Corrosion behaviour of AZ magnesium alloys with Al and Al–11Si thermal spray coatings

Abstract

The corrosion behaviour of AZ31, AZ80 and AZ91D Mg–Al–Zn alloys coated with Al and Al–11Si thermal spray was evaluated in 3.5 wt.% NaCl solution at 22 ºC by gravimetric and electrochemical impedance spectroscopy (EIS) measurements. The corrosion products were examined by scanning electron microscopy (SEM), energy dispersive X–ray analysis (EDX) and low–angle X–ray diffraction (XRD). The as–sprayed Al and Al–11Si coatings were porous and galvanic corrosion of magnesium substrates was observed at the substrate/coating interfaces. The application of a cold–pressing post–treatment smoothed and densified the coatings, improved the contact between coating and substrate and substantially increased the corrosion resistance. Cold–pressed Al coatings were an effective barrier against corrosion and revealed the same corrosion performance regardless of the composition of the magnesium substrate. However, cold–pressed Al–11Si coatings were permeable to the chloride–containing solution and galvanic corrosion of magnesium substrates was observed after a few days of immersion in 3.5 wt.% NaCl solution.