Electroless deposition and characterization of a double-layered Ni-B/Ni-P coating on AZ91D Mg alloy from eco-friendly fluoride-free baths

Electroless Ni deposition has attracted much interest in surface modification of Mg alloy owing to its good performance, especially in corrosion inhibition. In this work, an electroless double-layered Ni-B/Ni-P coating on AZ91D Mg alloy was fabricated successfully by plating of a Ni-B layer as primer and a Ni-P layer as outer layer from eco-friendly fluoride-free plating baths. The morphology, composition, and phase structure of the coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer (XRD), respectively. Several different electrochemical techniques such as linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopies (EIS) were used to analyze the deposition process and demonstrate the enhancement of corrosion inhibition of the Ni-B/Ni-P duplex coating. In addition, compared with Mg alloy and a single Ni-P coating, the new-designed duplex coating also exhibits a remarkable enhancement in micro-hardness. [Display omitted] •An electroless double-layered Ni-B/Ni-P coating on AZ91D Mg alloy is prepared.•The plating baths for both inner and outer layers are environment-friendly.•The deposition process involves displacement and auto-catalytic reactions.•The coating exhibits a remarkable enhancement in micro-hardness.