A nickel-underlayer/LDH-midlayer/siloxane-toplayer composite coating for inhibiting galvanic corrosion between Ni layer and Mg alloy

To obtain high corrosion protection and break the bottleneck arising from galvanic corrosion between Ni layer and Mg alloy substrate, we report herein a triple-layer composite coating approach, consisting of Ni (ENP) underlayer, NiAl-layered double hydroxide (LDH) middle layer, and silane (PFDTMS) toplayer, i.e., ENP/LDH/PFDTMS. This nanocomposite coating was prepared on the Mg alloy surface by a combination of simple electroless plating, hydrothermal-assisted in situ growth, and condensation reaction. The surface morphology, structure, and chemical composition of the resulting composite coating are characterized using several techniques, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The corrosion protection performance of the composite coating in a saline solution is evaluated by electrochemical impedance spectroscopy (EIS), Tafel curve, and immersion measurements. The results indicated that the ENP/LDH/PFDTMS composite coating possesses a much higher corrosion resistance than the single Ni coating and the LDH-grown Ni (ENP/LDH) coating. In contrast to the severe galvanic corrosion the Mg alloy with single Ni coating observed in two days of exposure, the substrate with either ENP/LDH/PFDTMS coating or ENP/LDH coating is shown to exbibit no galvanic corrosion phenomenon even after 30 days of exposure. Moreover, the result from tests including contact angle measurement of the ENP/DLH/PFDTMS coating demonstrates the superhydrophobic function to several typical liquids in daily life and the good self-cleaning capability after contamination by graphite powder. These findings provide a fresh insight into the design of advanced coatings for inhibiting galvanic corrosion between Ni layer and Mg alloy.