Cathodic reduction of the duplex oxide films formed on copper in air with high relative humidity at 60 °C

The cathodic reduction of duplex air-formed oxide film on copper was performed at a constant current density of i_c = −50 μA cm⁻² in deaerated 0.1 M KCl solution to investigate the sequence of cathodic reduction of each oxide layer and its mechanism. The single-phase thick CuO film on copper was also cathodically reduced at i_c = −50 μA cm⁻² or −2.5 mA cm⁻². The surface characterizations of the air-formed oxide film and single-phase CuO film before cathodic reduction and after partial or complete cathodic reduction were performed by XPS and X-ray diffraction, respectively.The two plateau regions appeared in the potential vs. time curve during cathodic reduction of the duplex air-formed oxide film on copper, while one plateau region was observed in the potential-time curve during cathodic reduction of the single-phase CuO film on copper. The potential in the first plateau region for the air-formed film coincided with that in the plateau region for the CuO film. The results of XPS and X-ray diffraction suggested that in the first plateau region, the outer CuO layer is directly reduced to metallic Cu, while in the second plateau region, the inner Cu₂O layer is reduced to metallic Cu.     

Ellipsometry of passive oxide films on nickel in acidic sulfate solution

Nickel passive film has been studied in acidic sulfate solutions at pH 2.3 and 3.3 by ellipsometry. During anodic passivation followed by cathodic reduction, the roughness increases with dissolution of nickel, being indicated by gradual decrease of reflectance. However, the ellipsometric parameters, Ψ (arctan of relative amplitude ratio) and Δ (relative retardation of phase), are relatively insensitive to the roughness increase. From the change of Ψ and Δ, δΨ and δΔ, during the anodic passivation and reduction, thickness of the passive oxide film was estimated with assumption of refractive index of n_f = 2.3 of the film. The thickness estimated is a range between 1.4 and 1.7 nm in the passive potential region from 0.8 to 1.4 V vs. RHE, having a tendency of thickening with increase of potential. Cathodic reduction at constant potential induces a change of the oxide film to an oxide film with lower refractive index of n_f = 1.7, accompanied by thickening of the film about 30% more in the initial stage of reduction for 30 s. The gradual decrease of thickness takes place for the oxide with the lower refractive index in the latter stage. The potential change from the passive region to cathodic hydrogen evolution region may initially cause hydration of the passive oxide of NiO, i.e., NiO + H₂O = Ni(OH)₂, and during the latter stage of reduction, the hydrated nickel oxide gradually dissolves.     

Study on initiation of localised corrosion on copper thin film electrode by combinational use of an EQCM with a liquid-phase ion gun

A new method allowing simultaneous measurements of anodic currents and small mass changes during initiation and early growth of a single localised corrosion site was discussed. An electrochemical quartz crystal microbalance (EQCM) was combined with a liquid-phase ion gun consisting of an Ag/AgCl microelectrode which produces chloride ions, causing local breakdown of passive film and pit growth. The method was applied to copper thin films polarised anodically at 0.6 or 0.8 V (SHE) in pH 8.4 borate buffer solution. It was found from comparison between coulometry and gravimetry that copper dissolves as Cu²⁺ during the pit growth. The shape of the pit was almost circular and the average pit current density, i_p=25 A cm⁻² was evaluated from the kinetics of 2D pit growth.     

In situ gravimetry of nickel thin film during potentiodynamic polarization in acidic and alkaline sulfate solutions

The passivation process of nickel thin films during potentiodynamic polarization in acidic and alkaline sulfate solutions was analyzed by an electrochemical quartz crystal microbalance (EQCM) to separate the partial current density of nickel dissolution through the passive film, i_Ni²⁺, and the partial current density of film growth, i_O^2-. The values of i_Ni²⁺ and i_O^2- during potentiodynamic polarization (20 mV s⁻¹) in the passive potential region could be separated by comparing the mass change rate and net polarization current as a function of electrode potential. It is found that i_Ni²⁺ is larger than i_O^2- in pH 3.0, 0.5 M sulfate solution, while the situation reverses in pH 12, 0.5 M sulfate solution. The sulfate concentration dependences of i_Ni²⁺ and i_O^2- in pH 3.0, sulfate solution were significant as compared to those in pH 12, sulfate solution.     

Determination of electrochemical parameters and corrosion rate for carbon steel in un-buffered sodium chloride solutions using a superposition model

Corrosion of carbon steel in un-buffered NaCl solutions was studied applying linear potential sweep technique to a rotating disk electrode. Current-potential curves were obtained from linear potential sweep at a rate of 1 mV s⁻¹ in solution with concentrations in the range 0.02-1 M NaCl and rotation rates in the range 170-370 rad s⁻¹, at 22 °C. Potential sweeps, which were conducted in the potential range −700 to −100 mV/SHE, were started from the cathodic limit in order to approach the measurement of corrosion under rust-free conditions. Polarization curves were analyzed with a superimposition model developed ad hoc and implemented in a computer program, which enabled determining the corrosion rate and kinetics parameters of the underlying anodic and cathodic sub-processes. The anodic sub-process, dissolution of iron, was well described in terms of a pure charge transfer controlled reaction, while the cathodic sub-process, oxygen reduction on iron, was well described in terms of mixed mass transfer and charge transfer control. Increase of electrode rotation rate increases the limiting current of oxygen reduction, which results in an enhanced corrosion rate of carbon steel. Increase of NaCl concentration has a dual effect: the limiting current of oxygen reduction decreases as a result of the influence of NaCl concentration on solution viscosity and the anodic dissolution of iron increases due to the influence of NaCl on pitting formation. However, this last mechanism predominates and a net increase in carbon steel corrosion rate is observed in this case.     

N-Phenylcinnamimide and some of its derivatives as inhibitors for corrosion of lead in HCl solutions

The anodic and cathodic polarization behavior of lead electrode was studied galvanostatically in HCl solutions of various concentrations. Increasing the acid concentration enhances the rate of the anodic dissolution of the metal and the rate of hydrogen evolution reaction, with anodic and cathodic Tafel slopes equal to 30 and 115 ± 5 mV decade⁻¹, respectively. The effect of addition of N-phenylcinnamimide and some of its derivatives, N-(p-nitrophenyl)cinnamimide, N-(p-methylphenyl)cinnamimide, and N-(p-methoxyphenyl)cinnamimide, as inhibitors on the kinetic of the anodic and cathodic reactions of lead in 0.1 M HCl solutions was also studied. These compounds inhibit both the rate of anodic dissolution and the rate of hydrogen evolution reactions without affecting the Tafel slopes. This result indicates that the used compounds were of mixed-type. The inhibition efficiency of these additives increases in the order: N-(p-nitrophenyl)cinnamimide < N-phenylcinnamimide < N-(p-methylphenyl)cinnamimide < N-(p-methoxyphenyl)cinnamimide. The equilibrium constant and the free energy of adsorption process have been calculated and discussed.     

Multicycle chronoammetry of RRDE in the investigation of the anodic oxide formation

The method of multicycle chronoammetry of RRDE makes it possible to obtain separately the partial currents of metal electrode ionization, anodic oxide formation and chemical oxide dissolution. The method is tested for Ag∣Ag₂O∣OH⁻(H₂O) system. In the range of low anodic potentials (0.48 ÷ 0.51 V) the process of active silver dissolution prevails; the phase formation current rapidly drops. At higher potentials (0.52 ÷ 0.53 V) the phase formation current prevails and noticeably exceeds the rate of the chemical oxide dissolution. The thickness of Ag₂O film rapidly increases; and the net phase formation current is close to 100%.     

Self-healing mechanism of a protective film prepared on a Ce(NO3)3-pretreated zinc electrode by modification with Zn(NO3)2 and Na3PO4

Self-healing mechanism of a protective film against corrosion of zinc at scratches in an aerated 0.5 M NaCl solution was investigated by polarization measurements, X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA). The film was prepared on a zinc electrode by treatment in a Ce(NO₃)₃ solution and addition of aqueous solutions containing 9.98 or 19.9 μg/cm² of Zn(NO₃)₂ · 6H₂O and 55.2 μg/cm² of Na₃PO₄ · 12H₂O. After the coated electrode was scratched with a knife-edge crosswise and immersed in the NaCl solution for many hours, polarization measurements, observation of pit formation at the scratches, XPS and EPMA were carried out. This film was remarkably protective and self-healing against zinc corrosion on the scratched electrode. The cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the surface except for scratches with a thin Ce₂O₃ layer containing a small amount of Ce⁴⁺ and the surface of scratches with a layer composed of Zn₃(PO₄)₂ · 4H₂O, Zn(OH)₂ and ZnO mostly.     

Electrochemical stability of anodic titanium oxide films grown at potentials higher than 3 V in a simulated physiological solution

The cathodic behaviour of oxides formed on titanium electrodes in physiological solutions at potentials between 3 and 5 V (vs. SCE) was studied by cyclic voltammetry. In case of anodic polarization at potentials higher than 3 V (vs. SCE), a cathodic peak at ∼0.4 V (vs. SCE) appears in the cathodic scan, which could be due to the reduction of unstable peroxides. The results show that this peak depends on the anodic potential and the oxidation time. This behaviour supposedly is due to the formation of unstable titanium peroxides like TiO₃ during anodization. Based on repetitive oxidation-reduction processes can be concluded that the created amount of TiO₃ inside of the TiO₂ surface layer seems to be constant.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

Electrochemical behaviour of Cu-40Zn in 3% NaCl solution polluted by sulphides: Effect of aminotriazole

The electrochemical behaviour of Cu-40Zn alloy, in 3% NaCl medium pure and polluted by 2 ppm of S²⁻ ions, has been studied in the absence and presence of the 3-amino-1,2,4 triazole (ATA) as corrosion inhibitor. Electrochemical measurements (polarisation curves and electrochemical impedance spectroscopy) showed that sulphides accelerate the alloy corrosion. The studies revealed that ATA inhibits both cathodic and anodic reactions, indicating a mixed type of inhibition. The inhibiting effect was higher in presence of S²⁻ ions than in its absence. Scanning electron microscopy analysis showed that the inhibitor acts by preventing the adsorption of S²⁻ ions, and formation of Cu₂S at the alloy surface. The inhibition efficiency reaches 98% at a concentration of 5 × 10⁻³ M.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.     

Corrosion inhibition during synthesis of Cu2O nanoparticles by 1,3-diaminopropylene in solution

Corrosion inhibition for Cu₂O nanoparticles in solution using the regulators of 1,3-dimorpholinpropylene and 1,3-diethylaminopropylene was studied by experimental and theoretical calculation methods. The inhibition mechanism of regulators was related to the arrangement of cetyltrimethylammonium (CTAB) molecules on the surface of Cu₂O nanoparticles. Control of the type and amount of regulators has been demonstrated to produce well-dispersed and active Cu₂O nanoparticles (∼100 nm). The oxidation temperature of Cu₂O nanoparticles decreased from 297.9-404.4 °C (blank) to 232.3-334.3 °C (containing regulator). For 1,3-dimorpholinpropylene, its arc structure and active sites (C_4C_5, N_2 and O_2) facilitate the formation of stable protective film over Cu₂O surface.     

On the fundamentals of electrochemical corrosion of X65 steel in CO2-containing formation water in the presence of acetic acid in petroleum production

Electrochemical corrosion behavior of X65 steel in CO₂-containing oilfield formation water in the presence of acetic acid (HAc) was investigated by various electrochemical measurements and analyses as well as thermodynamic calculations of ionic concentrations, reaction rate constants and equilibrium electrode potentials. A conceptual model was developed to illustrate corrosion processes of steel in oilfield formation water system. The anodic reactions of the steel contain a direct dissolution of Fe, Fe → Fe²⁺ + 2e, and the formation of corrosion scale, FeCO₃, by Fe + → FeCO₃ + H⁺ + 2e. The cathodic processes contain the reduction of H⁺, , H₂O and HAc, where reduction of HAc has the least negative equilibrium potential and thus dominates the cathodic process. With addition of HAc in the solution, both cathodic and anodic reaction rates increase remarkably. It is attributed to the fact that HAc inhibits or degrades the formation of protective scales due to the decrease of solution pH. Upon electrode rotation, the measured impedance decreases with the increase in HAc concentration. The FeCO₃ scale will not form on electrode surface. When HAc concentration is less than 1000 ppm, the adsorbed intermediate product is not significant, resulting in generation of a low-frequency inductive loop in EIS plots. When HAc concentration is more than 3000 ppm, the adsorption of intermediate product is significant, generating overlapped capacitive semicircles in EIS measurements.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Electrochemical pitting corrosion behaviour of α-brass in LiBr containing solutions

The potentiodynamic anodic polarization curve of α-brass (70% Cu-30% Zn) in 1 M LiBr solution showed an initial active region of the alloy dissolution followed by two well defined anodic current peaks then a narrow passivation region before the pitting potential (E_pit) is reached. The initial active anodic region exhibited Tafel slope with 90 mV dec⁻¹ attributed to the formation of CuBr₂⁻ complexes. The anodic current peaks were attributed to the formation of CuBr and Cu²⁺ ions, respectively. The change of pH values of LiBr solution did not affect the anodic polarization curves of α-brass. Increasing the solution temperature from 30 to 90 °C changed the corrosion type from pitting to general one. The addition of 10⁻² M benzotriazole (BTAH) to 1 M LiBr solution is completely inhibited the pitting corrosion at 30 °C while it did not inhibit the pitting at 90 °C. The inhibition effect was attributed to the adsorption of BTAH molecules on the alloy surface, which obeys Langmuir isotherm. The presence or absence of pitting corrosion was confirmed by using SEM.     

Corrosion of X65 steel in CO2-saturated oilfield formation water in the absence and presence of acetic acid

Electrochemical corrosion behavior of X65 steel in CO₂-saturated formation water in the absence and presence of acetic acid was studied by electrochemical measurements, scanning vibrating micro-electrode (SVME), localized electrochemical impedance spectroscope (LEIS) and surface analysis techniques. It is found that, when steel is immersed in formation water, the dissolution of Fe dominates the anodic process and the steel is in active dissolution state. Adsorption of intermediate product on the electrode surface results in generation of an inductive loop in the low frequency range of EIS plot. As corrosion proceeds, the concentration of Fe²⁺ in the solution increases. When the product of [Fe²⁺] × [] exceeds solubility product of FeCO₃, FeCO₃ will deposit on the electrode surface, and protects the steel substrate from further corrosion. The steel is in a “passive” state. When the electrode surface is completely covered with FeCO₃ film, the inductive loop in the low frequency range disappears. In the presence of acetic acid in formation water, the cathodic reaction will be enhanced due to the direct reduction of undissociated acetic acid. Addition of acetic acid degrades the protectiveness of corrosion scale, and thus, enhances corrosion of steel by decreasing the FeCO₃ supersaturation in solution.     

Evaluation of some non-toxic thiadiazole derivatives as bronze corrosion inhibitors in aqueous solution

The inhibiting effect of four innoxious thiadiazole derivatives (2-mercapto-5-amino-1,3,4-thiadiazole (MAT), 2-mercapto-5-acetylamino-1,3,4-thiadiazole (MAcAT), 2-mercapto-5-methyl-1,3,4-thiadiazole (MMeT) and 2-mercapto-5-phenylamino-1,3,4-thiadiazole (MPhAT)) on bronze corrosion in an aerated solution of 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ at pH 5 was studied by potentiodynamic voltammetry and electrochemical impedance spectroscopy.The corrosion parameters determined from the polarisation curves indicate that the addition of the investigated thiadiazole derivatives decreases both cathodic and anodic current densities, due to an inhibition of the corrosion process, through the adsorption of thiadiazoles on the bronze surface. The inhibiting effect of the investigated organic compounds appears to be more pronounced on the anodic process than on the cathodic one and, except for the case MPhAT, it is enhanced by the increases of the inhibitors’ concentration.The adsorption of the thiadiazole derivatives on bronze was confirmed by the presence of the nitrogen atoms in the EDX spectra of the bronze exposed to inhibitor-containing solutions.The magnitude of polarisation resistance values and, consequently, the inhibition efficiencies are influenced by the molecular structure of thiadiazole derivatives. The strongest inhibition was noticed in the presence of compounds with phenyl amino- or amino-functionalities in their molecules. The maximum protection efficiencies were obtained by addition of: 5 mM MAT (95.9%), 1 mM MAcAT (95.7%), 5 mM MMeT (92.6%) and 0.1 mM MPhAT (97%). EIS measurements also revealed that the inhibitor effectiveness of the optimal concentrations of thiadiazole is time-dependent.     

Tribological behaviour and residual stress of electrodeposited Ni/Cu multilayer films on stainless steel substrate

In the present study, [Ni (4.5 nm)/Cu (t_Cu = 2, 4 and 8 nm)] multilayers were pulse electrodeposited on stainless steel (AISI SS 304) substrate from sulphate based single bath technique. X-ray diffraction (XRD) was used to investigate the structure and stress of the Ni/Cu multilayer. The results from XRD analysis indicated that the deposited multilayers had a preferred crystal orientation of [111] and presence of satellite reflection suggested the formation of superlattice. The stress level within the deposited multilayers was found to be sensitive to the sublayer thickness. Sliding wear behaviour of electrodeposited Ni/Cu multilayer films has been investigated against a tungsten carbide (WC) ball as the counter body and compared with that of the constituents, Cu and Ni coatings. The wear tests were carried out by using a reciprocating ball-on-flat geometry at translation frequencies of 5 and 10 Hz, slip amplitude of 1 mm and at five different loads of 3, 5, 7, 9 and 11 N. Friction force was recorded on-line during the tests. At the end of the tests, the wear scars were examined by laser surface profilometry and scanning electron microscopy (SEM). Friction coefficient was found to be dependent on load and Cu layer thickness (t_Cu) and the values for multilayers were border between Ni and Cu. Among multilayers, sample with minimum t_Cu has shown the lowest friction coefficient and wear rate. With increasing t_Cu, the wear mechanism changes from pure abrasive wear at t_Cu = 2 nm, to particle entrapment at t_Cu = 4 nm to particle embedding at t_Cu = 8 nm. Detailed investigation of the wear scar morphology as well as wear rate measurement revealed that at low loads, (H/E) ratio and residual stress governed the wear rate and the principle wear mode was abrasive cutting. At intermediate loads, the role of residual stress became insignificant while wear was governed by (H/E) ratio and plastic deformation. However, at higher loads, plastic deformation played the major role.     

The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry

The dissolution of Mg has been investigated with atomic emission spectroelectrochemistry at open circuit and during potentiostatic control in chloride (NaCl) and sulfate (Na₂SO₄) electrolytes. A dissolution stoichiometry of approximately n = 2 is observed under all the conditions of these experiments with no evidence for the commonly invoked Mg⁺ intermediate. Nevertheless, in chloride electrolyte it is shown that anodic polarization results in a cathodic activation of the surface with increased hydrogen production during the polarization and an increased corrosion rate following the pulse. The formation of insoluble Mg oxides/hydroxides was also investigated in the sulfate electrolyte.