A Study On The Preparation And Properties Of Amorphous Nickel Phosphorous Electroplating Deposits

The processing parameters of the electroplating of amorphous nickel-phosphorous (Ni-P) alloy deposits from two aqueous systems (hypophosphite-nickel salts and phosphorous acid-nickel salts bath) and the properties of the electrodeposits are studied in this thesis.The bright amorphous Ni-P deposit with stability can be obtained by electroplating in the hypophosphite-nickel salts bath. And the effects of electrolyte composition, current density, temperature, pH values on the free corrosion potential and hardness of the deposits are studied by orthogonal tests, and the processing parameters to obtain deposits with optimum corrosion resistance and hardness are determined. The effect of every factor on the electrodeposition process is analyzed and the reasons for the forming of amorphous Ni-P deposits are indicated by the experiment results and X ray diffraction analysis of the deposits: the phosphorous content in the deposits increased with the phosphorous content in electrolyte, which causes the deformation of the nickel lattice and ultimately results in the amorphous structure.Different Ni-P deposits are obtained by changing the processing parameters of the hypophosphite-nickel salts bath. And the corrosion behavior and mechanism of these deposits in the neutral solution of 0.5M sodium chloride are studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The results show that: the corrosion resistance of electroplated Ni-P alloy increased with the decrease of pH values of the electrolyte and with the increase of hypophosphite content in the electrolyte, and reaches a maximum at NaH₂PO₂.H₂O cotent of 110g/l then decrease with the NaH₂PO₂.H₂O content. The electrodeposits obtained at low temperature (40℃) is more corrosion resistive than the deposits obtained at (60℃) .The bright amorphous Ni-P deposits can also be obtained stably in phosphorous acid-nickel salts bath, the effects of the components in electrolyte, solution, temperature, pH value on the free corrosion potential and microhardness are also studied by which the optimum processing parameters are selected. Also the corrosion resistance and the factors on the microhardness of the amorphous Ni-P alloys deposits are studied.The amorphous deposits obtained from two electrolyte system are studied in the solutions of 3.5wt% sodium chlorideN 1M hydrochloric acid> 0.5M sulphuric acid and 1M nitric acid medium by electrochemical impedance spectroscopy and potentiodynamic polarization techniques, it shows that the amorphous deposits obtained from phosphorous acid-nickel salts bath have a smaller free corrosion currents in the solutions of 3.5wt% sodium chloride> 1M hydrochloric acid^ 0.5M sulphuric acid medium, but the deposits obtained from hypophosphite-nickel salts are more corrosion resistive in 1M nitric acid.