Study On Nanocrystallization And Corrosion Mechanism Of Electroless Amorphous Ni-W-P Coatings

Using furnace and laser as heat sources to anneal the coatings, microstructures,hardness and corrosion resisitance of the coatings annealed by two different methods arecompared. Degree of crystallization, grain size and lattice strain are evaluated by XRDquantitative analysis technique, The corrosion resisitance of coatings before and afterannealing were examined by immersion and potentiodynamic polarization tests, andmicrohardness was measured by sclerometer. The surface topographies of coatingsbefore and after corrosion were observed by Scanning Electron microscope （SEM） andoptical microscope.The results show that the Ni-3.88%W-13.36%P deposit is amorphous. The ratio oflattice strain and volume fraction of amorphous phase are decreased with the increase intemperature and the decrease in laser scanning velocity, but relationship of whether thevolume fraction or grain size between Ni₃P,and Ni is reversely varied. The crystallizingreaction of Ni₃P occurs at400℃, and its grain size is larger than that of Ni phase. Whenthe temperature exceed500℃, the characteristic of grain sizes of two phases isreverse.The grain size of Ni₃P phase is always bigger than that of Ni phase during laserannealing. The grain sizes of both Ni₃P and Ni in coatings annealed by furnace and laserare kept in nano-scale. Microstructure characteristic of the coatings annealed by laser isintervenient that of the coatings annealed by furnace at400～500℃. The microhardnessincreases with the increase in temperature, and obtains maximum at500℃, declinewhen temperature is over500℃. The microhardness decreases with increase in laserscanning velocity, and achieves maximum at8mm/s. The range of microhardnessannealed by laser is equivalent to the range of microhardness annealed by furnacebetween300℃and400℃. The phenomenon of the shift in microhardness to lowervalue is due to overlap region of the laser scanning. Uniform corrosion characteristicwas present to all coatings, whether as-plated or annealed by furnace or laser. Thecorrosion resistance of as-plated coating is poorer because of larger lattice strain. Theincrease in corrosion resistance after annealing by furnace and laser is attributed to, onthe one hand, the decrease in lattice strain, on the other hand, the variation of grain sizeand degree of crystallization in coatings. Besides, the porosity of the coatings canaccelerate the uniform corrosion rate. The corrosion resistance of coatings annealed bylaser is higher than that of coatings annealed by furnace at400～500℃, and equivalentto that of the coatings annealed at500～600℃, the reason is that the number of Ni₃P phase in the coatings annealed by laser is more than that of Ni phase, accordinglydecreases the number of the corrosion micro-cell consisted of Ni₃P and Ni phase, andthe corrosion resistance is improved.