Avaliação do efeito das variáveis densidade de corrente, temperatura e pH do banho eletrolítico nas propriedades da liga Ni-W-Co obtida por eletrodeposição.

Abstract

Metallic coatings are extensively used for diverse applications. Pure metals or alloys formed by different elements with suitable characteristics for a specific purpose may be used. Corrosion is a spontaneous process where metals tend to react with the environment in which they are in, causing irreversible damages to the material. Electrodeposition is a technique widely used to obtain coatings applied against corrosion. In this paper, a complete factorial design (CFD) associated to surface response methodology (SRM) was performed to evaluate the effect of the following variables: current density, temperature and electrolyte bath pH in the Ni-W-Co electrodeposition process. Chemical composition, current efficiency, microhardness, microstructure, surface morphology and corrosion resistance were investigated. Chemical composition results showed that pH was the most influential variable. Nickel deposition was easier in alkaline conditions. Highest cobalt percentages were obtained in acid conditions. On the other hand, tungsten fractions did not largely change for the investigated conditions; its minimum value was 25.83 wt%. The highest current efficiency, 71.02%, was achieved in the following conditions: 40 mA/cm2, 55 oC, and pH 5. The highest value for microhardness was 754.8 HV, which was achieved in the optimized experiment in the following conditions: 70 mA/cm2, 42.5 oC, and pH 3.98; this experiment showed the highest Co content (63.94 wt%). All coatings presented different morphologies, which were, roughened, homogeneous, and with micro-nodules. Micro-crevices were also observed, specially in the coatings with the highest cobalt content. Alloys with different contents of tungsten showed amorphous characteristic. Corrosion tests showed that the Co54.57W38.85Ni6.58 (100 mA/cm², 55 °C and pH 5) alloy was considered the worst, because it exhibited highest corrosion current density (1.249x10-4 A/cm²) and the lowest polarization resistance (402.6 Ω.cm²). In the contrary, the Ni50,13W41,12Co8,75 (100 mA/cm², 55 °C and pH 8) was considered the best, because it showed the lowest corrosion current density (5.817x10-6 A/cm²) and the highest polarization resistance (5251 Ω.cm²).