Effect of accelerators and stabilizers on the formation and characteristics of electroless Ni-P deposits

The effect of thiourea, succinic acid and lead acetate on the formation and characteristics of electroless Ni-P deposits obtained from an acidic hypophosphite reduced electroless nickel bath is addressed in this paper. The rate of deposition of electroless Ni-P coating is found to be a function of concentration of these additives. Thiourea accelerates the rate of deposition up to 0.8 ppm and started to inhibit at a concentration of 1 ppm. Similarly, succinic acid increases the plating rate up to 12 g l-1 and exhibit an inhibiting effect at higher concentrations of the order of 15 g l-1. Addition of lead acetate inhibits the rate of deposition even at a concentration of 0.5 ppm and the extent of inhibition is increased when the concentration is higher than 1 ppm. Addition of these additives also caused a change in phosphorus content of the deposits; lead acetate (1 ppm) tends to increase the phosphorus content whereas thiourea (0.8 ppm) and succinic acid (12 g l-1) tends to decrease the phosphorus content. The X-ray diffraction patterns of electroless Ni-P coatings obtained in the absence of additives and in presence of 1 ppm of lead acetate exhibit a single broad peak centered at 44.5o 2θ, indicating the amorphous nature of these coatings. The peak broadening suggests a greater tendency to form amorphous structure when lead acetate is used as the additive. In contrast, for electroless Ni-P coatings obtained from thiourea and succinic acid containing baths, besides the reflection from Ni (1 1 1) plane, a weak reflection from Ni (2 0 0) plane is also observed. The X-ray diffraction patterns of electroless Ni-P coatings after annealing at 400 deg. C for 1 h exhibit the formation of fcc nickel and bct nickel phosphide (Ni3P) phases in all the cases with Ni3P (2 3 1) as the most intense reflection. Electroless Ni-P coatings obtained in presence of thiourea and succinic acid exhibit a nodular feature with a typical cauliflower like structure. The size of the nodules is relatively less in the latter case. In contrast, the electroless Ni-P coating obtained in the absence of additives and in presence of 1 ppm of lead acetate is relatively smooth. However, the deposit obtained in the absence of additives reveals the presence of fine particulates, attributed to the precipitation of Ni3P phases in the absence of stabilizers. The DSC traces of electroless Ni-P coatings exhibit a single well-defined exothermic peak in the temperature range studied in all the cases, which could be attributed to the precipitation of metallic nickel phase and formation of nickel phosphide (Ni3P) phase. The variation in the peak temperature and the energy evolved during the phase transition is due to the slight variation in the phosphorus content caused by the addition of thiourea, succinic acid and lead acetate. The study recommends that the choice of accelerators and stabilizers should be made only after a careful study