GUIMARÃES, T. L. M.; GUIMARÃES, Thalma Líbia de Melo.
Resumo:
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.