Effect of Al₂O₃ on Microstructure and Corrosion Characteristics of Al/Al₂O₃ Composite Coatings Prepared by Cold Spraying

Abstract

Cold spraying was used to prepare Al/Al₂O₃ composite coatings. The Al₂O₃ content was controlled to increase the mechanical property and corrosion resistance of the composite coating. The inclusion of Al₂O₃ particles results in considerable plastic deformation of Al particles and grain size refinement in the coating. Additionally, the coating’s surface roughness decreased from 24.63 μm to 9.02 μm, and the porosity decreased from 6.34% to 2.07%. The increase in microhardness of the composite coatings was attributed to the combined effect of residual compressive stress, second phase strengthening of Al₂O₃, and plastic hardening of Al particles. The electrochemical test results indicate that the mass fractions of Al₂O₃ significantly affected the corrosion resistance of the Al/Al₂O₃ composite coating. Compared to the Al coating, the composite coating exhibited improved corrosion resistance, with a reduction in corrosion current density from 1.09 × 10⁻³ A/cm² to 2.67 × 10⁻⁶ A/cm² and an increase in corrosion potential from −1.57 V to −1.14 V. However, when the alumina particle content exceeded 17.7%, it led to more Al₂O₃ particle breakage, increasing the weak bonding interfaces in the composite coating, and consequently reducing its corrosion resistance.

1. Introduction

Among the coating materials, aluminum coating is extensively utilized in aviation and space, transportation, marine engineering equipment, and electronics owing to its excellent anti-corrosion properties, and its electrical and thermal conductivity [1,2]. However, the application of aluminum coatings was limited due to its relatively low hardness and insufficient wear resistance. Therefore, the current research focus is on how to modify the Al coating to improve its overall performance. Aluminum metal matrix composite (Al-MMC) coatings containing ceramic reinforcements are a type of high-performance surface-modified coating [3,4]. Ceramic particles in Al-MMC materials can influence the degree of deformation of Al particles [5], resulting in a decrease in porosity [6] and roughness, and an increase in hardness [7], abrasion resistance [8], and corrosion resistance [9] of the coating, and it can also change the physical properties of the coating such as the strength-to-density ratio and high-temperature performance [10].

Cold spraying is a common method for preparing composite coatings [11]. Cold spraying technology is a surface coating technology developed based on the principles of gas dynamics and high-speed collision dynamics [12,13,14]. Cold spraying technology has the characteristics of high spraying rate, the high strength of the sprayed bonding layer, dense coating, and low temperature compared to preparation methods such as thermal spraying [15], electroplating [16], laser cladding [17], and chemical vapor deposition (CVD) [18]. In recent years, scholars have investigated the properties of cold-sprayed Al-MMC coatings. For instance, in their study, Wang et al. [19] investigated the corrosion characteristics of Al-MMC coatings and determined their anticorrosion mechanism. They found that the ceramic particle content and porosity of composite coatings have a great influence on corrosion resistance. However, the deposition efficiency and coating porosity of ceramic particles are related to the sizes of ceramic particles. Yang et al. [20] prepared an Al/Al₂O₃ composite coating by spraying the raw material containing Al₂O₃ particles onto the surface of the substrate using the cold spraying technique. The results show that the composite coating had the best corrosion resistance when the Al₂O₃ content was 20 vol. %. Xie et al. [21] prepared TiB₂ particle reinforced 7075Al composite coatings using the cold spraying technique, and reduced coating porosity while simultaneously improving the corrosion resistance of the coating. Qiu et al. [22] mixed spherical, irregular, and spherical–irregular Al₂O₃ particles into cold spray powder materials and studied the influence of Al₂O₃ morphology on coating performance. The research indicates that spherical particles have the best mechanical interlocking connection with the substrate and the lowest surface porosity. Wang et al. [23] utilized the cold spraying technique to prepare Al/Al₂O₃ composite coatings and investigated the microstructure and nanomechanical properties of the composite coatings in orthogonal and normal planes. The experimental results show that the degree of plastic deformation of the Al particles increases due to the tamping effect of the high-speed Al₂O₃ particles, leading to grain refinement along the grain boundaries of the composite coating, resulting in increased hardness and reduced friction coefficient. Many researchers have investigated the mechanical characteristics and corrosion resistance of cold-sprayed Al-MMC coatings [24,25], but comparatively little work has been done on the effect of the microstructure of Al-MMC coatings on the corrosion mechanism and process.

This paper aims to further explore and elucidate the effect of doping Al₂O₃ particles on the microstructure evolution and performance improvement of Al/Al₂O₃ composite coatings, building upon previous studies. The fabrication of Al/Al₂O₃ composite coatings on the surface of No. 45 steel was carried out using the cold spraying technique, with varying Al₂O₃ contents (mass fractions of 0%, 17.7%, 25.5%, 33.3%). A characterization of the microhardness and electrochemical corrosion behavior of the Al/Al₂O₃ composite coatings was conducted, along with an analysis of microstructure evolution.

2. Materials and Methods

2.1. Experimental Materials

Commercial spherical Al powder (diameter 20 µm) and Al₂O₃ powder (diameter 20 µm) were used in the cold spray experiment, as illustrated in Figure 1a,b. The substrate material selected was No. 45 steel, the composition of which is detailed in

Table 1. Chemical composition of No. 45 steel.

C	Mn	Si	Cr	Ni	Cu
0.42–0.50%	0.50–0.80%	0.17–0.37%	≤0.25%	≤0.30%	≤0.25%

. The dimensions of the substrate were 30 mm × 30 mm × 5 mm.

The carrier gas for cold spraying was compressed air at a pressure of 1.0 MPa and a temperature of 500 °C. The spray gun outlet was positioned 10 mm away from the substrate, while the traveling speed was set at 60 mm/s. The coating thickness obtained by cold spraying was about 400–500 μm.

2.2. Test Methods

The surface cross-section morphology of the cold-sprayed coatings was observed using a scanning electron microscope (SEM, Hitachi, Tokyo, Japan, SU5000). The chemical composition of the coatings was analyzed using an energy dispersive spectrometer (EDS, Ultimately Max40, Oxford Instruments, Abingdon, UK), and the microstructural evolution of the coatings was characterized using electron backscatter diffraction (EBSD, Symmetry, Oxford Instruments, Abingdon, UK). Porosity statistics of the coatings were obtained from SEM images of five different areas of the coating cross-section using Image J software (v1.52a, NIH, Rockville, MD, USA). The phase composition of the coatings was analyzed using an X-ray diffractometer (XRD, X’Pert PRO, Panalytical, Almelo, The Netherlands). Microhardness measurements of the samples were conducted using a Vickers hardness tester (HVS-1000Z, Shangcai, Shanghai, China) with a load of 50 g and a holding time of 15 s. Measurements were taken three times at 20 μm intervals along the depth direction. The surface roughness of the coating was measured using an ultra-depth-of-field three-dimensional microscope (OLYMPUS, Tokyo, Japan, DSX1000).

The electrochemical properties of the Al/Al₂O₃ composite coating were determined using an electrochemical workstation (VMP3, BioLogic, Seyssinet-Pariset, France) at 25 ± 1 °C in a 3.5 wt. % NaCl solution. In a three-electrode system, the working electrode (WE) was the coating, the reference electrode (SCE) was an Ag/AgCl electrode, and the counter electrode (CPE) was a platinum electrode. The reference electrode was positioned within 3 mm of the sample surface, and the counter electrode was placed at an equal distance from the other two electrodes. The open circuit potential (OCP) of the sample was measured. The electrochemical impedance spectroscopy (EIS) of the composite coating was evaluated at frequencies ranging from 10 mHz to 100 kHz, with the voltage amplitude was OCP ± 10 mV. Subsequently, potentiodynamic polarization curves of the samples were obtained. The electrochemical data of the cold-sprayed coatings were determined by fitting the potentiodynamic polarization curves by Tafel extrapolation. The tested area of the samples was 1 cm². All electrochemical tests were repeated at least three times for better accuracy.

3. Results

3.1. XRD Analysis

The XRD test results of the coating are shown in Figure 2. The results show that in the Al coating, the diffraction peaks were located at 38.5°, 44.7°, 65.1°, and 78.2°, which correspond to the (111), (200), (220), and (311) crystal planes, respectively. In addition, in the Al/Al₂O₃ composite coatings with three different Al₂O₃ contents, typical characteristic peaks of Al₂O₃ appeared at 52.6°, 57.5°, and 68.2°. These correspond to crystal planes (024), (116) and (300), respectively. As can be seen from the Figure 2, no new phases appeared during the composite coating-manufacturing process. The right side of Figure 2 shows a magnified partial XRD spectrum from 37° to 40°, indicating a shift of characteristic peaks toward higher angles. This phenomenon was due to stress changes within the coating. The compressive residual stress was caused by the shot peening effect of Al₂O₃ particles on Al coating. The presence of compressive residual stresses induced a reduction in the interplanar crystal spacing, accompanied by a shift in the characteristic peaks of Al phase to higher angles. Residual compressive stress is beneficial to the improvement of the mechanical and electrochemical properties of the composite coating by reducing the porosity of the coating and preventing microcrack propagation.

3.2. Surface Morphology of the Coating

Figure 3 presents the micro-morphology of the Al coating and the Al/Al₂O₃ composite coatings. In Figure 3a, the surface morphology of the Al coating reveals shallow craters and spherical Al particles. This occurrence is attributed to the relatively small plastic deformation experienced by the aluminum particles, allowing some particles to maintain their spherical shape. In contrast, the Al/Al₂O₃ composite coating does not exhibit a similar phenomenon. The peening action of the Al₂O₃ particles leads to the significant secondary plastic deformation of the co-deposited Al particles, resulting in the flattening of the spherical Al particles. Figure 3c illustrates that the coating surface displays deeper craters due to the presence of embedded spherical Al₂O₃ particles. Furthermore, Figure 3e demonstrates that an increase in the percentage of Al₂O₃ particles leads to the fragmentation of the embedded particles. Lastly, Figure 3g depicts the collision-induced breakage of some Al₂O₃ particles, causing the detachment of certain embedded Al₂O₃ particles from the coating.

As can be seen from Figure 3b,d,f,h, a small number of micropores developed between the particles. The process characteristics of cold spraying resulted in the formation of micropores in the deposited coating. In Figure 3h, it can be observed that with the increase in Al₂O₃ content in the raw material, the Al₂O₃ particles in the composite coating tended to change from internal cracks to fracture separation, which is consistent with the results observed in the surface microstructure. This indicates that the Al₂O₃ particles deposited in the coating are more susceptible to being broken by the impact of the subsequent Al₂O₃ particles with the increase in the Al₂O₃ content in the raw material in the spraying process.

The porosity of the coatings was calculated from the cross-sectional images using Image J software, and the porosity data are shown in

Table 2. Porosity of the cold spraying coatings.

Specimen	Al	Al/17.7% Al2O3	Al/25.5% Al2O3	Al/33.3% Al2O3
Porosity (%)	6.34 ± 0.08	2.07 ± 0.12	3.39 ± 0.07	3.59 ± 0.10

. The Al coating showed the highest porosity at around 6.34%. The composite coating’s porosity decreased to 2.07% with an increase in Al₂O₃ content to 17.7%. This decrease can be attributed to the compaction effect of the alumina particles. The hardness and density of Al₂O₃ particles are higher than those of Al particles. Including Al₂O₃ particles in the spraying process enhances the deformation of the Al particles, resulting in the sealing of micropores and cracks in the coatings. However, the porosity of the composite coatings increased slightly with increasing Al₂O₃ content. The porosity values of the Al/25.5% Al₂O₃ and Al/33.3% Al₂O₃ composite coatings were 3.39% and 3.59%, respectively. With increasing Al₂O₃ concentration in the composite coating, the probability of collision between the Al₂O₃ particles increases, resulting in more alumina particles being broken. Fractured Al₂O₃ particles increase microporosity and microcracking in Al/Al₂O₃ composite coatings, causing coating porosity to increase.

3.3. Coating Hardness and Roughness

The microhardness distribution of coatings with different Al₂O₃ contents is shown in Figure 4. With the increase in depth, the microhardness of the coating did not show a significant difference. This phenomenon is an indication that the Al₂O₃ particles are uniform throughout the coating. The average hardness of the Al coating is 21.62 HV, and the average hardness of the Al/17.7% Al₂O₃, Al/25.5% Al₂O₃, and Al/33.3% Al₂O₃ composite coatings are 47.25 HV, 48.54 HV, and 49.55 HV, respectively. The average hardness of the sprayed coating significantly increased when ceramic particles of Al₂O₃ were added. This phenomenon can be attributed to three reasons. Firstly, the Al₂O₃ particles enhanced the second phase-strengthening effect of the aluminum-based composite coating, thereby increasing its hardness. Secondly, the presence of ceramic particles created residual stress in the coatings, leading to changes in the crystal microstructure and grain morphology, consequently enhancing their hardness. Lastly, the addition of Al₂O₃ particles refined the Al grain and increased the number of grain boundaries. The presence of grain boundaries inhibited dislocation movement, increasing the yield stress in the composite and consequently the average coating hardness. The increase in coating hardness was relatively low after the Al₂O₃ content exceeded 17.7%. This was because, when a certain amount of reinforcement by ceramic particles occurred, the dislocation density significantly increased, leading to dislocation entanglement during dislocation movement. As a result, further dislocation movement is hindered, resulting in a relatively low increase in coating hardness.

Figure 5 shows the 3D surface morphology and surface root-mean-square roughness (Sq) of the Al coating, as well as the Al/17.7% Al₂O₃, Al/25.5% Al₂O₃ and Al/33.3% Al₂O₃ composite coatings, respectively. As the Al₂O₃ concentration increased, the surface roughness of the composite coatings tended to decrease and then increase. The Al coating showed a surface roughness of 24.63 μm. The Al/17.7% Al₂O₃ composite coating was 9.02 μm, the Al/25.5% Al₂O₃ composite coating was 15.63 μm, and the Al/33.3% Al₂O₃ composite coating was 16.94 μm. The incorporation of Al₂O₃ particles resulted in a smoother composite coating surface. The 3D morphology of the coatings shows that the surface of the Al coating was very rough and wavy and the Al/Al₂O₃ composite coating’s surface was relatively flat. The addition of Al₂O₃ particles caused the deformation of already coated Al particulates, gradually flattening the Al particles, and reducing the roughness of the coating. However, as the addition of Al₂O₃ particles increased, the cooperative deformation ability of Al particles decreased, and the probability of a mutual impact of Al₂O₃ particles rose, causing the Al₂O₃ particles to fracture. The broken Al₂O₃ particles were embedded in the coating surface, leading to a slight increase in the surface roughness of the coating.

3.4. Corrosion Experiment

3.4.1. Coating Electrochemical Corrosion Behavior

The potentiodynamic polarization curves of the cold spray coating measured in a 3.5 wt. % NaCl solution are shown in Figure 6a. The open circuit potentials (OCP) of the pure Al coating, and the Al/17.7% Al₂O₃, Al/25.5% Al₂O₃, and Al/33.3% Al₂O₃ composite coatings, were −1.46 V, −1.08 V, −1.25 V, and −1.20 V, respectively. The OCP values of the Al coatings were more negative compared to the Al/Al₂O₃ composite coatings, which can be a preliminary indication that the incorporation of Al₂O₃ particles improved the corrosion resistance of the coating. The corrosion potential (E_corr) and corrosion current density (I_corr) for different samples are presented in

Table 3. Results of Tafel extrapolation of potentiodynamic polarization curves and electrochemical data obtained via equivalent circuit fitting for cold-sprayed coatings.

Specimen	Ecorr (V)	Icorr (A/cm2)	Rp (Ω·cm2)	Rs (kΩ·cm2)	RI (kΩ·cm2)	Qc (μF·cm−2)	nc	Rct (kΩ·cm2)	Qdl (μF·cm−2)	ndl
Al	−1.57	1.09 × 10−3	139.4	1.72 × 10−2	0.071	229.7	0.71	2.20	596.9	0.62
Al/17.7%Al2O3	−1.14	2.67 × 10−6	21,447.7	1.58 × 10−2	7.82	35.1	0.87	3.41	487.4	0.92
Al/25.5%Al2O3	−1.26	2.12 × 10−5	2579.6	1.82 × 10−2	2.04	50.4	0.70	2.91	25.9	0.98
Al/33.3%Al2O3	−1.22	1.54 × 10−5	6967.8	1.84 × 10−2	3.49	45.6	0.83	3.01	164.2	0.61

. The E_corr and I_corr of the Al coating were −1.58 V and 1.09 × 10⁻³A/cm². For the Al/17.7% Al₂O₃, Al/25.5% Al₂O₃, and Al/33.3% Al₂O₃ composite coatings, the E_corr and I_corr were −1.14 V and 2.67 × 10⁻⁶ A/cm², −1.25 V and 2.12 × 10⁻⁵ A/cm², and −1.22 V and 1.54 × 10⁻⁵ A/cm², respectively. Notably, the E_corr of the Al/Al₂O₃ composite coating exhibited an increase compared to that of the Al coating, concomitant with a decrease in I_corr. The E_corr serves as an indicator of corrosion difficulty, while I_corr mirrors the rate of corrosion. The increase in E_corr and the decrease in I_corr indicate an improvement in the corrosion resistance of the composite coating. The E_corr values of the Al/25.5% Al₂O₃ and Al/33.3% Al₂O₃ composite coatings were higher than that of the Al coating but lower than that of the Al/17.7% Al₂O₃ composite coating. The I_corr was lower than that of the Al coating but higher than that of the Al/17.7% Al₂O₃ composite coating. The corrosion behavior of the coating is influenced by the content of Al₂O₃ particles. The addition of Al₂O₃ particles can make the coating denser and improve the corrosion resistance. However, the particle boundary between Al particles and Al₂O₃ particles provides a channel for the entry of NaCl solution. Al₂O₃ particles undergo fragmentation due to excessive addition form more particle boundaries. Therefore, the corrosion resistance of the coating increases and then decreases with the increase in alumina content.

The corrosion resistance of Al/Al₂O₃ composite coatings was evaluated using electrochemical impedance spectroscopy (EIS). Figure 6b depicts the Nyquist plots generated from tests conducted on cold-sprayed composite coatings immersed in a 3.5% NaCl solution. The diameter of the capacitive arc in the high-frequency region of the Nyquist diagram has a significant correlation with the corrosion resistance of the coatings. By adding ceramic particles, the arc diameter of the Al/Al₂O₃ composite coating is made considerably larger than that of the pure aluminum coating at a high frequency. This disparity signifies a noteworthy enhancement in corrosion resistance for the composite coating featuring added alumina, as opposed to its pure aluminum counterpart. Among the Al/Al₂O₃ composite coatings, the Al/17.7% Al₂O₃ coating has the largest capacitive arc diameter. The capacitive arc diameter of the Al/33.3% Al₂O₃ coating is slightly larger than the Al/25.5% Al₂O₃ coating. Figure 6c,d shows the Bode plot obtained after impedance testing. According to previous studies [26], the magnitude of low-frequency impedance |Z| is positively correlated with corrosion resistance. From Figure 6c, we see that the |Z| of the Al/17.7% Al₂O₃ composite coating was the largest, which also proves its best corrosion resistance. Figure 6d illustrates a phase diagram used for calculating the interfacial processes involved in impedance data. As shown in the figure, the phase diagrams of almost all coatings have two peaks throughout the test, i.e., two time constants (TC). One is in the mid-frequency region (MF), indicating the presence of an oxide film on the coating surface. The other is in the low-frequency region (LF). The low-frequency region is characterized by pitting formation, which is due to the relaxation process of adsorbed substances (e.g., Cl⁻) acquired in the vulnerable region [27], and the diffusion resistance in the system decreases with increasing Al₂O₃ content, which corresponds to the porosity of the composite coating in Table 2.

The obtained electrochemical impedance spectroscopy can be fitted using the ZSimpWin software (v3.60, AMETEK Scientific Instruments, Berwyn, IL, USA), which helps in reflecting the electrochemical processes at the interface between the sample and the electrolyte. Figure 6b shows the fitted equivalent circuit, which indicates that the oxide film on the coating surface is not uniform [28]. In the figure, R_s is the solution resistance, R_ct is the charge transfer resistance of the aluminum coating, and R_I is the added resistance of the electrolyte within the localized corrosion site. Considering the non-ideal nature of the system, we used a constant phase element CPE (abbreviated as Q) instead of a pure capacitor [29,30]. Q_C is an oxide film capacitor and Q_dl is a double-layer capacitor. The R_I and R_ct values derived from the fitting results are shown in Table 3. At low frequencies, the current cannot flow through the oxide film, only through the corrosion site. At high frequencies, the current can pass through the oxide film. The combined values of R_I and R_ct in the fitting results determine the corrosion resistance of the coating. A larger value of R_I + R_ct indicates the better corrosion resistance of the coating. The R_I + R_ct values of the coatings were 2.27 kΩ·cm², 11.23 kΩ·cm², 4.95 kΩ·cm² and 6.50 kΩ·cm², respectively. The coatings’ corrosion resistance were ranked, in descending, order as follows: Al/17.7% Al₂O₃ > Al/33.3% Al₂O₃ > Al/25.5% Al₂O₃ > Al. These results align with the potentiodynamic polarization curves. The mechanism of the improvement in the corrosion resistance of composite coatings is explained in detail in the discussion section.

3.4.2. Surface Morphology after Electrochemical Corrosion

Figure 7 depicts the surface morphology and chemical element composition of the Al coating and Al/Al₂O₃ composite coating after undergoing a corrosion test. The specific content of each element is presented in

Table 4. EDS analysis results of the coating surface after corrosion.

Specimen	Al (wt. %)	Cl (wt. %)	O (wt. %)
Al	79.22	2.66	18.12
Al/17.7%Al2O3	80.26	0.40	19.33
Al/25.5%Al2O3	73.01	1.18	25.82
Al/33.3%Al2O3	69.08	1.86	37.16

. Upon examination of Figure 7a,e,i,m, it was observed that the Al coating displayed significant corrosion holes, whereas the Al/Al₂O₃ composite coating exhibited smaller corrosion holes, which is mainly attributed to pitting corrosion. Additionally, the pure Al coating demonstrated the highest Cl content at 2.66 wt. %. On the other hand, the composite coatings, namely, Al/17.7% Al₂O₃, Al/25.5% Al₂O₃, and Al/33.3% Al₂O₃, showcased lower Cl contents at 0.40 wt. %, 1.18 wt. %, and 1.86 wt. %, respectively. The combination of morphology and data leads to the conclusion that the Al coating is more susceptible to corrosion. The Cl⁻ in the electrolyte reacted with the passivation film on the surface of the coating in a nucleophilic substitution reaction, resulting in the destruction of the passivation film. The reaction is as follows:

Al₂O₃ + 8Cl⁻ = 2AlCl⁴⁻ + 3O²⁻

(1)

O²⁻ + H₂O = 2OH⁻

(2)

AlCl⁴⁻ + 3OH⁻ = Al(OH)₃ + 4Cl⁻

(3)

The whole reaction is equivalent to Cl⁻ catalyzing the following reactions:

Al₂O₃ + 3H₂O = 2 Al(OH)₃

(4)

At the same time, the surface of the coating constituted a galvanic cell as the NaCl solution acts as an electrolyte. The cathode generated OH⁻, which led to a decrease in the PH value of the electrolyte, and the anode generated Al³⁺. The anode and cathode reacted as:

O₂ + H₂O + 3e⁻ → 4OH⁻

(5)

Al → Al³⁺ + 3e⁻

(6)

The corrosion resistance of the coating is negatively correlated with the content of Cl element. Cl⁻ tended to accumulate in corrosive areas. From Figure 7m,o, we see that the Cl element was mainly concentrated at the bonding interface between the Al matrix and Al₂O₃ particles. The bonding between Al particles and Al₂O₃ particles was weak, and a weak bonding surface was present. The weak binding surface provided a channel for the electrolyte to enter the coating. When the amount of alumina particles added exceeded 17.7%, more alumina broke due to collisions. The broken alumina particles created additional weak bonding surfaces in the coating, resulting in reduced corrosion resistance. The O contents of the Al coating and the Al/17.7% Al₂O₃, Al/25.5% Al₂O₃, and Al/33.3% Al₂O₃ composite coatings were 18.12 wt. %, 19.33 wt. %, 25.82 wt. %, and 37.16 wt. %, respectively. The surface oxygen contents of the coatings increased with the increase in Al₂O₃ content. The main sources of O on the surface of the coating were (1) Al₂O₃ particles containing the element O; also, (2) Al reacts with O₂ in the electrolyte to form Al₂O₃; (3) OH⁻ generated by the cathodic reaction reacts with Al³⁺ to form Al(OH)₃, and (4) Al(OH)₃ is further converted to insoluble Al₂O₃·3H₂O [31]. The reactions are as follows:

4Al + 3O₂ = 2Al₂O₃

(7)

Al³⁺ + 3OH⁻ → Al(OH)₃

(8)

2Al(OH)₃ → Al₂O₃·3H₂O

(9)

These reaction products covered the surface of the coating, making it more difficult for the coating to corrode further.

4. Discussion

To further investigate the microstructural evolution of coatings with different alumina contents, EBSD analysis was performed on Al coatings and Al/17.7% Al₂O₃ composite coatings. Figure 8a,d shows the IPF maps of the Al coating and Al/17.7% Al₂O₃ composite coatings, respectively, illustrating the grain orientation and microstructure within the coatings. The coatings underwent severe plastic deformation due to the high velocity impact. The surfaces were characterized by small grains surrounding large grains. The appearance of this structure is attributed to the localized distortion of particles concentrated at the particle interface during rapid collisions. In other words, a large amount of energy from particle impact was taken up by the particle surface, leading to particle refinement. High-speed particle impacts led to the accumulation of high dislocation densities, which further caused dynamic recrystallization, resulting in grain refinement. Crystal rotation is evident from the color gradient variations in Figure 8a,d, where the assorted color gradients represent different grain orientations. Furthermore, it can be observed that the introduction of Al₂O₃ particles led to a greater degree of deformation of Al particles in the coating, accompanied by an increase in the number of small-sized grains. Figure 8c,f displays the grain size distribution of the Al coating and Al/17.7% Al₂O₃ composite coating. The average grain size of the coating decreased from 1.68 μm to 1.47 μm after the addition of Al₂O₃ particles. Figure 8b,e shows the KAM plots of the Al coating and the Al/17.7% Al₂O₃ composite coating, respectively, which are used to check the residual compressive stress in the grains. It is evident from the plots that the residual compressive stress generated inside the grains of the Al/17.7% Al₂O₃ composite coating was higher than that of the Al coating. Moreover, in Figure 2, we see that the XRD peaks shifted towards higher angles, indicating a reduction in interplanar spacing, in accordance with Bragg’s law. This confirms the presence of residual compressive stress in the coating which induced crystal rotation and led to the creation of substructures in the coating. The presence of residual compressive stresses also contributed to the enhanced hardness of the composite coating.

Electrochemical tests showed that the incorporation of Al₂O₃ particles significantly reduced the degree of corrosion of the composite coating. Several factors contributed to the increase in corrosion resistance. Firstly, the inherent disadvantages of the cold spray technique resulted in a high porosity in the coating. The entry of electrolyte solution into these pores increases the specific surface area of the corrosion reaction. The addition of hard alumina particles can compact the coating, making the surface denser, reducing porosity. This makes it more difficult for the electrolyte to penetrate the interior of the coating, thereby enhancing the corrosion resistance of the composite coating. Secondly, after adding Al₂O₃ particles, the grains of the composite coating are refined, the grain size is reduced, and the grain boundary densities of the composite coatings are increased. The oxidation reactions preferentially occur at the grain boundaries due to their higher energy, and the increased density of these boundaries promotes the formation of Al₂O₃ from Al and O₂. This results in the formation of a barrier layer of Al₂O₃ on the coating surface, which improves its corrosion resistance. Thirdly the addition of Al₂O₃ particles improves the corrosion resistance of the composite coating. This is due to the reduction in the exposed area of the Al matrix after the addition. The corrosion resistances of Al/25.5% Al₂O₃ and Al/33.3% Al₂O₃ composite coatings are weaker than that of the Al/17.7% Al₂O₃ composite coating. This is because more Al₂O₃ particles will be broken due to collision when the amount of Al₂O₃ particles added exceeds 17.7%. The broken Al₂O₃ particles will create additional weak bonding surfaces in the coating, and the electrolyte solution can more easily enter the coating’s interior through the weak bonding surfaces. Figure 9 shows a schematic diagram of the corrosion principle of the coating before and after the addition of Al₂O₃ particles, depicting the grain refinement caused by the addition of Al₂O₃ particles as well as the fragmentation of Al₂O₃ particles. The addition of Al₂O₃ particles led to a large plastic deformation of the Al particles, resulting in the closure of the micropores in the composite coating and the densification of the coating. At the same time, the grains of the composite coating were refined. The corrosion resistance of the composite coating was improved by the combined effect of these changes.

5. Conclusions

The Al/Al₂O₃ composite coatings were deposited on No. 45 steel by cold spraying. The structure and characteristics of the Al/Al₂O₃ coatings were systematically investigated. The inclusion of Al₂O₃ particles led to a more significant plastic transformation of the previously deposited Al particles, resulting in a denser coating. Compared to the cold-sprayed Al coating, the grains in the Al/Al₂O₃ composite coatings were refined, and the residual compressive stress and hardness were increased. The porosity and surface roughness of composite coatings initially decreased and then increased with the amount of Al₂O₃ particles. The composite coating with Al/17.7% Al₂O₃ had the lowest porosity and showed the best corrosion resistance. The promotion of a corrosion-resistant layer on the surface of the Al/A₂O₃ composite coating was facilitated by the refinement of grains. The corrosion current density of the composite coating was 2.67 × 10⁻⁶ A/cm², which is significantly lower than the corrosion current density of the pure Al coating, at 1.09 × 10⁻³ A/cm². If the amount of added Al₂O₃ particles exceeds 17.7%, some of the Al₂O₃ will break. The broken Al₂O₃ particles result in a slight increase in the porosity and roughness of the composite coating, and will create more weak bonding surfaces in the composite coating. These weak bonding surfaces provide channels for the electrolyte to enter the interior of the coating, leading to a decrease in the corrosion resistance of the composite coating. The addition of Al₂O₃ particles to the cold spray Al coating will significantly impact the microstructure of the resulting Al/Al₂O₃ composite coating, affecting both its mechanical properties and corrosion behavior.