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Article

A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research

by
Shudi Zhang
1,*,
Linkun Liu
1,
Quanda Lei
1,
Tao Zhang
2,*,
Jiahui Bing
1 and
Jialin Dong
1
1
School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China
2
School of Materials Science and Engineering, Northeastern University, Shenyang 110016, China
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(5), 929; https://doi.org/10.3390/coatings13050929
Submission received: 1 April 2023 / Revised: 26 April 2023 / Accepted: 11 May 2023 / Published: 16 May 2023
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Abstract

:
The modified nano-CeO2/Zn–Mn phosphate composite coating was deposited on AZ91D magnesium alloy by chemical conversion to enhance its densification and corrosion resistance. The growth mechanism and corrosion resistance of the composite coating is clarified by adding different concentrations of ZnO and a certain amount of nano-CeO2 into the phosphate-plating solution. XRD and EDS show that the composite membrane is mainly composed of MgO, Mg(OH)2, Mn3(PO4)2·5H20, Zn, Zn3(PO4)2·4H2O and CeO2. Among them, AZ91D magnesium alloy matrix presents dispersed granule, clustered and petal-shaped under the action of different concentrations of ZnO. Under the optimum ZnO concentration, after adding nano-CeO2, dense grains appear, and cracks and pores in the riverbed are obviously reduced. Compared with single-layer phosphate coating, the performance of composite coating is significantly improved. The results show that the obvious double-layer structure is observed by SEM, and the thickness of the coating is about 48 μm. The LCSM shows that the surface roughness of composite coating is moderate. EIS shows that when the fitting impedance is 8050.43 Ω and PH = 3, the dropping time of copper sulfate in the composite coating is 58.6 s, which is better than that in the single-layer coating. The Tafel polarization fitting curve shows that the corrosion current density of the composite coating is obviously lower than that of the single coating, the corrosion current density is decreased from 1.86 × 10−6 A/cm2 to 9.538 × 10−7 A/cm2, and the corrosion resistance is obviously improved.

1. Introduction

Magnesium alloy is the lightest structural metal material in the world [1], which has a series of advantages such as high specific strength, high specific stiffness, thermal conductivity and vibration reduction performance, and is known as the “green engineering material in the 21st century” [2,3]. It is widely used in automobile, aviation, computer and electronic fields. AZ91D alloy is the most widely used die-casting magnesium alloy with excellent mechanical properties and castability. It provides a light substitute for traditional metal alloys and is becoming an extremely important structural application in automobile and light truck industries. However, due to some unfavorable properties of magnesium alloys, such as poor creep resistance, poor plastic deformation ability, high chemical reactivity and poor wear resistance, the development and application of magnesium alloys in industry are limited to some extent. China is rich in magnesium resources, but the application of magnesium alloys started late, and there is still a gap compared with developed countries [4,5,6]. Therefore, China should vigorously develop the value of magnesium alloys so that they can be better applied in different fields. With the more and more extensive application of magnesium alloys, the problem of poor corrosion resistance has become increasingly prominent [7]. In order to solve these problems, surface treatment and coating technology are two main methods to improve the corrosion resistance and mechanical stability of magnesium and its alloys and meet practical application requirements. In order to solve these problems, surface treatment and coating technology are two main methods to improve the corrosion resistance and mechanical stability of magnesium and its alloys and meet practical application requirements [8]. Chemical conversion membrane technology is one of the cheapest and most effective methods, which has the advantages of fast coat-forming speed, simple operation and being able to deal with uneven surfaces.
Magnesium alloy has a natural oxide coating on its surface, but the coating is thin, porous, loose and soft, which cannot provide good corrosion resistance [9]. Corrosion-resistant conversion coating is one of the most common and commonly used surface treatment technologies in the material chemistry industry, especially for magnesium–aluminum alloy and carbon steel [10]. Chen [11] introduced in detail various chemical conversion coatings on magnesium and magnesium alloys, such as chromate conversion coatings, fluoride conversion coatings, stannate conversion coatings, phosphate conversion coatings and rare earth conversion coatings. The research shows that the conversion coating has two main functions: first, it plays a role of corrosion resistance to the alloy matrix [12]; second, as a substrate to enhance the adhesion between the organic coating and the substrate, further enhancing the corrosion resistance [13]. In corrosive media, the conversion coating itself can play certain protective effects, but the corrosion resistance is limited, and today, most additives are known to be toxic and carcinogenic [14,15].
In recent years, nanoparticles have attracted extensive attention in the fields of material chemistry, biopharmaceuticals and environmental science because of their remarkable advantages in biocompatibility and environmental friendliness [16,17,18,19]. Especially, nano-SiO2, nano-TiO2, nano-ZnO and nano-ZrO2 have been applied to the corrosion and protection technology of alloy surfaces [20,21,22,23]. This is because the nanoparticles can play a very good role in the process of coating preparation and significantly improve the microstructure of the coating [24,25]. Ashassi [26] studied the corrosion resistance and hydrophobicity of nano-SiO2 in acrylic–silane polymer coating, which showed a significant improvement in corrosion resistance at 3.0 wt.% SiO2 with a maximum measured contact angle of 96.3°. Pommiers [27] prepared a nano-TiO2/Al2O3 composite coating on the surface of the alloy and studied and reported the hardness of the composite coating layer, which was significantly enhanced compared to the monolayer. Liang [28] investigated the effect of ZnO nanoparticles in Mg-rich epoxy coatings on the corrosion resistance and adhesion properties of AZ91D magnesium alloy, and the results showed that the corrosion resistance was improved by 1.2 orders of magnitude and the electrical conductivity was improved by more than 2 orders of magnitude. Previous studies have paid little attention to the effect of adding nanoparticles to the corrosion resistance of magnesium alloy conversion coating. In addition, it is not clear how the addition of nano-CeO2 changes the original coating growth mechanism. In addition, phosphate conversion coatings are an environmentally friendly surface treatment method that saves cost and energy for wear corrosion and lubrication to protect metal surfaces.
In this work, nano-CeO2/Zn–Mn phosphate composite coating was prepared using a chemical conversion method by adding nano-CeO2 particles into a phosphating solution. The corrosion resistance of the composite coating was evaluated using electrochemical impedance spectroscopy (EIS), Tafel curves and copper sulfate titration. The micro-morphology was observed with a scanning electron microscope (SEM), and the surface macro-morphology was observed by LCSM. The composition of the product was analyzed by XRD and EDS [29]. The target is to answer the following questions: (1) Can the composite coating formed by the addition of nano-CeO2 have more significant corrosion resistance than the single Zn–Mn coating? (2) If yes, what are the advantages of the composite coating prepared in this experiment compared with the traditional coating? In addition, the deposition mechanism and corrosion resistance of the composite coatings are revealed, which provides theoretical support and empirical methods for corrosion and protection of the alloy surfaces in this field.

2. Materials and Methods

2.1. Materials and Instruments

Main raw materials: AZ91D magnesium alloy (density 1.82 g/cm3); Marjiv salt (relative molecular weight: 248.91, white to grayish white or slightly reddish crystal, soluble in water to play the role of hydrolysis), Sinopharm Chemical Reagent Co, LTD; NaF (relative molecular weight: 41.99, white powder, soluble in water), Tianjin Damao Chemical Reagent Factory; EDTA-4Na (white crystalline powder, soluble in water, acid; insoluble in alcohol), Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China); Mn(NO3)2 (relative molecular weight: 251.01, pink crystal, soluble in water), Zhongtian Fine Chemical Co., Ltd. (Tianjin, China); ZnO (relative molecular weight: 81.38, white powder, becomes yellow after melting in acid heat and becomes white after cooling), Tianjin Beichen Founder Reagent Factory; CeO2 (relative molar mass: 172.115 g·mol−1, yellowish white solid, yellow deepens when heated).
Main experimental instruments: HH-S2 digital display constant temperature water bath; DZF vacuum-drying oven; 85-2 digital display constant temperature magnetic agitator; PHS-3C PH meter; CHINALAB 20 Electronic Balance; SB-3200DTNT Ultrasonic cleaning machine; CHI660E electrochemical workstation; VEGA3XMU scanning electron microscope; 3D confocal microscope; D8 ADVANCE X-ray diffractometer; 3 wt.% CuSO4 solution. The main experimental instruments are shown in Figure 1, below.

2.2. Sample Preparation

AZ91D magnesium alloy with the size of 10 mm × 10 mm × 20 mm was selected and polished with 600 #, 800 #, 1500 # and 2000 # sandpaper, respectively, until metallic luster appeared. Rinsed with deionized water for 2 min, washed with acetone to remove oil, taken out and rinse with deionized water for 3 min, put into pickling solution, pickled with 20 g/L H2PO4 and 12 g/LNa3PO4 for 25 s at room temperature, taken out, rinsed with deionized water for 5 min, put it into a vacuum-drying box, and then dried at room temperature for 20 min.

2.3. Experimental Content

The main components of the phosphide solution are 6.08 g/L Maziv salt, 1 g/L NaF, 4.6 g/L EDTA-4Na, 20 g/L Mn(NO3)2, 0–5 g/L ZnO, 2.4 g/L CeO2 and 0.5 g/L OP-10. Phosphating coating was prepared at a temperature of 70 °C and pH = 3 (pH adjusted by phosphoric acid and ammonia). Different phosphating coatings were prepared by changing zinc oxide content and zinc–manganese ratio in phosphating solution. Finally, the phosphating process with the best ZnO content was explored. The best ZnO content was selected, nano-CeO2 particles were added and then it was stirred at 45 °C for 25 min. Stirred at 800 RPM for 25 min until nano-CeO2 was in phosphating solution and there were enough colloidal mixture particles. Before phosphating, the phosphating solution was shaken for 20 min with an ultrasonic magnetic cleaner. Finally, the acid-washed AZ91D magnesium alloy matrix was put into phosphating solution for chemical transformation for 15 min. The optimum content of ZnO was explored by single factor experiment under the condition of changing the amount of ZnO first and keeping other conditions unchanged, and a single-layer zinc manganese phosphate coating was obtained. Finally, the modified nano-CeO2/Zn–Mn phosphate composite coating was prepared by adding nano-CeO2 particles on the basis of the optimal ZnO addition amount. After the deposition process, the deposited samples were washed with deionized water for 3 min and then dried in a vacuum-drying oven at 70° for 25 min.

2.4. Test and Analysis

(1)
Electrochemical test
The electrochemical impedance spectrum and polarization curve of the samples were tested by CHI660E electrochemical workstation. According to GB/T 24488-2009, the impedance and polarization curves were analyzed and tested. Traditional three-electrode system was used in the experiment. The sample was a working electrode (exposed area of 1 cm2), a saturated calomel electrode (SCE) was a reference electrode and a platinum electrode was an auxiliary electrode. Before each measurement, the working electrode was inserted into the electrolyte for a period of time in order to stabilize the open-circuit voltage (OCP). The test medium was 3.5% NaCl solution with a neutral PH value of deionized water, and the test temperature was (25 ± 5) °C. The scanning frequency of the electrochemical impedance spectroscopy was 0.01 Hz~100,000 Hz, the scanning voltage range of the polarization curve was ±0.5 V and the scanning rate was 5 mV/S. To ensure accuracy and repeatability, three groups of parallel tests were conducted for each test.
(2)
SEM and XRD detection under the morphology of the coating layer
According to GB/T 31563-2015 and GB/T 36422-2018, a VEGA 3 XMU scanning electron microscope (SEM) produced by TESCAN Company was used for testing and analysis. The supporting energy dispersive spectrometer (EDS) was used to test and analyze the thickness of the metal surface coating according to GB/T17722-1999, and the element concentration of the coating surface was determined and analyzed according to GB/T25189-2010. The surface morphology and cross-sectional morphology of the conversion coating were observed with a D8 ADVANCEX X-ray diffractometer (XRD) produced by AXS according to GB/T 36655-2018, and the elemental composition of the conversion coating was determined. The scanning voltage was 20 kV.
(3)
Analysis of macroscopic morphology of coating layer
A 3D confocal microscope (LCSM, Nanoscope System NS-3500) was used to characterize the coating thickness and surface features according to GB/T 33252-2016 nanolaser confocal MicroRaman spectrometer. The surface morphology of AZ91D magnesium alloy was characterized and analyzed by using 405 nm laser as the light source, and the thickness and roughness of the coating was determined.
(4)
Copper sulfate drop test
CuSO4 solution with a mass fraction of 3 wt.% was prepared at room temperature. According to the GB437-1993 copper sulfate test standard, the corrosion resistance of the coating prepared on the alloy surface was tested. Drops were carried out on the sample surface within the range of 1 cm2, and the time for the drops to change from blue to black was recorded after dropping CuSO4 solution. It was necessary to conduct three parallel experiments and record the average.

3. Results and Discussion

3.1. Characteristics of Zn–Mn Conversion Coatings

3.1.1. Microstructure and Chemical Composition Analysis

Figure 2 shows the X-ray diffraction spectra of different zinc oxide contents after a 15 min water bath in a phosphating solution. The results of pattern analysis and data show that the coatings are mainly composed of Mg, Zn, Mn and their compounds. However, the peaks and the intensities of different phases depend on the ZnO content. Firstly, there are five main peaks at 33.66°, 38.11°, 47.54°, 56.38° and 59.07° of 2θ; when ZnO is not present, the XRD pattern shows that the mixture is composed of MgO and Mg (OH)2. The PDF cards of XRD PDF#35-0821 and PDF#18-0787 are used for analysis. When the content of ZnO in a phosphating solution increases from 1 g/L to 3 g/L, the corresponding peak value of the main salt increases. According to the PDF cards of XRD PDF#04-0831 and PDF# 03-0426, it has been confirmed that there are some small grains of Zn and Mn 3(PO4)2·5 H2O and Zn3(PO4)2·4 H2O in the phosphide solution. When the content of ZnO is 4 g/L, the peak reaches the main peak. At this time, Mn3(PO4)2·5H2O and Zn3(PO4)2·4H2O are enriched, which can be seen from the scanning electron microscope images [30]. At the same time, the polyphase tends to be stable. When the ZnO content is further increased to 5 g/L, the peak value of elemental Zn continues to rise, and more phosphoric acid phases are obtained, resulting in the reduction of other mixed phases. It can be seen that the ZnO content has a significant effect on the formation of the coating. Therefore, the corrosion resistance of the coating decreases when the ZnO content exceeds 4 g/L.

3.1.2. Morphology of Zn–Mn Conversion Coatings with Different ZnO Contents

Figure 3 shows the microstructure of Zn–Mn conversion coatings with different ZnO contents after 15 min of water bath conversion. Three different surface morphologies of phosphating coatings were presented. The evacuation granules in Figure 3a,b; the clusters of Figure 3c,d; and those in Figure 3e,f are flower-shaped. As shown in Figure 3a,b, it can be seen that the particle diameter is about 15 μm. Large areas of aggregation did not occur, and cracks in the riverbed were still visible, making it difficult to completely cover the alloy substrate. When magnified to 5000 times, there are scratches on the grain surface, indicating poor corrosion resistance. In Figure 3c,d at 50 μm, the conversion coating has a cluster structure covering almost the entire substrate, which is attributed to the enrichment of the Zn phase in the phosphating solution. At the ratio of 10 μm, the cluster particles and tiny cracks in Figure 3c can be seen. In Figure 3d, the cluster particles have vilified and covered the whole crack of the riverbed, indicating that the corrosion resistance has been further improved. When ZnO content changes from Figure 3e, f, it can be clearly seen that flowers appear. In Figure 3e 4 g/L, the flower-like structure is obvious, and the small petals are closely connected and intertwined, showing a “blooming” shape, thoroughly blocking the cracks in the riverbed. At 5 g/L in Figure 3f, the morphology does seem to have a dendritic structure, but at this time, the membrane structure is still flower-like, and the flowers have been enriched; however, the gaps between small petals become larger. At 10 μm, it is difficult to cover the bottom completely, and the corrosion resistance becomes smaller. The test shows that it has the same composition. In addition, in the process of increasing ZnO content, the Zn–Mn conversion coating grows continuously and almost completely covers the substrate at 4 g/L. This indicates that the composition of the conversion coating is mainly a zinc phase mixture, which is attributed to the soft quality and high adhesion of zinc so that the gaps between the crystal particles on the surface of the substrate can be filled [31].

3.2. Corrosion Resistance of Zn–Mn Conversion Coatings

Corrosion Resistance of Zn–Mn Conversion Coatings with Different ZnO Contents

Figure 4 shows the polarization curves of ZnO with different concentrations in a 3.5% NaCl solution. Parallel experiments were carried out three times, excluding accidental error factors, and six groups of experiments were carried out each time. It can be seen from the figure that there are no obvious 10-anode arcs in the polarization curve without adding ZnO, and other anode arcs seem to show obvious pitting characteristics [32]. It can be concluded that this pitting potential is the penetration potential of the breakdown coating [33], and other parallel modes do not show pitting morphology. Pitting potential decreases as the ZnO concentration increases to 3 g/L, then sharply increases at 4 g/L, and then decreases again at 5 g/L. This indicates that there is an inflection point during the increase of ZnO content. The results show that the formation of these cracks in the coating with low ZnO content is due to the predominant manganese in the coating, which leads to the shrinkage of the coating. When the zinc oxide content is increased to 4 g/L and 5 g/L, the phosphate crystal changes into a flower-like structure due to the zinc phosphate in the coating. Small petals with flower-like structures grow on the gap of the crack boundary obtained with a ZnO content of 4 g/L. When the content of zinc oxide is 5 g/L, the membrane structure is similar to the flower structure of zinc oxide content of 4 g/L. However, the structural petals are large, and the spacing is large, so the cracks are difficult to cover. The tensile stress is caused by higher crystal hardness and more compact manganese phosphate. Therefore, the compressive stress of Mn coating is higher than that of Zn coating, and the advantage of Mn leads to many cracks on the surface of the substrate. On the contrary, when the content of ZnO is high, the weight percentage of zinc is greater than that of Mn, indicating that the coating is mainly composed of zinc compounds. Therefore, due to the high adhesion and low hardness of zinc, the advantages of zinc compounds in the coating can lead to the good filling of cracks between grain boundaries on the sample surface. It can be seen from Table 1 that the corrosion potential and corrosion current density (icorr) are both the smallest when the ZnO content is 4 g/L and the main salt concentration of coating forming remains unchanged, which are −1.420 V and 1.86 × 10−6 A/cm2. The results show that when ZnO content is 4 g/L, the corrosion resistance of the phosphating coating is the best, which is consistent with the analysis results of SEM morphology.
Figure 5 shows the electrochemical impedance spectra of ZnO and CeO2/Zn–Mn composite coatings with different concentrations in a 3.5% NaCl solution. As shown in the figure, except that no ZnO is added and no double arc of reactance occurs at 1 g/LZnO, the rest of the impedance spectrum is composed of two arcs of reactance, and the radius of the arc of reactance increases first and then decreases. When the ZnO concentration reaches 4 g/L, the arc radius of Zn–Mn reactance reaches the maximum, and the arc of CeO2/Zn–Mn composite coating is obviously larger than that of the Zn–Mn monolayer. The arc radius of the capacitive reactance is approximately equal to the electrochemical polarization resistance. When the conversion coating appears in the early stage, it indicates that it has certain corrosion resistance at this time, but it is easy to crack at the beginning of formation, resulting in local corrosion. With the passage of reaction time, the arc radius of the capacitive reactance increases, which indicates that the coating is growing continuously. When reaching a certain water bath time, the arc radius of capacitive reactance does not increase but decreases, and a double capacitive reactance arc appears. The former is the high-frequency capacitive reactance arc of charge transfer generated between the phosphating solution and coating layer [34], while the latter is the low-frequency capacitive reactance arc of the coating surface [35]. The low-frequency capacitive reactance arc has a larger radius, indicating excellent corrosion resistance. As can be seen from Figure 6, the longer the duration of blackening of Zn–Mn phosphating coating, the more significant the corrosion resistance effect. The results are consistent with the electrochemical polarization curve fitting results, indicating that the surface corrosion resistance is the best when pH = 3 and ZnO is 4 g/L.

3.3. Nano CeO2/Zn–Mn Composite Coating

3.3.1. Microstructure of Nano-CeO2/Zn–Mn Composite Coatings with Optimal ZnO Content

Figure 7 shows the SEM of CeO2/Zn–Mn composite coating in a phosphating solution for 15 min. In Figure 7a, the surface of the crystal particles is dense, covering the whole bottom of the riverbed. Almost no cracks can be seen, and the microstructure of crystal clusters is refined. Figure 7b shows that the surface cluster particles are rod-shaped crystal particles, which can be seen from the XRD diffraction pattern above and the EDS spectrum below. Rod-like grains are mainly composed of nano-CeO2 and some free cerium ions, which further refines the bed cracks on the surface of zinc manganese phosphate conversion coating. In Figure 7c, the surface grains are closely connected to each other at a ratio of 20 μm. Especially, in Figure 7d at 10 μm, the crystal grains have no scratches on the surface and effectively cover the bottom of the phosphating coating bed. At the same time, the existence of nano-CeO2 in the phosphating solution is easy to lead to negative overpotential migration during deposition, which is beneficial to the initiation of a crystal nucleus and the refinement of crystal grains. It shows that the addition of CeO2 is the key to improving the corrosion resistance, and the dispersion strengthening of the Zn–Mn monolayer is the core factor to improve the corrosion resistance.
Figure 8 shows the X-ray diffraction of the CeO2/Zn–Mn phosphate composite coating after 15 min of a water bath in a phosphating solution. The results show that the modified composite membrane is formed on the basis of the mixture of Mn3(PO4)2·5H2O, Zn, Zn3(PO4)2·4H2O, MgO and Mg(OH)2 in the Zn–Mn phosphate conversion membrane. The surface is mainly composed of CeO2, Ce and CeO2 nanoparticles. The relative atomic ratio of Ce to O is 1∶2 by comparing the diffraction peaks and EDS spectra of the composite coating surface in Figure 9. The results showed that the nano-CeO2 content accumulated on the surface of the composite membrane rather than infiltrated into the interior, and the surface was mainly composed of Zn, P, O, Mg and Ce elements. In addition, the fewer defects in the coating layer, the better corrosion resistance [36]. Obviously, the corrosion resistance of the composite coating is better than that of the Zn–Mn monolayer, which is consistent with the EIS and electrochemical polarization results in the figure below.

3.3.2. Corrosion Resistance of Nano CeO2/Zn–Mn Composite Coatings with Optimal ZnO Content

Figure 10 shows the electrochemical polarization curves of Zn–Mn phosphating coating and CeO2/Zn–Mn composite coating in a 3.5% NaCl solution. The anode region of the polarization curve shows obvious pitting corrosion characteristics. For magnesium alloy composite coating, the formation of this feature corresponds to the penetration potential of the coating [37]. Furthermore, the pitting/thin coating breakdown potential increased by about 100 mv, indicating that the small-amplitude sinusoidal potential signal was used to interfere with the system during the Tafel test analysis using an electrochemical workstation. Anode and cathode processes occur alternately on electrode 15, corresponding to anode and cathode arcs. The reason for the potential increase is that the addition of nano-CeO2 further refines the riverbed cracks on the surface of the Zn–Mn coating, making it look smaller under SEM. The composite coating deposited on the surface looks denser, and the corrosion potential increases. The more positive the corrosion potential, the better the corrosion resistance. According to the extrapolation method of the Tafel curve [38], when the conversion time is 25 min, the corrosion current density of the coating layer is the smallest. Corrosion potential reflects the corrosion resistance of the coating to some extent, but its priority is far less than the corrosion current density [39]. When the conversion time is 25 min, the corrosion current density is 9.538 × 10−7 A/cm2, which is one order of magnitude higher than that of the Zn–Mn monolayer. Figure 11 shows the 3D morphology of the coating layer under a laser confocal microscope. Figure 11a shows that the thickness of the single-layer coating is 27.743 μm, and Figure 11b shows that the thickness of the composite coating is 49.736 μm. The thickness of the 3D composite coating is consistent with SEM, which shows that it has obvious corrosion resistance, and the analysis results are consistent with SEM and EIS.
In the bode diagram Figure 12a, the ZnO value increases continuously when the ZnO content is not added to 4 g/L, and decreases at 5 g/L. The results show that when the content of ZnO is 4 g/L, the impedance performance of Zn–Mn phosphating coating is the best. The |Z| value of the CeO2/Zn–Mn composite coating is much higher than the |Z| value of the optimal ZnO content and has a stable passive response [40]. In Figure 12b, two peaks are obvious, indicating a bilayer structure. This is due to the fact that nano CeO2 makes up for the cracks in the bed of the Zn–Mn monolayer. That is to say, AZ91D alloy is combined with a proper amount of Zn–Mn phosphate, which makes the corrosion resistance stronger. In Figure 13, the fitting data is close to the experimental data, and the resulting error is small. Therefore, the equivalent circuit diagram can be used to fit the experimental data. Rs represents the resistance of the solution, while R1 and R2 represent the charge transfer resistance between the coating and the solution and the reaction resistance of the coating, respectively. Due to the rough surface and uneven electrochemical performance of magnesium alloy conversion coating, phase angle elements are often used instead of capacitors to explain the behavior of high-frequency capacitive arcs. CPE1 and CPE2, respectively, represent the phase angle elements of the corrosion products and the electric double layer at the reaction interface [41]. The larger the fitting resistance, the better the corrosion resistance, which is consistent with the results obtained by the polarization curve.

3.3.3. The Cross-Section Morphology of CeO2/Zn–Mn Nano-Composite Coatings with Optimal ZnO Content

Figure 14a shows the cross-sectional morphology of the CeO2/Zn–Mn composite coating prepared with the best ZnO content. Figure 12b is the corresponding EDS cross-section energy spectrum under SEM. Figure 14a has an obvious double-layer structure, and both the inner layer and the outer layer deposited on the surface of the inner layer have many micropore scratches. The distribution of elements in Figure 14b–d is clearly visible. In Figure 5, there are two capacitive reactance arcs in the prepared nano-CeO2/Zn–Mn composite coating, and the radius of the low-frequency capacitive reactance arc is larger, which indicates that the prepared composite coating is thicker. It can be seen from Figure 14a that the thickness of the composite coating is about 48 μm, which confirms the electrochemical impedance spectrum. It further shows that the corrosion resistance is enhanced by blocking the entry of corrosive media.

4. Conclusions

(1)
Based on the above analysis, the optimum technoligial formula of the composite coating is 6.08 g/L Mazhiv salt, 1 g/L NaF, 4.6 g/L EDTA-4Na, 20 g/L Mn(NO3)2, 4 g/L ZnO, 2.4 g/L CeO2 and 0.5 g/L OP-10. The preparation conditions are 70 °C, pH = 3 (adjusted by phosphoric acid and ammonia water), water bath conversion time of 15 min and conversion temperature of 60 °C;
(2)
In this work, when ZnO is not added, the Zn–Mn coating is composed of MgO and Mg(OH)2. When ZnO is added, the Zn phase appears. With the increase of ZnO concentration, the coating has a mixture of Mn3(PO4)2·5H2O, Zn and Zn3(PO4)2·4H2O. When the content of ZnO is 4 g/L, the phase tends to be in equilibrium. There are the fewest pores and cracks in the coating. When the content of ZnO is 5 g/L, a zinc-rich phase appears, and the monolayer coating has certain corrosion resistance;
(3)
The results show that the nano-CeO2/Zn–Mn composite coating has an obvious double-layer structure. The addition of nano-CeO2 refines the spalling on the surface of Zn–Mn phosphating coating, enhances the adhesion between surface grains and changes the size of the coating particles. The surface is denser, there are fewer cracks, the coating thickness is about 48 microns and the corrosion resistance is obviously improved;
(4)
According to the electrochemical impedance spectrum (EIS) and electrochemical polarization curve (Tafel), under the optimum ZnO concentration, the electrochemical impedance spectrum (EIS) value is obviously improved after adding nano-CeO2 particles into the phosphide solution. Nano-CeO2 acts as a strong oxidant, which helps to improve the potential of the REDOX potential solution, accelerate the formation of the coating and promote the increase of thickness. The corrosion current density decreased from 1.86 × 10−6 A/cm2 to 9.538 × 10−7 A/cm2, and the corrosion resistance of the coating was remarkable.

Author Contributions

Conceptualization, L.L. and S.Z.; methodology, L.L.; software, L.L.; validation, S.Z., Q.L. and L.L.; formal analysis, S.Z.; investigation, L.L.; resources, S.Z.; data curation, L.L.; writing—original draft preparation, L.L.; writing—review and editing, L.L.; visualization, L.L., S.Z. and J.D.; supervision, Q.L. and J.B.; project administration, T.Z. and S.Z.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Key project of the National Natural Science Foundation of China: Research basis of “corrosion-functional” integrated protective coating on magnesium alloys (U21A2045). the National Natural Science Foundation of China (NSFC), “Study of conductive-corrosion resistant chemical conversion coating of magnesium alloy” (51771050). Basic Research Project of Liaoning Provincial Education Department (LJKMZ20220600).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Macroscopic view of the main experimental apparatus: (a) scanning electron microscope; (b) X-ray diffractometer (XRD); (c) 3D confocal microscopy (LSCM).
Figure 1. Macroscopic view of the main experimental apparatus: (a) scanning electron microscope; (b) X-ray diffractometer (XRD); (c) 3D confocal microscopy (LSCM).
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Figure 2. Coating X-ray diffraction pattern of different zinc oxide content after water bath in phosphating solution for 15 min: (a) basic phosphating coating; (b) 1 g/L ZnO; (c) 2 g/L ZnO; (d) 3 g/L ZnO; (e) 4 g/L ZnO; (f) 5 g/L ZnO.
Figure 2. Coating X-ray diffraction pattern of different zinc oxide content after water bath in phosphating solution for 15 min: (a) basic phosphating coating; (b) 1 g/L ZnO; (c) 2 g/L ZnO; (d) 3 g/L ZnO; (e) 4 g/L ZnO; (f) 5 g/L ZnO.
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Figure 3. SEM images of different zinc oxide contents in phosphating solution after water bath for 15 min: (a) basic phosphating coating; (b) 1 g/L ZnO; (c) 2 g/L ZnO; (d) 3 g/L ZnO; (e) 4 g/L ZnO; (f) 5 g/L ZnO.
Figure 3. SEM images of different zinc oxide contents in phosphating solution after water bath for 15 min: (a) basic phosphating coating; (b) 1 g/L ZnO; (c) 2 g/L ZnO; (d) 3 g/L ZnO; (e) 4 g/L ZnO; (f) 5 g/L ZnO.
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Figure 4. Polarization curves of ZnO at different concentrations in 3.5% NaCl solution.
Figure 4. Polarization curves of ZnO at different concentrations in 3.5% NaCl solution.
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Figure 5. Electrochemical impedance spectra of ZnO and CeO2/Zn–Mn composite coatings with different concentrations in 3.5% NaCl solution.
Figure 5. Electrochemical impedance spectra of ZnO and CeO2/Zn–Mn composite coatings with different concentrations in 3.5% NaCl solution.
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Figure 6. Drip test of NaF with different mass concentrations at different pH values in 3% CuSO4 solution.
Figure 6. Drip test of NaF with different mass concentrations at different pH values in 3% CuSO4 solution.
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Figure 7. SEM of CeO2/Zn–Mn composite coating after 15 min water bath in phosphating solution: (a) 500 μm; (b) 200 μm; (c) 20 μm; (d) 10 μm.
Figure 7. SEM of CeO2/Zn–Mn composite coating after 15 min water bath in phosphating solution: (a) 500 μm; (b) 200 μm; (c) 20 μm; (d) 10 μm.
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Figure 8. X-ray diffraction of nano CeO2/Zn–Mn phosphate composite coating after 15 min water bath in phosphating solution: (a) 1.8 g/L CeO2; (b) 2.4 g/L CeO2; (c) 3 g/L CeO2.
Figure 8. X-ray diffraction of nano CeO2/Zn–Mn phosphate composite coating after 15 min water bath in phosphating solution: (a) 1.8 g/L CeO2; (b) 2.4 g/L CeO2; (c) 3 g/L CeO2.
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Figure 9. EDS spectra of nano-CeO2/Zn–Mn phosphate composite coating after 15 min water bath in phosphating solution.
Figure 9. EDS spectra of nano-CeO2/Zn–Mn phosphate composite coating after 15 min water bath in phosphating solution.
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Figure 10. Electrochemical polarization curves of Zn–Mn phosphating coating and CeO2/Zn–Mn composite coating in 3.5% NaCl solution.
Figure 10. Electrochemical polarization curves of Zn–Mn phosphating coating and CeO2/Zn–Mn composite coating in 3.5% NaCl solution.
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Figure 11. Surface morphology of Zn–Mn phosphate coating and CeO2/Zn–Mn composite coating layer under LCSM: (a) Zn–Mn phosphate coating; (b) CeO2/Zn–Mn composite coating.
Figure 11. Surface morphology of Zn–Mn phosphate coating and CeO2/Zn–Mn composite coating layer under LCSM: (a) Zn–Mn phosphate coating; (b) CeO2/Zn–Mn composite coating.
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Figure 12. Bode diagram of ZnO and CeO2/Zn–Mn composite coatings with different concentrations in 3.5% NaCl solution: (a) Phase Angle diagram; (b) Modulus diagram.
Figure 12. Bode diagram of ZnO and CeO2/Zn–Mn composite coatings with different concentrations in 3.5% NaCl solution: (a) Phase Angle diagram; (b) Modulus diagram.
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Figure 13. Equivalent circuit diagram of CeO2/Zn–Mn composite coating in 3.5% NaCl solution.
Figure 13. Equivalent circuit diagram of CeO2/Zn–Mn composite coating in 3.5% NaCl solution.
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Figure 14. SEM cross-section morphology and EDS spectra of CeO2/Zn–Mn composite coatings: (a) Nano-CeO2/zinc manganese composite coating; (b) EDS spectra of nano-CeO2/zinc-manganese composite coatings; (c) Distribution of O, Na, Ce and P elements in composite membranes; (d) Distribution of Zn, F, Mn and Mg in composite coating.
Figure 14. SEM cross-section morphology and EDS spectra of CeO2/Zn–Mn composite coatings: (a) Nano-CeO2/zinc manganese composite coating; (b) EDS spectra of nano-CeO2/zinc-manganese composite coatings; (c) Distribution of O, Na, Ce and P elements in composite membranes; (d) Distribution of Zn, F, Mn and Mg in composite coating.
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Table
Table 1. Ecorr, Icorr and Tafel curve extrapolation results of ZnO at different concentrations in 3.5% NaCl solution.
Table 1. Ecorr, Icorr and Tafel curve extrapolation results of ZnO at different concentrations in 3.5% NaCl solution.
List123
ZnO (g/L)Ecorr/Vicorr/(A/cm2)Rp (Ω)
0−1.5621.562 × 10−51308.2
1−1.5871.738 × 10−52659.47
2−1.5561.854 × 10−53220.16
3−1.5691.538 × 10−53898.27
4−1.4201.86 × 10−64653.85
5−1.5198.912 × 10−53401.08
CeO2/Zn–Mn−1.3279.538 × 10−78050.43
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Zhang, S.; Liu, L.; Lei, Q.; Zhang, T.; Bing, J.; Dong, J. A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research. Coatings 2023, 13, 929. https://doi.org/10.3390/coatings13050929

AMA Style

Zhang S, Liu L, Lei Q, Zhang T, Bing J, Dong J. A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research. Coatings. 2023; 13(5):929. https://doi.org/10.3390/coatings13050929

Chicago/Turabian Style

Zhang, Shudi, Linkun Liu, Quanda Lei, Tao Zhang, Jiahui Bing, and Jialin Dong. 2023. "A Nano-CeO2/Zn–Mn Composite Conversion Coatings on AZ91D Magnesium Alloy Surface of Corrosion Resistance Research" Coatings 13, no. 5: 929. https://doi.org/10.3390/coatings13050929

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