Effect of Oxidation Time on the Structure and Corrosion Resistance of Micro-Arc Oxidation Coating of AZ91D Magnesium Alloy in (NH₄)₂ZrF₆ Electrolyte System

Abstract

Micro-arc oxidation (MAO) coatings were obtained from an AZ91D magnesium alloy at different oxidation times (5, 10, 15, and 20 min), using a zirconium salt electrolyte system, with (NH₄)₂ZrF₆ as the main salt. The morphology of the coatings was studied using a scanning electron microscope (SEM) and confocal laser scanning microscopy (CLSM). Energy dispersive spectrometry (EDS), X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS) were employed to determine the type of element and the composition of its phase. The potentiodynamic polarization curve (PDP) was applied to illustrate the corrosion resistance of the coatings. We found the coatings had minor porosity and the best compactness when the MAO treatment time was 10 min. The coatings mainly comprised MgO, ZrO₂, MgF₂, and Zr₃O₂F₈ phases and amorphous magnesium phosphate. Among the MAO coatings prepared in this experiment, the 10 min coating had the lowest corrosion current density (I_corr), and the I_corr was 4.864 × 10⁻⁸ A/cm², which was three orders of magnitude lower than the uncoated AZ91D magnesium alloy.

1. Introduction

Magnesium alloys are suitable for application in aerospace, vehicles, electronic products, biomedical metals, and other fields, because of their low density, high specific strength and stiffness, excellent electromagnetic shielding performance, good shock absorption performance, and good biocompatibility [1,2,3]. Unfortunately, their applicability has always been constrained by the fatal flaw of weak corrosion resistance. Therefore, researchers have studied various anti-corrosion technologies to solve this problem, such as heat treatment [4], corrosion inhibition [5,6], chemical conversion [7], electroplating/electroless plating [8,9], micro-arc oxidation [10], vapor deposition [11], and laser surface treatment [12], etc.

Micro-arc oxidation (MAO), one of the many surface protection technologies, is regarded as one of the most promising surface treatment methods for magnesium alloys due to its straightforward procedure, environmental friendliness, capability to treat intricately shaped workpieces, and ability to create a coating with good corrosion resistance. MAO’s basic principle is to apply high voltage to a metal immersed in an electrolyte to cause a breakdown of micro-arc discharge on its surface. Under local high temperatures and high voltage, the metal surface is oxidized to generate ceramic coatings of matrix element oxides and electrolyte-element-doped oxides in situ [13]. After years of research by researchers, it is generally agreed that the performance of micro-arc oxidation films is affected by electrical parameters (voltage, current density, frequency, and duty cycle), oxidation time, electrolyte type, and electrolyte concentration [2,14,15,16].

Zirconium dioxide, which has stable chemical properties, high-temperature resistance, and corrosion resistance, can be introduced into the micro-arc oxidation coating as an oxide to improve the corrosion resistance of the coating. In recent years, numerous studies have reported the preparation of MAO coatings on magnesium alloys in zirconium salt electrolytes. Einkhah et al. [17] generated the zirconium-containing oxide layer on an AZ31 magnesium alloy using a two-step method, which significantly improved the corrosion resistance of the coating. Tang et al. [18] prepared a ZrO₂-Mg₂ZrO₁₂-MgO composite MAO coating on an AZ91D magnesium alloy by adding ZrO₂ nanoparticles to a phosphate solution containing 10 g/L K₂ZrF₆. The result showed that adding ZrO₂ nanoparticles further improved the coating’s thickness, compactness, and corrosion resistance. Selvi et al. [19] reported the effects of different electrolytes (NaSiO₃ and NaAlO₂ + Na₃PO₄ mixed with K₂ZrF₆ and Na₂ZrO₃ zirconium salts, respectively) and different voltages on the morphology and the property of an MAO coating on an AZ91D magnesium alloy.

Oxidation time is an essential parameter in controlling the structure and properties of coatings in the MAO process, which affect the surface morphology, roughness, friction and wear properties, and corrosion resistance of the coatings. Rehman et al. [20] prepared MAO coatings with different time intervals on an AZ31B magnesium alloy in a Na₂SiO₃-K₂ZrF₆ hybrid electrolyte. The coatings comprised MgF₂, ZrO₂, Mg₂SiO₄, and MgO phases. The hardness and corrosion resistance were significantly improved with the extension of the treatment time, and the best corrosion resistance was obtained at 15 min. Yong et al. [21] learned through orthogonal experimental research that the MAO coating of a ZK61M alloy had the best corrosion resistance at a 6 g/L (NH₄)₂ZrF₆ concentration, 450 V voltage, and 15 min oxidation time.

Researchers have researched the MAO process of a magnesium alloy in an electrolyte, using K₂ZrF₆ as the main salt. However, the research information on the role of oxidation time in the (NH₄)₂ZrF₆ electrolyte system was limited. In this paper, the MAO ceramic coating of an AZ91D magnesium alloy was fabricated in a (NH₄)₂ZrF₆ electrolyte, and the effect of different oxidation times on the microstructure and corrosion resistance of the coating was studied, which provided a valuable exploration for the micro-arc oxidation corrosion protection treatment of the magnesium alloy.

2. Materials and Methods

2.1. Sample Pretreatment and Coating Preparation

The size of the AZ91D magnesium alloys used in the experiment was 15 × 15 × 3 mm. The element content we tested is shown in

Table 1. Element content of AZ91D magnesium alloy.

Element	Mg	Al	Zn	Mn	Si	Fe	Cu	Ni
Content (wt.%)	Balance	9.01	0.65	0.21	0.013	0.0028	0.0018	0.00082

.

Before the MAO treatment, we used 800 grit, 1000 grit, 1500 grit, and 2000 grit abrasive paper to polish the surfaces of the samples until the surfaces were bright and without apparent scratches. Next, the pieces were ultrasonically cleaned with ethanol and acetone for 10 min, rinsed with deionized water, and dried in cold air for standby.

2.2. Experiment Process

The equipment used in the MAO treatment was a T-25Z thick film oxidation machine (Northwest Institute for Nonferrous Metal Research, Xi’an, China), mainly composed of a power supply, electrolytic cell, agitator, and refrigerant. The electrolyte components were: (NH₄)₂ZrF₆, NaH₂PO₄, NaF, and NaOH. NaOH was mainly used to adjust the pH value of the solution (7.5–8.0). The electrical parameters used in the MAO were a voltage of 450 V, a frequency of 500 Hz, a duty cycle of 15%, and different oxidation times of 5 min, 10 min, 15 min, and 20 min.

2.3. Characterization Method

A VEGA II XMU scanning electron microscope (SEM, Tescan, Oxford, UK) was used to observe the surface and cross-sectional morphologies of the coatings, and the energy dispersive spectrometer (EDS) was employed to analyze the element species and element distribution on the surfaces and cross-sections.

Zeiss LSM800 confocal laser scanning microscopy (CLSM, Zeiss, Oberkochen, Germany) was used to measure the three-dimensional surface morphology of the coatings, with a selected area of 255 × 255 μm.

The phase composition of the coatings was determined by a D/max 2200 PC X-ray diffractometer (XRD, RIGAKU, Tokyo, Japan) under the Cu Kα radiation of 45 kV and 200 mA, and the detection angle was set at a 2θ range of 10°–80° with a step size of 0.02° and a scanning speed of 2°/min.

Dimension Icon type X-ray photoelectron spectroscopy (XPS, Bruker, Billerica, MA, USA) analyzed the chemical states of the coating constituent elements.

The potentiodynamic polarization (PDP) curve was measured with a VersaSTAT 3F electrochemical workstation (AMETEK, Berwyn, PA, USA). The electrochemical workstation used the typical three-electrode system, with a saturated calomel electrode (SCE) as the reference electrode, a platinum grid as the counter electrode, and the sample as the working electrode. According to the set step time and step height (which determined the scanning rate), the PDP curve of the sample was obtained by scanning from the initial to the final potentials. Before measuring the PDP curve of each coating, the samples were sealed with modified acrylate adhesive, and the surfaces were exposed to a working area of 1 × 1 cm². Then, the samples were immersed in a 3.5 wt.% NaCl solution for 20 min, with the temperature kept at room temperature. The PDP curves were tested with a potential scanning region of −0.5–1.5 V and a scan rate of 2 mV/s.

3. Results and Discussion

3.1. Surface and Cross-Sectional Morphology of MAO Coatings

The surface morphologies of the MAO coatings prepared by different oxidation times are shown in Figure 1. The MAO coating surfaces showed an irregular porous convex morphology with deposits appearing in some pores, and the molten oxides were in a radially extending state. With the prolongation of the reaction time, the continuous stacking and covering of the molten oxides closed the reaction micropores, and the multi-layer pore structure of large pores covering tiny pores appeared on the surfaces of the coatings (Figure 1d).

The porosity of each coating was analyzed by ImageJ software, and the porosity of each coating showed apparent differences with the increase in oxidation over time (Figure 2). At 5 min, the porosity of the coating was the largest, and the coating had a minor porosity at 10 min. In the initial reaction stage, the MAO coating surfaces had violent oxidation reactions, accompanied by micro-discharge breakdown and gas release. Thus, some macropores, coupled with multiple discharge micropores, were generated on the MAO coating surfaces (Figure 1a). The molten oxides on the surfaces of the substrates were sprayed to the outside under local high temperature and electric voltage, and large sintering protrusions were formed by the cooling of the external electrolyte (Figure 1a) [22,23]. The pores on the coating surfaces became significantly smaller at 10 min, due to the accumulation of molten oxides and the sealing effect of the deposits on the pores (Figure 1b) [24]. When the time increased to 15 or 20 min, the oxidation degree of the coating became weakened, and the insulation was increased, resulting in partial micro-arc discharge on the surface. During this process, the discharge channels became smaller and energy accumulated gradually, causing the coatings to break and deteriorate. Therefore, the micropores of the coatings increased, and microcracks of different degrees accompanied the surfaces (Figure 1c,d) [25].

Three-dimensional surface morphologies of MAO coatings are displayed in Figure 3. The color differences of the images reflected the undulation of the coating surfaces to a certain extent. The more significant the color difference, the higher the undulation of the MAO coatings [26]. There were many blue areas on the surface of the micro-arc oxidation film at 5 min (Figure 3a), and many noticeably large particles could be seen, indicating that the coating surface fluctuated wildly. With the extension of the oxidation time, it can be seen that the color difference of the coatings gradually decreased, and the convex particles became smaller and flatter. The surfaces of the coatings gradually became smoother (Figure 3b–d).

The cross-sectional morphologies of the MAO coatings formed under different oxidation time conditions are illustrated in Figure 4. It can be shown that the increase in the thickness of the coatings was not apparent with the extension of the treatment time, the interfaces between coating and substrate were tightly bonded, and there was no evident cracking.

As we all know, a traditional MAO coating generally comprises a dense inner layer and an outer porous layer. However, it is evident from the cross-sections of the MAO coatings described in Figure 4 that pore defects were primarily distributed in the middle of the coatings, and some pores were internally mixed with deposits. The coatings could be roughly divided into a skinny internal barrier layer, a thick but porous intermediate layer, and a relatively dense outer layer (Figure 4c). We speculated that the formation of this structure was due to the filling and covering of the pores by molten materials during the growth of the micro-arc oxidation coatings. Still, some works from the literature believed that it was due to the difference in the transient temperature field of the inner and outer layers [20,27].

By comparing the coatings, it was found that the number of hole defects in the intermediate layer of the coating was relatively small when the treatment time was 10 min. There were no noticeable penetrating pores and microcracks, and the structure was pretty dense, which also coincided with the surface morphology shown in Figure 1b.

3.2. Composition Analysis of MAO Coatings

EDS surface scanning analysis was carried out on the MAO coatings under different time conditions to understand the main elements on the surfaces of the MAO coatings and their relative contents. The results are listed in

Table 2. EDS analysis data of MAO coating surface at different times.

Sample No.	O (wt.%)	F (wt.%)	Mg (wt.%)	Zr (wt.%)	P (wt.%)
5 min	49.17	13.63	23.70	6.44	7.06
10 min	47.78	12.89	24.75	7.66	6.92
15 min	45.63	15.68	27.26	5.91	5.52
20 min	44.68	20.78	26.19	3.69	4.67

. The coatings were mainly composed of O, F, Mg, Zr, and P elements, indicating that the components in the electrolyte were successfully incorporated into the layers. It was speculated that MgO, ZrO₂, MgF₂, phosphate, and other substances were generated in the coatings. Some in the literature revealed that the presence of the F element could improve the compactness of the coating, and MgF₂, as a stable product, played a dissolution inhibition role and reduced the pitting tendency, thus providing better protection for the MAO coating [28,29]. ZrO₂ has high strength and fracture toughness, a low coefficient of thermal expansion, and good corrosion resistance. Its chemical stability in neutral or acidic solutions is higher than that of MgO. The introduction of ZrO₂ has been beneficial in improving the corrosion resistance of the MAO coating [20,30,31].

The EDS line scan images of the 10 min coating are shown in Figure 5, which show the distribution of the various elements in the coating cross-section. It can be seen that the F, O, Zr, and P elements were mainly concentrated in the generated coating. The Zr and F elements showed more distribution in the loose intermediate layer, indicating that the ZrO₂ and MgF₂ contents around the coating defects were higher and played a role in filling the pores, which is consistent with previous literature studies [24,32].

3.3. Phase Analysis of MAO Coatings

To further determine the phase compositions of the MAO coatings, XRD analysis was carried out on the coatings. MDI jade software detected several diffraction peaks of ZrO₂, MgO, MgF₂, and Zr₃O₂F₈ (Figure 6), indicating that the coating consisted of ZrO₂, MgO, MgF₂, and Zr₃O₂F₈ phases.

The XPS spectra of the MAO coatings are described in Figure 7. According to the complete spectrum analysis, the main constituent elements of these surface layers were Mg, Na, F, O, Zr, and P (Figure 7a). The specific binding energies of the F, O, Mg, and Zr in each MAO coating are shown in Figure 7b–e. The F1s peak at 685.75 eV corresponded to the MgF₂; O 1s was divided into two peaks; the peak at 531.30 eV corresponded to the ZrO₂; and the peak at 532.10 eV corresponded to the MgO. The three peaks of Mg1s appeared at the binding energies 1303.90 eV, 1304.95 eV, and 1306.50 eV, of which 1306.50 eV and 1304.95 eV were attributed to the MgF₂, and 1303.90 eV was from the MgO. The Zr 3d spectrum was divided into three peaks. Two peaks of the Zr 3d3/2 were at 185.50 eV and 184.50 eV, and one peak of the Zr 3d5/2 was at 182.90 eV, which are all attributed to the ZrO₂. The phase detection results of the XRD were further verified by XPS spectrum analysis.

It is well known that many complex chemical oxidations, electrochemical oxidations, and plasma oxidations coexisted in the MAO reactions of the valve metals, which involved complex processes, such as chemistry, electrochemistry, thermodynamics, and metallurgy. Combined with the detection results of the XRD and XPS, we conjectured that the following reactions might have been involved in the formation stage of the MAO coatings [20,33]:

$${\mathrm{Mg}}^{2+}+2{\mathrm{OH}}^{-}\to {\mathrm{Mg}\left(\mathrm{OH}\right)}_2$$

(1)

$${\mathrm{Mg}\left(\mathrm{OH}\right)}_2\to \mathrm{MgO}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{Mg}}^{2+}+2{\mathrm{F}}^{-}\to {\mathrm{MgF}}_2$$

(3)

$${\mathrm{ZrF}}_6^{2-}+5{\mathrm{OH}}^{-}\to {\mathrm{Zr}\left(\mathrm{OH}\right)}_5^{-}+6{\mathrm{F}}^{-}$$

(4)

$${\mathrm{Zr}\left(\mathrm{OH}\right)}_5^{-}\to {\mathrm{ZrO}}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{OH}}^{-}$$

(5)

We observed that there were Zr₃O₂F₈ phases in the coatings, due to the fluorination of the ZrO₂ by NH₄F under the joint actions of reaction temperature, reaction time, and different electrolytes in the micro-arc oxidation process. The reaction formula is as follows [34]:

ZrO₂ + NH₄F→ZrO_xF_4−2x + NH₃ + H₂O

(6)

In addition, the presence of the P element in the coating was detected by EDS (Table 1), indicating that the electrolyte NaH₂PO₄ also participated in the reactions of coating formation. However, we did not find the diffraction peak of the related phosphate (Mg₃(PO₄)₂) through XRD detection, which indicated that the amorphous phases of magnesium phosphate were likely formed in the coatings [24,32,35].

3.4. Electrochemical Corrosion Test

We used the Tafel extrapolation method to deduce the corrosion parameters of the samples from the polarization curve, which included the corrosion potential E_corr, the corrosion current density I_corr, the anode Tafel slope β_a, and the cathode Tafel slope β_c. The polarization resistance R_p was calculated by the Stern–Geary formula, and described as follows:

R_p = β_a·|β_c|/[2.303·I_corr(β_a + |β_c|)]

(7)

The corrosion rate V_corr (expressed as penetration rate v_p) was calculated by the formulas described as follows, where A represents the molar mass of the main element Mg (24.30 g/mol), and ρ represents the density of the Mg (1.74 g/cm³) and z = 2 [36]:

v_p = 3.27 × (A/z)·(I_corr/ρ)

(8)

Figure 8 shows the polarization curves of the AZ91D magnesium alloy bare substrate and its MAO coatings, with the electrochemical data obtained by the Tafel extrapolation method and the Stern–Geary formula listed in

Table 3. Parameter values of potentiodynamic polarization curves of AZ91D magnesium alloy and MAO coatings in 3.5 wt.% NaCl solution.

Sample No.	Ecorr (V)	Icorr (A/cm2)	βa (mV/dec)	|βc| (mV/dec)	Rp (kΩ·cm2)	Vcorr (µm/year)
AZ91D	−1.479	2.829 × 10−5	144.27	129.03	1.045 × 103	645.96
5 min	−1.407	5.821 × 10−7	26.95	144.02	1.693 × 104	13.29
10 min	−1.485	4.864 × 10−8	110.9	100.62	4.710 × 105	1.11
15 min	−1.480	1.581 × 10−7	71.15	104.82	1.164 × 105	3.61
20 min	−1.582	2.345 × 10−7	174.12	153.35	1.510 × 105	5.35

.

Generally, we believe that high corrosion potential (E_corr) and low corrosion current density (I_corr) represent good corrosion resistances for the test samples. From the polarization curve and electrochemical data, we could see that the increase in oxidation time did not significantly improve the E_corr of the coatings, and the E_corr of some coatings was more negative than that of the uncoated bare substrate. This situation was attributed to the MAO coatings having a more significant inhibition effect on the cathode reactions than on the anode reactions, which makes the coatings exhibit a more negative corrosion potential [37]. Therefore, the I_corr and R_p could better reflect the change in the corrosion resistance of the MAO coatings.

From the data in Table 3, we can see that, compared with the AZ91D magnesium alloy bare substrate, the I_corr of the MAO coatings was generally reduced by 2–3 orders of magnitude, and the polarization resistance R_p increased by 1–2 orders of magnitude, and the corrosion rate V_corr reduced by 1–2 orders of magnitude. The samples under 10 min treatment had the lowest I_corr (4.864 × 10⁻⁸ A/cm²), the most extensive R_p (4.710 × 10⁵ kΩ·cm²), and the minimum V_corr (1.11 µm/year), which proved that the coating obtained under the 10 min treatment had the best corrosion resistance. The reason was that the 10 min coating with low porosity and fewer structural defects (Figure 1b) could more effectively block the penetration of corrosive media into the substrate [38]. In addition, the MgF₂ and zirconium-containing oxides in the coating also played a particular role in improving the corrosion resistance of the coating.

4. Conclusions

MAO ceramic coatings of the AZ91D magnesium alloy were prepared in the electrolyte (NH₄)₂ZrF₆ as the main salt under 5 min, 10 min, 15 min, and 20 min oxidation times. The following conclusions were drawn from the study of the microstructure, phase composition, and corrosion resistance of the coatings:

• The coating with the 10 min oxidation time had the lowest porosity, the fewest defects, and the best compactness.
• The MAO coatings of the AZ91D magnesium alloy formed in the (NH₄)₂ZrF₆ electrolyte were mainly composed of MgO, ZrO₂, MgF₂, Zr₃O₂F₈, and amorphous magnesium phosphate.
• The 10 min MAO coating had the best corrosion resistance. The corrosion current density of the 10 min MAO coating was 4.864 × 10⁻⁸ A/cm², which was three orders of magnitude lower than that of the uncoated magnesium alloy. The polarization resistance of the 10 min MAO coating was 4.710 × 10⁵ kΩ·cm², which was two orders of magnitude higher than that of the uncoated magnesium alloy.

In general, the experimental results clearly showed that the corrosion resistance of the MAO coatings was closely related to the oxidation time. Too short or too long of an oxidation time reduced the quality of the MAO coating. It was necessary to adjust the oxidation time in MAO treatments to obtain the best quality coating. In addition, although MAO coating significantly increased the corrosion resistance of the AZ91D magnesium alloy, it still had some limitations, so other coatings for hole sealing treatment should also be considered.