ピロりん酸浴からのニッケル電析

Abstract

The electrodeposition of nickel from pyrophosphate baths has been studied with reference to the effect of the pH buffer capacity (dB/dpH) of the bath on the purity of the nickel deposit. The dB/dpH vs pH curves of several nickel pyrophosphate baths were evaluated by pH titration, and showed broad peaks in the pH 5 to 9 region and sharp peaks at about pH 11. By comparison with simulated curves, it was found that the wide peaks were due to free pyrophosphate ion and the sharp peaks were due to precipitation equilibrium of nickel hydroxide. These curves provide significant information regarding the pH buffer capacity at the present pH and the allowable pH range of the electroplating bath.In baths with low pH buffer capacity, such as baths with low P2O7 concentration and/or highly basic baths, it was found that the partial current densities for nickel deposition were low and suppressed at more negative cathode potential. X-ray photoelectron analysis indicates that the electrodeposits from such baths contain oxygen and phosphorous, and the Ni 2p3/2 spectra suggest formation of nickel hydroxides. However, in the case of baths with high pH buffer capacity, such as baths with high P2O7 concentration and neutral pH or basic baths with low P2O7 concentration but containing B4O7, the nickel deposition current rose rapidly and remained high even when at higher potential Deposits from such baths contained no oxygen or phosphorous.These results indicate that pure nickel deposits can be obtained from such high pH buffer capacity baths.