Microstructural, mechanical and corrosion characterization of electroless Ni-P composite coatings modified with ZrO2 reinforcing nanoparticles

This study aims to develop medium P (MP, 6 wt%) and high P (HP, 11 wt%) Ni-P-ZrO2 nanocomposites on F22 steel substrate using an original lead-free and surfactant-free solution and to quantitatively relate nanocomposite characteristics to microhardness improvement and corrosion resistance. Incorporation, dispersion and distribu-tion of the nanosized reinforcing phase were evaluated quantitatively by coupling Scanning Electron Microscope (SEM) imaging acquisition with original procedure for image processing and analysis. X-ray Diffraction (XRD) analysis revealed that nanoparticles introduction does not alter microstructure, which remains amorphous for HP and nanocrystalline for MP. Significant differences in particles distribution are found between MP and HP nanocomposites, which are induced by the difference in plating rate of the two formulations. Faster growth of MP (approximate to 35 mu m/h) associates with greater enveloping capability, with higher incorporation and agglomeration phe-nomena that result in non-uniform microhardness across the thickness. Conversely, the slower growth of HP nanocomposites coatings (approximate to 20 mu m/h) relates to lower but uniform incorporation and good dispersion of the reinforcing nanoparticles. Effective dispersion strengthening was observed for nanoparticles concentration up to 13.5 g/l. Microhardness increase by >25 % was achieved for both MP and HP coatings. The combined effect of nanoparticles incorporation level and their agglomeration was systematically studied and a mathematical model was implemented. It was demonstrated that strengthening effectiveness depends on both the amount of embedded nanoparticles and the mean size of agglomerates, following a bi-liner relation that reliably predicts experimental microhardness. Potentiodynamic corrosion test revealed that introduction of ZrO2 nanoparticles enhance corrosion resistance of both MP and HP coatings. The presented hardening strategy for Ni-P coatings can be an efficient solution for midstream and downstream applications in the oil & gas industry, in order to increase service-life of components in contact with both wearing and corrosive media.