Porosity measurement of electroless Ni–P coatings reinforced by CNT or SiC particles

Abstract

Ni–P composite coatings containing SiC nanoparticles or carbon nanotubes (CNTs) were deposited on steel substrates via electroless plating technique. An electrochemical polarisation technique was employed to measure the porosity of Ni–P and composite coatings in as plated condition. The technique was based on the changes observed in the corrosion potential with varying cathode and/or anode area on a bimetallic corroding system. Electrochemical impedance spectroscopy (EIS) test was used to assess the corrosion resistance of the coatings. Results showed that incorporation of nanoscaled second phases could decrease the area fraction of porosity from 2··5×10⁻⁵ for Ni–P coating to 2×10⁻⁷ for Ni–P–CNT coating. Ni–P–CNT coating exhibited the least porosity that it could be attributed to the morphology of CNTs and their ability to fill out the pores more effectively as compared to SiC nanoparticles. EIS results were in agreement with the porosity results as Ni–P–CNT coatings illustrated the highest corrosion resistance.