Tailoring of Biodegradable Magnesium Alloy Surface with Schiff Base Coating via Electrostatic Spraying for Better Corrosion Resistance

Abstract

In this study, three new Schiff bases were synthesized from paeonol and amino acids to prepare a compound Schiff base coating on the Mg-Zn-Y-Nd alloy (ZE21B alloy) surface by electrostatic spraying, and these three single Schiff base coatings were prepared on the ZE21B alloy as control. The results of SEM and XPS confirmed the successful preparation of the coating. Immersion tests and electrochemical tests showed that both the single coating and the compound coating significantly improved the corrosion resistance of ZE21B alloy, and the compound coating could play a synergistic corrosion inhibition effect, thus showing the best corrosion resistance.

1. Introduction

Magnesium (Mg) alloy has a broad development prospect because of its good electrical conductivity, thermal conductivity, low density, high specific strength, and suitable elastic modulus, and is known as the green engineering material in the new century [1,2,3,4,5,6,7]. In recent years, Mg alloy has attracted much attention in the field of biomedical materials due to its good biocompatibility and biodegradability, but its poor corrosion resistance is the main limiting factor for its clinical application. The biomedical Mg alloys have a strong tendency of local corrosion in the in vivo environment, which will seriously affect their mechanical properties and cannot give full play to their expected biological functions [8,9,10,11,12,13,14]. The application of a corrosion inhibitor is a common and efficient method to prevent and reduce the corrosion of metal materials. Therefore, tailoring of the Mg alloy surface with corrosion inhibitor may be an effective strategy to enhance the corrosion resistance [15,16,17,18,19,20].

Schiff base is a kind of organic corrosion inhibitor with the advantages of high efficiency, non-toxicity, easy preparation, easy storage, and low cost. Its molecular structure contains functional groups (C=N−) which can combine with metal ions such as Mg²⁺ and Zn²⁺ to form stable complexes, and it has great potential in the field of metal material corrosion protection [21,22,23,24]. Ma et al. synthesized three kinds of amino acid Schiff bases using paeonol and amino acids and the study showed that Schiff bases formed an intact and dense film layer on the surface of Mg alloys by complexation reaction, which significantly decreased the corrosion rate and improved the corrosion resistance of the alloys [25]. However, for small Mg alloy implant devices, the immersion method is not suitable for preparing Schiff base coating, because the conventional immersion time will damage the device structure due to excessive corrosion, and the shorter immersion time cannot promise a complete and dense coating formed on the surface. The electrostatic spraying technology provides inspiration for the Schiff base coating preparation benefit from its advantages of the shorter time and higher effectiveness [26,27,28].

In this study, three kinds of novel Schiff bases were synthesized and a compound coating composed of these three Schiff bases were prepared on the Mg-Zn-Y-Nd alloy (ZE21B alloy) surface by electrostatic spraying. This compound coating was anticipated to endow the ZE21B alloy stronger corrosion resistance.

2. Materials and Methods

2.1. Experimental Materials

The ZE21B alloy (Mg-2.0Zn-0.46Y-0.5Nd alloy) was independently developed in Henan Key Laboratory of Advanced Magnesium Alloy (Zhengzhou, China), and the cylindrical ZE21B alloy were cut into small discs (Φ 10 mm × 3 mm), then polished with silicon carbide sandpaper and diamond polishing agent, and then ultrasonic cleaned in acetone and anhydrous ethanol successively for 3 min, and dried.

2.2. Synthesis of Schiff Base

Paeonol (natural organic matter extracted from peony root bark) was reacted with three kinds of amino acids (lysine, glycine, and methionine) to synthesize Schiff base containing amino group (−RC=N−) as described in the previous study [25], and the actual yield of all three Schiff bases can be beyond 80.0% theoretical yield. The molecular structures of the three synthesized Schiff bases: lysine Schiff base (PCLys), glycine Schiff base (PCGly), and methionine Schiff base (PCMet) are shown in Figure 1.

2.3. Preparation of Schiff Base Coatings on ZE21B Alloy

Schiff base coating was prepared on the surface of ZE21B alloy by electrospray technique. The synthesized Schiff base was dissolved in absolute ethanol and stirred vigorously at 1500 rpm for 36–48 h at a constant temperature of 30–40 °C using a magnetic heating stirrer (Hunan Saiwei Technology Co., Ltd, Changsha, China) to obtain a relatively homogeneous and stable Schiff base solution after being pumped into a dedicated syringe. Then, the Schiff base solution was dispersed into small droplets with very uniform fine microstructure using an applied high-voltage electrostatic field, and the small droplets were sprayed from the needle tip position and punched onto the ZE21B surface to form a uniform and dense layer of single Schiff base coating or compound Schiff base coating. In order to achieve a stable Taylor cone spray pattern for liquids sprayed from the needle tip position to form an ideal stable spray effect, the specific process parameters for electrospray were set as follows: spray distance of 35 mm, spray voltage of 7.0–9.0 kV, and spray flow rate of 10–20 µL/min (spray time 2 min). The concentrations of Schiff base solution used to prepare the single Schiff base coatings were designed as: PCLys = 0.05 M, PCGly = 0.07 M, PCMet = 0.07 M, and the concentration ratio of the 3 Schiff bases was PCLys: PCGly: PCMet = 1:1:2.

2.4. Characterization of Schiff Base Coatings on ZE21B Alloy

The micro-morphologies of the PCLys-, PCGly-, PCMet-, compound Schiff base-coated ZE21B, and the bare ZE21B were analyzed by scanning electron microscopy (SEM, FEIQuanta200, Eindhoven, Holland) [29], and the element content and distribution on each sample were analyzed by energy dispersion spectrometer (EDS, FEIQuanta200, Eindhoven, Holland) [30]. The functional groups of PCLys, PCGly, PCMet, and compound coatings on ZE21B were detected by Fourier transform infrared spectroscopy (FTIR, Nicolet IS50, Thermo Fisher Scientific, Waltham, MA, USA) [31], and the element composition was examined by X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos, Japan) [32]. The evaluation and validation of the data were carried out with the software CasaXPS (Version 2.3.24, Casa Software Ltd, Teignmouth, UK). Calibration of the spectra was done by adjusting the C1s signal to 284.5 eV.

2.5. Immersion Test

The non-working surface of the samples to be tested was sealed with silicone rubber before immersion experiments and weighed; The samples were transferred into a centrifuge tube and soaked with 25 mL RPMI 1640 cell culture medium (Art.No.:C3010-0500) and subsequently placed in a 37 °C constant temperature water bath for 1 day and 3 days. The pH value of each solution was measured every 24 h. At the 1st and 3rd day, the samples which retained the corrosion products were observed by SEM and EDS to investigate their pile up distribution and the deposition of calcium phosphorus salts, as well as analyze the degradation behavior of the coatings. The samples whose corrosion products were cleaned by ultrasonication in chromic acid solution (200 g/L CrO₃ + 10 g/L AgNO₃) for 2 min, sequentially cleaned with ultrapure water and 100% ethanol, and dried in oven at 37 °C. The weight loss was used to determine the average corrosion rate v and corrosion inhibition efficiency η (the calculation formula was shown as follow), and the corrosion of the sample surfaces was observed by SEM and EDS.

$$v=\frac{W_0-W_1}{s·t}$$

(1)

In the formula, W₀ and W₁ are the mass of the sample before and after soaking, s is the exposed area of the sample, and t is the soaking time. The formula for calculating the corrosion inhibition efficiency η is shown in Formula (2):

$$\eta =\frac{V_0-V_1}{V_0}\times 100\%$$

(2)

In the formula, V₀ and V₁ are the average corrosion rates of bare ZE21B and Schiff base-coated ZE21B, respectively. The formula for calculating the degradation rate CR_w of the sample is shown in Formula (3):

$$C{R}_w=3.65\frac{\Delta w}{D}$$

(3)

In the formula, CR_w is the average degradation rate, mm y⁻¹, Δw is the mass loss of the sample before and after immersion, and D is the material density.

2.6. Electrochemical Test

The electrochemical behavior of the sample in HBSS (Hank’s balanced salt solution, pH 7.4) was investigated by using RST5200 electrochemical workstation. The three-electrode mode was adopted, platinum electrode was used as counter electrode, saturated calomel electrode was used as reference electrode, and the exposing surface of test sample is used as the working electrode. The formula of HBSS used is shown in

Table 1. Formula of HBSS.

Component	Concentration (g/L)
NaCl	8.0
MgSO4·7H2O	0.1
KCl	0.4
MgCl2·6H2O	0.1
CaCl2	0.14
Na2HPO4·12H2O	0.152
KH2PO4·3H2O	0.06
C6H12O6	1.0
NaHCO3	0.35

.

The potentiodynamic polarization measurements were conducted for the samples at a scanning rate of 1 mV/s from −1.7 to −1.0 V (vs.) open circuit potential (OCP) in HBSS after 1 h immersion. The obtained curves were fitted to obtain corrosion potential E_corr and corrosion current density I_corr, which were used to analyze the anti-corrosion properties of the coatings. Electrochemical impedance spectroscopy (EIS, Zhengzhou Shi Ruisi Instrument Technology Co., Ltd., Zhengzhou, China) was performed with the frequency range of 100,000–0.1 Hz at an open circuit potential after 30 min exposure in HBSS, and the alternating current voltage amplitude was 5 mV. The obtained EIS spectra were fitted by ZSimpWin software (Version 2.0, AMETEK Scientific Instruments, Minneapolis, MN, USA). The sum resistance (R_sum) of the samples for EIS-fitted results was evaluated to compare with the results obtained from polarization curves and immersion tests. The calculation formula of sum resistance R_sum is as follows:

$$R_{sum}=R_1+R_2+R_3$$

(4)

R₁ represents the resistance of the Schiff base coating, R₂ represents the resistance of the corrosion product layer, and R₃ represents the charge transfer resistance.

3. Results and Discussion

Figure 2 displays the morphologies and element composition of PCLys-, PCGly-, PCMet-, and compound Schiff base-coated ZE21B samples, and the bare ZE21B. The ZE21B alloy showed a flat and uniform surface with many white secondary phase particles distributed on it, and EDS analysis results showed that the ZE21B contained a large amount of Mg (atomic percentage up to 95.77%) and a small amount of Zn, Y, and Nd on its surface. There were numerous long and short needle-like structures scattered disorderly on the surface of the four Schiff base coatings to form a layer of uniform and dense coating, and the discovery on the surface of N, which is a characteristic element in Schiff base, indicated that four coatings were successfully prepared on the ZE21B. In addition, four coatings also presented lower Mg ratios compared to the bare ZE21B surface.

The immobilization of organic functional molecules will cause changes in the content of functional groups and elements on the ZE21B surface. In order to furtherly confirm the successful preparation of Schiff base coatings, each sample surface was tested and analyzed by FTIR (Figure 3A). It could be seen that the FTIR curve of ZE21B alloy was relatively flat, the intensity of characteristic absorption peaks was relatively weak; the reason may be that there is no special chemical molecular structure on the surface of the bare matrix, so rare absorption peaks could be detected. The curve of Schiff base coatings showed a new peak with relatively high intensity at 1605 cm⁻¹ (PCLys and compound coatings), 1613 cm⁻¹ (PCGly coating), 1597 cm⁻¹ (PCMet coating), 1522 cm⁻¹ (compound coating), and 1413 cm⁻¹ (compound coating) corresponding to the stretching vibration absorption peak of the Cymene double bond, which was identified as the characteristic peak of the Schiff base molecule. Meanwhile, the Cymene double bond is also the key chemical group for the Schiff base molecule to show its corrosion inhibition function. In addition, an obvious absorption peak was observed near 3380 cm⁻¹, which was wide and blunt, and it belongs to the characteristic absorption peak of −OH.

The XPS full spectrum in Figure 3B presented the elements content on each surface: Compared with the four kinds of Schiff base coatings, the characteristic peaks of Mg KLL (eV), Mg 2s (eV), and Mg 2p (eV) in ZE21B alloy were more obvious, and the peak intensity was relatively higher. The Schiff base coatings presented obvious characteristic peaks of C 1s, N 1s, and O 1s with relatively high peak intensity due to their high content of C, N, and O elements.

Therefore, through the comparative analysis of the FTIR spectra and XPS full spectrum, it is sufficient to prove that the four kinds of Schiff base coatings were successfully prepared on the ZE21B alloy.

3.1. Immersion Test

The corrosion resistance properties of four kinds of Schiff base coatings on ZE21B alloy in RPMI 1640 cell culture medium were tested by static weight loss method.

Table 2. Static weightlessness test for 24 h.

Samples	Corrosion Rate v (mg· cm−2 ·h−1)	Degradation Rate CRw (mm·y−1)	Corrosion Resistance Efficiency η	pH
PCLys coating	0.0961	4.729	36.36%	7.09
PCGly coating	0.0794	3.908	47.40%	7.12
PCMet coating	0.0883	4.346	41.56%	7.03
Compound coating	0.0569	2.800	62.34%	7.48
ZE21B alloy	0.1511	7.436	-	7.88
Blank control	-			7.32

and

Table 3. Static weightlessness test for 72 h.

Samples	Corrosion Rate v (mg· cm−2 ·h−1)	Degradation Rate CRw (mm·y−1)	Corrosion Resistance Efficiency η	pH
PCLys coating	0.0935	4.601	17.34%	7.42
PCGly coating	0.0736	3.622	34.97%	7.24
PCMet coating	0.0288	1.417	74.57%	7.69
Compound coating	0.0281	1.383	75.14%	7.81
ZE21B alloy	0.1131	5.566	-	8.21
Blank control	-			7.45

presented that all the Schiff base coatings can significantly reduce the corrosion rate of ZE21B alloy in cell culture medium, suggesting effective improvement of corrosion resistance, wherein compound coating had the higher corrosion resistance efficiency and lower corrosion rate compared to the other coatings. In addition, the pH value of the bare ZE21B alloy rapidly increased from the 24th h to the 72th h, indicating a typical alkaline environment, which is not conductive to cell growth, while the pH values of Schiff base coated ZE21B alloy kept in stable, suggesting a neutral or weakly alkaline environment, which is conductive to cell growth [33].

The SEM images in Figure 4A depict the morphology of each sample with corrosion products after immersed in the RPMI 1640 cell culture medium for 24 h and 72 h. At the 24th h, corrosion occurred in the local area of the bare ZE21B alloy surface, pitting holes with different sizes unevenly distributed on its surface, and many clearly visible cracks appeared, while the four kinds of Schiff base coatings exhibited smoother and more uniform surfaces after soaking for 24 h, and there were no obvious corrosion pits on any coated samples. After soaking for 72 h, the local corrosion on the bare ZE21B alloy surface was serious, the number of corrosion pits on the surface increased and was deepened, the cracks and corrosion area increased, but the surfaces of the four Schiff base coatings were still relatively flat, and slight pitting corrosion could be observed, and there was no trend of corrosion deepening. The EDS results showed that the corrosion products on all the surfaces were composed of P, Ca, O, C, N, and Mg elements (Figure 4B,C), which indicated there may be calcium phosphate, MgO, or Mg(OH)₂ accumulated on each surface.

Figure 5A depicts the morphology of each sample without corrosion products after being immersed in the RPMI 1640 cell culture medium for 24 h and 72 h. After immersing for 24 h, it could be seen that there were a large number of pitting holes on the ZE21B alloy surface and the corrosion cracks were clearly visible. Up to 72 h, the ZE21B alloy surface seriously cracked and became uneven, suggesting intensified local corrosion, and the cracks even continued to extend to the entire surface, seriously damaging the Mg alloy. By comparison, the Schiff base-coated samples kept their surfaces smooth and uniform during the whole period, and there were only few and very small corrosion pits and cracks sparsely distributed on the surface. The EDS results (Figure 5B,C) showed that although all the samples presented higher Mg element and lower C, O, and N elements, the compound Schiff base coating still had higher N element compared to the other surfaces, suggesting more Schiff base molecules, which may be the reason of its stronger corrosion resistance.

3.2. The Inhibition Mechanism of Schiff-Base Coating

The magnesium alloy reacts in the corrosion solution and Mg²⁺ and H₂ are produced. When the Schiff base coating is destroyed by the corrosive medium, the released Schiff base molecule can be reacted with the Mg²⁺ released from magnesium alloy, and the reaction process includes the deprotonization reaction and complexation reaction of the Schiff base molecule, such as in Figure 6. The generated Schiff base complex is difficult to dissolve in water, which can stably adsorb the surface of the magnesium alloy, and form a dense uniform protective film layer together with the corrosion product such as MgO, Mg(OH)₂ to prevent corrosion. The medium further contacts the substrate to contribute to the substrate, thereby generating the effect of alleviating corrosion, reducing the corrosion rate of the alloy, and greatly increasing its corrosion resistance [25].

Compared to a single coating, the compound coating can synthesize the corrosion inhibition of three bakers, exhibiting a synergistic corrosion resistance, which exhibits stronger corrosion resistance. The mechanism is as follows: when single Schiff base coating is corroded, the formed protective film is not very dense, the erosion effect of the barrier corrosion medium is limited, and the synergistic effect of the three Schiff base molecules has strengthened the adsorption effect of Schiff base in magnesium alloy surface. The formed protective film layer is relatively denser, and can better block the corrosion medium erosion matrix and better protect the magnesium alloy, exhibiting higher corrosion inhibition efficiency.

3.3. Electrochemical Test

The self-corrosion potential (E_corr) and corrosion current density (I_corr) of the samples can be calculated from the polarization curves, as shown in

Table 4. Electrochemical parameters.

Samples	Ecorr (V)	Icorr (µA/cm2)
PCLys coating	−1.468	0.507
PCGly coating	−1.448	0.683
PCMet coating	−1.440	0.466
Compound coating	−1.427	0.440
ZE21B alloy	−1.501	5.560

. As can be seen from Figure 7A and Table 4, the E_corr value of ZE21B alloy is the lowest, −1.501 V, and the self-corrosion potential of each coating sample is significantly positive, which indicates that the corrosion tendency of each coating sample is higher than that of the single coating. The smaller the matrix, the more difficult it is to corrode. The I_corr value of ZE21B alloy is the highest, reaching 5.56 µA/cm². The current density of each coating sample has been significantly reduced, the corrosion rate has been greatly slowed down, and the corrosion resistance has been significantly improved. At the same time, it can be seen from Figure 7A that the shape of the polarization curves of different types of Schiff base coating samples did not change significantly, indicating that the corrosion inhibition mechanism of the Schiff base coating is not affected by the type of coating. Therefore, from the analysis results of the polarization curve, it can be seen that the Schiff base coating can significantly inhibit the corrosion of HBSS corrosion medium to magnesium alloys, and can play a better protective effect on ZE21B alloy, greatly improving its corrosion resistance and corrosion inhibition effect. The order from strong to weak is compound coating > PCMet coating > PCLys coating > PCGly coating, which is not much different from the results of the static weightless test.

In order to study the corrosion resistance mechanism of different coatings in HBSS in detail, electrochemical impedance spectroscopy (EIS) was used to track the performance of the surface films of different samples. The EIS spectrum data can be analyzed with the equivalent circuit in Figure 7E, and the corresponding EIS spectrum can be fitted with ZSimpWin software. The relevant parameters are shown in

Table 5. Equivalent circuit fitting parameters.

Samples	PCLys Coating	PCGly Coating	PCMet Coating	Compound Coating	ZE21B Alloy
Rs (Ω·cm2)	26.1	30.7	31.0	28.4	23.3
R1 (Ω·cm2)	84.4	64.7	233	16,580	24.3
Q-Y1 (cm2·sn·Ω)	0	0	0	0	0
n1	0.95	1	0.77	0.73	1
R2 (Ω·cm2)	12,660	11,000	12,430	1.3	77.3
Q-Y2 (cm2·sn·Ω)	0	0	0	0	0
n2	0.80	0.78	0.83	0.78	1
R3 (Ω·cm2)	3037	0.7	3500	53.5	115.8
Q-Y3 (cm2·sn·Ω)	0	0.71	0	0	0
n3	1	0.8	1	0.67	1
Rsum (Ω·cm2)	15,781.4	11,065.4	16,163	16,634.8	217.4

. The equivalent circuit model R(Q(R(Q(R(QR))))) is more suitable for simulating organic coatings, where R_s represents the resistance of the HBSS between the platinum electrode and the reference electrode, R₁ and Q₁ represent the resistance and capacitance of the Schiff base coating, respectively, R₂ and Q₂ represent the resistance and capacitance of the corrosion product layer, respectively, R₃ and Q₃ represent the charge transfer resistance and capacitance, respectively, and n₁, n₂, and n₃ are the capacitance coefficients associated with Q₁, Q₂, and Q₃, respectively. Figure 7B is the Nyquist diagram of different samples in HBSS. It can be seen from the figure that the Nyquist diagram is composed of three semicircular arcs in the high, medium, and low frequency region. The incomplete impedance arc is due to the corrosion product film formed on the surface of the magnesium alloy. It is caused by various reasons such as the electrochemical reaction of the layer and the surface of the electrode. There are three capacitive reactance arcs in the EIS spectrum of the bare substrate and the coated sample, of which the high frequency is the capacitive reactance arc related to the Schiff base film on the surface of the sample, and the intermediate frequency is the capacitive reactance related to the corrosion product film. Anti-arcing, at low frequencies is the capacitive anti-arcing associated with the Faraday charge transfer resistance. Compared with the bare substrate, the impedance arc shape of the coated sample did not change significantly, but the impedance arc radius increased significantly, indicating that the corrosion product film on the surface of the sample was more complete and dense, which increased the resistance to charge transfer on the surface of the magnesium alloy, the stability of the corrosion product film layer improved, the corrosion resistance greatly improved, the electrochemical reaction rate greatly reduced, the corrosion of the magnesium alloy was significantly inhibited, and the corrosion resistance of the alloy significantly improved. The composite coating sample had the best corrosion resistance. The composite coating can exert the effect of synergistic corrosion inhibition and greatly improve the corrosion resistance of the coating, which is basically consistent with the results of the polarization curve.

Figure 7C,D are the Bode-phase angle diagrams and Bode-impedance modulus diagrams of different samples in HBSS. From Figure 7C, it can be found that the phase angle curves of each coating sample appear. There are three peaks, which correspond to three time constants, one appears in the high frequency range, which can be attributed to the physical barrier properties of the Schiff base coating, and the other appears in the intermediate frequency region, which is caused by the corrosion product film layer, while the low frequency region is generated by the electric double layer capacitance and charge transfer resistance at the interface between the magnesium alloy substrate and the coating. It can be seen from Table 5 that the R_sum of the composite coating sample is the largest, which further shows that the corrosion resistance of the composite coating sample is the best, which is basically consistent with the results of the EIS spectrum and polarization curve.

4. Conclusions

In this study, the PCLys coating, PCGly coating, PCMet coating, and compound coating composed of these three Schiff base molecules were successfully prepared and applied onto the ZE21B alloy surface by electrostatic spraying. All the Schiff base coatings have a certain degree of corrosion resistance and can show different degrees of corrosion inhibition efficiency, and the corrosion inhibition efficiency of the composite coating is significantly better than that of the single coating, showing relatively good corrosion resistance performance. Compared to the single Schiff base coating, the compound coating can play a synergistic corrosion inhibition effect and better protect the magnesium alloy, so it is possible to show better corrosion resistance.