Corrosion properties of high-strength nanocrystalline ...

Journal of Alloys and Compounds 707 (2017) 63e67

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Corrosion properties of high-strength nanocrystalline Al84Ni7Gd6Co3 alloy produced by hot pressing of metallic glass Z. Wang a, b, *, S. Scudino c, K.G. Prashanth d, J. Eckert d, e a

School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China WPI Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan c IFW Dresden, Institut für Komplexe Materialien, Postfach 270116, D-01171 Dresden, Germany d Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria e €t Leoben, Jahnstraße 12, A-8700 Leoben, Austria Department Materials Physics, Montanuniversita b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 August 2016 Received in revised form 14 October 2016 Accepted 16 November 2016 Available online 16 November 2016

The corrosion properties of the nanostructured Al84Ni7Gd6Co3 alloy produced by hot pressing of the metallic glassy powder, have been investigated. The results reveal that the nanostructure Al84Ni7Gd6Co3 alloy shows different corrosion behavior compared to the pure aluminum, because of the presence of high volume fraction of Al-based intermetallic compounds. The corrosion resistance of Al84Ni7Gd6Co3 alloy is significantly influenced by the amount of aluminum and porosity in the inter-particle area. The main corrosion mechanisms observed in the Al84Ni7Gd6Co3 alloy are found to be the pitting corrosion (corrosion of aluminum in the consolidated particles) and crevice corrosion (corrosion of aluminum in the inter-particle area). © 2016 Elsevier B.V. All rights reserved.

Keywords: Aluminum alloys Pitting corrosion Crevice corrosion Nano-crystalline alloy Metallic glasses

1. Introduction For many years, high strength aluminium alloys, in the form of amorphous and/or nanostructure, have been developed by rapid solidification and powder metallurgy [1e5]. Compared with the traditional aluminium alloys, the nanostructured/amorphous aluminum alloys show extremely high strength but limited plasticity. The extremely high strength can promote the extensive use of aluminum alloys in structural, automotive and aerospace sectors. There has been a great interest in the production of bulk high strength aluminium alloys, since the amorphous and nanostructure aluminum alloys which were produced by rapid solidification, have restrictions in the maximum sizes that can be produced [5e7]. The evaluation of the corrosion properties of the aluminum alloys also attracts the attention, since some of the applications involve their exposure to acidic environments [8e13]. For instance, many of the industrial processes like manufacturing of nitric acid-based explosive, catalytic decomposition of ammonia, processing of ammonium nitrate, etc. uses aluminum alloys in one way or the

* Corresponding author. School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.11.212 0925-8388/© 2016 Elsevier B.V. All rights reserved.

other and they are frequently exposed to or gets in contact with the acid [12e17]. Pure aluminum exhibits excellent corrosion resistance in oxidizing environments due to the formation of a protective oxide film; however, aluminum suffers a severe corrosion in nitric acid with a corrosion rate in the order of 4.0 mm/y in the concentration range of 20e40% at room temperature [12,13]. Therefore, it is of great interest to study the corrosion resistance of the amorphous and nanostructure aluminium alloys. To our best knowledge, the study of corrosion properties of the amorphous and nanostructure aluminium alloys is very limited. Recently, we have developed a nanostructured Al84Ni7Gd6Co3 alloy by hot pressing of metallic glassy powder, which shows a superhigh strength of ~1.77 GPa in compression [1]. The selection of Al84Ni7Gd6Co3 composition, firstly, is because Al-rare earth metaltransition metal alloys have high glass-forming ability so that amorphous structure can be obtained [2,18,19]; secondly, Ni, Gd and Co elements have negligible solubility in Al therefore several intermetallics can be formed during the subsequent crystallization process [1,20]. In this work, we have studied the corrosion properties of the bulk Al84Ni7Gd6Co3 alloy samples in 0.1 M/1 M nitric acid at room temperature. For comparison, the corrosion resistance of pure aluminum samples, produced using the same conditions as the bulk Al84Ni7Gd6Co3 alloy, is also shown.

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Z. Wang et al. / Journal of Alloys and Compounds 707 (2017) 63e67

2. Experimental The bulk Al84Ni7Gd6Co3 samples from the gas-atomized (marked as HP0) and milled 100 h powders (marked as HP100) were obtained by uniaxial hot pressing. The detailed milling process and the microstructure evolution during milling has been reported elsewhere [20]. Hot pressing was performed using an electro-hydraulic universal axial pressing machine (WEBER PWV 30 EDS, Germany) with a capacity of 350 kN maximum load. In order to minimize the frictional effects during hot pressing, all the parts (i.e. compaction die and punches) were cleaned and sprayed with a thin layer of boron nitride. Approximately 2e3.5 g of powder was first placed in a die of 10 mm diameter and then preloaded to 20 kN. The chamber was evacuated to about 1 104 Pa in order to minimize the possible oxygen contamination during the hot pressing process. The desired pressure (446 or 637 MPa) was applied and subsequently the samples were heated to 773 K using an inductive coil. The hot pressing is performed for 3 min, once the desired hot pressing temperature is reached. The samples with the following dimension 3 mm 3 mm 2 mm were used for the weight-loss corrosion experiments. The samples were polished using SiC paper from 400 down to 4000 grit and subsequently polished using 3 mm and 0.25 mm diamond suspensions. The samples were cleaned with ethanol and the initial weight as well as the dimensions of the samples were measured. The samples were then immersed in 0.1 M and 1 M HNO3 solutions. The samples were taken out of the acidic solution every 24 h, subsequently rinsed with distilled water and dried in hot air. The samples were then weighted, and re-immersed in the acidic solution again for additional 24 h. This cycle was repeated for a total of 14 days. The corrosion products were not removed intentionally at any of the intermediate stages. They were removed only after the samples was examined using the microscope after 14 days. A Mettler Toledo AX205 analytical balance with the smallest increment of 0.01 mg was employed for all the weight measurements. Three independent trials were conducted for each condition and the results presented here are the average of the three data points. The surface of the samples after immersion corrosion tests were characterized by scanning electron microscopy (SEM) using a Gemini 1530 microscope. 3. Results and discussion It has been found that the morphology of the gas atomized Al84Ni7Gd6Co3 powders change significantly after 100 h milling, where the average diameter of the powder particles increase from ~4 mm to ~36 mm after milling. Accordingly, the HP0 (gas atomized powder e hot pressed) and HP100 (100 h milled powder e hot pressed) samples show different microstructure, where the amount of inter-particle areas in HP100 sample are drastically reduced compared to the HP0 sample (Fig. 1), because of the significant changes in the size of the particles before and after milling. Composition analysis reveals that the inter-particle areas observed in the HP0 and HP100 samples are rich in Al [1,21]. According the SEM observations, both HP0 and HP100 samples exhibit high volume fraction of nano-scale intermetallic compounds, homogenously distributed in the fcc-Al matrix, where the detailed characterization of the Al84Ni7Gd6Co3 samples has been reported elsewhere [1]. Fig. 2 displays the weight loss plots (expressed in mg/cm2) as a function of the immersion time (1e14 days). The weight loss increases with increasing corrosion time for all the three sample categories HP0, HP100 and pure aluminum. For 0.1 M HNO3, HP0 sample displays weight loss of 0.36 mg/cm2 after one day and 5.17 mg/cm2 after 14 days (Fig. 2a). After corrosion for 14 days, the

weight loss is 5.17, 7.20 and 8.27 mg/cm2 for HP0, pure aluminum and HP100 samples, respectively. Among the different samples, HP0 sample exhibits better corrosion resistance than the pure aluminum and HP100 exhibits the highest corrosion rate among these three samples. For 1 M HNO3, the samples show similar trend with HP0 exhibiting the best corrosion resistance and HP100 worst corrosion resistance (Fig. 2b). The weight loss curves show a nearly linear behavior, thus the linear fitting was performed for all curves in Fig. 2. For 0.1 M HNO3, the slope for HP0 sample, aluminum and HP100 sample in Fig. 2a is 0.33, 0.5 and 0.54, respectively. For 1 M HNO3, the slope for HP0 sample, aluminum and HP100 sample in Fig. 2b is 2.9, 4.4 and 5.9, respectively. The slope of the weight loss curves of HP100 sample is almost 2 times higher than the one of HP0 sample, confirming that the HP0 sample exhibits better corrosion resistance than the HP100 sample. The corrosion resistance of the Al84Ni7Gd6Co3 samples hot pressed at different pressures was also studied. Fig. 2c shows the weight loss plots of HP0 samples hot pressed at 446 MPa and 637 MPa, respectively. It is evident that the samples hot pressed at higher pressure exhibit lower weight loss. For example, HP0 sample displays weight loss of 10.3 mg/cm2 at 446 MPa and almost half (5.1 mg/cm2) at 637 MPa after 14 days. After linear fitting of the weight loss curves, HP0 sample has the slope of 0.73 at 446 MPa and 0.33 at 637 MPa, indicating significant improvement on the corrosion resistance of Al84Ni7Gd6Co3 samples by increasing the pressure during consolidation. However, for the HP100 samples, the pressure during consolidation do not have a significant influence in altering the corrosion resistance compared to the HP0 samples, where the slope of the weight loss curves for HP100 sample is 0.58 at 446 MPa and 0.54 at 637 MPa, respectively. Fig. 3 shows the corroded surfaces of the HP0 samples after immersion in 0.1 M HNO3. The corroded surfaces of the HP0 samples display two different corrosion behaviors at the inter-particle area and matrix area of the consolidated particles. Most of the inter-particle areas on the exposed surface were corroded, revealing that the aluminum in the inter-particle area is susceptible to corrosion (crevice corrosion). The remaining phases in the exposed surface of the matrix area are mostly the bright phases which are corresponding to the intermetallic compounds. Most of the dark aluminum areas are corroded out, indicating aluminum is the more susceptible to corrosion than the intermetallic phases. Similar pitting corrosion behavior is observed in the selective laser melted Al-based (Al-12Si) alloys [13]. Fig. 4 shows the corroded surfaces of the HP100 samples after immersion in 0.1 M HNO3. In contrast to the HP0 samples, the corrosion of aluminum in the inter-particle area becomes a serious problem, which show many corroded crevices in the inter-particle areas (Fig. 4(aeb)). With increasing exposure time in the acid solution, the inter-particle areas were severely corroded, resulting in de-cohesion of the consolidated particles (Fig. 4c). Meanwhile, the pitting corrosion leads to many pits on the surface of the consolidated particles (Fig. 4d). Fig. 4c shows the corrosion chimneys due to the deep corrosion of aluminum in the inter-particle areas. From the above results, the Al84Ni7Gd6Co3 alloy shows two typical corrosion behaviors, pitting corrosion on the surface of the consolidated particles and crevice corrosion in the inter-particle areas. The concentrations of HNO3 solution used in the present work (0.1 and 1 M) correspond to PH values of 1 and 2, respectively. According to the Pourbaix diagrams, aluminum will dissolve as Alþ3 in acidic conditions at these low PH levels. The microstructure of the Al84Ni7Gd6Co3 alloy has aluminum surrounded by the high volume fraction of intermetallic compounds, such as Al19Gd3Ni5, Al3Gd and Al9Co2 as described in detail in the reference [1]. The local structure leads to galvanic corrosion in the acid solution, where the intermetallic compounds are less active compared to the


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Fig. 1. Optical microscopy images of the (a) HP0 and (b) HP100 samples (b is adapted from Ref. [1]).

Fig. 2. Weight loss as a function of the corrosion time (a) HP0, HP100 and pure aluminum at 0.01 M HNO3 (b) HP0, HP100 and pure aluminum at 0.01 M HNO3 (c) HP0 produced as a function of applied load and (d) HP100 produced as a function of applied load.

aluminum and hence, can act as cathodic sites. This explains why, the nano-scale aluminum isolated by the intermetallic compounds is susceptible to pitting corrosion. In addition, the intermetallic particles can be etched out, resulting from the corrosion of aluminum surrounding the intermetallic particles. HP0 samples show better corrosion resistance than the pure aluminum which may be due to pitting corrosion of aluminum is limited due to the existence of the high volume fraction of intermetallic compounds (>50 vol%). The crevice corrosion is observed in the inter-particle areas of HP100 alloy, whereas the HP0 alloy shows limited crevice corrosion

because of the much higher volume fraction of inter-particle area exists in HP0 alloy than HP100 alloy (Fig. 1). It is known that the corrosion mechanism of aluminum is Dissolution-Precipitation Mechanism. The crevice corrosion is proved to be selfaccelerating, because the oxygen inside the crevice is consumed during corrosion and the crevice becomes more acidic via the precipitation reaction (Alþ3þ3H2O/3HþþAl(OH)3Y) [8]. The more hydrogen cations (Hþ) results in lower PH value and accelerate the corrosion rate in the inter-particle areas. Subsequently, the consolidated particles will be corroded out due to the severely crevice corrosion. Therefore, although pitting corrosion can be

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Fig. 3. Scanning electron micrographs of the HP0 samples surface after exposed to 0.1 M HNO3 solution at room temperature: for the 637 MPa hot pressed samples (a) 1 days and (bec) 14 and for 446 MPa hot pressed samples (d) for 14 days.

Fig. 4. Scanning electron micrographs of the surface of HP100 samples after exposure to 0.1 M HNO3 solution at room temperature: for (a) and (b) 1 days, and (c) and (d) 14 days which were produced at 637 MPa.

observed on the surface of the consolidated particles, the crevice corrosion of aluminum in the inter-particle area plays a major role on the corrosion resistance of HP100 alloy, which leads to the corrosion resistance worsen than HP0 alloy. By comparing the corrosion resistance between HP0 and HP100 samples, it can be observed that sufficiently high amount of aluminum is necessary in the inter-particle areas to limit the crevice corrosion and in turn to improve the corrosion resistance of the nanostructured aluminum alloy. Furthermore, HP0 samples hot pressed at lower pressure, show lower corrosion resistance (Fig. 2c). The relatively low dense result

in the presence of pores in the inter-particle area and acts as preferential sites for pitting to happen. This results in de-cohesion of the consolidated particles (Fig. 3d). The corrosion mechanism of pitting corrosion in the pores is similar with the crevice corrosion, therefore, the corrosion resistance of HP100 samples hot pressed at 637 MPa and 446 MPa is similar (Fig. 2d).

4. Conclusions We have studied the corrosion properties of the high strength nanostructured Al84Ni7Gd6Co3 alloy in 0.1 M/1 M HNO3 at room


temperature. The main corrosion mechanisms of these nanostructured aluminum alloys are pitting corrosion (along consolidated particles) and crevice corrosion (in the inter-particle areas). The crevice corrosion proceeds in a self-propagating fashion which severely deteriorates the corrosion resistance of the Al84Ni7Gd6Co3 alloy by causing the particle de-cohesion. A sufficient high amount of aluminum existing in the inter-particle can limit the crevice corrosion in the inter-particle area. The pores in the inter-particle area can severely decrease the corrosion resistance of the Al84Ni7Gd6Co3 alloy owing to the pitting corrosion preferentially occurred near the pores. When the crevice corrosion and pitting corrosion of aluminum in the inter-particle area was limited, the Al84Ni7Gd6Co3 alloy shows better corrosion resistance than pure aluminum owing to the existence of the high volume fraction of intermetallic compounds. It can also be observed that HP0 sample shows better corrosion resistance than pure aluminum and HP100, whereas HP100 shows the least corrosion resistance among the three. Acknowledgements Support from JSPS KAKENHI Grants: Number #16K18253 and the World Premier International Research Center Initiative (WPI) of MEXT, Japan is gratefully acknowledged. References [1] Z. Wang, R.T. Qu, S. Scudino, B.A. Sun, K.G. Prashanth, D.V. Louzguine-Luzgin, M.W. Chen, Z.F. Zhang, J. Eckert, Hybrid nanostructured aluminum alloy with super-high strength, NPG Asia Mater. 7 (2015) e229. [2] A. Inoue, Amorphous, nanoquasicrystalline and nanocrystalline alloys in Albased systems, Prog. Mater. Sci. 43 (1998) 365e520. [3] Y. Li, K. Georgarakis, S. Pang, J. Antonowicz, F. Charlot, A. LeMoulec, T. Zhang, A.R. Yavari, AlNiY chill-zone alloys with good mechanical properties, J. Alloy Compd. 477 (2009) 346e349. [4] K. Song, X. Bian, J. Guo, S. Wang, B.A. Sun, X. Li, C. Wang, Effects of Ce and Mm additions on the glass forming ability of AleNieSi metallic glass alloys, J. Alloy Compd. 440 (2007) L8eL12. [5] W.H. Wang, B.A. Sun, M.X. Pan, D.Q. Zhao, X.K. Xi, M.T. Sandor, Y. Wu,

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