Influence of Al11Ce3 Size and Distribution on the Electrochemical Properties of Sonoprocessed Al-10 wt.% Ce Alloy

Solidification of Al–Ce alloys under ultrasonic vibrations (Sonoprocessing) has a significant effect on the refining of Al11Ce3 intermetallic, which plays a key role in controlling the properties of this group of high-performance characteristics. In the current research, a group of as-received Al-10 wt% Ce alloys that were sonoprocessed under different temperatures was characterized. The influence of Al11Ce3 intermetallic size and distribution on the electrochemical properties of these alloys was investigated. Polarization tests in 3.5% NaCl solution were carried out and the recorded Tafel and impedance curves were studied. The corrosion test results were related to the microstructure characteristics as affected by sonoprocessing. Investigation of the as-received samples showed that ultrasonic vibrations broke the long lath-shaped particles of Al11Ce3 and obviously decreased their size and increased their surface area fraction. Sonoprocessing at the optimum temperature, 655 °C, reduced the Al11Ce3 particles size by 90% and enhanced their distribution in the matrix. This in its turn resulted in significant effects on their electrochemical behavior. Polarization tests showed that the corrosion rate of the un-sonoprocessed specimen decreased from ~ 0.00068 to 0.00006 mm/year after processing at the optimum condition (655 °C), and the polarization resistance increased from ~ 71 to 343 kΩ. By increasing the temperature of ultrasonic treatment beyond 655 °C, and the corresponding coarsening of the intermetallic particles, the corrosion rate slowly increased again, and concurrently, the polarization resistance decreased. The size and distribution of the intermetallic particles also influenced the formation of the corrosion pits, where the optimum sample showed shallow pits compared to those observed in the unprocessed specimen. This emphasizes the role of sonoprocessing in controlling the microstructure features and hence the electrochemical properties of Al-10 wt% Ce alloys.


Introduction
Al-Ce alloys have been reported to have the advantage of good high-temperature mechanical properties. 1,2 Having almost zero solubility in aluminum, Ce forms a highly stable intermetallic formulated as Al 11 Ce 3 . This intermetallic is the key factor in the special mechanical properties of these durable alloys. [3][4][5] Moreover, Al-Ce alloys are mainly prepared by metal casting which is an easy and economically competitive manufacturing process. In the first step of the casting process, the microstructure of Al alloys containing intermetallic particles (ex. Al-Si) can be controlled during the melting stage by adding modifying elements (Na, Sr, etc.) 6 which improves the morphology of the intermetallic particles. In the second step, which is the solidification stage, the microstructure features of Al-Ce alloys can be altered by adjusting the cooling rate as reported in the research of Belov 7 or by applying mechanical vibrations 8 or ultrasonic vibrations during the solidification process, which is known as ultrasonic treatment or sonoprocessing. Applying sonoprocessing can be done by introducing a horn into the crucible to generate ultrasonic vibrations while the alloy is in the liquid state. 9 The other way is to place the ultrasonic horn at the bottom of the casting mold to vibrate the melt during the solidification stage. 10 Whatever the method of application, excellent microstructure control, and improved mechanical properties have been reported for the Al alloys containing intermetallic particles after sonoprocessing. 11 Though reducing the size of Al 11 Ce 3 phase in Al-Ce alloys is of great importance in terms of mechanical properties, there are very limited researches that deals with this issue. A noticeable effect on the shape of the intermetallic was observed by Wang et al., 12 where the phase shape became fibrous instead of lamella after subjecting the Al-5%Ce alloy to stirring during the melting process. In a recent work by El-Hadad et al., 13 variety of size and distribution trends of the Al 11 Ce 3 intermetallic particles were obtained after ultrasonic treatment at different temperatures that changed between 645 and 665°C. The optimum temperature at which the finest intermetallic phase was achieved (3 lm) with the best articles distribution was optimized to be 655°C. At the optimum temperature, the best mechanical properties were therefore obtained.
However, in some applications, the mechanical properties of Al-Ce alloys are not of concern. It was recently suggested by Sims et al. 14 that Al-Ce alloys are characterized by their good compatibility with the anodization processes and show adhesion to passivated layer due to the presence of the Al 11 Ce 3 intermetallic, which has the ability to anchor the anodized product to unconverted bulk. Moreover, a research group at AMES Lab, 15 found that most of the binary intermetallic phases in Al-Ce system have high anodic potential which inhibits corrosion attack in severe environments. The most recent research 16 on Al-xCe alloys related the corrosion resistance of the alloys to the type and percentage of the intermetallic particles. However, there is very little research that deals with the issue.
Considering the above discussion, investigating the electrochemical properties of Al-Ce alloys as related to the size and distribution of the intermetallic phase is of great interest. In the current work, the influence of Al 11 Ce 3 size and distribution on the electrochemical properties of sonoprocessed Al-10 wt% Ce alloys was investigated. The alloys prepared in the previous work 13 were used for the study. A relationship between the Al 11 Ce 3 intermetallic characteristics and the electrochemical properties of the alloy was then established.

Characterization of Received Alloys
The investigation materials were received from the previous work published in ref. 13 The Al-10 wt% Ce alloy ingot was prepared by melting in a resistance furnace using pure Al and Ce metals. This ingot was then remelted and poured at different temperatures (645-665°C) relative to the liquidus of the alloy which was *643°C. 13 Pouring was done in a sand mold of sodium silicate-CO 2 . The sonotrode was placed right below the mold as shown in Figure 1. The ultrasonic system with 4.2 KW power and 21.4 kkHz frequency was used. The ultrasonic vibrations were applied right before pouring the melt into the mold cavity for a period of 100 s then the sonoprocessing system was switched off. The melt was then allowed to cool until solidification. Investigation specimens were cut at 10 mm far from the sonotrode position and coded from 1 to 6C as shown in Table 1, where 1C is the reference sample that was processed without ultrasonic vibrations for  comparison. All the samples were ground and polished with alumina paste to be ready for corrosion tests.

Polarization Tests
In this test, a potentiostat of model (Metrohm AUTOLAB PGSTAT302N) was used to evaluate the electrochemical properties of the prepared Al-Ce samples presented in Table 1. All the measured values (volt, current, electrochemical impedance, etc.) were recorded and extracted through NOVA software that fits with the AUTOLAB module. The sample was considered as the working electrode, a platinum electrode was acting as the cathode, and the reference electrode was Ag/AgCl. The area of exposure of the sample was 0.38 cm 2 and the test media was a solution of 3.5% NaCl. In the beginning, the experiment is set, however, does not start till the steady-state open circuit potential (OCP) is confirmed. A scan rate of 0.001 V/s was applied and the tests were all carried out at room temperature. The frequency ranged between 100 kHz and 0.1 Hz with 0.01V amplitude. The Tafel plots electrochemical impedance and bod phase curves were recorded. The corroded samples were then washed with distilled water and photographed using a stereoscope and the percentage of the corroded part was calculated relative to the circle surface area of the test (0.38 cm 2 ). All the tested specimens were investigated by SEM and EDX to understand their corrosion behavior.

Microstructure Investigation Out of the Corroded Area
The micrographs of the specimens out of the corroded area are shown in Figure 2. The quantitative analysis of the intermetallic particle size (length) is also presented in Figure 3 as an average of three micrographs for each condition. The microstructure simply constitutes an aluminum matrix with dispersed Al 11  The results shown in Figure 2 can be understood in the scope of the reported works. 11,17 It is clear from the literature that there is an optimum temperature at which the sonoprocessing achieves the maximum benefit of structure refinement (smallest particle size). When ultrasonic waves are applied at temperatures lower than the optimum temperature (655°C, in the current study), ultrasonic vibrations induce few nucleation sites which encourages the formation of coarse second phase particles. Besides, the high melt viscosity restricts the fragmentation of these intermetallic particles. On the other side, sonoprocessing at temperatures higher than the optimum temperature does not give enough time for the created nuclei to survive and perform their role in growing fine particles. Therefore, the temperature of 655°C was decided to be the optimum based on the intermetallic particle size. Hence the electrochemical properties of the metals and alloys are very related to their microstructure constituents, interesting results of corrosion tests were expected.

Microstructure Inside the Corroded Area
In order to understand the corrosion manner of the current Al-10 wt% Ce alloy samples, the surface inside the corrosion regions was deeply investigated. Figures 4 and 5 show SEM micrographs of the as corroded surfaces after being washed with distilled water and dried. The corroded positions appear as dark black areas (arrowed) in the figure.
Generally, corrosion was more observed on the grain boundaries of a-Al. Also, several corrosion pits were detected which varied in their depth and distribution depending on the specimen condition. It has been reported that there is a galvanic cell between the intermetallic particles and the aluminum matrix. 18 This cell comes from the difference in potential between a/Al and of Al 11 Ce 3 phase, which encourages pitting of the surface at different positions. Considering the current study, fragmenting the intermetallic particles under the influence of ultrasonic vibrations was believed to affect the distribution and size of the pits.
In the magnified micrographs of Figure 5, the corroded area where pitting was observed is emphasized. Contrasting sample 1C to that of 4C and 6C, it is clear that the pits that are formed in both the 1C (reference) and 6C samples were deep while shallow pits were observed in 4C (optimum  condition). It appears that the presence of the elongated lath-like intermetallic particles in the case of samples 1C and 6C, Figure 2a, f, supports the formation of deep pits compared to the fine-well distributed particles that were found at the optimum sonoprocessing condition, 6C, Figure 2d.
Here, it is interesting to note that the Al 11 Ce 3 phase particles anchor the pits, which can be observed clearly in Figure 5f. This phenomenon is very important in anodization applications, where a good adhesion of the anodized layer to the substrate containing Ce can be obtained due to the presence of multiple pits in the  substrate which causes adhesion with the anodized layer. 15 Figures 6 and 7 present the EDX elemental maps of the corroded 1C (reference) and 4C (optimum) samples.   which indicate the impedance of the different samples are also shown in Figure 9. All the recorded values that indicate the electrochemical behavior, corrosion rate, polarization resistance, etc. during the polarization test are all summarized in Table 2.

Evaluation of Electrochemical Properties
Comparing OCP value of sample 1C which was conventionally solidified to the other specimens 2C to 6C that were sonoprocessed, significant differences can be primarily detected. The OCP of the reference alloy, 1C, was -0.75 and then decreased clearly to -0.69 in 4C sample, however, the value of OCP elevated again to -0.78 in 6C, which is an indication that the alloy treated at the optimum condition has less susceptibility to corrosion. The results of polarization tests also showed obvious differences, where the corrosion rate of 1C specimen contrasting to 4C, decreased from 0.00068 to 0.000067 mm/year and the polarization resistance increased from * 71 to 343 kX. However, by increasing the temperature of ultrasonic treatment to 660°C, the corrosion rate increased slowly, and concurrently, the polarization resistance decreased (see Figures 8 and 9). Interestingly, the corrosion rates of 2C and 3C samples (645 and 650°C) were even higher than that of the untreated 1C sample. These results can be explained through particle fragmentation under the influence of ultrasonic vibrations. When the particles fragmented, the number of galvanic cells between the intermetallic particles and the matrix increases, which hence increases the corrosion rate. In case of 2C and 3C samples, the particles were smaller in size than the untreated specimen, 1C, however, they were not well distributed. At the optimum condition, sample 4C showed very small fragmented particles which were expected to increase the corrosion rate, however, the good distribution of these particles at this processing temperature overcome the effect of the increased corrosion cells.
The observed variation in the corrosion rate among the different samples can be also explained in the scope of the changes in microstructural features due to ultrasonic  treatment. It is notable that the refining of Al 11 Ce 3 particles in 1C sample increased gradually by sonoprocessing at 645°C to 655°C, and then the particles started to retain their long lath morphology upon increasing the treatment temperature to 665°C. This means that intermetallic fragmentation is the key factor in controlling the electrochemical properties of Al-10 wt% Ce alloy. This is in agreement with the available literature that discusses different Al-Ce alloys. 16,18

Conclusions
In the current work, the effect of size and distribution of Al 11 Ce 3 intermetallic particles on the electrochemical properties of sonoprocessed Al-10 wt% Ce alloys was investigated. The findings could be summarized as follows: 1. Solidification of Al-10 wt% Ce alloy under ultrasonic vibrations could successfully decrease the size of Al 11 Ce 3 intermetallic by 90 % at the ideal treatment temperature of 655°C. 2. Fragmentation of the intermetallic particles significantly affected the corrosion behavior of the different samples depending on the size of Al 11 Ce 3 intermetallic phase. 3. The corrosion rate of the un-sonoprocessed specimen was 0.00068 compared to 0.00006 mm/year for the sample that was processed at the optimum condition (655°C), and the polarization resistance increased from 71 to 343.8 kX. 4. By increasing the temperature of ultrasonic treatment beyond 655°C, and the corresponding coarsening of the intermetallic particles, the corrosion rate slowly increased again, and concurrently, the polarization resistance decreased. 5. The size and distribution of the intermetallic particles also influenced the formation of the corrosion pits, where the optimum sample showed shallow pits compared to those observed in the unprocessed specimens.