Crystallization Characteristics And Corrossion Behavior Of Electroless Ni-Mo-P And Ni-P Alloy Coatings

Due to its excellent performance,electroless plating has been widely applied in production,and whether it can be adopt to the harsh environment,especially corrosive environment,also has got widespread attention.This paper was prepared by chemical deposition of Ni-P and Ni-Mo-P alloy coating,a comparative study after the furnace annealing and laser annealing,crystallization of its organizational characteristics and corrosion behavior of alloy coatings of different composition.Using SEM/EDS,optical microscopy and optical interferometry,the coating composition were measured,the surface morphology observed and roughness analyzed;By the XRD,the diffraction pattern of platings before and after heat treatment were obtained.By using Jade to calculate and proposed merger,the coating microscopic structure of organizations,which including the degree of crystallization,crystallization phase grain size and relative content,were quantitatively analyzed;combining corrosion test weight loss,potentiodynamic polarization and electrochemical impedance spectroscopy,the corrosion behavior and corrosion resistance of the coating were studied,the main conclusions are as below:The Ni-9.19P?wt%?and Ni-4.63Mo-12.39P?wt%?coatings are amorphous structures,and the Ni-4.11Mo-6.5P?wt%?is mixed crystal structure.After the furnace annealing at temperatures below 400 ?,Ni-4.11Mo-6.5P only had Ni alloy coating phase crystallization;at 400 ? or exceeds 400 ?,Ni3 P crystallization reaction and the formation of Ni-Mo solid solution appeared;crystallization degree reached 88.13% at 600 ?.When the heat treatment temperature of 300 ?,the Ni-P alloy coating begin precipitated Ni3 P phase,when the temperature reached 600 ?,the crystallization degree of 91%.Under the conditions of the same heat treatment temperature,Ni-Mo-P alloy coating grain size is less than Ni-P alloy coating.When the scanning speed in the laser annealing process is higher than 10 mm/s,Ni-4.63Mo-12.39 P coating only had Ni phase crystallization,the scanning speed is lower than or equal to 10 mm/s,the coating had Ni3 P phase precipitated,and simultaneously Ni/Ni-Mo phase crystallized,grain size of Ni3 P precipitation phase is always greater than that of Ni/Ni-Mo phase size.When scanning speed of 6 mm/s,Ni-Mo-P plating layer crystallization degree of 82.11%.For Ni-P coatings,the change of crystallization processes which follow scanning speed,except Ni-Mo phase,all are similar to the Ni-Mo-P coating,when the scanning speed of 6mm/s,the degree of crystallization of Ni-P reaches was 84.85 %.The changes in grain size of each phase of Ni-P coating was under scanning speed from 10mm/s to 8mm/s,the grain size of Ni3 P phase is larger than the size of Ni phase,but when the scanning speed of 6mm/s,in turn,grain size of Ni phase is greater than that of Ni3 P phase.In 0.5 M H₂SO₄ solution,the Ni-Mo-P and Ni-P coatings after furnace annealing or laser annealing showed uniform corrosion in the form of corrosion.The corrosion resistance of the coatings mainly depend on microscopic structural characteristics after annealing,such as the formation of Ni/Ni-Mo phase,the size and relative quantity of Ni3 P phase.For the Ni-Mo-P plating,the corrosion mechanism is mainly related to that Mo is dissolved from the Ni-Mo solid solution in H₂SO₄ electrolyte solution.All coatings which had occurred justly Ni3 P phase crystallization reaction had the worst corrosion resistance,further increase the annealing temperature or decrease the scanning speed,the coating corrosion resistance is improved,the Ni-Mo-P coating corrosion resistance is better than that of Ni-P coating.In the HCl and NaCl solution which contained Cl-,regardless of furnace annealing or laser annealing,the forms of Ni-Mo-P and Ni-P plating corrosion are both pitting corrosion,the corrosion resistance of coating will depend mainly on its surface morphology state,the size and dense of the“nodular structure”,and the pores in the coating.After 600 ? high temperature annealing,the Ni-4.11Mo-6.5P alloy coating exhibits excellent corrosion resistance in 10% HCl solution;the EIS study in 3.5% NaCl solution showed that,at the same scanning speed conditions Ni-4.63Mo-12.39 P has a greater charge transfer resistance than Ni-P plating coating,when the scanning speed of 12 mm/s,the best pitting corrosion resistance ability been got.