Surface Technology最新文献

The effects of various operating and bath variables on the electrodeposition of Ni-P alloy coatings were studied. The amount of phosphorus in the coating and the cathode current efficiency were functions of the H₃PO₃, H₃PO₄ and NiCO₃·2Ni(OH)₂·4H₂O contents, the temperature and the current density. The maximum amount of phosphorus in the alloy coating was approximately 35 wt.%.

A comprehensive survey of the existing literature on the spongy electrogrowth of zinc is given and discussed. Some conclusions relating to the appearance of mossy zinc and its features are also drawn. It is shown that spongy zinc deposits represent natural crystallization at lower current densities and consist of small microcrystals of the same lattice characteristics as those of the compact electrogrowth modes. Some impurities and additives enhance whereas others rather retard or even suppress the mossy growth of zinc, creating various induction periods for spongy electrogrowth (the synergetic amplification and counteraction effect). It is shown that spongy deposits grow whenever the exchange current approaches or exceeds the limiting current and hence the appearance of mossy zinc is a feature of chloride rather than of sulphate or alkaline media. The spongy growth appears to be mass transport controlled. The situation becomes even more complex because the mossy deposit frequently accompanies black zinc oxide as excess metallic zinc, while the crystalline forms at and close to the limiting current appear to be of the same nature even under microscopy analysis.

Pure iron was oxidized in air at room temperature for 3–14 days and was then immersed in a deaerated 0.1 mol l^-1 phosphate solution at pH 7.0. The corrosion potential E_corr and the rate of dissolution of the oxide were measured as functions of the immersion time. It is proposed that the oxide film formed in air, which consists of an outer γ-FeOOH layer and an inner Fe₃O₄ layer, dissolves according to the following cell reactions: an outer cathodic reaction γ-FeOOH+H₂PO₄^-+3H⁺+e→FeH₂PO₄⁺+2H₂O with E°=1.049 V, an inner cathodic reaction Fe₃O₄+3H₂PO₄^-+8H⁺+2e→3FeH₂PO₄⁺+4H₂O with E°=1.177 V, and the anodic reactions Fe+2H₂O→Fe(OH)₂+2H⁺+2e with E°=-0.104 V and/or 3Fe+4H₂O→Fe₃O₄+8H⁺+8e with E°=-0.085 V in which the cathodic reaction determines E_corr. The cathodic reaction occurs at the oxide-solution interface by accepting electrons transported through the oxide, and the anodic reaction occurs at the metal-oxide interface to form oxide by reacting with OH^- and/or O^2- ions migrating through the oxide layer. Thus the proposed mechanism is rather different from the local action cell model. After the dissolution of the oxide film, the iron dissolves according to the following cell reactions: Fe+H₂PO₄^-→FeH₂PO₄⁺+2e with E°=-0.505 V and 2H⁺+2e→H₂ with E°=O V, where the anodic reaction determines E_corr.

We combined optical and impedance measurements with conventional electrochemical techniques to investigate the photoprocesses that occur on polycrystalline TiO₂ electrodes in the presence of reducible adsorbates in solution. The semiconductor-electrolyte junction is treated as a simple Schottky barrier. The photopotential, photocurrent and capacitance are described using this model. The observed anomalous large cathodic photoeffects on n-type TiO₂ result from either anodic photoproduction or the diffusion of the dissolved oxygen adsorbed on the electrode surface. Anomalous photocurrents occur together with relatively large dark currents, which are caused in n-type semiconductors by electron transfer from the conduction band to the interface via tunnelling through the space charge layer. The influence of the surface states on the determination of the flat-band potential by measurements of either the photocurrent or the impedance was also examined. It was found that reliable results could be obtained by scanning from cathodic to anodic potentials after prepolarization at -1.5 V. The value determined in this way for the flat-band potential was -0.5±0.05 V.

The dissolution of ZnO both in the dark and under illumination at 365 nm and the flat-band potential display a linear pH dependence which can be explained by a thermodynamic model based on the linear relationship between either the rate of the reaction or the current and the electrochemical reaction energy used in a proton attack followed by desorption of Zn²⁺ and OH^- from the charged oxide surface. The experimental data supporting this hypothesis are taken from the literature and from some new investigations of the photodissolution of ZnO which are described in this paper. The photocurrent was studied potentiostatically and its potential dependence was interpreted using Gärtner's theory for illuminated metal-semiconductor contacts. The linear relation between the photocurrent and the pH corresponds to a stationary state under constant illumination in which the current is equal to the hole oxidation rate of H₂O at pH 9.2 and of H₂O plus ZnO at pH less than 9.2. The decomposition of the ZnO lattice is thermodynamically controlled by the hole oxidation of desorbing OH^- ions to OH radicals which are assumed to form H₂O₂ by combination.

The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H⁺ and Mg²⁺ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg²⁺] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature.

(1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg²⁺ of OH^- depending on the value of [Mg²⁺]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H⁺→Mg²⁺ + H₂O

(2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration.

(3) At pH > 7 the rate-controlling step was OH^- adsorption followed by Mg²⁺ and OH^- desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H₂O→Mg²⁺ + 2OH^- The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH)₂ precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol^-1) and the overall reactions are then limited by proton and OH^- diffusion.

The rates of electropolishing of inclined copper cylinders in H₃PO₄ were studied by measuring the limiting current of the process. The variables studied were the angle of inclination of the cylinder and the H₃PO₄ concentration. The limiting current increased as the cylinder was inclined from the vertical position, reached a maximum value at an angle of 45°, and then decreased slightly on further inclination towards the horizontal position. The difference between the maximum limiting current at 45° and the minimum limiting current at 90° (vertical position) ranges from 30% to 47% depending on the H₃PO₄ concentration.

Structurally sensitive in situ methods such as photopolarization and impedance were used to examine the passivation process and the properties of the protective oxide layers on titanium. The kinetics of anodic oxidation and the non-stoichiometry of the surface oxide were correlated. The composition of the anodic film on titanium changes with the relative potential from lower to higher oxidation stages according to TiH₂+TiO→nTi₂O₃·nTiO₂→Ti₅O₉orTi₆O₁₁→Ti₃O₅→TiO₂

The characteristic behaviour of titanium as a member of the valve metal group can easily be seen at higher anodic potentials (approximately +1.5 V (SCE) in 5 mol H₂SO₄ dm^-3) when the electrode is covered with a nearly stoichiometric TiO₂ layer. The semiconducting properties of TiO₂ were investigated using an anodic film stabilized at +2 V(SCE) and it was found that TiO₂, like the lower titanium oxides, is an n-type semiconductor under anodic polarization. The flat-band potential for the stabilized TiO₂ film was -0.2 V, as determined by the two methods in 5 mol H₂SO₄ dm^-3. The donor concentration was 2×10²⁰cm^-3 under the same conditions (a value of 60 was used for the dielectric constant).

The conditions required for obtaining electrolytic Ni-Pb-P alloy layers were investigated. Alkaline citrate-ammonia solutions typical of those used in electroless nickel plating containing less than 10 mM lead(II) were used. The electrolysis of these solutions at temperatures of 25–85 °C and current densities of 25–400 A m^-2 produced smooth Ni-Pb-P films containing 4–50 wt.% Pb and 2–8 wt.% P. The films containing 5–8 wt.% and 2–4 wt.% P were very bright and exhibited good corrosion resistance.

Investigations of the effect of changes in the concentration of the solution components and in other Ni-Pb-P deposition parameters showed that lead was deposited preferentially. Under these conditions the cathodic deposition of lead was diffusion controlled. The lead(II) inhibited the partial deposition of nickel and phosphorus. The efficiency of the current depended strongly on the deposition temperature and varied from 30% to 50% at 25 °C and from 100% to 150% at 75 °C. These results proved that the contribution of electroless nickel and phosphorus deposition increased with increasing temperature. Electroless nickel and phosphorus deposition at the cathode in the presence of such strong inhibitors as lead(II) salts proved that a continuous renewal of the catalyst surface, i.e. metallic nickel, was the most important factor in simultaneous electroless processes.

Perovskite catalysts of the type La₂MnMO₆ (M  Co, Ni, Cu) were synthesized by means of a ceramic technique. Single-phase formation was checked by means of X-ray diffraction using Cu Kα radiation. Electrical conductivity measurements were carried out with a standard two-probe cell. The catalytic activity of the perovskites was tested for the decomposition of 2-propanol in the temperature range 200–300 °C in a fixed-bed flow type of reactor. All the perovskites promote dehydrogenation of the alcohol. A possible mechanism for the reaction is proposed by studying the effect of products on the reaction rate and from the variations in the electrical conductivity of the pelletized catalysts in the reactant and product atmospheres.