Superhydrophobic Corrosion-Resistant Coating of AZ91D Magnesium Alloy: Preparation and Performance

Abstract

This research presents the development of a surface treatment for AZ91D magnesium alloy that exhibits both superhydrophobic and anticorrosive properties. Initially, a zinc-based phosphate film was deposited on the magnesium alloy surface. Subsequently, a composite coating with superhydrophobic properties was produced by surface modification using a fluorosilane-ethanol solution. The composite coating’s microstructure, chemical composition, wettability, self-cleaning, and anti-corrosion properties were evaluated using scanning electron microscopy, a contact angle measurement instrument, and an electrochemical workstation. The results demonstrated that the main components of the composite coating were P, O, Zn, F, and C. The static contact angle reached 158°, providing superior self-cleaning and acid and alkali corrosion resistance. Additionally, the charge transfer resistance and coating resistance of the composite coating were significantly higher than those of the magnesium alloy substrate, effectively preventing corrosion and preserving the surface from fouling.

1. Introduction

Magnesium (Mg) and its alloys possess advantageous characteristics, including light weight, exceptional heat dissipation, high strength, anti-electromagnetic shielding properties, and recyclability [1,2,3,4]. They are used in numerous fields, including aerospace, medicine, the automotive industry, military defense, and electronics [5,6,7,8]. However, the active chemical nature of Mg results in poor corrosion resistance, significantly limiting its use in critical equipment. Consequently, enhancing the corrosion resistance of magnesium alloys is crucial for their practical applications. Since the surface of metal materials is typically the first area to corrode, implementing appropriate surface treatment technologies represents one of the most effective solutions to address the corrosion issues of magnesium alloys [9].

From nature and life, it can be found that, for example, lotus leaves, strider legs, butterfly wings, moth eyes, etc., almost all show hydrophobic or superhydrophobic properties. This is related to the micro-nano structures of their surfaces [10,11,12,13], and these surfaces have been extensively observed owing to their probable applications in anticorrosion [14], self-cleaning [15], and so on.

In recent years, scientists have developed numerous methods for preparing structures that imitate the superhydrophobic surfaces found in nature. Ludmila B. Boinovich et al. have studied recent advances in applying superhydrophobic coatings for corrosion protection of magnesium substrates and analyzed the corrosion protection mechanism of superhydrophobic coatings regarding the durability of corrosion protection [16]. Some techniques can be employed, including anodizing, electrodeposition, and chemical conversion [17,18,19]. Among these methods, chemical conversion is particularly favored by researchers due to its simplicity, affordability, and efficiency. In the chemical conversion process, preparing a chromate conversion film [20] involves using chromate, which is highly toxic. Consequently, chromate conversion film has generally been discontinued for several years. In contrast, phosphate has the advantages of low cost and reduced environmental impact, and it is gradually becoming an essential method of surface protection for magnesium alloy. G. Y. Li et al. [21] investigated the growth of zinc phosphate coating on the surface of AZ91D magnesium alloy. R. Amini and A. A. Sarabi [22] obtained a phosphate coating on an AZ31 alloy surface by varying the promoter. In a study conducted by Trong-Linh Nguyen et al. [23], the growth mechanism and characterization of zinc-manganese phosphate film were investigated by varying the Zn/Mn ratio in the solution. Nevertheless, the corrosion resistance of the samples treated by the above research methods is unsatisfactory and requires further modification. Based on the above study, the superhydrophobic and anti-corrosion composite coating was produced, which weakened the magnesium alloy’s surface wettability while enormously improving its corrosion resistance.

In this study, the phosphate conversion coating (PCC) was originally prepared on the surface of AZ91D magnesium alloy with chemical conversion, and then the superhydrophobic composite film was achieved through surface modification with fluorosilane-ethanol solution. The entire process was completed quickly, efficiently, and at low cost. In particular, the investigation encompassed an examination of the surface composition, morphology, self-cleaning capabilities, and wettability characteristics of the composite film layers. Furthermore, the coating’s resistance to corrosion was assessed through electrochemical testing procedures. The findings from this study have significant implications for enhancing the corrosion resistance of magnesium alloys.

2. Experimental Procedures

2.1. Materials

The AZ91D magnesium alloy (according to manufacturer’s data, composition: 89.788 wt.% Mg; 9.3 wt.% Al; 0.52 wt.% Zn; 0.28 wt.% Mn; 0.06 wt.% Si; 0.025 wt.% Fe; 0.025 wt.% Cu; 0.001 wt.% Ni; 0.001 wt.% Be) was utilized as the substrate and was purchased from Dongguan Kuangyu Metals Materials Co. Anhydrous ethanol (AR, 99.7%) was obtained from Tianjin Fuyu Chemical Co. The sodium hydroxide and sodium phosphate were procured from Sichuan Xilong Science Co., while the sodium fluoride was purchased from Ron Reagent Co. Zinc oxide, zinc sulfate, phosphoric acid, citric acid, and 1H, 1H, 2H, 2H -perfluorooctyltriethoxysilane (FAS, 97%) were supplied by Aladdin. The chemicals employed in this article were all analytical standards and did not require additional purification steps.

2.2. Pretreatment

The AZ91D magnesium alloy was cut into pieces measuring 20 mm × 20 mm × 5 mm. The samples were sequentially sanded with 600, 1000, 1500, and 2000# SiC sandpaper and then chemically cleaned in a mixed solution of sodium phosphate and sodium hydroxide for five minutes to achieve surface degreasing. This process involved a water wash followed by acid activation using dilute phosphoric acid. Afterwards, the material was washed and air-dried, making it ready for use.

2.3. Preparation

The experimental flow chart is shown in Figure 1. The conversion solution was formulated using zinc oxide and zinc sulfate as the main salts, and citric acid and sodium fluoride as the accelerator. The main components were zinc oxide (4 g/L), zinc sulfate (4 g/L), phosphoric acid (32 g/L), citric acid (3 g/L), and sodium fluoride (1.6 g/L). The pre-treated magnesium alloy specimens were fully immersed in the prepared conversion solution and converted at 65 °C for 5 min to form a layer of zinc phosphate film on their surface. Subsequent to washing and air-drying, the specimens were submerged in a 1% FAS ethanol solution for 45 min, and maintained at ambient temperature [24]. The specimens underwent removal and were subsequently dried for 10 min at a temperature of 120 °C [25]. Finally, a composite film layer with superhydrophobic properties was obtained on the surface of the magnesium alloy.

2.4. Characterization

The measurement of the water contact angle (CA) on the surface of the sample was conducted utilizing an optical contact angle meter (OCA35, Dataphys obtained on ics, Dongguan, China) at ambient temperature with 6 μL drops of deionized water. The value was calculated as the mean of three measurement points taken on the sample surface. The micro-morphological features of the samples were analyzed and characterized using a field emission scanning electron microscope (SEM, Nova NanoSEM 430, FEI, manufacture, Waltham, MA, USA). The film phases’ composition was analyzed using X-ray diffraction (XRD, D8 Advance, Bruker AXS, Karlsruhe, Germany). Chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, Waltham, MA, USA) and Fourier transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker, Karlsruhe, Germany). The XPS scan rate was 1ev/s and the data obtained were analyzed using Avantage 6.1 software.

2.5. Self-Cleaning Performance and Chemical Stability Test

The specimens were positioned at an angle on slides, graphite powder was employed to simulate pollutants, and deionized water was added to the top of the specimens using a dropper. The effect of the water droplets on the graphite powder was then observed as they moved along the slopes under the influence of gravity. The phenomenon was recorded with a camera to assess whether the composite membrane layer exhibited self-cleaning properties. A series of solutions was prepared, namely a hydrochloric acid solution with pH = 2, a neutral solution with pH = 7, and a sodium hydroxide solution with pH = 12. The specimens were submerged in these solutions for 3 h. Afterwards, they were rinsed with deionized water and left to dry. Each sample was immersed 3 times for 3 h each time to calculate the average value. The durability of the composite film under acidic, neutral, and alkaline conditions was evaluated by this method.

2.6. Corrosion Performance

The measurement of potentiodynamic polarization and electrical impedance in the specimens was carried out with the aid of a CS2350H electrochemical workstation (Kost, Wuhan, China). The platinum electrode served as the auxiliary electrode, while the reference electrode comprised an Ag/AgCl electrode immersed in a saturated KCl solution. To simulate the similarity of the actual application environment, at ambient temperature, potentiodynamic polarization measurements of electrochemical kinetics were performed utilizing a 3.5 wt.% NaCl solution, with the test area of the samples fixed at 1 cm². The data were recorded at a scanning range of −0.5 V to 1.5 V and a scanning rate of 1 mV/s, with the impedance spectra being performed in the test range from 1 × 10^{− 1} Hz to 1 × 10⁵ Hz. The data were then analyzed using the software (Corr Test. CS Studio 5).

3. Results and Discussion

3.1. Chemical Composition

X-ray diffraction analyses were deployed to gain a more comprehensive understanding of the superhydrophobic composite coatings’ elemental composition and phase structure. As displayed in Figure 2, the diffraction peaks corresponding to Mg (PDF#35–0821) and Mg₁₇Al₁₂ (PDF#01–1128) can be unequivocally identified in the AZ91D substrate. In the XRD pattern of the experimental sample, the diffraction peaks of Mg (PDF#35–0821), Zn (PDF#04–0831), and Zn₃ (PO₄)₂‧4H₂O (PDF#33–1474) can be observed. The strongest signal of the Zn peak can be observed at the 43° position, which provides compelling evidence that the coating consisted of a mixed phase of Mg, Zn, and their compounds.

To further confirm the chemical composition of the composite film surface, XPS analysis was carried out. The scanned full spectrum of the FAS-modified sample is indicated in Figure 3a, revealing the presence of oxygen, magnesium, phosphorus, zinc, carbon, and fluorine. A strong peak of F1s can be observed in the full spectrum, indicating that the outermost surface has been saturated with fluorouracil [26]. After the above analyses, it is clear that the main elements present on the composite film layer are P, Zn, O, F, and C. Among them, Zn, O, and P elements are derived from the zinc phosphate film. The characteristic peak of P appearing at a binding energy of 134.53 eV in Figure 3b corresponds to the phosphorus-related compounds [27], which may be attributed to the presence of Zn₃ (PO₄)₂‧4H₂O, which is the main component of the phosphate film on the surface of the AZ91D alloy [28]. Furthermore, the O 1s spectrum appears as the peaks at a binding energy of 532.33 eV for the peak component (Figure 3c), corresponding to the O element in P–O. In addition, the peak of Zn 2p at 1022.88 eV (Figure 3d) can be attributed to Zn₃ (PO₄)₂‧4H₂O and the Zn-containing component [29]. In addition, the C1s spectrum was divided into five components (Figure 3e), specifically –CH₂–CH₂ (284.80 eV), –C–O (286.06 eV), –CH₂–CF₂ (289.85 eV), –CF₂ (291.56 eV), and –CF₃ (293.75 eV) [30]. Among them, –CF₂ and –CF₃ are the essential components responsible for the superhydrophobic properties. The findings from the XPS analyses exhibit a strong consistency with the outcomes of the XRD results. Figure 3f presents the FTIR spectra of the samples modified by FAS. In the spectra, peaks located at approximately 1492, 1402, 1240, and 1141 cm⁻¹ can be observed, which are attributed to the C–F stretching vibrations of the –CF₃ and –CF₂– groups in the FAS molecule. The presence of these functional groups provides further evidence of FAS covering the surface of magnesium alloys. FAS has a very low surface energy, which makes it effective in reducing the free energy on the surface of the samples [31,32]. The results of the FTIR analyses indicate that fluorine-containing, long-chain alkyl chemicals have covered the surface of the samples, forming a low surface energy layer. This layer interacts with the rough micro-nanostructures to generate a superhydrophobic surface.

3.2. Growth Process of Zinc Phosphate Film

The following is the growth process of the zinc-based phosphate film in the phosphate bath, in which several reactions occur [33,34]:

ZnSO₄ →Zn²⁺ + SO4²⁻

(1)

NaF → Na⁺ + F⁻

(2)

ZnO + 2H₃PO₄ → Zn(H₂PO₄)₂ + H₂O

(3)

ZnO + 2H⁺ → Zn²⁺ + H₂O

(4)

Upon contact with the mixed solution, the AZ91D magnesium alloy sample initiates a chemical reaction involving the release of electrons by Mg and Al. The following reaction is observed [33,35]:

Mg → Mg²⁺ + 2e⁻

(5)

Al → Al³⁺ + 3e⁻

(6)

As illustrated in Reactions (7) and (8), water molecules and hydrogen ions obtain electrons to form H₂ [35,36].

2H₂O + 2e⁻ → H₂ + 2OH⁻

(7)

2H⁺ + 2e⁻ → H₂

(8)

The combination of magnesium ions and fluoride ions results in the formation of magnesium fluoride, which exhibits slight solubility in dilute acid, as the Reaction (9) shows. This compound serves as an initial protective layer and can impede the corrosion of the magnesium alloy substrate and facilitate the growth of the phosphate film.

Mg²⁺ + 2F⁻ → MgF₂

(9)

Finally, crystals such as Zn and Zn₃ (PO₄)₂‧4H₂O, which are the dominant components of the phosphate film, are precipitated [33,37].

Zn²⁺ + 2e⁻ → Zn

(10)

Zn (H₂PO₄)₂ → ZnPO₄⁻ + H₂PO₄⁻ + 2H⁺

(11)

3Zn²⁺ + 2H₂PO₄^- + 4H₂O + 4e⁻ → Zn₃ (PO₄)₂‧4H₂O + 2H₂

(12)

3.3. Microscopic Morphology of PCC at Different Phosphating Times

To gain insight into the phosphate film’s crystallization and crystal growth process, the substrate was treated in a phosphate bath for 40, 90, 150, and 300 s. Figure 4 reveals the changes in the surface morphology of the film layers at different phosphating times. As can be seen in Figure 4(a₁–a₃), after 40 s of phosphating treatment, tiny nuclei have formed on the surface of the substrate, which is the initial nucleation stage, and have appeared initially on some of the surfaces. At 90 s, more aggregates of nuclei have appeared and continue to grow, as exhibited in Figure 4(b₁–b₃) [38]. At 150 s of phosphating, most of the substrate surface is covered, and a fine structure has progressively developed on the surface, which forms a dense coating, as revealed in Figure 4(c₁–c₃). After 300 s of phosphating, as indicated in Figure 4(d₁–d₃), the intricate dense structure has steadily expanded in a coordinated fashion until it nearly completely envelops the substrate. The results demonstrate that after 300 s of phosphating treatment, an almost dense phosphate film has been produced with only a few cracked areas. Figure 4(e₁–e₃) displays the surface microscopic morphology of the surface after the phosphating transformation and modification [39]. FAS adheres to rough structures to form topographical surfaces with superhydrophobic properties.

3.4. Wettability

Initially, the bare magnesium alloy surface has a contact angle value of 20°, which exhibits significant hydrophilic properties, as shown in Figure 5(c₁). Figure 5(c₂) demonstrates the surface contact angle of the phosphate film with a contact angle value of 72°, which is lower than 150°, resulting in water droplets still contacting the magnesium alloy substrate. Figure 5(c₃,c₄) indicate the water contact angle after different phosphating treatment times and modifications. Strikingly, the surface CA values after phosphating for 150 s and 300 s and chemical modification are 152° and 158°, respectively, which are greater than 150°, suggesting that the surface exhibits superhydrophobicity, as manifested in Figure 5(c₅,c₆). It should be noted that the main reason for the change in wetting behavior is the formation of surfaces with micro- and nanostructures, which have low surface energy and weak affinity for water after the adhesion of the modified fluoro-silanes [40], resulting in the inability of the water droplets to spread on their surfaces.

3.5. Self-Cleaning Performance and Chemical Stability Test

The graphite powder was evenly spread on the sample surface before conducting the drip test. The corresponding results are shown in Figure 6. When a water drop falls on the AZ91D magnesium alloy substrate spread with graphite powder, the water drop adheres to the surface instead of rolling down. This causes a change in the morphology of the drop, and also accumulates the graphite powder around it. The surface reflects hydrophilicity, so it does not possess self-cleaning properties. On the other hand, when the water droplets fall on the surface of the composite coating spread with graphite powder, the pollutants are easily carried away from the superhydrophobic surface with the rolling water droplets, leaving obvious traces, which are distinctly different from the surrounding area. Because of its superhydrophobic composite coating and low surface energy, it repels dust effectively. As the water droplets flow down, they easily remove surface pollutants, giving the composite coating excellent self-cleaning performance and efficiently preventing contamination of the magnesium alloy substrate.

The static contact angles of the magnesium alloy substrate and different film layers after immersion in hydrochloric acid, neutral solution, and sodium hydroxide solution separately for 3 h are illustrated in Figure 7. The corrosion of acid and alkaline solutions leads to dissolution of the magnesium alloy substrate and phosphate film surfaces, thus significantly changing their static contact angles. Nonetheless, even under these conditions, the static contact angle of the composite coating remains essentially constant and remains above 150°. This result indicates that the composite coating exhibits strong resistance to both acid and alkaline corrosion and maintains its superhydrophobicity in acidic and alkaline environments.

3.6. Anti-Corrosion Test

Figure 8 displays the kinetic potential polarization curves for different samples in 3.5 wt.% NaCl solution, and the values of the polarization parameters are shown in

Table 1. Detailed parameters of polarization curves of AZ91D substrate and experimental samples.

Sample	Icorr (A/cm2)	Ecorr (V)	η (%)	Corrosion Rate (mm/a)
AZ91D	6.5121 × 10–5	−1.5154	− −	1.3813
PCC	2.7325 × 10–5	−1.0722	58.0396	0.5791
Composite coating	9.1224 × 10–7	−0.4181	98.5991	0.01933

. The corrosion potential (E_corr) reflects the thermodynamic corrosion tendency of the system, while the corrosion current density (I_corr) provides information about the kinetics of the corrosion process [40]. The corrosion inhibition rate η was calculated according to the following equation [41], where i_0corr is the corrosion current density of the Mg matrix and i_corr is the corrosion current density of the specimen.

$$\eta ={\displaystyle \frac{{\mathrm{i}}_{0 \mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}-{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}{{\mathrm{i}}_{0 \mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}}\times 100\mathrm{\%}$$

(13)

In this experiment, the E_corr of the substrate, PCC, and composite coating surfaces are −1.5144, −1.0722, and −0.4181, respectively, and the I_corr is derived from Tafel extrapolation to be 6.5121 × 10^–5, 2.7325 × 10^–5, and 9.1224 × 10^–7, respectively. The specimen with composite coating has the highest E_corr, lowest I_corr, and best corrosion resistance. It is clear that, due to the absence of a protective film, the substrate has the lowest E_corr and the I_corr is the highest. The composite-coated specimen exhibits the highest E_corr, and lowest I_corr, and demonstrates the best corrosion resistance. Compared with the substrate, the I_corr of the composite coating specimen is two to three orders of magnitude lower than the substrate. The results clearly demonstrate that the composite coating sample significantly enhances the corrosion protection of the samples.

Figure 9 illustrates the EIS measurements, where the scatters are the measured data points and the curves depict the fitted data. The results of the EIS fitting are presented in

Table 2. Equivalent circuit fitting data for AZ91D substrate and experimental samples.

Sample	Rs·(Ω·cm2)	Rc·(Ω·cm2)	CPEd·(μF/cm−2·S(α − 1))	Rct (Ω·cm2)
AZ91D	24.29	− −	0.93	285
PCC	56.91	2811	0.91	2115
Composite coating	62.36	6337	0.88	4380

. In Nyquist plots, conventionally, the wider the radius of the capacitive arc, the greater the resistance to charge transfer, and for metals, the greater the charge transfer resistance, the better the corrosion resistance of the material [42,43]. The figure shows that the radius of the substrate capacitive arc is the smallest, while the radius of the composite film layer sample is the largest and much larger than that of the AZ91D magnesium alloy substrate. In the Bode plot, the higher the |Z| value, the better the corrosion protection [44]. It can be seen from the plot that all the experimental samples have higher |Z| values than the AZ91D substrate and the composite film layer sample has the largest |Z| value.

Figure 10 shows the equivalent circuit model for fitting the AZ91D magnesium alloy substrate and composite-coated sample. The term Rs signifies the resistance that the solution itself poses, while Rc stands for the resistance emanating from the coating, and Rct pertains to the resistance encountered during the charge transfer process. A constant phase element (CPE) to replace the ideal capacitor was applied in the circuit simulation, as the CPE could provide a better fit by more accurately modeling the characteristics of a non-ideal capacitor in the equivalent circuit [45]. The available data from Table 2 indicate that the Rct of the AZ91D magnesium alloy substrate is 285 Ω, and the Rct of the PCC and composite film layer sample are 2115 Ω and 4380 Ω, respectively, whereas the charge transfer resistance of the composite film layer samples is 15 times higher than that of the magnesium alloy matrix. The results indicate that the composite coating specimen has better corrosion resistance than the magnesium alloy substrate.

3.7. Anti-Corrosion Mechanism

A schematic diagram of the coating against corrosion is presented based on the results of the hydrochloric acid immersion test (Figure 11). In this process, the AZ91D magnesium alloy comes into direct contact with the corrosive medium without coating protection, and the substrate dissolves directly, forming a layer of uneven corrosion products. Mg²⁺ reacts with Cl⁻ to form soluble MgCl₂, which does not act as a protective layer, and the corrosion continues to spread internally. In contrast, the PCC-coated sample acts as an isolation layer to a certain extent, slowing down the contact between the corrosive medium and the substrate, but its protective ability is limited. Nevertheless, the cluster structure generated on the surface of the composite-coated sample can effectively reduce the direct contact between Cl⁻ and the substrate, thus significantly improving the corrosion resistance of the substrate.

4. Conclusions

A composite coating exhibiting superhydrophobic and anti-corrosion properties was fabricated on the magnesium alloy surface through a combination of chemical conversion and subsequent modification processes. This coating also possesses excellent self-cleaning properties. The modified fluorosilane-ethanol solution adhered to the surface of the zinc phosphate film, resulting in a composite coating with low surface energy and P, O, Zn, F, and C elements. The static contact angle of the coating was 158°. In addition, the corrosion current density of the composite coating was reduced by two to three orders of magnitude compared with the magnesium alloy substrate, while charge transfer resistance was considerably improved, and it had a significant barrier effect on chloride ions. Consequently, it demonstrated favorable self-cleaning performance and corrosion resistance, effectively slowing the corrosion rate of the magnesium alloy substrates and protecting them from contamination. This process offers a novel approach to corrosion protection for magnesium alloys.