Preparation of corrosion-resistant MgAl-LDH/Ni composite coating on Mg alloy AZ31B

Abstract

Compared to single coatings, multiple-layer coatings or composite coatings could provide better and more durable corrosion protection to magnesium alloys. In this paper, a MgAl-LDH/Ni composite coating was successfully prepared on magnesium alloy AZ31B via preparation of a LDH primer, an electroless-deposited Ni-B (ENB) intermediate layer, and an electroless-deposited Ni-P(ENP) top layer. The surface morphologies, chemical compositions, and corrosion resistance of the composite coating were characterized and discussed. The results showed that nano-scale Ag particles acting as the activation sites were introduced onto the surface of the LDH layer after the activation process. Ni particles were deposited in the areas where Ag particles exist, and an intact Ni-B layer formed after 15 min of ENB plating. The total thickness of the composite coating is about 38 µm, in which the LDH layer is ~13 µm, and the EN coating is ~25 µm. The composite coating does not show any perforation after exposure to a 3.5 wt% NaCl solution for 210 h or 0.1 mol·dm−3 HCl solution for 30 h, attributed to the amorphous structure of the electroless-deposited nickel layer and the P-rich layer formed at the surface to hamper nickel oxidation. In comparison, obvious corrosion holes appeared on the LDH coating after immersion in a 3.5 wt% NaCl solution for 20 h or in a 0.1 mol·dm−3 HCl solution for 30 min. Furthermore, the EIS test results confirmed the durable corrosion protection of the composite coating to the magnesium alloy substrate.