Preparation Of Ni-Mo Coating By Pulse Electrodeposition And Its Tribology And Corrosion Behavior

In this paper,Ni-Mo coating was prepared by unidirectional pulse electrodeposition.Orthogonal test was used to optimize the process parameters.Based on the optimized combination of the parameters obtained by orthogonal test,Ni-Mo coatings with different current density were prepared.The microstructure and phase structure of the coating was analyzed and characterized by SEM,EDS,XRD and laser confocal(3D)microscope.The friction wear tester and electrochemical workstation were used to study the sliding wear behavior and electrochemical corrosion behavior of the coatings,and the effect of electrodeposition current density on the wear resistance and electrochemical corrosion resistance of the coating were discussed,the regularity similarly.(1)In the orthogonal test,the microhardness of the coating was taken as an index,and the average current density,the time,the temperature and the content of sodium molybdate were chosen as the main factors.The results of the orthogonal test showed that the average current density had the most significant impact on the microhardness of the coatings,followed by time,then the content of sodium molybdate and temperature.The optimized combination of the parameters is average current density of 6 A/dm~2,sodium molybdate content of 4g/L,temperature of 30?and time of 45 min.Based on the process parameters,the Ni-Mo coatings were prepared by electrodeposition under current density of 2A/dm~2,4A/dm~2 and 6A/dm~2,respectively.The microstructure and composition of the coating was analyzed by SEM and EDS.It was found that the content of Mo increased with the increase of electrodeposition current density,and the bract characteristics of the coating tissue became more and more obvious.At the same time,the thickness of the coating increased with the increase of the current density of the electrodeposition.The XRD map showed that the main phase of the coatings was Ni-Mo solid solution,which grew all along(111)crystal surface.With the increase of electrodeposition current density,the grain size became smaller,but the increase of electrodeposition current density also made the surface roughness of the coating worse.Hardness test showed that the hardness of the coating increased with the increase of current density,and the microhardness of the three coatings reached 216.550HV,453.125HV and 512.450HV respectively.(2)A sliding friction and wear test was carried out on the deposited coating,and the morphology of the wear scar was observed by SEM and EDS compone nt analysis.The results showed that the wear mechanism of the three coatings was different.When the current density was 2A/dm~2,the wear mechanism of the coating was mainly adhesive wear,while there was plastic deformation and oxidation wear.When 4A/dm~2,the wear mechanism of the coating was mainly abrasive wear and accompanied with slight adhesive wear and oxidation wear.When 6A/dm~2,the wear mechanism of the coating was still abrasive wear,which accompanied by partial adhesive wear and oxidation wear,and even slight fatigue wear occurred.Therefore,the wear mechanism of the coating changed from adhesive wear to abrasive wear in general,as the current density of electrodeposition increased.(3)Using the 50g/L sulfuric acid solution,the 50g/L sodium hydroxide solution,and the 35g/L Sodium Chloride solution as the electrochemical corrosion medium,the potentiodynamic polarization curve and the impedance spectrum of the coating were measured respectively.The conclusion was that in the sulfuric acid solution of 50g/L,the passivation stability of the Ni-Mo coating decreased with the increase of electrodeposition current density,and local corrosion resistance got worse.Conversely the corrosion rate of the coating and its passivation film both decreased,so the corrosion resistance was improved totaly.In the sodium hydroxide solution of 50g/L,the coating had self-passivation ability.Although with the increase of electrodeposition current density,the corrosion rate of the coating increased,the alkali resistance of the coating was still good.In the 35g/L Sodium Chloride Solution,the self-passivation and pitting corrosion occurred in the coating.With increase of electrodeposition current density,the corrosion rate of the coating increased.The corrosion resistance decreased,but the pitting corrosion resistance increased.The above research results indicate that Ni-Mo coating under suitable technological conditions has potential application in the field of surface protection for copper and copper alloy.