β Scribed by F.M. Donahue; C.U. Yu
No coin nor oath required. For personal study only.
THE rate-controlling step for the electroless deposition of nickel is not completely understood. The rate law for the process shows a first-order dependence on the hypophosphite-ion concentration (at constant pH)lsa and a negative order with respect hydrogen-ion concentration with variable PH.~ Catalytic mechanisms," thermodynamic arguments (ULZ common ion effects),3 and slow transport of hydrogen ions3** have been offered to explain the observed results. The experiments reported here are intended to clarify the controlling step in the electroless deposition of nickel and to offer a different approach to the study of electroless plating processes in general. The basic premise of this study is that electroless plating processes are mixed electrode systems6-7 wherein the anodic partial process is the oxidation of hypophosphite and the cathodic partial processes are hydrogen evolution and nickel deposition. Thus, at low pH (<4),)the hydrogen-evolution reaction would proceed at a rate comparable to that of nickel deposition, leading to low measured plating rates; while at high pH (4 < pH < 6), the rate of hydrogen evolution would be small compared to nickel deposition, leading to larger measured plating rates. In order to test the hypothesis of the applicability of the mixed potential theory to electroless plating, it was necessary to measure simultaneously the rate of nickel plating and the electrode potential of the sample being plated. Nickel foil or wire samples were used as one arm of a Wheatstone bridge in order to detect changes in resistance of the foil during the plating operation. A second nickel sample used in the Wheatstone bridge circuit was coated with epoxy and immersed in the plating bath in order to obviate temperature corrections for the plated sample. The experimental sample was cleaned in distilled water, cathodically treated in 6 N HCI, rinsed in water, given a nickel strike, and immersed in the plating bath. The plating bath contained O-5 M sodium acetate, O-5 M acetic acid, and variable amounts of sodium hypophosphite (O-05 to l-0 M) and nickel chloride (O-05 to 1-O M). The bath was stirred by a Teflon paddle and thermostated at 90Β°C. In order to minimize pH changes, the ratio of solution volume to surface area was large and no experiment lasted longer than 4 h. The change in thickness of the sample was calculated from the measured resistance change?, and the plate thickened linearly with time for all experiments.
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π SIMILAR VOLUMES
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