Portugaliae
Electrochimica Acta

The inhibition behavior of chromate ions towards corrosion and dezincification of α-brass (Cu/Zn: 64/36) in acetate buffer solutions (pH 2.4, 7.0 and 8.5) containing 3% sodium chloride was studied. Weight loss and solution analysis were used for a long period (four weeks) and electrochemical polarization for a short period (one hour). Chromate accelerated corrosion and dezincification at pH 2.4 by acting as depolarizer but chromate suppressed efficiently the corrosion and dezincification at pH 8.5. At pH 7.0 the corrosion and dezincification suppressed in the presence of chromate but the inhibition action started to deteriorate at chromate concentration less than 0.005 mol/L. The inhibition effect is attributed to adsorption of chromate ions on the corroding surface and improvement of the passivity. Chromate led to an increase in the concentration of copper species in solution due to the formation of the more soluble CuCrO4 than Cu2O, which contributed to the apparent suppression of dezincification.

The electrochemical behavior of Cu-(0.5-13 wt.%)Al alloys in K2SO4 0.5 mol dm-3 (m = 1.5 M, pH 2-5) was studied using open-circuit potential (Eoc) measurements and polarization curves, in order to know the conditions of copper oxides formation, influence of Al concentration and solution pH on the electrode processes. The open-circuit potential measurements with time indicated that the Eoc decreases with the Al content, due to the lower Al potential compared to Cu. The electrode potentials decrease ca. 20 mV per unity of solution pH as increase pH of the solution. The polarization curves showed changes with the increase of Al concentration. For alloys with up to 3 wt.%Al the obtained Tafel anodic coefficient was 40 mV/decade for low potentials region (E < -320 mV) and 130 mV/decade for the other region (-320 < E/mV < -250), indicating that up to this concentration there is no considerable change in the electrochemical behavior of the alloys, when compared to pure Cu. For alloys with an Al content from 8 to 13 wt.% the Tafel anodic coefficient was 60 mV/decade. No influence of the solution pH on the Tafel anodic coefficient was observed.

The electrochemical behaviour of coated Cr3C2-NiCr steel in aerated 0.5 M H2SO4 solution was studied by means of electrochemical ac and dc measurements. The structural characterisation of the coated steel, before and after electrochemical tests, was also performed in order to identify the mechanism of the electrolyte penetration through the coating up to the steel substrate, causing its corrosion. This characterisation may also help to explain electrochemical results. Three types of Cr3C2-NiCr coatings performed by a High Velocity Oxy-Fuel Spraying system (HVOF) were analysed. The facility for the electrolyte penetration through the coating and the corresponding electrochemical behaviour of the samples were strongly influenced by the spray parameters used and heat treatment applied. It was observed that heat-treated coatings (Q1 and Q3) showed better corrosion resistance than the as-sprayed coating (A). For coatings Q1 and Q3, the electrolyte did not reach the steel substrate during the measurement, leading to a better protection of the steel substrate against corrosion.

Cyclic voltammetry and surface analysis techniques (EDX microanalysis, mapping of elements and SEM images) have been used to study the solid state reactions of Ir in blank solution or Hg(I)-containing solution. The studies in KNO3/HNO3 solution permit to suggest the formation of a thin oxide layer film deposited in the substrate. The RCV process exhibits the decrease of capacitive current associated to the substrate surface restructuring. The electro deposition of Hg occurs initially in UPD condition from Hg(I)-containing solution, C[Hg(I)] = 3.51x10-7 mol/L. The electro deposition of bulk mercury occurs from Hg(I)-containing solution, C[Hg(I)] = 3.51x10-5 mol/L. It was observed the deposition of Hg in the substrate in open circuit for all employed conditions.

A new amperometric biosensor for lactose determinaion in raw milk was developed through the simultaneous immobilization of β-galactosidase and galactose oxidase on a derivatised polyethersulphone membrane. β-galactosidase catalyses the hydrolysis of lactose into galactose and glucose and galactose oxidase catalyses the oxidation of galactose into galactonic acid and H2O2. The membranes with the two immobilized enzymes were then used in an amperometric sensor, by oxidation of the H2O2 formed, at a Pt electrode of an Universal Sensors electrode base system. The sensitivity and the reproducibility of the biosensor thus formed were found to be 6.81 and 0.72 nA.M-1, respectively. Biosensors were found to be stable for 20 days.

The zinc electrochemical behaviour from sulphate and sulphate-gluconate baths was studied both on a vitreous carbon and a copper electrode. The voltammetric response depends on the complexing agent concentration, the electrode material, the pH and the cathodic potential limit. The reduction of zinc ions proceeds with hydrogen evolution, and at less negative potentials on the copper electrode than on the vitreous carbon electrode, specially in absence of gluconate. Gluconate decreases the efficiency of the process. On the other hand, gluconate favours zinc electrodeposition on vitreous carbon but retards it on copper.

The electrochemical intramolecular cyclisation of allyl 2-bromophenyl ethers in N,N'-dimethylformamide at constant current in a diaphragmless cell has been developed using Ni(II) and Co(II) complexes as electron-transfer mediators. Cyclic compounds are obtained in good yields under appropriate experimental conditions.