Desenvolvimento e otimização de um banho para eletrodeposição de ligas CO-MO resistentes à corrosão.

Abstract

The corrosion process is constantly transforming the metallic materials in such a way that their durability and performance stop satisfying the purposes to which they are destined. A way to minimize the corrosion of metallic materials is to cover them with other materials. The electrodeposition is a method often applied to obtain metallic coverings resistant to corrosion and mechanical stress. Through electrodeposition it is possible to obtain metal alloys, which are usually prepared with the purpose of enhancing the properties of its initial constituents. Inside this class, the formation process of alloys through molybdenum (Mo) is considered a process of induced electrodeposition. An example of these alloys is the one of cobalt-molybdenum (Co-Mo) and its applicability may vary, depending on the composition of cobalt and molybdenum used to obtain the alloy. Thus the main purpose of the proposed study is to optimize the components of the bath for electrodeposition of alloys Co-Mo as a function of the corrosion resistance and deposition efficiency. The electrochemical bath used for the electrodeposition of alloy of Co-Mo was composed of the following reagents: cobalt sulphate, sodium molybdate, sodium citrate, cobalt sulphate, ammonium sulphate and sodium 1-dodecyl sulphate. The pH values were tuned adding ammonium hydroxide and sulphuric acid. The Co-Mo alloys were electrodeposited over a copper substrate. The anode was a cylindrical mesh of platinum. To optimize the electrodeposition process of the alloys, a full-factorial experimental design was performed. The influence of the input variables over cathodic efficiency and corrosion resistance of the system, and its possible interactions as well were quantitively assessed through the minimum amount of experiments. The optimum values found for the corrosion resistance of Co-Mo alloys were obtained by increasing the concentrations of cobalt sulphate and sodium molybdate. In such conditions the best value of polarization resistance (Rp) of 4728 Ohm was achieved, with a corrosion potencial (Ecorr) of -0.711 V and a deposition efficiency (Eff) of approximately 44%. The optimized values for the deposits as a function of the deposition efficiency were obtained by the greater concentration of cobalt sulphate and the smaller concentration of sodium molybdate, achieving a deposition efficiency of approximately 55 %, though the Rp value was smaller. The deposits have shown the presence of nodules and microcracks at its surface. The Co-Mo alloy does not have good corrosion resistance when compared to chromium. The impedance results confirmed that alloy obtained by the greater concentrations of cobalt sulphate and sodium molybdate was the most resistance of corrosion.