V. Ezhil Selvi

Electrochemical behaviour of high phosphorus electroless Ni–P–Si3N4 composite coatings

Abstract A high phosphorus electroless nickel bath was used to prepare plain Ni–P and composite coatings containing submicrometre size silicon nitride particles. Deposits were characterised for their composition, morphology and electrochemical behaviour. Codeposition of particles in a Ni–P matrix has not influenced the phosphorus content (10 wt-%). Surface morphology of plain Ni–P deposits was smooth; the composite deposits became slightly rough with small nodules due to particle incorporation. Cross-sectional examination of composite coating revealed that the particles were uniformly distributed throughout the thickness of the coating. Potentiodynamic polarisation and electrochemical impedance studies were carried out in 3·5 wt-% sodium chloride solution in non-deaerated condition. Potentiodynamic polarisation studies showed that the corrosion current density value obtained for composite coatings is lower than that for plain Ni–P coatings. Electrochemical impedance spectroscopy studies showed that the coating resistance of the composite coating is higher than that of plain Ni–P coating. This was further confirmed by SEM analysis of corroded samples.

Electrochemical behavior of nanocrystalline Ni–P alloys containing tin and tungsten

Autocatalytic ternary Ni-Sn-P, Ni-W-P and quaternary Ni-W-Sn-P films were prepared using an alkaline bath. Plain Ni-P films were also prepared for comparison. Corrosion resistance of the films was evaluated in 3.5% sodium chloride solution in non-deaerated condition by potentiodynamic polarization and electrochemical impedance spectroscopy methods. Deposits were also immersed in 3.5% sodium chloride solution for 7 days. All the coatings attained stable equilibrium potential within 30 minutes in NaCl medium. Lower corrosion current density values were obtained for ternary Ni-Sn-P coatings compared to the plain Ni-P coatings. Ternary Ni-W-P and quaternary Ni-W-Sn-P alloys did not show improved corrosion resistance compared to the ternary Ni-Sn-P coatings. Similar behavior of these coatings was further confirmed by the electrochemical impedance studies. After the potentiodynamic polarization test deposits were examined by scanning electron microscope. It was found that more corrosion occurred for the quaternary deposit compared to other deposits. Energy dispersive analysis of X-ray results indicated that more amount of Fe present on NiWP and NiWSnP coated samples. Similar behavior was confirmed from the optical images of the surfaces obtained for the deposits after the immersion test. The article is published in the original.