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Article

Effect of Carbohydrates on the Formation Process and Performance of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloy

by
Yingxiu Du
†,
Mingyue Hu
†,
Xiaohua Tu
*,
Chengping Miao
,
Yang Zhang
and
Jiayou Li
*
College of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing 314001, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Technologies 2023, 11(5), 139; https://doi.org/10.3390/technologies11050139
Submission received: 14 September 2023 / Revised: 4 October 2023 / Accepted: 7 October 2023 / Published: 10 October 2023
(This article belongs to the Section Innovations in Materials Processing)
Browse Figures
Versions Notes

Abstract

:
An environmentally friendly alkaline electrolyte of silicate and borate, which contained the addition of carbohydrates (lactose, starch, and dextrin), was applied to produce micro-arc oxidation (MAO) coatings on AZ31B magnesium alloy surfaces in constant current mode. The effects of the carbohydrates on the performance of the MAO coatings were investigated using a scanning electron microscope (SEM), an X-ray diffractometer (XRD), energy-dispersive spectroscopy (EDS), the salt spray test, potentiodynamic polarization curves, and electrochemical impedance spectroscopy (EIS). The results show that the carbohydrates effectively inhibited spark discharge, so the anodized growth process, surface morphology, composition, and corrosion resistance of the MAO coatings were strongly dependent on the carbohydrate concentration. This is ascribed to the surface adsorption layer formed on the surface of the magnesium alloy. When the carbohydrate concentration was 10 g/L, smooth, compact, and thick MAO coatings with excellent corrosion resistance on the magnesium alloy were obtained.

1. Introduction

Magnesium alloys have always been recognized as the material of choice for industrial applications due to their low densities, high strength-to-weight ratios, and good electrical shielding compared with traditional aluminum alloy materials. Nevertheless, the application of magnesium alloys is seriously restricted due to the considerable drawback of being impressionable to corrosion [1]. Surface modification is an important approach to improve the corrosion resistance of magnesium alloys without influencing their initial properties. Currently, many surface technologies, such as micro-arc oxidation (MAO), organic coating, and chemical conversion, are widely studied [2,3,4]. Among them, as mentioned above, MAO technology has proved to be a promising way to create a protective oxide layer on the surface of a magnesium alloy [5].
The DOW17 and HAE processes are outstanding representatives of the practical application of MAO technology in the protective treatment of magnesium alloys [6,7]. MAO technology for magnesium alloys has been continuously developed, which is mainly manifested in the development of and changes in electrolyte compositions and electrical parameters [8,9,10,11,12]. The composition of electrolytes plays an important role in improving the behavior of the MAO process and the properties of the oxide film. Electrolytes containing chromium compounds, fluorides, permanganates, and phosphates have been previously investigated [13,14,15,16,17,18,19,20]. Recently, environmentally friendly electrolytes, such as phosphorus-free, fluorine-free, and chromium-free electrolytes, have been employed in the MAO process on magnesium alloys, which has mainly focused on alkaline electrolytes usually containing silicates, aluminum salts, and/or borates. Wu et al. reported that the corrosion resistance of MAO coatings on a magnesium alloy was significantly improved with the addition of borates and silicates into an alkaline electrolyte compared with that of an MAO coating obtained using HAE processes [21]. Hsiao et al. [22] found that the addition of Al(NO3)3 to an alkaline electrolyte system can make the oxide film more uniform with a reduction in the thickness of the oxide film. Xue et al. [23] reported that an oxide film with excellent performance can be obtained in an alkaline silicate system, and the composition and corrosion resistance of the oxide layer are greatly affected by Na2SiO3.
However, the MAO treatment of magnesium alloys in an alkaline electrolyte system composed of inorganic substances is prone to destructive electric sparks, which makes it difficult to control the voltage during the MAO process. It has been shown in the literature that some organic additives can effectively control the spark discharge, increase the resistance of the anode surface, and stabilize the MAO process on magnesium alloys. It has been found that some nitrogen-containing organic compounds can improve the MAO process and obtain an oxide film with excellent corrosion resistance [24,25]. In addition, some organic compounds with oxygen-containing lone-pair electrons can also effectively improve the MAO process and obtain oxide films with excellent performance [26,27,28]. Carbohydrates are a kind of non-toxic, harmless, and renewable natural compound. These compounds have more oxygen lone-pair electrons on their molecular chains and have good potential to form surface adsorption layers, thereby improving the MAO process on magnesium alloys [29,30]. In this paper, the MAO coating formation process and the mechanism of influence on the corrosion resistance of magnesium alloys were investigated in a NaOH-Na2SiO3-Na2B4O7 electrolyte system with various concentrations of lactose, dextrin, and starch.

2. Experimental Section

2.1. Materials

The AZ31B magnesium alloy (Hongdi Metal, Dongguan China) was cut into 30 × 20 × 2 mm samples, which were sequentially polished with 240#, 360#, 600#, and 1000# sandpaper. Then, the sample pieces were washed with distilled water and acetone. All chemical reagents used in the experiments were of analytical grade.

2.2. Preparation of MAO Coatings

The MAO of the magnesium alloy was carried out using an MAO-50-type oxidation power source. The electrolyte consisted of NaOH (45 g/L), Na2SiO3 (60 g/L), Na2B4O7 (90 g/L), and lactose, starch (soluble starch was selected in this study), and dextrin with different concentrations (5, 10, and 15 g/L). The MAO treatment was carried out in constant current mode with the following electric parameters: current density, 1.5 A/dm2; frequency, 200 Hz; duty cycle, 10%; and oxidation time, 15 min.

2.3. Characterization of MAO Coatings

The thickness and roughness of the MAO coatings were measured with an MP20E-S eddy current coating thickness gauge and a TR200 roughness tester, respectively. Each sample was measured five times, and the average values were taken. The microscopic morphologies and elemental compositions were characterized with a scanning electron microscope (HITACHI S-4800) and energy-dispersive spectrometer (HORIABA EX-250), respectively. The coating phase compositions were analyzed using an X-ray diffractometer (Rigaku D/max-2400) with CuKα as the radiation source with a scanning speed of 0.035°/s.

2.4. Corrosion Resistance Test of MAO Coatings

Electrochemical impedance spectroscopy, potentiodynamic polarization curves, and the salt spray test were used to evaluate the corrosion resistance of the micro-arc oxidation coatings. The salt spray test was carried out with the CASS salt spray method in a ZYQ025 salt spray corrosion test chamber. The working conditions were as follows: saturator temperature, 58.5 °C; test chamber temperature, 49.5 °C. The solution used for the salt spray test was NaCl 50 g/L and CuCl2 · 2H2O 0.26 g/L, pH 3.1–3.3, adjusted using glacial acetic acid and sodium hydroxide. Each condition was measured with three samples, and the average values were taken. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves were carried out in a three-electrode system in a 3.5 wt% NaCl solution at 25 °C. Prior to immersion in the NaCl solution, the sides and one face of each anodized sample were coated with epoxy resin. The working electrode was the MAO coating with an area of 1 cm2, the auxiliary electrode was a large-area Pt sheet, and the reference electrode was a saturated calomel electrode (SCE). The scanning rate of the potentiodynamic polarization curves was 1 mV/s. The frequency range of the electrochemical impedance test was 0.01–10,000 Hz, and the amplitude of the disturbance voltage was 5 mV. In order to achieve a steady state, samples treated with anodized films were pre-immersed in the electrolytes for 10 min.

3. Results and Discussions

3.1. MAO Process

Figure 1 shows the voltage–time relationship curves of the magnesium alloy in the electrolytes with and without lactose, starch, and dextrin (10 g/L). According to the change in the slope on the voltage–time curve, the MAO process can be divided into three stages, which is consistent with the trend reported in many studies [21,31,32]. In stage I (0 to 90 s), many bubbles were released from the surfaces of each sample, and the surface of the magnesium alloy was dissolved. It was estimated that an extremely thin, transparent, and dense oxide layer was rapidly formed. As the anodization time increased, the voltage increased almost linearly. The corrosion resistance of the transparent oxide coating at this stage was poor, but when the transparent coating was used as the inner layer, it provided better corrosion resistance.
When the breakdown voltage was reached, one or two bright small sparks appeared on the surface of the magnesium alloy. This indicated that the MAO was entering stage II (90 to 600 s). In this stage, the voltage continued to increase with the MAO time, but the increasing trend was reduced compared with stage I. Moreover, the breakdown voltage of the MAO coatings formed in the electrolytes with lactose, starch, and dextrin was higher than that of the oxide coating obtained without the carbohydrates. This was ascribed to the adsorption of lactose, starch, and dextrin on the surface of the magnesium alloy, which led to an increase in the resistance between the surface of the magnesium alloy and the electrolyte [30,33,34].
In stage III (600 to 900 s), a steady-state spark occurred on the surface of the magnesium alloy. The white sparks turned into separate orange ones, which represented entering into the arc discharge process of MAO. At this stage, the spark generated by the MAO discharge gradually became larger. The voltage fluctuated frequently within a certain range, resulting in pits and cracks on the surface of the coating, which seriously affected the corrosion resistance of the coating. However, after adding lactose, starch, or dextrin to the electrolyte, the spark discharge was obviously inhibited, indicating that lactose, starch, or dextrin promoted the inhibition of the arc discharge in the MAO process.

3.2. Properties of MAO Coatings

The effects of the concentration of carbohydrates on the thickness, roughness, and corrosion resistance of the MAO coatings are shown in Figure 2. The corrosion resistance was measured using the salt spray test, and the time when the first corrosion point appeared on the surface of the MAO coating was used as the evaluation basis. It can be seen in Figure 2a that after adding lactose, starch, and dextrin to the electrolyte, the thickness of the coating first increased and then decreased. When the concentration of carbohydrates was 10 g/L, the thickness of the oxide film reached the maximum, with values of 16.4, 16.6, and 15.9 μm, respectively. This phenomenon could be ascribed to the saturation of the surface adsorption layer. Hence, excessive additive carbohydrates on the anode surface may affect the transfer between coating-forming ions and magnesium ions in the electrolyte, resulting in influencing the formation process of the coating [35].
It is determined from Figure 2b that with the increase in the carbohydrate concentration, the roughness of the coating gradually decreased. When the additive concentration was 15 g/L, the roughness values of the oxide films obtained by adding lactose, starch, and dextrin were 0.724, 0.705, and 0.714 μm, respectively, which were significantly improved compared with the oxide coating (1.414 μm) obtained in the electrolyte without carbohydrates. This was ascribed to the arc suppression effect of the carbohydrates reducing the roughness of the oxide coatings, and this reduction in roughness promoted the improvement of the corrosion resistance of the MAO coatings [18,36].
Corrosion resistance is the main means to evaluate the MAO performance of magnesium alloys. Therefore, the relationship between the concentration of carbohydrates and corrosion resistance was further investigated. Figure 2c shows the results of the salt spray test of the oxide films obtained with different concentrations of additives. Compared with the MAO coating obtained in the electrolyte without carbohydrates, the corrosion resistance of the oxide coatings obtained with lactose, starch, and dextrin was effectively improved. This could be ascribed to the formation of an adsorption layer on the surface of the magnesium alloy with carbohydrates, which contributed to building a corrosion-resistant Si-O-Si network on the surface of the alloy [37,38].
It can be seen in Figure 2c that when the additive concentration was 10 g/L, the corrosion resistance of the oxide films was obtained using lactose, starch, and dextrin with times of 13 h, 14 h, and 12 h, respectively. It was concluded that the performance of the coating obtained with the addition of starch was better than those with the additions of lactose and dextrin. This can be ascribed to the growth in the carbon chains of the carbohydrates, which promotes the improvement of the effect of the surface adsorption layer during the MAO process, thereby raising the corrosion resistance of the oxide coating [30]. However, the poor water solubility of dextrin compared with lactose and soluble starch may lead to a decrease in the adsorption capacity on the surface of the magnesium alloy, which may hinder the formation of a stable surface adsorption layer [29]. Therefore, it can be considered that the improvement of the MAO process on magnesium alloys with carbohydrates is closely related to the carbon chain length and water solubility.

3.3. Morphological Characteristics

Figure 3 shows the SEM morphologies of the MAO coatings obtained with and without various concentrations of carbohydrates in the alkaline electrolyte. Figure 3a is the SEM image of the MAO coating obtained in the electrolyte without carbohydrates, and Figure 3(b-1–b-3), Figure 3(c-1–c-3), and Figure 3(d-1–d-3) are the SEM images of the coatings obtained with various concentrations (5, 10, and 15 g/L) of lactose, starch, and dextrin, respectively. It is shown that micropores, cracks, and molten oxides were observed on the surfaces of the MAO coatings of all samples, which were mainly caused by plasma discharge and gas escape during the oxidation process. As shown in Figure 3a, the oxide film obtained in the electrolyte without carbohydrates had a rougher surface appearance. Some micropores with a diameter of 5–10 μm and several cracks were distributed irregularly on the surface. This open structure is more easily penetrated by corrosive ions, which accordingly leads to corrosion. It can be seen in Figure 3b–d that the distributions of micropores on the surfaces of the MAO coatings obtained with carbohydrates were more uniform, and the diameters of the micropores were reduced. Moreover, with the increases in the concentrations of lactose, starch, and dextrin, the surface morphologies of the MAO coatings were improved. These results are basically consistent with the roughness variation trend depicted in Figure 2.
The cross-sectional morphologies of the MAO coatings in the electrolytes with and without carbohydrates are presented in Figure 4. It is determined that all the MAO coatings had a rough structure that contributed to the surface roughness. In the presence of the carbohydrates (Figure 4b–d), the compactness and thickness of the MAO coatings were improved. Furthermore, as seen in Figure 4, the interfaces between the MAO coatings and magnesium substrates were indistinct, indicating a good bond between them.

3.4. Phase Analysis

The XRD patterns of the MAO coatings obtained with and without 10 g/L of lactose, starch, and dextrin are presented in Figure 5. The MAO coating formed in the electrolyte without carbohydrates was mainly composed of MgO, MgSiO3, and Mg2SiO4. With the additions of lactose, starch, and dextrin to the electrolyte, the compositions of the MAO coatings were almost similar to that obtained in the electrolyte without carbohydrates. The diffraction peaks associated with the Mg substrate are also observed in all the XRD patterns due to the low thicknesses and porous surfaces of the MAO coatings. Furthermore, the elemental compositions of the MAO coatings obtained in the electrolytes with and without lactose, starch, and dextrin were characterized with energy spectra, as shown in Table 1. The main elements of the MAO coatings obtained in the electrolytes with lactose, starch, and dextrin were Mg, Si, and O. Both XRD and EDS analysis failed to find the characteristic element C of the carbohydrates, so it can be inferred that lactose, starch, and dextrin participated in the MAO process, but they did not participate in the film-forming reaction [25].

3.5. Electrochemical Properties of MAO Coatings

3.5.1. Polarization Curves

The potentiodynamic polarization curves of the MAO coatings obtained in the electrolytes with and without lactose, starch, and dextrin in a 3.5% NaCl aqueous solution are shown in Figure 5. The parameters related to the corrosion properties were extracted directly from the potentiodynamic polarization curves presented in Table 2, where Ecorr is the corrosion potential, and jcorr is the corrosion current density. In general, a positive shift in the corrosion potential and a reduction in the corrosion current density are beneficial to improving the corrosion resistance of MAO coatings [39]. As can be seen in Table 2, the corrosion current density and corrosion potential of the MAO coating prepared in the electrolyte without carbohydrates were 1.34 × 10−7 A/cm2 and −1.45 V, respectively. After the additions of lactose, starch, and dextrin to the electrolyte, the corrosion current densities of the obtained oxide films were 2.78 × 10−8 A/cm2, 2.43 × 10−8 A/cm2, and 6.08 × 10−8 A/cm2, respectively. Meanwhile, the corrosion potentials were −1.28 V, −1.27 V, and −1.31 V, respectively. Therefore, it is determined that the corrosion current densities and corrosion potentials of the MAO coatings were obviously improved with the additions of lactose, starch, and dextrin. It is noted that the corrosion current density of the MAO coating with the addition of starch reached the minimum value, and the associated corrosion potential shifted to the most positive value. In summary, compared with lactose and dextrin, starch is the proper addition to the electrolyte for the improvement of the anticorrosion performance of MAO coatings. It is determined that the validity of the corrosion rate with the Tafel extrapolation method is limited under several conditions [40]. As seen in Figure 6, both branches of the polarization curves of carbohydrates were under activation control. Meanwhile, changes in the electrode potential in this corrosion system did not induce additional electrochemical reactions. Thus, this indicated the rationality of the corrosion current density in this experiment.

3.5.2. Electrochemical Impedance Spectroscopy

The EIS diagrams of the MAO coatings formed in the electrolytes with and without lactose, starch, and dextrin in a 3.5% NaCl aqueous solution are depicted in Figure 7. As shown in Figure 7a, there is a low-frequency capacitive loop, which is mainly caused by the pitting corrosion of aggressive chloride ions in the electrolyte. With the addition of different concentrations of lactose, starch, and dextrin to the electrolyte, the low-frequency capacitive loop disappeared, and the diameters of the capacitive loops increased, as shown in Figure 7b–d, indicating that the carbohydrates improved the anticorrosion performance of the MAO coatings. Figure 8 shows that Zmod Bode diagrams can also estimate the corrosion protection of the anodized films formed in electrolytes with different concentrations of carbohydrates. It is determined that at a fixed frequency of 0.01 Hz, the order of the values of |Z| for different concentrations of carbohydrates were 10 g/L > 15 g/L > 5 g/L. This indicates that the MAO coatings obtained in an electrolyte with 10 g/L of carbohydrates offer good corrosion resistance [41]. These results are in accordance with the results of the Nyquist diagrams.
The equivalent circuits in Figure 9a,b were used to fit the EIS of the MAO coatings formed in the electrolytes with and without the addition of carbohydrates [17,30,33,42,43,44,45]. The simulated results of the EIS experiments are shown in Table 3, where Rs is the solution resistance, C is the electric double-layer capacitance at the interface between the MAO coating and the solution, Rp is the resistance of the porous layer of the MAO coating, CPE1 and n1 represent the constant phase element and dispersion effect of the porous layer, respectively, CPE2 and n2 represent the constant phase element and dispersion effect of the barrier layer, respectively, and Rct is the charge transfer resistance of the electrode reaction. The value of Rct represents the anticorrosion performance of the MAO coatings [35,46].
The Rct values of the MAO coatings formed in the electrolyte with carbohydrates were significantly improved compared with that of the MAO coating formed in the electrolyte without carbohydrates, as shown in Table 3. Moreover, with increasing concentrations of lactose, starch, and dextrin, the Rct values show a trend of first increasing and then decreasing.
When the concentration of lactose, starch, and dextrin was 10 g/L, the Rct reached the maximum values. It was indicated that the best anti-corrosion performance could be obtained when the concentration of lactose, starch, and dextrin was 10 g/L. Furthermore, the Rct values of the MAO coatings formed in the electrolyte with lactose, starch, and dextrin were 1.62 × 107 ohm·cm−2, 1.68 × 107 ohm·cm−2, and 1.47 × 107 ohm·cm−2, respectively. This indicates that starch is a more suitable additive compared with lactose and dextrin, which is consistent with the results of the potentiodynamic polarization tests.
Based on the results of the salt spray test, potentiodynamic polarization, and EIS, it can be concluded that the anti-corrosion performance of the MAO coatings had an important relationship with the carbohydrates. This may be ascribed to the differences in morphologies, film thicknesses, roughness, and compositions of the MAO coatings formed in the electrolytes. The surface of the MAO coating obtained in the electrolyte without carbohydrates was distributed with many cracks and micropores with a low coating thickness. This defect structure is more likely to adsorb corrosive Cl−, causing the chemical dissolution and corrosion of the coating components under neutral conditions, as shown in Equation (1). Furthermore, it is also easier for Cl− to reach the magnesium substrate through the defect region of the coating layer to form electrochemical corrosion, as shown in Equation (2) [47].
MgO + 2 H + → Mg 2 + + H 2 O
Mg → Mg 2 + + 2 e −
On the other hand, with the addition of the carbohydrates, the thicknesses, roughness, and surface structures of the oxide coatings were effectively improved, which was conducive to the inhibition of the above-mentioned corrosion reaction.

4. Conclusions

(1)
Spark discharge was suppressed with the additions of lactose, starch, and dextrin into the alkaline electrolyte in the MAO process. Also, the thickness and corrosion resistance of the MAO coatings with a smooth surface structure were improved.
(2)
The additives of lactose, starch, and dextrin affected the MAO process but did not participate in the film-forming reaction. The XRD results show that the MAO coatings were mainly composed of MgO, MgSiO3, and Mg2SiO4.
(3)
Through the polarization curves and EIS test, it was found that the addition of lactose, starch, and dextrin to the alkaline electrolyte significantly improved the corrosion resistance of the MAO coatings. When the concentration of lactose, starch, and dextrin additives was 10 g/L, the MAO coatings had the best corrosion resistance. In this framework, the corrosion potentials of the MAO coatings were −1.28 V, −1.27 V, and −1.31 V, respectively; the corrosion current densities were 2.21 × 10−8 A/cm2, 1.90 × 10−8 A/cm2, and 3.22 × 10−8 A/cm2; and the Rct values were 1.62 × 107 ohm·cm−2, 1.68 × 107 ohm·cm−2, and 1.47 × 107 ohm·cm−2, respectively.

Author Contributions

Conceptualization, X.T. and J.L.; formal analysis, X.T., C.M. and J.L.; investigation, Y.D., M.H., Y.Z. and C.M.; resources, X.T. and J.L.; writing—original draft preparation, Y.D., M.H. and X.T.; writing—review and editing, X.T. and J.L.; funding acquisition, X.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Zhejiang Provincial Natural Science Foundation of China under grant no. LY16E010003 and LGN22C200009, and the National College Student Innovation and Entrepreneurship Training Program under grant no. 202310354003.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no financial or commercial conflict of interest.

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Figure 1. Relationship between voltage and time in electrolytes containing different carbohydrates during anodizing process.
Figure 1. Relationship between voltage and time in electrolytes containing different carbohydrates during anodizing process.
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Figure 2. Effects of carbohydrate concentration on properties of MAO coatings: (a) thickness, (b) roughness, and (c) corrosion resistance.
Figure 2. Effects of carbohydrate concentration on properties of MAO coatings: (a) thickness, (b) roughness, and (c) corrosion resistance.
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Figure 3. SEM images of MAO coatings in electrolytes with different concentrations of carbohydrates: (a) 0 g/L; lactose: (b-1) 5 g/L, (b-2) 10 g/L, and (b-3) 15 g/L; starch: (c-1) 5 g/L, (c-2) 10 g/L, and (c-3) 15 g/L; and dextrin: (d-1) 5 g/L, (d-2) 10 g/L, and (d-3) 15.
Figure 3. SEM images of MAO coatings in electrolytes with different concentrations of carbohydrates: (a) 0 g/L; lactose: (b-1) 5 g/L, (b-2) 10 g/L, and (b-3) 15 g/L; starch: (c-1) 5 g/L, (c-2) 10 g/L, and (c-3) 15 g/L; and dextrin: (d-1) 5 g/L, (d-2) 10 g/L, and (d-3) 15.
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Figure 4. Cross-sectional morphologies of MAO coatings in electrolytes with and without carbohydrates (10 g/L): (a) none, (b) lactose, (c) starch, and (d) dextrin.
Figure 4. Cross-sectional morphologies of MAO coatings in electrolytes with and without carbohydrates (10 g/L): (a) none, (b) lactose, (c) starch, and (d) dextrin.
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Figure 5. XRD patterns of MAO coatings in electrolytes with different concentrations of carbohydrates with concentration of 10 g/L.
Figure 5. XRD patterns of MAO coatings in electrolytes with different concentrations of carbohydrates with concentration of 10 g/L.
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Figure 6. Potentiodynamic polarization curves of MAO coatings on magnesium alloy obtained with different carbohydrates (10 g/L) in 3.5% NaCl solution.
Figure 6. Potentiodynamic polarization curves of MAO coatings on magnesium alloy obtained with different carbohydrates (10 g/L) in 3.5% NaCl solution.
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Figure 7. Nyquist plots of MAO coatings obtained at different concentrations of carbohydrates in 3.5% NaCl solution. (a) None, (b) lactose, (c) starch, and (d) dextrin.
Figure 7. Nyquist plots of MAO coatings obtained at different concentrations of carbohydrates in 3.5% NaCl solution. (a) None, (b) lactose, (c) starch, and (d) dextrin.
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Figure 8. Bode plots of MAO coatings obtained at different concentrations of carbohydrates in 3.5% NaCl solution. (a) Lactose, (b) starch, and (c) dextrin.
Figure 8. Bode plots of MAO coatings obtained at different concentrations of carbohydrates in 3.5% NaCl solution. (a) Lactose, (b) starch, and (c) dextrin.
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Figure 9. Equivalent circuits for modeling the behavior of MAO coatings formed in electrolytes without (a) and with 5−15 g/L of carbohydrates (b).
Figure 9. Equivalent circuits for modeling the behavior of MAO coatings formed in electrolytes without (a) and with 5−15 g/L of carbohydrates (b).
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Table 1. Elemental compositions of MAO coatings.
Table 1. Elemental compositions of MAO coatings.
Addition/10 g/LElement (wt%)
ONaMgSiAl
None55.76.228.39.10.7
Lactose51.63.132.69.21.0
Starch53.83.132.79.60.9
Dextrin54.33.532.29.10.9
Table 2. Electrochemical parameters related to potentiodynamic polarization curves.
Table 2. Electrochemical parameters related to potentiodynamic polarization curves.
Addition/10 g·L−1Ecorr/Vjcorr/A·cm−2
None−1.447 ± 0.0021.34 × 10−7 ± 0.24 × 10−7
Lactose−1.284 ± 0.0042.78 × 10−8 ± 0.18 × 10−8
Starch−1.273 ± 0.0032.43 × 10−8 ± 0.16 × 10−8
Dextrin−1.307 ± 0.0056.08 × 10−8 ± 0.28 × 10−8
Table 3. Simulated results of EIS diagrams of MAO coatings on magnesium alloy.
Table 3. Simulated results of EIS diagrams of MAO coatings on magnesium alloy.
Additionc/g·L−1Rs/ohm·cm−2C/μF·cm−2Rp/ohm·cm−2CPE1/
(μF·cm−2)1/n		n1CPE2/
(μF·cm−2)1/n		n2Rct/107 ohm·cm−2
None091.57--24850.55 × 10−20.910.33 × 10−20.930.13
Lactose5117.91.32 × 10−28391----8.92 × 10−20.670.83
10153.10.82 × 10−24432----7.96 × 10−20.711.62
15168.54.76 × 10−210,680----7.12 × 10−20.721.21
Starch5128.45.44 × 10−24827----9.54 × 10−20.600.71
10142.71.02 × 10−24262----0.12 × 10−20.681.68
15122.23.94 × 10−25586----0.10 × 10−20.721.33
Dextrin598.15.63 × 10−27752----9.37 × 10−20.701.09
10139.31.13 × 10−26831----1.23 × 10−20.661.47
15103.25.19 × 10−25101----9.34 × 10−20.701.18
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MDPI and ACS Style

Du, Y.; Hu, M.; Tu, X.; Miao, C.; Zhang, Y.; Li, J. Effect of Carbohydrates on the Formation Process and Performance of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloy. Technologies 2023, 11, 139. https://doi.org/10.3390/technologies11050139

AMA Style

Du Y, Hu M, Tu X, Miao C, Zhang Y, Li J. Effect of Carbohydrates on the Formation Process and Performance of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloy. Technologies. 2023; 11(5):139. https://doi.org/10.3390/technologies11050139

Chicago/Turabian Style

Du, Yingxiu, Mingyue Hu, Xiaohua Tu, Chengping Miao, Yang Zhang, and Jiayou Li. 2023. "Effect of Carbohydrates on the Formation Process and Performance of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloy" Technologies 11, no. 5: 139. https://doi.org/10.3390/technologies11050139

APA Style

Du, Y., Hu, M., Tu, X., Miao, C., Zhang, Y., & Li, J. (2023). Effect of Carbohydrates on the Formation Process and Performance of Micro-Arc Oxidation Coatings on AZ31B Magnesium Alloy. Technologies, 11(5), 139. https://doi.org/10.3390/technologies11050139

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