Abstract
The layered double hydroxides (LDHs) were prepared on 6061Al alloy by in-situ synthesis. The effects of LDHs on the corrosion behavior of 6061Al alloy in 3.5wt% NaCl solution with different dissolved oxygen (DO) contents (1~16 mg/L) were analyzed through the polarization curves (Tafel) and electrochemical impedance spectroscopy (EIS). The results indicate that the polarization processes of 6061Al and LDHs/6061Al alloys are controlled by DO diffusion. With increasing the DO content, a large number of pits appear on the 6061Al alloy surface, and the pit is gradually deepened. The corrosion current is increased from 1.273 μA to 1.743 μA, and the pit depth is increased from 12 μm to 19 μm. However, no pits can be observed in LDHs/6061Al alloy at different DO contents, which is attributed to the inhibition of DO diffusion and the polarization process. In addition, the resistance of 6061Al alloy is increased with increasing the DO content, which is related to the formation of Al2O3. However, the resistance of LDHs/6061Al alloy has no obvious change at different DO contents, showing excellent isolation performance against DO.
Keywords
Science Press
The 6061Al alloy is a heat-treated aluminum alloy with excellent processing properties and good mechanical properties, which has been widely used for shipbuilding, marine equipment, transportation, and other industrie
In recent years, the effect of DO on the corrosion behavior of metal attracts widespread attention. Dexter et a
To slow down the corrosion of aluminum alloy in seawater, a series of surface treatment techniques, such as anodic oxidation, electroplating, and spraying, have been adopted for corrosion protectio
LDHs are the environmentally-friendly material, which can be used as the protection film to inhibit the corrosion of aluminum alloy. At present, the principle of the corrosion resistance of LDHs is still ambiguous. It is proved that LDHs have a trapping effect on C
In this research, LDHs were prepared on the surface of 6061Al alloy by in-situ synthesis. Different DO contents (1~16 mg/L) were used according to the distribution of DO in marine environmen
The ultra-pure water with a resistivity greater than 18 MΩ·cm was used in this research. All reagents were directly used without further purification. The 6061Al alloy (0.15wt%~0.4wt% Cu, 0.15wt% Mn, 0.80wt%~1.2wt% Mg, 0.25wt% Zn, 0.4wt%~0.8wt% Si, 0.15wt% Ti, and balanced Al) was cut into specimens with the size of Φ14 mm×5 mm, and then polished with silicon carbide sandpaper of 320#, 600#, 1200#, and 2000# and by diamond of 0.1 μm in size.
LDHs were prepared by conventional in-situ synthesis metho
X-ray diffraction (XRD, D/MAX 2500 V, Rigaku, Japan) was used to analyze the crystal structure of the films. The monochromator was operated at 40 kV and 150 mA. The scanning rate was 8°/min. The surface and cross-section morphology characteristics of LDHs/6061Al alloy were studied by scanning electron microscope (SEM, SU8220, Hitachi, Japan). The elements of LDHs/6061Al alloy films were measured by energy disperse spectroscopy (EDS, QUANTAX, Bruker, Germany). The corrosion morphology was characterized by optical microscope (OM, DVM6s, Leica, Germany) and SEM (Pro X, Phenom, Netherlands). Moreover, Fourier transform infrared spectroscopy (FTIR, IRTracer-100, Shimadzu, Japan) was used to analyze the molecular structure and chemical bond.
The electrochemical experiments were conducted to study the influence of different DO contents on the corrosion behavior of LDHs/6061Al and 6061Al alloys. The schematic diagram of the determination device is shown in
Fig.1 Schematic diagram of experiment device
NaCl solution. The DO content was determined by DO analyzer (J650s, LeiCi, China).
To obtain the stable open circuit potential, the solution was placed still for 30 min before each measurement. The electrochemical resistance spectroscopy (EIS) and Tafel polarization curves were measured at different DO contents (1~16 mg/L), and three measurements were performed at each DO content to ensure the accuracy. The polarization curve was tested with a potential scanning rate of 5 mV/s under the scanning range from -1.0 V vs. SCE to 0 V vs. SCE. EIS was tested with the amplitude of 10 mV under the frequency from 1
XRD patterns of 6061Al and LDHs/6061Al alloy are shown in Fig.2. It can be seen that there are Al and Mg2Si phases in 6061Al alloy.
XRD results of LDHs/6061Al alloy films show the characteristics of hydrotalcite type clay, corresponding to the typical diffraction peaks of (003), (006), and (100) plane
Fig.3 shows FTIR spectrum of LDHs/6061Al alloy. The broadband centered at 3440 c
SEM images and EDS analysis of LDHs/6061Al alloy are shown in Fig.4. LDHs film with the thickness of tens of nanometers is composed of hexagonal nanosheets of different sizes (2~5 μm). The film grows perpendicularly to 6061Al alloy substrate, presenting an obvious orientation relationship. The surface of 6061Al alloy substrate is completely covered by LDHs, and there is no phenomenon of aggregation growth. Fig.4c shows the cross-sectional morphology of the LDHs/6061Al alloy. The LDHs film has no breakpoint and shows the height inhomogeneity, indicating that the film is well bonded with the substrate with an average film thickness of 3.2 μm. Fig.4d shows EDS results of point A in Fig.4b. The atomic ratio of Zn/Al in LDHs/6061Al alloy is 1.93, which is basically consistent with Ref.[
The schematic diagram of in-situ synthesis process of LDHs/6061Al alloy is shown in
| CO(NH2)2+3H2O→2NH | (1) |
| Al2O3+2O | (2) |
| Al→A | (3) |
| O2+ H2O+4 | (4) |
| Z | (5) |
| Al2O3+3H2O→2Al(OH)3 | (6) |
| Al(OH)3+2Zn(OH | (7) |
Fig.5 Schematic diagram of in-situ synthesis process of LDHs film on 6061Al alloy substrate
The hydrolysis of urea produces CO2 and O
The cathodic polarization is widely used to study the polarization behavior of films. Fig.6 shows the cathodic polarization curve of 6061Al and LDHs/6061Al alloys in 3.5wt% NaCl solution with different DO contents.
With increasing the DO content, the corrosion potential Ecorr of 6061Al alloy is increased, while that of LDHs/6061Al alloy does not change significantly, indicating that LDHs show an effective inhibitory effect against DO. The cathodic polarization curves of LDHs/6061Al alloy shift towards the region of low current density, and the corrosion potential also moves towards the positive direction. At the same potential, the current of LDHs/6061Al alloy is obviously less than that of 6061Al alloy, which indicates that the LDHs film enhances the corrosion resistance of 6061Al alloy substrate.
Tafel slope can be used to judge the initiation difficulty of electrode reaction. The larger the slope, the greater the resistance against the electrode reaction process.
In addition, there are two regions in the polarization curves of 6061Al alloy. In region II, the difference in polarization curves of 6061Al alloy at different DO contents is caused by the fact that the increase in DO content is conducive to the oxygen reduction reaction (ORR), and with ORR proceeding, the DO diffusion is not sufficient to supply the cathode consumption, which is manifested by the gradually increased Tafel slope. In region I, the difference in polarization curves results from the DO diffusion. The corrosion current density Jcorr of 6061Al alloy is positively correlated with the DO content, because the diffusion of oxygen plays a major role in the electrochemical reaction and the cathodic reaction rate is increased with increasing the diffusion rate. The corrosion current is increased from 1.273 μA to 1.743 μA. However, the polarization behavior of LDHs/6061Al alloy is independent of the initial DO content, which indicates that LDHs film has an excellent inhibition effect against ORR.
Fig.7 shows the anodic polarization curves of 6061Al and LDHs/6061Al alloys at different DO contents. The electrochemical parameters of 6061Al and LDHs/6061Al alloys at different DO contents were obtained by Tafel extrapolation method (
The corrosion potential of 6061Al alloy shifts positively with increasing the DO content, because oxygen is a depolarizer for aluminum alloy
Fig.8 shows the Bode diagrams of 6061Al and LDHs/6061Al alloys in 3.5wt% NaCl solution with different DO contents. There are three and two time constants in LDHs/6061Al and 6061Al alloys, respectively. The time constants in low frequency region (1
Fig.8e shows the equivalent circuit models for EIS spectra of 6061Al and LDHs/6061Al alloys. The constant phase-angle element (CPE) is used to compensate for the non-uniformity in the system instead of capacitors. The resistance of CPE is defined as ZCPE=[Y(jω
The fitting parameters of the equivalent circuits of 6061Al and LDHs/6061Al alloys are presented in
Fig.9 OM corrosion morphologies of 6061Al (a~e) and LDHs/6061Al (f~j) alloys after polarization with different DO contents: (a, f) 1 mg/L, (b, g) 4 mg/L, (c, h) 8 mg/L, (d, i) 12 mg/L, and (e, j) 16 mg/L
From the analysis of 3D corrosion morphologies in
Fig.10 3D corrosion morphologies of 6061Al (a~e) and LDHs/6061Al (f~j) alloys after polarization with different DO contents:
(a, f) 1 mg/L, (b, g) 4 mg/L, (c, h) 8 mg/L, (d, i) 12 mg/L, and (e, j) 16 mg/L
1) The cathodic polarization process of 6061Al and layered double hydroxides (LDHs)/6061Al alloy is controlled by the diffusion of dissolved oxygen (DO), and the polarization behavior of 6061Al alloy is also related to the initial content of DO. In the initial stage of the reaction, the increase in DO content will accelerate the reaction rate of 6061Al alloy.
2) In the strong polarization region, the cathodic reaction slope of LDHs/6061Al alloy is greater than that of 6061Al alloy at different DO contents, due to the inhibition of the cathodic polarization process caused by LDHs/6061Al alloy.
3) The LDHs film significantly enhances the corrosion resistance of 6061Al alloy, and the resistance value of LDHs/6061Al alloy is 2~3 orders of magnitude higher than that of 6061Al alloy. The resistance value of LDHs/6061Al alloy has no obvious change at different DO contents, indicating the stable protection for 6061Al alloy
4) The corrosion degree of 6061Al alloy is increased with increasing the DO content. The corrosion current is increased from 1.273 μA to 1.743 μA, and the pit depth is increased from 12 μm to 19 μm. However, the corrosion current of LDHs/6061Al alloy does not change significantly at different DO contents, and no pits can be observed.
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