microstructure and corrosion behavior of al ti/adc12...

International Journal of Minerals, Metallurgy and Materials Volume 25, Number 7, July 2018, Page 840 https://doi.org/10.1007/s12613-018-1633-4

Corresponding author: Hong Yan E-mail: [email protected]

© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Microstructure and corrosion behavior of Al3Ti/ADC12 composite

modified with Sr

Bao-biao Yu, Hong Yan, Qing-jie Wu, Zhi Hu, and Fan-hui Chen

School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China

(Received: 12 July 2017; revised: 3 April 2018; accepted: 4 April 2018)

Abstract: In this study, we investigated the effect of the addition of Sr (0wt%, 0.1wt%, 0.2wt%, and 0.3wt%) on the microstructure and cor-rosion behavior of Al3Ti/ADC12 composite by optical microscopy, X-ray diffraction, scanning electron microscopy, and energy diffraction spectroscopy. The results reveal that the α-Al phases were nearly spherical and 40 μm in size and that the eutectic Si phases became round in the composite when the Sr content reached 0.2wt%. The Al3Ti particles were distributed uniformly at the grain boundary. The results of the corrosion examination reveal that the Al3Ti/ADC12 composite exhibited a minimum corrosion rate of 0.081 gm–2h–1 for an Sr content of 0.2wt%, which is two thirds of that of unmodified composite (0.134 gm–2h–1). This improved corrosion resistance was due to galvanic cor-rosion, which resulted from the low area ratio of the cathode to anode regions. This caused a low-density corrosion current in the composite, thereby yielding optimum corrosion resistance.

Keywords: Sr modification; Al3Ti/ADC12 composite; microstructure; corrosion behavior

1. Introduction

With the rapid industrial development around the world, particle-reinforced metal matrix composites (PRMMCs) have attracted the attention of many researchers due to their simple processing, low-cost preparation, and excellent me-chanical properties. Zymański [1] successfully fabricated an Al3Ti/Al composite using an in-situ method and found the mechanical properties of PRMMCs to be superior to those of the entire matrix. Wu et al. [2] and Jiang et al. [3] report-ed that ADC12 reinforced by SiC and Al2O3 particles sig-nificantly improved its tensile strength and hardness. How-ever, improving the corrosion properties of the composite has been proved to be a challenge due to defects such as coarse dendrites and the uneven distribution of the rein-forcements, which substantially restricts their further appli-cation [4–6].

The results from previous studies have indicated that the corrosion property of a composite depends on its micro-structure, particularly on the amount and distribution of the reinforcements and the grain size [7–9]. On the basis of the-

se studies, modification treatments have been widely ap-plied to improve the dispersion of reinforcements and the corrosion resistance and mechanical properties of compo-sites [10–12].

However, most studies on aluminum matrix composites have focused on their mechanical properties. Chen et al. [13] found Al3Ti/2024 Al composites to be significantly strengthened by ultrasonic vibration and the even distribu-tion of Al3Ti particles. The authors reported yield strength, ultimate tensile strength, and elongation-to-fracture values of 361 MPa, 449 MPa, and 3.16%, respectively. Wang et al. [14] reported that the element La significantly improved the me-chanical properties of A356–2.5wt%TiB2 composites, espe-cially their elongation value, which was nearly twice as large as that of the matrix composite. The corrosion behav-ior of aluminum matrix composites with modification treatment has remained largely unexplored despite the fact that corrosion resistance is an important material perfor-mance factor in aluminum matrix composites. Statistically, the annual economic loss due to corrosion and corrosion damage can account for 1.5%–4.2% of a national economy’s

B.B. Yu et al., Microstructure and corrosion behavior of Al3Ti/ADC12 composite modified with Sr 841

annual gross domestic product [15]. Therefore, more atten-tion should be paid to improving the corrosion behavior of aluminum matrix composites.

There have been many studies of the corrosion behavior of aluminum alloys with modification, but few have ad-dressed the corrosion behavior of aluminum matrix compo-sites. These studies have also slight differences in approach. Osório et al. [16–17] investigated the corrosion behavior of Al–Si alloys by electrochemical impedance spectroscopy and polarization tests, whereas Cardinale et al. [12] and Arrabal et al. [18] studied the corrosion properties of Al–Si alloys using the long-term immersion method. In this paper, we investigated the microstructures of Al3Ti/ADC12 com-posites with different Sr contents. To simulate long-term corrosion in actual application environments, we also con-ducted a long-term immersion corrosion experiment.

2. Experimental

2.1. Sample preparation

In this study, we used ADC12 aluminum alloy as raw material, the chemical composition of which is shown in Table 1. We prepared the alloy modification material by drying Na3AlF6 and titanium (Ti) reactant powders in an electrical resistance furnace at 100°C for 1 h, for which the mass ratio of Na3AlF6 to Ti was 1:1. Meanwhile, we melted ADC12 alloy in a graphite crucible at 800°C in an electrical resistance furnace. After the alloy was completely melted, the mixed powder (the amount of Ti powder addition is 3.0wt%) was divided to several parts and added to the ADC12 alloy melt with aluminum foil coated. Then, the melt was maintained isothermally at 800°C for about 10 min to ensure a complete reaction between the alloy and the Ti powder. After the in-situ reaction, the composite melt was cooled to 750°C. The Al–10%Sr master alloys were placed into the melt at 750°C to fabricate composites with various quantities of Sr (0wt%, 0.1wt%, 0.2wt%, 0.3wt%). The melt was held at 820°C for about 10 min to ensure a complete reaction. Finally the melt was poured into the metal pre-heated mold (200°C).

Table 1. Chemical composition of ADC12 alloy wt%

Si Cu Mg Zn Fe Mn Al

9.6 2.2 0.25 0.06 0.8 0.4 Bal.

2.2. Microstructural characterization

First, we ground, polished, cleaned, and dried samples of the cast ingot. Then, we etched the samples using a 0.6% hydrofluoric acid solution and examined the microstructures

of the polished samples with an optical microscope (OM; Nican M200, Nican Tokyo, Japan) and a NOVA NANOSEM 450 scanning electron microscope (SEM; FEI Quanta 200F, FEI Company, Hillsboro, Oregon, and JSM-6701F, JEOL Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS; INCA 250X-MAX 50). We characterized the phases by X-ray diffraction (XRD; OXFORD X-act) analysis.

We used Image Pro-Plus 6.0 software to measure the primary aluminum grain size. To do so, we first determined the area of interest for the grains, and then performed area calibration. We determined the area of the grains using Eq. (1), as follows:

1

4

π

nn

n

S

Dn

(1)

where D, Sn, and n are the mean size, area, and number of grains, respectively. We then calculated the mean grain size. We determined the total area of the α-Al in the OM micro-graph using Eq. (2), as follows:

n

nn

S S

1

(2)

where S is the total area of the α-Al in the OM micrograph and Sn and n are the mean area and number of grains, re-spectively.

2.3. Corrosion tests

Heavier specimens, which showed smaller corroded sur-faces, lost a small amount of weight due to corrosion, which prevented a fully accurate morphological examination. To remedy this situation, in place of the heavier specimens, in our corrosion tests we used 10 mm ×10 mm cylindrical specimens of Al3Ti/ADC12 composites modified with Sr. Prior to the tests, we ground the top surface of the Sr modi-fied Al3Ti/ADC12 composite with 1000-grit SiC abrasive paper and degreased it in ethanol. Then, we immersed the specimens in 1 M HCl solution for 22 h, with the HCl solu-tion concentration of 30 mL/cm2. We then observed and measured the corroded top-surface morphologies and quali-ties at 2 h intervals. In each group of experiments, we per-formed measurements using an electronic balance (Shi-madzu, AUY200; Shimadzu, Tokyo, Japan) and average values represent at least three measurements. Prior to SEM observation, we cleaned the specimens with distilled water for 5 min and allowed them to dry in air. We also used SEM and EDS to observe and analyze the corroded surface mor-phology and corrosion products. We determined the corro-sion rate of each specimen based on its weight loss, under the same set of corrosion parameters. We calculated the av-

842 Int. J. Miner. Metall. Mater., Vol. 25, No. 7, Jul. 2018

erage corrosion rate after the removal of corrosion products in an aqueous mixture of 20 g/L chromic oxide and 50 mL/L phosphoric acid at 80C for 2–3 min. We calculated the corrosion rate using Eqs. (3) and (4), as follows [19]:

1 2W WV

A t

(3)

2ππ

2

DA H D (4)

where V (g·m–2·h–1) is the corrosion rate, W1 (g) is the qual-ity of the specimen before corrosion, W2 (g) is the quality of the specimen after corrosion, A (m2) is the total area of the specimens in contact with the corrosion solution, t (h) is the corrosion time, D (m) is the diameter of the specimen, H (m) is the height of the specimen, and π is the circularity ratio.

3. Experimental results

3.1. Microstructural characterization

Fig. 1 shows our XRD analysis results for the matrix and the Al3Ti/ADC12 composite fabricated without Sr modifica-tion, in which we see that there is an Al3Ti peak as well as diffraction peaks of the elements Al and Si. In addition, Fig.

2(a) shows an SEM image of the Al3Ti/ADC12 composite fabricated by in-situ reaction, in which we can see that a large number of polygon-like phases appear. According to the EDS analysis result shown in Table 2, the main compo-sitions of the polygon-like phase are Al and Ti and the mo-lecular ratio of Al to Ti is about 3:1. Combined with Fig. 1, we can confirm that the Al3Ti/ADC12 composite was suc-cessfully fabricated via the in-situ reaction and that the pol-ygon-like phases were Al3Ti.

Fig. 1. XRD pattern of the Al3Ti/ADC12 composite.

Fig. 2. SEM image (a) and EDS analysis results (b) of the composite material.

Table 2. Compositions of reinforcing particles

Element Mass fraction / wt% Atomic fraction / at%

Al K 52.16 62.24

Si K 11.83 13.56

Ti K 36.01 24.20

Total 100.00 100.00

Fig. 3 shows the optical microstructures of Al3Ti/ADC12 composites prepared with different amounts of Sr. In Fig. 3(a), we can see that when the Sr content was 0wt%, the eu-tectic silicon particles exhibited a long needle-like structure and were irregularly distributed along the boundaries of the α-Al primary phases. We can also see that the Al3Ti particles in the α-Al grain have a block shape. The microstructures of the Al3Ti/ADC12 composites were significantly improved

by increasing the Sr content, as shown in Figs. 3(b) and 3(c). When the Sr content was 0.1wt%, the Al3Ti particles and eutectic silicon were obviously more refined, with their shapes changing from long needle or blocks to short rods. The α-Al primary phases also display a finer structure, as we can see in Fig. 3(b). In Fig. 3(c), when the Sr addition was 0.2wt%, we can see that the morphologies of the α-Al pri-mary phases have approximately spherical and elliptical shapes and the eutectic silicon changed from long sticks into granular crystalline shapes. In addition, the Al3Ti phases were evenly distributed and about 5 μm in size. However, when the Sr content reached 0.3wt%, we can see from the microstructure that the roundness of the α-Al primary phas-es is significantly decreased and the needle-like eutectic sil-icon and plate-like Al3Ti phases reappear (Fig. 3(d)).


Fig. 3. Microstructures of Al3Ti/ADC12 composites: (a) 0wt% Sr; (b) 0.1wt% Sr; (c) 0.2wt% Sr; (d) 0.3wt% Sr.

3.2. Corrosion resistance

To determine the effect of the addition of Sr on the corro-sion resistance of Al3Ti/ADC12 composites, we used the weight loss method to determine the corrosion rate. As we can see in Fig. 4, the immersion time and Sr content had significant effects on the corrosion rate of the composites samples. The corrosion rate increased gradually with incre-ments of immersion time. When the immersion time was 22 h, the values of the corrosion rates of the Al3Ti/ADC12 composites modified with 0wt%, 0.1wt%, 0.2wt%, and 0.3wt% Sr were 0.134, 0.177, 0.073, and 0.116 g·m–2·h–1, respectively, which represent increases of 43.2%, 38.3% and 21.9%, respectively, compared with that of the samples im-mersed for 2 h.

Fig. 4. Corrosion rate, immersion time, and strontium con-tent curves.

Fig. 5 shows an SEM micrograph of the corroded top surface of Al3Ti/ADC12 composites with different Sr con-tents after immersion in 1 M HCl solution for 22 h. We ob-served many corrosion products on the corroded surfaces of the tested samples, which exhibited silvery white phases around the corrosion holes. With increases in the amount of Sr added, the influence of the solution on the formation of corrosion holes decreased, as shown in Fig. 5. In Fig. 5(a), we can see that the Al3Ti/ADC12 composite sample without Sr modification exhibited the largest corrosion with many corrosion holes on the corroded surface. In Figs. 5(b) and 5(c), we can clearly see that the depth of the holes on the surface of the composite modified with 0.1wt% Sr has de-creased. However, the corrosion coverage area remains ba-sically unchanged. When the Sr content reached 0.2wt%, many of the corrosion holes on the surface had nearly dis-appeared and the existing holes had very small diameters (about 15 m) compared with those in the other samples. In Fig. 5(d), we can see that when the Sr content reached 0.3wt%, the number of the corrosion holes increased and the propor-tion of corroded areas on the surface showed an increasing trend. This result indicates that the composite modified with 0.2wt% Sr demonstrated the best corrosion resistance.

4. Discussions

Fig. 6 shows the SEM morphologies and EDS analyses of Al3Ti/ADC12 with 0.2wt% Sr. As we can see, Sr did not


Fig. 5. SEM micrographs of corroded top surface of the Al3Ti/ADC12 composites with different Sr contents after immersion in 1 M HCl solution for 22 h: (a) 0wt%; (b) 0.10wt%; (c) 0.20wt%; (d) 0.30wt%.

Fig. 6. SEM micrographs (a, c) and EDS analyses (b, d) of the matrix (top) and solid–liquid interfaces (bottom).


dissolve into the Al matrix but was present around the Al3Ti phases. This phenomenon leads to supercooling and eventu-ally leads to a refinement of the Al3Ti phases. Moreover, the addition of Sr reinforced the degree of undercooling crystal-lization, which also resulted in the refinement of the Al3Ti phase [20]. Additionally, the presence of Sr improved the chemical potential of α-Al and reinforced the capacity for nucleation and growth during eutectic solidification, which constrained the accumulation of Al3Ti particles. Conse-quently, we can conclude that the microstructure of Al3Ti/ADC12 composite can be optimized effectively by the Sr modification method.

Table 3 shows that the open circuit potentials of Si, Al3Ti, Al2Cu and α-Al are 0.26, 0.485, 0.621 and 0.85 V, re-spectively [21–24]. As we know, these composites are mainly composed of α-Al, eutectic Si, and small amounts of Al2Cu and Al3Ti particles. The value of the open circuit po-tential of α-Al is smallest, in contrast. Therefore, in the cor-rosion test, the cathodes are the phases of Si, Al3Ti, and

Al2Cu and the anode is α-Al, due to the differences in the open circuit potential.

Table 3. Open circuit potentials of intermetallic phases

Chemical compo-sition

Open circuit potential vs. SCE / V

References

Si −0.26 [21]

Al3Ti −0.485 [22]

Al2Cu −0.621 [23]

ɑ-Al −0.85 [24]

Note: SCE—Saturated calomel electrode.

Fig. 7 shows an SEM image and EDS analyses of the corrosion residue of the Al3Ti/ADC12 composites modified with 0.1wt% Sr. We can see that the corrosion residue is the incompletely refined dendritic eutectic silicon phase. This phenomenon further substantiates our finding that the cath-ode was composed mainly of eutectic silicon in the galvanic corrosion when the Al3Ti/ADC12 composites were im-mersed in HCl solution.

Galvanic corrosion is generally related to the anode and cathode areas of alloys. The surface area of the anode re-duces or that of the cathode increases, which causes an ac-celeration of the corrosion rate of the anode. The galvan-ic-corrosion anode current is always equal to the cathode

current. Therefore, the smaller the anode area, the higher is the current density in the solution, which results in a greater corrosion rate of the anode. This is commonly known as ‘large cathode–small anode’ galvanic corrosion [25], a schematic diagram of which is shown in Fig. 8.

Fig. 7. Top surface (a) and EDS (b, c) analyses of Al3Ti/ADC12 composite modi-fied by 0.1wt% Sr after corrosion for 22 h.


Fig. 8. Schematic diagram of ‘large cathode–small anode’ galvanic corrosion.

I i S 1 1 1 (5)

2 2 2I i S (6)

Where i2 is the cathode current density, i1 is the anode cur-rent density, S2 is the cathode zone area, S1 is the anode zone area, I2 is the cathode current, and I1 is the anode current. The galvanic current can be expressed as shown in Eq. (7).

2 2 1 1i S i S (7)

The cathode current density is restricted by the limiting diffusion current density (iL), and the galvanic corrosion is controlled by the H+ diffusion in the solution. As such, the cathode current density (i2) is as follows:

2 Li i (8)

and the anode current density (i1) is as follows:

21 L

1

Si i

S (9)

In fact, the limiting diffusion current density iL is one of the constants in the corrosion solution. Hence, the galvanic current density is proportional to the area ratio (S2/S1) in ac-cordance with Eq. (9), whereby a higher area ratio produces more severe galvanic corrosion and results in a faster corro-sion rate.

When the Al3Ti/ADC12 composite is modified with Sr, the cathode and anode areas reveal a significant difference, as we can see in Table 4 and Fig. 9. As we know from our calculation, the value of the area ratio tends to decrease with increasing Sr content. When the Sr content was 0.2wt%, the area ratio of the composite and the corrosion rate were at their lowest.

As discussed above, the galvanic current density is pro-portional to the area ratio. When the Sr content was 0.2wt%, the Al3Ti/ADC12 composite showed the best corrosion re-sistance and the lowest corrosion rate.

Table 4. Cathode and anode areas of the Al3Ti/ADC12 com-posites with various Sr contents

Sr content/ wt% Scathode / μm2 Sanode / μm2 Scathode / Sanode

0 56520 46711 1.21

0.1 62789 40442 1.55

0.2 48043 55188 0.87

0.3 53361 49870 1.07

Fig. 9. Anode areas of Al3Ti/ADC12 composites with different Sr contents (the marked parts of the image are the anode region): (a) 0wt%; (b) 0.1wt%; (c) 0.2wt%; (d) 0.3wt%.


5. Conclusions

(1) When the Sr content of the Al3Ti/ADC12 composite was 0.2wt%, the α-Al primal phases exhibited nearly spher-ical and elliptical shapes with about 40 μm in size and the eutectic Si phase was refined from long needle-like particles into round particles.

(2) When the Sr-modified alloys were immersed in 1 M HCl corrosion solution, after 22 h, the corrosion rates of the 0.2wt% Sr Al3Ti/ADC12 composite and the unmodified Al3Ti/ADC12 composite were 0.081 and 0.134 g·m–2·h–1, respectively. The corrosion resistance of the 0.2wt% Sr Al3Ti/ADC12 composite was the best. The corrosion rate of the Al3Ti/ADC12 composite modified with 0.2wt% Sr was about two thirds of that of the unmodified composite.

(3) The results obtained from the immersion test indi-cate that the corrosion resistance was improved by the ad-dition of Sr. During the corrosion process, the corrosion current direction originated from a small area of the anode region to a large area of the cathode region, and the corro-sion rate of the composite was proportional to the area ratio (Scathode/Sandoe). In other words, the smaller the area ratio, the lower is the degree of corrosion. The significant reductions in the surface areas of the cathode and significant increases in those of the andoe were caused by the addition of the el-ement Sr, resulting in the eventual improvement of the cor-rosion resistance of the composite. The Al3Ti/ADC12 com-posite modified with the 0.2wt% Sr exhibited the best cor-rosion rate with the lowest area ratio (Scathode/Sandoe = 0.87).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51364035), and by the Natural Science Foundation of Jiangxi Province (No. 20171BAB206034).

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