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Article

The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods

by
Chao Wang
1,
Shineng Sun
2,
Yunhe Ling
1,
Haifeng Tan
1 and
Chunlin He
1,*
1
Liaoning Provincial Key Laboratory of Advanced Materials, Shenyang University, Shenyang 110044, China
2
Shenyang Key Laboratory of Micro-Arc Oxidation Technology and Application, Shenyang University, Shenyang 110044, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(6), 733; https://doi.org/10.3390/coatings14060733
Submission received: 1 May 2024 / Revised: 3 June 2024 / Accepted: 6 June 2024 / Published: 8 June 2024
(This article belongs to the Special Issue Advanced Corrosion Protection through Coatings and Surface Rebuilding)
Browse Figures
Versions Notes

Abstract

:
Tartaric-sulfuric acid anodic (TSA) films were prepared on the surface of the 2024 Al alloy. These films were sealed with cerium salts at 25 °C and 65 °C, hot water, and dichromate. The morphology and corrosion resistance of the anodic films were investigated using a field emission scanning electron microscope/energy-dispersive spectrometer, an electrochemical workstation, an acidic spot test, and an immersion test. The results indicated that the surface of the TSA film sealed with cerium salt at 65 °C had a slightly lower cerium content compared to the TSA film sealed at 25 °C. It was found that increasing the sealing temperature of cerium salt could enhance the corrosion resistance of the TSA film. After immersion in a 3.5 wt.% NaCl solution for 336 h, no obvious corrosion pits were observed on the surface of the TSA film sealed at 65 °C, whereas many larger corrosion pits appeared on the surface of the TSA film sealed at 25 °C. The improved corrosion resistance of the TSA film sealed at 65 °C could be attributed to the synergistic effect of cerium oxide deposition and the hydration reaction. The corrosion resistance of the TSA film sealed at 65 °C was significantly better than that of the film sealed with hot water, but it was still lower than that of the TSA film sealed with dichromate.

1. Introduction

The 2024 Al alloy is widely used in construction and aerospace applications due to its excellent mechanical properties [1]. It has excellent mechanical properties due to its complex microstructure [2,3]. However, alloying can deteriorate the corrosion resistance of the alloy, although it can improve its mechanical properties. For the 2024 Al alloy, the existence of the Cu-rich CuAl2 phases and Cu-poor regions [4,5,6,7] enhances the local electrochemical activity in the alloy [8,9,10,11,12], resulting in severe localized corrosion. Ramirez et al. [13] also found that localized corrosion pits are generally formed in the vicinity of Cu-rich intermetallic compounds. Therefore, aluminum alloys need to be protected to enhance their corrosion resistance and thus increase their service life. Among all the protection methods, anodizing is widely used because of its simple process, low cost, and excellent protection.
Currently, the most widely used anodizing process for aluminum alloys is sulfuric acid anodizing (SAA) [14]. This process offers advantages such as easy operation and the formation of a thick film. However, it can also increase the probability of corrosion fatigue in the workpiece [15]. Additionally, the SAA process consumes a significant amount of sulfuric acid, which can have a considerable impact on the environment. Therefore, it is necessary to develop an anodizing process that provides excellent corrosion resistance while minimizing its environmental impact. A new process, known as tartaric-sulfuric acid anodizing (TSA), was developed by Airbus in France [16,17]. This process significantly reduces the amount of sulfuric acid used compared to the SAA process. Furthermore, Curioni et al. [18] found that the addition of tartaric acid could reduce the dissolution rate of the anodic film in an acidic environment, thereby significantly enhancing the corrosion resistance of aluminum alloys. Boisier et al. [19] found that the pores of the TSA film were smaller than those of the SAA film, which led to the former showing superior corrosion resistance.
In order to further enhance the corrosion resistance of the anodic film on aluminum alloys, it is commonly sealed. There are many sealing methods, and hot water sealing (HWS) [20,21] is one of the most widely used methods, which involves the use of boiling water to seal the anodic film. HWS can cause the pore wall of the porous layer to swell due to the formation of hydrated alumina, γ-AlO(OH) [19,21,22,23,24,25]. The swelling of the pore wall is attributed to the fact that hydrated alumina typically has a larger volume than aluminum oxides, resulting in pore blockage. However, this method has disadvantages, such as high energy consumption and low efficiency. Dichromate sealing is an effective method. Hydroxy aluminum dichromate (AlOHCrO4) or hydroxy aluminum chromate ((AlO)2CrO4) [21], produced by the reaction of dichromate with aluminum oxide, can inhibit the corrosion of the anodic film. However, Cr6+ is extremely hazardous to the environment and human health. The current research trend is to find environmentally friendly sealing methods [26,27,28,29]. Cerium salts are effective corrosion inhibitors [30] and have been widely used to improve the corrosion resistance of aluminum alloys [31,32,33,34,35,36]. Moreover, cerium is abundant in nature and is environmentally friendly. The application of cerium in anodic film sealing has attracted much attention [37,38,39,40,41,42]. Terada et al. [41] found that adding cerium salt to hot water could enhance the corrosion resistance of the TSA film on the 2024 Al alloy. Cerium ions not only formed a cerium conversion layer on the surface of the anodic film but also generated Ce(OH)4 or CeO2 precipitates in the micropores of the porous layer [23], which played a significant role in the corrosion protection of the anodic film [43]. In previous studies, it was found that increasing the temperature of the cerium salt sealing solution was beneficial for enhancing the corrosion resistance of the anodic film [13,37]. In the present study, the TSA films on the 2024 Al alloy were sealed with cerium salts at 25 °C and 65 °C, hot water, and dichromate. This research focused on examining the corrosion resistance and morphology of the sealed films.

2. Materials and Methods

2.1. Sample Pretreatment

The material used was the 2024 Al alloy plate, and its nominal chemical composition is listed in Table 1. The plate was first cut into samples with dimensions of 40 mm × 25 mm × 2 mm. Subsequently, the samples were ground step by step using 240#–1000# silicon carbide sandpapers. Before anodizing, the samples underwent the following treatments [40,44]: ultrasonic cleaning (acetone, 10 min; anhydrous ethanol, 20 min) using an ultrasonic cleaner (KQ3200B, Kunshan, China), followed by an alkaline wash (50 g/L NaOH, 60 °C, 3 min), and then desmutting (30% (v/v) HNO3, 25 °C, 1 min). The samples were rinsed with deionized water after each step.

2.2. Anodizing and Sealing

The electrolyte for anodizing consisted of 40 g/L H2SO4 and 80 g/L C4H6O6. During the anodizing process, the anode material was a 2024 Al alloy plate, and the cathode material was a lead plate. Anodizing was conducted at a constant voltage of 14 V for 25 min using a DC-regulated power supply, while the electrolyte was magnetically stirred at a temperature of 37 ± 1 °C. After anodizing, the samples were rinsed with deionized water and then thoroughly dried with cold air.
Cerium salt, hot water, and dichromate were used to seal the anodized samples, and H2O2 was added to the cerium salt solution within two minutes before sealing. The composition of the sealing solution as well as the sealing conditions are listed in Table 2.

2.3. Morphological Observation

The morphology and composition of the anodized samples were analyzed using a Hitachi S-4800 (Hitachi, Tokyo, Japan) field emission scanning electron microscope (FESEM) with an accompanying energy-dispersive spectrometer (EDS, Oxford, UK) [45]. The secondary electron (SE) mode was used to observe the surface of the sample, while the backscattered electron (BSE) mode was utilized to examine the cross-section. The cross-section for FESEM testing was prepared as follows: the sample was cut to the appropriate size using a hacksaw, embedded in epoxy resin, and then ground and polished using emery paper and diamond paste. Platinum was sputtered onto the surface of the samples using a Hitachi E-1045 ion sputter before observation.

2.4. Corrosion Resistance Test

The acidic spot test was conducted on the sealed samples using an acidic spot solution composed of 3 g of K2Cr2O7, 25 mL of HCl (ρ = 1.18 g/mL), and 75 mL of deionized water at room temperature. A drop of the solution was placed on the surface of the anodic film, and the time (t) was recorded when the droplet changed color from orange to green. The corrosion resistance of the anodic film was evaluated in terms of t. The longer the t, the better the corrosion resistance of the anodic film.
The corrosion resistance of the anodic film in the 3.5 wt.% NaCl solution was evaluated using electrochemical techniques. The test equipment used was an electrochemical workstation (CHI604E, Chenhua, Shanghai, China) [45], with a saturated calomel electrode (SCE) as the reference electrode, a graphite electrode as the auxiliary electrode, and an anodized sample (16.39 cm2) as the working electrode. Before taking measurements, the open-circuit potential (OCP) of the samples should be stable. The scanning rate for potentiodynamic polarization was 0.5 mV/s, and the potential scanning range was −1.0 V~−0.25 V. The frequency range of electrochemical impedance spectroscopy (EIS) was 105 Hz~10−2 Hz, and a sinusoidal perturbation voltage of 10 mV was applied. In the present paper, EIS of anodized samples immersed in a 3.5 wt.% NaCl solution for 336 h was continuously monitored to evaluate the change in the corrosion resistance of the anodic film.

3. Results and Discussion

3.1. Morphology and Composition

The surface morphologies and composition of the TSA films sealed using different methods are shown in Figure 1. It can be seen that many spherical particles formed on the surface of the TSA film after sealing with Ce 25 (Figure 1a), which is consistent with the findings of Yu et al. [46]. After sealing with Ce 65, the surface of the TSA film was slightly rough (Figure 1c), relative to the case of Ce 25, which is associated with cerium oxide deposition and the hydration reaction of aluminum oxide [19]. Petal-like structures formed on the surface of the TSA film that was sealed with hot water (Figure 1e), which resulted from the precipitation of aluminum salt, as shown in reaction (1) [19]. As for dichromate sealing, the surface of the TSA film was almost the same as the unsealed surface (Figure 1g,i).
Al 2 O 3 + H 2 O → 2 AlO OH
According to the results of the EDS analysis, it is evident that the TSA films sealed with Ce 25 and Ce 65 contain elements such as O, Al, S, and Ce. However, the Ce content is lower for the film sealed with Ce 65 (Figure 1b,d). This is because an increase in temperature accelerates the decomposition of H2O2, which reduces the amount of H2O2 available for the deposition of cerium oxide, resulting in a decrease in cerium content. The effect of H2O2 is demonstrated by its ability to hydrolyze to produce OH−, which increases the pH value at the reaction site [37]. This contributes to the production of more Ce(OH)4 (reaction (2) and (3)) with a solubility product lower than that of Ce(OH)3 (reaction (4) and (5)) [38,47,48]. Hot water-sealed and -unsealed TSA films contain only Al, O, and S elements (Figure 1f,j). Element Cr was detected in the TSA film sealed with dichromate (Figure 1h) because chromate can react with aluminum oxide at high temperatures to form hydroxy aluminum dichromate (reaction (6)) and hydroxy aluminum chromate (reaction (7)) [49], which adhere to the surface and pores of the anodic film. The presence of element S in all samples indicates that the sulfate radical also participates in the anodizing reaction, which probably produces Al2O3·Al(OH)x(SO4)y [44].
2 Ce 3 + + H 2 O 2 + 2 O H − → 2 Ce OH 2 2 +
Ce OH 2 2 + + 2 OH − → Ce OH 4 → CeO 2 + 2 H 2 O
Ce 3 + + 3 OH − → Ce OH 3
2 Ce OH 3 → Ce 2 O 3 + 3 H 2 O
Al 2 O 3 + 2 HCrO 4 − + H 2 O → 2 AlOHCrO 4 + 2 OH −
Al 2 O 3 + HCrO 4 − → AlO 2 CrO 4 + OH −
The cross-sectional morphologies of the TSA films sealed by different methods are shown in Figure 2. The darker part is the TSA film, and the thickness of the film is approximately 3 μm, which is consistent with the results reported [13]. The EDS line scanning results of the cross-section of the TSA film sealed with Ce 25 and Ce 65 are shown in Figure 3, the scanning route is from the surface to the substrate. It can be seen that the thickness of the TSA film is approximately 3 μm (Figure 3a), and Ce oxides precipitate in the porous layer (Figure 3a,b) [37]. Figure 3b also clearly shows that after sealing with Ce 65, the content of cerium in the TSA film is slightly lower than that of the TSA film sealed with Ce 25. On the one hand, higher temperatures not only promote cerium oxide deposition but also accelerate H2O2 decomposition, leading to a reduction in the amount of cerium oxide deposited in the film [50]. On the other hand, hydration reactions may also occur to a greater or lesser extent at higher temperatures, resulting in the swelling of the pore walls and the narrowing of the pores in the porous layer. In narrower pores, it is difficult for the sealing solution to diffuse, resulting in a lower amount of cerium oxide being deposited.

3.2. Corrosion Resistance

The time, t, used to change the acidic spot droplet from orange to green in the acidic spot test, is listed in Table 3. It can be seen that the t of the TSA film sealed with Ce 25 and Ce 65 is 21.02 min and 35.38 min, respectively. The longer t is, the better the corrosion resistance of the anodic film. Therefore, the corrosion resistance of the TSA film sealed with Ce 65 is significantly improved compared to Ce 25 [13]. In comparison, it was found that among the four sealing methods listed in Table 3, the corrosion resistance of the TSA film sealed with Ce 65 was significantly better than that of the TSA films sealed with hot water and Ce 25 but weaker than that of the TSA film sealed with dichromate.
Figure 4 shows the EIS of TSA films sealed using different methods after immersion in a 3.5 wt.% NaCl solution for 1 h. The EIS plot displays the impedance modulus |Z| against frequency f. In this graph, the high-frequency range reflects the resistance and capacitance of the porous layer of the anodic film, and the low-frequency range reflects the resistance and capacitance of the barrier layer. The EIS of the TSA film can be fitted using the equivalent circuit shown in Figure 5, and the fitting results are listed in Table 4. Here, Rs represents the solution resistance; Rp and Rb represent the resistances of the porous layer and the barrier layer of the anodic film, respectively; and CPEp and CPEb represent the constant phase angle elements of the porous layer and the barrier layer, respectively. In addition, Y stands for admittance, and n represents the dispersion coefficient. It can be seen that the Rp values of the TSA films sealed with Ce 25 and Ce 65 were 0.45 MΩ·cm2 and 1.71 MΩ·cm2, respectively, indicating that higher-temperature sealing is favorable for increasing the Rp [13]. The TSA film sealed with dichromate had the largest Rp, indicating that the porous layer is the most resistant to aggressive Cl− and offers the best corrosion resistance. The larger the impedance modulus value at a low-frequency region, the better the corrosion resistance. Therefore, the sealed film exhibited better corrosion resistance than the unsealed film.
To evaluate the differences in the corrosion resistance of anodic films sealed using different methods, the EIS of anodic films immersed in a 3.5 wt.% NaCl solution was continuously monitored for 336 h, as shown in Figure 6. As depicted in the figure, in the low-frequency range, the |Z| values of TSA films sealed with Ce 25 and Ce 65 decrease with increasing immersion time, but the decrease is more significant in the former. In the mid-frequency range, the phase angle and capacitive arc width of the films sealed with Ce 25 and Ce 65 are both reduced with increasing immersion time, but the former reduction is more obvious (Figure 6a,b). In the high-frequency range, the phase angle and |Z| values of the TSA film sealed with Ce 25 increase up to 120 h and then decrease with increasing immersion time. On the contrary, the phase angle and |Z| values of the TSA film sealed with Ce 65 show an increasing trend all the time. The phase angle and |Z| values of TSA film sealed with dichromate show a similar trend to TSA film sealed with Ce 65, and the increase is more significant (Figure 6d). During immersion, the barrier layer of the TSA film sealed with hot water gradually loses its capacitance and becomes more resistive with increasing immersion time (Figure 6c) [4]. In the low-frequency region, its phase angle decreases significantly, and the time constant gradually shifts to higher-frequency regions [4]. The EIS results indicate that the film sealed with Ce 65 provides better protection than Ce 25 and hot water, but it is still lower than that sealed with dichromate.
Figure 7 shows the curves of polarization resistance versus immersion time for TSA films sealed using different methods. During the immersion process, the polarization resistance of the TSA film sealed with Ce 65 was consistently larger than that of the TSA film sealed with Ce 25, revealing that the former has stronger corrosion resistance since the higher polarization resistance represents the stronger corrosion resistance. Although the polarization resistance of the TSA film sealed with hot water was greater than that of the film sealed with Ce 65 in the early stage of immersion (~40 h), it decreased much more than that of the TSA film sealed with Ce 65. Therefore, the corrosion resistance of the film sealed with Ce 65 is better than that of the film sealed with hot water. After 366 h of immersion, the polarization resistances of the films sealed with Ce 25, Ce 65, hot water, and dichromate were 0.65 MΩ·cm2, 3.88 MΩ·cm2, 1.44 MΩ·cm2, and 29.12 MΩ·cm2, respectively. Thus, dichromate sealing can provide the best corrosion resistance, followed by Ce 65 sealing, while Ce 25 sealing provides the worst corrosion resistance.
Figure 8 shows the polarization curves of the TSA films after immersion in a 3.5 wt.% NaCl solution for 1 h (Figure 8a) and 336 h (Figure 8b), respectively. The corresponding fitting results are listed in Table 5. After immersion for 1 h, the corrosion current density (Icorr) of the film sealed with dichromate is 5.16 × 10−10 A/cm2, Icorr for the film sealed with hot water is 3.51 × 10−9 A/cm2, and Icorr for films sealed with Ce 25 and Ce 65 are 4.25 × 10−9 A/cm2 and 4.06 × 10−9 A/cm2, respectively. This shows that dichromate sealing can result in the smallest Icorr, whereas there is almost no distinction for the other three methods. When the immersion times increase from 1 h to 336 h, the corrosion potential (Ecorr) of the films sealed with Ce 25 and hot water both decrease by ~0.2 V, and the Ecorr of the film sealed with dichromate decreases by ~0.1 V. Interestingly, the Ecorr of the film sealed with Ce 65 stays almost unchanged, and it is about 0.19 V~0.25 V greater/more positive relative to the other films. As for the Icorr, the film sealed with Ce 65 is 5.03 × 10−8 A/cm2, which is the smallest, except for that of the film sealed with dichromate. In general, the smaller the Icorr and the more positive the Ecorr, the better the corrosion resistance. Therefore, the corrosion resistance of the TSA film sealed with Ce 65 is much better compared with films sealed with Ce 25 and hot water. The more positive the pitting potential (Epit), the stronger the resistance to film breakdown. At 1 h, the Epit values of the films sealed with Ce 65, hot water, and chromate are very close, which are much more positive than that of Ce 25 sealed film. However, after immersion for 336 h, the Epit values of the films sealed with Ce 25 and hot water decrease by 0.2 V, and the Epit value of the film sealed with Ce 65 only decreases about 0.05 V, while the Epit value of the film sealed with chromate increases about 0.07 V. This indicates that Ce 25 and hot water-sealed films were more easily broken down. In contrast, the films sealed with chromate and Ce 65 are more difficult to pit.
The macroscopic morphology of the TSA films after immersion in a 3.5 wt.% NaCl solution for 336 h is shown in Figure 9. Macroscopically, there appear to be corrosion pits on the surfaces of the films sealed with Ce 25 and hot water (Figure 9a,c), showing that the films are thoroughly damaged near the pits. However, the number of pits is relatively fewer for the latter. No obvious corrosion pits were observed on the surfaces of the TSA films sealed with Ce 65 and dichromate (Figure 9b,d). The microscopic morphology of the TSA films after immersion in a 3.5 wt.% NaCl solution for 336 h is shown in Figure 10. Microscopically, cracks appear on the surface of all TSA films. Except for the large pits (Figure 10a,c), corrosive media can also penetrate to the substrate through cracks, causing substrate corrosion. In addition, the cracks on the surface of the film sealed with Ce 65 are slightly wider compared to the film sealed with dichromate (Figure 10b,d). This shows that the TSA film sealed with Ce 65 possesses better corrosion resistance than the films sealed with hot water and Ce 25, although it is still lower than the film sealed with dichromate.

4. Conclusions

(1)
The surface morphologies of the TSA films sealed with Ce 25 and Ce 65, hot water, and dichromate were different. The Ce content on the film sealed with Ce 65 was slightly lower compared to the film sealed with Ce 25.
(2)
The experimental results of the acidic spot test indicated that the corrosion resistance of the films, from strongest to weakest, was as follows: sealed by dichromate, Ce 65, hot water, and Ce 25.
(3)
After immersion for 366 h, the polarization resistances of the films sealed with Ce 25, Ce 65, hot water, and dichromate were 0.65 MΩ·cm2, 3.88 MΩ·cm2, 1.44 MΩ·cm2, and 29.12 MΩ·cm2, respectively. The Ecorr of the film sealed with Ce 65 was almost unchanged, and it was about 0.19 V~0.25 V greater/more positive relative to the other films. As for the Icorr, the film sealed with Ce 65 obtained a 5.03 × 10−8 A/cm2 value, which was the smallest, except for that of the film sealed with dichromate.
(4)
No obvious corrosion pits were observed on the surfaces of the TSA films sealed with Ce 65 and dichromate, whereas more corrosion pits appeared on the surfaces sealed with Ce 25 and hot water.
(5)
The TSA film sealed with Ce 65 exhibited superior corrosion resistance compared to the films sealed with hot water and Ce 25, although it was still inferior to the film sealed with dichromate. The enhanced corrosion resistance of the TSA film sealed with Ce 65 could be attributed to the combined effect of cerium oxide deposition and the hydration reaction.

Author Contributions

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

Funding

This work was financially supported by the Key R&D Projects of Liaoning Province (2020JH210100011), Liaoning Provincial Department of Education Basic Research Program (JYTMS20231163).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank H. B. Long and X. Y. Wang from the Liaoning Provincial Key Laboratory of Advanced Materials for the SEM analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 14 00733 g001aCoatings 14 00733 g001b
Figure 1. Surface FESEM images (50,000×) and EDS analysis results of TSA films sealed with (a,b) Ce 25, (c,d) Ce 65, (e,f) hot water, and (g,h) dichromate or left (i,j) unsealed.
Figure 1. Surface FESEM images (50,000×) and EDS analysis results of TSA films sealed with (a,b) Ce 25, (c,d) Ce 65, (e,f) hot water, and (g,h) dichromate or left (i,j) unsealed.
Coatings 14 00733 g001aCoatings 14 00733 g001b
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Figure 2. Cross-section FESEM images (10,000×) of TSA films sealed with (a) Ce 25, (b) Ce 65, (c) hot water, and (d) dichromate or left (e) unsealed.
Figure 2. Cross-section FESEM images (10,000×) of TSA films sealed with (a) Ce 25, (b) Ce 65, (c) hot water, and (d) dichromate or left (e) unsealed.
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Coatings 14 00733 g003
Figure 3. EDS line scanning results of (a) Al, O, and (b) Ce elements on the cross-section of the TSA film sealed with Ce 25 and Ce 65, where the line scanning route is from the film surface to the substrate.
Figure 3. EDS line scanning results of (a) Al, O, and (b) Ce elements on the cross-section of the TSA film sealed with Ce 25 and Ce 65, where the line scanning route is from the film surface to the substrate.
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Figure 4. EIS of the TSA films sealed using different methods in 3.5 wt.% NaCl solution.
Figure 4. EIS of the TSA films sealed using different methods in 3.5 wt.% NaCl solution.
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Figure 5. Equivalent circuit for EIS fitting of the TSA films.
Figure 5. Equivalent circuit for EIS fitting of the TSA films.
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Figure 6. EIS change of the films sealed with (a) Ce 25, (b) Ce 65, (c) hot water, and (d) dichromate as a function of immersion times in a 3.5 wt.% NaCl solution.
Figure 6. EIS change of the films sealed with (a) Ce 25, (b) Ce 65, (c) hot water, and (d) dichromate as a function of immersion times in a 3.5 wt.% NaCl solution.
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Figure 7. The changes in polarization resistance of the films with immersion times.
Figure 7. The changes in polarization resistance of the films with immersion times.
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Figure 8. Potentiodynamic polarization curves for the TSA films immersed in 3.5 wt.% NaCl for (a) 1 h and (b) 336 h, respectively.
Figure 8. Potentiodynamic polarization curves for the TSA films immersed in 3.5 wt.% NaCl for (a) 1 h and (b) 336 h, respectively.
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Figure 9. Macroscopic morphology (5×) of TSA films sealed using different methods after 336 h of immersion: (a) Ce 25; (b) Ce 65; (c) hot water; (d) dichromate.
Figure 9. Macroscopic morphology (5×) of TSA films sealed using different methods after 336 h of immersion: (a) Ce 25; (b) Ce 65; (c) hot water; (d) dichromate.
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Figure 10. Microscopic morphology of TSA films sealed using different methods after 336 h of immersion: (a) Ce 25 (500×); (b) Ce 65 (1000×); (c) hot water (500×); (d) dichromate (1000×).
Figure 10. Microscopic morphology of TSA films sealed using different methods after 336 h of immersion: (a) Ce 25 (500×); (b) Ce 65 (1000×); (c) hot water (500×); (d) dichromate (1000×).
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Table 1. Element composition of 2024 Al alloy.
Table 1. Element composition of 2024 Al alloy.
ElementFeSiMnNiCuTiZnMgAl
Content (wt.%)0.450.500.700.094.700.130.251.75Bal.
Table 2. Sealing solutions and conditions.
Table 2. Sealing solutions and conditions.
Sealing MethodsSolution CompositionTime (min)Temperature (°C)pH
Ce 2540 g/L Ce(NO3)3·6H2O
+ 3 g/L H2O2 (30%)		12025 ± 14~5
Ce 6540 g/L Ce(NO3)3·6H2O
+ 3 g/L H2O2 (30%)		12065 ± 14~5
Hot waterDeionized water 4099 ± 16.5~7.5
Dichromate50 g/L K2Cr2O71593 ± 15~6
Table 3. The color change time vs. sealing methods in acidic spot test.
Table 3. The color change time vs. sealing methods in acidic spot test.
Sealing MethodsCe 25Ce 65Hot WaterDichromateUnsealed
Time (min)21.0235.3830.5139.4912.31
Table 4. Fitting results of the EIS for TSA films sealed using different methods.
Table 4. Fitting results of the EIS for TSA films sealed using different methods.
Sealing MethodsRp
(MΩ·cm2)		CPEpRb
(MΩ·cm2)		CPEb
Y (μS·sn·cm−2)nY (μS·sn·cm−2)n
Ce 250.4501.760.9613.853.500.941
Ce 651.711.710.9425.502.870.969
Hot water0.03140.1150.82310.10.9430.916
Dichromate7.400.8090.97423.50.06280.998
Unsealed0.4021.420.9151.1110.50.902
Table 5. Fitting results of the polarization curves in Figure 8.
Table 5. Fitting results of the polarization curves in Figure 8.
Sealing MethodsCe 25Ce 65Hot WaterDichromate
Ecorr (V)1 h−0.603−0.601−0.581−0.705
336 h−0.802−0.593−0.840−0.784
Icorr (A/cm2)1 h4.25 × 10−94.06 × 10−93.51 × 10−95.16 × 10−10
336 h1.14 × 10−75.03 × 10−85.27 × 10−86.17 × 10−10
Epit (V)1 h−0.581−0.527−0.519−0.513
336 h−0.781−0.576−0.775−0.446
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MDPI and ACS Style

Wang, C.; Sun, S.; Ling, Y.; Tan, H.; He, C. The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods. Coatings 2024, 14, 733. https://doi.org/10.3390/coatings14060733

AMA Style

Wang C, Sun S, Ling Y, Tan H, He C. The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods. Coatings. 2024; 14(6):733. https://doi.org/10.3390/coatings14060733

Chicago/Turabian Style

Wang, Chao, Shineng Sun, Yunhe Ling, Haifeng Tan, and Chunlin He. 2024. "The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods" Coatings 14, no. 6: 733. https://doi.org/10.3390/coatings14060733

APA Style

Wang, C., Sun, S., Ling, Y., Tan, H., & He, C. (2024). The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods. Coatings, 14(6), 733. https://doi.org/10.3390/coatings14060733

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