Corrosion Resistance Improvement of Ni-P Coatings: Surface Cracks with Different Temperatures Repaired by Laser Remelting

Amorphous nickel-phosphorus (Ni-P) coatings formed by electrochemical deposition (ECD) show high corrosion resistance, but surface microcracks limit their further application. In this study, surface microcracks in Ni-P coatings prepared by ECD were removed by picosecond-laser remelting (LR) at different initial coating temperatures (−20, 25, and 60 °C). The effects of LR at different initial temperatures on the surface quality, microstructure, and electrochemical corrosion behavior of the Ni-P coatings were analyzed in detail. Compared with that of the raw coating after ECD, the surface roughness increased after LR. As the initial temperature during LR increased, the surface defects and roughness of the coating decreased. It was found that LR induced crystallization of the coating, resulting in the surface phase changing from Ni/Ni-P to Ni-PO_x compounds. The corrosion rate of the coatings decreased by 2.7-3.9 times after LR, which was attributed to the microcrack closure. In addition, stress was replaced by pitting as the coating corrosion mechanism because of the surface morphology evolution induced by LR. This study provides specific guidance for effective LR surface modification at different ambient temperatures to improve the corrosion resistance of Ni-P coatings.