| Because magnesium alloys have outstanding properties such as high strength and stiffness to weight ratio, high damping capacity and good electromagnetic shielding effectiveness, they are extensively applicated in automotive, electronics and aerospace industries, et al. However, they have high chemical affinity to form unprotective oxide film or hydroxide film on their surface in water. This restrains their widespread applications. In order to improve the corrosion resistance of magnesium alloys, it is necessary to apply special surface modification treatments. Currently, a number of methods were used to prevent corrosion of magnesium alloys, such as anodizing, electroless nickel plating, electroplating, chemical conversion coatings and organic coatings. However, among these various treatments, chemical conversion coatings-organic coatings are the most used technique. Recently, metal coatings, especially electroless nickel plating layers have become a research focus because of extensive demand.Currently, most of electroless nickel phosphorus platings on magnesium alloys have following disadvantages. First, it has been suffering from corrosion issues caused by acidic electroless bath. Second, in order to increase the adhesion strength of Ni-P coating to the Mg substrate, the pretreatments for electroless plating employ hydrofluoric acid (HF) or hexavalent chromium (Cr6+) which can cause pollution and health problems. In this paper, a HF-free and alkaline pre-electroless bath and Ni-P deposition process for AZ91D magnesium alloys which avoided the use of HF were developed.And environment-friendly molybdate conversion coatings were employed as the pretreatment for pre-electroless Ni-P plating to replace conventional chromium conversion coatings.Molybdate conversion coatings coated AZ91D Mg was immersed in pre-electroless nickel bath,as a result,obtaining Ni-P coatings which show good adhesion and corrosion resistance. Moreover, it is feasible for other metals to further plate on pre-electroless plated Mg.EIS, Tafel polarization, immersion and evolved hydrogen volume measurements were used to characterize corrosion resistance of molybdate conversion coatings on AZ91D Mg. SEM and XRD were employed to observe micromorphology and study composition of conversion coatings, respectively. The conditions to prepare molybdate conversion coatings on AZ91D Mg were determined. AZ91D Mg coated in40g/L concentration of NaMoO4·2H2O solutions at pH5.9adjusted by HNO3at60℃for lh showed best corrosion resistance. Conversion coatings are composed of Mg2Mo3O8and a small amount of MgMoO4. Polarization measurements indicated that the corrosion current density and corrosion depth per year of Mo-coated Mg is decreased by two orders of magnitude and27times when compared to that of bare Mg, respectively. EIS, immersion and hydrogen volume evolved measurements also indicated that molybdate conversion coatings could provide good protection for Mg.Effects of bath composition, bath temperature and bath pH on the deposition rate of nickel were studied by the mass gain method. As a result, an optimum pre-electroless bath was determined and an empirical equation was formulated to predict the Ni-P deposition rate. Micromorphology and cross-section micromorphology of Ni-P coated Mg exhibited a classic nodular cauliflower-like structure and there were no pores and cracks in the interface between Ni-P coatings and the substrate. EDS analysis indicated that Ni-P coatings contained5.69wt%phosphorus. Tafel polarization measurements indicated that the corrosion potential of Ni-P coated sample was shifted positively804mV compared to that of AZ91D Mg. EIS measurements indicated that the charge transfer resistance of Ni-P coatings coated samples increased by two orders of magnetitude compared to that of bare AZ91D Mg. EIS and Tafel polarization measurements showed that corrosion resistance was improved after pre-electroless Ni plating layers were applied on AZ91DMg. |
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