Electrodeposition of Ni-Fe-Mo-W Alloys - 15th Quarterly Report
Yujia Zhang and Prof. E.J. Podlaha-Murphy*
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the 15th quarter of project work (July-September 2016). Progress on the previous quarters has been published in summary in the NASF Report in Products Finishing and in full at www.pfonline.com. A list of past project reports is available at the end of this paper. A printable PDF version of the current report is available by clicking HERE.
Beginning in January 2013, the NASF, through the AESF Foundation Research Board, funded a three-year project on alloy plating at Northeastern University, in Boston, under the direction of Dr. Elizabeth Podlaha-Murphy, with emphasis on nickel-molybdenum-tungsten deposits. In 2016, a one-year extension was granted to examine the influence of oxide particulates codeposited with different combinations of Ni, Mo and tungsten alloys on the resulting deposit composition in the generation of novel alloy composites. What follows is the third report on this new work.
In the last report, we examined the effect of titania particle concentration on Ni-W electrodeposition from an ammonia-free electrolyte with precise temperature control. With an increase in the electrolyte particle concentration, there was an associated higher deposit concentration. However, beyond an electrolyte particle concentration of 12.5 g/L, there was no further enhancement in the deposit particle content. A simple model following that of Guglielmi,1 can capture this behavior.
Together with an increase in the amount of titania in the deposit, there was also a lower weight percentage of tungsten in the deposit. The decrease in the tungsten content was not due to a decrease in its deposition rate, or partial current density, but interestingly due to a larger nickel deposition rate. Furthermore, inspecting the polarization curves generated from the deposition electrolyte, and determining the side reaction partial current densities, suggested that the addition of the particle in a Ni-W-TiO2 composite coating might be a better catalyst for hydrogen evolution compared to Ni-W alone, owing to the higher side reaction values when the particle was present.
In this report, pulse current (PC) electrodeposition was investigated to compare with direct current (DC) electrodeposition results on deposit composition and partial current density. The pulse frequency was varied to determine how it affected deposit composition and deposition rate.
Pulse current electrodeposition has several advantages over direct current electrodeposition, such as influencing adsorption and desorption phenomena. Significant grain refinement results from a larger instantaneous applied current density, and thus overpotential, when compared to an average, direct current value.2-3
Goldasteh and Rastegari4 investigated PC deposition of Ni-W-TiO2 composite coatings at a fixed cathodic current density of 100 mA/cm2, using a tungsten-rich, ammonium electrolyte at an elevated temperature of 75°C, with a cationic surfactant hexadecyltrimethylammonium bromide (CTAB). They found that the amounts of tungsten and TiO2 in the PC plated coatings were slightly higher than those in DC plated coatings (2 wt% and 0.5 wt% higher for tungsten and TiO2, respectively) when the pulse frequency was 1000 Hz. They also reported that an increased pulse frequency corresponded with increased titania content in the deposit as well as reduced surface roughness.
Kumar, et al.5 also compared PC and DC plated Ni-W-TiO2 deposits at a fixed cathodic current density (150 mA/cm2), pulse frequency (14 Hz) and duty cycle (0.6), using an ammonia-containing a dimethyl sulfoxide (DMSO)-water electrolyte, which contained equal molar concentrations of nickel and tungsten, as well as a sodium lauryl sulfate (SLS) surfactant, and 0.6mM of the additive 2-butyne, 1,4-diol (BD) and at 70°C. A more uniform surface was observed for the PC plated deposits. Goldasteh4 and Kumar5 both obtained deposits with a low tungsten content (~25 wt%).
The influence of current density and the effect of pulse frequency on the Ni-W-TiO2 composite coating composition, as well as the metal reaction rate have not yet been reported. In addition, composites with high tungsten content have not yet been examined.
Our goal for this report was to determine the effect of pulse frequency on the deposit composition, rate of metal reduction and morphology, using an ammonia-free electrolyte and with deposits having high tungsten content.
An ammonia-free electrolyte containing 0.15M nickel sulfate, 0.1M sodium tungstate, 0.285M sodium citrate, 1M boric acid was used as the plating electrolyte. The electrolyte pH was adjusted to a value of 8 with sodium hydroxide. Titanium dioxide microparticles (-325 mesh, Acros Organics) was then loaded to the electrolyte with a concentration of 12.5 g/L.
Electrodeposition was carried out on copper covered brass cylinder electrodes, having a diameter of 0.6 cm and length of 8 cm. A rotating Hull cell was used to quickly survey the different current density conditions. A plastic cylinder placed between the anode and the cathode was used to create the current distribution. The average applied pulse current density was -33 mA/cm2 during the on-time (ton), and was zero during the off-time (toff). Pulse frequency was varied at 0.2 Hz, 2 Hz, 20 Hz and 200 Hz in this study, with a fixed duty cycle of 0.6. In all cases, the total on-time was 30 minutes. A double-jacked cell using a water bath was employed to maintain the electrolyte temperature at 25°C. Samples of direct current deposition were also prepared using the same experimental conditions (plating for 30 minutes at 25°C). A potentiostat (PINE Instrument Company, model AFCBP1), controlled by a function generator (AMEL, model AMEL 568) was used to generate the pulse current.
The composition and thickness were measured using x-ray fluorescence (XRF), (Kevex, model Omicron), in an air environment.
The morphology of the deposits was examined using scanning electron microscopy (SEM), (Hitachi, model 4800). SEM images were taken at a low current density region (~6-8 mA/cm2) and a high current density region (~50-80 mA/cm2). The rotating Hull cell electrode positions that corresponded to these regions were estimated by using a primary current distribution.
The rotating Hull cell provides a current distribution along the working electrode, as long as the current flow is governed by ohmic effects, described as a primary current distribution. In our previous study,6 the Wagner number, a measure of kinetic to ohmic resistance, was determined to be 0.003 (<< 1), in a non-pulsing, DC situation, indicating that the current distribution was indeed nearly primary and that the local current density will change from a low to high value from end-to-end of the electrode. However, during pulse deposition the repetition of “on” and “off” times is expected to change the species’ surface concentration which can then influence the reaction rate, and thus the kinetic resistance, which in turn affects the Wagner number in a dynamic way. Therefore, in this report, all data were plotted versus a dimensionless length L = x/h instead of an estimated local current density as done in our previous report,6 with x being the distance from the low current density end of the working electrode and h being the total length of the working electrode.
Figure 1 compares the difference in deposit composition along the working electrode of the DC deposition deposits and the PC deposition deposits at different frequencies. Figure 1(a) shows that, for the DC condition, the tungsten content in the deposit increased from 45 to 55 wt% as the dimensionless cathode length L increased from 0.1 to 0.4, beyond which it remained almost constant. In cases corresponding to PC deposition, the amount of tungsten was consistently lower compared to DC deposition, except at the very high current density end of the electrode where the tungsten content in the deposit was almost the same as in the DC case.
Figure 1(b) shows that with DC deposition, the particle content was constant (~4-5 %) over a large range of current densities. The deposit particle composition during PC deposition was similar to that for DC deposition at the very low and very high current density regions, but was slightly higher (~6-7 %) between these regions. As a result, PC deposition tended to increase slightly the amount of titania particles in the deposit at higher current densities, at the expense of lowering the amount of tungsten. This same observation was made in the previous report when the deposit was DC deposited, as noted above. With an increase in the particle electrolyte content and a subsequent increase in the deposit particle concentration, the tungsten content also decreased. Thus, the parameters that help the incorporation of the particles seem to hinder the tungsten deposition.