**1. Introduction**

Magnesium (Mg) and its alloys (with a low density of about 1.8 g/cm3) have become a hot topic of research because of the potential engineering applications. Moreover, magnesium alloys show outstanding potential in automotive, aerospace, and electronic industries because of their high strength-to-weight ratio, high stiffness, outstanding electromagnetic shielding ability, and remarkable damping performance, etc. In recent years, Mg alloys have been receiving ascending attention as biodegradable implant materials, as well. Regrettably, the inferior wear and corrosion performances of Mg alloys severely limit their extensive applications. The most commonly employed method for improving the surface properties of a substrate is surface treatment. In this regard, various conversion coatings [1], anodization process, plasma electrolytic oxidation (PEO), physical vapor deposition (PVD), electro-less, before processes: annealing processes, electroplating and ions implantation methods and thermal spray processes have been employed to modify the surface of Mg alloys for improving their wear and corrosion resistances [2–10].

An approximate new coating technology that deserves particular attention is cold spray process (as an environmentally friendly method) which doesn't involve toxic fumes or other harmful emissions [11,12]. Compared to the high velocity oxy-fuel (HVOF) thermal spray process which uses a combination of thermal and kinetic energies, cold spray

**Citation:** Daroonparvar, M.; Kasar, A.K.; Farooq Khan, M.U.; L. Menezes,P.; Kay, C.M.; Misra, M.; Gupta, R.K. Improvement of Wear, Pitting Corrosion Resistance and Repassivation Ability of Mg-Based Alloys Using High Pressure Cold Sprayed (HPCS) Commercially Pure-Titanium Coatings. *Coatings* **2021**, *11*, 57. https://doi.org/ 10.3390/coatings11010057

Received: 16 December 2020 Accepted: 28 December 2020 Published: 6 January 2021

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utilizes only kinetic (dynamic) energy to deposit the powder particles [12–15]. Likewise, the microstructural degeneration of heat-susceptible substrates such as Mg alloys which is frequently seen in the substitute thermal spray methods could be prevented by means of cold spray process [4,16–18]. In contrast to thermal spray technologies such as electric arc wire spray, plasma spray, flame spray and HVOF spray processes which partially and/or fully melt particles during the spray process; CS can avert the thermal effects including oxidation, porosity, grain growth and phase transformation during spray process [12,13,19,20].

It was reported that corrosion resistance of Mg alloys can be improved with the cold sprayed coatings (in comparison with counterpart coatings made by other techniques e.g., anodizing, E-plating, conversion coating and etc.), in 3.5 wt % NaCl solution [21]. Aluminum (with good corrosion resistance, low density, and having low standard electrode potential difference with Mg alloys) is used (as protective coating) to reduce the corrosion rate of Mg substrates [2,22–24]. Current cold sprayed (N2 as propellant gas) Al-based coatings (as single layer) on Mg alloys lack acceptable hardness, wear resistance and are highly susceptible to localized corrosions in severe corrosive atmospheres [22,25,26]. These coatings also showed low repassivation ability [27]. In fact, the passive film has a weak propensity to repair itself (or passivate) in corrosive environment.

Compared to Al and its alloys, Ti and its alloys can be extensively used in severe corrosive environments such as offshore (salt water), aerospace, automotive, etc. This was attributed to the good mechanical properties and excellent corrosion resistance (due to the formation of a firm protective oxide film on the metal surface) [28–30]. Low standard reduction potential mismatch between coating and substrate makes Ti coating (from group 4B) as a subsequent candidate for the corrosion protection of Mg and its alloys [31,32]. In this regard, warm sprayed (WS) Ti coatings couldn't noticeably enhance the corrosion potential and lower the corrosion current density of AZ91E Mg alloy [31]. These coatings disclosed poor corrosion resistance and finally led to the fast degeneration of Mg alloy. The poor performance of WS Ti coatings was due to the presence of through-thickness porosities which simply conducted the chloride containing solution towards the substrate surface. The untimely tear of titanium coatings (after only 24 h of immersion) in 3.5 wt % NaCl electrolyte was eventually observed [31]. This was mainly related to the corrosion products formation and accumulation at the interface between WS Ti coating and Mg substrate [31]. The MS (magnetron sputtered) Ti-coated AZ91D Mg alloy showed even much inferior performance than the uncoated AZ91D Mg alloy after 1 day in NaCl solution [32]. Most part of MS Ti coatings came off the Mg alloy substrate which had undergone the severe corrosion [32].

In this research, we developed a fairly compact cold sprayed titanium coating on Mg alloy using HPCS system. It is anticipated that high pressure cold sprayed commercially pure-Ti coating could alleviate the problems associated with current cold sprayed Al coatings on Mg alloys and exceptionally increase the repassivation ability of Mg alloys. Moreover, immersion test for 11 days was performed to further elucidate the effectiveness and corrosion protection performance of HPCS titanium coating on magnesium alloys in corrosive environment.
