The Deposition Process and the Properties of Direct Electroless Nickel-Phosphorous Coating with Chromium-Free Phosphate Pickling Pretreatment on AZ31 Magnesium Alloy

摘要

A direct electroless nickel-phosphorous (Ni-P) coating was obtained on AZ31 magnesium alloy afterdipping in phosphate pickling bath. The deposition process and the properties of Ni-P coating onmagnesium alloy were investigated by measuring open potential-time curves in situ and polarizationcurves in combination with X-ray fluorescene spectrometry, X-ray diffractometer and scanningelectron microscopy. The results showed that after phosphate pickling pretreatment, the Mg-Mnphosphate film formed on magnesium alloy surface made the electrode potential of magnesium alloysshift to the positive direction. The Ni-P coating was compact, uniformly covered the magnesium alloysurface, with high phosphorus content (for about 10.3 wt%). The hardness of electroless Ni-P coatingfor 60 min was 552.4 HV, which is far higher than that of the Mg alloy substrate. The corrosionpotential of electroless Ni-P coating for 60 min was -0.323 V vs. SCE, and shifted to the positivedirection by 1198 mV, as compared to magnesium alloy substrate. The corrosion resistance of themagnesium alloy could be greatly increased by the formations of Mg-Mn phosphate film andelectroless Ni-P coating.