Strontium phosphate conversion coating as an economical and environmentally-friendly pretreatment for electroless plating on AM60B magnesium alloy

Corrosion resistance of the magnesium alloys must be improved before any outdoor applications by an appropriate technology such as electroless plating. In this study, application of Ni-P electroless coating on AM60B magnesium alloy via strontium phosphate (Sr-P) conversion coating as economical and complete environmentally-friendly pretreatment has been investigated. Morphology, composition, microstructure, and corrosion behavior of the coatings were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and electrochemical techniques respectively. After conversion coating process, flake-like crystals were formed on some parts of the substrate while the remaining areas were covered by an amorphous, composite and porous film. After immersion in the Ni-P bath, nickel nuclei were mainly formed through the pores of the conversion coating at initial plating times and then were grown in the horizontal and vertical directions to cover whole substrate. Uniform and pore-free electroless coatings with compact grain boundaries were formed on the 2 and 5 min Sr-P treated samples while the coating on the 15 min treated sample showed granular structure with obvious inter-granular space. All of the coatings showed mixed crystalline-amorphous structure with moderate amounts of the phosphorous. The amount of the phosphorous increased with decreasing the time of the Sr-P treatment led to disappearance of the grain boundaries and formation of more compact structure. The electroless coating obtained by 2 min Sr-P treatment showed the best corrosion protection in 3.5 wt% NaCl due to its compact structure while the coating applied on 15 min treated sample was failed when exposed to the corrosive solution.