Double-layered Ni-P/Ni-P-ZrO2 electroless coatings on AZ31 magnesium alloy with improved corrosion resistance

• Electroless Ni-P/Ni-P-ZrO2 coatings have been coated on AZ31 substrate.
• Utilize ZrO2 sol to co-deposit ZrO2 nanoparticle in Ni-P-ZrO2 coating.
• Hardness of coating significantly improved due to Ni-P-ZrO2 layer.
• Corrosion resistance improved greatly due to double-layered coating structure.

The double-layered electroless deposition of Ni-P/Ni-P-XZrO2 coatings (where X refers to the amount of ZrO2 sol (mL/L) added to the plating solution) on AZ31 magnesium alloy was investigated. The inner Ni-P layer contained high phosphorus content while the outer layer was a sol-enhanced Ni-P-ZrO2 composite coating with a lower phosphorus content. The morphology, phosphorus content, hardness and corrosion resistance of the coating were investigated. The results show that the corrosion resistance of coating has been improved greatly due to the high phosphorous inner coating layer. The corrosion potential moves positively to more than 1000 mV, and the corrosion current density is reduced by one order of magnitude. Double-layered Ni-P/Ni-P-XZrO2 coatings can resists salt spray test for more than 480 h. In addition, the hardness of coating is improved significantly from 640 HV to 820 HV due to the sol-enhanced nano ZrO2 co-deposited in the outer layer.