Estudos sobre banhos para eletrodeposição de ligas de Ni-Mo-B.

Abstract

Electroplating is among the most widely spread chemical based technologies, applied in a wide range of scales in thousands of facilities. Surprisingly, many aspects of the field have not significantly changed over the years, and many current plating processes and facilities still closely resemble their old origins. Recent requirements, for new processes and tighter specifications from the electronic and other high technology industries, and due to severe restrictions imposed by environmental considerations, have considerably stimulated the field of electrodeposition. At present hundreds of binary and ternary alloys have been investigated to a greater or less extent to help solve the problem of alloy production by electrolysis. At the same time, only a few of these alloys have been used industrially. This is because the electrodeposition of alloys is a much more complex process than the electrodeposition of pure metais and often needs continuous control and regulation. An adequate control of such a complex process requires a correct understanding of its mechanism. Therefore, it is necessary to realize a comprehensive study which can assist in predicting the conditions for depositing an alloy of a required composition and characteristics. The electrolytic baths contain complex and polyanionic species. To know exactly the composition and characteristics of the deposit, it is necessary that the actual form of the species in the bath is known. For this reason, the work with electrometric techniques such as potentiometry, conductometry and the Job's method of continuous variation has been realized, which reveals the stoichiometric composition of the species. This study showed that in the basic pH the molibdate is present as a simple ion. With the successive diminuition of pH, in the ranges 5.4-5.6 and 4.0-4.2, in the ratios 7Mo:8H+ and 2Mo:3H+ , exist hepta- M07O246 " and octo-Mo0264 " ions, respectively. The next part of the work has been devoted to development and optimization of the baths for electrodeposition of the Ni-Mo-B alloys, explaining the effects of the different variables involved in the deposition, such as the quantity of the constituents of the bath, complexing composition of the deposits. The alloy of nickel-molybdenum with small quanties of boron was electrodeposited on a copper substrate from the aquous solutions containing 0.175M nickel sulfate, 0.0175M sodium molybdate, 0.4M sodium citrate, 0.0728M boron phosphate, 0.13M ammonium sulfate, 0.35 g/l saccharin and 0.035 g/l sodium dodecyl sulfate. The pH of the bath was controlled by ammonium hidroxide. The adequate values of the operational parameters were found to be: mechanical rotation 15 rpm, temperature 75°C and current density 100 mA/cm2 . The next step, after the study of the operational and composition of the bath variables, was to study the properties of the deposit of the bath developed, gave thick, sound and hard deposits with an average mass composition 79%Ni-20%Mo-1 %B.