Evaluation of microstructures and properties of laser-annealed electroless Ni-P/Ni-Mo-P duplex coatings

Duplex Ni-P (inner)/Ni-Mo-P (outer) coatings on carbon steel were prepared by electroless deposition using dual baths, and then subjected to laser annealing to achieve crystallization. The microstructures in terms of degrees of crystallization and crystalline sizes were characterized using a quantitative XRD method. The evaluation of the properties of the Ni-P/Ni-Mo-P coatings, including hardness and corrosion behavior, was carried out using nanoindentation, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 10% HCl solution. It was found that the microstructures of the duplex coatings laser-annealed were determined by the laser scanning velocities when the power and beam dimension of the laser beam were kept constant. The hardness of the duplex coatings was dependent on the crystallization degree, relative sizes between Ni3P and Ni/Ni-Mo phases. The results of the electrochemical measurements revealed that the duplex coatings provided better corrosion resistance than the single Ni-P coating. The observation of the corrosion morphologies by SEM/EDS showed that the duplex coatings exhibited a combined corrosion attack of uniform corrosion and pitting corrosion. A loss of protection ability of the duplex coatings to substrate was found when the pitting corrosion reached the steel surface, leading to spallation of the coatings. The best corrosion protection of the duplex coatings was achieved when the coatings were laser-annealed at the scanning velocity of 14 mm/s.