Electrodeposition of Ni-Fe-Mo-W Alloys - 16th Quarterly Report
Prof. E.J. Podlaha-Murphy*and Yujia Zhang
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the 16th and final quarter of project work (October-December 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 all project reports is available at the end of this paper. A printable PDF version of this 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 fourth and final report on this new work, involving Ni-W-TiO2 composite plating with the additive 2-butyne-1,4-diol under direct current, pulse current and pulse reverse modes of deposition.
In our previous report, Ni-W-TiO2 composite electrodeposits were fabricated under pulse current (PC) conditions, and compared with deposits electrodeposited under direct current (DC) conditions, to examine the effect of PC on deposit composition and partial current density. With PC deposition, the deposit tungsten content was slightly decreased, with an accompanying slight increase in titania and nickel content. Interestingly, under PC deposition, the partial current densities of nickel and tungsten were both enhanced, with a larger enhancement of nickel than tungsten, consistent with the metal compositional changes. In the DC deposition of Ni-W alloys, the addition of particles into the alloy created a duller and rougher surface finish. At the microscopic level, as observed by SEM, PC deposition did not significantly alter the nodular size of the surface features, although it did have an effect of reducing small pits caused by the co-evolving hydrogen side reaction. Preliminary studies combining the additive 2-butyne-1,4-diol (BD) with PC deposition, however, showed an improvement of the surface aspect.
In this report, the influence of BD was further examined. The addition of BD was studied in Ni-W electrodeposition, without TiO2 particles, in our previous 8th and 9th quarterly reports.1,2 Others have noted a brighter and smoother appearance of Ni-W alloys with BD in an ammonium ion-containing electrolyte,3,4 and we similarly found a shinier deposit in an ammonia-free electrolyte. Wu, et al.3 noted that BD enhanced the water reduction side reaction, and not only did we similarly observe an increase in hydrogen evolution, but found that the metal reduction rates were also reduced, at a similar pH of 8, leading to substantially lower current efficiency. It was thought that the lower metal reduction rates were inhibited not only by a blocking effect of adsorbed BD, but perhaps more substantially by the adsorbed hydrogen that it promotes. Since there is this unwanted trade-off, PC deposition may be able to counteract the inhibition effect of the metal reduction reactions, by allowing for the desorption of hydrogen intermediates during the “off” or relaxation part of the pulse cycle. A pulse reverse deposition, with a small anodic component, is also expected to facilitate the desorption of adsorbed hydrogen intermediates.
In an effort to improve the surface morphology of the Ni-W-TiO2 composites, we report here on the use of BD in our former electrolyte comparing DC, PC and PR deposition. A goal of this study was to see if the addition of TiO2 particles in the electrolyte, and co-deposited with the alloy, changed the beneficial improvements in morphology by BD. The amount of BD in the electrolyte was varied. The composite was plated using a rotating Hull cell, followed by composition and thickness analyses to determine how BD effected deposit composition, partial current density, side reaction and current efficiency.
Similar to tungsten, molybdenum cannot reduce alone in aqueous solution, but requires iron group elements, such as nickel, to induce its deposition.5 In order to examine if our results could be translated to the Ni-Mo system, a preliminary investigation of the effect of titania particles on Ni-Mo induced codeposition behavior is also presented in this report. Sodium tungstate was simply replaced with the same amount of sodium molybdate in the electrolyte, while keeping the other conditions the same as our previous study.
A boric acid (1.0M), sodium citrate (0.285M), ammonium-free electrolyte was used for the electrodeposition. The Ni-W electrolyte contained 0.1M nickel sulfate and 0.15M sodium tungstate, and the Ni-Mo electrolyte contained 0.1M nickel sulfate and 0.15M sodium molybdate. The electrolyte pH was 8, adjusted with sodium hydroxide, and the titania particle (particle diameters less than 44 microns, -325 mesh) concentration was 12.5 g/L. To examine the effect of BD concentration on the Ni-W-TiO2 composite deposition, 0, 1 and 5 mM BD were added to the electrolyte. To examine the cooperative effect of BD with PC and PR on Ni-WTiO2 composite deposition, a constant BD concentration of 5 mM was used. A double-jacketed cell using a water bath was employed to maintain the electrolyte temperature at 25°C in all cases.
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 with an electrode rotation rate of 500 rpm to quickly survey different current densities. A plastic cylinder was placed between the anode and the cathode to create the current distribution. When studying the effect of BD concentration on Ni-W-TiO2 composite deposition, the average applied cathodic current density was 33 mA/cm2 for 30 minutes. When studying the effect of titania particle on Ni-Mo deposition, the average applied cathodic current density was 50 mA/cm2 for 30 minutes.
Polarization curves were measured for both investigations, using a rotating cylinder electrode. The polarization measurements were performed by increasing the applied potential from the open circuit potential to a large enough potential to achieve the current density range that was examined in a rotating Hull cell configuration. The scan rate was 2 mV/sec, and the electrode rotation rate was the same value used in the rotating Hull cell experiments. When studying the cooperative effect of BD with PC and PR on Ni-W-TiO2 composite deposition, the average deposition current density was 33 mA/cm2 for all cases, with the off-time current density being zero for the PC deposition and the reverse current density being +5 mA/cm2 for the PR deposition.
The pulse frequency was 20 Hz and the duty cycle was 0.6. The total applied pulsing time was 200 minutes, and the total on-time, where deposition occurred was 120 min.
A potentiostat/galvanostat current generator (Solartron, model 1287A) was used for both DC electrodeposition and linear sweep voltammetry. The pulse or pulse reverse waveforms were generated with a function generator (AMEL, model AMEL 568) connected to a potentiostat (PINE Instrument Company, model AFCBP1). The deposit composition and thickness were measured using x-ray fluorescence (XRF), (Kevex, model Omicron), in an air environment.
Variable BD concentration
Polarization curves. Figure 1 shows the polarization curves of Ni-W-TiO2 with different concentrations of BD in the electrolyte. With increasing amounts of BD, the polarization curves were shifted to more negative potentials. Srinivasan and Bapu6 also observed a similar trend of inhibiting the total current density with an increasing concentration of BD, in a nickel-based electrolyte.