Application of Dynamic Impedance Measurements for Adsorbed Plating Additives
The 1986 AES Gold Medal Award was given to J.C. Farmer and H.R. (Rudy) Johnson for the Best Paper appearing in Plating and Surface Finishing in 1985. Their work presents important work on electrochemical impedance measurements, then a new technique in analyzing how plating baths and their additives work.
J.C. Farmer and H.R. Johnson
Sandia National Laboratory
Editor’s Note: Originally published as J.C. Farmer and H.R. Johnson, Plating and Surface Finishing, 72 (9), 60-66 (1985), and was awarded the 1986 AES Gold Medal for Best Paper published in Plating and Surface Finishing in 1985. A printable PDF version is available by clicking HERE.
Two-phase, multi-frequency AC-cyclic voltammetry (AC-CV), a novel dynamic impedance technique, was used to study the inhibition of nickel electrodeposition by 0.26 mM rhodamine-B and 0.83 mM sodium saccharin, which differed greatly in their effect and capacitance. Rhodamine-B was more strongly adsorbed over the potential range of 500 to -1000 mVSCE, during nickel electrodeposition and anodic stripping. Hydrogen evolution at a high overpotential resulted in surface roughening, as evidenced by dramatic increases in capacitance.
Organic compounds are added to plating baths to alter the surface finish and mechanical properties of electrodeposits and probably function by preferential adsorption or electroreduction at sites of highest activity on the cathode. These additives are very dilute and are consumed by incorporation in the deposit, anodic oxidation, or cathodic reduction. Therefore, it is necessary to periodically monitor and adjust additive concentrations.
Because the effects of additives on surface finish and mechanical properties are related to changes in the double-layer structure and electrokinetic rate, precise electronic determinations of the double-layer capacitance and charge transfer resistance as quantitative measures of additive concentrations are useful. This study was undertaken to determine the sensitivity of these measured quantities to the presence of either rhodamine-B or saccharin over a broad range of potential and to gain fundamental knowledge of the electric double layer during deposition and dissolution.
The effects of rhodamine-B and saccharin on the double-layer capacitance and charge transfer resistance were measured during both anodic dissolution and electrodeposition of nickel from aqueous solutions of boric acid and nickel chloride. A novel dynamic impedance technique - two-phase, multi-frequency, AC-cyclic voltammetry (AC-CV) - was employed. Boric acid and nickel chloride were chosen for study because they are ingredients in the popular Watts bath.1 Boric acid is used as a buffer and the chloride anion assists in the corrosion of passivated anodes. Saccharin is a commonly used stress-reducing additive for nickel baths.2 Rhodamine-B has been found to be an effective brightener for lead and nickel baths and has been studied on the electrode surface by spectroscopic ellipsometry.3
Bond4 has used single-phase, single-frequency AC-CV to elucidate sequential reaction steps on an electrode and Ohsaka and coworkers5,6 have used a dynamic impedance technique similar to AC-CV to study hydrogen adsorption. Nickel deposition has been investigated with AC impedance by Epelboin and coworkers, and important conclusions were drawn regarding the effects of chloride and sulfate anions during electrodeposition.7,8
The dynamic impedance measurements presented here are more limited in frequency range than Epelboin's potentiostatic impedance measurements; however, the dynamic impedance technique allowed investigation of a much broader potential range that included both deposition and dissolution. Despite limitations in frequency, precise measurements of the double-layer capacitance were possible. The charge transfer resistance was also determined but with less relative accuracy. The cyclic method employed here is novel and represents the first reported application of a dynamic impedance technique to the study of adsorbed organic additives during electrolytic deposition.
Electrolyte concentrations were 0.5M H3BO3, 0.25M NiCl2, and either 0.26 mM (0.13 g/L) rhodamine-B or 0.83 mM (0.2 g/L) sodium saccharin. Stock solutions had a pH of 4.5 and were prepared from reagent-grade chemicals and deionized, distilled water. The cell volume was about 200 mL and was purged with N2. The cathode was a platinum rotating disc electrode (RDE) with 0.5 cm2 of active area and driven at 2000 rpm by a Pine Instruments Model ASR analytic rotator. Nickel-reduction kinetics are slow and this rotation rate was sufficient to insure that the deposition was not mass-transfer controlled.
The circular platinum sheet was embedded in Teflon and polished mechanically with 0.05-μm abrasive particles. The anode was 99.999% nickel and had a surface sufficiently large that its impedance could be neglected. All potentials were referenced to a saturated calomel electrode (SCE) that had a porous plug junction (Fisher Cat. No. 13-639-51). Anode purity was verified by flame emission, stock solution purity by a differential pulse polarography unit,* and deposit purity by energy-dispersive analysis of x-rays (EDAX).
The RDE potential was controlled using a potentiostat equipped with a digital coulometer. A programmer applied a ramp potential (500 to -1000 mV at 50 mV/sec) to one signal input of the potentiostat while a Schlumberger Model SG1271 variable-frequency/variable-amplitude oscillator applied a sinusoidal modulation (100 to 1000 Hz at 10 mV rms) to the second signal input (Fig. 1). The resulting wave form of the RDE potential was that of a high-frequency modulation superimposed on a slowly swept ramp. Because faradaic and capacitive current responses to the modulation were separated in phase by 90 degrees, they could be detected simultaneously with a dual-channel, lock-in amplifier and recorded as the potential was ramped. Several cycles, each at a different modulation frequency, provided impedance data that were fit to simple equivalent circuit models (Fig. 2). Data points were digitized and fit to the model at 100-mV intervals, and more frequently where abrupt changes in response occurred.