Corrosion Behavior of High-Pressure Cold-Sprayed Zn30Al Alloy Coating on Q235 Steel

Abstract

This study employed a high-pressure cold spray to apply a Zn30Al alloy coating to Q235 steel substrates to provide corrosion protection for steel in marine environments. The corrosion resistance of the coatings was investigated through full immersion tests, and the corrosion mechanisms were further analyzed using electrochemical experiments. The results were compared with those of traditional flame-sprayed Zn30Al alloy coating. The findings indicate that the high-pressure cold-sprayed Zn30Al alloy coating possesses a dense microstructure with a porosity of only 0.32%, providing effective cathodic protection to the substrate during the immersion tests. The cold-sprayed Zn30Al alloy coating maintained good integrity after 720 h immersion in 3.5 wt.% NaCl solution, whereas the flame-sprayed Zn30Al alloy coating exhibited significant pitting corrosion.

1. Introduction

Steel’s exceptional mechanical qualities and advanced production and processing techniques make it a highly valued material in diverse fields, including in the automotive industry and for aerospace equipment and marine engineering [1,2,3,4]. Steel must withstand intense corrosion in both marine environments and certain rivers, which can lead to component deterioration, financial losses, and safety hazards [5,6]. As a result, significant research efforts are underway to develop effective corrosion protection strategies and prolong the service life of steel [7,8,9].

Thermal spray coatings, notably Zn and Al, represent practical and potent solutions for safeguarding steel against corrosion. Due to its lower electrode potential, Zn functions as a sacrificial anode to the steel [10,11]. On the other hand, Al coatings create a dense Al₂O₃ passive film, ensuring prolonged corrosion resistance [12]. Zn-Al alloy coatings combine the advantages of both Zn and Al coatings, making them well-suited for corrosion protection in marine environments [13,14]. The Al content in Zn-Al coatings is critical, as evidenced by the superior corrosion resistance displayed by Zn-Al coatings with a high Al content [15]. Hu et al. [16] examined the microstructure and corrosion behavior of arc-sprayed Zn-xAl (x = 15, 30, 50) alloy coatings in NaCl solution, and the findings revealed that the Zn-30Al coating exhibited the highest long-term corrosion resistance. However, the increased brittleness of alloy feedstock poses a challenge when preparing Zn-Al alloy coatings with an Al content exceeding 15% using traditional thermal spray methods, like arc spraying, as the process becomes difficult to execute [17,18].

The high temperatures necessary during thermal spray processes can lead to oxidation and increased porosity within Zn-Al alloy coatings, potentially impacting performance and durability. Cold spray (CS), an innovative solid-state deposition method, employs high-velocity gas to deposit coatings onto a substrate below the materials’ melting point. Its benefits include minimal heat input, low oxygen content, and dense coating microstructure, making it suitable for producing Zn-Al alloy coatings [19,20,21]. Xu et al. [22] used low-pressure CS (LPCS) to prepare pure Zn and Zn-15Al alloy coatings, finding that the addition of Al promoted the formation of dense corrosion products such as Zn₆Al₂(OH)₁₆CO₃·4H₂O and Zn₅(OH)₈Cl₂·H₂O while inhibiting the formation of less corrosion-resistant products like ZnO. Zhao et al. [23] used high-pressure CS (HPCS) to produce Zn-Al composite coatings with a 1:1 weight ratio of Zn and Al. The coating exhibited uniform corrosion after long-term neutral salt spray testing.

Overall, there has been limited research on preparing CS Zn-Al coatings, particularly for high-Al content Zn-Al alloy coatings with superior corrosion resistance. This study demonstrates the successful application of HPCS technology to deposit a Zn30Al alloy coating (CS-Zn30Al) onto Q235 steel. An additional Zn30Al coating was fabricated using flame-spraying (FS-Zn30Al) for comparison. Both coatings underwent thorough phase composition and microstructure analysis and immersion tests to evaluate their corrosion resistance. The corrosion mechanisms were analyzed to provide a theoretical basis for their application in marine environments.

2. Materials and Methods

2.1. Raw Powder and Substrate Materials

The commercial Zn30Al alloy powder (70 wt.% Zn, 30 wt.% Al) used in this study was prepared by a gas atomization process with a particle size distribution of 10~35 µm. As shown in Figure 1, the Zn30Al alloy powder exhibits a predominantly spherical morphology. Previous studies have shown that coatings made from spherical powders have lower porosity due to enhanced flow during feeding and more uniform resistance in high-velocity gas streams [24,25]. The EDS test indicates that the alloy powder’s Zn to Al mass ratio is close to 7:3.

Before the experiment, the Q235 substrate was sandblasted using eight-mesh brown corundum at a pressure of 0.6 MPa to enhance surface roughness. Subsequently, the substrate was cleaned with acetone to remove contaminants from the surface.

2.2. Coatings Preparation

CS-Zn30Al coating was prepared using an HPCS system (PCS-H10, Plasma Giken Co., Ltd., Saitama, Japan). Nitrogen served as the carrier gas, with a powder delivery gas pressure of 4 MPa and a temperature of 300 °C. To prevent clogging of the gun barrel, a small powder-feeding rate (0.3 g/s) was adopted in the experiment. The spray gun was mounted on an ABB robot with a standoff distance of 15 mm and moved in raster trajectory at a speed of 40 mm/s.

For flame spray, FS-Zn30Al was applied using Zn30Al alloy wire and an oxygen-acetylene flame spray system (QX-2, Shanghai Ouya Spraying Machinery Co., Ltd., Shanghai, China). Nitrogen was used as the carrier gas to reduce the oxide level in the coating. During flame spraying, the acetylene and oxygen pressure was maintained at 0.13 MPa and 0.6 MPa, with a standoff distance of 200 mm.

2.3. Characterization

The microstructural morphology of the samples was observed using a scanning electron microscope (SEM, Tescan VEGA 3, Brno, Czech Republic). The elemental distribution was characterized using an energy dispersive spectroscopy (EDS) scanner attached to the SEM. Phase analysis of the coatings was conducted using an X-ray diffractometer (XRD, Bruker D8 Advance, Billerica, MA, USA) with a Cu target and Kα radiation. The scan step was set at 0.02° over a 10 to 90° range, with an operating voltage of 20 kV and a current of 50 mA. The porosity of the coatings was quantified using ImageJ software (Version 1.54i), calculating the average value from 1000× magnified SEM images taken from five different locations on the coating. Microhardness measurements were carried out using a microhardness tester (HV-1000Z, Bangyi Precision Measuring Instruments Corp., Ltd., Shanghai, China) at a load of 50 gf applied for a duration of 10 s.

2.4. Corrosion Resistance Test

Before the experiment, the remaining surfaces of the samples were encapsulated with epoxy resin, ensuring an exposed coating area of 1 cm². For the immersion tests, a 3.5 wt% NaCl solution was selected to simulate seawater conditions. The embedded samples were then fully submerged in the solution for a total immersion period of 720 h.

Electrochemical tests, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), were conducted using an electrochemical workstation (CS2350M, Wuhan Corrtest Instrument Corp., Ltd., Wuhan, China). The electrochemical evaluation employed a three-electrode cell setup in which the coating samples served as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode. The PDP data were fitted using the CS Studio6 software, while the EIS data were fitted using the ZView software (Version 3.5i) to establish an equivalent circuit model and analyze the measurement results.

3. Results and Discussion

3.1. Microstructure of Zn30Al Alloy Coatings

Figure 2 presents the XRD spectra of two Zn30Al coatings and the feedstock powder. The diffraction of the Zn30Al alloy powder shows distinctive Zn and Al peaks. The phase composition of the CS-Zn30Al coating remains unchanged as the powder is deposited in a solid state. The XRD pattern of FS-Zn30Al coating also reveals no obvious presence of oxides or other impurities due to the protective effect of nitrogen as the carrier gas.

Figure 3 illustrates the surface morphology of two Zn30Al alloy coatings, in which CS-Zn30Al coating exhibits a dense surface without obvious pores or cracks. Additionally, several spherical particles can be observed on the coating’s surface. This is due to the absence of hammering effects from subsequent particles, leading to incomplete plastic deformation of the particles within the surface layer and consequently increasing the surface roughness. In contrast, the surface micrograph of the FS-Zn30Al coating reveals an uneven distribution of surface morphology. A dense and smooth microstructure is observed near the central region of the flattened surface particles. However, defects in the form of pores can be found at the edges of the particles, as shown in Figure 3c,d, likely attributable to the splashing of the molten particles during the deposition process. The pores, particularly those that develop vertically, can potentially serve as pathways for corrosive media to penetrate and affect the material.

The cross-sectional morphology of the Zn30Al alloy coatings is displayed in Figure 4, indicating that the surface preparation was adequate since a clean coating/substrate interface can be seen and no cracks, porosity, or detachment can be observed in it. During CS, the hardness of the steel substrate is higher than that of the ZnAl alloy particles, causing the deposited particles to undergo significant plastic deformation while the substrate experiences little deformation. The bonding mechanism between the particles and the substrate is primarily mechanical interlocking, accompanied by a small amount of physical or metallurgical bonding. Our previous study [26] indicates that the adhesion of the CS-Zn30Al coating can reach 46.38 ± 1.10 MPa, while thermal sprayed ZnAl coatings in other studies generally exhibit adhesion of less than 10 MPa [27]. Furthermore, Figure 4a reveals a highly dense cross-sectional microstructure in CS-Zn30Al coating with a porosity of only 0.32 ± 0.10%. In contrast, the FS-Zn30Al coating possesses a much higher porosity of roughly 1.8 ± 0.47% (Figure 4c). It is suggested that even though the two alloy coatings share similar phase compositions, their microstructure differs significantly. The lower particle velocity and severe volume change during the flame spray process cause the high porosity in the FS-Zn30Al coating [28]. The porosity decreases remarkably due to pronounced plastic deformation during the CS process, resulting in no discernible pores or cracks between the particles within the CS-Zn30Al coating. The microhardness of the coatings was tested to verify the differences in the microstructure. The result shows that the CS-Zn30Al coating exhibits a higher hardness (68 $\pm$ 0.5 HV_0.05) than the FS-Zn30Al coating (61.5 $\pm$ 2.5 HV_0.05) while maintaining a much smaller standard deviation. This slightly higher hardness is a consequence of cold working and the higher density of discontinuities generated by CS [29]. Figure 4b,d show that both coatings are composed of uniformly distributed bright and dark regions. The EDS examination suggests the bright and dark regions are Zn-rich and Al-rich, respectively.

3.2. Corrosion Morphology of Zn30Al Alloy Coatings

The XRD patterns of two Zn30Al alloy coatings after 720 h immersion experiments are shown in Figure 5. The corrosion products in both coatings contain Zn₅(OH)₈Cl₂·H₂O and Zn₆Al₂(OH)₁₆CO₃·4H₂O. Zn₅(OH)₈Cl₂·H₂O can reduce the current between the coating and the steel substrate, benefiting the corrosion protection of steel [30,31]. At the same time, Zn₆Al₂(OH)₁₆CO₃·4H₂O can form a protective film on the metal surface and seal pores and defects within the metal oxide layer [32]. Moreover, several studies suggest that Al(OH)₃ is considered a precursor, where the position of Al³⁺ is replaced by Zn²⁺ to form Zn₆Al₂(OH)₁₆CO₃·4H₂O [33,34]. The Al(OH)₃ peaks are visible in the XRD spectrum of the FS-Zn30Al coating but absent in the CS-Zn30Al coating. This occurs because the FS-Zn30Al coating has a higher porosity, and Zn₆Al₂(OH)₁₆CO₃·4H₂O formed on the surface cannot entirely seal the pores, facilitating the penetration of corrosive media into the interior of the coating and the continuous generation of fresh corrosion products. The dissolution of metals and the formation of corrosion products occur as follows:

Zn = Zn²⁺ + 2e

(1)

Al = Al³⁺+ 3e

(2)

5Zn²⁺ + 8OH⁻ + 2Cl⁻ + H₂O = Zn₅(OH)₈Cl₂·H₂O

(3)

Al³⁺ + 3OH⁻ = Al(OH)₃

(4)

2Al(OH)₃ + 6Zn²⁺ + 10OH⁻ + CO₃²⁻ + 4H₂O = Zn₆Al₂(OH)₁₆CO₃·4H₂O

(5)

Figure 6 illustrates the surface morphology of the two coatings after the immersion test. The corrosion products covering the surface of CS-Zn30Al coating are evenly distributed (Figure 6a). High-magnification images reveal that the products comprise tiny flakes, with some agglomerating into clusters. In addition, microcracks are also observed on the coating surface, which various factors, such as the growth strength of the corrosion product film, may cause. Another reasonable explanation is the faster corrosion rate of Zn within the coating, whereas Al remains relatively stable, resulting in preferential Zn corrosion and subsequent crack formation [19]. In contrast, the corrosion product film on the surface of the FS-Zn30Al coating is broken and can no longer provide effective corrosion protection. High-magnification images reveal that although the products are also composed of flakes, the size of the flakes and clusters is significantly larger than that of CS-Zn30Al coating. This can lead to the formation of a loose surface structure, making it more susceptible to the penetration of corrosive media. The EDS results obtained on the surfaces of two coatings are presented in

Table 1. EDS scanning of the surfaces of two coatings following immersion tests.

Coating	Elemental Weight Percentages/%
	Zn	Al	O	Cl	Na	Si
CS-Zn30Al	34.49	17.01	40.30	0.87	6.66	0.67
FS-Zn30Al	39.80	9.64	36.58	2.24	5.21	6.54

.

The cross-sectional SEM images of two coatings after the immersion test are displayed in Figure 7. Notably, the FS-Zn30Al coating exhibits a markedly wider dark corrosion region at its top compared to the CS-Zn30Al coating. Chemical composition analyses were performed on both the corroded and uncorroded areas of two coatings, and the results are shown in

Table 2. EDS scanning of four regions in Figure 7.

Region	Elemental Weight Percentages/%
	Zn	Al	O	Cl
A	15.70	54.30	27.57	2.43
B	62.69	37.31	-	-
C	21.51	61.36	15.92	1.21
D	62.84	33.91	3.25	-

. The corrosion products were mainly composed of Zn, Al, O, and Cl, which is consistent with the XRD results. The mass ratio of Zn to Al in the corrosion layers is much smaller than that of as-deposit coatings (7:3), indicating that Zn is corroded preferentially in a 3.5% NaCl solution. In addition, O was detected in region D inside the FS-Zn30Al coating, indicating that the corrosive media has penetrated the coating. The CS-Zn30Al coating showed superior corrosion resistance as no oxygen was detected inside the coating (region B).

To quantify the corrosion resistance of the coating, the changes in the mass and thickness of the coating before and after corrosion were measured. After 720 h of immersion, the average thickness reduction for the CS-Zn30Al and FS-Zn30Al coatings was 33.3 µm and 76.9 µm, respectively, while the weight losses of the two coatings were 11.6 mg and 35.1 mg. Assuming a density of 4.78 g/cm³ for the Zn30Al alloy, the discrepancy between the material loss of the FS-Zn30Al coating as measured by thickness reduction and weight loss is less than 5%. However, the thickness reduction of the CS-Zn30Al coating has been overestimated. This error may arise from the higher surface roughness and difficulty defining the boundary between the CS-Zn30Al coating and the dense corrosion product. Thus, the CS-Zn30Al coating exhibits a corrosion rate of approximately one-third of that of the FS-Zn30Al coating, highlighting the remarkable corrosion resistance properties of CS coatings.

3.3. Electrochemical Performance of Zn30Al Alloy Coatings

To gain a deeper understanding of the corrosion behavior of the Zn30Al alloy coating, PDP curves were plotted at various immersion intervals (24, 120, 240, and 720 h) and are presented in Figure 8. During the initial immersion period, both coatings showed signs of passivation, which intensified markedly after 120 h. Notably, after 720 h of immersion, only the CS-Zn30Al coating exhibited a distinct passivation region, suggesting the breakdown of the passivation film on the FS-Zn30Al coating surface due to its inferior corrosion resistance. The corrosion current density (I_corr) obtained from the polarization curves is presented in

Table 3. The corrosion current density and potential of different coatings after various immersion times.

Coating	Ecorr/V (vs. OCP)				Icorr/(10−5A/cm2)
	24 h	120 h	240 h	720 h	24 h	120 h	240 h	720 h
CS-Zn30Al	−1.365	−1.414	−1.326	−1.258	3.875	4.416	3.991	2.812
FS-Zn30Al	−1.315	−1.391	−1.157	−0.969	7.987	5.883	11.970	9.061

. The initial I_corr of the CS-Zn30Al coating is 3.875 × 10⁻⁵ A/cm², less than half of the FS-Zn30Al coating (7.987 × 10⁻⁵ A/cm²). Following fluctuations, the final I_corr of the CS-Zn30Al coating after 720 h immersion was 2.812 × 10⁻⁵ A/cm², representing even less than a third of that of the FS-Zn30Al coating (9.061 × 10⁻⁵ A/cm²). The results verify the superior corrosion resistance of the CS-Zn30Al coating, consistent with the findings from weight-loss tests.

Figure 9 and Figure 10 depict the results of electrochemical impedance spectroscopy (EIS) conducted on coatings after varying immersion durations, along with the corresponding selected fitting equivalent circuit. In the equivalent circuit, Rc and CPE1 represent the coating’s resistance and capacitance. Meanwhile, Rs, Rct, CPE2, and W stand for the solution resistance, charge transfer resistance, double layer capacitance, and Warburg impedance, respectively. All capacitors are replaced by constant phase elements (CPE), and the impedance of a CPE can be defined as the following [9]:

$$Z_{CPE}={\displaystyle \frac{1}{Y_0(\mathrm{jw}{)}^{\mathrm{n}}}}$$

(6)

where j² = −1, Y₀ is the frequency-independent admittance, n is the CPE power, and w = 2Tf is the angular frequency. The factor n is an adjustable parameter that lies between 0 and 1. The CPE element represents a capacitance when n = 1.

Throughout the 720 h immersion period, the EIS analysis of the FS-Zn30Al coating only exhibited a single capacitive loop, modeled by the equivalent circuit R(R.Q.). This pattern suggests that the primary corrosion mechanism of the FS-Zn30Al coating involves the active dissolution of the metal coating. This observation aligns with SEM findings, which reveal a disrupted corrosion product film, resulting in continuous exposure of the coating material to corrosive environments. As a result, the FS-Zn30Al coating maintains a single capacitive loop throughout the immersion test. In contrast, the equivalent circuit for the CS-Zn30Al coating after a 24 h immersion period is described as R(Q(R.W.)), while a small capacitive loop in the high-frequency range of the EIS indicates the formation of an initial passivation film on the coating surface. Previous studies suggest that this feature points to the development of a compact oxide layer on the coating surface [35,36]. As the immersion duration extends to 120 h, the equivalent circuit evolves into R(Q(R(Q.R.))), suggesting a degradation in the corrosion product layer. Furthermore, the progressive reduction in the capacitive loop’s radius throughout the immersion process confirms the gradual decline in the coating’s protective barrier effectiveness against corrosion.

4. Conclusions

In this study, Zn30Al alloy coatings were prepared on the surface of the Q235 steel substrate using HPCS equipment. The microstructure and corrosion resistance of the coatings were investigated, leading to the following conclusions:

• Cold-sprayed and thermal-sprayed Zn30Al alloy coatings are composed of Zn and Al without obvious oxide impurities. Owing to the advanced solid-state deposition, the CS-Zn30Al coating shows a much denser microstructure with a porosity of only 0.32%. In contrast, the porosity of the FS-Zn30Al alloy coating is significantly higher, reaching 1.8%.
• The dominant corrosion products in both Zn30Al coatings are Zn₅(OH)₈Cl₂·H₂O and Zn₆Al₂(OH)₁₆CO₃·4H₂O, and take the form of flakes and clusters. Notably, the CS-Zn30Al coating exhibits significantly smaller flake sizes, forming a dense corrosion product layer that offers enhanced protection against corrosive media.
• Upon immersion in a 3.5 wt.% NaCl solution for 720 h, the weight loss of the CS-Zn30Al coating was less than one-third of that of the FS-Zn30Al coating. Furthermore, electrochemical tests have validated the significantly lower corrosion current of the CS-Zn30Al coating, demonstrating the remarkable corrosion resistance offered by cold-sprayed coatings.