High-Temperature Oxidation Properties of Micro-arc Oxidation Film on 2A12 Aluminum Alloy

Abstract

The high-temperature oxidation experiments were conducted on the micro-arc oxidation film on 2A12 aluminum alloy surface. Results show that the high-temperature oxidation resistance of the micro-arc oxidation film is decreased with increasing the temperature. However, the oxidation index is above 2, indicating that the micro-arc oxidation film has a protective effect for the 2A12 aluminum alloy and can effectively prevent the oxygen diffusion during high-temperature oxidation. The air-cooled thermal shock resistance property of the micro-arc oxidation film layer is better than its water-cooled thermal shock resistance property. After 60 thermal shock cycles, the film falls off at the corners of alloy after water-cooled thermal shock, while only cracks appear on the surface of alloy after air-cooled thermal shock without film shedding. The failure of micro-arc oxidation film after thermal shock is mainly caused by the difference in coefficients of thermal expansion between film and substrate and the formation of hydroxides and oxides through the reactions of film with H₂O and oxygen, respectively. The CeO₂ inside the film can reduce the pore size and the impact of thermal shock on the film at the initial reaction stage. However, with increasing the thermal shock cycles, the subcarbonate and hydroxide are generated, leading to the failure of micro-arc oxidation film.

Keywords

high-temperature oxidation resistance property; micro-arc oxidation film; thermal shock resistance; rare earth oxide additive

Science Press

Aluminum alloys have been widely used in the aerospace industry, automobile industry, and electronic products, owing to their high specific strength, great machinability, and excel-lent heat and electric conductivity. However, the low hardness and toughness, inferior abrasion and shock resistance, rela-tively high heat conductivity coefficient and thermal deforma-tion, and poor thermal stability all restrict the application of aluminum alloys. In order to apply aluminum alloys on engine manufacture, the preparation of film with high-temperature resistance and thermal shock resistance on the aluminum alloy surface is of great importance.

Micro-arc oxidation (MAO) technique can form a layer of ceramic membrane, which mainly consists of the matrix metal oxides on the metal surface. By regulating the electrolyte and corresponding electrical parameters, the instantaneous high temperature and pressure are generated by the arc strike on the metal matrix and the alloy surface^[1,2]. The generated ceramic membranes usually have better comprehensive mechanical and chemical properties, such as high hardness and excellent corrosion and abrasion resistance. Thus, the high-temperature oxidation resistance and thermal shock resistance of MAO layer were investigated in this research.

Rare earth elements are commonly used in the surface treat-ment to enhance the properties of oxidants or catalysts. Tradi-tionally, the alloy is simply immersed in the rare earth saline solution for MAO film preparation, which leads to few rare earth elements on the matrix surface, i.e., the advantages of rare earth element addition cannot be revealed. Therefore, the surface density and corrosion resistance of this kind of MAO film should be further improved^[3].

In this research, the rare earth oxide CeO₂ was added into the electrolyte for MAO film preparation on the surface of 2A12 aluminum alloy. The mechanisms of high-temperature oxidation and thermal shock resistance of MAO film were investigated to provide guidance for the protection of 2A12 aluminum alloy.

1 Experiment

The chemical composition of 2A12 aluminum alloy is shown in Table 1^[4]. The specimens were uniformly cut into rectangle plates with the size of 25 mm×20 mm×1.5 mm by spark wire cutting. Punch a hole on the specimen to connect the wire. Firstly, the specimen was mechanically polished and then polished with water-based sandpaper from 400# to 1500#. After the polishing process, the ultrasonic cleaner was used to clean the specimen in absolute ethanol for 20 min. Then, a hair dryer was used to dry the specimen. The stainless steel was used as the negative electrode of the experiment. The power supply was the pulse power, and the related parameters are shown in Table 2. The electrolyte was prepared with deionized water, and its composition is shown in Table 3. The low-temperature thermostat was used to control the reaction temperature at 25±5 °C.

Table 1  Chemical composition of 2A12 aluminum alloy^[4] (wt%)

Si	Cu	Mg	Zn	Mn	Ti	Ni	Fe	Al
≤0.50	3.80~4.90	1.20~1.80	≤0.30	0.30~0.90	≤0.15	≤0.10	≤0.50	Bal.

Table 2  Power supply parameters

Positive voltage/V	Negative voltage/V	Positive pulse width/ms	Negative pulse width/ms	Pulse interval/ms	Time/s
300	80	1000	1000	300	900

Table 3  Composition of electrolyte (g‧L^-1)

Na2SiO3	KOH	CeO2
15	5	3

The high-temperature oxidation experiments were conducted in a muffle furnace. Firstly, the ceramic crucible was put into the resistance furnace for preheating, and then the specimens were weighed and put into the crucible. The heating duration was 2, 4, 6, 8, and 10 h. The temperature was kept at 400, 450, and 500 °C. After the specimens were air-cooled to room temperature, the specimen mass was measured to calculate the mass gain. The surface morphology was observed by scanning electron microscope (SEM)^[5].

The thermal shock experiments were conducted under the conditions of air-cooling and water-cooling separately. The reaction occurred in the resistance furnace, and the thermal shock stability was evaluated by the number of thermal shock cycles when the alloy surface was damaged. Under the air-cooling condition, the specimen was put into a preheated ceramic crucible, kept for 5 min, and then taken out. After air-cooling for 3 min, the specimen was placed into the ceramic crucible again. The experiment temperature was 550 °C. Under the water-cooling condition, the specimen was put into a preheated ceramic crucible,kept at 550 °C for 5 min, and then immersed in water for quenching.

2 Results and Discussion

2.1 Surface microstructure after high-temperature oxida-tion

Fig.1 shows SEM surface morphologies of the MAO films after oxidation at 400 °C for different durations. After oxidation at 400 °C for 2 h, no obvious changes can be observed on the MAO film surface, which is mainly attributed to the less severe oxidation condition and uniformly distributed crater-formed pores, whose size is relatively consistent^[6]. With prolonging the oxidation duration to 4 h, a large number of microcracks appear and the surface is obviously rougher than the original surface. However, due to the relatively low oxidation temperature, no obvious propagation of the microcracks occurs. With prolonging the oxidation duration to 6 and 8 h, the cracks on the surface are further increased and the crack width is also increased. With prolonging the oxidation duration to 10 h, the surface of MAO film begins to fracture.

Fig.1  SEM surface morphologies of MAO films after oxidation at 400 °C for 2 h (a), 4 h (b), 6 h (c), 8 h (d), and 10 h (e)

Fig.2 shows SEM surface morphologies of the MAO films after oxidation at 450 °C for different durations. After oxidation at 450 °C for 2 h, the cracks appear on the surface and pass through the pores^[7-9]. The longer the oxidation duration, the larger the crack width. The fracture occurs after oxidation for 10 h^[10].

Fig.2  SEM surface morphologies of MAO films after oxidation at 450 °C for 2 h (a), 4 h (b), 6 h (c), 8 h (d), and 10 h (e)

Fig.3 shows SEM surface morphologies of the MAO films after oxidation at 500 °C for different durations. Because the temperature is higher, the surface morphology of MAO film changes more easily than that oxidized at relatively lower temperature does. It can be seen that the surface changes from white (original surface) to sepia accompanied by the appearance of black spots^[11-13]. With prolonging the oxidization duration, the roughness and the number of black spots on MAO film surface are increased, and the width and length of the microcracks are also increased. This phenomenon results from the sustaining stress during the high-temperature oxidation. Besides, during the air- and water-cooling processes, the air and water enter the film through the pores on the surface and react with the film layer and aluminum alloy matrix to form granular oxide, which grows into lamellar oxide with prolonging the oxidation duration. The stacking occurs when the number of lamellar oxides is increased, resulting in the rougher surface of MAO film. Therefore, reducing the pore size is a key factor to improve the oxidation resistance of MAO film at high temperatures^[14,15].

Fig.3  SEM surface morphologies of MAO films after oxidation at 500 °C for 2 h (a), 4 h (b), 6 h (c), 8 h (d), and 10 h (e)

By adding CeO₂ particles into the electrolyte, the pore size of the MAO film surface is effectively reduced, thus decreasing the amount of air or water which is in contact with the matrix. With the oxidation proceeding, the rare earth oxide gradually becomes subcarbonate, thereby losing the efficacy^[16]. The related reaction is as follows:

CeO2+CO2+H2O→Ce(CO3)2‧Ce(OH)3

(1)

The rare earth oxide can also react with H₂O to produce hydroxide, as follows:

CeO2+H2O→CeO(OH)

(2)

CeO2+H2O→Ce(OH)3

(3)

The generated subcarbonate and hydroxide accumulate with prolonging the oxidation duration. When the generated products reach a certain amount, the internal thermal stress will form and expand the microcracks. When the oxidation duration reaches 10 h, the surface film layer is separated by microcracks, which is connected to the crater-formed pores. Therefore, the H₂O and CO₂ easily enter the inner layer, the accumulation of lamellar oxides is accelerated, and more rare earth oxides are generated at high temperatures. Due to the internal stress, the MAO film layer falls off.

2.2 Mass gain after high-temperature oxidation and kinetic analysis

Fig.4 illustrates the mass change curves of MAO films after oxidation at different temperatures. It can be seen that after oxidation at 400 °C for 2 h, the mass does not change, which is consistent with the morphology observation results. With prolonging the oxidation duration to 4 h, the mass is increased by 0.000 04 g/cm². The mass gain is 0.000 06 g/cm² when the oxidation duration is 6 h. When the oxidation duration is 8 h, the mass gain is 0.000 04 g/cm². When the oxidation duration is 10 h, the mass declines obviously due to the shedding of the surface film.

Fig.4  Mass change curves of MAO films after oxidation at different temperatures

After oxidation at 450 °C for 2 h, the mass gain is 0.000 02 g/cm². The mass gain is increased with prolonging the oxidation duration, and it tends to be stabilized when the oxidation duration is 6 h. After oxidation for 10 h, the mass is decreased due to the shedding of the film layer.

After oxidation at 500 °C for 2 h, the mass gain is 0.000 03 g/cm². The mass gain is increased with prolonging the oxidation duration, and it tends to be stabilized when the oxidation duration is 6 h. After oxidation for 10 h, the mass is slightly decreased due to the shedding of the film layer.

The oxidation kinetics of MAO aluminum alloy film layer can be expressed by the parabolic velocity equation^[17], as follows:

W2=2Kt

(4)

where W represents oxidation mass gain, K is oxidation rate constant, and t is the oxidation time.

After revising the Wagner's empirical formula of oxidation, Eq.(5) can be obtained, as follows:

Wn=Kt

(5)

where n is the oxidation index.

Thus, the relationship between oxidation mass gain and oxidation time and the kinetic relationship of high-temperature oxidation of MAO film layer can be obtained. If 1<n<2, the oxidation reaction depends not only on the reaction between MAO film and oxygen, but also on the reaction velocity between oxygen and Al^[18]. If n>2, it can be inferred that MAO film layer can efficiently prevent the high-temperature oxidation. Therefore, in order to determine the difficulty degree of oxidation reaction at high temperature and to quantitatively judge the antioxidant performance, Eq.(6) can be obtained, as follows:

K=K0exp(-Q/RT)

(6)

where K₀ is a constant, Q is the oxidation activation energy, R is gas constant, and T is the temperature. The higher the oxidation activation energy, the higher the energy for O diffusion across MAO film surface, i.e., the better the antioxidation performance of MAO film^[19].

Table 4 shows the high-temperature oxidation index of MAO film at different temperatures. All the oxidation indexes are above 2, indicating that MAO films show excellent antioxidation performance at high-temperature, and can effectively prevent the O diffusion.

Table 4  High-temperature oxidation indexes of MAO films at different temperatures

Temperature/°C	Oxidation index, n	Oxidation activation energy, Q/kJ‧mol-1
400	3.2	620.4
450	2.7
500	2.3

2.3 Surface morphology of MAO film after thermal shock

The thermal shock resistance can directly reflect the influence of periodically changing temperature on the binding force between MAO film and matrix. The higher the thermal shock resistance, the higher the stability of MAO film. The thermal shock failure consists of thermal shock fracture and thermal shock damage, which occurs on the MAO surface in instantaneous heating, resulting in the surface crack or crazing after multiple heating-cooling cycles. The thermal shock failure of MAO film is mainly thermal shock damage.

Fig.5 shows SEM microstructures of MAO films after thermal shock cycles with air-cooling and water-cooling. After 12 thermal shock cycles with water-cooling at 500 °C, the black spots appear on MAO film, which are inferred as the oxidation products inside MAO film, and their sizes are basically the same with increasing the thermal shock cycles. MAO film does not fall off at the spot positions. After 30 thermal shock cycles with water-cooling at 500 °C, the cracks appear on MAO film surface due to the thermal stress caused by the difference in the thermal expansion coefficients of MAO film layer and the aluminum alloy matrix. The water enters the surface pores and reacts with the film and matrix, forming the lamellar oxide. With the thermal shock cycles further proceeding, the lamellar oxide multiplies and the stacking phenomenon occurs, which facilitates the crack expansion. Besides, MAO film is sensitive to the thermal stress, which further promotes the formation and growth of the internal microcracks. After 60 thermal shock cycles with water-cooling at 500 °C, MAO film on the matrix corners begins to fall off. More water enters the inner part of MAO film, which directly destroys the integrity of MAO film, resulting in more cracks and fracture. Due to the difference in the thermal expansion coefficients of MAO film layer and the aluminum alloy matrix, the upper and lower layers are damaged and then MAO film peels off. The rare earth oxide CeO₂ inside the film layer reduces the amount of water entering the film layers by greatly decreasing the pores during the initial reaction stage, thus releasing partial stress produced by thermal shock and impeding the formation of cracks to some extent. However, with increasing the number of thermal shock cycles, the stress release is not timely, resulting in the film cracking and damage. Moreover, when the specimen is immersed in water, lots of bubbles are generated when the water boils, which produces a transient cavitation effect on the MAO film, thereby promoting the shedding of MAO film layer.

Fig.5  SEM microstructures of MAO films after thermal shock cycles with air-cooling (a) and water-cooling (b)

MAO films after thermal shock cycles with air-cooling show better performance than the ones with water-cooling do. After 30 thermal shock cycles with air-cooling, small black spots and small cracks appear on MAO film surface, which is caused by the entrance of water and oxygen in the air into the inner layer of MAO film. After 60 thermal shock cycles with air-cooling, the surface cracks are expanded, the number of black spots is increased, and no obvious damage or peeling occurs, indicating that the thermal shock resistance of MAO films after thermal shock cycles with air-cooling is better than that with water-cooling.

2.4 Thermal shock failure models of MAO film

The thermal shock resistance of MAO films is mainly influenced by the difference in thermal expansion coefficients between MAO film and the matrix, phase transformation, oxidation reaction, and thermal corrosion during the high-temperature oxidation. Among them, the difference in thermal expansion coefficients between MAO film and the matrix is the most influential factor, which generates the thermal stress between layers and induces the crack formation. Thus, MAO film peels off along the direction parallel to the aluminum alloy matrix. The film layer and the matrix can react with H₂O and oxygen, continuously producing a series of new substances, such as oxide and hydroxide, with the thermal shock cycles proceeding. When the production accumulates to a certain level, the film cracks and falls off.

Fig.6 shows the schematic diagram of the thermal shock failure models of MAO film. It can be seen that the initial MAO film is closely connected to the aluminum alloy matrix with high bonding strength and the flat uniform surface. After several thermal shock cycles, because the inner layer is more sensitive to the thermal stress, the internal tensile stress and external compressive stress are exerted jointly onto the interface. Meanwhile, H₂O and O₂ enter the inner film layer, and then the corresponding oxides and hydroxides are formed. With the thermal shock cycles further proceeding, the internal stress concentration occurs and the produced oxides and hydroxides are continuously accumulated due to the continuous internal tensile stress and the influence of external pressure. The cracks appear on MAO film surface and then begin to expand. Finally, when the crack extends across the entire surface, MAO film peels off^[20,21].

Fig.6  Schematic diagram of thermal shock failure models of MAO films

3 Conclusions

1) The high-temperature oxidation resistance of the micro-arc oxidation film on 2A12 aluminum alloy is decreased with increasing the temperature. The oxidation indexes are all above 2, indicating that the micro-arc oxidation films have ex-cellent antioxidation performance during the high-temperature oxidation and can effectively prevent the oxygen diffusion.

2) The thermal shock resistance of micro-arc oxidation film after thermal shock cycles with air-cooling is better than that with water-cooling. After 60 thermal shock cycles, the micro-arc oxidation film layer suffering water-cooling on the matrix corners begins to fall off, while that suffering air-cooling only has surface cracks without film layer shedding.

3) The thermal shock failure of the micro-arc oxidation film is mainly caused by the difference in thermal expansion coeffi-cients between the film and the matrix and the formation of oxides and hydroxides on the film due to the reaction with H₂O and O₂. CeO₂ in the film layer reduces the pore size, which effectively reduces the influence of thermal shock on the film layer at the initial stage of thermal shock cycles. However, with the thermal shock cycles further proceeding, the subcarbonate and hydroxide are generated, leading to the film failure.