High corrosion resistance of electroless Ni/Ni-B coating from fluoride-free baths on AZ31 magnesium alloy

Electroless nickel/nickel-boron (Ni/Ni-B), Ni/alkaline Ni-P and Ni/acidic Ni-P double-layer coatings were deposited on AZ31 magnesium (Mg) alloy prepared by environmental fluoride-free solution under low temperatures. The morphologies, compositions, and crystal structures of three different double-layer coatings were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistance of the different double-layer coatings on Mg alloy surface was studied by electrochemical polarization and electrochemical impedance spectroscopic (EIS) tests, which indicated that the Ni/Ni-B coating presented the more superior corrosion resistance than the Ni/alkaline Ni-P and Ni/acidic Ni-P coatings. Meanwhile, the Ni/Ni-B coating exhibited good repeatability through the electrochemical polarization tests for three Ni/Ni-B samples that prepared under the same processing conditions. In addition, the optimal plating time for the Ni-B coating was further determined by electrochemical polarization tests of samples with different plating times.