Effects of applied anodic potential and pH on the repassivation kinetics of pure aluminium in aqueous alkaline solution

The repassivation kinetics of pure aluminium have been explored in aqueous alkaline solutions as functions of applied anodic potential and pH by using an abrading electrode technique and a rotating disc electrode. The repassivation rate of the abraded bare surface of pure aluminium increased with increasing applied anodic potential in aqueous alkaline solutions, while it decreased with increasing pH. These results revealed that the growth rate of the passivating oxide film is enhanced by an applied electric field, but it is lowered due to the chemical attack by hydroxyl ions. A potentiostatic anodic current decay transient obtained from the abraded electrode surface showed a constant repassivation rate in neutral and weakly alkaline solutions. In contrast, in concentrated alkaline solutions it was observed to consist of three stages: a high repassivation rate in the initial stage due to a high formation rate of the oxide film on the abraded bare surface; a zero value of the repassivation rate in the second stage due to the dissolution of the oxide film by the attack of OH⁻; a high repassivation rate in the third stage due to a lowered dissolution rate of the oxide film. The dissolution rate of the passivating oxide film was observed to depend on the removal rate of aluminate ions from the oxide/solution interface. Received: 1 April 1998 / Accepted: 3 July 1998     

Experimental and theoretical studies of acridine orange as corrosion inhibitor for copper protection in acidic media

The corrosion process commonly limits the use of copper in practical applications. The use of corrosion inhibitors is one of the effective methods to reduce the corrosion rate of copper. In this research, the inhibition effect of acridine orange (3,6-bis(dimethylamine)acridine) (AcO) for the protection of copper in 0.5 ?M ?H₂SO₄ solution was studied. For this aim, the change of open circuit potential with exposure time (E_ocp-t), electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), anodic and cathodic potentiodynamic polarization measurements (PP) and chronoamperometry (CA) techniques were used. Some quantum chemical parameters (E_HOMO, E_LUMO and dipole moment) were calculated and discussed. The AcO film formed over the copper surface was examined by SEM, EDX, AFM and contact angle measurements. The electrochemical data showed that AcO is an effective corrosion inhibitor even at low concentrations (ranging between 99.1% and %99.4 ?at concentrations from 0.01 ?mM to 1 ?mM). The corrosion rate of copper decreases in the presence of the inhibitor by reducing both anodic and cathodic rates, which is depended on its concentration. This compound behaves as mixed-type corrosion inhibitors with predominantly cathodic type. Its adsorption on the copper surface obeys Langmuir adsorption isotherm. The value of adsorption equilibrium constant (K_ads) and the standard free energy of adsorption were ΔG_ads 1.298 x 10³ ?M^?1 and -27.71 ?kJ/mol in the case of 0.5 ?M ?H₂SO₄ solution containing 1.0 ?mM AcO, which shows the adsorption is high and spontaneous. The adsorbed inhibitor film over the metal increase contact angle of the surface, which suggests the more hydrophobic properties of the surface are increasing coming from the orientation of hydrophobic sites to the electrolyte. The zero charge potential (E_pzc) studies showed that the surface charge of the metal is positive in the corrosive media containing the inhibitor. Quantum chemical calculations showed that the binding of inhibitor molecules to the metal surface takes place through N atoms of the inhibitor.     

Application of Polytyramine Nanoparticles to the Corrosion Protection of Copper

Electrochemical polymerization of tyramine was achieved on copper electrode surface from tyramine in 0.3 M oxalic acid (pH=1.2) solution by using cyclic voltammetry technique. The formation of polytyramine nanoparicles (PTN) were characterized by cyclic voltammetry (CV), fourier transform infrared‐attenuated total reflectance (FTIR‐ATR) spectroscopy and scanning electron microscopy (SEM). The corrosion behavior of PTN coated copper was investigated by means of the change of open circuit potential with exposure time (E_ocp t), electrochemical impedance spectroscopy (EIS) and anodic polarization curves in 3.5 % NaCl solutions at room temperature. The obtained results showed the shift of corrosion potential toward positive values for electropolymerized copper and a significant decrease in corrosion current and corrosion rate in comparison with bare copper, so that the PTNs coating could be used as an important protection against corrosion of copper.     

Growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions

The present work was conducted to explore the growth mechanism of anodic oxide films on pure aluminium in aqueous acidic and alkaline solutions by using a.c. impedance spectroscopy and a beam deflection technique. From the analyses of a.c impedance data, it was found that the reciprocal capacitance of anodic oxide film on pure aluminium increased linearly with increasing film formation potential in both acidic and alkaline solutions, indicating a linear increase in the film thickness with film formation potential. However, as the film formation potential increased, the resistance of anodic oxide film decreased in acidic solution, while it increased in alkaline solution. From the measurements of the deflection, the deflection was observed to move towards only a compressive direction with time in acidic solution, but it showed a transition in the direction of movement from compressive to tensile in alkaline solution. Based upon the above experimental results, it is suggested that the movement of oxygen vacancy through the oxide film contributes to the growth of anodic oxide film on pure aluminium in acidic solution, but the movement of both aluminium vacancy and oxygen vacancy accounts for that oxide film growth in alkaline solution. Received: 12 August 1997 / Accepted: 9 October 1997     

Pitting of Al and Al–Si alloys in KSCN solutions and the effect of light

The pitting corrosion susceptibility of pure Al and three Al-Si alloys, namely (Al-6%Si), (Al-12%Si) and (Al-18%Si) has been studied in 0.04 M KSCN solution. Measurements were carried out under the effect of various experimental conditions using cyclic polarization, potentiostatic and galvanostatic techniques. In all cases, the potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of a thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion, at a certain critical potential, pitting potential (E_pit), as a result of breakdown of the passive film by SCN^? anions. Cyclic polarization measurements allowed the determination of the pitting corrosion parameters, namely the pitting potential and the repassivation potential (E_rp). Alloyed Si decreased the passive current (j_pass) and shifted both E_pit and E_rp towards more positive values. Thus alloyed Si suppressed pitting attack. The effect of illumination on passivity and the initiation of pitting corrosion on Al in KSCN solutions was also studied. It is observed that illumination of Al leads to an increase in its pitting corrosion resistance-apparent from j_pass, E_pit, and E_rp measurements in aggressive KSCN solutions.     

The influence of different variables on the electrochemical behavior of mild steel in circulating cooling water containing aggressive anionic species

Cyclic voltammetric, potentiodynamic anodic polarization and current–time transient studies were carried out on mild steel in circulating cooling water containing Cl⁻ and SO₄⁻² ions under the effect of different variables such as coolant flow, the availability of oxygen, cooling system temperature, and cooling system pH. The anodic excursion span of mild steel in cooling corrosive solution was characterized by the occurrence of a well-defined anodic peak (A1), while the reverse sweep was characterized by the appearance of two cathodic peaks (C1 and C2). The presence of Cl⁻ and SO₄⁻² ions in cooling water enhance the active dissolution of mild steel and tend to breakdown the passive film and induce pitting attack. The data reveal that increasing flow rate and temperature of cooling solution enhances the anodic peak current density (j _A1) and shifts the pitting potential (E _pit) towards more active values. It is seen that the peak current density of the anodic peak A1 increases and the pitting potential (E _pit) displaced in the noble direction in the aerated solution compared that in de-aerated solution. The pitting corrosion of mild steel by Cl⁻ and SO₄⁻² ions initiates more readily in acidic medium (pH 2.0). It was found that the incubation time (t _i) increase and in turn the pitting corrosion decrease in the order: pH 10 > pH 6.8 > pH 2.0.     

Multicycle chronoamperometry at rotating ring-disc electrode as a method of current separation into partial currents of silver ionization,Ag<Subscript>2</Subscript>O formation and its chemical dissolution in alkaline medium

A method of multicycle chronoamperometry at rotating ring-disc electrode is suggested for experimental separation of the disc polarization current into its components that correspond to the substrate metal ionization, an oxide formation, and the oxide chemical dissolution. The method was validated by the example of the Ag|Ag₂O|OH^?(H₂O) system. At moderate anodic potentials of Ag-disc (0.48–0.51 V), silver active dissolution from open areas of its surface and through film’s pores dominates; the phase-forming current, hence, the current efficiency of this process drops down rapidly. At the potentials of the maximum at voltammograms (0.52–0.53 V), when the silver active dissolution current is suppressed, the phase-forming currents dominate; they exceed the oxide chemical dissolution rate significantly. The Ag₂O film thickness increases rapidly, the current efficiency of the oxide formation process approaches 100% during the entire disc polarization period. The Ag(I)-oxide chemical dissolution rate constant practically does not depend on the anodic phase-formation potential; however, it somewhat varies depending on the oxide film thickness, thus reflecting changes in the film structure and, possibly, chemical composition (from AgOH to Ag₂O).     

Electrochemical performance of Mg–9Al–1Zn alloy in aqueous medium

A systematic study on the corrosion and passivation behavior of AZ91D alloy in relation to the influence of concentration, temperature, pH, and immersion time was made in aqueous sulfate solution using electrochemical techniques including open-circuit potential, potentiodynamic polarization and impedance spectroscopy. It was found that the corrosion and pitting potentials (E _corr and E _pit) of the alloy drift to more active values with increasing either concentration (0.01–1.0 M) or temperature (278–338 K) of the test solution, suggesting that sulfate solution enhances the alloy dissolution, particularly at higher temperatures. On the other hand, values of the total film resistance (R _T) indicate that neutral solution (pH 7.0) supports the formation of a better protective layer on AZ91D surface than alkaline (pH 12.5) or acidic (pH 1.0) medium. The growth of a protective film on the alloy surface at short immersion times (up to ∼50 h) is evinced by a rapid positive evolution of E _corr and fast decrease in the corrosion rate (i _corr). However, for a long-term exposure (up to 500 h) E _corr drifts negatively and i _corr increases due to breakdown of the protective film, which causes a decrease in the alloy stability. Fitting the impedance data to equivalent circuit models suitable to each behavior assisted to explore the mechanism for the attack of the sample surface at various testing times. The results obtained from the three studied electrochemical techniques are in good agreement.     

Electrochemical behavior of CoCrMo implant in Ringer's solution

CoCrMo has been successfully employed as an orthopedic and orthodontic material because of its excellent corrosion resistant and suitable biocompatibility. The purpose of this research was to investigate the susceptibility of CoCrMo in Ringer's solution at three different temperatures: 22 °C, 37 °C, and 60 °C. The corrosion behavior of CoCrMo was carried out by using common electrochemical methods such as open circuit potential, potentiodynamic measurement, electrochemical impedance spectroscopy (EIS), and Mott–Schottky measurements. CoCrMo was passivated by the air, forming a passive film, which was not destroyed during the immersion in electrolytes under different temperatures. Results from potentiodynamic tests showed that at high anodic potentials, the passive film did not demonstrate significant localized corrosion and rather exhibited overall passive film degradation corresponding to the general corrosion of the alloy in Ringer's solution at the three temperatures. EIS measurements showed the presence of the stable passive film on the alloy surface when tested at open circuit potential. Mott–Schottky test indicated that the preformed passive film is an n‐type semiconductor due to the presence of a donor species. This is implied by the existence of oxygen vacancies and interstitial metallic cations. As the potential increased, the Cr³⁺ oxidized and produced soluble Cr⁴⁺ species. This resulted in the film changing to a p‐type semiconductor owing to the dissolution and creation of cation vacancies (acceptor species). The passive film rupture was not due to p‐type characteristics but rather was a result of the considerable oxidative dissolution of the film at high anodic potential. Copyright © 2013 John Wiley & Sons, Ltd.     

硫酸根离子对铁在弱碱性溶液中阳极溶解和钝化行为的影响

采用浸泡实验, 电化学测试和表面分析技术研究了硫酸根离子浓度对铁在稀碳酸氢钠溶液中开路状态和阳极极化行为的影响. 在无硫酸根离子及含有少量硫酸根离子的碳酸氢钠溶液中, 铁的开路电位约为(-0.225±0.005) V, 并呈现钝化状态, 其电化学阻抗很大, 腐蚀速率较低. 在含有较高浓度硫酸根离子的碳酸氢钠溶液中, 铁的开路电位为(-0.790±0.010) V并呈现活性溶解状态, 其电化学阻抗较小, 腐蚀速率较高, 同时阳极极化曲线上能观察到活化-钝化转变现象. 由于铁在含有较高浓度硫酸根离子的碳酸氢钠溶液中处于活化状态, 阳极极化曲线上存在数个电流峰. 足够高的硫酸根离子浓度会导致铁表面预先形成或转变而成的氧化膜失效. 相比于自然曝氧状态, 在除氧条件下较低的硫酸根离子浓度即可引起铁在碳酸氢钠溶液中由钝态向活性溶解态的转变.     

Development of CeO<Subscript>2</Subscript>- and TiO<Subscript>2</Subscript>-incorporated aluminium metal-composite matrix with high resistance to corrosion and biofouling

Cerium oxide (CeO₂) is a potential corrosion inhibitor for aluminium, and titanium oxide (TiO₂) is an efficient anti-fouling agent in the marine environment. The present study explored the possibility of incorporating CeO₂ and TiO₂ in aluminium to prepare a metal matrix composite that could have high corrosion and biofouling resistance under marine conditions. Such incorporation of CeO₂ and TiO₂ in pure aluminium offered high resistance to corrosion and biogrowth under marine conditions as evidenced during different tests. The specimens exhibited more anodic and stable open circuit potential throughout the period of the study. The optimum concentration of CeO₂ and TiO₂ was found to be 0.2 and 0.1%, respectively. The present results lay emphasis on the potential scope of the use of CeO₂- and TiO₂-incorporated aluminium in marine environments.     

Electrooxidation of the hypophosphite ion on a palladium electrode

Anodic oxidation of sodium hypophosphite on smooth Pd and Pd/Pt electrodes and a Pd membrane is studied. Thei vs.E curves for the Pd electrode exhibit two anodic current peaks. One is caused by oxidation of H₂PO ₂ ^- , and the other, by simultaneous ionization of Pd and oxidation of H₂PO ₂ ^- . The hypophosphite ion adsorbed on the Pd surface hinders the formation of the passive film. This brings about a rapid dissolution of Pd in the oxygen region and its subsequent deposition with the formation of palladium black. The oxidation probably includes a slow heterogeneous chemical reaction, specifically, a cleavage of the P-H bond of the hypophosphite ion. The change in the reaction stoichiometry following an increase in solution pH and in anodic polarization is probably due to changing conditions of the H₂PO ₂ ^- adsorption and the number of adsorption sites occupied by H₂PO ₂ ^- on the surface. Following an increase in polarization, the phosphite ion may undergo oxidation to phosphate. Deceased.     

Effect of acetic acid on CO2 corrosion of carbon steel in NaCl solution

Potentiodynamic sweep and electrochemical impedance spectroscopy measurements were applied to investigate the effects of both temperature and acetic acid (HAc) on the anodic and cathodic reactions in CO₂ corrosion of P110 steel in 3.5% NaCl solution. The temperatures were controlled at 30 and 60 °C. The concentrations of HAc were controlled at 0, 1000, 3000 and 5000 ppm. In this work, the corrosion parameters of polarization curves, such as corrosion potential (E_corr), corrosion current density (i_corr), and anodic and cathodic branch slopes (b_a and b_c), are presented and discussed in detail. In addition, the equivalent circuit models and ZsimpWin software were utilized to discuss the Nyquist plots. The plots showed that the E_corr values shifted in the positive direction as the HAc concentration increased. The i_corr values increased with the increase in HAc concentration, indicating that HAc could accelerate the corrosion. The impedance spectra measured at 30 and 60 °C have different time constants and characterization. The coverage fraction θ and the thickness L of corrosion film are two most important controlled variables that influence and control the CO₂ corrosion mechanisms. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of albumin adsorption on physico-chemical properties of alumina surfaces

The effect of albumin adsorption on neutral active aluminium oxide was investigated in the presence of polar and non-polar liquids. The adsorbed values were highest near the isoelectric point of albumin and varied in the range 5–10 and 3–11 mg g^–1 with phosphate buffer and potassium chloride respectively after 2 and 24 h. In the case of aluminium oxide the effect of albumin adsorption on total heterogeneity of adsorbents is not explicit. On the one hand, the modified samples showed decreasing surface area with increase of surface coverage with albumin. On the other hand, modifications under the same conditions but without albumin caused similar changes. These effects suggest the strong influence of medium pH on surface properties (due to surface polarization) and competitive co-adsorption of ions on the process. The volumetric fractal dimensions of the studied materials change in the range 2.25–2.32 for pure aluminium oxide and BSA modified from the phosphate solution. E _d,max values (desorption energy in the maximum of distribution function) diminish (in the range 40–45 kJ mol^–1) compared with pure aluminium oxide (E _d,max=52 kJ mol^–1) for water thermodesorption at modified surfaces to the increase of a number of active centers of hydrophobic character, and weakening of the adsorbent–adsorbate increases.     

An EQCM study on the influence of saccharin on the corrosion properties of nanostructured cobalt and cobalt-iron alloy coatings

Nanostructured cobalt (Co) and cobalt-iron (CoFe) alloy coatings were electrodeposited from sulfate solutions in the presence and absence of saccharin. The effects of saccharin on the corrosion behavior of Co and CoFe alloy coatings were investigated using the electrochemical quartz crystal microbalance (EQCM) technique coupled with cyclic voltammetry (CV) measurements. Saccharin was added to the electrolyte as a grain refiner and brightener. Interestingly, opposite corrosion behaviors were found for all nanostructured coatings in 0.1 M H₂SO₄ and 0.1 M NaOH. The use of saccharin as an additive in the plating solution accelerated the anodic reaction for all deposits in acidic medium. The mass decreases while dissolution rate increased with higher saccharin concentration. Meanwhile, formation of a thick passive film on the Co electrode surface were enhanced while a hindering effect was observed for CoFe alloy coatings deposited in the presence of saccharin in alkaline solution. The anodic and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results.     

The electrochemical properties of Ni<Subscript>3</Subscript>C carbide

The electrochemical properties of Ni₃C was studied. In acidic sulfate solutions, the carbide is characterized by high overpotential of its oxidation as compared with nickel. In the case of carbide oxidation, the anodic reaction orders with respect to anions are low, indicating a weak dependence of the rate of the anodic process on the solution composition. Significant differences in the kinetics of the anodic processes indicate different mechanisms of the oxidation of nickel and its carbide. The rate and kinetic parameters of the hydrogen evolution reaction are comparable on Ni and Ni₃C. In neutral and alkaline solutions, the metal and carbide samples were similar in their electrochemical characteristics. The anodically grown oxide film is thinner on nickel carbide than on nickel metal, and the oxide formed on the carbide is more readily reduced under cathodic polarization. This film is also more resistant to the pitting attack than the oxide film on nickel metal.     

Correction to: The effect of small silver inclusions on the palladium activity in formicacid oxidation reaction and corrosion stability

A PdAg deposit containing ~ 25 at.% Ag is obtained by the electrochemical codeposition from an aqueous solution of Pd and Ag sulfates (Au support, 0.5 M H₂SO₄). The deposit is characterized by means of various physical, physicochemical, and electrochemical methods. The PdAg deposit demonstrates the ~ 2 times higher specific activity (per the electrochemically active surface area (EASA) of Pd) in the formic acid oxidation reaction (FAOR) as compared with the individual Pd deposit prepared under the same conditions. The effect of silver additions on the palladium activity depends on many factors. The corrosion stability of PdAg is studied in 0.5 M H₂SO₄ solution based on the overall cyclic voltammograms (CVAs) and also on anodic and cathodic half-cycles in the region E = 0.3 − 1.25 V (vs. reversible hydrogen electrode (RHE)). The electrochemical estimates are compared with the results of direct analytical determination of dissolution products in solution after anodic polarization of deposits. The total amounts of Pd dissolved substantially increase with incorporation of Ag, which is associated, first of all, with the considerable increase in the EASA; at the same time, the specific dissolution of Pd also substantially increases. The possible factors determining the active dissolution of PdAg deposits are discussed; in particular, the specific mechanism of their dissolution via silver adatoms is proposed.

     

Investigation by electrochemical and deflectometric techniques of silicon dissolution and passivation in alkali

The effects of anodic polarization of p-Si electrodes in alkaline medium have been investigated by the probe beam deflection (PBD) or `mirage' technique and the optical cantilever or bending beam method (BBM). The PBD technique permits a monitoring of dissolution and passivation processes and provides an estimate of the oxide etchback times during open circuit corrosion. The BBM technique has been used to estimate the stress of the thin oxide layer, which appears to be in the order of 180 MPa for very thin oxide films (nm range). The result is discussed in comparison with literature properties of thermally generated oxides.     

Electrochemical corrosion kinetics of tin and tin amalgams in NaCl aqueous solutions

Electrochemical techniques were used to determine the corrosion rate of pure tin metal as compared to 80 Sn/20 Hg tin amalgam. X-ray diagrams showed that this amalgam was a crystalline γ₂ phase, whereas a 50 Sn/50 Hg amalgam contained liquid alloy embedded in the same γ₂ phase. Open circuit potential measurements, combined with narrow range potential scanning voltammetry, lead to the conclusion that amalgamation resulted in enhancement of the corrosion current, mainly by increasing the cathodic electron transfer reaction kinetics both in deaerated and in oxygen-saturated NaCl solution. When maintained at zero current potential in a solution containing dissolved O₂ gas, the samples were gradually covered with an insulating oxide layer which was identified by a series of electrochemical impedance diagrams recorded at different time intervals. The oxide layer was firmly adherent to the bulk tin metal but was poor at protecting the amalgam electrode. Finally, at potential values where the anodic current reached a few mA/cm², the pure tin metal surface was suddenly deteriorated by the formation of extremely deep pinhole corrosion pits, while this effect was smoothed down by amalgamation. Electronic Publication     

The inhibition mechanism of pitting corrosion of pure aluminum by nitrate and sulfate ions in neutral chloride solution

The inhibition mechanism of pitting corrosion of natural oxide film-covered pure aluminum by NO⁻ ₃ and SO²⁻ ₄ ions has been examined in 0.1 M NaCl solution using potentiodynamic polarization experiments, a.c. impedance spectroscopy, Auger electron spectroscopy and a combination of the potentiostatic current transient method and optical microscopy. It was found that NO⁻ ₃ ions can be incorporated into the natural oxide film on pure aluminum at open-circuit potential, but the incorporation of SO²⁻ ₄ ions into the film hardly occurs. The incorporation of NO⁻ ₃ ions lowered the pitting susceptibility of pure aluminum in 0.1 M NaCl solution. Based upon the experimental results, it is suggested that the pitting corrosion inhibition mechanism by anions can be classified into two different groups: inhibition by competitive adsorption of anions (SO²⁻ ₄) with Cl⁻ ions and inhibition by the incorporation of anions (NO⁻ ₃) into the film rather than competitive adsorption. Both cases may impede the incorporation of Cl⁻ ions into the film, thus inhibiting pitting corrosion of natural oxide film-covered pure aluminum in chloride solutions. Received: 16 October 1998 / Accepted: 6 January 1999