Electrodeposition of Ni-Fe-Mo-W Alloys - 11th-12th Quarter Report
Ahn Phong Tran and Prof. E.J. Podlaha-Murphy*
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the 11th and 12th quarters of project work (July-December 2015). 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 is available by clicking HERE.
Novel, electrodeposited Ni-Mo-W alloys were developed in our past work1 and continued in Quarterly Report #7,2 with variable composition, and with interest in tailoring properties. For example, small amounts of tungsten in a deposit can improve the deposit hardness, wear resistance and corrosion resistance, while molybdenum can also offer similarly improved corrosion resistance, but also imparts catalytic behavior of interest for generating clean hydrogen.3-5 From a fundamental point of view, combining both molybdenum and tungsten elements in a deposit helps to probe their unusual induced codeposition behavior. Molybdenum and tungsten alloy deposition behavior is characterized by the observation that in aqueous solutions molybdenum and tungsten ions cannot be fully reduced to a metallic state, but can be completely reduced in the presence of certain elements, such as nickel, and first reported by Brenner.6 Experimentally, the codeposition of different binary combinations of molybdenum and tungsten alloys with cobalt, nickel and iron has been widely examined in a variety of aqueous electrolytes. In many of these electrolytes, ammonium hydroxide is a key component to achieve high current efficiencies in excess of 60%, although with relatively low amounts of the reluctant metal (i.e., molybdenum or tungsten) in the deposit. Eliminating the amount of ammonia in the electrolyte can be used as a strategy to increase molybdenum7,8 and tungsten9 in deposits when codeposited with nickel, however, with an appreciable drop in current efficiency. The use of ammonium hydroxide in aqueous solutions can also yield aqueous ammonia depending on the pH, which can be problematic in a plating line, as the ammonia readily volatilizes, and can be oxidized at the anode,10 thus its concentration is not easy to maintain.
In our past work in electrodepositing Ni-Mo-W alloys, an ammonia-free electrolyte with excess boric acid was used and deposits, with a reflective, smooth aspect were produced. The current efficiency was fairly low, ~10%, and could be improved with elevated electrolyte temperature to 20-30%.2 Interestingly, even with equivalent amounts of molybdate and tungstate ions in the electrolyte, there was consistently more molybdenum in the deposit. In this report we take a closer look at comparing Ni-W, Ni-Mo and the ternary alloy Ni-Mo-W, with the use of rotating cylinder electrodes. The electrodes have a small recess to promote a uniform current distribution.
Ni-W, Ni-Mo and Ni-Mo-alloys were electrodeposited onto copper cylinder electrodes at a rotation rate of 517 rpm. The ammonia-free electrolyte contained 0.375M sodium citrate, 1.0M boric acid, and was maintained at pH 7 at room temperature. Polarization data were collected using a three electrode cell with a saturated calomel reference electrode (SCE), a sweep rate of 10 mV/sec, and corrected for ohmic drop using impedance spectroscopy. Galvanostatic deposition for one hour was used to deposit films over a large range of cathodic current density (13-300 mA/cm2). The composition of the alloys was analyzed using x-ray fluorescence (XRF). The XRF results only reflect the heavy elements and lighter elements such as sodium, carbon and oxygen were not characterized. The weight of the cylinder electrodes was measured before and after each experiment. When present, sodium molybdate and tungstate had a concentration of 0.075M, while the amount of nickel sulfate was varied between values of 0.05M and 0.2M. Six different electrolyte conditions were examined: (a) elemental nickel (0.1M), (b) a binary alloy of nickel-tungsten with 0.1M nickel species, (c) a binary alloy of nickel-molybdenum with 0.1M nickel species, (d) a binary alloy of nickel-tungsten with 0.05M nickel species, (e) a binary alloy of nickel-molybdenum with 0.05M nickel species and (f) a ternary alloy nickel-tungsten-molybdenum with 0.1M nickel species.
Results and discussion
Figure 1 shows the polarization curves for the electrolytes used in depositing elemental nickel and the binary alloys of Ni-W and Ni-Mo (Fig. 1(a)) having the same electrolyte composition of nickel, and for the binary electrolytes having a lower amount of nickel species in the electrolyte (Fig. 1(b)) and when they are all combined in a ternary electrolyte, (Fig. 1(c)). All polarization curves see a sharp increase in the region between -1.3 and -1.4 VSCE. In this region, the deposition of metal or alloy occurs. There is little difference in the total polarization when the amount of nickels ions is changed.