Preparation and Corrosion Resistance of Hydrothermal Coatings on LZ91 Mg–Li Alloy

Abstract

A corrosion-resistant coating was fabricated on the surface of LZ91 Mg–Li alloy via a one-step hydrothermal method under varying reaction temperatures (70, 90, 110, and 130 °C). This involved immersing bare Mg–Li alloy substrates in a 10 wt.% Na₂CO₃ aqueous solution for 3 h. The microstructure, elemental distribution, and phase composition of the as-prepared coatings were systematically characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization measurements. The results revealed that the hydrothermal treatment led to the formation of a dense nanostructured coating composed of fine nanosheets, with their morphology and population density being highly dependent on the reaction temperature. Phase analysis confirmed that the coating primarily consisted of Mg(OH)₂, MgCO₃, and Li₂CO₃. The electrochemical tests demonstrated that the coatings substantially enhanced the corrosion resistance of the alloy. Additionally, the corrosion resistance decreased in the following order: 130 °C > 110 °C > 90 °C > 70 °C > bare LZ91 substrate.

1. Introduction

Mg–Li alloys—the lightest metallic engineering material—exhibit outstanding characteristics, including high specific strength, excellent thermal and electrical conductivity, and remarkable electromagnetic shielding performance, demonstrating broad application prospects in aerospace and 3C (computer, communication, and consumer electronics) products [1,2,3,4]. However, the introduction of Li substantially deteriorates their corrosion resistance, severely limiting the development and practical applications of these alloys [5,6,7]. In dual-phase Mg–Li alloys, the galvanic couple formed between the α-Mg phase (anodic) and the β-Li phase (cathodic) due to their potential difference markedly increases corrosion rates [8,9,10]. Therefore, it is imperative to implement appropriate anti-corrosion measures for Mg–Li alloy products.

Current strategies employed for enhancing the corrosion resistance of Mg–Li alloys primarily focus on surface treatments, such as electroplating, chemical conversion coatings, and micro-arc oxidation [11,12,13]. Although these methods can moderately improve corrosion resistance, they are limited by high energy consumption, environmental concerns, and suboptimal protective performance. Compared to the above methods, hydrothermal treatment emerges as a superior alternative, offering distinct advantages such as environmental friendliness, low-temperature processing efficiency, and exceptional coating adhesion strength, which are particularly advantageous for precision components and sensitive materials [14,15,16].

Several studies have investigated the use of hydrothermal treatment for protecting Mg–Li alloys. Wang et al. [17] prepared a superhydrophobic film with high corrosion resistance on a Mg–9Li alloy using a stearic acid precursor medium. They comparatively studied the effects of hydrothermal reaction processes on the microstructure, hydrophobicity, and corrosion resistance of the film. Zhang et al. [18] utilized a hydrothermal method to grow a layered double hydroxide (LDH) Mg–Al film in situ on an LA43M Mg–Li alloy and evaluated the corrosion resistance and wear resistance of the film. Mou et al. [19] employed a hydrothermal method to synthesize a Li₂CO₃/Mg(OH)₂ composite film on an LA141 Mg–Li alloy and investigated the growth process and failure mechanism of the film. Zhang et al. [20] synthesized an Al/Li LDH film on an LA51 Mg–Li alloy and reported that the film increased the surface area, noticeably improved the chloride capture efficiency, and enhanced the protective effect of the film. Recently, the preparation of carbonate coatings has attracted considerable research attention in the field of Mg alloy surface treatment [21,22,23]. An aqueous solution of MgCO₃ or Li₂CO₃, which are sparingly soluble in water, is considered an ideal green protective layer. However, although research on the preparation of surface coatings for Mg alloys has been initiated, surface coatings for LZ91 Mg–Li alloy and their corrosion resistance characterization remain scarcely documented.

In this study, a one-step hydrothermal method was employed to prepare a protective coating on the surface of LZ91 Mg–Li alloy. This method offers several advantages, such as ease of operation and minimal equipment requirements. A Na₂CO₃ solution was selected as the hydrothermal reaction medium because it is easy to prepare, cost-effective, and ecofriendly. The effects of different reaction temperatures on the coating morphology, phase composition, and electrochemical properties were investigated. Furthermore, the formation and corrosion resistance mechanisms of the coating were discussed. This study advances the surface protection methods for Mg–Li alloy products and provides valuable guidelines for promoting the widespread application of ultra-lightweight Mg alloys.

2. Materials and Methods

2.1. Materials

The substrate material used in the experiment was a hot-rolled LZ91 Mg–Li alloy sheet. Its chemical composition is provided in

Table 1. Chemical composition of LZ91 Mg–Li alloy.

Mg	Li	Zn	Mn	Fe	Cu	Ni	Si
Balance (wt.%)	8.70	0.99	0.018	0.0034	0.0003	0.0003	0.0054

. The sample dimensions were 10 mm × 10 mm × 1.5 mm. The substrate surface was sequentially ground using metallographic sandpaper with grit sizes of 400#, 600#, 1000#, 1200#, 2000#, 3000#, and 5000#. The polished samples were then cleaned with ethanol and deionized water, followed by low-temperature drying.

2.2. Hydrothermal Treatment

One-step hydrothermal method refers to a coating preparation method that does not require pre-treatment or post-treatment of the LZ91 substrate but achieves coating preparation solely through a single hydrothermal reaction step. Regarding the parameter settings for temperature and time, we have determined their optimal range through preliminary experiments. At room temperature, a 10 wt.% Na₂CO₃ solution was prepared and ultrasonically agitated for 5 min. The prepared samples were immersed in the solution and then placed in a hydrothermal reaction autoclave, which was tightly sealed and transferred to a constant-temperature blast drying oven. Hydrothermal reactions were conducted at temperatures of 70, 90, 110, and 130 °C for 3 h to fabricate corrosion-resistant coatings. After the reaction, the autoclave was allowed to cool to room temperature before the samples were removed. The treated samples were thoroughly rinsed with deionized water and stored in ethanol in a sealed container for subsequent characterization. The exposed area of the coating sample is 1 cm², and the non-exposed areas are sealed with epoxy resin. After curing, the edges of the seal are wiped with ethanol to ensure that only a 1 cm² area of the coating surface comes into contact with the 3.5 wt.% NaCl solution. Figure 1 illustrates the schematic process flow of the one-step hydrothermal coating preparation on the LZ91 Mg–Li alloy surface.

2.3. Characterization and Testing

The surface and cross-sectional morphologies of the samples were examined using scanning electron microscopy (SEM, FEI Scios, Hillsboro, OR, USA). Elemental composition analysis was performed using an METEK energy-dispersive X-ray spectroscopy (EDS) system attached to the SEM. Phase identification was conducted via X-ray diffraction (XRD) using a D/MAX-2500/PC diffractometer (Rigaku, Akishima, Tokyo, Japan) with Cu Kα radiation. The samples were scanned in the range of 10–80° at an accelerating voltage, current, and scanning speed of 40 kV, 30 mA, and 2°·min⁻¹, respectively.

The corrosion resistance of the coatings was evaluated using a CHI660E electrochemical workstation using a 3.5 wt.% NaCl aqueous solution at ambient temperature. A standard three-electrode system was employed, with the coated sample serving as the working electrode (1 cm² exposed area), a platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The open-circuit potential was monitored until stabilization was achieved, which typically requires approximately 30 min. Subsequently, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10⁻²–10⁵ Hz at an AC amplitude of 10 mV. Following the EIS measurements, potentiodynamic polarization tests were conducted at a constant scanning rate of 2 mV·s⁻¹ to evaluate the corrosion behavior of the coatings.

Weight loss analysis was conducted by suspending the samples in beakers containing 3.5 wt.% NaCl solution, with all exterior surfaces exposed to the solution, all are 10 mm × 10 mm × 1.5 mm. The weight loss rate (V_m) was calculated based on the mass difference before and after immersion according to the formula V_m = (W₀ – W)/W₀ × 100%, where V_m, W₀, and W denote the weight loss rate of the sample, initial mass of the sample prior to immersion, and mass of the sample after immersion, respectively.

3. Results and Discussion

3.1. Coating Morphology Characterization

Figure 2 presents the surface morphologies of the coatings prepared at different hydrothermal reaction temperatures. As shown in Figure 2a,b, at lower temperatures, light polish marks that are not completely covered by the coating are visible on the surface of the sample, and the coating appears to be loosely bonded to the substrate, with noticeable protrusions. With increasing reaction temperature (Figure 2c,d), the extent of the reaction improves, and the traces are completely covered after the coating densification, resulting in shallower scratch grooves and larger flake-like structures. These observations suggest that an appropriate reaction temperature improves the uniformity of the coating morphology.

Table 2. Surface elemental composition (at.%) of hydrothermal coatings at different positions shown in Figure 2.

Point	C	O	Zn	Mg
#1	1.02	22.52	0.85	75.61
#2	1.24	22.88	0.96	74.91
#3	0.36	26.24	1.45	71.95
#4	0.63	24.35	1.75	73.27
#5	0.61	16.70	1.31	81.38
#6	0.37	14.45	0.99	84.19
#7	0.00	27.36	1.34	71.30
#8	0.34	28.77	1.19	69.70

displays the elemental composition of the coating at different locations. The higher oxygen content indicates the formation of oxide and hydroxide layers on the alloy surface, which is consistent with the typical hydrothermal reaction products formed on Mg alloys. The morphological evolution with temperature can be attributed to enhanced nucleation and growth kinetics at elevated temperatures, leading to more compact and uniform surface coverage.

Figure 3 shows the cross-sectional morphologies of the hydrothermal coatings prepared at different temperatures. The micrographs clearly demonstrate excellent adhesion between the coatings and the LZ91 substrate, with no visible gaps or delamination at the interfaces. A distinct temperature-dependent trend in coating thickness is observed: the coating thickness progressively increases with rising hydrothermal temperature, following the sequence 70 °C < 90 °C < 110 °C < 130 °C.

Table 3. Cross-sectional elemental composition (at.%) of hydrothermal coatings at different positions shown in Figure 3.

Point	C	O	Mg
#1	1.15	1.71	97.14
#2	0.43	0.42	99.15
#3	1.92	7.06	91.02
#4	0.00	0.01	99.99
#5	10.18	5.48	84.34
#6	1.11	0.54	98.35
#7	3.61	16.26	80.13
#8	0.03	0.14	99.83

presents the results of the EDS point analysis at specific positions across the coating–substrate interface. These point analysis results reveal the elemental composition at localized regions, showing a gradual transition in elemental distribution from the coating to the substrate. For instance, the oxygen content is relatively higher in the coating region and significantly lower in the substrate region, which indirectly reflects an oxygen gradient from the coating surface to the substrate. This compositional variation further confirms the strong metallurgical bonding between the coatings and the Mg alloy substrate, suggesting diffusion-controlled oxide formation during hydrothermal processing.

3.2. Phase Composition

Figure 4 depicts the XRD patterns of the samples treated at different hydrothermal temperatures. The diffraction pattern of the bare substrate reveals that the Mg–Li alloy primarily consists of β-Li and α-Mg phases. After hydrothermal treatment, diffraction peaks corresponding to Mg(OH)₂, MgCO₃, and Li₂CO₃ are observed, indicating that the coating layer is mainly composed of these three compounds.

3.3. Corrosion Resistance of the Coating

3.3.1. EIS

Figure 5 presents the EIS results of the substrate and samples subjected to hydrothermal treatment at different temperatures. As observed from the phase-angle Bode plot (Figure 5a), the high-frequency phase angles of the coatings formed at 70, 90, and 110 °C are close to zero. This indicates that the corrosive medium has penetrated the coating and reached the substrate surface, thereby reducing the electrochemical response of the coating. By contrast, the coating treated at 130 °C exhibits a markedly higher high-frequency phase angle, effectively blocking the infiltration of the corrosive solution. Additionally, the impedance-magnitude Bode plot (Figure 5b) reveals that the low-frequency (f < 1 Hz) impedance modulus |Z| can also reflect the corrosion resistance of the samples [24]. The low-frequency impedance values of the LZ91 substrate and coatings formed at 70, 90, and 110 °C remain low, whereas the |Z| value of the coating formed at 130 °C is noticeably higher than that of the substrate. The Nyquist plot (Figure 5c) shows a distinct inductive loop in the low-frequency region, which is typically attributed to the adsorption of intermediate corrosion products ( $\mathrm{M}{\mathrm{g}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{\mathrm{2}\mathrm{+}}$) on the surface [25]. The radii of the impedance arc are larger for all hydrothermally treated samples than for the substrate. Additionally, the radius increases with rising hydrothermal temperature, indicating progressive improvement in corrosion resistance. The equivalent circuit in Figure 5d was used to fit the EIS data of the Mg–Li alloy substrate and hydrothermally treated coatings. In the equivalent circuit, CPE represents a constant phase element for the double-layer capacitance, R_ct denotes the charge transfer resistance, and L and R_L correspond to inductance and inductive resistance, respectively [26,27]. The fitted parameters are listed in

Table 4. Electrochemical data obtained via equivalent circuit fitting of EIS curves.

Sample	Rs (Ω·cm2)	CPE × 10−5 (S·sn)/cm2	n	Rct (Ω·cm2)	RL (Ω·cm2)	L (Ω·cm2)
LZ91	10.14	12.7	0.8964	75.9	27.02	689
70 °C	8.206	3.337	0.8865	450.6	70.04	840.3
90 °C	7.5	2.415	0.8988	726.1	129.5	214.1
110 °C	7.652	2.427	0.7468	1057	223.5	4516
130 °C	10.84	3.397	0.6806	1434	36.3	10,690

. A higher R_ct value indicates higher corrosion resistance. The coating formed at 130 °C exhibited the highest resistance.

3.3.2. Potentiodynamic Polarization Analysis

The polarization curves in Figure 6 and the corresponding electrochemical parameters in

Table 5. Fitted parameters from polarization curves.

Sample	Ecorr (V/SCE)	icorr (A/cm2)
LZ91	−1.562	1.653 × 10−4
70 °C	−1.492	8.995 × 10−6
90 °C	−1.490	8.517 × 10−6
110 °C	−1.479	5.727 × 10−6
130 °C	−1.464	2.257 × 10−6

demonstrate the corrosion behavior of the specimens. Typically, a more positive corrosion potential (E_corr) or lower corrosion current density (i_corr) indicates higher corrosion resistance [28]. The bare LZ91 Mg–Li alloy exhibited poor corrosion resistance with an E_corr of −1.562 V and i_corr of 1.653 × 10⁻⁴ A·cm⁻². In comparison, all hydrothermally treated coatings showed improved corrosion resistance, as evidenced by positive shifts in corrosion potentials (with the maximum E_corr reaching −1.464 V) and reduced corrosion current densities (minimum i_corr of 2.257 × 10⁻⁶ A·cm⁻²). Notably, the sample treated at 130 °C demonstrated the highest corrosion resistance among all tested specimens. These results are consistent with the EIS data, confirming the correlation between corrosion current density and impedance behavior.

Figure 7 presents the SEM images of uncoated and coated alloys after 168 h of immersion in 3.5 wt.% NaCl solution. As shown in Figure 7a, the uncoated alloy surface suffers severe damage after immersion. The compactness of the 110 °C membrane layer is between 90 and 130 °C, and a small amount of Cl⁻ penetration induces the formation of fine Mg(OH)₂ particles. By contrast, the coated alloy exhibits only morphological changes compared to its pre-immersion state (Figure 2). However, no distinct corrosion pits or cracks were observed on the surface. As shown in Figure 7f, all samples contain high concentrations of O and Mg, indicating the formation of magnesium oxide corrosion products.

To further characterize the corrosion products, the coatings were analyzed via XRD. Figure 8 displays the XRD patterns of both bare and coated alloys after 168 h of immersion in 3.5 wt.% NaCl solution. Although Cl⁻ ions initially degrade the protective coating, the OH⁻ ions generated during the corrosion process increase the local pH, leading to the redeposition of Mg(OH)₂ (which exhibits a low-density structure and limited protective properties). During prolonged immersion, Mg(OH)₂ dehydrates to form magnesium oxides, while basic magnesium chloride compounds are formed in Cl⁻-enriched regions.

Figure 9 presents the weight loss measurements of the LZ91 substrate and various coated specimens after 168 h of immersion in 3.5 wt.% NaCl solution. The weight loss tests were conducted in triplicate at each temperature, showing narrow deviations and good reproducibility. The bare substrate exhibits the highest weight loss rate, whereas the coating treated at 130 °C demonstrates the lowest weight loss. Quantitative analysis revealed weight loss rates of approximately 4.95% for the bare LZ91 substrate, and 4.85%, 4.37%, 1.67%, and 0.44% for the coatings prepared at 70, 90, 110, and 130 °C, respectively. Compared to the substrate, all coated specimens showed improved corrosion resistance, with the coating treated at 130 °C exhibiting exceptional performance. The enhanced corrosion resistance of the coating treated at 130 °C can be attributed to its dense lamellar nanostructure.

3.4. Formation and Degradation Mechanisms of Coatings

3.4.1. Coating Formation Mechanism

When the LZ91 Mg–Li alloy substrate is immersed in the hydrothermal solution, Mg and Li from the alloy dissolve under high-temperature and high-pressure conditions, generating Mg²⁺ and Li⁺ ions. In the presence of $\mathrm{C}{\mathrm{O}}_3^{2-}$, solid reaction products such as Mg(OH)₂, MgCO₃, and Li₂CO₃ form on the alloy surface. Notably, the presence of Li influences the stability of the coating [29,30]. Additionally, the trace amounts of Zn in the alloy may lead to the formation of ZnO or Zn(OH)₂, which can fill pores in the coating and enhance its compactness. Figure 10 illustrates the formation of the coating, which follows the chemical reactions given below:

$${\mathrm{M}\mathrm{g}+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}\mathrm{g}\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{H}}_2\uparrow$$

(1)

$${\mathrm{M}\mathrm{g}\left(\mathrm{O}\mathrm{H}\right)}_2{+\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_3\to {\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}$$

(2)

$$2\mathrm{L}\mathrm{i}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\uparrow$$

(3)

$$2\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}{+\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_3\to {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}$$

(4)

3.4.2. Coating Degradation Mechanism

When the coating is immersed in a NaCl solution, its lamellar structure grows vertically on the substrate surface, forming an interwoven physical barrier that temporarily impedes contact between the corrosive medium and the substrate. However, Cl⁻ ions gradually degrade the coating through their strong penetrability and corrosiveness, leading to localized pitting, coating dissolution, and accelerated galvanic corrosion. This degradation process converts the primary coating components (Mg(OH)₂, MgCO₃, and Li₂CO₃) into soluble MgCl₂ and LiCl. The principal transformation reactions are as follows.

$$\mathrm{M}\mathrm{g}(\mathrm{O}\mathrm{H}{)}_2+2{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2+2{\mathrm{O}\mathrm{H}}^{-}$$

(5)

$${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3+2{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2+{\mathrm{C}\mathrm{O}}_3^{2-}$$

(6)

$${\mathrm{L}\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3+2{\mathrm{C}\mathrm{l}}^{-}\to 2\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l}+\mathrm{C}{\mathrm{O}}_3^{2-}$$

(7)

With prolonged immersion time, the dissolution zones of the coating gradually expand, providing penetration channels for corrosive media (e.g., Cl⁻ and H₂O). When the solution contacts the substrate, the exposed Mg immediately reacts with OH⁻ ions formed from water electrolysis. This leads to the formation of a new Mg(OH)₂ protective layer, demonstrating the self-healing characteristics of the coating. However, highly corrosive Cl⁻ ions persistently degrade the newly formed protective layer through competitive reactions.

4. Conclusions

(1)

A corrosion-resistant coating composed primarily of Mg(OH)₂, MgCO₃, and Li₂CO₃ was successfully prepared on an LZ91 Mg–Li alloy surface via a one-step hydrothermal method.

(2)

The hydrothermal temperature heavily influenced the surface morphology of the coating. As the reaction temperature increased, the coating thickness gradually increased in the following order: 70 °C < 90 °C < 110 °C < 130 °C.

(3)

The coatings markedly improved the corrosion resistance of the Mg–Li alloy substrate. The corrosion resistance of the coatings prepared at different hydrothermal temperatures decreased in the following order: 130 °C > 110 °C > 90 °C > 70 °C > bare LZ91.