3.1 Surface morphology and elemental composition of organic composite coatings
The macroscopic morphology of all samples is shown in Fig. 2. It can be observed that the surface of all samples is smooth, and there are obvious color differences between the surface of sample S0 and the surface of the other three samples. Among them, the surface color of S0 is grayish white (Fig. 2(a)), because the surface of the substrate only contains MAO coating, and the color of the MAO coating is grayish white. The surface color of S1 (Fig. 2(b)), S2 (Fig. 2(c)) and S3 (Fig. 2(d)) is jujube red, which is due to the spraying of a jujube red SiO2@α-Fe2O3 organic coating on the MAO coating. The SiO2 nanoparticles are white, and the color of the α-Fe2O3 nanoparticles is jujube red, so the color of the coating solution after mixing is also jujube red, that is, the color of the prepared organic composite coating is jujube red.
Figure 3 reflects the surface morphology of S0, S1, S2 and S3. The MAO coating prepared in this study is a typical porous structure (Fig. 3(a)), and the micropores on the surface of the MAO coating are small. The porosity of MAO coating on S0 surface was calculated to be 0.998% by Image J software. The results showed that the MAO coating prepared in this study had low porosity and good compactness. After spraying the organic coating, the microstructure of the sample surface is shown in Fig. 3(b, c and d). It is not difficult to observe that the micropores on the surface of S1, S2 and S3 are completely covered, and there are uniform and dense SiO2@α-Fe2O3 nanoparticles on the surface of the organic composite coating.
The organic composite coating is mainly composed of C, Fe, O, Si, Mg, Nd, Na, Zn, P and Yb elements (Fig. 4). The percentage of coating elements on the surface of all samples (Fig. 4) showed that the content of C and Fe elements on the surface of S0 was 0 wt.%, which was due to the fact that the surface of S0 was not coated with SiO2@α-Fe2O3 organic coating. Comparing the percentage content of each element on the surface of S1, S2 and S3, it can be observed that with the increase of the mass ratio of SiO2 and α-Fe2O3, the content of C and O elements decreases, and the content of Fe and Si elements increases. The contents of Si and Fe on the surface of S1, S2 and S3 are 11.66%, 12.81% and 14.06%, respectively, which is consistent with the increasing trend of the mass ratio of SiO2 and α-Fe2O3.
3.2 Three-dimensional morphology and cross-sectional morphology of organic composite coatings
Figure 5 shows the three-dimensional morphology of all sample surfaces. It can be seen from Fig. 5(a) that the fluctuation degree of S0 surface is large, which can be attributed to the porous structure of MAO coating on S0 surface. When the sample surface is sprayed with an organic coating (Fig. 5(b, c and d)), the degree of surface fluctuation gradually decreases. During the preparation of organic composite coatings on S1, S2 and S3 surfaces, the coating solution needs to be sprayed onto the MAO coating. Due to the fluidity of the liquid, it will automatically level after a period of time, and a smooth and flat organic coating can be obtained after drying. Compared with the MAO coating, the fluctuation of the surface of the organic composite coating is significantly reduced. And with the increase of the mass ratio of SiO2 and α-Fe2O3, the smoothness of the coating is further improved because SiO2 can make α-Fe2O3 nanoparticles more evenly dispersed in the organic coating solution. The surface roughness values (Fig. 6) of S0, S1,S2 and S3 are 0.9387 µm, 0.1774 µm, 0.1771 µm and 0.1033 µm, respectively. It is not difficult to see that the roughness value of the organic composite coating is significantly reduced, which is consistent with the trend of surface fluctuation reflected in Fig. 5. Therefore, with the increase of the mass ratio of SiO2 and α-Fe2O3, the surface of the coating becomes smoother.
The cross-sectional morphology of S0, S1, S2 and S3 is shown in Fig. 7. It is not difficult to see that the surface of S0 is only covered by a layer of MAO coating, and the thickness of the coating is thin. The surface of S1, S2 and S3 contains a MAO coating and an organic coating. The organic coating is thick and dense, and the thickness of the organic coating is about 40 mm. The thick and dense coating can block the invasion of external harmful ions to a certain extent 50,51. In addition, it is observed that there is a partial mechanical interlocking between the SiO2@α-Fe2O3 organic coating and the MAO coating, which is caused by the penetration of the coating liquid into the micropores on the surface of the MAO coating. The mechanical interlocking between MAO coating and SiO2@α-Fe2O3 organic coating can effectively improve the adhesion between the composite coatings 48,49.
3.3 Phase composition of organic composite coatings
The phase composition and crystal structure of the samples were analyzed by X-ray diffractometer. Figure 8 shows the XRD patterns of S1, S2, S3, and S4. It can be seen that the phase of MAO coating is mainly composed of MgO, Yb2O3 and ZnO. MgO is formed by the dehydration of Mg(OH)2 due to the instantaneous high temperature generated by spark discharge during micro-arc oxidation 50,52. Yb2O3 and ZnO are mainly derived from Yb2O3 and ZnO nanoparticles added in the electrolyte. The diffraction peaks of MgO are mainly concentrated on the (220) crystal plane, the diffraction peaks of Yb2O3 are concentrated on the (111) crystal plane, and the diffraction peaks of ZnO are mainly concentrated on the (102) crystal plane. The phase of the organic composite coating is mainly composed of SiO2 and α-Fe2O3. SiO2 and α-Fe2O3 are mainly derived from SiO2@α-Fe2O3 nanoparticles added in the organic coating solution. The diffraction peaks of α-Fe2O3 appear at 2θ = 24.1 °, 33.1 °, 35.6 °, 40.8 °, 49.4 °, 54.0 °, 57.5 °, 62.4 °, 63.9 °, 71.8 °, 75.1 °, 84.8 ° and 88.4 ° for S1, S2 and S3, corresponding to the (012), (104), (110), (113), (024), (116), (018), (214), (300), (1010), (217), (134) and (226) crystal planes of face-centered cubic α-Fe2O3, respectively. The diffraction peaks of SiO2 appear at 2θ = 49.2 ° and 51.9 ° for S1, corresponding to the (202) and (210) crystal planes of SiO2, respectively. S2 and S3 are the diffraction peaks of SiO2 at 2θ of 37.4 ° and 49.2 °, corresponding to the (103) and (202) crystal planes of SiO2, respectively. The peak intensity of SiO2 at 49.2 ° increases with the increase of the mass ratio of SiO2 to α-Fe2O3.
3.4 Electrochemical test of organic composite coating
The OCP curves of S0, S1, S2 and S3 surface coatings in natural seawater are shown in Fig. 9(a). Interestingly, the four OCP curves show four trends. The OCP curve of S0 surface coating showed a downward trend as a whole, and the change range was − 0.52 V ~ -0.45 V. And the curve decreases faster in the first 200 s, and the rate of decline between 200 s and 1200 s slows down. The OCP curve of the S1 surface coating fluctuates around − 0.1 V, and the overall trend is relatively stable, and the change range is -0.19 V ~ 0 V. The OCP curve of the coating on the surface of S2 showed a trend of rising first and then decreasing, showing an upward trend in the first 50 s and then gradually decreasing, with a range of-0.02 V ~ 0.18 V. The OCP curve of S3 surface coating is the most stable, showing a downward trend, and the change range is 0 V ~ 0.02 V. According to the change of OCP value, the surface change of S3 is the smallest, indicating that the electrochemical activity of S3 surface is the lowest.
Figure 9(b) shows the Tafel curves of all coating samples in natural seawater, and it is not difficult to observe that the polarization curves show obvious stratification. The calculation results of corrosion voltage \({E}_{\text{c}\text{o}\text{r}\text{r}}\)(V), corrosion current density \({i}_{\text{c}\text{o}\text{r}\text{r}}\)(µA·cm− 2) and average corrosion rate \({P}_{i}\)(µm·y− 1) are shown in Table 1. The corrosion voltage and corrosion current density are directly calculated by Tafel extrapolation method in Tafel region of cathodic polarization curve. The average corrosion rate is calculated by substituting the corrosion current density into the following formula 53,54:
Table 1
Corrosion voltage, corrosion current density and corrosion rate of coatings on S0, S1, S2 and S3.
| Sample | Ecorr(V) | icorr (µA·cm− 2) | Pi (µm·y− 1) |
| S0 | -0.5768 | 4.1811×10− 2 | 0.9554 |
| S1 | -0.2961 | 7.9771×10− 3 | 0.1823 |
| S2 | -0.0757 | 2.3435×10− 3 | 0.0535 |
| S3 | -0.0435 | 3.0447×10− 4 | 0.0069 |
$${P}_{i}=22.85{i}_{corr}$$
1
The corrosion voltages of S0, S1, S2 and S3 coatings are-0.5768 V, -0.2961 V, -0.0757 V and − 0.0435 V, respectively. It can be observed that the corrosion voltage gradually becomes positive with the increase of the mass ratio of SiO2 and α-Fe2O3 in the organic coating. The smaller the corrosion current density is, the lower the corrosion rate is, and the better the corrosion resistance of the coating is 55. The corrosion current density of all coating samples are 4.1811×10− 2 µA·cm− 2, 7.9771×10− 3 µA·cm− 2, 2.3435×10− 3 µA·cm− 2 and 3.0447×10− 4 µA·cm− 2, respectively. It is obvious that the corrosion current density of the sample surface coated with organic coating is reduced by at least one order of magnitude, and the performance is better than that of Ba et al. 56. The corrosion resistance of different samples from large to small is S3 > S2 > S1 > S0. It can be seen that the corrosion resistance of the organic composite coating increases with the increase of the mass ratio of SiO2 and α-Fe2O3. The reason is that the increase of the content of SiO2 nanoparticles in the organic coating makes α-Fe2O3 more uniformly dispersed in the coating solution and more effectively resists the invasion of corrosive ions.
Electrochemical impedance spectroscopy is also a means to characterize the corrosion characteristics of coating samples 57. The capacitance ring in the electrochemical impedance spectroscopy is positively correlated with the corrosion resistance. The larger the radius of the capacitor ring, the stronger the resistance of the exchange electron, and the lower the electron exchange capacity. The lower electron exchange capacity represents better corrosion resistance 58–60. As shown in Fig. 9(c), the radius of the capacitor ring of S3 is the largest at the same frequency, indicating that S3 has the best corrosion resistance. On the contrary, the radius of capacitance change ring of S0 is the smallest and the corrosion resistance is the worst. This phenomenon is further verified in the Bode diagram (Fig. 9(d)).
The obtained EIS data are fitted by the equivalent circuit diagram shown in Fig. 10. The data after fitting are shown in Table 2. Among them, Rs is the solution resistance, Rf and Qf are the resistance and constant phase elements of the coating, Cdl is the electric double layer capacitance, and Rct is the charge transfer resistance 61,62. The greater the Rct value, the greater the resistance of charge transfer and the better the corrosion resistance 63. The Rct values of S0, S1, S2 and S3 are 3.02 × 106 Ω cm2, 7.07 × 106 Ω cm2, 1.08 × 107 Ω cm2 and 7.73 × 107 Ω cm2 respectively. It can be observed that the Rct value of the organic composite coating is one order of magnitude higher than that of the MAO coating. According to the size of the Rct value, the corrosion resistance of different coating samples from large to small is S3 > S2 > S1 > S0. This is consistent with the conclusion of the Tafel curve.
Table 2
EIS equivalent circuit parameters of coatings on S0, S1, S2 and S3.
| Sample | Rs (Ω cm2) | Qf (Ω−1cm−2s− n) | n | Rf (Ω cm2) | Cdl (F cm− 2) | Rct (Ω cm2) |
| S0 | 1.19×104 | 6.44×10− 9 | 0.98 | 1.84×104 | 2.57×10− 6 | 3.02×106 |
| S1 | 9.82×103 | 5.58×10− 10 | 0.97 | 2.94×104 | 3.75×10− 7 | 7.07×106 |
| S2 | 1.02×104 | 3.98×10− 10 | 0.93 | 3.37×104 | 1.71×10− 7 | 1.08×107 |
| S3 | 1.67×104 | 3.64×10− 10 | 0.90 | 3.63×104 | 1.62×10− 7 | 7.73×107 |
3.5 Potential analysis of organic composite coating
Figure 11 shows the SKPFM test results of all coating samples. The surface morphology of MAO coating and organic composite coating is shown in Fig. 11(a3 and a4). It can be seen that the surface of MAO coating is rough, and the surface fluctuation is 0.0 µm ~ 0.5 µm. The surface of the organic composite coating is relatively smooth, and the surface fluctuation degree is 0.12 µm ~ 0.22 µm, which is consistent with the results reflected in Fig. 5 and Fig. 6. Since there is a positive correlation between the Volta potential measured by SKPFM and the corrosion potential, SKPFM measurement is used to evaluate the protective performance of the coating on the metal substrate. According to the Volta potential of the coating samples (Fig. 11(a1, b1, c1, and d1)), the Volta potential range of S0 is-0.45 V ~ 0.15 V, the Volta potential range of S1 is 0.08 V ~ 0.13 V, the Volta potential range of S2 is 0.0 V ~ 0.3 V, and the Volta potential range of S3 is 0.03 V ~ 0.25 V. It can be seen from Fig. 11(a2, b2, c2, and d2) that the line scan curves of S0, S1, and S2 fluctuate around − 0.2 V, 0.09 V, and 0.15 V, respectively, while the line scan curve of S3 continues to rise and the potential continues to become positive. Combining the Volta potential range and the change trend of the line scan curve, it can be concluded that the corrosion resistance of the coating samples is S3 > S2 > S1 > S0. S1, S2 and S3 were soaked for 120 h and compared with the unsoaked coating samples. The Volta potential range of S1 after soaking was-0.15 V ~ 0.15 V (Fig. 11(e1)), the Volta potential range of S2 after soaking was-0.15 V ~ 0.17 V (Fig. 11(f1)), and the Volta potential range of S3 after soaking was-0.07 V ~ 0.25 V (Fig. 11(g1)). The line scan curves after soaking are shown in Fig. 11(e2, f2 and g2). The line scan curves of S1 and S2 showed a trend of increasing first and then decreasing, and the line scan curve of S3 showed an upward trend. Therefore, the organic composite coating after soaking for 120 h is relatively stable, and the stability is S3 > S2 > S1. In summary, the corrosion resistance and stability of the organic composite coatings increase with the increase of the mass ratio of SiO2 to α-Fe2O3.
3.6 Absorbing performance test of organic composite coating
The absorbing performance of the material depends on the impedance matching characteristics and attenuation characteristics at the same time. There are electrical loss and magnetic loss, and one item cannot be emphasized unilaterally. It is necessary to comprehensively evaluate the absorbing performance of the absorbing material. At present, the performance of absorbing materials is evaluated by reflection loss (\({R}_{\text{L}}\)). The vector network analyzer 64 is used to measure S1, S2, and S3. The measured permeability (µ) and dielectric constant (ε) are calculated according to the formula (2–5)65:
$$\begin{array}{c}{\mu }_{\text{r}}={\mu }_{\text{r}}^{,}-j{\mu }_{\text{r}}^{,,}\#\left(2\right)\end{array}$$
$$\begin{array}{c}{\epsilon }_{\text{r}}={\epsilon }_{\text{r}}^{,}-j{\epsilon }_{\text{r}}^{,,}\#\left(3\right)\end{array}$$
$$\begin{array}{c}{Z}_{\text{X}}=\sqrt{\frac{{\mu }_{\text{r}}}{{\epsilon }_{\text{r}}}}\text{tanh}\left(\text{j}\frac{2{\pi }\text{d}\text{f}\sqrt{{\mu }_{\text{r}}{\epsilon }_{\text{r}}}}{\text{c}}\right)\#\left(4\right)\end{array}$$
$$\begin{array}{c}{R}_{\text{L}}\left(\text{d}\text{B}\right)=20\text{lg}\left|\frac{{Z}_{\text{X}}-1}{{Z}_{\text{X}}+1}\right|\#\left(5\right)\end{array}$$
Among them, \({\mu }_{\text{r}}\) represents the relative permeability, \({\mu }_{\text{r}}^{,}\), and \({\mu }_{\text{r}}^{,,}\) represent the real and imaginary parts of the relative permeability, respectively, \({\mu }_{\text{r}}^{,}\) represents the degree of magnetic field polarization, and \({\mu }_{\text{r}}^{,,}\) represents the magnetic loss capability. \({\epsilon }_{\text{r}}\) represents the relative dielectric constant, \({\epsilon }_{\text{r}}^{,}\), and \({\epsilon }_{\text{r}}^{,,}\) represent the real and imaginary parts of the relative dielectric constant, respectively, \({\epsilon }_{\text{r}}^{,}\) represents the electric field polarization ability, and \({\epsilon }_{\text{r}}^{,,}\) represents the electric loss ability. \({Z}_{\text{X}}\) is the normalized input impedance, \(\text{f}\) is the frequency of the incident wave, \(\text{d}\) is the thickness of the absorbing material, \(\text{c}\) is the speed of light, and the \({R}_{\text{L}}\left(\text{d}\text{B}\right)\) result is generally negative. The simulated reflection loss curves at different frequencies and different material thicknesses are plotted in two-dimensional and three-dimensional, as shown in Fig. 12. At the integer thickness (Fig. 12(a, c and e)), the minimum values of the reflection loss curves of S1, S2 and S3 are − 0.5032 dB, -6.4987 dB and − 11.2638 dB, respectively. It can be seen from the reflection loss 3D diagram (Fig. 12(b, d and f)) that S1 has a prominent loss peak at a frequency of 12.27 GHz and a thickness of 2.2 mm, and the peak value of the prominent loss peak is -5.1963 dB. S2 has a prominent loss peak at a frequency of 16.98 GHz and a thickness of 0.3 mm, and the prominent loss peak-to-peak value is -12.6630 dB. S3 has a prominent loss peak at a frequency of 17.82 GHz and a thickness of 0.5 mm, and the peak-to-peak value of the prominent loss peak is -33.2162 dB. When the reflection loss value is smaller, the absorbing performance is better 66,67. Therefore, the absorbing performance of S3 surface coating is the best, the absorbing performance of S1 surface is the worst, and the absorbing performance of S2 is in the middle. The reason is that the α-Fe2O3 nanoparticles in the organic coating have high resistivity, which can effectively make the electromagnetic wave pass through. SiO2 can reduce the agglomeration of α-Fe2O3 nanoparticles, make them more evenly distributed in the organic coating, and have higher wave absorption efficiency 43,44,68. In summary, the microwave absorption performance of the organic composite coating increases with the increase of the mass ratio of SiO2 and α-Fe2O3.
3.7 Corrosion mechanism of organic composite coating
The corrosion mechanism of all coating samples is shown in Fig. 13. When the MAO coating surface is not coated with SiO2@α-Fe2O3 organic coating (Fig. 13(a)), although the MAO coating with Yb2O3 has a good sealing effect, the sealing is not complete. There are still a small number of micropores on the surface of the coating. It is precisely because these micropores provide a channel for corrosive ions to enter the metal matrix, or local corrosion will occur. When the MAO coating surface is coated with SiO2@α-Fe2O3 organic coating (Fig. 13(b)), the coating liquid will flow into the micropores on the MAO coating surface to achieve a complete sealing effect. At the same time, the prepared organic composite coating is thick and dense, and the corrosive ions are well separated, which greatly improves the corrosion resistance of the coating. Due to the infiltration of the coating liquid into the micropores on the surface of the MAO coating, the mechanical interlocking between the coatings is caused. This phenomenon can effectively improve the bonding force between the composite coatings 48,49.