V.D. Jović

Characterization of electrochemically formed thin layers of binary alloys by linear sweep voltammetry

It has been demonstrated that linear sweep voltammetry can be used as anin situ technique for characterization of electrodeposited thin layers of binary alloys. The anodic dissolution characteristics of linear sweep voltammograms are very sensitive to the type of electrodeposited alloy. If both metals do not passivate in the investigated solution, eutectic type alloys are characterized by two sharp dissolution peaks, indicating no miscibility between components in the solid phase. From the ideal solid solutions, components of the alloy dissolve simultnaeously, while in the case of intermediate phases and intermetallic compounds each phase, or compound, has its own peak of dissolution.

EIS and differential capacitance measurements onto single crystal faces in different solutions: Part I: Ag(111) in 0.01 M NaCl

Abstract Differential capacitance versus frequency curves simulated for different equivalent circuits representing different cases of anion adsorption have been analyzed. The analysis showed that in all cases with an ideal, homogeneous electrode surface, the real value of the differential capacitance, representing the sum of the double layer and adsorption capacitances (independent of frequency) could be obtained only at frequencies lower than about 2 Hz. In the case of real surfaces, presented by introduction of a constant phase element instead of a double layer capacitance, differential capacitance versus frequency curves were found to be more complex, with no possibility to obtain values of the double layer capacitance at high and adsorption capacitance at low frequencies, since the differential capacitance was found to depend on frequency in the whole range of frequencies (10 −4 –10 5 Hz). Chloride adsorption onto the Ag(111) face in 0.01 M NaCl solution was found to follow the approach initiated in this work by introducing a constant phase element instead of the double layer capacitance. It is shown that impedance diagrams deviate from the ‘ideal double layer behavior’ and that differential capacitance versus potential curves depend on frequency. It is confirmed by the analysis of differential capacitance versus frequency curves that this is the consequence of adsorption of chloride anions presented by adsorption resistance and capacitance connected in series, as well as of inhomogeneity of the real single crystal face.

Kinetics of chloride ion adsorption and the mechanism of AgCl layer formation on the (111), (100) and (110) faces of silver

Abstract The kinetics of chloride anion adsorption on the (111), (100) and (110) faces of silver are investigated by linear sweep voltammetry, potentiostatic pulse and impedance (admittance) measurements. It is shown that the first voltammetric (capacitive) peak corresponds to the adsorption of randomly arranged chloride anions with complete charge transfer between the anions and the substrate ( l ≈ 1) and a relatively slow rate of adsorption: k s = (8−9) × 10 −6 cm s −1 . The second (third) voltammetric (capacitive) peak is found to be very sharp, representing (according to the earlier findings) the formation of an ordered AgCl adlayer formed with the topmost Ag layer. At more positive potentials a three-dimensional (3D) growth of AgCl takes place, characterized by the anodic “crystallization loop” and the j − t transients typical of 3D nucleation. It is shown that after prolonged polarization in the potential region of 3D AgCl formation, irreversible formation of AgCl takes place on the electrode surface.

Corrosion Behavior and Passive Film Characteristics Formed on Ti, Ti3SiC2 , and Ti4AlN3 in H 2 SO 4 and HCl

We report on the chemical and electrochemical corrosion of Ti, Ti 3 SiC 2 , and Ti 4 AlN 3 in HCl and H 2 SO 4 solutions. The chemical corrosion of Ti 3 SiC 2 under conditions similar to those used industrially for the electrolysis of HCl (15% HCl, 65°C) is significantly better than that of pure Ti, indicating that it could be a promising substrate for dimensionally stable anodes. The enhanced resistance is attributed to the formation of a thin, passivating SiO 2 -based layer. The same layer protects Ti i SiC 2 during its anodic oxidation. After a 4 day period, during which the rate of growth of the SiO 2 -based layer is initially high, but then slows to a steady rate of about 130 μm/yr cm 2 is established, which translates to a Ti dissolution rate of ∼9 × 10 -4 mol/yr cm 2 . Analysis of Mott-Schottky plots and electrochemical impedance spectra show that the passive films that form onto Ti and Ti 3 SiC 2 in 1 M HCl and 1 M H 2 SO 4 at room temperature are n-type semiconductors dominated by space-charge capacitances. The films forming on Ti 3 SiC 2 have significantly lower space-charge-layer thicknesses and higher donor densities than those that form on Ti. Preliminary results for Ti 4 AlN 3 indicate that the passive film formed is a p-type semiconductor.

The corrosion behavior of Ti3SiC2 in common acids and dilute NaOH

Hot isostatically processed bulk, fine (3–5 μm) grained samples of Ti3SiC2 were immersed in concentrated and dilute hydrochloric, HCl, sulphuric, H2SO4, nitric, HNO3, dilute hydrofluoric, HF, acids and sodium hydroxide, NaOH, solutions at room temperature. Based on six-months weight changes the dissolution rates of Ti3SiC2 in concentrated and dilute HCl, H2SO4 and dilute NaOH were found to be negligible (<2 μm/yr). In dilute HF and concentrated HNO3 the corrosion rates were, respectively, ≈5 μm/yr and 13 μm/yr respectively. In contrast to Ti metal, the weight losses of Ti3SiC2 in dilute HNO3 were higher (≈250–320 μm/yr) and depended on concentration. Post-immersion scanning electron microscopic micrographs of samples immersed in HNO3 indicated that an oxygen rich Si-based layer forms on the surface of the samples. This implies that the Ti atoms are leached out into the HNO3 solution, leaving behind a Si-rich layer that is ultimately oxidized. Cyclic polarization and potentiostatic i–t transients in dilute HCl and H2SO4 acids, strongly suggest that a thin irreversible electrically insulating layer forms on the surface of Ti3SiC2. Exposing a sample to a constant current density of 0.6 mA/cm2 for two days resulted in the formation of a 5 μm thick SiO2-based layer on the surface. The presence of such a layer would explain the excellent corrosion resistance of Ti3SiC2 in these acids.

The influence of adsorbing substances on the lead UPD onto (111) oriented silver single crystal surface. I

Abstract Effect of anions present in solution on the UPD of lead onto (111) oriented silver surface has been investigated. Linear sweep voltammetry was used. It was established that organic anions, such as acetate and citrate, shift the UPD phenomena towards the reversible potential of lead. Adsorption isotherms of lead indicate first order phase transition. Addition of molecular thiourea in solution, which is known to be strongly adsorbed, justified the assumption that the observed effects are due to the adsorbing properties of the anions.

Anion absorption on the (111) face of silver

Abstract Differential capacity (admittance-impedance) measurements for a (111) face of silver in supra-pure solutions of NaF, Na 2 SO 4 and NaCH 3 COO are analysed to obtain data on the specific adsorption of the above-mentioned anions. It is shown that the frequency dependence of the impedance of the electrochemical interface cannot be represented by an equivalent circuit consisting of the ohmic resistance ( R OHρ ) and double-layer capacitance ( C dl ) connected in series in the presence of specific adsorption. The addition of a series combination of a resistance ( R ct ) and capacitance ( C ad ) in parallel with C dl gives a reasonable representation of the data. R ct permits the evaluation of some semiquantitative information about the adsorption kinetics.

Electrochemical Formation of Laminar Deposits of Controlled Structure and Composition II . Dual Current Pulse Galvanostatic Technique

A method of obtaining laminar metals by electrochemical deposition is described. It is based on a pulsating current regime, which provides total concentration polarization with respect to metal 1, as a prerequisite for simultaneous deposition of both metals. Equations are derived for thickness of both layers, as well as for the composition of the layer containing both metals. The regime was applied to two systems: copper‐lead and copper‐nickel, representing two extremes of total immiscibility and total miscibility in the solid phase, respectively. In the first system, the replacement reaction takes place, which leads to a virtual dissolution of the less noble metal during the off period. The kinetics of the replacement reaction are considered.

Identification of intermetallic compounds in electrodeposited copper-cadmium alloys by electrochemical techniques

Abstract Anodic sweep voltammetry was used to identify different intermetallic compounds and assess their amount in copper-cadmium alloys. Four compounds are found in thin (1–8 μm) electroplated alloy films: CuCd 3 , Cu 5 Cd 8 , Cu 4 Cd 3 and Cu 2 Cd. Their presence is reflected in four peaks recorded in the voltammograms, the fifth peak corresponding to dissolution of copper remaining after dissolution of cadmium. For quantitative assessment potential-step technique was also used, avoiding the problem of overlap of peaks. Indications were obtained that the first peak is a mixture of pure cadmium and the cadmium rich compound. Since changes in voltammograms with time of resting of the alloy in an inert atmosphere were found, they were ascribed to solid state reactions of formation of compounds richer in copper from the one richer in cadmium (or pure cadmium) and pure copper. Thermodynamics of alloy formation was considered and approximate values of the standard potentials and standard free energies of the compounds are evaluated.

EIS and differential capacitance measurements onto single crystal faces in different solutions. Part II: Cu(111) and Cu(100) in 0.1 M NaOH

Abstract The electrochemical behavior of Cu(111) and Cu(100) in 0.1 M NaOH solution in the underpotential region of Cu2O formation has been investigated by cyclic voltammetry and EIS (differential capacitance) measurements. By the analysis of differential capacitance versus potential and differential capacitance versus frequency curves it is shown that the process of OH− species adsorption/desorption, takes place on both faces of the copper substrate. It is also shown that, on Cu(111), slow reconstruction of the original surface occurs during the process of OH− species adsorption/desorption, while on Cu(100), this process was found to be very fast involving a much greater amount of adsorbed OH− species.