electroless Ni–P coating increased due to the intensified anodic dissolution process; in 15% H₂SO₄ solution, electroless Ni–P coating shows obvious anodic passivation effect.     

Novel corrosion experiments using the wire beam electrode. (I) Studying electrochemical noise signatures from localised corrosion processes

This series of papers presents four novel experiments that were designed to study localised corrosion phenomena using an electrochemically integrated multi-electrode array namely the wire beam electrode (WBE). This present paper reports a WBE based experimental method that has been employed, for the first time, to study electrochemical noise patterns (called noise signatures) from localised corrosion processes. The objective of this work is to demonstrate the applicability of the WBE for investigating the origin of spontaneous electrode potential/current fluctuations and their effects on electrochemical processes. The key strategy of this work is to apply the WBE in a novel experimental set-up to simultaneously measure electrode potential noise and WBE current distribution maps--an approach that allows the direct comparison and correlation of electrochemical noise and corrosion events. Preliminary experiments have been carried out using a classic pitting corrosion system: stainless steel in a solution containing FeCl₃. A large number of anodic sites were found to exist on WBE surface at the very beginning of its exposure to the corrosion environment. Correlation between characteristic patterns in electrode potential noise and corrosion behaviour has been observed. More specifically, the characteristic sharp peaks in potential noise data (called noise signature I) were found to correlate with the sudden disappearance of single unstable anode in WBE current distribution maps. The characteristic noise pattern of quick potential changes followed by partial or no recovery (called noise signature II) was found to correspond with the massive disappearance of minor anodes in WBE current distribution maps. This result suggests that, in the corrosion system under study, electrode noise activities were associated with the disappearance of minor anodic sites, which lead to the eventual disappearance of most anodic sites. Localised corrosion was the result of the accelerated anodic dissolution of a small number of remaining anodic sites. The characteristics features in electrochemical noise and in WBE maps were reproducible.     

Electrochemical corrosion behavior of X80 pipeline steel in a near‐neutral pH solution

In this work, the electrochemical corrosion behavior of X80 pipeline steel was investigated in a near‐neutral pH solution using electrochemical impedance spectroscopy (EIC) and photo‐electrochemical (PEC) measurements as well as X‐ray photo‐electron spectroscopy (XPS) technique. The effects of hydrogen‐charging and stress were considered. The results show that the steel is in an active dissolution state, and a layer of corrosion product is formed and deposited on the electrode surface, which is subjected to further oxidation to form ferric oxide and hydroxide. Photo‐illumination enhances anodic dissolution of the steel when it is under anodic polarization due to destroying of the corrosion product film. When the steel is under cathodic polarization, the cathodic current density decreases upon laser illumination due to the photo‐oxidation of hydrogen atoms generated during cathodic reactions, which behaves as an anodic reaction to offset the cathodic current density. Hydrogen‐charging and stress decrease the corrosion resistance of the steel and enhance the dissolution rate of the steel.     

Electrochemical Behaviour of Platinum in Oxygenated Solutions of Sodium Hydrogen Carbonate at Elevated Temperatures

Abstract

The behaviour of the Pt electrode in oxygenated 0.1 M NaHCO₃ solution at 162° and 238°c has been investigated using open-circuit and potentiostatic polarisation techniques. The effects of temperature and concentration of O₂ in the solution on the anodic and cathodic polarisation curves are discussed, and the parameters which control the rest potential of the electrode are identified. The electrode is not thermodynamically reversible but can provide a useful reference potential for electrochemical studies of corrosion processes in oxygenated solutions at elevated temperatures. Limiting currents for the diffusion of O₂ were determined for several concentrations of O₂ between 20° and 238°c and used to determine the activation energy of diffusion of O₂.     

Corrosion protection properties of hydroxamic acid self-assembled monolayer on carbon steel

Self assembled monolayers (SAMs) of hydroxamic acids CH₃(CH₂)_nCONHOH with different alkyl length were formed on the carbon steel electrode surface. The corrosion protection properties of the monolayers were examined and characterized by electrochemical polarization curves, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS) and contact angle measurements. XPS results showed that the hydroxamic acid molecules adsorbed on the carbon steel surface, and the contact angle values on the modified surface supported the formation of hydrophobic hydroxamic acid SAMs. The results of electrochemical studies showed that the values of the corrosion potential shift towards the positive direction, and anodic currents of the carbon steel dissolution significantly decreases, indicating that hydroxamic acids are anodic inhibitors. However, the chain length and assembling time influence the protection efficiency.     

High-speed anodic dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: II. Nitrate solutions

Anodic dissolution of two heat-resistant chrome-nickel alloys containing 12% (weight) tungsten and 8% tungsten with 6% rhenium in a 2 M NaNO₃ solution was investigated using the rotating disk electrode method at current densities up to 30 A/cm². It is shown that anodic dissolution of these alloys occurs in the transpassive region of potentials with transition of the components in solution to forms with the highest oxidation level. Various mechanisms of alloy dissolution determining the processing speed (together with electrochemical dissolution) are proposed, including disintegration of the hardening phase, chemical oxidation of low-valence intermediates by solution components, and electrochemical formation of surface oxide layers. The results of change in the chemical composition of surfaces depending on the processing regimes are presented. Some variants of control by regimes of electrochemical dimensional processing (ECDP) of details from these alloys to achieve the optimal parameters of ECDP are proposed.     

On the influence of microstructure and carbide content of steels on the electrochemical dissolution process in aqueous NaCl‐electrolytes

The electrochemical dissolution behaviour of armco‐iron and of the steels C15, C45, C60 and 100Cr6 in concentrated sodium chloride media has been investigated. Anodic metal dissolution experiments have been carried out using the flow channel cell (parallel plate reactor), the rotating cylinder electrode (RCE) and the capillary cell. The microstructure of the steel has been varied through variation of carbon content and heat treatment (e.g. soft annealed with globular carbides or pearlitic). Current‐efficiency values have been obtained by gravimetric measurements in the current‐density range from i = 5 to 60 A/cm². For the soft annealed steels, the divalent ferrite dissolution in combination with electroless cementite removal dominates. For the pearlitic steels, the occurrence of oxygen evolution at electronically conductive metal carbides or trivalent ferrite dissolution, depending on the current density applied, was detected. Microstructure dependent potentiostatic current transients and potentiodynamic polarization curves have been presented. Polarization resistances, R_pol, were measured in dependence on NaCl concentration and the applied anode potential. For pearlitic steels (with carbon contents ≥ 0.45%) R_pol exceeds that of the analogous soft annealed steels. The topographies of the steel surfaces after anodic dissolution show microscopic structures, based on inert metal carbides, which are the result of preferential ferrite dissolution. Qualitative metal dissolution models explain the electrochemical dissolution behaviour of soft annealed and pearlitic steels on the basis of the formation of solid films at the substrate surfaces and recognizing the role of the inert metal carbides in the steel matrix. In these models, the role of a polishing layer forming between the solid particles has been taken into account.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.     

Effect of the Scan Rate on the Kinetic Parameters of Active Dissolution and Passivation of Iron in a Neutral Solution

The effect of the potential scan rate (V = 0.2–100 mV/s) of a rotating disk electrode (rotational speed = 6000 rpm) on the kinetics of active anodic dissolution and active-passive transition of Armco iron in a deaerated borate buffer (pH 7.40) was studied by cyclic voltammetry. The rates of active dissolution and the formation of a prepassive film were found to be determined, under free-diffusion conditions, by slow electrochemical steps of electron transfer throughout the V range studied; the cyclic voltammogram is a transient, thermodynamically nonequilibrium curve. The anodic current peak linearly grows and its potential is shifted in the positive direction with an increase in logV. The apparent coefficients of electron transfer for active anodic dissolution depend on V in the whole range studied. This can provide explanation to a large scatter of literature data on the active anodic dissolution and active-passive transition of iron in neutral media.     

The Limiting Dissolution Current of a Spherical Electrode in Acid

Anodic dissolution of a spherical electrode in acid solution is analyzed. An equation derived implicitly describes the dependence of the limiting current of anodic metal dissolution on the acid concentration. The consideration is exemplified by silver anodic dissolution in nitric acid solution. The dependence of the limiting current of silver anodic dissolution on the acid concentration is numerically calculated by using tabulated data on the Ag solubility in AgNO₃.     

Anodic oxide growth on tungsten studied by EQCM, EIS and AES

Anodic oxide growth on tungsten in 0.1 M H₂SO₄ + 0.4 M Na₂SO₄ has been studied on stationary and rotating electrodes using the electrochemical quartz crystal microbalance (EQCM), Auger electron spectroscopy (AES) depth profiling, electrochemical impedance spectroscopy (EIS) and the rotating ring disc electrode (RRDE). The sputter rate of tungsten oxide was calibrated with respect to a tantalum oxide reference film using Rutherford backscattering spectroscopy (RBS). Long time polarisation experiments at constant potential showed that at a stationary electrode the oxide thickness determined by AES increased with time, whereas a steady state was reached after a few seconds under rotating conditions. On the other hand, EIS indicated a time independent oxide thickness on both stationary and rotating electrodes. Both methods yielded a higher absolute thickness for stationary electrodes. The oxide growth behaviour at short times was studied with the EQCM by monitoring the current and the mass change resulting from a potential step. From these data the film growth fraction and the oxide thickness change were calculated. After an initial period of a few seconds dominated by film growth, the rotating electrodes exhibited a significant mass loss contrary to stationary electrodes. RRDE experiments confirmed dissolution during anodic polarisation of a rotating electrode. The results of this study demonstrate the necessity to combine several experimental methods when studying anodic film growth and they illustrate the usefulness of the EQCM which can monitor directly and with sub-second time resolution the mass change of the electrode together with the current density. They show the decisive effect of convection conditions on anodic oxide growth on tungsten.     

Role of ClO4 in breakdown of tin passivity in NaOH solutions

The anodic behaviour of a tin electrode in NaOH solutions containing different concentrations of NaClO₄ was studied by employing potentiodynamic, potential transient under constant current density methods and complemented with scanning electron microscopy (SEM). In perchlorate-free NaOH solutions, the E/i response exhibits active/passive transition. The active region involves two anodic peaks corresponding to the formation of Sn(II) and Sn(IV) species respectively. The permanent passive layer is duplex and consists of SnO and SnO₂. Additions of NaClO₄ to the alkali solution, accelerates the active dissolution of tin and tends to breakdown the duplex passive layer at a certain breakdown potential. SEM examination confirms the occurrence of film breakdown. The breakdown potential decreases with an increase in ClO₄⁻ concentration, but increases with increasing both OH⁻ concentration and scan rate. The potential-time transients display that the incubation time for pit initiation decreases with increasing both ClO₄⁻ concentration and anodic current density.