Scholarly article on topic 'Improvement of pitting corrosion resistance for Al-Cu alloy in sodium chloride solution through equal-channel angular pressing'

Improvement of pitting corrosion resistance for Al-Cu alloy in sodium chloride solution through equal-channel angular pressing Academic research paper on "Materials engineering"

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{"Al-Cu alloy" / "equal-channel angular pressing" / "pitting corrosion resistance" / "ultrafine-grained particle"}

Abstract of research paper on Materials engineering, author of scientific article — Dan SONG, Ai-bin MA, Jing-hua JIANG, Ping-hua LIN, Liu-yan ZHANG

Abstract Significant corrosion resistance improvement was achieved in solid-solution treated (T4) Al-Cu alloy after severe grain refinement through equal-channel angular pressing. The bulk ultrafine-grained Al-Cu alloy with grain sizes of 200-300 nm has higher pitting potential (elevated by about 34 mV, SCE) and lower corrosion current density (decreased by about 3.88 µA/cm2) in polarization tests than the as-T4 alloy, and increased polarization resistance (increased by about 5.7 kΩ·cm2) in electrochemical impendence spectrum tests, along with alleviated corrosion damage in immersion tests. Two factors responds to the improved corrosion resistance of the alloy: the first is the refinement of residual θ-phase (Al2Cu) particles leading to the lower micro-galvanic currents and reduced susceptibilities to pitting corrosion, the second is the mass of strain-induced crystalline defects providing more nucleation sites for the formation of more volume fractions of stable oxide film.

Academic research paper on topic "Improvement of pitting corrosion resistance for Al-Cu alloy in sodium chloride solution through equal-channel angular pressing"

Improvement of pitting corrosion resistance for Al-Cu alloy in sodium chloride solution through equal-channel angular pressing

Dan SONG, Ai-bin MA, Jing-hua JIANG, Ping-hua LIN, Liu-yan ZHANG College of Mechanics and Materials, Hohai University, Nanjing 210098, China Received 2 March 2011; accepted 26 July 2011

Abstract: Significant corrosion resistance improvement was achieved in solid-solution treated (T4) Al-Cu alloy after severe grain refinement through equal-channel angular pressing. The bulk ultrafine-grained Al-Cu alloy with grain sizes of 200-300 nm has higher pitting potential (elevated by about 34 mV, SCE) and lower corrosion current density (decreased by about 3.88 ^A/cm2) in polarization tests than the as-T4 alloy, and increased polarization resistance (increased by about 5.7 kfl-cm2) in electrochemical impendence spectrum tests, along with alleviated corrosion damage in immersion tests. Two factors responds to the improved corrosion resistance of the alloy: the first is the refinement of residual 0-phase (Al2Cu) particles leading to the lower micro-galvanic currents and reduced susceptibilities to pitting corrosion, the second is the mass of strain-induced crystalline defects providing more nucleation sites for the formation of more volume fractions of stable oxide film.

Key words: Al-Cu alloy; equal-channel angular pressing; pitting corrosion resistance; ultrafine-grained particle

1 Introduction

Equal-channel angular pressing (ECAP), one of the most effective methods for fabricating bulk ultrafine-grained (UFG) materials with grain sizes in the range of 10-1 000 nm, has attracted great attention in the last decade [1]. ECAP has a number of advantages compared with traditional metal process technologies [2-3]. Most investigations of UFG materials fabricated by ECAP have been focused on the structural characterization, thermal stability and mechanical properties while there are limited studies on the corrosion behaviors of the ECAP processed (ECAPed) materials [4-7].

Several researches have been performed on the Al-Cu alloy processed by ECAP, while most of them were focused on the texture and mechanical properties [8]. It was found that the tensile strength of Al-0.63%Cu and Al-3.9%Cu (mass fraction, %) were improved from 78 MPa and 128 MPa to 225 MPa and 285 MPa through ECAP process, respectively [9], i.e. ECAP significantly enhanced the mechanical properties of Al-Cu alloy. The corrosion resistance of ECAPed Al-Cu alloy also needs special attention, because the ultrafined-material may be

endowed different corrosion resistances compared to the coarse-grained counterpart. As well known, Al and Al alloys have good passivation ability, but they are inclined to pitting corrosion in Cr-containing solution. In view of the binary structure of Al-Cu alloy, two factors decide its corrosion resistance: one is the corrosion resistance and stability of the oxide film of the alloy, the other is the morphology and distribution of the 8 phase of the alloys which acts as a cathode in the galvanic corrosion [10]. ECAP process will severely change the detailed microstructure of the a(Al) matrix, as well as the morphology and distribution of the 8 phase, and may endow the alloy with changed corrosion resistance. In the present work, the corrosion resistance of ECAPed Al-Cu alloy in the chloride-containing solution was investigated via electrochemical methods, immersion test, surface analysis and microstructure observation techniques. In particular, the effect of deformed microstructure and refined residual 8 phase particles on the corrosion resistance of the UFG Al-Cu alloy was discussed.

2 Experimental

2.1 ECAPed specimen preparation

Foundation item: Project (SBK201020174) supported by the Natural Science Foundation of Jiangsu Province, China; Project (2011B08214) supported by the Fundamental Research Funds for the Central Universities; Project (2009425411) supported by the Natural Science Foundation of Hohai University, China; Project (20101146) supported by the Program for Transformation & Industrialization of Scientific and Technological Achievements in the industrial field by Changzhou City of Jiangsu Province, China; Project (AMM201007) supported by the Opening Project of Jiangsu Key Laboratory of Advanced Metallic Materials, China Corresponding author: Ai-bin MA; Tel/Fax: +86-25-83787239; E-mail: aibin-ma@hhu.edu.cn

The material investigated was cut from cast Al-Cu alloy ingots, which has the chemical composition (mass fraction, %) of Al-5Cu-0.11Fe-0.12Ti. Before ECAP process, the billets with the size of 19.5 mm*19.5 mm* 40 mm were solid-solution treated at 798 K for 16 h, then water quenched at room temperature. The schematic illustration of the ECAP process is shown in Fig. 1, the billets were pressed for 4 passes with a plunger speed of 0.5 mm/s at room temperature. Graphite was used as lubricant to reduce the friction coefficient between the billet and the die inner wall. After each pass, the billets would be inverted, then rotated by 180° in the same direction.

Fig. 1 Schematic illustration of ECAP process

2.2 Microstructure observation

The samples used for optical microstructure observation were cut parallel to the pressing direction, and then etched by keller solution (2.5% HNO3 + 1.5% HCl+1% HF+95% H2O, volume fraction). Transmission electron microscopy (TEM, Tecnai F20, USA) was conducted to observe the grain size, grain boundary and dislocation of the UFG Al-Cu alloy. The thin foils for TEM were parallel to the pressing direction and prepared by ion milling.

2.3 Corrosion tests

The corrosion resistance of the ECAPed Al-Cu alloy was investigated by two methods, namely constant immersion test and electrochemical test. The as-T4 Al-Cu alloy samples were also studied for comparison. All the experiments were conducted in NaCl solution at room temperature, and the solution was obtained via dissolving NaCl (AR grade) in distilled water.

For constant immersion tests, the initial specimens were grinded with emery papers up to 1 000 level and then mechanically polished by diamond paste. The prepared specimens were immersed in the 3.5% (mass fraction) NaCl solution for 3 days. After immersion, their corrosion morphologies were observed by a BX51M optical microscope (Olympus, Japan) and S3400N

scanning electron microscope (Hitachi, Japan).

Electrochemical tests were conducted in the 0.01 mol/L NaCl solution via a Parstat 2273 advanced potentiostat (USA) with the traditional three-electrode system. The system contains a saturated calomel reference electrode and a Pt counter electrode. The electrochemical tests procedure was quite similar to the electrochemical experiments in the Ref. [11]. Herein, all samples were cut from the core of UFG billets perpendicular to the pressing direction by electric discharging machine, and then molded in the epoxy with an exposed surface of 1 cm2. For a good reproducibility, all the samples were polished, cleaned with acetone and dried in warm air, and then pre-immersed in the solution for 15 min to achieve the stable open circuit potential before all the electrochemical tests. Two kinds of electrochemical tests, namely electrochemical impedance spectroscopy test (EIS) and potentiostatic polarization test, were carried out. For good accuracy, the potentiodynamic polarization tests were performed at a relative low scan rate of 0.5 mV/s. The frequency range of EIS tests were from 10 kHz to 10 mHz and the amplitude of sinusoidal potential signal was 10 mV with respect to the open circuit potential.

3 Results

3.1 Microstructures of UFG Al-Cu alloy

Figure 2 shows optical microstructures of the as-T4

Fig. 2 Optical microstructures of Al-Cu alloys: (a) As-T4 sample; (b) UFG sample

and UFG Al-Cu alloy. The typical microstructure of the as-cast Al-Cu alloy is composed of a(Al) matrix and net-like 8 phase. It can be seen from Fig. 2(a) that most of the 8 phase has been dissolved into a(Al) matrix during the solid solution treatment, but there are still some residual 8 phase distributed in the a(Al) matrix. Due to severe plastic deformation during the ECAP process, the morphology of the residual 8 phase was severely changed. The UFG sample after 4 passes ECAP has much finer and uniformly distributed residual 8 phase than the as-T4 sample. The detailed microstructure of the deformed a(Al) matrix of the UFG Al-Cu alloy should be observed by TEM.

Figure 3 shows TEM microstructure of the deformed a(Al) matrix of the UFG Al-Cu alloy. According to the equivalent strain equation in the Ref. [12], the sample stored nearly more than 4.0 equivalent strains, which leads to a substantial reduction in grain size, from the size of 50-100 ^m of the as-T4 alloy to 300-500 nm of the UFG sample in average. The imposed strain leads to not only reducing grain sizes but also forming many crystalline defects, such as grain

boundaries and dislocations. The ultra-fined grains are surrounded by the clear and regular shaped boundaries. The SAED pattern of the UFG Al-Cu alloy presents distinct ring and thus can be characterized as high-angle grain boundaries [13]. The high-angle grain boundary is non-equilibrium grain boundary, which stores a lot of internal energies. The severe strain also creates lots of intragranular dislocations. Although the a(Al) matrix has low stacking fault energy, and dynamic recovery may happen during the ECAP process, there are still mass of dislocation stored due to the low pressing temperature.

3.2 Corrosion resistance in constant immersion testing

Constant immersion test is a direct and illustrative way to detect corrosion behavior of a material. Figure 4 shows optical micro-morphologies of the as-T4 and UFG Al-Cu alloy samples after immersion in 3.5% NaCl solution for 3 days. The main corrosion behavior of the as-T4 and UFG Al-Cu alloy in the NaCl solution is pitting corrosion with the form of passive film breaking. The pits on the as-T4 sample surface are much larger and darker than those of the UFG sample, which indicates that the pits on the as-T4 sample surface have already corroded into the matrix while the pits on the UFG sample are still restricted on the surface. Different to the UFG sample, there is also some irregular corrosion along the coarse residual 8 phase beside the corrosion pits. From the pits number and size, and corroded area of the tested Al-Cu alloy sample, it is easy to find that the UFG

Fig. 3 TEM microstructure (a) of UFG Al-Cu alloy and corresponding SAED pattern (b)

Fig. 4 Optical corrosion morphologies of Al-Cu alloy: (a) As-T4 sample; (b) UFG sample

sample has alleviated corrosion damage than the as-T4 sample. As seen from SEM corrosion morphologies shown in Fig. 5, lots corrosion induced pits and micro-crack can be found on the as-T4 sample surface, while the surface of the UFG sample still keeps complete relatively. The relatively complete surface film of the UFG sample after immersion test can provide better corrosion protection compared to the that of as-T4 one, showing elevated corrosion resistance.

Fig. 5 SEM corrosion morphologies of Al-Cu alloy: (a) As-T4 sample; (b) UFG sample

3.3 Corrosion resistance in electrochemical tests

Electrochemical tests were performed to get accurate electrochemical data of the UFG Al-Cu alloy. ESI test was conducted to study the electrochemical characteristic of the passive film of the Al-Cu alloy in the NaCl solution. Figure 6(a) shows Nyquist plots of the impedance spectra of the UFG Al-Cu alloy in 0.01 mol/L NaCl solution. According to the Ref. [14], passive film forming is the major reaction on the sample surface when the Al and Al alloys were immersed in water solution for the initial period. The passive film is mainly composed of Al2O3-H2O, whose density and stability correlate with the corrosion resistance of Al and Al alloys. From the plots, both samples have a capacitive arc in the middle and low frequencies, and only a little inductive arc (or tail) is found at the end of the plot of the as-T4 sample. This indicates that the passive films on both the sample surfaces are intact, and there are no stable pits happen in the initial immersion stage in the tested NaCl solution. Several references (for example, Ref. [15]) relate this

capacitive arc with metal dissolution in the corrosion process, whose diameter is associated with chargetransfer resistance, i.e., corrosion resistance. The larger diameter of the capacitive arc corresponds to better corrosion resistance. As shown in Fig. 6(a), the diameter of the capacitive arc of the UFG sample is much larger than that of the as-T4 one. This indicates the larger resistance of the passive film on the UFG sample surface, which can provide better corrosion resistance of the UFG Al-Cu alloy against Cl-. Randles circuit is very common for electrochemical systems of metal/oxide layer/ electrolyte, where Rs, Rp and C represent the electrolyte resistance, the charge-transfer resistance of corrosion reaction and the capacitance of oxide film, respectively. The equivalent circuit, as illustrated in Fig. 6(b), was used to fit the experimental Nyquist plots of the as-cast and UFG samples by ZSimpwin commercial software (USA) in order to get Rp—C circuit parameters for the capacitive arc. The Rp value of the UFG sample is 17.5 k^-cm2, much higher than that of the as-cast one (12.8 k^-cm2). From the larger diameter of the capacitive arc and higher fitted Rp value, the resistance of the passive film on the UFG sample surface is much higher than that of the as-T4 one. Better resistance and stability of the passive film will provide improved corrosion resistance for the UFG Al-Cu alloy compared to that of the as-T4 one.

~4 8 12 16 20 Z'/(kfl-cm2) C

(b) _| |_

Fig. 6 Electrochemical characteristic curves of Al-Cu alloy: (a) EIS Nyquist plots; (b) Equivalent circuit

Figure 7 shows potentiodynamic polarization curves of the UFG and as-T4 Al-Cu alloy samples. Both samples have similar polarization curves, indicating similar electrochemical corrosion behavior in the tested NaCl solution. According to Beehencourt's research [16],

the electrochemical corrosion behavior of the Al-Cu alloy in NaCl solution is pitting corrosion. Under the condition of strong polarization of the electrochemical test, Al-Cu alloy suffers pitting corrosion immediately at the very beginning of the anode polarization, and the pitting potential, ^pit, can be characterized by corrosion potential (^corr). Pitting potential can be used to estimate the passivation effect and corrosion resistance of a metal in aggressive environments. The more positive the ^pit value is, the better the pitting corrosion resistance is. The ^pit of the UFG sample (about -443.1 mV, SCE) is much more positive than that of the as-T4 sample (about -477.2 mV, vs SCE). This implies that the UFG Al-Cu alloy has lower susceptibility to pitting corrosion than the as-T4 one. Besides the more positive ^pit value, the UFG sample also shows decreased corrosion current density, Jcorr, which can represent the corrosion rate of the metal in the tested solution. Benefited from the significant larger anodic polarizability and smaller cathodic polarizability, the UFG sample has much smaller Jcorr, value (3.68 ^A/cm2) than the as-T4 sample (7.56 ^A/cm2), and shows lower corrosion rate in polarization test.

Fig. 7 Potentiodynmaic polarization curves of Al-Cu alloy

4 Discussion

As well known, two major factors determine the corrosion resistance of the Al-Cu alloy in aggressive solutions [10]. One is the thickness, density and insulation of the passive film. The passive film on the surface can provide the protection against Cl- penetration. Any methods, that improve the thickness, density and insulation of the passive film, will benefit the corrosion resistance of the Al-Cu alloy. The other is the quantity, size and distribution of the 8 phase. 8 phase acts as a cathode to accelerate the galvanic corrosion of the Al-Cu alloy. From the obtained experiment results, the ECAP fabricated UFG Al-Cu alloy in NaCl solution is still

inclined to pitting corrosion, but its corrosion resistance is obviously improved compared to the as-T4 one. The improvement should be derived from the combined action of the passive film and the 8 phase.

4.1 Effect of ECAP induced crystalline defects on

passive film

Al is an active metal, but it has favorable corrosion resistance in many media due to the compact passive film on the surface. When Al contacts with water solution, a rapid hydrated oxide film (AlOOH, also can be regarded as Al2O3-H2O) will form on the surface. The hydrated oxide film formation process is as follows [17]:

Al+H2O^AlOH+H++e (1)

AlOH+H2O^Al(OH)2+H++e (2)

Al(OH)2^AlOOH+H++e (3)

Then, the total electrode reaction is Al+2H2O^AlOOH+3H++3e

Although this hydrated oxide film has some pores, it has good resistance against ion current. Composed with hydrated oxide film (outside) and nature oxide film (inside), the composite oxide film was formed on the Al surface, which endows Al matrix good passivity in the water solution. When Al contacts with Cl--containing solution, the passive film will be attacked by anodic dissolution reaction. The anodic dissolution products (AlOHCl, AlOHCl2) have good water solubility, and they promote the anodic dissolution of the passive film [14].

As well known, the oxide films or other passive films are prone to nucleate at the crystalline defects, such as grain boundaries and dislocations [18]. From the present TEM observation, ECAP process brings to the UFG Al-Cu alloy not only significant grain refinement but also lots of crystalline defects with high internal energy, such as large fractions of high-angle grain boundaries and dislocations. Those energetic crystalline defects provide the UFG Al-Cu alloy more nucleation sites for passive film forming. When the UFG sample is soaked in the water solution, rapider and more drastic passive film forming reaction will happen, and more volume fractions of oxide film will be obtained on the UFG sample surface. This result can be verified exactly by the elevated ^corr value of the UFG alloy in the polarization curves, since the electrode potential of the tested sample is decided by both the noble oxide films (outer side) and less noble matrix (inner side), and thus more volume fractions of noble oxide film endows the sample elevated ^corr value. The formation of more volume fractions of oxide film were also found in the ECAPed pure Mg [19] and AZ91D Mg alloys in our former researches [20]. But different to the metastable

oxide film obtained on the ECAPed pure Mg and AZ91D Mg alloys, the more volume fractions of oxide film observed on the surface of UFG Al-Cu alloy in this research presented better stability and provided better protection against Cl- compared to that of the as-T4 one. Two reasons can explain the better performance of the enhanced oxide film on the UFG sample. On one hand, the more volume fractions of passive film will cost longer time for Cl- to deteriorate. On the other hand, even if the metastable pits forms in some where, more volume fractions of stable passive film will retard the migration of Cl- and corrosion products, which leads to the increase of ohmic drop of the solution inside the pits and makes easier repassivation of the metastable pits. Similar phenomenon can also be found in the ECAPed UFG Pure Ti [21].

The EIS results are forceful to verify the effect of energetic crystalline defects on the passive film. The passive film formation is the principal reaction in the initial immersion stage, and the anodic dissolution is quite slow. From the single capacitive arc one can know that the passive films on the both sample surfaces are relative integrated, and there are no obvious anodic dissolution. The UFG sample has much larger diameter of capacitive arc and fitted value compared to those of the as-T4 one. Those improved electrochemical characters should be caused by the rapid formation of the more volume fractions of stable oxide film, which is benefited from the large fractions of high-angle grain boundaries and dislocations of the UFG Al matrix.

4.2 Effect of 0 phase refinement on pitting corrosion resistance of Al-Cu alloy

BAUMGARTNER et al [22] indicated that the 8 phase was the major cause for pitting corrosion of the Al-Cu alloy. The quality, size and distribution of the 8-phase particles will greatly affect the corrosion resistance of the Al-Cu alloy. The harmful effect of the 8-phase particles is that, it acts as a cathode to induce pitting corrosion and promotes the anodic dissolution in a(Al) matrix. Benefited from the severe plastic deformation, the morphology and distribution of the second phases will be seriously changed with the grain refinement. Fang's research indicated that the 8 phase of the cast Al-Cu alloy was extremely refined by ECAP, and its distribution was more uniform than that of as-cast one [9]. Herein, mass of 8 phase have already been dissolved into the a(Al) matrix during the solid-solution treatment. The decrease in the volume fraction of the 8 phase can benefit the corrosion resistance of the Al-Cu alloy. Due to the severe plastic deformation, the residual coarse 8 phase was extremely refined into much smaller 8-phase particles and uniformly distributed in the deformed a(Al) matrix during the following multi-pass

ECAP process. The refinement of the residual 8 phase will benefit the improvement of the UFG sample's corrosion resistance. It will reduce the susceptibility of pitting corrosion as "small cathode, large anode" mechanism. The elevated pitting potential of the UFG alloy verifies its effect on enhancing pitting corrosion resistance.

5 Conclusions

1) UFG Al-Cu alloy has a finer grain size of 200-300 nm in comparison with as-T4 sample (50-100 |im), and the residual coarse 8 phase is also refined into much smaller 8-phase particles. The severe plastic deformation has enhanced the pitting corrosion resistance of the UFG sample in NaCl aggressive solution, resulting in larger fitted values in EIS plots, more positive ^pit and lower Jcorr values in potentiodynmaic polarization curves, and obviously alleviated corrosion damage in constant immersion test.

2) Severe strain induced energetic crystalline defects, such as large fractions of high-angle grain boundaries and dislocations are one of the major factors improving the pitting corrosion resistance of the Al-Cu alloy. Mass of crystalline defects in the deformed a(Al) matrix provide the UFG sample more volume fractions of passive film (composite oxide film). This passive film presents better stability in NaCl solution, and enhances the corrosion resistance of the UFG Al-Cu alloy.

3) ECAP induced residual 8-phase refinement is another major factor improving the pitting corrosion resistance of the Al-Cu alloy. The refinement in 8-phase particles reduces the susceptibility to pitting corrosion of the Al-Cu alloy as the "small cathode, large anode" mechanism.

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