Corrosion behavior of Ni–P–Ag and Ni–P–Al2O3 composite coatings

Abstract

The aim of this research is to study the co-deposition behavior of Ag and Al2O3 particles within Ni–P coating and the corrosion resistance of Ni–P–Ag and Ni–P–Al2O3 composite coatings. The structure of the composite deposits was evaluated by XRD and the morphology and cross-section of the coatings were analyzed by SEM and TEM. 3D optical profilometer was also used to determine the surface roughness of the coatings and the co-deposition percentage was calculated by the image analysis of micrographs. The electrochemical behavior of the coatings were evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization and also electrochemical impedance spectroscopy (EIS) techniques. The results showed that the Al2O3 particles were co-deposited more than that of Ag particles in the existence of CTAB cationic surfactant whereas maximum co-deposition were achieved about 20 and 34 vol% in Ni–P–Ag and Ni–P–Al2O3 composite deposits. Moreover, the phosphorus content and especially the deposition rate of Ni–P matrix coating were strongly influenced by the concentration of the particles in plating solution. The different co-deposition behavior of these two types of reinforcements have been directly correlated to the density, particle size and zeta potential of the particles. The EIS results revealed that the incorporation of Ag and alumina powder in Ni–P matrix cause the lower corrosion resistance rather than that of conventional Ni–P coating. This behavior has been attributed to the pore formation in the composite deposits as well as the galvanic corrosion especially in Ni–P–Ag composite coating.