Corrosion resistance of electroless Ni-P deposits and its relation to P contents

By measuring polarization curves and alternating-current impedances, the corrosion resistance of electroless Ni-P deposits with P contents from 16.2% to 23.4%(x) was investigated in 5% (w) NaCl solution. It was found that there was a maximum of polarization resistance (R-p) when the P content was at 21%similar to 22%(x) and the peak temperature of crystallization(T-p) of electroless Ni-P alloys, determinated by differential scanning calorimetry (DSC), also showed a maximum at such a P content. X-ray diffraction (XRD) experiments indicated that the electroless Ni-P deposits were amorphous. The maxima for both polarization resistance (Rp) and the peak temperature of crystallization (Tp) were explained in terms of the rhomb unit structure model (RUSM) for amorphous structures. The corrosion resistance of electroless Ni-P deposits was related to the changes of the bond numbers formed between metal (Ni) and metalloid element (P) with the P contents. The rationalization to adopt RUSM was also discussed by comparing the measured and theoretically calculated densities for electroless Ni-P deposits with various P contents.