Microstructure characteristics and corrosion resistance of Ni–Co alloy coating prepared on AZ91D magnesium alloy

Abstract

In this article, a protective Ni–Co alloy coating was prepared on AZ91D magnesium alloy which was pretreated with electroless plating. The surface morphologies, microstructure and chemical composition of the coatings were studied using environmental scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy respectively. The surface morphologies and microstructure analysis showed that the Ni–Co alloy coating possessed cone shaped grain congeries and formed face centred cubic solid solution. The energy dispersive spectroscopy analysis revealed that the content of cobalt element in the alloy coating was ∼31 wt-%. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy were employed to investigate the corrosion resistance of different corrosive systems in neutral 3·5 wt-%NaCl and 0·5 mol L^–1 Na₂SO₄ solutions, which were chosen to simulate the effects of marine and industrial environments respectively. The results showed that the corrosion resistance of the AZ91D magnesium alloy was obviously improved by the Ni–Co alloy coating.

Keywords:

• Magnesium alloys
• Electroless plating
• Ni–Co electroplating
• Potentiodynamic polarisation
• Electrochemical impedance spectroscopy