Compositionally modulated CGDS+MAO duplex coatings for corrosion protection of AZ91 magnesium alloy

Abstract

• First of its kind attempt was made to duplex radically different viz., CGDS-MAO techniques. • AZ91 Mg alloy having an Al top coat was subjected to MAO process for obtaining duplex coatings. • Duplex coatings exhibit compositionally graded phase distribution across their thickness. • Corrosion current density is reduced by 1 to 2 orders of magnitude compared to the bare substrate. • Polarization resistance of duplex coating is about 14 times higher than the plain MAO coating. The micro arc oxidation (MAO) technology is being increasingly recognized as a non-traditional and novel approach for enhancing the corrosion and wear protection of Al, Ti, Mg and their alloys. Since the MAO coatings formed on Mg in general tend to be porous, several methods of duplexing the MAO with other conventional and non-conventional techniques is gaining popularity for enhancing the corrosion protection of Mg and its alloys. The Cold Gas Dynamic Spray (CGDS) technique was employed for the first time as a pre-MAO treatment to deposit Al on AZ91 Mg alloy. The Mg alloy having an Al top coat was subsequently subjected to MAO process for obtaining duplex coating. The coating morphology was studied by employing scanning electron microscopy (SEM), the elemental concentration and their distribution across the coating thickness was evaluated using energy dispersive spectroscopy (EDS) coupled with SEM. The coating is not only duplex in terms of combining two diverse and altogether different methods namely CGDS and MAO, but also by virtue of the elemental distribution across the coating thickness exhibiting a compositionally graded nature. The potentiodynamic polarization studies conducted in 3.5 wt.% NaCl solution revealed that the overall corrosion current density is reduced by 1 to 2 orders of magnitude compared to the bare magnesium substrate.