Electrodeposition of Ni-Fe-Mo-W Alloys - 14th Quarterly Report
14th Quarterly Report - AESF Research Project #R-117. This NASF-AESF Foundation research project report covers the 14th quarter of project work (April-June 2016).
Yujia Zhang and Prof. E.J. Podlaha-Murphy*
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the 14th quarter of project work (April-June 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 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 second report on this new work.
In our previous report, the effect of titania microparticles on Ni-W electrodeposition from an ammonia-free electrolyte was examined. In the presence of 12.5 g/L particles in the electrolyte, only 4-6 wt% titania particles were captured into the Ni-W alloy, and the alloy deposit composition ratio was not significantly disturbed at low cathodic current densities (<40 mA/cm2), although there was a slight decrease in the amount of tungsten at higher cathodic current densities. In addition, adding particles did not significantly change the current efficiency. However, there was a large change in the surface morphology, with the deposit surface becoming significantly rougher when the particles were present in the electrolyte. In this report, the addition of different amounts of titania microparticles was investigated to examine the effect of particle concentration on deposit composition, partial current density, side reaction, current efficiency and the deposit appearance. In our prior report the temperature was not maintained and was at room temperature. To reduce fluctuations in composition and current efficiency, on account of fluctuations in temperature, in this work, we modified the cell holder to include a double-jacketed cell so that a precise temperature of 25°C was maintained.
Composite electrodeposition refers to the strategy of reducing metal ions and at the same time capturing solid particles from the electrolyte to achieve a two-phase coating, imparting tailored properties to the materials. There are many reports on properties of different particle types electroplated with metals.1-3 Guglielmi4 first reported a two-step adsorption mechanism for the incorporation of particles, which was employed in many studies to predict the amount of particles in the deposit. In the first step, particles are loosely adsorbed onto the electrode in a manner similar to Langmuir adsorption. Therefore, the deposit particle content should increase with an increasing amount of particles in the electrolyte, and reach a maximum level at a large particle electrolyte concentration. In the second step, particles strongly incorporate into the metal film assisted by the electric field, consequently are influenced by the electrochemical process. As a result, the incorporation rate of particle may follow a Tafel-like behavior. While there are other more comprehensive models, the Guglielmi model is a conveniently simple way to quantitatively summarize the amount of particles incorporated into the deposit with plating parameters, under a fixed hydrodynamic environment.
Previous studies on electrodeposited Ni-W alloys with titania particles have been reported in the literature5-7 using a fixed amount of particles in electrolytes with a focus on deposit properties and structure, and with the particle loadings studied being relatively low (below 20 g/L). These studies did not examine the influence of changing the particle concentration in the electrolyte. However, the effect of particle concentration has been reported for other types of particles with Ni-W plating. In ammonium containing electrolytes, Yari and Dehghanian8 reported that the incorporated alumina particle content in the Ni-W deposits first increased with an increasing amount of particles in the electrolyte, and then became almost constant with additional particle electrolyte loading. When there were more tungstate ions than nickel ions in the electrolyte, the maximum amount of particles in the deposit occurred with 5 g/L of particles in the electrolyte. A clear maximum amount of particles in the deposit with the amount of particles in the solution was not observed when there was an excess amount of nickel ions. In our study here there is a small excess of nickel ions compared to tungstate ions. Additionally, the amount of tungsten in the deposits obtained by Yari and Dehghanian was independent of the electrolyte particle concentration. Yao, et al.9 reported on the amount of SiC in electroplated Ni-W-SiC coatings and showed that there was an increase with increasing SiC loading in the electrolytes, without reaching a maximum value.
Our goal for this report is to determine how the particle concentration (over a relatively wide range) affects the alloy composition, amount of particles in the deposit, rate of deposition, side reaction, current efficiency and the deposit appearance, using an ammonia-free electrolyte in order to obtain deposits with high amounts of tungsten.
An ammonia-free electrolyte containing 0.15M nickel sulfate, 0.1M sodium tungstate, 0.285M sodium citrate, 1.0M boric acid and variable amounts of titanium dioxide particles was used to plate the composite coating. The electrolyte pH was adjusted to a value of 8 with sodium hydroxide. In order to investigate the effect of particle concentration on Ni-W deposition, 0, 2.5, 7.5, 12.5, 25 and 50 g/L titanium dioxide microparticles (-325 mesh, Acros Organics) were respectively loaded in the electrolytes. A double-jacketed cell using a water bath was employed to maintain the electrolyte temperature at 25°C.
Polarization curves and deposition were carried out on copper covered brass cylinder electrodes, having a diameter of 0.6 cm. A rotating cylinder electrode was employed to measure the polarization curves. The polarization measurements were performed by increasing the applied potential from the open circuit potential value to a large enough potential to achieve a current density that was examined in a rotating Hull cell configuration. The scan rate was 2 mV/sec and the electrode rotation rate was 500 rpm. During the polarization scans, a three-electrode cell configuration was used with a platinum mesh anode and a saturated calomel reference electrode (SCE). The curves were corrected for ohmic drop with impedance spectroscopy (not shown). A rotating Hull cell was used to quickly survey the applied current density conditions. The rotating Hull cell used rods of the same diameter, with a length of 8 cm. A plastic cylinder placed between the anode and the cathode was used to create the current distribution. The average applied current density was 50 mA/cm2, and the rotation rate was 500 rpm.
A Solartron 1287A potentiostat/galvanostat current generator was used for both linear sweep voltammetry and deposition. The composition and thickness were measured using Kevex x-ray fluorescence (XRF).
Figure 1 shows the polarization curves of Ni-W with different concentrations of titania particles in the electrolytes. With the addition of different amounts of particle electrolyte loading, all polarization curves were shifted to more positive potentials at the low current density region (see inset, Fig. 1). However, no trend in the shift of the polarization curve by varying the particle electrolyte concentration was observed at higher current densities, perhaps due, in part, to the stochastic response generated by the large water reduction, hydrogen evolution side reaction.