High corrosion protection performance of the LDH/Ni-P composite coating on AM60B magnesium alloy

Electroless nickel-phosphorus deposit was applied on the AM60B alloy using the LDH coating as underlayer. The SEM micrographs indicated that the magnesium alloy was coated by the LDH film which showed flake-like morphology by increasing the hydrothermal processing time from 2 to 4 and 8 h. A uniform, compact, and non-porous Ni-P coating with cauliflower nodular morphology was obtained on the LDH underlayer. Formation of a regular nodular structure is ascribed to the formation of infinite number of Ni nuclei on the entire surface of the LDH-treated sample. Based on the AFM analysis, the Ni-P coating showed low surface roughness (R-a = 199.7 nm) due to the balanced growth of the nickel nodules in a direction perpendicular to the surface. The electroless coating had mixed amorphous-crystalline microstructure due to the medium phosphorus content (9.1 wt%) which was confirmed by the EDS and XRD methods. According to the EIS results, the corrosion resistance of the LDH layer enhanced from 2 to 32 k Omega cm(2), when the treatment time extended from 2 to 8 h. The PDP results also showed that the sample treated for 8 h had a corrosion current density (J(corr)) of approximately one seventh of the sample obtained after 2 h of treatment. Corrosion protection of the LDH film was attributed to its barrier properties and ion exchange capacity. Also, the Ni-P coating on the LDH underlayer showed excellent corrosion resistance during 6 h testing in 3.5 wt% NaCl, so that the polarization resistance value was about 93 k Omega cm(2) at the end of the immersion. Moreover, pore-free structure of the electroless coating was confirmed by the porosity test. The Ni-P coating showed suitable micro-hardness value (about 554 HV) and the thermal shock test confirmed its good adhesion.