Pulsed Potential Dissolution to control anode pitting during nickel electroplating
Burke C. Barlow and Ian J. Burgess
ABSTRACT
The electrodissolution of nickel pellets with different controlled electrical variables was investigated in an effort to mitigate anodic residue in Watts bath based electroplating cells. During galvanostatic dissolution the anode potential was seen to oscillate between passive and transpassive potentials. In-situ microscopy and ex-situ characterization has shown that lacy-pit formation and large, partially perforated, pit covers form during the potential decay from transpassive levels. In general this type of covered pitting is thought to contribute significantly to anode residues.
The use of pulsed potential electrodeposition is one way to maintain greater control over surface pitting by creating a higher density of pit nuclei while at the same time controlling their growth. The growth of pits initiated by an active-to-transpassive potential step was examined ex situ using SEM and WLIM for a series of pulse times and three distinct stages were identified in the current transient. An alternative method of anode dissolution was devised using potential pulses and duty cycles that allowed average current densities comparable to industrial plating conditions to be achieved. By restricting the time spent at large transpassive potentials to 50 ms, anode residue was reduced more than six-fold in comparison to an equivalent galvanostatic dissolution run at a comparable current density.
In addition to pulsed potentiostatic experiments this ideology was extended to pulsed current regimes. As well, various aspects of the cathode deposits generated by each method will be discussed.
Figure- Current transient after stepping the anode potential from 0.0 V to 1.2 V in chloride-containing (thick black line) and chloride-free (thin black line) Watt’s bath. Red lines are fits for t2 (0.25-0.75 s) and t1/2 (>1.0s).
KEYWORDS
Nickel dissolution, Pitting corrosion, Anode residue, Watts bath, Electroplating