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Electrodeposition and corrosion characterization of epoxy/polyaniline coated AZ61 magnesium alloy

  • Vigneshkumar Murugesan ORCID logo EMAIL logo and Ashoka Varthanan Perumal
Published/Copyright: July 4, 2022
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Corrosion Reviews
From the journal Corrosion Reviews Volume 40 Issue 5

Abstract

An epoxy with 7% polyaniline (PANI) powder coating by electrostatic deposition is proposed to enhance the corrosion resistance of AZ61 magnesium alloys. The coating thickness is varied from 85 to 130 microns, and the results are compared to the corrosion resistance of epoxy coated and uncoated AZ61 magnesium alloy. The surface characterization and corrosion behaviour of AZ61 magnesium alloy + epoxy/PANI coating are examined using X-ray photoelectron spectroscopy, salt spray test, potentiodynamic test and Fourier transform infrared spectroscopy. It is observed that the polymer coating protects the base metal against corrosion and the corrosion resistance increases with coating thickness. The corrosion rate of the uncoated, 130 µm coating thickness of epoxy coated and epoxy/PANI coated AZ magnesium alloy is observed to be 0.5379, 0.0869 and 0.0372 mm/year, respectively. The corrosion resistance of epoxy/PANI coating is superior to the epoxy coating. The increase in corrosion resistance of the epoxy/PANI coated AZ61 alloy is attributed to the physical insulation and suppression of magnesium dissolution.

Keywords: epoxy; FTIR; magnesium alloy; polyaniline; XPS

1 Introduction

Magnesium alloys offer various desirable properties, such as low density, high stiffness, superior castability, excellent machinability and less environmental impact (Phuong et al. 2017; Zhao et al. 2018). The commercial utilization of magnesium has increased rapidly in response to the requirement for light-weight in the transportation and electronics industry in the last 10 years (Gusieva et al. 2015). However, its poor corrosion resistance has limited its employment in a wide range of applications (Zhang et al. 2010). Magnesium is a group 2 alkaline metal that has two valence electrons in its outermost shell and low ionization energy. Hence it is thermodynamically unstable by often reacting with other atoms leading to the formation of an oxide, hydroxide or sulphide.

Magnesium is currently mostly used as cast magnesium components, rather than as an alloying element in other metal alloys. Several studies have found that corrosion on magnesium alloys is impacted by many factors, including chemical composition (Gusieva et al. 2015), grain size (Argade et al. 2012), intermetallic compounds (Cain et al. 2014) and distribution of α-Mg matrix phases (Shuai et al. 2019). Magnesium alloys are highly susceptible to under film and pitting corrosion than other forms of corrosion. The lower open corrosion potential of Mg matrix phases is attributable to the pitting corrosion of magnesium alloys (Zeng et al. 2014).

Mg is generally unstable and highly electroactive in aquatic conditions. The electrode potential of magnesium is less (−2.37 V). But the presence of higher electrode potential impurities (Fe = −0.41 V, Al = −1.66 V, Cu = 0.6 V) increases the micro-galvanic corrosion in the magnesium base metal. In magnesium alloys, water reduction is the major cathodic reaction that occurs in association with anodic dissolution in harsh external environments (Liu et al. 2016), as indicated in Equations (1) and (2).

(1)Mg(S)→Mg2++2e−–(anodic reaction)
(2)2H2O+2e−→H2+2OH−–(cathodic reaction)

The overall reaction for the degradation of magnesium is defined as in Equation 3

(3)Mg+2H2O→Mg(OH)2+H2

During the cathodic reaction, a hydroxide coating forms around the surface of the magnesium. Many variables have been observed to impact the stability of the oxide coating, including no homogeneity in the microstructure of magnesium alloys, the orientation of atoms, and the presence of impurities (Dai et al. 2020). Several techniques for improving the corrosion resistance such as acid conversion coating (Liu et al. 2006), anodizing (Song and Chi 2014), chemical vapour deposition (Tański et al. 2014), physical vapour deposition (Sivapragash et al. 2018) and plasma electrolytic oxidation (Hussein et al. 2013), have been analyzed in recent works of magnesium alloys. Polymeric coatings are commonly utilized to prevent corrosion in metals and alloys. These polymeric coatings can safeguard the substrates by acting as a shield through corrosion inhibitors included in the coating. Because of hydroxyl and ether groups in the chemical structure, epoxy resin exhibits remarkable adhesion capability among polymeric materials (Siva et al. 2014).

Polyaniline (PANI) is a p-type semiconductor that has attracted a lot of attention due to its versatility and beneficial mechanical and chemical characteristics. However, a conducting polymer such as PANI can preserve the substrate for a limited amount of time (Mostafaei and Nasirpouri 2014). To achieve realistic and enhanced corrosion resistance for magnesium alloys, in the long run, it is essential to obtain the advantages of organic coating with conducting polymer. It has been reported that the blended PANI functions as an anion reserve, discharging anions when the coating has been damaged. It also serves as a second physical barrier, limiting aggressive ions from penetrating the coating (de Souza 2007; Sathiyanarayanan et al. 2007; Zhang et al. 2011). The previous study also revealed that the addition of PANI in the epoxy matrix could fill the micropores or microcracks and significantly increase the hardness (Xu et al. 2022). PANI is recommended for Mg coating where superior corrosion resistance is required. However, epoxy coating alone can be used if the components are not subjected to harsh corrosive environments for reducing the coating cost. In extensive oil–water filtration experiments representing oily sewage, PANI coating materials show exceptional separation capacity, including continuous emulsion separations (Li et al. 2020). Hence they can also be used in the presence of lubricating oil such as in a gear box. Radhakrishnan et al. (2009) investigated the corrosion resistance of stainless steel using different percentages of PANI inhibitor in epoxy powder and concluded that 7% PANI in epoxy is the most effective. Hence the effect of 7% PANI in epoxy powder on the corrosion rate of AZ61 magnesium alloy and the anticorrosive mechanism is analyzed in the present study.

2 Materials and methods

2.1 Materials

AZ61 magnesium alloy, electrostatic epoxy powder RAL9005 and aniline are used as received from the Indiamart. In addition, degreasing solution (degreasers) and derusting solution (SEDERUST-GL) from Mathur Tech Pvt Ltd. are used for the surface preparation for the coating. The chemical composition of the AZ61 magnesium alloy as per the weight percentage is presented in Table 1.

Table 1:

Chemical composition of AZ61 magnesium alloy (wt%).

Al Zn Mn Cu Si Fe Ni Mg
6.5 1.3 0.35 0.05 0.1 0.005 0.004 91.69

2.2 Electrostatic deposition of epoxy/PANI on AZ61 substrate

The base metal (AZ61) surface is initially cleaned with ammonia solution and degreasing solution to remove the dust, dirt, oil, grease and residue on its surface. Otherwise, the presence of dirt and dust affect the adhesion of coating powder on the metal substrate. The surface debris such as rust and scaling are initially removed using blasting with compressed air at 2 bar pressure followed by the derusting solution. The drier removes the moisture from the chemical solutions in the specimen, and it is heated to 100°C since most of the thermosetting powders have the melting interval of 80–100°C (Gherlone et al. 1998). The PANI powder is prepared from the polymerization of aniline, dried and mixed with epoxy powder in the weight proportion of 7%. The epoxy and epoxy mixed with PANI powder is deposited on the prepared specimen using electrostatic spray deposition. The epoxy + PANI powder acquires an electrical charge in the spray gun, and it is applied on the AZ61 metal substrate, as shown in Figure 1. The coated AZ61 specimen is finally cured in the powder curing oven at 320°C for 1 h. The curing enables the powder particles to melt and distribute uniformly over the entire surface of the substrate, forming a continuous film.

Figure 1: Schematic diagram of electrostatic deposition of epoxy/PANI on AZ61 magnesium alloy.
Figure 1:

Schematic diagram of electrostatic deposition of epoxy/PANI on AZ61 magnesium alloy.

2.3 Characterization of epoxy/PANI coating

The coating thickness of epoxy/PANI on AZ61 alloy is measured using the metallurgical microscope (Dewinter Tech) as per ASTM B487-20 standard. The elemental composition of the coated structure are analyzed using an X-ray photoelectron spectrometer (ESCALAB 250XI, Thermo Scientific) using XR6 micro-focused monochromator. Fourier transform infrared (FTIR) spectroscopy measurements are conducted using the PerkinElmer spectrum two FT-IR spectrometer to characterize the existence of different chemical groups on the coating.

2.4 Corrosion characterization

2.4.1 Salt spray corrosion test

According to the ASTM B117-19 standard, the salt spray corrosion test is conducted in a controlled corrosive environment. A 5% sodium chloride solution with a pH value of 7.1 is atomized by spray nozzles with an air pressure of 15 psi is maintained. A total of five AZ61 salt spray coupons with different coating conditions such as uncoated, epoxy coated 130 μm and epoxy/PANI coated with the thickness (85 μm, 110 μm and 130 μm) were used for the salt spray test. The potentiodynamic polarization tests were repeated thrice to ensure the reproducibility of the data. The specimens are exposed to dense saltwater fog. The specimens are weighed before and after the test and the corrosion characteristics are analyzed.

2.4.2 Potentiodynamic corrosion test

The uncoated, epoxy coated and epoxy/PANI coated AZ61 magnesium alloy sheets surfaces are initially cleaned with acetone and followed by ethanol. The corrosion vessel is then cleaned with 95% ethanol and placed under a fume hood for 30 min to allow the ethanol to evaporate completely. The dry cleaned corrosion vessel is rinsed with 10 mL of sodium chloride solution, and the corrosion test is carried out using de-aerated 0.1 M sodium chloride solution at 25°C room temperature. The working, reference and counter electrode for our research is epoxy/PANI coated AZ61 specimen, standard Ag/Agcl and activated titanium mesh, respectively. The working electrode is placed in the middle, and the screws are tightened to prevent electrolyte leaking. The corrosion test is carried out using deaerated 3.5% weight sodium chloride solution at 25 °C room temperature as per ASTM G5 standard (Tozkoparan et al. 2020). A flat circular disc specimen of a 1 cm radius is mounted in the specimen. The presence of PANI increases the electrical conductivity of epoxy/PANI coating which makes it more suitable for conducting polarization tests (Vanga-Bouanga et al. 2016). The electrodes are connected with a potentiostat, and the steady-state open circuit potential is established in 1 h. The corrosion potential and current readings are analyzed using the electrochemical software package (Gamry Framework V7.4). The potentiodynamic polarization tests were repeated thrice to ensure the reproducibility of the data.

3 Results and discussion

3.1 Salt spray corrosion test

Specimens are tested in a chamber consisting of chloride medium for 96 h to determine the corrosion resistance as shown in Figure 2. The specimens are weighed after the specified time, and the extent of corrosion attack is determined using the corrosion rate as per Equation (4).

(4)Corrosion rate(mm/year)=(K×W)(A×T×D)

where K is the corrosion rate unit constant, T is the exposure period in hours, A is the area (cm2), W is mass loss in grams and D is mass density of the material (g/cm3). For AZ61 magnesium alloy, the density is 1.80 g/cm3. The constant K for the corrosion rate (mm/year) is 8.76 × 104. The area of specimen (A) is 12.25 cm2. The above equation is used to compute the specimen’s corrosion rate. and the results are presented in Table 2. It is observed that the corrosion resistance of coated AZ61 specimens are superior to the uncoated specimens (0.5379 mm/year). The corrosion rate is decreased from 0.2482 mm/year to a negligible 0.0372 mm/year when the epoxy/PANI coating thickness is increased from 85 to 130 µm. Thus it is superior to the epoxy coating of 130 µm thickness, which has a corrosion rate of 0.0869 mm/year.

Figure 2: Salt spray test results of AZ61 magnesium alloy uncoated and epoxy/PANI coated specimens.
Figure 2:

Salt spray test results of AZ61 magnesium alloy uncoated and epoxy/PANI coated specimens.

Table 2:

Salt spray test results of epoxy/PANI coated AZ61 magnesium alloy.

Magnesium AZ61 substrate Epoxy/PANI coating Epoxy coating
85 μm 110 μm 130 μm 130 μm
Initial weight (g) 13.2892 13.4092 13.4126 13.4132 13.4122
After 48 h (g) 13.2826 13.406 13.411 13.4128 13.4113
After 96 h (g) 13.2764 13.4036 13.4098 13.4122 13.4098
Corrosion rate (mm/year) 0.5379 0.2482 0.1241 0.0372 0.0869

3.2 Potentiodynamic test

The potentiostat is turned on, and before the application of ramp potential, the current value between the working electrode (positive potential) and counter electrode (negative potential) is set between −0.1 and 0.01 μA in the open circuit phase. The potential is monitored using the measurement view of the Gamry software to ensure the stabilized conditions with no fluctuations in electrode potential. Tafel plot interprets the excessive voltage of the specimen, which is above the corrosion potential. The characteristic equation of the Tafel plot is denoted in Equation (5).

(5)η=β  logiiCORR

where η is the excessive voltage, β is the slope of the curve, and iCORR is the corrosion current at η (mA/cm2).

The initial details for the conduction of the experiment such as scan rate are set up as 0.167 mV/s, the sample area being 3.14 cm2, the density of 1.8 g/cm3 and initial delay of 3600 s for the stabilization of the corrosion test are entered in the software. The stabilized specimens are polarized at about 250 mV both anodically and cathodically to obtain the Tafel plot as per ASTM G3-14 standard. The Tafel spectra of the different samples, such as uncoated AZ61 Mg alloy, epoxy coated and epoxy/PANI coated AZ61 Mg alloy with different coating thicknesses of 85 μm, 110 μm, and 130 μm as shown in Figure 3. The readings of corrosion potential and corrosion current for the coated and uncoated AZ61 mg alloy are specified in Table 3.

Figure 3: Potentiodynamic polarization curves of bare AZ61 Mg alloy, epoxy and epoxy/PANI coated specimens of different coating thicknesses.
Figure 3:

Potentiodynamic polarization curves of bare AZ61 Mg alloy, epoxy and epoxy/PANI coated specimens of different coating thicknesses.

Table 3:

Potentiodynamic polarization test results of epoxy/PANI coated AZ61 magnesium alloy.

Magnesium AZ61 substrate Epoxy/PANI coating Epoxy coating
85 μm 110 μm 130 μm 130 μm
Corrosion potential, η (V) −2.1 −1.6 −1.2 −0.84 −0.9
Corrosion current, iCORR (mA/cm2) 1.25 × 10−2 7.943 × 10−5 5.011 × 10−6 1.368 × 10−6 1.584 × 10−6

The Tafel plot exhibits the total current, which is the addition of anode and cathode current. As the current changes its direction from cathodic to anodic or vice versa, it is observed as the distinct intersecting vertical line in the Tafel plot. The uncoated AZ61 magnesium substrate has a corrosion potential of −2.1 V and a corrosion current of 1.25 × 10−2 mA/cm2. However, the corrosion potential of the coated samples exhibits much higher corrosion potential values. The corrosion potential curve of epoxy/PANI coated specimen of 85 μm thickness is increased by +0.5 V (magnitude of −1.6 V) with the reduction in corrosion current to 7.943 × 10−5 (mA/cm2). Thus the corrosion resistance of the epoxy/PANI coated AZ61 alloy is remarkable than the uncoated AZ61 magnesium alloy. The Tafel curve of the epoxy/PANI coated specimens with higher coating thicknesses such as 110 and 130 μm is shifted rightwards with higher corrosion potential values of −1.2 and −0.84 V, respectively. Thus it implies that the corrosion potential of the material increases with increasing coating thickness. Also, it is found that the magnitude of corrosion current reduces with an increase of coating thickness. The uncoated specimen has the largest corrosion current value of 1.25 × 10−2 mA/cm2, and it is decreased to 1.368 × 10−6 mA/cm2 for the epoxy/PANI coating thickness of 130 μm. However, the corrosion resistance and corrosion potential of 130 μm epoxy coated AZ61 is found to be −0.9 V and 1.584 × 10−6 mA/cm2, respectively. The smaller the magnitude of corrosion current and larger the magnitude of corrosion potential, the better the corrosion resistance of the material (Choi et al. 2007). Thus both the salt spray tests and the potentiodynamic test indicated the corrosion resistance of the epoxy/PANI coated material is superior to the epoxy coated material.

3.3 Cross section morphology

The surface characteristics of coated specimens are examined in order to fully understand the anticorrosive mechanism. A linear scanning of elements is performed across the cross-section of 130 μm epoxy/PANI coated AZ specimen using scanning electron microscope as shown in Figure 4. The intersection of carbon and magnesium element curves is observed in the 15–20 μm zone. Beyond 25 μm, the carbon and oxygen elements are dominant for the entire coating thickness. The carbon and oxygen elements are derived from the epoxy/PANI coating, and magnesium elements are derived from the AZ61 magnesium alloy. The interference region is observed in the 12–24 μm zone. Thus the minimum coating thickness of the magnesium + epoxy/PANI for imparting corrosion resistance is 24 μm.

Figure 4: Linear scanning of elements across the cross-section of epoxy/PANI coated AZ61 Mg alloy.
Figure 4:

Linear scanning of elements across the cross-section of epoxy/PANI coated AZ61 Mg alloy.

3.4 FTIR spectroscopy

FTIR is utilized to examine the presence of organic compounds in the epoxy/PANI coated sample, as shown in Figure 5. It can be seen that the compounds of MgO, MgCO3, MgH2, benzyl oxide and CH bending are observed in the FTIR spectroscopy. The broadband (range of 3200–3500 cm−1) in the magnesium alloy AZ61 substrate and the epoxy/PANI coated substrate is attributed to the formation of MgO compounds (Balakrishnan et al. 2020). The skeletal vibrations of the benzene ring are mainly responsible for the absorption band at ∼1560 cm−1 (Zhang et al. 2011). The absorption band at ∼1464 cm−1 is assigned to OH bending (Ambrosio et al. 2020). The stretching vibration of benzyl oxide in the epoxy main chain is attributed to the peak at ∼1235 cm−1 (Zhang et al. 2018). Thus the existence of PANI enclosed in the epoxy resin is confirmed. According to these observations, the principle of epoxy curing by PANI involves the curing reaction of PANI amine groups with epoxy resin epoxide groups (Sivapragash et al. 2018) as indicated in Figure 6. The absorption bands at ∼1109 cm−1 and ∼1160 cm−1 are undoubtedly attributed to MgCO3 and MgH2, respectively, based on the previous investigation (Zhang et al. 2018). The variation in the absorption band at ∼1353, ∼902 and ∼761 cm−1 is attributed to the aromatic CH bending (Djebaili et al. 2015). It can be inferred from the FTIR spectra that the epoxy/PANI coating has been successfully coated on the AZ61 magnesium alloy.

Figure 5: FTIR spectra of bare AZ61 and epoxy/PANI coated AZ61 magnesium alloy.
Figure 5:

FTIR spectra of bare AZ61 and epoxy/PANI coated AZ61 magnesium alloy.

Figure 6: Chemical curing reactions between epoxy and PANI compound.
Figure 6:

Chemical curing reactions between epoxy and PANI compound.

3.5 XPS analysis

The X-ray photoelectron spectroscopy (XPS) analysis is used to reveal the elemental composition at the surface by emitting X-rays on the coated specimen and measuring the energy emitted from the top surface. The surface morphology of the epoxy/PANI coated AZ61 magnesium alloy is examined using XPS analysis, as shown in Figure 7a–e. The bonding condition of C 1s, Mg 1s and O 1s is analyzed using a deconvolution process with a Gaussian profile. According to the XPS survey results, the epoxy/PANI conversion coating on the AZ61 alloy surface features Mg, C, O, Zn and Al components. The C 1s spectrum, shown in Figure 6b, exhibits three forms of carbon bonding: C at 291.39 eV, C-OH at 289.9 eV and CO3 at 289.23 eV (Cui et al. 2013; Feliu Jr. et al. 2014; Guo et al. 2020; Liao et al. 2013). The peak at 289.23 eV is attributed to the formation of magnesium carbonate. The peak at 289.90 eV is accountable for the adsorption of alcohol (C-OH) from the epoxy compound. The peak of 291.39 eV is attributed to the carbon–carbon (C–C) bonding components resulting from the reaction of epoxy/PANI compounds.

Figure 7: XPS characterization of epoxy/PANI coated AZ61 magnesium alloy: (a) Al 2p, (b) C 1s, (c) O 1s, (d) Zn 2p and (e) Mg 1s.
Figure 7:

XPS characterization of epoxy/PANI coated AZ61 magnesium alloy: (a) Al 2p, (b) C 1s, (c) O 1s, (d) Zn 2p and (e) Mg 1s.

Moreover, the O 1s spectra exhibit three peaks at 533.32, 531.82 and 533.76 eV (Zhang et al. 2011), indicating that MgCO3, MgO and O/(–C6H5–N–H–)n are detected in the surface of the coating. The existence of O/(–C6H5–N–H–)n suggests the PANI oxygen reduction process and the development of a passive film beneath the epoxy/PANI coating. It suppresses magnesium dissolution and increases corrosion resistance (Zhang et al. 2011). The existence of Al2O3 and Zn is identified in the Al 2p and Zn 2p bands. The aluminium and zinc elements must have been derived from the AZ61 substrate (Zhang et al. 2018). Amorphous MgO and MgCO3 might make up the majority of the coating’s surface. The XPS analysis does not reveal the presence of magnesium hydride on the surface. Therefore, it can be inferred that the AZ61 magnesium alloy interacts with the byproducts of epoxy/PANI, resulting in the emergence of Al2O3, MgO, MgCO3 and O/(–C6H5–N–H–)n on the substrates. The introduction of PANI to the epoxy modifies chemical composition of the corrosion film, thereby imparting the protective mechanism. Hence the epoxy/PANI coated AZ61 magnesium alloy can be considered for replacing aluminium in the automobile applications.

4 Conclusions

The corrosion resistance of AZ61 magnesium alloys can be significantly enhanced with the epoxy/PANI coating. Based on the corrosion testing, XPS and FTIR analysis, it is observed that the surface of the coating comprises MgO, MgCO3, Al2O3, O/(–C6H5–N–H–)n and C–OH components resulting from the reaction of magnesium substrate and organic coating elements. The salt spray test and the potentiodynamic polarization curves reveal that the epoxy/PANI coating has a superior corrosion resistance when compared to the uncoated AZ61 substrate (0.5379 mm/year). It is also observed that the increase of epoxy/PANI coating thickness from 85 to 130 µm reduces the corrosion rate from 0.2482 to 0.0372 mm/year and it is better than the 130 µm epoxy coating thickness which has the corrosion rate of 0.0869 mm/year. The physical insulation and suppression of magnesium dissolution are responsible for the epoxy/PANI coated AZ61 alloy’s increased corrosion resistance.

Corresponding author: Vigneshkumar Murugesan, Department of Mechanical Engineering, Sri Krishna College of Engineering and Technology, Coimbatore, Tamilnadu641008, India, E-mail:

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: None declared.

  3. Conflicts of interest: The authors declare no conflicts of interest regarding this article.

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Received: 2021-09-01
Accepted: 2022-04-28
Published Online: 2022-07-04
Published in Print: 2022-10-26

© 2022 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Reviews
  4. An overview of anti-corrosion properties of ionic liquids for corrosion of carbon steel in acidic media
  5. Corrosion monitoring techniques for concrete in corrosive environments
  6. Graphene-based coatings for magnesium alloys: exploring the correlation between coating architecture, deposition methods, corrosion resistance and materials selection
  7. Original Articles
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  9. The third-generation biodiesel blends corrosion susceptibility of oxide particle-reinforced Si-rich aluminum alloy matrix composites
  10. Electrodeposition and corrosion characterization of epoxy/polyaniline coated AZ61 magnesium alloy
  11. Novel anticorrosive coating of silicone acrylic resin modified by graphene oxide and polyaniline
https://doi.org/10.1515/corrrev-2021-0069
Keywords for this article
epoxy; FTIR; magnesium alloy; polyaniline; XPS

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Reviews
  4. An overview of anti-corrosion properties of ionic liquids for corrosion of carbon steel in acidic media
  5. Corrosion monitoring techniques for concrete in corrosive environments
  6. Graphene-based coatings for magnesium alloys: exploring the correlation between coating architecture, deposition methods, corrosion resistance and materials selection
  7. Original Articles
  8. Decision support system to evaluate a vandalized and deteriorated oil pipeline transportation system using artificial intelligence techniques. Part 1: modeling
  9. The third-generation biodiesel blends corrosion susceptibility of oxide particle-reinforced Si-rich aluminum alloy matrix composites
  10. Electrodeposition and corrosion characterization of epoxy/polyaniline coated AZ61 magnesium alloy
  11. Novel anticorrosive coating of silicone acrylic resin modified by graphene oxide and polyaniline
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