A.R. Pierna

Electrooxidation of phenol on amorphous Ni–40Nb–(1−x)Pt–xSn alloys

Abstract The electrochemical oxidation of phenol at room temperature in aqueous solutions on amorphous Ni–40Nb–(1−x)Pt–xSn anodes has been studied. After activation in hydrofluoric acid, the alloys show a high activity for the anodic oxidation of phenol, the activity being affected by the presence of tin in the alloy and by the activation time in HF. The HF treatment leads to a platinum enrichment of the alloy surface and to an increase in the roughness of the electrode. Best results were obtained at low current densities and after long activation times in HF.

Electrochemical treatment of toxic compounds on the surface of amorphous Ni–Nb–Pt–Sn alloys

Abstract Acid aqueous solutions of phenol, hydroquinone, and p -benzoquinone have been treated by electrochemical methods to obtain the oxidation of these toxic compounds to CO 2 and water. The electrochemical reaction was occurred at the surface of different Ni–Nb–Pt–Sn alloys. The properties of the electrodes shows that the electrochemical oxidation occurs at Pt in the amorphous state and tin oxide layers that are supported in a nickel–niobium matrix. The results show that the optimum composition is Ni–40Nb–0.6Pt–0.4Sn (at.%) and 36% phenol, 29% hydroquinone and 23% of benzoquinone are reduced to CO 2 and H 2 O after 3 h of electrochemical oxidation in conditions of 100 A m −2 . The synergistic effect of Sn and Pt as metallic particles is demonstrated by these results.

Hydrogen evolution reaction with Finemet–Co amorphous alloys in acid solution

Abstract The aim of this work was to study the effect of cobalt addition on the rate of hydrogen effusion reaction, in Fe–Co based amorphous alloys. Different techniques were used. Polarization measurements show that the addition of cobalt to Finemet alloys results in an increase of the catalytic activity of the base material. On the other hand, comparative measurements have been carried out by electrochemical impedance spectroscopy (EIS), on different amounts of cobalt in electrodes. Despite the instability of the system, Volmer–Heyrovsky mechanism is proposed to explain the process.

Influence of corrosion on surface magnetic density in amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloys

The variation of surface magnetization with corrosion in Fe73.5Cu1Nb3Si13.5B9 in the amorphous and nanocrystalline states is investigated. The results show that in the amorphous samples when the oxidized layer increases the surface saturation magnetization decreases. However, in the nanocrystalline samples the surface saturation magnetization does not change significantly with the corrosion process but the approximation to saturation is different in the non-oxidized and oxidized nanocrystalline samples. In the case of non-oxidized samples, the surface saturation magnetization is reached at a low applied magnetic field (1000 Am−1), whereas in the oxidized samples it is reached at a higher applied magnetic field of 7000 and 9000 Am−1 for the samples annealed at 520°C for 60 min and 30 min, respectively.

Analysis of the nanocrystallization of Finemet type alloy by temperature-modulated differential scanning calorimetry

Abstract In this paper, the changes in the crystallization of Fe 73.5 Si 13.5 B 9 Cu 1 Nb 3 Finemet type alloy, has been studied by temperature-modulated differential scanning calorimetry (MDSC) method. The main advantage of this method is the separation of reversing heat flow from non-reversing heat flow. It is demonstrated that the analysis of the reversible processes taking place in the course of the amorphous to crystalline transformation of the alloy should be used to separate the glass transition of the alloy and the Curie temperature of the Fe(Si) phase with nanometer-size crystallites, hidden under the exothermic nanocrystallization peak of the heat flow curve. Therefore, this technique is an addition to the DSC method that offers additional information in the study of thermal properties of materials.

Compositional dependence of the stress plus field induced anisotropy in CoNiSiB and CoFeNiSiB amorphous alloy ribbons

Abstract Compositional dependence of the stress plus field induced magnetic anisotropy and the compositional dependence of the index, n, of proportionality between the induced magnetic anisotropy and the saturation magnetization in (Co1−xNix)75Si15B10 (x = 0, 0.08, 0.15 and 0.22) and [Co1−x(Fe0.5Ni0.5)x]75Si15 B15 (x = 0.20, 0.50 and 0.80) have been measured. To explain the microscopic origin of this anisotropy several contributions have been proposed, namely: (1) atomic pair ordering and, (2) tetrahedral holes surrounded by 3Co1Fe and 1Co3Fe, while the contributions of tetrahedral holes such as 3Co1Ni and 1Co3Ni seem to be negligible. The compositional variation of the index, n, for CoNiSiB system is similar to that obtained for the induced anisotropy and could be satisfactorily interpreted if it is assumed the non-magnetic character of Ni atoms which dilutes the interactions of Co atoms as the Ni content increases. Moreover, the variation of index, n, for CoFeNiSiB system is found to be similar in overall compositional range and it shows different behavior to that of the induced anisotropy. The influence of pre-annealing on this anisotropy when the magnetic field is applied either longitudinally or transversely to the ribbon axis for the composition (Co0.50Fe0.25Ni0.25)75Si15B10 has been also measured.

Structural, magnetic, and magnetostriction behaviors during the nanocrystallization of the amorphous Ni5Fe68.5Si13.5B9Nb3Cu1 alloy

The changes in the magnetic properties (coercive field and saturation magnetostriction) of Ni5Fe68.5Cu1Nb3Si13.5B9 amorphous alloy after thermal treatment are reported. In order to develop a nanocrystalline structure, the thermal annealing was carried out by Joule heating currents of 1.7 and 1.8 A during different annealing times. Additionally, a microstructural characterization of the treated samples was performed by x-ray diffraction, determining the partial volume of the two, crystalline grains and residual amorphous matrix, existing phases. They seem to be correlated to the softening of the nanocrystallized samples in comparison to the as-cast one.

A comparative study, by EIS and Tafel experiments, of the electrochemical behavior of FINEMET alloys modified with chromium

The electrochemical corrosion behavior of Fe (73.5 x)Cu1Nb3Si13.5B9CrX(0 5) FINEMET type alloys have been investigated in 0.5 N KOH solutions using potentiodynamic polarization and AC impedance spectroscopic techniques. Kinetic parameters were obtained after holding the samples at open-circuit potential for 20 min. The open-circuit potential of all specimens decreases with inmersion time, and a shift of stationary potential in the anodic direction was observed with the Cr addition. The present study attempts to correlate impedance data with polarization data and the results indicate that both values are comparable for passivated alloys. In the case of FINEMET type alloys, the addition of chromium results in the formation of a passive film which prevents the corrosion of the alloy and which can be represented by a single time constant equivalent circuit.

Electrochemical behaviour of Fe40Ni38Mo4B18 amorphous alloys in aqueous solution at high pH. Influence of the structural relaxation

Abstract Electrochemical properties of Fe 40 Ni 38 Mo 4 B 18 metallic glass in aqueous KOH solution, both as-quenched and relaxated, have been studied. The relaxation was induced by thermal treatment at 100°C, 200°C, 300°C, and 350°C for 30, 60 and 120 min. The results obtained show that the electrochemical properties of the samples relaxed at 200°C, 300°C and 350°C can be connected with short range ordering and with migration diffusion phenomenon of metalloid atoms. The structural changes of the samples produced at high temperatures, differ from those observed by techniques of bulk measurements and, therefore, mechanisms related to the surface properties may be inferred.

Influence of glass diaphragms and anionic membranes in the electrochemical oxidation of aromatic aqueous solutions

The present work studies the electrochemical oxidation of solutions of hydroquinone (Hy) and p-benzoquinone (Bz). Both compounds are found in the electrochemical oxidation of phenol and they can be considered as a redox couple, so that the main objective is to study the influence of different separators in the global electrochemical reaction of these compounds. Different glass separators as diaphragms, and different ionic exchange membranes (Nafion 350 and 450) have been used as well as one undivided cell, but the results obtained with this last were not very satisfactory. Working with the two different diaphragms, the best behaviour is shown by the glass separator, which has the greatest hydraulic radius. Testing the ionic exchange membranes using Nafion 350, a bigger amount of organic matter is eliminated. The voltage loss due to the separators in the best of the cases represented one 11% of the total potential applied to the electrochemical reactor and the amount of organic matter eliminated is one 25%.