Portugaliae
Electrochimica Acta

The effect of some thiourea derivatives on the corrosion inhibition of copper has been investigated in 2 M HNO3 solution. Weight-loss and polarization measurements were applied to analyse the metal corrosion behaviour in the absence and presence of the inhibitor. The results show that all the studied thiourea derivatives inhibit the corrosion of copper in the solution and that the inhibiting efficiency increases with a decrease in temperature or an increase in concentration of the thiourea derivatives. The obtained results show that the adsorption of the additives used follow the Temkin adsorption isotherm. Moreover, the thermodynamic activation parameters of the copper corrosion reaction were calculated and discussed.

Studies on the initial stages of the atmospheric corrosion of copper exposed to the urban atmosphere of the city of Lisboa, at a site called Campo Grande, from November 2000 till February 2001 (winter period), are presented in this paper. Average corrosion rates of 0.139 and 0.105 g/(m2 day) were obtained of copper samples after one and three months of exposure, respectively. From SEM/EDS data it was concluded that during the first month of exposure corrosion products were mainly composed by copper oxides and minor amounts of copper chlorides, while exposures of two and three months lead to the formation of copper sulphates apart from copper oxides and copper chlorides. A uniform and adherent film was visible on the surface after three months of exposure. During the period under evaluation concentrations of NO2 were between 32 and 36 ppb and deposition rates of SO2 between 17 and 20 mg/(m2 day), without significant changes during the three month period. The degree of humidity and the average precipitation were both quite high (winter season). The SO2 present in the atmosphere with a high degree of humidity explains the formation of copper sulphates.

An ideal low temperature synthetic route to prepare vanadates such as LiMnVO4 and LiNiVO4 with high purity, better morphology and good electrochemical behaviour was attempted via, Self Propagating High Temperature (SPHT) method using an internal fuel. Conventional solid-state fusion method and acetate precursor method were also carried out at high temperatures such as 1073 K and 873 K respectively and the results were compared with that of SPHT method. Samples obtained from these three methods were examined for their FTIR spectra, XRD pattern, surface area, particle size analysis and SEM. Both the compounds viz. LiMnVO4 with an orthorhombic spinel-related structure and LiNiVO4 with an inverse spinel structure were found to be synthesized stoichiometrically at a lower temperature (573 K) through SPHT method. Charge-discharge studies carried out at a constant current drain (0.1 mA) showed that these vanadates were electrochemically active. To understand the role of fuels in SPHT method, three fuels namely Urea (U) (FCV=6) and two other fuels with a high fuel calorific value viz. Glycine (FCV=9) and Pthalyl DiHydrazide (PDH) (FCV=37) were employed in the synthesis. Among the three fuels better products were obtained for Glycine and Pthalyl DiHydrazide (PDH) rather than Urea in terms of surface homogeneity and better morphology. Advantages of the SPHT method of synthesis and the role of fuels were discussed in detail.

The acidic dissociation constants of the protonated form of 2-amino-(2-hydroxymethyl)-propane-1,3-diol (tris) and 2-amino-2-methylpropane-1,3-diol (bis) in a series of water/1,2 dimethoxy ethane mixtures at 5 ºC temperature intervals from 25 to 45 ºC are determined using a cell without liquid junction potential. The corresponding thermodynamic parameters (ΔGº, ΔHº, and ΔSº) of the protonated form of these buffer indicators in the various solvent mixtures are evaluated. The variation of the standard free energy change of transfer with solvent compositions for both indicators indicates a characteristic minimum which is attributed to selective solvation of the individual species participating in the equilibrium process. Solubility measurements of these indicators at different temperatures are determined in order to evaluate the thermodynamic functions of the transfer of the free bases which may provide a better insight on the nature of the ion-solvent interactions and the relative basicity of the solvent.

This investigation deals with the stress corrosion cracking of AISI 304 stainless steel in concentrated boiling H3BO3 solution with NaCl additions. The characterization of the passive film was carried out using samples submitted to the corrosive environment, in both the annealed and cold worked states. The study shows that plastic deformation of the metallic material reduces polarization resistance and film stability (in terms of cathodic reduction). Furthermore, from the capacitance measurements, using the Mott-Schottky approach, it can be concluded that the influence of plastic deformation is higher when the passive film reveals p-type conductivity. Based on the electronic band structure of n-type and p-type semi-conductors, a representation for a possible correlation of charges of opposite sign between space charges and near surface dislocation is given.