Estudo de Eletrodeposição de Ligas Cu-Sn em Meio de Citrato de Sódio

Abstract

Coatings produced from metallic Cu-Sn alloys show great interest due to their properties, creating a large capacity of industrial application, especially in electronics components and materials industries. Such alloys have also been commercially used as coatings on different metals, such as steel, to protect them against corrosion and improve their appearance. In the present work, cyclic voltammetry, chronoamperometry and anodic linear sweep voltammetry were used to study the processes of deposition of Cu and / or Sn from two sets of solutions containing CuCl2.2H2O and SnCl2.2H2O in the ratios of Cu:Sn = 1:10 to 10:1, and Na3C6H5O7 1,00 mol / L at pH = 6,0. The cyclic voltammetric curves carried out on graphite substrate were used to calculate the equilibrium constant of the complexes of both metals with sodium citrate, as well as in the determination of the cathodic potentials applied in the chronoamperometry test. After the deposition of the alloys at the potential established for each solution, the Cu-Sn coatings were dissolved in a 0,5 mol/L NaNO3 solution using linear anodic scanning, for the evaluation of their chemical compositions. Initial results showed that the changes in Cu (II) concentration and in the applied potential influenced the copper content in the alloy. However, it was noticed that the tin content has not undergone major variations, independent of concentrations of the solutions and the applied potential. In the solution with the highest copper concentration it was achieved metals contents in the alloy very close to the commercial bronze alloy. Based on these results, coatings were produced on carbon steel substrate from the bath containing CuCl2.2H2O SnCl2.2H2O in the 10:1 ratio, by employing the potentiostatic chronoamperometry technique. Four different potential values (-0,39 V, -0,67 V, -1,00 V, -1,20 V and -1,94 V vs. Ag / AgClsat.) were applied, obtaining layers which ranged from pale pink colorations to dark yellow. The cathodic current efficiency (Ef) decreased as the applied potential became more negative. In the same direction, there is an increase of tin content in the alloy deposited and smaller grain sizes. However, when the potential became more negative (-1,20 V and -1,94 V) it was observed the presence of crystallites of different sizes and dendrites. Coatings with improved corrosion resistance in a 0,5 mol/L NaCl solution were produced in the potential of -1,00 V and -1,20 V, which compositions were 99,40% m/m Cu / 0,60% m/m Sn and 97,70% m/m Cu / 2,30% m/m Sn, respectively. The XRD analysis permitted to verify that these coatings consisted mainly of the α- CuSn phase