Dual-layer coatings on magnesium alloys: corrosion and degradation performance

ABSTRACT

To prevent rapid degradation causing plugging failure in degradable magnesium alloy packer slips under high-temperature, high-pressure, corrosive conditions, a composite dual-coated magnesium alloy was developed. The coating (hereafter referred to as Black Composite Coating, BCC) utilized micro-arc oxidation (MAO) and spray coating. SEM, XRD, FTIR, polarization curves, and EIS characterized microstructure and corrosion performance pre- and post-modification. Results showed the MAO-treated surface formed a porous structure, increasing specific surface area and roughness. This enhanced base/auxiliary material adhesion and corrosion resistance. The added black composite coating significantly reduced corrosion current density and increased charge transfer resistance (Rct). MAO pores provided mechanical interlocking sites. The black coating's low porosity and sealing blocked corrosive media penetration. Compared to soluble magnesium alloys, the coating reduced corrosion current density by ~five orders of magnitude (from ~10⁻⁴ to ~10⁻⁹ A/cm²) and increased Rct by ~six orders (from ~10² to ~10⁶ Ω·cm²). Corrosion rates were suppressed >91.8%, stabilizing between 0.1~1.0 mm/year. Per ISO 9223:2012, this moderate corrosion resistance (Class C3-C4) indicates suitability for mildly aggressive environments like indoor industrial areas, low-chloride rural zones, or oil wells. This technique enhances degradable magnesium alloy corrosion resistance downhole, providing valuable industrial guidance.

Keywords:
Soluble magnesium alloy; Composite coating; Microstructure; Corrosion resistance; Micro-arc oxidation (MAO)

1. INTRODUCTION

Magnesium alloys, renowned for their low density, high strength, efficient heat dissipation, and excellent electromagnetic shielding properties, play a crucial role in lightweight structural design [¹]. Compared to traditional materials such as aluminum alloys, magnesium alloys have found increasingly widespread applications in aerospace [²] and biomedical fields [³]. Notably, their advantages as degradable materials in downhole tool manufacturing have become more pronounced, primarily due to their biocompatibility and degradability in physiological environments [⁴, ⁵]. AZ81 alloy, with a relatively high yield strength of approximately 120 MPa [⁶], exhibits good plasticity and forgeability, making it widely used in the automotive manufacturing [⁷] and petrochemical industries [⁸]. However, the strength and degradation behavior of magnesium alloys remain insufficient to meet practical requirements [⁹].

In the study of soluble downhole tools, room-temperature high-plasticity soluble magnesium alloys have been employed in the oil and gas extraction industry for downhole tools such as bridge plugs [¹⁰], fracturing balls [¹¹], and packer devices [¹²]. Since 2009, Xu et al. [¹³] developed a novel controlled electrolytic metallic (CEM) ball that can dissolve rapidly in fracturing fluid post-operation without requiring drilling, reducing production costs and improving operational efficiency. Chen et al. [¹⁴] invented a highly temperature-resistant soluble magnesium alloy for fracturing materials. Qian et al. [¹⁵] improved the surface properties of magnesium alloys using micro-arc oxidation (MAO) technology. However, existing studies still have limitations. The primary drawback of magnesium alloys as downhole tool materials lies in structural instability caused by rapid degradation [¹²]. The standard electrode potential of magnesium alloys is −2.37 V, indicating high electrochemical activity and susceptibility to oxidation, with their porous and loose oxide films further compromising corrosion resistance [¹⁶, ¹⁷]. Therefore, developing effective surface treatment technologies to enhance the corrosion resistance of magnesium alloys is critical for their application in downhole tools [¹⁸].

Surface modification techniques, particularly MAO, have proven effective in significantly enhancing the corrosion resistance of magnesium alloys without altering the base composition. Currently, surface modification methods for magnesium alloys include chemical conversion [¹⁹], anodizing [²⁰], metal electroplating [²¹], and cold spraying [²²]. However, these techniques face challenges such as poor adhesion between the coating and substrate, uneven thickness, complex processes, high costs, and environmental pollution, limiting their development. MAO, also known as plasma electrolytic oxidation (PEO) [²³], generates ceramic oxide films on magnesium alloy surfaces through spark discharge in an electrolyte solution [²⁴]. A porous architecture is always inevitable for PEO coatings, which is a result of a series of complex plasma-chemical-electrochemical reactions occurring on the metal surface [²⁵]. The PEO is a less complicated environmentally friendly technique to produce coatings, which provides the creation of durable ceramic-like coatings at a lower cost [²⁶]. This process typically involves four stages: anodizing, spark discharge, micro-arc discharge, and arc discharge [²⁷]. Factors affecting the performance of MAO coatings include voltage [²⁸], current density [²⁹], duty cycle [³⁰], frequency [³¹], and the type and concentration of electrolyte [³⁰]. MAO coatings are at the forefront of surface treatment technologies for magnesium alloys due to their superior adhesion, bonding, wear resistance, corrosion resistance, electrical insulation, and optical properties [³²]. Moreover, MAO is a simple, efficient, and environmentally friendly process, regarded as one of the most promising surface treatment methods for magnesium alloys [³³, ³⁴].

Despite its advantages, the density of MAO coatings still requires improvement. Surface micropores and microcracks can serve as pathways for corrosive media to penetrate the substrate, making it crucial to address coating defects to enhance corrosion resistance [³⁵]. Researchers have developed one-step and two-step approaches to improve the integrity of MAO coatings. The one-step method incorporates nanoparticles during the MAO process to increase film density [³⁶, ³⁷], while the two-step method constructs composite coatings on MAO films to repair surface defects [³⁸].

Addressing the limitations of existing techniques, this study proposes a composite coating modification process to improve the corrosion resistance of magnesium alloys for downhole tool applications. By employing MAO technology, this study analyzes the coatings using XRD, SEM, FTIR, and electrochemical testing to investigate the effects of different MAO fabrication methods on the corrosion resistance of AZ81 alloy coatings. The findings aim to provide scientific insights and technical guidance for practical applications.

2. EXPERIMENTAL DETAIL

2.1. Materials and preparation methods

Pre-treatment of Magnesium Alloy: AZ81 cast magnesium alloy was selected as the substrate, with a chemical composition of 7.77 wt.% Al, 0.56 wt.% Zn, 0.02 wt.% Ni, and the balance being Mg. The alloy was cut into blocks with dimensions of 10 mm × 10 mm × 2 mm using a wire-cutting technique. The samples were polished sequentially with water-resistant abrasive papers of grit sizes 600#, 800#, 1000#, 1500#, 2000#, and 5000#, followed by polishing to achieve a smooth surface. Polished samples were ultrasonically cleaned in deionized water and anhydrous ethanol for 30 minutes, repeated three times. The cleaned samples were dried at 60°C in an oven and stored for further use.

Micro-Arc Oxidation Treatment: AZ81 magnesium alloy samples served as the anode in the micro-arc oxidation (MAO) process, with a stainless steel plate as the cathode. The electrolyte system consisted of 10 g/L sodium silicate (Na₂SiO₃·9H₂O), 10 g/L potassium hydroxide (KOH), 10 g/L potassium fluoride (KF), and 5 mL/L glycerol (C₃H₈O₃). The MAO process was conducted under constant voltage conditions (350 V), with a frequency of 500 Hz and a duty cycle of 10%, ensuring that the operating temperature did not exceed 30°C. Upon completion, the samples were ultrasonically cleaned alternately in ultrapure water and anhydrous ethanol three times each. After cleaning, the samples were dried and vacuum-sealed for preservation.

Preparation of Black Composite Coating: The spray coating materials for the black composite coating (BCC) consist of a base layer and a top layer, as illustrated in the accompanying figure. The fabrication process initiates with the spraying of a polyamide-imide (PAI) resin base layer onto the substrate, followed by solidifying at 120°C for 20 minutes. After cooling to 60°C, a polytetrafluoroethylene (PTFE) top layer is applied via spraying; the PTFE emulsion, a thermoplastic polymer synthesized via the emulsion polymerization of tetrafluoroethylene, exhibits outstanding corrosion resistance attributable to the high bond energy of the C-F bonds, though it demonstrates relatively poor adhesion to substrates. For the experimental PTFE coating formulation, PTFE served as the primary material, blended with binders (pure acrylic emulsion and KH-560 silane coupling agent), a wear-resistant agent (Al₂O₃), and a film-forming agent (Texanol). This mixture was combined in a beaker at the designated ratios and homogenized using a magnetic stirrer at 1000 rpm for 1 hour. Subsequently, the prepared coating was applied using an air spray gun connected to an air compressor via an air hose, with the pressure adjusted to approximately 0.25 MPa; spraying onto test specimens was conducted at a distance of approximately 20 cm from the nozzle for a fixed duration of approximately 5 seconds per specimen. The sprayed specimens were then placed in a muffle furnace and subjected to a programmed heating cycle, raising the temperature to 380°C over 20 minutes to prevent cracks or edge lifting during curing induced by rapid temperature rise. Upon reaching 380°C, an isothermal hold was maintained for 30 minutes, followed by switching off the furnace to allow gradual cooling within the chamber, thereby minimizing the risk of coating delamination caused by differential thermal stresses resulting from rapid cooling. Finally, a post-solidifying step was performed at 390°C for 50 minutes, yielding a composite coating with a total thickness of approximately 25 µm. The BCC preparation process initiates as shown in Figure 1.

Figure 1
Preparation flowchart of the Black Composite Coating (BCC).

2.2. Experimental methods

SEM Analysis: The surface morphologies of the magnesium alloy, MAO-treated samples, and black composite-coated samples were observed using a Phenom PRO scanning electron microscope (SEM). As the samples exhibited poor conductivity, gold sputtering was performed prior to observation.

XRD Analysis: Phase composition analysis was conducted using a Bruker D8 ADVANCE X-ray diffractometer with a copper target Kα radiation source. Continuous scanning was performed using a 2θ/θ mode, with the voltage and current set to 40 kV and 40 mA, respectively. The scan range was 10°–80° with a step size of 0.01°. Data analysis was conducted using MDI Jade software.

FTIR Analysis: The functional groups of the black composite coating were analyzed using a VERTEX 70 Fourier-transform infrared (FTIR) spectrometer.

Coating Adhesion Testing: Adhesion strength was evaluated following the GT9286-2021 standard. A cutting tool was used to create a 1 mm × 1 mm grid pattern on the coating surface by applying uniform pressure and speed. Subsequently, a 600-HC33 adhesive tape was applied over the grid, adhered for 5 minutes, and then peeled off at a 60° angle within 1 second. The grid area was inspected under a magnifying glass, and adhesion was rated on a scale from 0 to 5, with 0 indicating the highest adhesion.

Contact Angle Measurement: Samples were placed on clean glass slides, and a 10 μL droplet of simulated downhole fluid was deposited onto the surface using a micro-syringe. The contact angle was measured using a JC2000D1 contact angle goniometer, and the droplet shape was recorded via photography. Surfaces exhibiting contact angles of 90°–150° were classified as hydrophobic.

Tafel Polarization and Electrochemical Impedance Spectroscopy (EIS): Electrochemical measurements were conducted using a CHI660E electrochemical workstation with a three-electrode system. The working, reference, and counter electrodes were the coated magnesium alloy sample, a saturated calomel electrode, and a platinum electrode, respectively. The exposed area of the working electrode in the electrolyte was 1 cm². The electrolyte was a simulated downhole fluid with a chloride ion mass fraction of 5%, adjusted using NaCl, as described in Table 1.

Table 1
Simulate the composition of the well fluid.

INGREDIENT	CONTENT (G/L)
Na2SO4	0.64
CaCl2	21.1
MgCl2	3.15
NaHCO3	0.24
KCl	11.47
NACl	42.31

Open-circuit potential (OCP) testing was performed initially to stabilize the working electrode surface. Once the OCP was stable, EIS testing was conducted with a frequency range of 100 kHz–0.01 Hz and a perturbation amplitude of 5 mV. Data analysis and fitting were performed using ZView software. All tests were repeated at least three times to ensure accuracy.The scanning rate for the Potentiodynamic polarization (PDP) curve test was set at 1 mV/s, and the test interval was set at ±250 mV relative open-circuit.

3. RESULTS AND DISCUSSION

3.1. Microstructure characterization

3.1.1. Scanning electron microscopy (SEM)

Figure 2 shows the SEM micrographs of AZ81 magnesium alloy, the magnesium alloy after micro-arc oxidation (MAO), and the magnesium alloy treated with a black composite coating [³⁹]. The analysis reveals distinct microstructural features for the three samples, providing clear insights into their surface morphology and texture. The AZ81 magnesium alloy exhibits a relatively smooth surface with discontinuous secondary phases. After MAO treatment, the substrate surface becomes porous and uneven, exposing a larger specific surface area. Following the application of the composite coating, the surface displays no distinct boundaries, with varying-sized pores and a trend toward smoothness.

Figure 2
SEM images of (a) AZ81, (b) MAO-AZ81, and (c) BCC-AZ81.

3.1.2. X-Ray diffraction (XRD)

Figure 3(a) illustrates the XRD patterns of AZ81 magnesium alloy and the MAO-treated magnesium alloy. The results indicate that the primary peaks correspond to the Mg matrix and Mg₁₇Al₁₂, with no other secondary phases detected in the magnesium alloy. The MAO layer predominantly comprises Mg, MgO, MgF₂, and Mg₂SiO₄ phases.

Figure 3
(a) XRD pattern of AZ81 and MAO-AZ81, (b) FTIR spectrum of BCC-AZ81.

The AZ81 magnesium alloy consists mainly of an Mg matrix and the intermetallic compound Mg₁₇Al₁₂, formed during solidification due to the presence of aluminum and zinc. The MAO process, performed in a specific electrolyte environment, determines the final phase composition of the coating through electrochemical reactions. During MAO, the magnesium alloy substrate acts as an anode, forming compounds like MgO, MgF₂, and Mg₂SiO₄. MgO enhances corrosion resistance, MgF₂ improves wear resistance, and Mg₂SiO₄ contributes to structural stability [⁴⁰]. The intensity ratio of Mg peaks in the MAO-AZ81 sample differs significantly from the untreated AZ81, indicating texture reorientation induced by micro-arc oxidation. This suggests preferential growth of MgO/Mg₂SiO₄ phases during plasma discharge, altering the crystalline orientation of the underlying Mg matrix.

3.1.3. Fourier-transform infrared spectroscopy (FTIR)

Figure 3(b) presents the FTIR spectrum of the black composite coating. Absorption peaks at 487, 545, and 611 cm⁻¹ correspond to the bending vibrations of F-C-F bonds [⁴¹], while peaks at 1,147 and 1,203 cm⁻¹ correspond to symmetric and asymmetric stretching vibrations of F-C-F, respectively, consistent with the characteristic peaks of polytetrafluoroethylene (PTFE) groups. The peak at 3,388 cm⁻¹ represents the stretching vibration of N-H in amide bonds, while the peaks at 1,776 and 1,714 cm⁻¹ represent symmetric and asymmetric stretching vibrations of C=O in imide groups associated with the polyacrylamide component [⁴²]. Additionally, the peak at 1,365 cm⁻¹ is associated with the stretching vibration of C-N in imide structures, and the peak at 1,652 cm⁻¹ corresponds to the stretching vibration of C=O in amide and imide groups [⁴³]. These absorption peaks are consistent with the characteristic groups of PAI resin.

3.1.4. Contact angle and coating adhesion

Figure 4 shows the contact angle and adhesion test results for the MAO and black composite coatings. According to the GT9286-2021 standard, the MAO coating demonstrates a complete and smooth cut line with no peeling, indicating grade 0 adhesion. The black composite coating shows minor detachment at the crosscut intersections, affecting less than 5% of the area, indicating grade 1 adhesion.

Figure 4
The test images of contact angle and binding force of (a) MAO-AZ81 and (b) BCC-AZ81.

Figure 4(b) reveals a contact angle of 124.7° for the black composite coating, suggesting hydrophobic properties.The hysteresis phenomenon increases with the increase of PTFE content [⁴⁴].

3.2. Electrochemical performance evaluation

3.2.1. Potentiodynamic polarization test

Potentiodynamic polarization testing is a critical method for evaluating the corrosion characteristics of materials by measuring the corrosion current density (I_corr) and corrosion potential (E_corr). As shown in Figure 5, the polarization curves of AZ81 magnesium alloy, micro-arc oxidation-treated AZ81 (MAO-AZ81), and black composite coating-treated AZ81 (BCC-AZ81) are presented.

Figure 5
Polarization curve of AZ81, MAO-AZ81 and BCC-AZ81.

Among these in Table 2, BCC-AZ81 exhibited the highest corrosion potential (E_corr = −0.259 V) and the lowest corrosion current density (I_corr = 1.28E-08 A/cm²), indicating superior corrosion resistance. MAO-AZ81 showed a corrosion potential of −1.392 V and a corrosion current density of 1.49761E-06 A/cm², demonstrating improved corrosion resistance compared to AZ81 but inferior to BCC-AZ81. In contrast, AZ81 exhibited the lowest corrosion potential (−1.429 V) and the highest corrosion current density (I_corr = 6.13903E-05 A/cm²), signifying the poorest corrosion resistance. These results indicate that both micro-arc oxidation and black composite coatings significantly enhance the corrosion resistance of AZ81 magnesium alloy, with the black composite coating providing the most pronounced improvement.

Table 2
Fitting data of potentiodynamic polarization curve for BCC-AZ81, MAO-AZ81 and AZ81.

ANODE	BCC-AZ81	MAO-AZ81	AZ81
Ecorr(V)	−0.259	−1.392	−1.429
Βcm(V/dec)	−2.78387	−4.35981	−7.95685
−βam(V/dec)	16.37922	13.02702	8.08242
Icorr(A/cm2)	1.28E-08	1.49761E-06	6.13903E-05
Rp(Ω·cm2)	1389.6	1570.9	4143.4

3.2.2. Electrochemical impedance spectroscopy (EIS) test

Electrochemical impedance spectroscopy (EIS) is a non-destructive technique used to investigate electrode interface processes and corrosion mechanisms. Figure 6 illustrates the Nyquist plots, Bode plots, and equivalent circuit models for AZ81, MAO-AZ81, and BCC-AZ81.

Figure 6
(a) Nyquist diagram, (b) Bode diagram Phase diagram, (c) Bode diagram parting diagram and (d) equivalent circuit diagram of AZ81, MAO-AZ81 and BCC-AZ81.

Nyquist plots reveal semicircles for all samples, with BCC-AZ81 exhibiting the largest semicircle diameter, indicating the highest charge transfer resistance (Rct). This result aligns with the polarization test, confirming that BCC-AZ81 possesses the best corrosion resistance. The semicircle diameter of MAO-AZ81 is smaller than that of BCC-AZ81 but larger than that of AZ81, indicating intermediate corrosion resistance.

The Bode plots further corroborate the Nyquist plot findings, showing that BCC-AZ81 exhibits the highest modulus values in the low-frequency region, which signifies superior corrosion resistance. In contrast, MAO-AZ81 and AZ81 exhibit lower modulus values, reflecting inferior corrosion performance.

Based on the fitting data in Table 3, BCC-AZ81 demonstrates the lowest series resistance (Rs = 164.1 Ω·cm²) and the highest charge transfer resistance (Rct = 9.13 × 10⁶ Ω·cm²), consistent with its excellent corrosion resistance. MAO-AZ81 exhibits Rs and Rct values of 67.8 Ω·cm² and 5932 Ω·cm², respectively, indicating that the micro-arc oxidation treatment enhances the corrosion resistance of AZ81. In comparison, AZ81 has the lowest Rs (5.1 Ω·cm²) and a relatively low Rct (632.4 Ω·cm²), which align with its poor corrosion performance.

Table 3
Fitting data of electrochemical impedance spectroscopy of BCC-AZ81, MAO-AZ81 and AZ81.

DATA	BCC-AZ81	MAO-AZ81	AZ81
Rs/(Ω·cm2)	164.1	67.8	5.1
CPE1(Ω−1·cm−2·sn)	3.21 × 10−8	3.06 × 10−7	9.69 × 10−6
n1	0.89	0.77	0.93
R1/(Ω·cm2)	516620	20611	–
Rct/(Ω·cm2)	9.13 × 106	5932	632.4
CPE2/(Ω−1·cm−2·sn)	1.09 × 10−7	7.33 × 10−7	–
n2	0.59	0.98	–
RL/(Ω·cm2)	–	1661	2901
L/(H·cm2)	–	1539	11044

In summary, the results from both potentiodynamic polarization and EIS tests confirm that the black composite coating significantly improves the corrosion resistance of AZ81 magnesium alloy, followed by the micro-arc oxidation treatment. These findings provide critical experimental evidence for surface modification of AZ81 magnesium alloy.

3.2.3. Corrosion test

In this study, the corrosion resistance of AZ81 magnesium alloy and black composite coating-treated magnesium alloy (BCC-AZ81) was evaluated by immersing the samples in simulated well fluids of varying pH values at 56°C. The surface morphology after 21 days of immersion was observed using scanning electron microscopy (SEM).

Place the sample holder in a 100 mL polytetrafluoroethylene bottle, and attach 3 sample thin slices to the sample holder. The injection volume of the simulated solutions with different pH values is 65 mL. Place it in the reaction vessel, put it in the oven, and maintain a temperature of 56°C to simulate the underground pressure environment. There are 3 groups of samples in each environment. The corrosion duration is 3 weeks. After 3 weeks, collect the samples and clean the surface corrosion products with 200g CrO; then weigh again. Due to the excessive complexity of the data, this experiment only compared the corrosion rates of AZ81 and BCC-AZ81, as shown in Figure 7. Immersion.The corrosion rate of the experiment is calculated according to the ASTMG31-72 Formula 1:

Figure 7
Variation of corrosion rate of AZ81 and BCC-AZ81 with PH.

$$\text{V}\text{=}\text{87600ΔW/A}\text{×}\text{T}\text{×}\text{D}$$

Where: V is the corrosion rate, mm/y; ΔW is the mass loss, g; A is the exposed area of the test piece, cm²; T represents the soaking time, in hours; D represents the density of the test piece, in g/cm³.

Figure 8 illustrates that in simulated well fluid with a pH of 3.0, the surface of AZ81 magnesium alloy exhibited deeper corrosion pits and a larger corroded area. Conversely, in simulated well fluid with a pH of 9.0, the corroded area was relatively smaller, and more flat magnesium matrix surfaces were preserved. These results indicate that the degradation of magnesium alloy increases with decreasing pH of the simulated well fluid. Small pits observed on the corroded surface were caused by the detachment of the second phase, due to the lower potential of the magnesium matrix compared to the Mg₁₇Al₁₂ second phase. This potential difference, coupled with the chloride-rich corrosive environment, promotes micro-galvanic corrosion, accelerating the degradation of the magnesium matrix surrounding the second phase and leading to its detachment.

Figure 8
SEM images of AZ81, MAO-AZ81 and BCC-AZ81 after 21 days.

The micro-arc oxidation (MAO) treatment forms a dense oxide layer on the surface of the magnesium alloy, which improves its corrosion resistance to a certain extent. In simulated well fluid with a pH of 3.0, the corrosion pits on the surface of MAO-AZ81 were relatively shallow, indicating that the MAO layer.

4. DISCUSSION

The AZ81 magnesium alloy comprises a matrix interspersed with discontinuous secondary phases, such as the intermetallic compound MG₁₇AL₁₂, formed during casting or heat treatment due to segregation or phase transformation of alloying elements like aluminum and zinc. This uneven distribution of phases results in a smooth yet heterogeneous surface. MAO treatment creates a ceramic layer on the magnesium alloy surface through intense chemical reactions and physical changes at high temperatures. Initially, a thin barrier oxide film forms, followed by micro-arc discharge under high voltage, causing material redistribution and resulting in a porous, uneven surface with increased specific surface area. During this process, magnesium atoms migrate to the surface, promoting film growth. The black composite coating adheres to the magnesium alloy surface via physical or chemical interactions, filling the pores and uneven regions to create a smoother surface. The coating materials, likely containing oxides and carbides, enhance uniformity and density during curing, reducing porosity and improving corrosion and wear resistance [³⁹]. The porous MAO layer (Figure 2b) increased surface area for mechanical interlocking, enhancing BCC adhesion (grade 1, Figure 4b). FTIR-confirmed PTFE/PAI (Figure 3b) sealed micropores, reducing I_corr by 5 orders vs. AZ81. BCC’s Rct = 9.13 × 10⁶ Ω·cm² (Table 3) indicates superior barrier properties due to PTFE’s hydrophobicity (124.7°, Figure 4b).

The BCC (MAO+PTFE/PAI) developed in this study reduced the corrosion current density of AZ81 alloy to the level of 10^–9 A/cm², which was three orders of magnitude lower than that of the pure MAO coating (10^–6 A/cm²). It was significantly superior to the LDH/8HQ composite coating (10^–6 A/cm²) reported by Liu et al. [³⁸] and the SIO₂ sealed MAO coating (10^–7 A/cm²) reported by Zhu et al. [³⁵]. In terms of charge transfer resistance, the Rct (10⁶ ω·cm²) of the BCC coating reached 102 times that of the triple coating by Lee et al. [⁴], indicating that the dense sealing effect of PTFE is more effective in inhibiting electrochemical corrosion. Furthermore, the corrosion rate of the coating is stable at 0.1⁻1.0 mm/year, which meets the C3-C4 grade requirements of ISO 9223 and is comparable to the MAO salt spray test result (1.2 mm/year) of Huang et al. [³²]. However, this system has greater application potential in high-pressure acidic underground environments. In the future, the nano-filler strategy of wang et al. [⁴⁰] can be drawn upon to further optimize the porosity.

5. CONCLUSION

This study demonstrates that a dual-layer Black Composite Coating (BCC), comprising a micro-arc oxidation (MAO) base layer and a PTFE/PAI top coat, significantly enhances the corrosion resistance of AZ81 magnesium alloy for downhole applications. Key findings include:

1. Microstructure: MAO formed a porous ceramic layer (MgO/MgF₂/Mg₂SiO₄, XRD) with high surface roughness, enabling mechanical interlocking for BCC adhesion (Grade 1, GT9286).

2. Electrochemical Performance:

   • Corrosion current density (I_corr) reduced from 6.14 × 10⁻⁵ A/cm² (bare AZ81) to 1.28 × 10⁻⁸ A/cm² (BCC), a ~4-order decrease.

   • Charge transfer resistance (Rct) increased from 632 Ω·cm² (AZ81) to 9.13 × 10⁶ Ω·cm² (BCC), confirming superior barrier properties.

3. Corrosion Resistance:

   • Immersion tests (21 days, pH 3.0) showed corrosion rates of 0.1–1.0 mm/year (BCC) vs. 12.3 mm/year (AZ81), suppressing degradation by >91.8%.

   • Classified as ISO 9223 C3-C4 (moderate resistance), suitable for mildly aggressive environments (e.g., oil wells).

4. Hydrophobicity: BCC achieved a contact angle of 124.7°, impeding electrolyte penetration.

This MAO-BCC synergy offers a viable pathway to mitigate rapid degradation of magnesium alloy downhole tools. Future work will optimize porosity sealing via nano-fillers for high-pressure acidic conditions. In summary, the composite coating technology developed in this study not only enhanced the corrosion resistance of AZ81 magnesium alloy but also provided scientific evidence supporting its practical application in harsh oil and gas well environments. Future work will focus on further optimizing the coating process to improve its durability and adaptability while exploring the coating’s performance in actual oil and gas well conditions. These findings provide crucial experimental evidence for the application of magnesium alloys in downhole tools and suggest promising pathways for further improvement in corrosion resistance.

6. ACKNOWLEDGMENTS

This work is supported by the General Project of the Key Research and Development Program of Shaanxi Province: “Research on Thermal Storage Technology of Wide Temperature Range Nickel-Titanium-based Phase Change Materials in Underground Service” (2025CY-YBXM-129) and the National Natural Science Foundation of. China “Phase Transformation Mechanism and thermal Cycle Stability of Titanium-zirconium-niobium based High temperature shape memory Alloy” (No.: 52071261).

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Publication Dates

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

History

• Received
  22 Mar 2025
• Accepted
  12 Sept 2025