**1. Introduction**

Magnesium (Mg) alloys have potential use in aircraft and automobile industries due to their excellent specific strength [1]. They are also used to protect steel structures from corrosion in freshwater and soil environments as sacrificial anodes [2]. Casting is the main technique to produce AZ91D alloy workpieces; however, it is not suitable to fabricate large or complex-shaped products [3]. In addition, AZ91D work pieces are difficult to be repaired using traditional techniques (tungsten inert gas (TIG) welding [4], metal inert gas (MIG) welding [5], and thermal spraying [6], etc.) due to their high-temperature activity.

As an emerging coating building and rebuilding technique, cold spraying has advantages to elaborate Mg alloy workpieces [7,8]. In cold spraying, particles are accelerated to a high velocity (300~1200 m·s−1) by a high-pressure thermal gas through a de Laval-type nozzle. Then, the particles can deposit onto a substrate and form a coating once the particle velocity exceeds the critical velocity of the material [9,10]. In the view of bonding, the jet formation of impact couples is viewed as the necessary condition.

In cold spraying, the main gas temperature is lower than the melting point of the sprayed material; thus, oxidization can be avoided. In the past decade, cold spraying has been used to deposit oxidation-sensitive alloys, such as Ti [11], Al [12,13], and Mg [8] alloy coatings. In addition, cold spraying was successfully used to repair Al alloy gearbox components [14]. However, few publications on cold-sprayed AZ91D coatings and AZ91D-based composite coatings were reported. Based on this fact, AZ91D powder was used to elaborate coatings using cold spraying in this paper, and SiC powder was used as reinforcing particles to deposit the composite coatings. In consideration of the application of cold-sprayed Mg alloy workpieces, compressed air, not noble gas, was used as the accelerating gas. Different gas temperatures were conducted to study the feasibility of fabricating cold-sprayed Mg alloy work pieces, and then ceramic particles were added as reinforcement to improve the performance of the coatings. The effects of gas temperature and the addition of ceramic particles on the coatings' microstructure and electrochemical behaviors were evaluated.
