Systematic comparison of microstructure, coating quality and corrosion resistance of arc-sprayed Zn, Al and Zn15Al coatings

ABSTRACT

This study systematically analyzed the microstructure, quality, and corrosion resistance of arc-sprayed Zn, Al, and Zn15Al coatings commonly used on structural components in port equipment. The arc-sprayed Zn and Zn15Al coatings exhibited a lamellar structure, where the Zn coating consisted of a single Zn-rich phase while the Zn15Al coating comprised a Zn-Al eutectic. In contrast, the arc-sprayed Al coating displayed a relatively dense internal morphology without distinct lamellar features. The Al coating exhibited the highest bond strength at 10.22 MPa, significantly exceeding those of the Zn (7.51 MPa) and Zn15Al (7.35 MPa) coatings. The Al coating had the strongest corrosion resistance with a self-corrosion current density of 1.622 × 10⁻⁶ A·cm⁻². With prolonged immersion time, the corrosion products on the Al coating surface evolved from an intermittent to a continuous distribution pattern, identified as Al(OH)₃. Conversely, the Zn and Zn15Al coatings exhibited flocculent and granular corrosion products, characterized as Zn₅(OH)₈Cl₂·H₂O and Zn₆Al₂(OH)₁₆CO₃·4H₂O, respectively. These findings offer valuable reference data for corrosion protection of structural components in port equipment.

Keywords:
Arc-sprayed Coating; Zn; Al; Zn15Al; Corrosion resistance

1. INTRODUCTION

Port equipment serves as essential infrastructure for port operations, where structural integrity directly impacts operational safety. Structural components operate at the berthing front, continuously exposed to aggressive marine environments characterized by high humidity, wide temperature fluctuations, and salt spray [¹, ², ³]. This exposure accelerates corrosion damage, leading to section loss and degraded mechanical properties. Furthermore, the large-scale, complex geometry of these components combined with variable loading conditions creates critical stress-corrosion synergies. Consequently, comprehensive corrosion prevention strategies during operation and maintenance are imperative for mitigating safety risks [⁴, ⁵, ⁶]. Common corrosion-resistant coatings include Ni-based, Al-based, and Zn-based systems. VIPRA et al. [⁷] arc-sprayed Ni-Al and Fe-Cr coatings on SS316 steel, demonstrating through slurry erosion testing that coated specimens exhibited significantly reduced mass loss compared to uncoated substrates when exposed to abrasive particles. SHARMA et al. [⁸] evaluated the corrosion behavior of WAAM-CMT fabricated Inconel 625 in 3.5wt% NaCl solution, finding that furnace cooling after 1100°C/2h heat treatment yielded optimal corrosion resistance. JHA et al. [⁹] deposited equiaxed crystalline AlN coatings on aluminum substrates via RF sputtering; contact angle measurements confirmed hydrophobic behavior that delayed corrosion progression, reducing substrate erosion by ~1.5× versus uncoated controls. SINGH et al. [¹⁰] investigated PTFE-modified HVOF-sprayed TiC coatings on stainless steel, with cyclic corrosion testing revealing sustained protection through seven cycles. Laser texturing further enhanced performance by inducing super-hydrophobicity.

Arc spraying is widely employed for port equipment corrosion protection due to its low cost, high efficiency, field applicability, and excellent performance [¹¹, ¹²]. Common arc-sprayed coating materials for marine environments include Zn, Al, Zn-Al alloys, and pseudo-alloy coatings. When applied to steel substrates, these metallic coatings (particularly Zn and Al) function sacrificially as anodes. They provide dual protection: 1) forming a physical barrier against corrosive media, and 2) delivering cathodic protection to extend component service life [¹³, ¹⁴]. Zn higher electrochemical activity than Fe enables effective sacrificial anode behavior. However, this reactivity accelerates coating consumption. In contrast, Al offers lower density, lower cost, and water-insoluble corrosion products that enhance corrosion resistance in acidic and marine environments. Al coatings form relatively dense structures with stable passivation films that inhibit substrate corrosion, but these insulating films create high resistance to anodic dissolution, reducing Al cathodic protection capability [¹⁵]. PANOSSIAN et al. [¹⁶] demonstrated that Zn coatings maintain sacrificial activity in natural atmospheres, whereas Al coatings provide cathodic protection only in high-chloride environments. Zn-Al alloy coatings [¹⁷] combine advantages of both: though their corrosion potential and current density resemble Zn (promoting dissolution), they match Al protective lifespan while retaining Zn sacrificial protection for steel substrates.

However, when Al content exceeds 15 wt.% in Zn-Al alloys, increased brittleness inhibits wire drawing and formability, constraining coating development. In contrast, Zn-Al pseudo-alloy coatings offer distinct advantages: (1) Zn and Al exist as separate homogeneous phases with minimal intermetallic formation; (2) Compositional flexibility enables Al content >15 wt.%; (3) Synergistic corrosion protection combines Zn’s cathodic protection with Al’s barrier effect [¹⁸]. Consequently, these pseudo-alloy systems show significant promise for steel protection. Nevertheless, arc-sprayed Zn-Al pseudo-alloy coatings exhibit inherent porosity that permits corrosive media ingress, substantially increasing substrate corrosion risk [¹⁹].

Previous studies have been limited to examining either single coatings or pairwise comparisons (e.g., CHOE et al. [¹²] investigated only pure Zn versus pure Al coatings), lacking systematic analysis across pure Zn, pure Al, and Zn15Al coatings. This work comprehensively characterizes three distinct coatings through comparative assessment of: Microstructure, Porosity, Oxidation rate, Hydrophobicity, Bonding strength, Corrosion resistance, Corrosion product formation. The systematic evaluation of these coatings provides critical guidance for selecting optimal corrosion protection systems for harbor structures.

Arc-sprayed Zn, Al, and Zn15Al coatings effectively enhance the corrosion resistance of harbor equipment. Consequently, a systematic comparison of the microstructures and corrosion resistance of these widely used coatings is necessary to provide valuable reference data for the port construction industry. Building on prior research into the arc spraying process for different coatings, this study utilized pure Zn, pure Al, and Zn15Al arc-spraying wires to deposit Zn, Al, and Zn15Al coatings, respectively, onto a Q235 steel substrate. For simplicity, these coatings are hereafter referred to as Zn, Al, and Zn15Al coatings. The microstructure, coating properties, and corrosion resistance of these coatings were characterized and evaluated. This work systematically analyzes the influence of the arc-sprayed wire material on the coating microstructure and quality, elucidates the corrosion behavior of the different coatings, and compares their corrosion resistance.

2. EXPERIMENTAL DETAILS

2.1. Experimental materials and methods

This study utilized 40 × 80 × 4 mm low-carbon steel (Q235) substrates. Arc-sprayed coatings were deposited using 2 mm diameter Zn, Al, and Zn-15Al wires (compositions in Table 1). Substrates underwent grit-blasting with 0.85–1.5 mm copper mineral slag at 0.5–0.6 MPa pressure, 60–200 mm standoff distance, and 70–90° incidence angle to enhance coating adhesion through mechanical interlocking. Surfaces were subsequently cleaned with anhydrous ethanol to remove abrasive residues. Spraying was completed within 2 hours post-treatment using a DH-600A arc spray system, maintaining coating thicknesses at approximately 200 μm. Process parameters are detailed in Table 2. The spray parameters were initially determined based on a literature survey [²⁰] and subsequently optimized empirically.

Table 1
The composition of arc-sprayed wires (wt.%).

SPRAYED WIRE	ZN	AL
Zn	>99.6	–
Al	–	>99.6
ZnAl	85	15

Table 2
Process parameters for arc-sprayed Zn, Al and Zn15Al coatings.

ARC SPRAY COATING	AIR PRESSURE (MPa)	SPRAYING DISTANCE (mm)	OPERATING VOLTAGE (V)	OPERATING CURRENT (A)
Zn	0.80	150	26	160
Al	0.80	150	29	170
Zn15Al	0.80	150	26	160

2.2. Coating characterization

Surface morphology, cross-sectional morphology, and composition of the coatings were characterized using scanning electron microscopy (SEM; TESCAN VEGA 3 XMU) equipped with energy-dispersive spectroscopy (EDS). Phase analysis was performed by X-ray diffraction (XRD; Rigaku Smart Lab 9 kW) under the following conditions: 4 kW power, 10–90° 2θ range, 10°/min scan rate. Coating cross-sections were examined using an OLYMPUS metallographic microscope at 200× magnification. Porosity was quantified through grayscale analysis of these micrographs using ImageJ software, identifying pores as regions of darker contrast. Three-dimensional morphology and surface roughness measurements were obtained using an OLYMPUS metallurgical microscope. To observe the surface morphology of the coatings, coated specimens were sectioned into 10 × 10 mm samples, cold-mounted in epoxy resin (exposing the coating surface), cleaned sequentially with distilled water, anhydrous ethanol, and acetone, then dried for storage. For cross-sectional morphology observation and porosity determination, samples were similarly sectioned and mounted. The cross-sections were then ground using SiC sandpaper (progressing through 200, 600, 800, 1200, and 2000 grit) until a uniform surface was achieved, followed by polishing with a 1 μm diamond suspension. Finally, samples were ultrasonically cleaned in alcohol for 10 minutes and dried under compressed air.

2.3. Coatings properties testing

Static contact angles of 3.5 wt.% NaCl solution droplets (3–5 μL) on each coating were measured at room temperature using an optical contact angle goniometer (OCA20, Data Physics, Germany). Measurements were taken at multiple surface locations and averaged. The RJTC-10 Coating Adhesion Measuring Instrument was used to determine the bonding strength of the coatings. Specifically, 20 mm spindles were selected, and 5 measurement points were designated on each coating specimen. These spindles were bonded to the surface of the coating specimens using AB glue (DP420), followed by a 24-hour curing period. After curing, the sleeve of the drawing apparatus was coupled with the spindle, and the coated area around each designated point was subjected to tensile force using the drawing apparatus until the coating separated from the substrate. The strength value was then directly read from the LCD display of the instrument. For each coated specimen, five sets of valid data were recorded, and the average value was calculated as the final result.

Electrochemical testing of the specimens was conducted using a Shanghai Chen hua CHI-660E electrochemical workstation, with the tests performed at room temperature in a 3.5 wt.% NaCl solution as the corrosion medium. The sample preparation process for electrochemical experiments was as follows: coated specimens were cut into 10 mm × 10 mm blocks using a wire-cutting machine, and surface oil stains and impurities were removed via ultrasonic cleaning in alcohol for 10 minutes. Copper leads were then affixed to the back of each sample, which was subsequently sealed with epoxy resin (leaving only the coated surface exposed). At least 3 independent parallel tests were performed under identical experimental conditions to ensure the reliability of the results. For full immersion experiments, the coatings were also encapsulated using the same method described above. Each coated specimen was immersed in a 3.5 wt.% NaCl solution, and after different immersion periods (7 days, 14 days, and 28 days), the surface corrosion morphology of the specimens was characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) patterns of the corrosion products formed on the surfaces of different coatings were obtained using an XRD instrument, and their phase contents were calculated based on the reference intensity ratio (RIR) method.

3. RESULTS AND DISCUSSION

3.1. Microstructure of arc-sprayed Zn, Al, Zn15Al coatings

Figure 1 presents the macroscopic morphology of the Q235 steel plate surface after sandblasting, along with the macroscopic morphologies of the Zn, Al, and Zn15Al coatings and spray paint following application. From Figure 1a, it can be observed that the surface of the Q235 steel is clean and free of rust after sand blasting treatment. Additionally, combined with the micromorphology of the sandblasted Q235 steel shown in Figure 2, the surface is found to be rough and uniform, with a surface roughness (Sa) of 14.67 ± 1.45 μm. Energy-dispersive X-ray spectroscopy (EDS) analysis reveals that the oxygen content on the sandblasted Q235 steel surface is only 2.82 wt.%. These results indicate that the sandblasting process employed in this study was effective for rust removal and surface roughening of the Q235 steel plate. Furthermore, as evident in Figure 1, all three coatings exhibit an off-white and bright appearance. Each coating uniformly covers the substrate surface, with relatively flat surfaces displaying a subtle frosted texture. No obvious spraying defects or loose spray particles were observed.

Figure 1
Macroscopic morphology of sandblasted as well as arc-sprayed Zn, Al, and Zn15Al coatings: (a) matrix; (b) Zn coating; (c) Al coating; (d) Zn15Al coating.

Figure 2
Micro-morphology and roughness of Q235 steel plate surface after sandblasting treatment: (a-b) surface morphology; (c) surface roughness of the substrate.

Figure 3 illustrates the surface and cross-sectional micromorphologies of the Zn coating. As observed in Figures 3a and 3b, the coating exhibits a relatively flat surface with only minimal porosity; however, the overall structure is homogeneous and dense. This dense structure provides effective shielding of the substrate, which is beneficial for reducing porosity and enhancing the coating’s corrosion resistance. Additionally, the Zn coating surface primarily consists of a large molten zone (MZ) and localized semi-molten zones (SMZ), characterized by a lamellar stacking morphology. Energy-dispersive X-ray spectroscopy (EDS) analysis of the entire area in Figure 3a reveals that the oxygen content on the Zn coating surface is 2.31 wt.%. Figures 3c and 3d display the cross-sectional micromorphology of the arc-sprayed Zn coating. As shown, the dark gray region at the bottom of the images corresponds to the Q235 substrate, while the upper region is the Zn coating. The thickness of the Zn coating is approximately 200 μm, and EDS analysis of the cross-sectional area indicates an oxygen content of only 1.28 wt.%. During the coating formation process, deformed particles impact the substrate at high velocity and accumulate in a wave-like pattern on the Q235 surface, forming a layered structure with only a few scattered voids. This results in a high overall coating density. Furthermore, the cross-sectional micromorphology clearly demonstrates good bonding at the interface between the Zn coating and the substrate.

Figure 3
Microscopic morphology of Zn coatings: (a, b) surface morphology; (c, d) cross-section morphology.

Figure 4 presents the surface and cross-sectional micromorphologies of the Al coating. Observations of the surface morphology in Figures 4a and 4b reveal that, compared to the Zn coating, the Al coating surface contains more voids and exhibits lower surface density. Correspondingly, energy-dispersive X-ray spectroscopy (EDS) analysis of the surface region in Figure 4a shows a significant increase in oxygen content, reaching 4.84 wt.%. In contrast to the Zn coating, the Al coating surface displays a marked reduction in the area of the molten zone (MZ) and an increase in the area of the semi-molten zone (SMZ). Figures 4c and 4d illustrate the cross-sectional micromorphology of the arc-sprayed Al coating. The coating, with a thickness of approximately 200 μm, contains fewer pores near the substrate interface but shows a significant increase in porosity toward the surface. EDS analysis of the cross-sectional region in Figure 4d indicates that the oxygen content within the coating is significantly lower, measuring 1.96 wt.%. Additionally, when compared to the cross-sectional morphology of the Zn coating, the Al coating does not form a distinct lamellar structure and appears more compact overall.

Figure 4
Microscopic morphology of Al coatings: (a, b) surface morphology; (c, d) cross-section morphology.

Figure 5 shows the micromorphology of the arc-sprayed Zn15Al coating. As observed, the surface morphology of the Zn15Al coating is relatively rough, with increased surface pores and poor densification. Compared to both the Zn and Al coatings, the Zn15Al coating surface exhibits a significant reduction in the area of the molten zone (MZ) and a corresponding increase in the area of the semi-molten zone (SMZ). The cross-sectional morphologies in Figures 5c and 5d reveal that the Zn15Al coating has a higher overall pore distribution compared to the Zn and Al coatings. However, higher-magnification observations of the cross-sectional micromorphology show that the internal structure of the coating is relatively dense. The Zn15Al coating consists of a lamellar eutectic structure, with an overall thickness of up to 200 μm. Notably, the lamellar Zn15Al eutectic exhibits distinct variations: most regions feature a fine eutectic structure, while local areas display a coarser eutectic morphology. EDS analysis indicates that the oxygen contents of the coating surface and cross-section are 2.2 wt.% and 1.62 wt.%, respectively.

Figure 5
Microscopic morphology of Zn15Al coatings: (a, b) surface morphology; (c, d) cross-section morphology.

Figure 6 presents the X-ray diffraction (XRD) patterns of the four coatings. No oxide phases were detected in any of the coatings: the Zn coating consists primarily of the Zn phase, the Al coating is composed of the Al phase, and the Zn15Al coating contains the Zn15Al phase. These results are consistent with the aforementioned analysis.

Figure 6
XRD patterns of Zn, Al, and Zn15Al coatings.

3.2. Coating quality of arc-sprayed Zn, Al, Zn15Al coatings

The porosity of the coatings was determined by identifying pore locations in cross-sections of different coatings using ImageJ software with a grayscale analysis method, as illustrated in Figure 7. Comparative analysis revealed that the porosities of the Zn, Al, and Zn15Al coatings are all less than 5%. Among them, the Zn coating exhibits the lowest cross-sectional porosity at 2.57%. In contrast, the cross-sectional porosities of the Al and Zn15Al coatings are significantly higher, measuring 3.85% and 4.62%. Additionally, when combined with the surface morphology characterization of each coating in Section 3.1, it was observed that the Zn coating has the largest proportional area of molten zone (MZ) on its surface and the lowest number of surface pores. This indicates that Zn particles are more prone to spreading uniformly across the substrate surface, resulting in higher surface quality, whereas the other coatings exhibit inferior surface quality compared to the Zn coating.

Figure 7
Porosity of Zn, Al, and Zn15Al coatings.

Observation of the micromorphology of the aforementioned coatings reveals that the surfaces of the different arc-sprayed coatings are not smooth. Given the frequent rainy weather in harbor environments, the hydrophobicity of coatings directly influences their corrosion resistance, as the ability to repel water affects the interaction with corrosive media. Notably, many researchers have focused on developing hydrophobic coatings on steel substrates to effectively prevent the penetration of corrosive agents [¹, ²]. Therefore, we analyzed the hydrophobic properties of the different coatings. The contact angles of the coatings with 3.5 wt.% NaCl solution are presented in Figure 8. All three coatings exhibit good hydrophobicity: the Zn coating surface shows a contact angle of up to 113.1° with the 3.5 wt.% NaCl solution, while the Al and Zn15Al coatings achieve contact angles of up to 130°.

Figure 8
3.5 wt.% NaCl solution contact angle on Zn, Al, and Zn15Al coatings surfaces.

The main reasons why hydrophobicity affects the corrosion resistance of a coating include:

• (1)

  Self-cleaning effect (lotus leaf effect): The hydrophobic coating makes it difficult for water to wet the surface and tends to form water droplets that roll off. During the roll-off process, the water droplets carry away dust, dirt, salt and other contaminants from the surface. This greatly reduces the adhesion and accumulation of dirt on the surface of the coating.

• (2)

  Corrosion (especially galvanic corrosion) occurs when water and electrolytes (such as dissolved salts) come into contact with the metal substrate. Hydrophobic coatings greatly reduce the residence time and contact area of liquid water on the coating surface, effectively blocking the path for water, dissolved oxygen and ions (electrolytes) to penetrate the coating/metal interface.

• (3)

  Reduced Water-Induced Coating Degradation: Moisture is one of the main factors that cause swelling, hydrolysis, blistering, and peeling (loss of wet adhesion) of many organic coatings. Hydrophobicity directly reduces the extent and duration of the coating’s contact with water, helping to maintain the coating’s physicochemical stability and adhesion to the substrate, and extending the service life of the coating itself.

The differences in contact angles are primarily attributed to variations in the arithmetic mean surface roughness (Sa) of the different coatings. To investigate this, we characterized the surface roughness of each coating, as shown in Figure 9. The Zn coating exhibits the smallest Sa at 10.76 µm, which can be attributed to the low melting point of zinc: this allows zinc wires to melt fully during spraying, resulting in better atomization. Consistent with this, comparisons of the surface micromorphologies across coatings reveal that the Zn coating has the largest proportional area of molten zone (MZ), which contributes to its minimal Sa. Additionally, the Zn coating is uniformly consistent, with no obvious cracks, bulges, or other defects. In contrast, the Sa of the Al coating increased significantly to 13.26 µm, primarily due to the high melting point of aluminum wires, which leads to uneven melting and atomization of particles during the spraying process. The Sa of the Zn15Al coating is comparable to that of the Al coating, measuring 13.32 µm. Overall, the 3.5 wt.% NaCl solution exhibits high contact angles on all three coating surfaces, indicating that the coatings possess good hydrophobicity— a property that is beneficial for enhancing their corrosion resistance.

Figure 9
Surface roughness of the different coatings: (a) Zn; (b) Al; (c) Zn15Al.

Figure 10 presents the bond strength results of the Zn, Al, and Zn15Al coatings. The Zn coating exhibits a bond strength of up to 7.51 MPa, while the Zn15Al coating shows a comparable bond strength of 7.35 MPa. In contrast, the Al coating achieves the highest bond strength at 10.22 MPa. The superior bond strength of the Al coating may be attributed to the better fluidity of molten aluminum droplets, which enhances droplet atomization and thereby improves the coating’s bonding strength to the substrate.

Figure 10
Bonding strength of Zn, Al and Zn15Al coatings.

The higher coefficient of thermal expansion (CTE) of Zn coatings relative to steel generates significant residual tensile stresses at the interface during post-spray cooling, as Zn contracts substantially more than the substrate. This stress state frequently causes coating cracking or delamination, severely compromising bond strength. Zn-15Al coatings exhibit comparable bond strength to pure Zn coatings due to their nearly identical metallurgical bonding mechanism.

Figure 11a-c shows the macroscopic tensile fracture morphologies of the three coatings. It can be observed that the fracture location for all three coatings occurs at the interface between the coating and the Q235 substrate. This indicates that the coatings themselves are relatively dense, with strong bonding between their internal lamellar structures. Since the interfaces between the three arc-sprayed coatings and the substrate are mechanically bonded—an inherently weaker interface—when an external load is applied to the coating surface, the interface is the first to reach its load-bearing limit, leading to fracture. Figure 11d-f further illustrates the microscopic morphologies of the tensile fracture surfaces of the three coatings. The silver-white spotted regions correspond to residual coating material on the fracture surfaces. Comparing the microscopic fracture surfaces across the coatings reveals that the residual coating content on the fracture surface of the Al coating is significantly higher than that of the Zn and Zn15Al coatings. This observation confirms that the Al coating exhibits stronger bonding with the substrate.

Figure 11
Drawing fracture morphology of Zn, Al and Zn15Al coatings. (a-c) represent the macroscopic morphologies of Zn, Al, and Zn15Al respectively; (d-f) represent the micrographs of Zn, Al, and Zn15Al respectively.

3.3. Corrosion resistance of arc-sprayed Zn, Al, and Zn15Al coatings

Figure 12 presents the open-circuit potential (OCP) plots and dynamic potentiodynamic polarization curves of the Q235 substrate, Zn, Al, and Zn15Al coatings. The results of Tafel fitting for the polarization curves are summarized in Table 3. Open-circuit potential measurements were conducted by immersing the samples in a 3.5 wt.% NaCl solution for 600 seconds, during which all samples reached a steady state. The self-corrosion potential (E_corr) and self-corrosion current density (i_corr) were derived via Tafel extrapolation directly from the Tafel regions of the potentiodynamic polarization curves (PPCs). It was observed that the self-corrosion potentials of all specimens shifted negatively after arc spraying, which is attributed to the chemical properties of the sprayed metals. Compared to the bare substrate, all arc-sprayed specimens exhibited a reduction in self-corrosion current density. Notably, the Al coating showed the most significant decrease—nearly one order of magnitude—with a self-corrosion current density of 1.622 × 10⁻⁶ A·cm⁻², indicating it possesses the strongest corrosion resistance among the tested coatings.

Figure 12
Q235, Zn, Al and Zn15Al coatings. (a) Open circuit potential plot. (b) Dynamic potential polarization plot.

Table 3
Tafel fitting results of polarization curves for Q235, Zn, Al and Zn15Al coatings.

SAMPLE	Q235	Zn	Al	Zn15Al
Open-circuit potential (V)	–0.5719	–1.057	–0.7377	–1.054
Ecorr (V vs. SCE)	–0.616	–1.151	–0.738	–1.126
icorr (A·cm–2)	1.002 × 10–5	5.365 × 10–6	1.622 × 10–6	4.387 × 10–6
βa (mV/dec)	13.962	20.691	13.635	22.511
βc (mV/dec)	−3.970	–16.510	–4.800	–7.53

Figure 13 displays the surface corrosion morphologies of the three coatings after 7 days of full immersion in a 3.5 wt.% NaCl solution, along with the chemical elemental compositions of selected regions. Overall, significant differences were observed in the corrosion morphologies of the arc-sprayed Zn, Al, and Zn15Al coatings. The corrosion morphologies of the Zn and Zn15Al coatings were more similar, primarily exhibiting a granular appearance. Specifically, the corrosion products on the Zn coating were more aggregated, whereas those on the Zn15Al coating were more dispersed. High-magnification observations revealed that the corrosion products of the Zn coating mainly existed in a flocculent form, composed primarily of Zn and O, with a small amount of Cl also detected. In contrast, the corrosion products of the Zn15Al coating included both flocculent and lamellar structures, consisting mainly of Zn, Al, C, and O. Additionally, under high magnification, the corrosion products of the Zn coating appeared relatively dense, while those of the Zn15Al coating contained obvious pores. Compared to the Zn and Zn15Al coatings, no significant corrosion products were observed on the surface of the Al coating. High-magnification inspection revealed an intermittent thin layer on the Al coating surface, which was primarily composed of Al and O.

Figure 13
Surface corrosion morphology and EDS analysis results of different coatings after 7 days of total immersion in 3.5 wt.% NaCl solution for (a) Zn; (b) Al; (c) Zn15Al.

Figure 14 shows the surface corrosion morphologies of the three arc-sprayed coatings (Zn, Al, and Zn15Al) after 14 days of full immersion in a 3.5 wt.% NaCl solution. The surface corrosion morphology of the Zn coating has not changed significantly, with large areas of granular corrosion products still distributed across its surface. High-magnification observation reveals that these corrosion products consist of flocculent structures and tiny particles, composed primarily of Zn and O, with a small amount of Cl also detected. The surface corrosion products of the Al coating have increased significantly, and the area of the oxide thin layer on its surface—primarily composed of Al and O elements—has expanded substantially. For the Zn15Al coating, dispersed granular corrosion products remain visible on the surface. Upon magnification, these products mainly include lamellar and flocculent structures, with relatively dense packing between the corrosion products. Notably, the contents of O and C elements in these products have increased significantly.

Figure 14
Surface corrosion morphology and EDS analysis results of different coatings after 14 days of total immersion in 3.5 wt.% NaCl solution for (a) Zn; (b) Al; (c) Zn15Al.

The lamellar structures of Zn and Zn15Al alloys significantly enhance their corrosion resistance through a variety of synergistic mechanisms, including a physical barrier effect, strengthened sacrificial anode protection, promotion of protective corrosion product coverage, refinement of corrosion morphology to inhibit pitting, and utilization of the passivation capability of the aluminum phase. In particular, the fine lamellar structure within the Zn15Al eutectic alloy prioritizes sacrificial anode protection, enabling it to exhibit superior corrosion resistance compared to pure zinc across a range of corrosive environments.

Figure 15 presents the surface corrosion morphologies of the three coatings after 28 days of full immersion in a 3.5 wt.% NaCl solution. The corrosion products on the surface of the Zn coating are overall more dense, consisting primarily of flocculent and granular structures composed of Zn, O, and Cl elements. The surface corrosion morphology of the Al coating has changed significantly: instead of the intermittent thin layers observed earlier, a dense oxide layer has formed that completely covers the surface. For the Zn15Al coating, the granular corrosion products on the surface have become more aggregated. High-magnification observation reveals that its flocculent and lamellar corrosion products have completely fused into a single structure with no obvious pores present.

Figure 15
Surface corrosion morphology and EDS analysis results of different coatings after 28 days of total immersion in 3.5 wt.% NaCl solution for (a) Zn; (b) Al; (c) Zn15Al.

The corrosion products of the Zn15Al coating have largely transitioned from a flocculent to a more continuous, needle-like structure. This continuous corrosion product layer exhibits density, stability, good adhesion, and a degree of self-repair capability. Simultaneously, during the corrosion process, the Zn15Al coating forms a layered double hydroxide (LDH) structure. This LDH structure effectively traps corrosive anions (such as chloride ions, Cl^-) from the environment within its interlayers, thereby inhibiting their further penetration towards the substrate. Furthermore, corrosion of the Al component generates Al(OH)₃. This compound subsequently forms a highly dense, gelike film on the coating surface. Characterized by extremely low permeability, this film effectively impedes the transport of water, oxygen, and corrosive ions to the interface between the coating and the substrate.

Figure 16 presents the cross-sectional morphology and energy-dispersive X-ray spectroscopy (EDS) analysis results of the Zn coating after 28 days of immersion in a 3.5 wt.% NaCl solution. The corrosion products on the surface of the Zn coating are relatively dense, with a thickness of up to 30 μm. These products are enriched in a large amount of O, along with small amounts of Zn and Cl elements.

Figure 16
Cross-sectional corrosion morphology and EDS analysis results of Zn coating after 28 days of total immersion in 3.5 wt.% NaCl solution for (a) Morphology; (b) O element; (c) Zn element; (d) Cl element.

Figure 17 shows the cross-sectional morphology and energy-dispersive X-ray spectroscopy (EDS) analysis results of the Al coating after 28 days of immersion in a 3.5 wt.% NaCl solution. As depicted in Figure 17a, the corrosion product layer adjacent to the coating exhibits distinct microcracks, whereas the layer farther from the coating is more compact. The total thickness of the corrosion product layer is approximately 40 μm. This observation is consistent with the previously noted changes in the surface morphology of the Al coating’s corrosion products. EDS analysis reveals that the corrosion product layer is enriched in O elements and contains a small amount of Al.

Figure 17
Cross-sectional corrosion morphology and EDS analysis results of Al coating after 28 days of total immersion in 3.5 wt.% NaCl solution for (a) Morphology; (b) O element; (c) Al element.

Figure 18 presents the cross-sectional morphology and energy-dispersive X-ray spectroscopy (EDS) analysis results of the Zn15Al coating after 28 days of immersion in a 3.5 wt.% NaCl solution. The corrosion product layer on the surface of the Zn15Al coating is highly dense, with a thickness of approximately 30 μm. This layer is enriched with a large amount of O, along with some Zn and a small amount of Al elements.

Figure 18
Cross-sectional corrosion morphology and EDS analysis results of Zn15Al coating after 28 days of total immersion in 3.5 wt.% NaCl solution for (a) Morphology; (b) O element; (c) Zn element; (d) Al element.

To further identify the compositions of corrosion products on the surfaces of different coatings, X-ray diffraction (XRD) analyses were performed on the three coatings after 28 days of full immersion in a 3.5 wt.% NaCl solution, as shown in Figure 19. By calibrating the XRD patterns of each coating, the corrosion products were determined as follows: Zn₅(OH)₈Cl₂·H₂O for the Zn coating, Al(OH)₃ for the Al coating, and Zn₆Al₂(OH)₁₆CO₃·4H₂O for the Zn15Al coating. Additionally, no iron-containing oxides were detected in any of the three coated specimens, indicating that the Q235 substrates remained uncorroded. This protection is attributed to the effective physical barrier provided by the coatings as well as their sacrificial anode effect.

Figure 19
XRD patterns of Zn, Al, and Zn15Al coatings after 28 days of total immersion in 3.5 wt.% NaCl solution.

According to the reference intensity method [²¹], the percentage of different phases in different coatings can be quantified, as shown in Formula 1:

(1) $$W_J=\frac{I_J/K_S^J}{{\displaystyle {\sum }_{i=1}^n{I}_i/K_S^i}}$$

Among them, IJ is the peak intensity value of phase J, and ${\text{K}}_{\text{s}}^{\text{J}}$ is the K value of phase J, from which the mass percentage of phase J can be calculated.

By consulting the PDF cards for Zn₅(OH)₈Cl₂H₂O and the Zn phase in the Zn coating (PDF numbers 97-009-5365 and 04-003-7274, respectively), it is determined that the reference intensity (RIR) values for Zn₅(OH)₈Cl₂H₂O and the Zn phase are 4.46 and 8.49, respectively. In the XRD spectrum of the Zn coating, the peak intensities of Zn₅(OH)₈Cl₂H₂O and Zn phase are 794 CPS and 9616 CPS, respectively.

Based on the reference intensity method, the mass fractions of Zn₅(OH)₈Cl₂H₂O and Zn phase can be calculated by Formula 2:

(2) $$W_{Zn}=\frac{9616/8.49}{9616/8.49+749/4.46}$$

The mass fractions of Zn₅(OH)₈Cl₂H₂O and the Zn phase are 13.58% and 86.42%, respectively. The main phases present in the Al coating after immersion are Al(OH)₃ and Al. By consulting the Powder Diffraction File (PDF) cards for the Al(OH)₃ and Al phases (PDF numbers 97-002-6830 and 98-000-0062, respectively), the reference intensity ratio (RIR) values were determined to be 1.41 for Al(OH)₃ and 4.47 for Al. In the XRD spectrum of the Al coating, the peak intensities of the Al(OH)₃ and Al phases are 612 counts per second (CPS) and 7796 CPS, respectively. Using these values, the calculated mass fractions of Al(OH)₃ and Al are 19.93% and 80.07%, respectively. The Zn15Al coating primarily consists of Zn₆Al₂(OH)₁₆CO₃·4H₂O, Zn, and Al phases. After reviewing the PDF cards for these phases (PDF numbers 97-015-5052 for Zn₆Al₂(OH)₁₆CO₃·4H₂O, 04-003-7274 for Zn, and 98-000-0062 for Al), their respective RIR values were identified as 6.78, 8.49, and 4.47. From the XRD patterns, the peak intensities of Zn₆Al₂(OH)₁₆CO₃·4H₂O, Zn, and Al are 1643 CPS, 4636 CPS, and 1134 CPS, respectively. Based on these data, the calculated mass fractions of Zn₆Al₂(OH)₁₆CO₃·4H₂O, Zn, and Al are 23.25%, 52.40%, and 24.35%, respectively.

Figure 20 presents the open-circuit potential (OCP) plots and potentiodynamic polarization curves of the Zn, Al, and Zn15Al coatings after 28 days of immersion in a 3.5 wt.% NaCl solution. The results of Tafel fitting for the polarization curves are summarized in Table 4. Open-circuit potential measurements were conducted by immersing the samples in the 3.5 wt% NaCl solution for 600 seconds, during which all samples reached a steady state. From the potentiodynamic polarization curves, it is evident that after 28 days of immersion in the corrosive solution, the Al coating retained the strongest corrosion resistance, with a self-corrosion current density of 2.261 × 10⁻⁶ A·cm⁻². This was followed by the Zn15Al coating (3.593 × 10⁻⁵ A·cm⁻²) and the Zn coating (9.919 × 10⁻⁵ A·cm⁻²).

Figure 20
Zn, Al and Zn15Al coatings after 28 days immersion in 3.5 wt.% NaCl solution. (a) Open circuit potential graph. (b) Dynamic potential polarization graph.

Table 4
Tafel fitting results of polarization curves of Zn, Al and Zn15Al coatings after 28 days immersion in 3.5 wt.% NaCl solution.

SAMPLE	Zn	Al	Zn15Al
Open-circuit potential (V)	−1.043	−0.8509	−0.9712
Ecorr (V vs. SCE)	−1.102	−0.93	−1.151
icorr (A·cm−2)	9.919 × 10−5	2.261×10−5	3.593 × 10−5
βa (mV/dec)	4.451	4.004	9.857
βc (mV/dec)	−7.297	−6.088	−7.332

3.4. Discussion on the corrosion resistance mechanism of arc-sprayed Zn, Al and Zn15Al coatings

Corrosion Mechanism of the Zn Coating: In a 3.5% NaCl solution, numerous tiny anodic and cathodic regions form on the surface of the Zn coating, constituting a corrosion cell. In the anodic regions, zinc atoms oxidize and dissolve into Zn²⁺ ions while releasing electrons. In the cathodic regions (primarily areas with sufficient dissolved oxygen), O₂ accepts electrons and is reduced to OH⁻ ions. The released Zn²⁺ ions combine with the OH⁻ ions generated at the cathode and the Cl⁻ ions in the solution, forming a layer of corrosion products dominated by alkaline zinc chloride near the metal/solution interface.

Zn→Zn²⁺+2e⁻

O₂+2H₂O+4e⁻→4OH⁻

5Zn²⁺+8OH⁻+2Cl⁻+H₂O→Zn₅(OH)₈Cl₂·H₂O

Corrosion Mechanism of the Al Coatings: Initially, the coating contains defects that allow aggressive media to penetrate and initiate corrosion. Coatings with surface defects or pores begin to dissolve after a period of exposure to the corrosive environment.

Al→Al³⁺+3e⁻

Al³⁺+3H₂O→Al(OH)₃+3H⁺

Al³⁺+3Cl⁻→AlCl₃

AlCl₃+3H₂O→Al(OH)₃+3Cl⁻+3H⁺

Corrosion Mechanism of the Zn15Al Coating: The corrosion of the Zn15Al coating in a 3.5% NaCl solution is a typical electrochemical process involving the dissolution of zinc and aluminum phases, oxygen reduction, and the formation of corrosion products. The core reaction equations describing this corrosion mechanism are as follows:

Since Zn (⁻0.76 V) is more active than Al (⁻0.71 V), it preferentially ionizes under microgalvanic effect.

Zn→Zn²⁺+2e⁻

Al rapidly forms a thin amorphous Al₂O₃/Al(OH)₃ passivation film in near-neutral environments, thereby inhibiting its own dissolution.

The hydrolysis of Zn²⁺ causes a decrease in pH at the interface.

Zn²⁺ + 2H₂O → Zn(OH)₂ + 2H⁺

Oxygen reduction occurs at the cathode region.

O₂+2H₂O+4e⁻→4OH⁻

The high pH (>8) in the cathode region dissolves the Al passivation film.

Al₂O₃ + 2OH⁻ → 2AlO₂⁻ + H₂O

Dissolved Al³⁺/AlO₂^-reacts with Zn²⁺ at the pH gradient interface.

Zn²⁺+Al³⁺+OH⁻+ CO₃²⁻→ Zn₆Al₂(OH)₁₆CO₃·4H₂O(LDH)

4. CONCLUSIONS

In this study, three commonly used arc-sprayed coatings—Zn, Al, and Zn15Al—were prepared. The micromorphology, coating quality, and corrosion resistance of these coatings were systematically investigated and compared.

• (1)

  The arc-sprayed Zn and Zn15Al coatings exhibited a lamellar structure: the Zn coating consisted of a single Zn phase, while the Zn15Al coating was composed of a Zn-Al eutectic. In contrast, the arc-sprayed Al coating had a denser internal structure consisting of the Al phase, with no obvious lamellar organization.

• (2)

  The porosities of the Zn, Al, and Zn15Al coatings were all less than 5%. Among them, the Zn coating displayed the lowest cross-sectional porosity at 2.57%. The cross-sectional porosities of the Al and Zn15Al coatings were significantly higher, measuring 3.85% and 4.62%.

• (3)

  All three coatings demonstrated good hydrophobicity. The 3.5 wt.% NaCl solution showed a contact angle of up to 113.1° on the Zn coating surface, while contact angles of up to 130° were observed on the Al and Zn15Al coating surfaces.

• (4)

  The Zn coating achieved a bond strength of up to 7.51 MPa, and the Zn15Al coating exhibited a comparable bond strength of 7.35 MPa. The Al coating displayed the highest bond strength at 10.22 MPa. Fracture in all three coatings occurred at the interface between the coating and the substrate.

• (5)

  The Al coating showed the strongest corrosion resistance, with an initial self-corrosion current density of 1.622 × 10⁻⁶ A·cm⁻². From the potentiodynamic polarization curves, it was observed that after 28 days of immersion in the corrosive solution, the Al coating retained the strongest corrosion resistance, with a self-corrosion current density of 2.261 × 10⁻⁶ A·cm⁻². This was followed by the Zn15Al coating (3.593 × 10⁻⁵ A·cm⁻²) and the Zn coating (9.919 × 10⁻⁵ A·cm⁻²).

• (6)

  With increasing immersion time, the corrosion products on the Al coating surface transitioned from an intermittent to a continuous distribution, consisting primarily of Al(OH)₃. The Zn coating and Zn15Al coating formed corrosion products in flocculent and granular forms, identified as Zn₅(OH)₈Cl₂·H₂O and Zn₆Al₂(OH)₁₆CO₃·4H₂O, respectively.

• (7)

  The excellent cathodic protection properties of Zn coatings make them ideal for safeguarding steel structures in port environments against localized damage or for mitigating the effects of microporous defects in coatings. For Al coatings, their core advantage lies in the formation of a dense, stable aluminum oxide protective layer, which provides exceptional barrier protection. This characteristic makes them particularly suitable for protecting the exterior surfaces of port equipment that are exposed to prolonged high levels of salt spray and humidity. The Zn15Al alloy coating combines the cathodic protection capability of Zn with the superior barrier protection of Al, exhibiting a synergistic effect. It is especially well-suited for protecting critical port equipment components operating under complex working conditions, where multiple corrosion factors overlap and high demands are placed on protection longevity.

5. BIBLIOGRAPHY

• ^[1] ZHANG, P., GAO, Z., CAO, L., et al, “Marine systems and equipment prognostics and health management: a systematic review from health condition monitoring to maintenance strategy”, Machines, v. 10, n. 2, pp. 72, 2022. doi: http://doi.org/10.3390/machines10020072.
  » https://doi.org/10.3390/machines10020072
• ^[2] ALAMRI, A.H., “Localized corrosion and mitigation approach of steel materials used in oil and gas pipelines-an overview”, Engineering Failure Analysis, v. 116, pp. 104735, 2020. doi: http://doi.org/10.1016/j.engfailanal.2020.104735.
  » https://doi.org/10.1016/j.engfailanal.2020.104735
• ^[3] PRICE, J.S., FIGUEIRA, B.R., “Corrosion protection systems and fatigue corrosion in offshore wind structures: current status and future perspectives”, Coatings, v. 7, n. 2, pp. 25, 2017. doi: http://doi.org/10.3390/coatings7020025.
  » https://doi.org/10.3390/coatings7020025
• ^[4] BURSTEIN, G.T., VINES, S.P., “Repetitive nucleation of corrosion pits on stainless steel and the effects of surface roughness”, Journal of the Electrochemical Society, v. 148, n. 12, pp. B504, 2001. doi: http://doi.org/10.1149/1.1416503.
  » https://doi.org/10.1149/1.1416503
• ^[5] SHAW, B., KELLY, R., “What is corrosion”, The Electrochemical Society Interface, v. 15, n. 1, pp. 24–26, 2006. doi: http://doi.org/10.1149/2.F06061IF.
  » https://doi.org/10.1149/2.F06061IF
• ^[6] XU, X., LIU, D., ZHANG, X., et al, “Influence of ultrasonic rolling on surface integrity and corrosion fatigue behavior of 7B50-T7751 aluminum alloy”, International Journal of Fatigue, v. 125, pp. 237–248, 2019. doi: http://doi.org/10.1016/j.ijfatigue.2019.04.005.
  » https://doi.org/10.1016/j.ijfatigue.2019.04.005
• ^[7] VIPRA, R., SINGH, V., SINGH, I., et al, “Slurry erosion behavior of Ni-Al and Fe-Cr coatings deposited by arc wire spray on hydro machinery SS316 steel”, Journal of Materials Engineering and Performance, v. 33, n. 21, pp. 11670–11681, 2024. doi: http://doi.org/10.1007/s11665-023-08818-0.
  » https://doi.org/10.1007/s11665-023-08818-0
• ^[8] SHARMA, H., SINGLA, J., SINGH, V., et al, “Influence of post heat treatment on metallurgical, mechanical, and corrosion analysis of wire arc additive manufactured inconel 625”, Journal of Materials Research and Technology, v. 27, pp. 5910–5923, 2023. doi: http://doi.org/10.1016/j.jmrt.2023.11.074.
  » https://doi.org/10.1016/j.jmrt.2023.11.074
• ^[9] JHA, S., SINGH, V., KUMAR, V.J., et al, “Microstructure, wettability, cavitation and corrosion performance of aluminum (Al6061) coated with RF-sputtered AlN thin film”, Surface and Coatings Technology, v. 489, pp. 131168, 2024. doi: http://doi.org/10.1016/j.surfcoat.2024.131168.
  » https://doi.org/10.1016/j.surfcoat.2024.131168
• ^[10] SINGH, V., KUMAR, V., BANSAL, A., et al, “Investigating the effect of poly-tetra-fluoro-ethylene on the cavitation-erosion and corrosion resistance of HVOF sprayed tic coatings on stainless steel”, Journal of Materials Engineering and Performance, v. 33, n. 23, pp. 12990–13003, 2024. doi: http://doi.org/10.1007/s11665-024-10238-7.
  » https://doi.org/10.1007/s11665-024-10238-7
• ^[11] LEE, H.S., SINGH, J.K., ISMAIL, M.A., et al, “Corrosion mechanism and kinetics of Al-Zn coating deposited by arc thermal spraying process in saline solution at prolong exposure periods”, Scientific Reports, v. 9, n. 1, pp. 3399, 2019. doi: http://doi.org/10.1038/s41598-019-39943-3. PubMed PMID: 30833629.
  » https://doi.org/10.1038/s41598-019-39943-3
• ^[12] CHOE, H.B., LEE, H.S., SHIN, J.H., “Experimental study on the electrochemical anti-corrosion properties of steel structures applying the arc thermal metal spraying method”, Materials, v. 7, n. 12, pp. 7722–7736, 2014. doi: http://doi.org/10.3390/ma7127722. PubMed PMID: 28788271.
  » https://doi.org/10.3390/ma7127722
• ^[13] KATAYAMA, H., KURODA, S., “Long-term atmospheric corrosion properties of thermally sprayed Zn, Al and Zn-Al coatings exposed in a coastal area”, Corrosion Science, v. 76, pp. 35–41, 2013. doi: http://doi.org/10.1016/j.corsci.2013.05.021.
  » https://doi.org/10.1016/j.corsci.2013.05.021
• ^[14] SONG, Q.H., ZHANG, Y.F., LI, Q., et al, “Corrosion electrochemical behavior of arc sprayed Al coatings”, Digest Journal of Nanomaterials and Biostructures, v. 17, n. 3, pp. 825–837, 2022. doi: http://doi.org/10.15251/DJNB.2022.173.825.
  » https://doi.org/10.15251/DJNB.2022.173.825
• ^[15] PAPAVINASAM, S., ATTARD, M., ARSENEULT, B., et al, “State-of-the-art of thermal spray coatings for corrosion protection”, Corrosion Reviews, v. 26, n. 2/3, pp. 105–145, 2008. doi: http://doi.org/10.1515/corrrev.2008.105.
  » https://doi.org/10.1515/corrrev.2008.105
• ^[16] PANOSSIAN, Z., MARIACA, L., MORCILLO, M., et al, “Steel cathodic protection afforded by zinc, aluminum and zinc/aluminum alloy coatings in the atmosphere”, Surface and Coatings Technology, v. 190, n. 2/3, pp. 244–248, 2005. doi: http://doi.org/10.1016/j.surfcoat.2004.04.023.
  » https://doi.org/10.1016/j.surfcoat.2004.04.023
• ^[17] XIAO, Y., JIANG, X., XIAO, Y., et al, “Research on Zn-Al15 thermal spray metal coating and its organic painting composite system protection performance”, Procedia Engineering, v. 27, pp. 1644–1653, 2012. doi: http://doi.org/10.1016/j.proeng.2011.12.632.
  » https://doi.org/10.1016/j.proeng.2011.12.632
• ^[18] SURYANARAYANA, C., Development of zinc and low carbon steel coatings on ABs plastic by electric ARC spraying, SAE Technical Papers, 2022-01-0057, Warrendale, SAE International, 2022.
• ^[19] WU, W.P., SUN, G.Q., WANG, Q.Q., et al, “Preparation, wear resistance, and corrosion performance of arc-sprayed Zn, Al, and Zn-Al coatings on carbon steel substrates”, Journal of Materials Engineering and Performance, v. 32, n. 23, pp. 10542–10555, 2023. doi: http://doi.org/10.1007/s11665-023-07889-3.
  » https://doi.org/10.1007/s11665-023-07889-3
• ^[20] LEE, H.S., SINGH, J.K., PARK, J.H., “Pore blocking characteristics of corrosion products formed on Aluminum coating produced by arc thermal metal spray process in 3.5 wt.% NaCl solution”, Construction & Building Materials, v. 113, pp. 905–916, 2016. doi: http://doi.org/10.1016/j.conbuildmat.2016.03.135.
  » https://doi.org/10.1016/j.conbuildmat.2016.03.135
• ^[21] YU, Y., TIAN, J.T., DAI, J.H., et al, “Corrosion resistance of three-layer superhydrophpbic composite coating on carbon steel in seawater”, Electrochimica Acta, v. 97, pp. 409–419, 2013. doi: http://doi.org/10.1016/j.electacta.2013.03.071.
  » https://doi.org/10.1016/j.electacta.2013.03.071

Publication Dates

• Publication in this collection
  27 Oct 2025
• Date of issue
  2025

History

• Received
  08 Apr 2025
• Accepted
  25 Aug 2025