Electrochemical behavior of low phosphorus electroless Ni–P–Si3N4 composite coatings

In the present investigation the electroless Ni–P–Si3N4 composite coatings were prepared by using a low phosphorus bath containing submicron size silicon nitride particles. Plain Ni–P deposits were also prepared for comparison. The phosphorus contents present in electroless plain Ni–P and Ni–P–Si3N4 coatings are 3.7 and 3.4 wt.%, respectively. Scanning electron microscope (SEM) images obtained for composite coatings (cross-sections) showed that the second phase particles are uniformly distributed throughout the thickness of the deposits. It was found that nodularity increased with particle codeposition in Ni–P matrix. To find out the electrochemical behavior of plain Ni–P and composite coatings, potentiodynamic polarization and electrochemical impedance (EIS) studies were carried out in 3.5 wt.% sodium chloride solution in non-deaerated condition. Second phase particle incorporation in Ni–P matrix indicated a marginal decrease in corrosion current density compared to the plain Ni–P deposits. This was further confirmed by EIS studies and SEM analysis of the corroded samples.