Study On Direct Electroless Ni-P Alloy Plating On Magnesium Alloy Into Neutral Plating Bath And Properties Of Plating Layer

Magnesium and magnesium alloy(such as AZ91D)had low density,high strength,good size stability and good magnetic shield,which became the most hopeful material in modern high-tech fields.Its performance was superior to aluminum alloy.Because of its mechanical properties and physical properties, it was widely used in aerospace,automotive industry,electronics industry and so on.However,poor corrosion resistance and wear resistance of magnesium and magnesium alloy constraint its use.How to solve this problem,had been a subject of concern to people all the time.Chemical nickel plating was widely used in recent years as a surface treatment methods.Its coating had the benefits of tight structure,uniform thickness,hardness,good corrosion resistance and good wear resistance.However,in the processes of chemical nickel on magnesium alloy,acidic or alkaline solution would cause serious corrosion to magnesium alloy matrix,impacting on the performance of coating directly.So how to reduce corrosion of solution to matrix was also a very important research direction.However,the neutral chemical bath which had small corrosions to magnesium matrix are not widely reported.In this paper,we studied the magnesium alloy electroless Ni-P process in the PH value of 6.5 to 7 neutral bath,the coating performance and structure,and we added none of the CrO₃ to pre-treatment bath,not producing Cr⁶⁺which was harmful to human body and reducing the pressure on the environment.We had studied the pretreatment on the chromium-free and neutral in the chemical bath Ni-P alloy plating process and use of metallographic microscope and Axiovert25CA(Zeiss)optical image to analyize and observe the coating surface topography and microstructure,finding that the coating surface with the ball cellular accumulation,cellular and cellular combine closely,every large cell was consitiuted of several deformed small cells.From the WS-97-AE-scratch coating for scratch test of the combination of strength,we could see that the matrix of magnesium alloy plating on the chemical coating of Ni-P FC approximately 40-60 N,coating and the matrix combining well.Using electronic weighing scales and calculate plating rate,we found main salt concentration,reductant concentration chelators and temperature would affect the coating deposition rate.Useing of GDS(glow discharge spectrometer)to determine the coating composition and distribution while using of X-ray diffraction(XRD)detect Structures,we came to the conclusion that the phosphorus content in the Coating is in the range of 9.5%to 11%,X-ray diffraction analysis curve was like steamed bread,possessing diffraction pattern of the typical of amorphous alloy,showing that the coating was amorphous structure. By wear tester we found the friction coefficient of Ni-P coating and Steel ball's mutual frictions was larger than magnesium alloy AZ91D.After tempering coating hardness reduce,friction coefficient reduced more.Wear track morphology indicate that under the conditions of the dry friction the carrying capacity of magnesium alloy matrix was poor,friction was more serious,trace of friction was wider.Furthermore various small spalling and a large number of parallel plow ditch.After chemical nickel phosphorus,there were slightly more pittings trace of friction is shallow and narrow,plow ditch shallow and small, wear resistance had increased.After tempering 400℃,plow ditch and loss of adhesion was not obvious, only minor abrasions,showing after tempering the wear resistance of Ni-P coating had increased.Electrochemical tests showed that the sample of magnesium alloy which was done after chemical nickel processing whose corrosion resistance has been greatly improved.Nickel layer of corrosion potential was-615,which had increase by 843 mv compared with magnesium alloys.Compared with the original coating,after tempering heat Treatment,the corrosion potential at first rise after the sharp decline characteristics.After tempering at 200℃,the corrosion potential increase,Passivation was also very clear,corrosion resistance has strengthened.Aftertempering at 300℃and 400℃the corrosion potential decline,corrosion resistance significantly weakened。Salt spray test found that magnesium alloy of nickel plating chemical spray for eight hours with no significant corrosion spots,well corrosion resistance.When the spray time was up to 20 hours,Corrosion appeared in local coating,especially in the edge of the sample,a lot of corrosion product like white cotton appeared.