The Research On Electroless Nickel Plating Process Of Magnesium Alloy

Magnesium alloys can be widely used in aerospace,automotive,electronics and other fields, but the corrosion potential of magnesium alloys is extremely low, greatly limited the application of magnesium alloys. The problem can be solved by electrolss nickel plating.Corrosion factors of magnesium alloys in plating solution which is the simplest electroless plating nickel formulation was studied by the method of corrosion weight loss.The results show that corrosion speed of magnesium alloys is fast which is influenced by pH value, temperature, sulfate radical and nickel radical,but corrosion speed of magnesium alloys is slow when it is immersed into distilled water or sodium hypophosphite solution.So,three processes of electroless nickel plating on magnesium alloys directly were studied in this paper.Pretreatment of magnesium alloys which is the first process were studied, acid pickling formula and activation formula were defined by contrast experiment.The properties of electroless plating Ni-P were detected by SEM, XRD and electrochemical polarization curves.The results indicated that the Ni-P coating is very meticulous and uniform,and the structure of Ni-P coating is amorphous,electrochemical polarization test indicated that corrosion potential of magnesium alloy was improved about 1.1V,and corrosion current density dropped obviously.Electroless plating nickel solution using nickel hydroxide as main salt was studied which is the second poccess.The best formula was defined by single factor experiment and orthogonal experiment.The beginning growth procedure of electroless plating nickel was tested and analyzed by SEM and EDS,procedure of growth can be divided into two stages,the first stage is exchange reaction between nickel ions and magnesium,the average activation energy is about 103.3KJ·mol-1;the second stage is Ni-P co-deposition reaction,the activation energy is about 78.3KJ·mol-1.The adhesion of magnesium alloys and Ni-P coating is perfect which tested by file test and thermal shock test.The number of MTO can be up to two when nickel hydroxide dissolved in lactic acid,and the number of MTO can be up to four when nickel hydroxide dissolved in hydrofluoric acid.The process of electrolss nickel plating which inhibitors were added into plating solution of nickel sulfate as main salt was studied which is the third poccess.The efficiency of inhibitors was evaluated by corrosion efficiency,the best formula was defined by orthogonal experiment.The beginning growth procedure of electrolss nickel plating was tested and analyzed by SEM and EDS.The results shown that magnesium alloys was protected by inhibitors,at the same time,exchange reaction was happen between nickel ions and magnesium alloys at the beginning of electrolss nickel plating. when magnesium alloys and inhibitors were isolated by coating,inhibitors will not be working for magnesium alloys,but Ni-P co-deposition can be go on continuously.