Corrosion behavior of Ni-P-nano-Al₂O₃ composite coating in the presence of anionic and cationic surfactants

Ni-P-nano-Al₂O₃ composite coatings on the mild steel were obtained by adding Al₂O₃ nanoparticles into the traditional electroless Ni-P bath. Cationic and anionic surfactants were used to prevent the agglomeration of the incorporated nano-Al₂O₃. Surface morphology, compositions and microstructures of the coating were studied using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) as well. Corrosion resistances of the modified mild steel samples were evaluated in 3.5 wt.% NaCl solution by electrochemical impedance spectroscopy (EIS). The results showed that compared to the pure Ni-P coating on the mild steel, the coating formed in the bath with 6 g/L nano-Al₂O₃ performed the highest resistance. Under the optimal conditions, the adding of anionic surfactant (SDBS) improved the anti-corrosion performance of the samples further, while the addition of cationic surfactant led to a worse corrosion resistance. The potential mechanism was that anionic surfactant molecules would contact with active sites of Ni-P composite coating and nano-Al₂O₃ in the bath, which would improve the dispersion of nano-Al₂O₃, thus leading to a homogeneous coating with better corrosion resistance.