Electrodeposition of Ni-Fe-Mo-W Alloys - Part 8
A. Kola and Prof. E.J. Podlaha-Murphy,*
Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the eighth quarter of project work (October - December 2014). 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.
The project, initiated in January 2013, addresses the induced codeposition of molybdenum and tungsten alloys with nickel and iron having a focus on developing a toolbox of plating conditions to deposit different combinations of Ni, Fe, Mo and W. This paper covers progress made during the eighth quarter. Work was focused on the effect of the additive 2 butyne-1,4-diol (BD) in an acidic Ni-W electrolyte.
Through NASF-AESF Foundation funding, several students, both graduate and undergraduate, have gained experience in surface finishing research. In this period, Graduate student Avinash Kola has continued work on the influence of deposition conditions on the properties of the Ni-W alloys.
Effect of 2 butyne-1,4-diol (BD) on Ni-W deposition
Nickel-tungsten is a promising alloy that may be of interest to replace the environmentally unfavorable hard chromium coatings, due to the satisfactory appearance of the coating, and the mechanical and anti-corrosion properties.1,2 The corrosion behavior of Ni-W alloys is well documented. For example, Yao, et al.3 found that Ni-W is more corrosion resistant than stainless steel 304 in acidic media. Stepanova and Purovskaya4 investigated the dissolution of different compositions of Ni-W alloys and found that an alloy having a composition of 28 wt% tungsten had the most favorable results. It is also well-known that tungsten cannot be deposited from an aqueous solution and codeposition of tungsten with other elements, such as those of the iron group (i.e., Fe, Ni, Co) permits deposition of tungsten in the alloy.5
Corrosion protection by organic inhibitors is mostly based on the modification of the surface properties of the metal alloy through adsorption of inhibitor molecules and the subsequent formation of a blocking layer.6,7 Acetylenic compounds are known to be strong corrosion inhibitors in acidic solutions, and one such widely examined compound for corrosion inhibition is 2-butyne-1,4-diol (BD).8 The inhibitive effects of BD on mild steel in sulfuric acid were investigated by Hosseini and Arshadi.9 The introduction of BD can lead to the formation of a thin inhibitor film on the steel surface, which effectively protects it from further corrosion. The addition of BD can also reduce the deposit stress,10,11 depending on its concentration, and act as a brightener for nickel alloys.12,13 In addition, it has been shown to improve the corrosion resistance of Ni-W deposits.13,14 However, little understanding is present on the effects of BD on the deposition rates of nickel, tungsten and the side reaction during reduction. In this study, we examine the effect of different concentrations of BD during the electrodeposition of Ni-W alloys, and its influence on the nickel and tungsten deposition behavior using a conventional Hull cell.
The electrolyte used in this study contained 0.1M nickel sulfate, 0.15M sodium tungstate and 0.285M trisodium citrate at a pH of 2 and at room temperature. The BD concentration was varied from 0 to 5 mM. The applied current density was -50 mA/cm2 on a copper plate in a Hull cell configuration for 30 min at an agitation rate of 3 L/min (air). Polarization data was obtained by placing the electrodes parallel to each other, at a scan rate of 10 mV/sec, with a Ag/AgCl reference electrode. The curves were corrected for ohmic drop with impedance spectroscopy. Composition and thickness were measured using x-ray fluorescence.
Figure 1 shows the polarization curves of (a) Ni-W and (b) Ni deposition with varying amounts of BD. A shift of the polarization curves toward more positive potentials was observed in the Ni-W electrolyte as the BD concentration increased. This unusual polarization behavior is opposite to what is expected in a nickel electrolyte during nickel electrodeposition. Srinivasan and Bapu15 showed an inhibiting effect of the total current density with an increase in BD concentration in a nickel-containing electrolyte. In the low pH region, citrate electrolyte presented here when tungsten is removed, Fig. 1(b), the nickel polarization behavior is indeed similar to prior literature reports, with a decrease in the total current density with increasing amounts of BD. Thus, the introduction of tungstate to the electrolyte creates this unusual behavior. In order to investigate the source of the polarization shift in the Ni-W electrolyte, the partial current densities were determined. The partial current densities can help to indicate if the observed increase in the total current density at a given potential with BD is due to an increase in the metal deposition rate or due to an increase in the side reaction. In order to determine the partial current densities of the overall reactions, both composition and mass measurements are needed over a range of current densities.