Research on Surface Protection Technology of Aluminum Alloy Heat Sinks in Lithium Battery with Composite Anodic Oxide Film

Aluminum alloy sheets are immersed in a mixed acid electrolyte for anodic oxidation to prepare different types of anodic oxide films. The mass concentration of TiO2 nanoparticles in the electrolyte is appropriately increased, and under the action of liquid flow disturbance, the number of nanoparticles adsorbed on the surface of aluminum alloy specimens increases, which plays a good role in homogenizing current and filling, promoting more uniform film formation and inhibiting local excessive corrosion and dissolution of the anodic oxide film, Gradually generate a composite anodized film with good density. The composite anodized film constructs a protective barrier on the surface of the aluminum alloy heat sink in lithium batteries, which can effectively block corrosive media and has a better protective effect on the heat sink than conventional anodized film and mixed acid anodized film, thereby significantly improving the corrosion resistance of the heat sink.


Introduction
A heat sink is a functional component used to dissipate heat from easily heating components in household appliances, electronic products, and batteries.It can be divided into plate, column, and finned tubes, all of which increase surface area through heat conduction to achieve rapid heat dissipation [1][2][3].The commonly used materials for manufacturing heat sinks are aluminum alloy and copper.Copper has good thermal conductivity but high manufacturing difficulty, resulting in expensive copper heat sinks and limited large-scale applications.Aluminum alloy heat sinks are easy to manufacture and have good heat dissipation performance.They have a high cost performance ratio and are currently widely used.However, the corrosion resistance of aluminum alloy heat sinks is not ideal.Although a natural oxide film is easily formed on the surface of aluminum alloy, the protective effect of this film is limited in humid atmospheric or salt spray environments [4][5].Therefore, it is of great significance to further enhance the corrosion resistance of aluminum alloy heat sinks through surface treatment.
Anodizing is a commonly used surface treatment process for aluminum alloys, commonly using sulfuric acid electrolyte.In recent years, in order to further improve the corrosion resistance of anodized films, researchers have developed mixed acid electrolytes by adding certain organic acids or compounds to sulfuric acid electrolytes [6][7][8], achieving good results.Other studies have shown that adding nanoparticles to the electrolyte can also significantly improve the corrosion resistance of the anodic oxide film.If nanoparticles are added to the mixed acid electrolyte, it is expected to significantly improve the corrosion resistance of the anodic oxide film without reducing the number of processes through subsequent sealing treatment, thereby making the aluminum alloy heat sink better meet the requirements of use.
In view of the rare reports on related research in this area, this article selects 6063 aluminum alloy plates to be immersed in sulfuric acid electrolyte, mixed acid electrolyte, and mixed acid electrolyte containing TiO2 nanoparticles for anodizing experiments.By comparing the microstructure, composition, phase, thickness, and corrosion resistance of different anodizing films, excellent corrosion resistant anodizing films are selected, And further verify the protective effect of the anodic oxide film on the aluminum alloy heat sink used in a certain type of lithium battery.

Experimental Materials and Anodization
Select 6063 aluminum alloy sheet for basic experiments, with a specimen size of 50 mm × 25 mm × 2 mm, followed by polishing, degreasing, activation, cleaning, and drying treatment.Polish using 1000 # and 2000 # metallographic sandpaper, degrease using a mixed solution of sodium hydroxide (45 g/L) and sodium carbonate (12 g/L), heat to 65 ℃, and then soak the test piece for 10 minutes.Activate using 15% volume fraction hydrochloric acid and soak the test piece for 1 minute.Clean with deionized water and assist with ultrasonic oscillation.
Plate type heat sink made of 6063 aluminum alloy material (size 50 mm) × 25 mm × 10 mm) for validation experiments, with the same preprocessing steps as before.
Using MP3002D DC stabilized power supply, with lead plate as cathode, aluminum alloy test piece and heat sink as anode, both immersed in electrolyte for anodizing experiment.Water bath heating controls the electrolyte temperature to remain constant, with a variation range of ± 0.5 ℃, while continuously stirring the electrolyte.Design four sets of experiments using electrolyte composition and process conditions.During the experiment, set the stirring speed and current density to 200 r/min and 2 A/dm 2 respectively, and the oxidation time to 50 minutes.Before each experiment, continuously stir the electrolyte for 2 hours.For ease of expression, the aluminum alloy heat sink after anodizing in sulfuric acid electrolyte is referred to as a conventional anodized heat sink, and the resulting anodized film is referred to as a conventional anodized film; The aluminum alloy heat sink after anodizing in a mixed electrolyte of sulfuric acid and citric acid is called a mixed acid anodized heat sink, and the resulting anodized film is called a mixed acid anodized film; The aluminum alloy heat sink after anodizing in a mixed acid electrolyte containing TiO2 nanoparticles is called a composite anodized heat sink, and the resulting anodized film is called a composite anodized film.

Electrochemical Corrosion Experiment and Salt Spray Corrosion Experiment
Aluminum alloy and different anodized film specimens are used as working electrodes (with an exposed area of 1 cm 2 ), saturated calomel electrodes are used as reference electrodes, platinum sheets are used as auxiliary electrodes, and 3.5% sodium chloride solution is used as the corrosion medium.Simulate electrochemical corrosion experiments using the Parstat2273 electrochemical workstation, and test the electrochemical impedance spectrum after the open circuit potential stabilizes.The starting frequency is 105 Hz, the ending frequency is 10-2 Hz, and the disturbance signal amplitude is 5 mV.By using ZSimpWin software to fit electrochemical impedance spectroscopy test data, the charge transfer resistance (Rct) and low-frequency impedance modulus (|Z| 0.01 Hz) were obtained, and the corrosion resistance of aluminum alloy and different anodic oxide films was analyzed.
The untreated and anodized aluminum alloy heat sink is suspended and placed in the LRHS-108-RY type salt spray test box.A mixed solution of sodium chloride (50 g/L) and copper chloride (0.3 g/L) is used as the corrosion medium, which is atomized by compressed air and continuously sprayed.The salt spray settling amount is 1-2 mL/(80 cm 2 • h).The ambient temperature in the salt spray test box is set at 35 ± 2 ℃, and the experimental period is 120 hours.The protective effect of different anodic oxide films on the surface of the heat sink is analyzed based on the degree of corrosion.Figure 1 shows the microstructure of 6063 aluminum alloy and different anodic oxide coatings.The impedance modulus values of aluminum alloy, conventional anodized film, mixed acid anodized film, and composite anodized film all show an increasing trend with decreasing frequency, but there are differences in their low-frequency impedance modulus values.Research has shown that the low-frequency impedance modulus can reflect the strength of the protective effect of the oxide film or coating layer on the substrate.The low-frequency impedance modulus values of aluminum alloy, conventional anodized film, and mixed acid anodized film are ranked from high to low as follows: mixed acid anodized film>conventional anodized film>aluminum alloy.This indicates that both conventional anodized film and mixed acid anodized film can provide certain protective effects on aluminum alloy, and the protective effect of mixed acid anodized film is better than that of conventional anodized film, which is consistent with the research conclusions drawn by relevant scholars.The low-frequency impedance modulus of the composite anodized film shows a trend of first increasing and then decreasing with the increase of the mass concentration of TiO2 nanoparticles in the electrolyte.When the mass concentration of TiO2 nanoparticles in the electrolyte is 2.5 g/L, the composite anodized film has the highest low-frequency impedance modulus of 7.62 × 103 Ω• cm2, increased by about 4.17 compared to conventional anodized film and mixed acid anodized film, respectively × 103 Ω• cm2, 2.60 × 103 Ω• cm2 further indicates excellent corrosion resistance.

Corrosion Status of different Aluminum Alloy Heat Sinks
Figure 2 shows the original appearance of untreated and anodized aluminum alloy heat sinks.Comparing figures 2(a) to (d), it is found that the original appearance of untreated heat sinks, conventional anodized heat sinks, mixed acid anodized heat sinks, and composite anodized heat sinks are all silver white with no significant difference in glossiness.From the overall appearance and localized magnified appearance of the untreated heat sink, it can be seen that there are large areas of dark brown rust spots on the surface of the ribs, with severe overall corrosion.From the overall appearance and locally enlarged appearance of the conventional anodized heat sink, it can be seen that there are large areas of dark brown rust spots on the surface of the ribs, indicating that the protective effect of the conventional anodized film on the heat sink is not ideal.From the overall appearance and local magnified appearance of the mixed acid anodized heat sink, it can be seen that the surface of the ribs is densely distributed with small dark brown rust spots, and no large areas of rust spots appear.Although the protective effect of mixed acid anodized film on heat sinks is better than that of conventional anodized film, as the salt spray corrosion time prolongs, the corrosive medium will gradually penetrate into the pores and pits of the mixed acid anodized film and accumulate, leading to an increase in the degree of corrosion.From the overall appearance and local magnified appearance of the composite anodized heat sink, it can be seen that there are only a few small dark brown rust spots on the surface of the ribs, and some small rust spots appear at the root of the ribs.
Compared with conventional anodized heat sinks and mixed acid anodized heat sinks, the overall corrosion degree of composite anodized heat sinks is the lightest, which confirms that the protective effect of composite anodized film on heat sinks is better than that of conventional anodized film and mixed acid anodized film.Due to the small number of pores and irregular pits on the surface of the composite anodized film, its uniformity and density are good.In addition, the composite anodized film is relatively thicker.Therefore, a protective barrier is constructed on the surface of the rib of the heat sink, effectively blocking the corrosive medium and increasing the resistance of the corrosion reaction, thereby providing better protection for the heat sink and significantly improving its corrosion resistance.

Conclusion
A composite anodized film with porous structure was prepared on the surface of 6063 aluminum alloy sheet.The mass concentration of TiO2 nanoparticles in the electrolyte has a certain influence on the density, TiO2 particle content, thickness, and corrosion resistance of the composite anodized film.Properly increasing the mass concentration of TiO2 nanoparticles in the electrolyte promotes the formation of a dense and thick composite anodic oxide film, gradually improving corrosion resistance.However, excessive concentration of TiO2 nanoparticles in the electrolyte may exacerbate the tendency of the composite anodic oxide film to excessively corrode and dissolve, leading to a decrease in its density and corrosion resistance.
Adding 2.5 g/L TiO2 nanoparticles to a mixed electrolyte of sulfuric acid and citric acid resulted in the best surface density of the composite anodic oxide film, with a thickness of 16.7 μm.Exhibiting excellent corrosion resistance.The composite anodized film constructs a protective barrier on the surface of the aluminum alloy heat sink in lithium batteries, which can effectively block corrosive media and has a better protective effect on the heat sink than conventional anodized film and mixed acid anodized film, thereby significantly improving the corrosion resistance of the heat sink.

Figure 1 .
Figure 1.Microstructure of 6063 aluminum alloy and different anodized films.

Figure 2 .
Figure 2. Original image of aluminum alloy heat sink after anodizing.