Enhanced galvanic corrosion protection of nickel-phosphorus-coated magnesium alloy by an ambient-pressure-prepared LDH primer

Deposition of a nickel-phosphorus (Ni-P) coating with excellent galvanic corrosion protection and robust adhesion onto magnesium (Mg) alloy via an environmentally-friendly pretreatment remains a challenge. This issue is addressed in this study by the in situ preparation of a phosphate anion-modified layered double hydroxide (LDH-P) film at ambient pressure throughout the process. The LDH-P layer, when sufficiently thick, efficiently separates the subsequent Ni-P alloy from the magnesium alloy, thereby electrically isolating the coating from the substrate. Moreover, phosphate anions intercalated in the LDH-P interlayer can be released via ion exchange with corrosive anions, acting as corrosion inhibitors and effectively extending the penetration time of the corrosive electrolyte, thereby decreasing the risk of galvanic corrosion. The Ni-P coating with LDH-P pretreatment (LDH-P/Ni-P) exhibited a high low-frequency impedance modulus, a low corrosion current density ( j corr , 0.480 μA cm −2 ), and a positive corrosion potential ( E corr , −399.0 mV), and remained macroscopically intact after 28 days of exposure in 3.5 wt .% NaCl solution or 240 h of neutral salt spray exposure. Furthermore, the specific surface configuration with innumerable cavities and vertically oriented nanosheets provides abundant mechanical interlocking sites for the nucleation and growth of Ni grains, thereby endowing the Ni-P coating with good adhesion. These results demonstrate that the smart and eco-friendly LDH-P primer, developed under ambient pressure, offers a promising strategy for deposition of Ni-P coatings on Mg alloys with good adhesion and durable corrosion protection.