Microstructure and Corrosion Resistance of an HVAF-Sprayed Al-Based Amorphous Coating on Magnesium Alloys

Abstract

An Al₈₆Ni₆Y_4.5Co₂La_1.5 amorphous coating was prepared on a ZM5 magnesium alloys substrate by using high-velocity air fuel (HVAF) spray. The coating contained a 75.8% amorphous phase (volume fraction) in addition to the crystallization phases of α-Al, Al₄NiY, and Al₉Ni₅Y₃. The microhardness reached 420 HV_0.05 for the coating. The coating could endure 500 h neutral salt spray tests without apparent corrosion. Moreover, the coating exhibited a much nobler corrosion potential and two orders of magnitude smaller corrosion current density compared to the substrate. These improvements can be attributed to the compact coating structure and the passive film formed during corrosion.

1. Introduction

Magnesium (Mg) alloys have good thermal and electrical conductivity, high dimensional stability, and good machinability, as well as easy-recycling capability. Therefore, Mg and its alloys are increasingly being used in various industrial fields [1,2,3]. However, the corrosion performance of Mg-based metallic materials in acidic environments and in salt-water conditions is poor due to the fact that they degrade spontaneously in an aqueous solution, which limits their widespread application. Over the past two decades, a great effort has been made to improve the corrosion resistance and understand the corrosion mechanism of Mg alloys. An effective way to improve the surface properties of Mg alloys is coating technology. Many coating technologies (such as micro-arc oxidation coatings, electrochemical plating, gas-phase deposition, anodizing, laser surface alloying/cladding, organic coatings, etc.) are suitable for Mg and its alloys [4,5,6,7,8]. Importantly, recent studies [9,10,11] have shown that thermal spray coatings have an enormous potential to improve the surface properties of Mg alloys.

The high-velocity air/oxygen fuel (HVAF/HVOF) spray is popular due to its flexibility, cost-effectiveness, and superior coating quality [12,13,14]. Different coating materials, such as Ni-based alloys [4,15], Co-P [6], Mg-O [7] composite materials, and Fe-based amorphous alloys [9], were sprayed on Mg alloys using the HVAF/HVOF process to achieve good corrosion resistance performance. The coating materials could keep the original structure and mechanical properties due to the fact that the HVAF/HVOF spray exhibits ultra-high flying velocity and relatively low temperature [16,17]. Therefore, the HVAF/HVOF spray technology is suitable for the production of amorphous coatings with dense structure and superior properties. Benefiting from its sound corrosion resistance performance, Al-based amorphous coatings can provide effective surface protection to the underlying substrates [18,19,20]. Gao et al. [18] reported the superiority in corrosion resistance of the uniform and dense Al-based amorphous coating fabricated by high-velocity air fuel (HVAF) spraying. They found that the coating exhibited an increased pitting corrosion potential and a lower corrosion potential than that of the AA 2024 substrate. Similar findings on the improved corrosion resistance by Al-based amorphous coatings were reported by Tailleart et al. [19] and Zhang et al. [20]. However, few studies have applied the HVAF spray to successfully deposit Al-based amorphous coatings on highly chemically active Mg alloys, with the aim to endow Mg alloys with high hardness and corrosion resistance.

In this study, a representative Al-based amorphous coating was fabricated on a ZM5 Mg alloy substrate using HVAF spray. The thermal stability and phase structure were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimeter (DSC). The microstructure of the powder and coating were characterized using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Finally, the corrosion behavior of the coating was studied compared with the ZM5 Mg alloy substrate.

2. Materials and Methods

A commercial ZM5 Mg alloy (the compositions are given in

Table 1. Alloy compositions (wt.%) of the ZM5 Mg substrate and amorphous powder.

ZM5 Alloy	Al	Mn	Zn	Mg	-
	7.5~9.0	0.2~0.8	0.15~0.5	Bal	-
Powder	Al	Ni	Y	Co	La
	68.1	10.4	11.8	3.4	6.3

) ingot was used as the substrate material. The ingot was cut into specimens with 25.4 mm diameters and 10 mm lengths. Each specimen was ground using the 1000 grit SiC paper and cleaned with ethanol before spray. The Al-based amorphous powders with nominal composition of Al₈₆Ni₆Y_4.5Co₂La_1.5 (it has the best glass-forming ability in the Al-based amorphous alloys [21]) were prepared by gas atomization method. The powders with a diameter smaller than 45 μm, which exhibited almost 100% amorphous phase, were chosen as the feeding particles (see Figure 1). The AK 07 HVAF (Kermetico, Benicia, CA, USA) spray guns were used. The spray distance, spray speed, air pressure, and feed rates for fabricating the amorphous coating were deliberately adjusted, and the values were 120 mm, 3600 mm/s, 78 PSI, and 21 g/min, respectively. The phase composition and microstructure were characterized by X-ray diffraction (Rigaku D/max2400, Tokyo, Japan), transmission electron microscopy (TEM, Tecnai G2 F30), differential scanning calorimetry (Netzsch-404C, Selb, Germany), scanning electron microscopy (FEI Quangta 600, Eindhoven, Holland), and X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, Waltham, MA, USA) with an energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, Oxford, England), respectively. The Vickers hardness was measured by the MVK-H3 Vickers hardness tester using a load of 50 g holding for 20 s. The values reported were the average of 10 measurements. Neutral salt spray tests were carried out in a sealed test chamber (YWX/Q–250, China). The polished samples were placed in the chamber and exposed to a neutral salt spray environment in accordance with the national standard GB/T 2423.17-93. Each sample was the same distance from the salt spray generator. The open-circuit potential and the subsequent potentiodynamic polarization tests were carried out in a 3.5 wt.% NaCl solution. The initial pH value of the solution was 6.2 at room temperature.

3. Results and Discussion

Figure 2 shows the X-ray diffraction (XRD) patterns for the Al₈₆Ni₆Y_4.5Co₂La_1.5 atomized powder and the HVAF-sprayed coating. Only a broad halo characteristic peak is found around 2θ = 38° for the powder, indicating a fully glassy structure of the feeding particles. In addition to the halo peak, some diffraction peaks can be observed on the coating, indicating that the amorphous powders are partially crystallized during HVAF spraying. Specifically, the coating possesses an amorphous matrix and crystallized phases of α-Al and Al₄NiY. Figure 3 shows the TEM bright-field images and selected-area electron diffraction (SAED) patterns of the coating. Four different phases existed in the coating. In addition to the three phases of α-Al, Al₄NiY, and the amorphous phase, the Al₉Ni₅Y₃ intermetallic phase is also detected by the SAED. The rod-shaped crystal phase is in the 110 direction.

Figure 4 shows the DSC traces of Al₈₆Ni₆Y_4.5Co₂La_1.5 amorphous powder and the HVAF-sprayed coating. The glassy structure is confirmed by the DSC result of the amorphous powder. During DSC measurement, heat is applied to the powder, resulting in the phase transition from an amorphous phase to the crystalline phases. In Figure 4, crystalline phases develop in the powder, the onset temperature of the glass transition T_g is 497 K, and the developed crystals correspond to the three exothermic peaks (T_X1, T_X2, and T_X3). The first two characteristic temperatures of the two crystallization events (T_x1 and T_x2) are 522 K and 603 K, respectively. Accordingly, the undercooled liquid region (ΔT_x = T_x1 − T_g) is about 25 K. According to a previous study [22], the first two exothermic peaks are both related to the precipitation and growth of nano-scale fcc-Al. The third peak (T_x3 = 666 K) corresponds to the formation of the intermetallic compound of Al₄NiY. In contrast with the powder, the coating exhibits an inapparent solid-state glass transition because the heat interaction between the high-velocity air–fuel and the towing particles is weak, resulting in the direct occurrence of the growth process of coating layers instead of the endothermic process to form the nucleation cluster. The first peak located at low temperatures moves to a higher temperature, and the other two peaks stay at similar temperatures to the powder. The three peaks for the sprayed coating have significantly weaker intensity than that of the amorphous powder. Additionally, according to the DSC curves, the volume fraction of the amorphous phase for the sprayed coating is 75.8%, as revealed by a comparison of the enthalpy of crystallization (ΔH in Figure 4) of the powder and the sprayed coating [23].

Figure 5a shows a cross-sectional SE-SEM micrograph of the microstructure of the coating. No obvious cracks and voids can be found within the sprayed coating. Since the substrate is a relatively hard Mg alloy, no significant deformation of the substrate takes place due to the continuous impact of Al particles. Thus, a clear line-shaped and flat interface between the coating and the substrate forms. Additionally, the sharp contrast of the interface materials indicates that little elemental diffusion occurs between the coating and the substrate, as shown in Figure 5b in the SE-SEM image. An EDS line scan across the coating and the substrate to analyze the elemental constitutional distribution is presented in Figure 5b. The uniform distribution of alloy elements can be observed in the sprayed coating and the substrate. The dilution is slight and only occurs in the interface region, with a predominant mixing distance of less than 0.1 μm.

Figure 6 shows the microhardness of the sprayed coating at the depths from the coating surface to the substrate. The microhardness rises sharply across the coating–substrate region and reaches 420 HV_0.05 for the coating, suggesting a rather distinct interface with a very narrow mixing zone, consistent with the EDS results in Figure 5.

Figure 7 shows the corroded surfaces of the coating and the substrate at different times in a neutral salt spray environment. The coating after the 500 h test reveals an intact surface without apparent corrosion damage. By comparison, the substrate degrades greatly after 24 h, showing many pits on the corroded surface. With time, the pitting corrosion propagates horizontally, and the developed corrosion products cover the whole surface after the 500 h test. The corrosion products are mainly magnesium oxides/hydroxides [3], which are loose in structure and incapable of offering corrosion protection to the substrate alloy. However, the Al-based amorphous coating endures a long-term salt spray process, indicating an excellent corrosion barrier for the Mg alloy substrate.

Figure 8 shows the open circuit potential (OCP) for 1800 s immersion and the potentiodynamic polarization curves of the coating and the substrate after OCP tests.

Table 2. Electrochemical parameters of the substrate and coating.

Samples	Ecorr V (SHE)	icorr A/cm2	ba	bc
ZM5 alloy (average)	−1.306	6.74 × 10−4	34.6	150.6
Al-based coating (average)	−0.430	7.53 × 10−6	221.9	−132.3

shows the electrochemical parameters extracted from the polarization curves. After 1800 s immersion, the coating and substrate reach a steady state, showing a flat OCP curve in Figure 8a. The static potential for the substrate is about −1.361 V vs. SHE, which is much less noble compared to the coating (−0.496 V vs. SHE). The nobler potential of the coating indicates that the coating can reduce the damage of the substrate when galvanic corrosion occurs. The polarization curves in Figure 8b exhibit more noble values of corrosion potential and lower corrosion current density of the coating compared to the ZM5 substrate. Specifically, the corrosion potential of the coating is about −430 mV vs. SHE, which is 676 mV bigger than that of the substrate, consistent with the OCP results. At the same time, the corrosion current density of the coating is almost two orders of magnitude lower than that of ZM5 magnesium alloy (Table 2). These results confirmed that the HVAF-sprayed Al-based coating offers excellent protection for the Mg alloy. Moreover, the coating has an apparent passivation region (the relatively big b_a in Table 2) right after the corrosion potential, which is absent on the Mg alloy substrate. The passivation characteristic of the coating can be attributed to the formation of oxides during corrosion.

With the aim to characterize the passive oxides on the coating, the SEM morphology, EDS, and XPS measurements were analyzed for the coating samples after polarization. The SEM image is captured on the condition of secondary electron mode with 20.00 kV acceleration voltage, 11.1 mm working distance, 1000× magnification, and 5.0 spot. Figure 9a shows the uniformly corroded surface of the coating without apparent pits. The SEM image is obtained after the passive region of the coating; the absence of pitting corrosion indicates the repassivation of the coating material. Figure 9b shows the EDS results of zone 1 in Figure 9a. In addition to the elements in the coating, oxygen is abundant on the corroded surface, confirming the formation of oxides during polarization.

The XPS results in Figure 10 show that the Al 2p spectrum is composed of an ionic Al³⁺ spectrum peak (74.8 ± 0.1 eV) and a metallic Al⁰ spectrum peak (72.6 ± 0.1 eV). The spectrum peak of metallic Al⁰ is significantly lower than that of ionic Al³⁺. The significant ionic state Al³⁺ in the surface layer may combine with O²⁻ ions to form Al₂O₃, which is the main compound of the passive film. Moreover, there are peaks of the metallic state and oxidation state of other elements at the same time, as shown in Figure 10b–e. These ionic states also develop oxides. On the other hand, the metallic states of these elements indicate that the passive film is not 100% intact. Thus, pitting corrosion is possible at higher anodic polarization (see the pitting potential on the polarization curve of the coating in Figure 8b). Overall, the XPS analysis confirms the passivation film on the coating surface. Additionally, the main components of the passive film are Al₂O₃, Y₂O₃, and La₂O₃, which endow the Mg substrate with excellent corrosion resistance.

4. Conclusions

An Al₈₆Ni₆Y_4.5Co₂La_1.5 amorphous coating was successfully fabricated on a ZM5 Mg alloy substrate using high-velocity air fuel (HVAF) spraying. The microstructure, micro-hardness, and corrosion behavior of the coating were investigated. The volume fraction of the amorphous phase of the coating was 75.8%, in addition to the other three crystalline phases of α-Al, Al₄NiY, and Al₉Ni₅Y₃. The micro-hardness rises sharply across the coating/substrate interface and reaches 420 HV_0.05 for the coating. The corrosion potential of the coating is −430 mV, which is much nobler than that of the substrate. The corrosion current density of the coating is two orders of magnitude smaller than the substrate due to the compact coating structure and the oxide film (mainly Al₂O₃, Y₂O₃, and La₂O₃) formed during corrosion. Overall, the amorphous coating can endow Mg alloys with improved corrosion resistance.