Improving surface characteristic and corrosion inhibition of coating on Mg alloy by trace stannous (II) chloride

Highlights: •An effective additive is demonstrated for preparing Ni coating on Mg alloy. •Trace stannous chloride (SnCl₂) is helpful to obtain smoother coating surface. •Trace SnCl₂ is powerful in enhancing coating corrosion resistance. •The coating is Sn-free but properties improved. -- Abstract: Duplex Ni coatings consisting of Ni–P deposits as primer layer and Ni–Sn–P deposits as outer layer were prepared on Mg alloy. These coatings exhibit uniform surface and improved corrosion resistance after adding trace stannous (II) chloride (SnCl₂) into the outer layer plating bath. X-ray photoelectron spectroscopy proved the non-existence of Sn in the coating. The coating surface was characterized by scanning electron microscopy and atomic force microscopy. X-ray diffractometer patterns show that SnCl₂ is favourable for amorphisation of the coating. The corrosion protection of the coating was confirmed by potentiodynamic polarisation and electrochemical impedance spectroscopy measurement.