*2.3. Characterization*

Optical microscopy (IX70, Olympus) was used to analyze the polished cross-sectional microstructures of as-sprayed coatings on the AZ31B. For this purpose, coated AZ31B samples were cut, mounted and polished using with standard metallographic procedures on an Allied Metprep 3TM grinder/polisher system. ImageJ software was utilized to analyze the porosity level of the as-sprayed coatings (using ASTM E2109-01) [33]. The surface morphology, polished cross-sectional microstructures and chemical composition (elemental analysis) of developed coatings before and after immersion test were studied using a LYRA-3 model XUM integrated variable pressure focused ion beam-field emission scanning electron microscope (FIB-FESEM; TESCAN), and a TM-3030 Scanning Electron Microscope (SEM; Hitachi,) equipped with Energy Dispersive X-ray spectroscopy (EDS) capability. Moreover, the structural phases of as-sprayed coatings, bare AZ31B Mg alloy substrate and feed stock powders were analyzed using an Ultima IV X-ray machine (Rigaku), after grinding up to 1200 grit size sandpaper (for only bare and coated Mg alloys). The X-ray tube emits Cu-Kα radiation with an excitation voltage of 40 kV and excitation current of 35 mA. The samples were scanned at a rate of 1 degree/min with a step width of 0.04 degree. The data were analyzed using X'Pert HighScore Plus software with ICDD database. Furthermore, 2θ (as diffraction angle) range of 30◦–90◦ was employed to collect diffraction patterns of the different samples.

A Vickers hardness tester (Beuhler-Wilson Tukon 1202), was used to measure the microhardnesses of the substrate and the coatings, under the load of 0.245 N. It should be noted that the substrate hardness measurements were performed at the regions away from the interface between coating and the Mg alloy substrate. Additionally, ten (10) measurements were done on each sample and the average was reported as micro-hardness value.

Average surface roughness (*R*a) of as-cold sprayed coatings were inspected during profilometry using an Alicona Infinite Focus, a 3D measurement system which has a noncontact, optical measurement principle based on focus-variation. Prior to profilometry, the surface was cleaned with DI water using an ultrasonic cleaner. The brightness and contrast were adjusted at a range of focus to make sure all the features are within focus during the scan. The lateral resolution was set at 50 nm. *R*a was measured using line scans across the IFM scan. At least five readings were collected for *R*a to minimize standard deviation.

Dry reciprocating sliding tests (according to ASTM G133-05 [34]) were performed using a Rtech-Tribometer at room temperature (~25 ◦C and 40–50% relative humidity). Prior to sliding tests, all the coated surfaces were polished to achieve an average surface roughness (Ra) of 0.2 ± 0.05 μm. *R*a is defined as the arithmetic mean of the absolute values of the vertical deviation from the mean line through the profile [35]. E52100 steel ball with 6.35 mm diameter was used as a counterpart. All the reciprocating sliding tests were carried out with a track length of 15 mm and 1 mm/s velocity under a normal load of 4 N for a total distance of 1000 mm. The 1000 mm of sliding distance was selected based on the stabilized wear depth during sliding. The wear depth was recorded to measure the wear volume. The specific wear rate was then measured using the following formula [36]:

$$\text{Wear rate } \left(\mu\text{m}^3/\text{Nm}\right) = \frac{\text{Wear volume } \left(\mu\text{m}^3\right)}{\text{Normal load } \left(\text{N}\right) \times \text{Selling distance } \left(\text{m}\right)}\tag{1}$$

### *2.4. Sample Preparation for Corrosion Tests*

The surface area of as-cold sprayed coatings which have a highly active surface [37] is increased. This is attributed to the rough surface of as-cold sprayed coatings. As a general practice, the rough and porous surface layer of as-cold sprayed coatings should be removed before corrosion tests [28]. Hence, the samples surface was ground up to 1200 US grit size sandpaper (SiC abrasive papers) and then cleaned with ethanol using an ultrasonic cleaner for 5 min before cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS) and long term immersion tests.

### *2.5. Cyclic Potentiodynamic Polarization (CPP) Tests in 3.5 wt % NaCl Solution*

Cyclic potentiodynamic polarization tests were performed in a three-electrode setup using a flat cell and a Bio-logic potentiostat (per ASTM standard G61 [38]). The standard calomel electrode, platinum electrode, and sample under test were connected as a reference electrode, counter electrode, and working electrode, respectively. Before the CPP test, the open circuit potential (OCP) was tracked for 1 h to allow the system to achieve an equilibrium in the electrolyte. PDP tests were done in 3.5 wt % NaCl (pH 6.7) at a scan rate of 1 mV/s from 200 mV below OCP to a current limit of 10 mA/cm<sup>2</sup> or a potential limit of 2.5 VSCE and reversed back to the same starting potential of 200 mV below OCP at room temperature. The pitting potential was determined by intersecting the line coming from extending the passive current density region and the linear increase in the current density region after passivation region.

### *2.6. Electrochemical Impedance Spectroscopy (EIS) in 3.5 wt % NaCl Solution*

A three-electrode setup was used in a flat cell where standard calomel electrode, platinum electrode, and sample were connected as a reference electrode, counter electrode, and working electrode, respectively. The sample was monitored for 1 h observing the OCP in 3.5% NaCl (pH 6.7) exposing an area of 1 cm<sup>2</sup> at room temperature. Likewise, 100 kHz to 10 mHz (as frequency range) at OCP was selected for performing EIS test. For each EIS scan, ten measurements were recorded per decade, with an average of at least three points per measurement. Furthermore, sinusoidal AC perturbation with an amplitude of 10 mV (rms) was considered for all EIS tests. EC-lab 11.21 provided in the Bio-logic potentiostat was utilized to analyze CPP as well as the EIS data. Electrochemical corrosion tests were carried out three times to substantiate the repeatability of the obtained results. After the immersion test for 11 days, the samples were rinsed with DI water and subsequently dried in air.
