Electrodeposition of Ni-Fe-Mo-W Alloys - Part 7
by Prof. E.J. Podlaha-Murphy,* A. Kola and Rui Wu, Northeastern University, Boston, Massachusetts, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the seventh quarters of project work (July-October 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 seventh quarter. Work was focused on the change of composition of Ni-Mo-W alloys from non-ammonium containing electrolytes with an eye towards improving the current efficiency.
Through NASF-AESF Foundation funding, several students, both graduate and undergraduate, have gained experience in surface finishing research. In this period, there was a change in student participation. A new student, Rui Wu, has joined our research effort on Ni-Mo-W alloys. Graduate student Avinash Kola has continued work on the influence of deposition conditions on the properties of the Ni-W alloys.
Effect of operating variables on Ni-Mo-W current efficiency
From our past work, one detracting feature noted is that without the use of ammonium-containing species in the electrolyte, the current efficiency was particularly low, ~10%. In an effort to improve the current efficiency in our boric acid-citrate electrolyte, two variables were examined: the electrolyte temperature and tungstate composition. These two variables were identified previously as having a significant change on the deposit thickness, hence current efficiency.
Two types of electrode experimental set-ups were used: (a) a rotating cylinder electrode (RCE) and (b) a rotating cylindrical Hull cell (RHC) as the working electrode. The counter electrode in both cases was a DSA anode. The electrolyte contained 0.15M nickel sulfate, 0.005M sodium molybdate, 0.375M sodium citrate and 0.1M boric acid with various amounts of sodium tungstate (0.01, 0.05 and 0.1M). Five different electrolyte temperatures were examined: 25, 35, 45, 55 and 65°C. The electrolyte pH was maintained at 7.0 with sodium hydroxide and sulfuric acid additions.
Rotating cylinder electrode (RCE)
The RCE was used to assess the current-potential relationship (i.e., polarization curves). The use of an RCE ensures uniform mixing near the electrode surface with a fully developed turbulent flow profile. The electrode diameter was 0.6 cm, and the electrode length was 1 cm. The rotation rate was constant at 500 rpm and the sweep rate was 10 mV/sec.
Rotating cylindrical Hull cell (RCHC)
A rotating Hull cell was used to survey the current density for the different electrolyte tungstate concentrations and temperatures. In contrast to a conventional Hull cell, the cylindrical design provides a better control of the mixing environment and hence boundary layer thickness. The RCHC was first introduced by Madore and Landolt,*** and was designed to mimic the current distribution of the original Hull cell. The current distribution is created by placing the anode outside a plastic shield that surrounds the cathode. Figure 1 is a sketch of the RCHC used here with the dimensions of the cell. Hence, this design merges the advantages of the rotating cylinder electrode with a controlled, non-uniform current distribution as in the Hull cell. Deposits were analyzed by x-ray fluorescence (XRF).