Research On Pretreatment Process Of Electroless Nickel Plating On Magnesium Alloys

Lightweight material is a necessary trend in the development of modern materialscience. Magnesium alloy is a typical lightweight alloy that found with suchadvantages as lightweight, high strength, strong electromagnetic shielding, goodconductivity, as well as easy recycling，which bring great applications in the field ofelectronics, cars and aerospace. However, poor corrosion is a bottleneck thathampered the scale applications of magnesium alloy. Surface treatment of magnesiumalloy is the primary means of improving the corrosion resistance, electroless nickelplating, of which the resulting coating share a uniform thickness, good adhesion,excellent corrosion resistance and other characteristics, has become a hot in thesurface treatment of research and application of magnesium alloy in recent years.After decades of scholars’efforts, two processes—direct electroless nickel plating anddipping zinc have been developed for the surface treatment of magnesium alloy,which greatly promoted the development and application of magnesium alloy.However, the use of strong carcinogenic hexavalent chromium compounds and easyvolatile hydrofluoric acid in its pre-treatment process is unfavorable forenvironmental protection. What’s more, direct electroless nickel plating generally leadpoor adhesion to fail the purpose of protection, while dipping zinc ease the immersionof much Zn2+into the electroless nickel solution lead to deteriorated performance andgreatly reduced bath life as well as affected performance of the nickel plating layer.To solve this problem, we studied every step in pre-treatment process of reportedelectroless nickel plating procedures of magnesium alloys to find out a citric acidpickling procedure to replace the traditional chromic acid pickling. And throughanalysis, improvement and optimization of the pre-treatment process’ activation anddipping zinc subscription, we set a sound experimental basis for the exploration ofenvironment-friendly, bath-damage-free pre-treatment processes.We developed a citric acid based pickling solution, by SEM we observed thechange of surface morphology after pickling, and found that addition of a smallamount of NaNO3and corrosion inhibitors KF can lead an uniform surface corrosion,which not only avoided the use of strong carcinogen but also diminished the impact ofash hanging in pickling process. The influences of F-concentration in alkali-activated solution was also learned and by observing surface morphology after base activationand through analysis of binding capacity of dipped zinc coating, we determine theconcentration range of F-. The effects of NaH2PO2and NiSO4in zinc-dipping solutionon zinc coating properties were also investigated, and found to significantly improvethe uniformity and density, zinc coating layer corrosion potential shiftedapproximately0.06V positively with compare to traditional liquid.We came out with a new pre-treatment process named "acid activation–zincdipping", identified the operating parameters of alkali activation process anddetermined the dip zinc liquid formulations. The process makes use of zinc andfollowing zinc-leaching procedure to avoid the Zn2+pollution of nickel platingsolution effectively. After thermal shock at220℃and48hours of neutral salt spraytest, the results indicated that through the developed pre-treatment process we can getnickel-plated coating good adhesion and corrosion resistance.