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In recent years, electroless nickel coatings have been extensively used for various purposes in many fields of industry and a wide variety of functions have been required of the electroless nickel deposits. In this study, an electroless nickel plating bath operated at relatively low temperature has been developed, and the effects of certain factors (temperature, pH, and concentration of acetate and hypophosphite) on the deposition rate and phosphorous content of the electroless nickel deposits have been investigated in some detail. Deposits with phosphorus contents higher than 9wt% were found to be amorphous and to have improved properties with respect to tensile strength and corrosion resistance. The addition of an unsaturated compound such as maleate having a double bond will reduce the molar ratio of hydrogen gas to deposited Ni-P alloy, and also increase the phosphorus content of the deposits.

A computer-aided coulostatic method was employed for the in-situ monitoring the deposition rate of electroless nickel. The relationship between deposition rate (ielp) and polarization resistance (Rp) was given by the equation; ielp=K/Rp. The K-values were empirically determined from pairs of ielp and Rp values, which were measured both by weight gain due to electroless plating and also by the coulostatic method. The K-value relating to the electroless nickel deposition was found to be 10-20mV depending on temperature, pH and acetate concentration as well as stirring, but it was substantially independent of hypophosphite concentration. Once a K-value was established, the deposition rate could be successfully monitored by this method. The empirical K-value agreed fairly well with theoretical one which was calculated from Tafel slopes (ba, bc) measured by the galvanostatic trangent and a modified coulostatic method by using the equation; K=ba·bc/2.3 (ba+bc). The increase in K-value with rising pH was essentially ascribed to an increase in Tafel slopes. The pH dependence of the Tafel slopes was more pronounced in cathodic reactions than in partial anodic reactions. The coulostatic method is found to be useful not only for monitoring the deposition rate of electroless nickel, but also for elucidating the mechanisms of the deposition process.