Effect of nano-Al 2O 3 particles on the NiP/nano-Al 2O 3 coatings' properties

Composite coatings were prepared using electroless nickel bath containing different concentrations of Al2O3 nano-particles. The analyses of coating compositions, carried out by EDS, showed that there is marginal difference between phosphorus contents of NiP and NiP/nano-Al2O3 deposits. The structure of the coatings was examined by scanning electron microscopy (SEM), and X-ray diffraction (XRD). It has been found that the co-deposition of nano-Al2O3 particles with Ni disturbs the NiP coating's regular surface structure and increases its surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 3.5wt.% solution of NaCl in order to evaluate their corrosion resistance. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests both showed that, the corrosion resistance of NiP-Al2O3 coatings firstly increases and then decreases when Al2O3 concentration in electroless bath is increasing, but the corrosion resistance of NiP-Al2O3 composite coating is better than that of amorphous NiP coating.