Characterization of pitting corrosion of 7A60 aluminum ... pitting resistance of 7150 alloy can be improved

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  • Trans. Nonferrous Met. Soc. China 24(2014) 3907−3916

    Characterization of pitting corrosion of

    7A60 aluminum alloy by EN and EIS techniques

    Xue-hui WANG1,2, Ji-hui WANG1,2, Cong-wei FU2

    1. State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin 300072, China; 2. Tianjin Key Laboratory of Composite and Functional Materials,

    School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

    Received 17 October 2013; accepted 17 November 2014

    Abstract: The pitting corrosion behaviors of 7A60 aluminum alloy in the retrogression and re-aging (RRA) temper were investigated by electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques, and the microstructure and the second phase content of the alloy were observed and determined by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results show that there exist two different corrosion stages for 7A60 alloy in 3.5% NaCl solution, and the corrosion process can be detected by the appearance of EIS spectrum with two capacitive time constants and the wavelet fractal dimension D extracted from EN. SEM and EDS results also demonstrate that severe pitting corrosion in 7A60 alloy is mainly caused by electrochemical active MgZn2 particles, secondly by Al2MgCu and Mg2Si. Al7Cu2Fe particles make little contribution to the pitting corrosion of 7A60 alloy. Key words: 7A60 aluminum alloy; constituent particles; pitting corrosion; electrochemical impedance spectroscopy; electrochemical noise; wavelet fractal dimension 1 Introduction

    7xxx series aluminum alloys are widely used in military and aerospace industries for their low density and favorable mechanical properties [1]. However, these alloys are often suffered from pitting corrosion and stress corrosion cracking (SCC) when subjected to aggressive environments such as salt water spray and/or salt fog [2,3]. It has been reported that a superior balance of SCC resistance and strength can be obtained by retrogression and re-aging (RRA) treatment [4−6]. And our previous study has also proved that the SCC resistance of 7A60 aluminum alloy in 3.5% NaCl solution is improved by RRA treatment [7].

    Al−Zn−Mg−Cu alloys contain numerous constituent particles with electrochemical potentials different from those of the matrix, then corrosion pits can readily develop in these particles [8,9]. The initiation of SCC cracks can be basically associated with intensive localized corrosion around constituent particles [10,11].

    The electrochemical characteristics of intermetallic phases in Al−Zn−Mg−Cu series alloys have been studied by BIRBILIS and BUCHHEIT [12]. It is concluded that Al7Cu2Fe and Al2Cu phases are noble particles while MgZn2 phase is active particle with high self-dissolution rates. Nevertheless, the dealloying and incongruent dissolution of Al2CuMg particles may lead to polarity reversal. The pitting behaviors of 7150 and 7075 alloys under solution heat treatment and laser surface treatment have been discussed [9,10]. It is demonstrated that the pitting resistance of 7150 alloy can be improved due to the removal of active constituent particles such as η(MgZn2) and S(Al2MgCu) phases.

    7A60 alloy is a new developed ultra-high strength aluminum alloy, and its SCC behaviors in RRA tempers have been studied [7]. In this work, the pitting corrosion behavior of 7A60 alloy in the RRA temper was investigated by using electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) methods, and their corroded micrographs were observed and determined by scanning electron microscopy (SEM)

    Foundation item: Project (13JCZDJC29500) supported by the Key Project of Tianjin Natural Science Foundation, China; Projects (2011CB610505,

    2014CB046801) supported by the National Basic Research Program of China; Project (20120032110029) supported by the Specialized Research Fund for the Doctoral Program of Higher Education, China

    Corresponding author: Ji-hui WANG; Tel/Fax: +86-22-27890010; E-mail: jhwang@tju.edu.cn DOI: 10.1016/S1003-6326(14)63550-0

  • Xue-hui WANG, et al/Trans. Nonferrous Met. Soc. China 24(2014) 3907−3916

    3908

    and energy dispersive spectrometer (EDS). 2 Experimental 2.1 Material and heat treatment

    7A60 alloy used was composed of 8.0%−9.0% Zn, 2.3%−3.0% Mg, 2.0%−2.6% Cu, 0.1%−0.2% Zr, 0.09% Fe, 0.06% Si, 0.002% Be and balanced Al. The alloy was prepared by conventional casting, hot rolling and annealing at 470 °C for 2 h. And then, the hot rolled plates with a thickness of 2 mm were RRA-treated in the following sequences: 1) pre-aged at 120 °C for 24 h; 2) retrogressed at 195 °C for 180 min and 3) re-aged at 120 °C for 24 h. 2.2 Immersion test

    7A60 specimens were mounted in epoxy resin with an exposed area of 2 cm2. Then, they were abraded with silicon carbide paper (from 400 to 2000 grade) and polished. The immersion test was carried out in 3.5% NaCl solution for 72 h at room temperature to evaluate the pitting corrosion behavior. The microstructure and corroded surface were observed by a TDCLS4800 scanning electron microscope, and the chemical compositions of the constituent particles were analyzed by energy dispersive spectrometer. 2.3 EIS measurements

    EIS measurements of 7A60 alloy in 3.5% NaCl solution at different immersion time were carried out by using a VersaSTAT 4 electrochemical workstation with three-electrode system at open circuit potential with a 10 mV sine perturbation to the cell. The working electrode was 7A60 alloy with an exposed area of 2 cm2. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a platinum plat. The measuring frequency range was 105−10−2 Hz. The experimental data of the impedance were analyzed in terms of an appropriate equivalent circuit using the ZSimpWin program, and the values of the parameters were determined by the simulation. In the circuit, capacitance was mathematically modeled using a constant phase element Q in order to obtain a better simulation between the model and the experimental data. Then, the impedance was defined by [13,14]

    nYZ −−= ωω j)()j( 10 (1)

    where Y0 is the Q-constant; j is the imaginary unit; ω is the angular frequency (ω=2πf, f is the frequency); n the Q-power ranging from −1 to 1. 2.4 EN measurements

    Electrochemical noise data of 7A60 alloy in 3.5% NaCl solution was measured by using two nominally

    identical 7A60 alloys with an exposed area of 2 cm2 as working electrodes and a saturated calomel electrode as reference electrode. The EN system used was specifically described in Ref. [15]. The sampling frequency used in this study was 2 Hz and each time record consisted of 512 s. The standard deviations (STD) of the potential and current noise were given to evaluate the corrosion activity. Before STD analysis, the direct current (DC) component was removed from the original EN data by a 5-order polynomial fitting every 512 s interval.

    Wavelet transformation was also used to analyze EN signals [16]. By using the wavelet transformation technique based on orthogonal db2 wavelet, the collected electrochemical potential noise data were decomposed to seven levels (d1−d7) [17]. Then, for orthonormal discrete wavelet decomposition, the following power law was used [18,19]:

    β σσ

    − =

    )( j j 2

    2 2 (j=1, 2, …, 7) (2)

    where σ2 is the variance of EN signal; 2jσ is the variance of detail crystal dj, which can be calculated by the following equation:

    ∑ =

    − −

    = jN

    k jkjjj ddN

    2/

    1

    2 ,

    2 )( 12/

    1σ (j=1, 2, …, 7) (3)

    where N is the number of data record, k=1, 2, …, N/2j.

    The slope β was obtained from the plot 22log jσ versus level j:

    2

    2 2

    2 loglog σβσ += jj (4) Finally, the fractal dimension D was obtained by

    β5.05.2 −=D (5) 3 Results 3.1 Surface morphology

    The microstructure and EDS spectra of 7A60 alloy before immersion are shown in Fig. 1. From Fig. 1 it can be observed that the round constituent particles were rich in Mg and Si elements (33.34% Mg and 15.7% Si in Fig. 1(b)), and could be related as β(Mg2Si) phase. The constituent particles with irregular shape were composed of 76.15% Al, 15.87% Cu and 7.98% Fe (Fig. 1(c)), and could be referred to as ternary phase Al7Cu2Fe. The oval particles should be S phase (Al2MgCu) with the composition of 50.17% Al, 24.59% Mg and 20.93% Cu (Fig. 1(d)). The particles with high contents of Mg and Zn in Fig. 1(e) could be related as η phase (MgZn2) [9,12,20].

    The surface morphology of 7A60 alloy after immersion is shown in Fig. 2. During the initial 2 h of immersion, there were no visible pits. However, it was

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    clearly seen that the oxide layer rupture (pits nucleation) was mainly localized at the constituent particles (Fig. 2(a)). As immersion time increased to 3 h, the corrosion pits with an average size of 20 μm were observed, and