- Email: [email protected]
Electronic structure and pitting behavior of 3003 aluminum alloy passivated under various conditions
Electrochimica Acta 54 (2009) 4155–4163
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electronic structure and pitting behavior of 3003 aluminum alloy passivated under various conditions Y. Liu a , G.Z. Meng a,b , Y.F. Cheng a,∗ a b
Dept. of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 December 2008 Received in revised form 19 February 2009 Accepted 20 February 2009 Available online 3 March 2009 Keywords: Aluminum alloy Passivity Pitting corrosion Chloride ions Electrochemical measurements
a b s t r a c t Passivity of aluminum (Al) alloy 3003 in air and in aqueous solutions without and with chloride ions was characterized by electrochemical measurements, including cyclic polarization, electrochemical impedance spectroscopy (EIS), localized EIS and potential of zero charge, Mott–Schottky analysis and secondary ion mass spectroscopy (SIMS) technique. Stability, pitting susceptibility and repassivation ability of Al alloy 3003 under various film-forming conditions were determined. Results demonstrated that passive films formed on 3003 Al alloy in air and in Na2 SO4 solution without and with NaCl addition show an n-type semiconductor in nature. The passive film formed in chloride-free solution is most stable, and that formed in chloride-containing solution is most unstable, with the film formed in air in between. Pitting of Al alloy 3003 passivated both in air and in aqueous solutions is inevitable in the presence of chloride ions. There is the strongest capability for the air-passivated Al alloy 3003 to repassivate, and the weakest repassivating capability for Al alloy 3003 passivated in chloride-containing solution. The resistance of the passivated Al alloy 3003 to pitting corrosion is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). According to the differences between corrosion potential and potential of zero charge, passive film formed in air has the strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Chloride ions causing pitting of passivated Al alloy 3003 in air and in chloride-free solution come from the test solution, while those resulting in pitting of passivated Al alloy 3003 in chloride-containing solution mainly exist in the film during film-forming stage. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Aluminum (Al) alloys of 3xxx series, due to their favorable strength-to-weight property, high thermal conductivity, excellent formability, as well as good corrosion resistance, have been widely used in automobile heat exchange systems, replacing more traditional materials like stainless steels and copper alloys [1,2]. However, Al alloys are prone to experience pitting corrosion during service in cooling system [3–5]. It has been acknowledged [6,7] that corrosion resistance of aluminum (Al) alloy depends on formation of a layer of passive film on its surface. However, halide ions, especially chloride ions (Cl− ), show a strong attack to passive film, resulting in pitting corrosion of Al alloy. It was reported [8,9] that 3xxx series Al alloys containing 1–1.5% manganese (Mn) and Al/Mn intermatellic compounds might undergo the attack of chloride ions at vulnerable defect sites. The role of Cl− in pitting processes and its interaction with passive film have been studied extensively, and
∗ Corresponding author. Tel.: +1 403 220 3693; fax: +1 403 282 8406. E-mail address: [email protected] (Y.F. Cheng). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.058
models have been developed to illustrate pitting corrosion [10–13]. In particular, point defect model (PDM) is a relatively mature model to describe the growth mechanism and kinetics of passive film as well as pit initiation and growth in the presence of Cl− [14–16]. Passive films formed on Al alloy under various conditions are associated with different structures. For example, a thin layer of Al oxide film formed immediately in air is observed to be amorphous, while the passive film formed in aqueous solution is usually dense, coherent and compact [6]. It is expected that there are significant effects of the structure of passive film on its electrochemical and semiconducting properties, and thus the pitting corrosion resistance. To date, there has been limited work to investigate and compare mechanistically the electrochemical and semiconducting properties and pitting susceptibilities of passive films formed under the various conditions [17–20]. For example, Bockris and Kang [17] measured Mott–Schottky plots of the passive-film-covered pure Al and its alloys to categorize the passive film on Al and Al alloys are n-type semiconductors. Fernandes et al. [18] investigated the electronic properties of oxide film formed on 99.5% Al and 2024-T3 Al alloy in a sulphuric-boric bath. The results indicated that the film shows an n-type semiconductive behavior, with bandgap energies
4156
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
and semiconducting characteristics depending on the environmental conditions. Levinea et al. [19] employed Mott–Schottky analysis to study the properties of oxide film of Al alloy 2024-T3, and determined that it is a p-type semiconductor, with non-stoichiometric defects or substitutions existing in the ultra-thin layer. Kobotiatis et al. [20] studied the electronic properties of passive layer grown anodically on Al 70775 in chromate and oxalate solutions using electrochemical impedance spectroscopy. It was found that the oxide developed in the presence of chromate (good inhibitor) exhibits a less-noble flat-band potential and a lower average density of state. It has been acknowledged [14–16] that the electronic structure and properties of passive films were responsible for the film breakdown and the initiation of pitting. A high pit density occurring on the metal surface was generally associated with an n-type oxide. However, the actual correlation between the semiconductive behavior of passive films and the pitting susceptibility was nonexistent. Furthermore, passive films formed under different conditions are expected to show different semiconducting properties and have distinct electronic structures, which would result in different pitting susceptibilities. In this work, passive films formed on Al alloy 3003 either in air or in aqueous solutions without and with Cl− were characterized by various electrochemical techniques, including cyclic polarization, electrochemical impedance spectroscopy (EIS), localized EIS (LEIS) and potential of zero charge (PZC), Mott–Schottky analysis, and secondary ion mass spectroscopy (SIMS). Electrochemical corrosion behavior of the passivated 3003 Al alloy electrode was determined, and the composition and electronic structure of the film was studied. The adsorption, penetration and distribution of Cl− in passive film and the role of Cl− in pitting of Al alloy 3003 were discussed. It is anticipated that this research provides an essential insight into the mechanistic understanding of passive film formation and breakdown as well as pitting corrosion of Al alloy 3003 under various conditions. 2. Experimental 2.1. Electrodes and solutions Specimens for electrochemical tests were cut from a round bar of 3003 Al alloy supplied by DANA Canada Corporation, with the chemical composition (wt%): Cu 0.20, Fe 0.70, Si 0.60, Mn 1.50, Mg 0.05, Cr 0.05, Zn 0.10, Ti 0.05 and Al balance. Specimens were machined and embedded in epoxy resin manufactured by LECO, leaving a circular working area of 0.4 cm2 . The working surface was ground with emery papers up to 1200 grit, cleaned by deionized water and degreased in acetone. 2.2. Formation of passive films on 3003 Al alloy Three types of passive film were formed on Al alloy 3003 under controlled conditions. The first type was formed in air naturally when 3003 Al alloy electrode was exposed in air. The second and third types were formed in 0.25 M Na2 SO4 solution, without and with 0.5 M NaCl, respectively. Preparation of oxide film in solution was not simply to immerse the air-exposed 3003 Al alloy electrode to the aqueous solution. The electrode surface was ground first with a 1200-grit emery paper that is installed inside the film-forming solution in order to remove completely the air-formed film before the new film was generated in the solution. The ground electrode continued to stay in solution for 2 h, and there was external potential applied at this stage. After then the film-covered electrode was transferred rapidly to the test solution for electrochemical characterization.
Fig. 1. Cyclic polarization curves of the passivated 3003 Al alloy electrode in 0.25 M Na2 SO4 + 0.5 M NaCl solution (potential scanning rate: 0.333 mV/s).
All solutions were made up from analytical grade reagents and ultra-pure deionized water (18 M cm in resistivity). 2.3. Electrochemical measurements Electrochemical measurements were performed through a Gamry Reference 600 electrochemical system by using a threeelectrode cell, with 3003 Al alloy as working electrode (WE), a saturated calomel electrode (SCE) as reference electrode (RE) and a Pt wire as counter electrode (CE). All electrochemical tests were conducted in 0.25 M Na2 SO4 + 0.5 M NaCl solution. Prior to cyclic polarization measurement, 3003 Al alloy WE was immersed in test solution at least 1 h until corrosion potential (Ecorr ) reached a steady-state value. Anodic polarization scan was performed at a potential sweep rate of 0.333 mV/s, with a reverse in scan direction when anodic current density reached 0.1 mA/cm2 . Pitting potential (Epit ) was determined when anodic current density deviated abruptly from the stable passive current density, as indicated in Fig. 1. The conventional EIS measurements were conducted on the macroscopic Al alloy 3003 WE at Ecorr or Epit , with the measuring frequency ranging from 20 kHz to 0.001 Hz and an applied AC disturbance signal of 10 mV. LEIS measurements were performed on WE through a PAR Model 370 scanning electrochemical workstation, which was comprised of a scanning Pt microprobe with a 10 m tip, a 370 scanning control unit, an M236A potentiostat, an M5210 lock-in amplifier and a video camera system. For LEIS mapping, the Pt microprobe, which was set above the electrode surface at 50 m, was stepped over a designated area of the electrode. The probe scanning took the form of a raster in x–y plane. The step size during LEIS scanning was controlled to obtain a plot of 32 lines × 24 lines with a scanning area of 1000 m × 750 m. An AC disturbance signal of 10 mV was applied to WE that was at Ecorr . The measurement frequency was fixed at 10 Hz. In measurements of PZC of 3003 Al alloy electrode, a frequency of 18 Hz and an AC disturbance signal of 10 mV were applied. Doublecharge layer capacitance was obtained from the measured EIS. All the tests were performed at ambient temperature (∼22 ◦ C) and open to air. 2.4. SIMS characterization Negative and positive SIMS characterizations were performed through a ToF-SIMS IV instrument manufactured by IonTOF GmbH.
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
A chopped 15 keV Ga+ ion beam of about 200 nm in diameter was used to generate secondary ions, which were then separated by the time-of-flight mass analyzer. In the imaging mode, maps of the lateral distribution of elements across the target surface were collected from an area of 40 m × 80 m. For the depth profiling, a dual ion beam technique was used, where the Ga analytical ion gun was scanned over an area of 30 m × 30 m near the centre of a crater of 200 m × 200 m, created by another, sputter ion gun. The sputter guns used either Cs+ or O− ion beams of 80 nA at 1 keV initial energy for negative and positive secondary ion profiles correspondingly. In order to characterize the permeation and diffusion of chloride ions in passive films formed under various conditions, i.e., in air and in aqueous solutions without and with chloride ions, the films were polarized at an anodic potential of −0.65 V (SCE) in 0.25 M Na2 SO4 + 0.5 M NaCl solution for 3 h, and then characterized by SIMS. For comparison, a blank specimen that was filmed in 0.25 M Na2 SO4 solution without further anodic polarization in chloride-containing solution was also under SIMS characterization. 3. Results 3.1. Cyclic polarization measurements Fig. 1 shows the cyclic polarization curves measured on Al alloy 3003 with passive films formed in air and aqueous solutions without and with chloride ions in 0.25 M Na2 SO4 + 0.5 M NaCl solution, where the solid arrows indicated the potential scan direction. It is seen that all passivated electrodes showed a stable passive region where the low passive current density was independent of potential. Current density then increased abruptly at Epit , followed with a positive hysteresis loop during reverse potential scanning. The values of Epit were −0.60 V, −0.53 V and −0.50 V (SCE) for electrodes with passive films formed in air, in Na2 SO4 solution, and in chloridecontaining Na2 SO4 solution, respectively. There was similar Erp of about −0.70 V (SCE) for all electrodes. Furthermore, although the current density was set at 0.1 mA/cm2 for scan reversion, it did not decrease immediately after the potential was reversely scanned, but continued to increase. The different current densities resulted in different sizes of the hysteresis loop for the three types of passive film, which were correspondent with different repassivation abilities of the film. There was the biggest loop for passive film formed in chloride-containing solution, while the smallest loop for the film formed in air. 3.2. Conventional EIS measurements on the macroscopic electrode Fig. 2 shows the Nyquist diagrams measured on Al alloy 3003 electrodes with passive films formed in air and in solutions without and Cl− and solution, respectively, in 0.25 M Na2 SO4 + 0.5 M NaCl solution (at Ecorr ). There was a common characteristic for all curves, i.e., a capacitive semicircle in the high-frequency range and a diffusive tail in the low-frequency range. There was the biggest semicircle for passive film formed in solution without chloride ions, and the smallest semicircle for the film formed in chloridecontaining solution. Fig. 3 shows the Nyquist diagrams of the passivated 3003 Al alloy electrodes at Epit . It is seen that, at Epit , there was a significant decrease of the semicircle size. Moreover, an inductive loop was observed in low-frequency range in all diagrams. Furthermore, there was the biggest diameter of the semicircle for passive film formed in chloride-free solution (∼1300 ), and the smallest one for passive film formed in chloride-containing solution (∼12 only). Observation of electrode morphologies after LEIS
4157
Fig. 2. Nyquist diagrams measured on 3003 Al alloy with passive films formed in air (a), chloride-free solution (b) and chloride-containing solution (c) in 0.25 M Na2 SO4 + 0.5 M NaCl solution at individual corrosion potential.
measurements in Fig. 4 showed that the electrode passivated in chloride-containing solution suffered severe pitting corrosion with deep pits (Fig. 4c), while the passivated electrode in air and in chloride-free solution had slight pitting with small and shallow pits (Fig. 4a and b). The EIS measurements were performed in aerated solutions and thus described the sum of cathodic and anodic processes proceeding on heterogenous surface of oxide and pits. However, at Epit , anodic reaction including pitting corrosion dominated the electrode behavior, and the cathodic response was too small to be ignored. 3.3. LEIS measurements Fig. 5 shows the LEIS maps measured over 3003 Al alloy electrodes with passive films formed in air and the solutions without and with chloride ions, respectively, at Ecorr in 0.25 M Na2 SO4 + 0.5 M NaCl solution. In the x–y–z three-dimensional space, |Z| represents the measured impedance amplitude, which usually refers to the resistance of electrode to localized corrosion at individual measuring point. Thus, the fluctuating plane in the 3D figure represents the distribution of local impedance over the scanned surface of the electrode. The 3D impedance distribution was also projected on x–y plane, where the impedance amplitude of individual point was represented with different colors. It is seen that there were frequent fluctuations of impedance value measured on passive film formed in air (Fig. 5a). The impedance distribution was the most uniform on electrode passivated in chloride-free solution (Fig. 5b). 3.4. PZC measurements Fig. 6 shows the double-charge layer capacitance of passive films formed in air, 0.25 M Na2 SO4 , and 0.25 M Na2 SO4 + 0.5 M NaCl solutions, respectively, as a function of applied potential. It is seen that there is a common feature for the three curves, i.e., a minimum of double-charge layer capacitance that is considered as PZC of the electrode was observed. In addition, steady-state corrosion potential (Ecorr ) of the passivated 3003 Al alloy electrode was also included in each diagram. Generally, PZC ware more negative than Ecorr for all passivated electrodes. The differences between Ecorr and PZC (E = Ecorr – PZC) for passive films formed in air, Na2 SO4 solution and Na2 SO4 + NaCl solution were 0.119 V, 0.093 V and 0.036 V,
4158
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 4. Surface morphology of 3003 Al alloy with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Epit .
Fig. 3. Nyquist diagrams of 3003 Al alloy with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Epit .
respectively. Thus, there was a smaller potential difference, E, for passive film formed in aqueous solutions than that formed in air, and further, passive film formed in chloride-containing solution had a smaller potential difference than that formed in chloride-free solution.
3.5. Capacitance measurements and Mott–Schottky analysis Potential dependence of the capacitance of space-charge layer (Csc ) is expressed by Mott–Schottky relationship [21]: for n-type semiconductor 1 2 CSC
=
2 eεr ε0 ND
E − ϕfb −
T e
(1)
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 5. LEIS maps measured on 3003 Al alloy electrodes with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Ecorr in 0.25 M Na2 SO4 + 0.5 M NaCl solution.
for p-type semiconductor 1 2 CSC
=−
2 eεr ε0 NA
E − ϕfb −
T e
4159
Fig. 6. Relationship between double-charge layer capacitance vs. applied potential for 3003 Al alloy with passive films formed in air (a), chloride-free solution (b) and chloride-containing solution (c).
(2)
where e is electron charge (1.6 × 10−19 C), εr is dielectric constant of Al oxide, taken as 10 [22], ε0 is the vacuum permittivity (8.85 × 10−14 F cm−1 ), ND is the donor density, NA is the acceptor
density, E is the applied potential, ϕfb is flat-band potential, is Boltzmann constant (1.38 × 10−23 J K−1 ) and T is absolute temperature. ND and NA can be determined from the slope of the linear −2 relationship between CSC and E, while ϕfb is obtained from the −2 = 0. extrapolation to CSC
4160
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 7. Mott–Schottky curves for three types of passive film formed on 3003 Al alloy measured at 1000 Hz in 0.25 M Na2 SO4 + 0.5 M NaCl solution.
Fig. 7 shows the Mott–Schottky curves for the three types of passive film measured at 1000 Hz in 0.25 M Na2 SO4 + 0.5 M NaCl solution. To demonstrate the consistence of capacitances measured by EIS and Mott–Schottky at Ecorr , the film formed in chloridecontaining solution was used as an example. The capacitances were 0.36 F m−2 and 0.5 F m−2 , respectively, indicating that the measured capacitance corresponded to the capacitance of passive film. It is seen Fig. 7 that all passive films behaved like an n-type semicon−2 ductor, with a positive slope of the linear CSC ∼ E. The curved form of the lines indicated the highly disordered nature of passive film where highly localized states existed between the valence and the conduction bands [23,24]. The fitted values of flat-band potential and donor density for three passive films are shown in Table 1. It is clear that passive film formed in air had a more negative ϕfb and a higher ND , and there were similar values of ϕfb and ND for passive films formed in aqueous solutions. 3.6. SIMS characterization Fig. 8 shows the chloride concentration profiles of the three passive films formed under various conditions and a blank 3003 Al alloy specimen measured by SIMS. It is seen that the concentration of chloride ions decreased continuously with the sputter depth in the electrode. As expected, there was the lowest or even zero chloride concentration for passive film formed in Na2 SO4 solution without a further anodic polarization in chloride-containing solution. The permeation depth of chloride ions into passive film followed the order: film formed in chloride-free solution < film formed in air < film formed in chloride-containing solution. Despite the slight irregularity of chloride concentration determined by SIMS for passive film formed in chloride-free solution, generally, the concentration of permeated chloride ions was ranked as: film formed in air < film formed in chloride-free solution < film formed in chloridecontaining solution.
Fig. 8. Depth profiles of chloride ions on passive films measured by SIMS.
4. Discussion 4.1. Passive films formed on 3003 Al alloy in air and in aqueous solutions The present work shows clearly (Fig. 2) that there are quite different stabilities of passive films formed on 3003 Al alloy electrode under various conditions. The measured EIS plots at corrosion potential, i.e., a high-frequency capacitive semicircle and a low-frequency diffusive tail, are fitted with an electrochemical equivalent circuit shown in Fig. 9a [25], where Rs is solution resistance, CPE is constant phase element, Rf is charge-transfer resistance of passivated 3003 Al alloy electrode, and W is Warburg diffusive impedance. The high-frequency capacitive semicircle represents the charge-transfer reaction of passivated 3003 Al alloy, while the low-frequency diffusive impedance is associated with the oxygen diffusion. Under stable passivation, the film formation achieves an equilibrium state. Thus, the film formation rate is equal to the dissolution rate of 3003 Al alloy. Electrochemical parameters fitted from EIS data are listed in Table 2. Apparently, there is the largest resistance (thus the most stable) for passive film formed in
Table 1 Flat-band potential, ϕfb , and donor density, ND , for passive films formed under various conditions. Film formation medium
ϕfb vs. SCE (V)
ND (×1027 m−3 )
In-air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
−1.509 −0.833 −0.815
32.39 7.73 7.48
Fig. 9. Electrochemical equivalent circuits used for fitting EIS data measured at individual Ecorr (a) and at Epit (b).
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
4161
Table 2 Electrochemical parameters fitted from EIS data measured at individual Ecorr . Film-forming condition
Rs ()
CPE-Y0 (S sn )
n
Rf (×105 )
W (S s0.5 )
In air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
7.4 9.6 9
7.14 7.48 1.58
0.93 0.80 0.91
1.91 5.71 1.03
226 24.9 247
chloride-free solution, the smallest resistance (most unstable) for passive film formed in chloride-containing solution, and the passive film formed in air in between. In general, passive film formed in aqueous solution is usually associated with a compact, uniform structure because of hydration process occurring on 3003 Al alloy electrode. It is acknowledged [26,27] that the hydrated passive film always shows a higher stability than that without hydration. For passive film formed in air, the film structure is usually non-uniform, with different thickness and compositional distribution. The LEIS mapping on air-formed passive film shows significant fluctuations of local impedance on the film (Fig. 5a), demonstrating the structural non-uniformity. As a comparison, the LEIS mapping on passive films formed in aqueous solutions (Fig. 5b and c) is quite uniform. LEIS has been demonstrated as a unique alternative to characterize the localized corrosion behavior of metal at a microscopic scale [28–31]. While the conventional EIS reflects an averaged impedance response of a macroscopic electrode, LEIS provides information specific to the individual microscopic site. Therefore, a LEIS mapping is capable of detect local active spots where a low impedance is usually identified. It is thus concluded from Fig. 5 that passive films formed in aqueous solutions are much more uniform, with fewer local defects, than that formed in air. For passive film formed in chloride-containing solution, it is expected that chloride ions get involved in the film formation process, as demonstrated by SIMS characterization results (Fig. 8) that there is the deepest chloride sputter depth and the highest chloride concentration at individual depth. It is generally acknowledged [6,7] that Cl− plays an important role in initiation and propagation of pitting corrosion. The high concentration of Cl− existing in the passive film formed in chloride-containing solution results in the difficulty of film to be repassivated, as seen in cyclic polarization measurement in Fig. 1. Upon initiation of the corrosion pit, Cl− also contributes to the rapid propagation of pitting.
4.2. Pitting susceptibility of 3003 Al alloy electrodes passivated in air and in aqueous solutions Electrochemical cyclic polarization measurement is capable of predicting the susceptibility of passivated metal to pitting corrosion [6,32]. Generally, if the reverse anodic curve is shifted to lower currents, i.e., negative hysteresis, or if the reverse curve essentially retraces the ascending curve, i.e., neutral hysteresis, no pitting corrosion will occur on the target metal or alloy. In contrast, if the reverse anodic curve is shifted to higher currents than the forward curve, i.e., positive hysteresis, pitting corrosion will occur. It is apparent from Fig. 1 that positive hysteresis loops are measured on 3003 Al alloy passivated under various conditions, suggesting that pitting of 3003 Al alloy passivated in air and in aqueous solutions is inevitable in the test system. The values of Epit show that
it is earliest for 3003 Al allow passivated in air to occur pitting, while it is relatively most difficult for 3003 Al alloy passivated in chloride-containing solution to initiate pitting, with that passivated in sulfate solution in between. Thus, in accordance with the measured Epit , The pitting susceptibility of passivated 3003 Al alloy is ranked as: in air > in chloride-free solution > in chloride solution. Furthermore, the area of the measured positive hysteresis loop indicates the repassivity capability of the metal or alloy, and a smaller area indicates a stronger ability for metal or alloy to repassivate. Therefore, there is the strongest capability for the air-passivated 3003 Al alloy to repassivate, and the weakest repassivating capability for 3003 Al alloy passivated in chloride-containing solution. The resistance of a passivated metal or alloy to pitting is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). Although the airformed passive film is easy to initiate pitting in chloride solution (the lowest Epit ), it has the strongest capability to repassivate, i.e., to self-repair after pitting initiation. Thus, the overall ability of passivated 3003 Al alloy to pitting is in moderate state. The passive film formed in chloride-containing solution has the relatively most positive Epit , but the weakest repassive ability. Consequently, it shows the most active state. The passive film formed in chloride-free solution is the most stable, which is attributed to the moderate Epit and repassivating ability. The relative stability of passive films formed under various conditions is demonstrated by EIS measurements on passivated 3003 Al alloy electrodes at their individual Epit (Fig. 3). Upon pitting, the roughness of the electrode surface increases, and the electrode state thus becomes more non-uniform. As a consequence, an inductive loop is observed in the low-frequency range, which is one of the typical features indicating pitting corrosion or electrode roughening [33,34]. The EIS feature is fitted with the electrochemical equivalent circuit in Fig. 9b, where L is inductance and RL is inductive resistance. The fitted electrochemical parameters are shown in Table 3. It is seen that there is the highest charge-transfer resistance for passive film formed in chloride-free solution (Fig. 3a), and the lowest charge-transfer resistance for the film formed in chloride-containing solution (Fig. 3b).
4.3. Pitting mechanism of passivated 3003 Al alloy electrodes The present work demonstrates that passive films formed in air and in aqueous solutions behave like an n-type semiconductor, as indicated by a positive slope of Mott–Schottky relationship in Fig. 6. According to point defect model [35], the main electron donors in an n-type semiconductor are oxygen vacancies. Chloride ions would occupy the positions of oxygen vacancies to generate cation vacancies at solution/film interface, which transport towards the film/metal interface to produce cation vacancy condensate, resulting in local depart of passive film and thus pitting. A complete
Table 3 Electrochemical parameters fitted from EIS data measured at Epit . Passivation condition
Rs ()
CPE-Y0 (×10−5 S sn )
n
Rf ()
RL ()
L (H)
In air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
39 35 2.9
126 1.93 1.64
0.80 0.71 1
970.1 1573 10.15
687 821 0.97
106 123 K 2.22
4162
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
to pitting corrosion. The present work shows that there is the highest donor density in passive film formed in air, as seen in Table 1, providing potential sites for chloride ions to occupy. Moreover, it is determined that there is the strongest capability for chloride ion adsorption on passive film formed in air, it is expected that the passive film formed in air has the lowest resistance to pitting, as demonstrated by a lowest Epit . It is realized that the size, shape and distribution of second phase intermetallic particles influence the pitting corrosion behavior. For example, it was found [39] that the adsorption of Cl− in passive film prefers at or around inclusions and second phase particles due to weaker oxide film on these sites. This relevant subject will be explored in more detail in the further work. 5. Conclusions
Fig. 10. Schematic diagrams of electric field distributions at the electrode/solution interface when electrode is at Ecorr (a) and PZC (b).
description about the interfacial electrode reactions and masstransport processes are proposed by Macdonald based on PDM [35]. Therefore, adsorption and permeation of chloride ions into passive film is usually the first step to cause pitting. To understand fundamentally the sources of chloride ions to result in pitting in passive film under different forming conditions, the potential of zero charge is measured and shown in Fig. 6. The potential of zero charge at which the excess charge at the electrode/electrolyte interface could be eliminated usually acts as a reference in determining the type and amount of ions adsorbed on the electrode surface [36]. If an electrode under its open-circuit potential is positively charged and thus adsorbed with anions, the PZC is a more negative potential applied to counteract the excess charges at the interface, as schematically represented in Fig. 10. The potential differences between Ecorr and PZC, E, for Al passive films formed under various conditions show positive values, suggesting that the electrode surfaces are positively charged at Ecorr for all types of passive films. Consequently, chloride ions are expected to adsorb on electrode surface. Furthermore, from the value of E, it is deduced that passive film formed in air has the largest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Thus, passive film formed in aqueous solution, especially the chloride-containing solution, has a weak capability in anions adsorption, which is attributed to the mutual repulsion among anions. It is expected that a high concentration of chloride ions exists in passive film formed in chloride-containing solution, and a further adsorption of chloride ions from the solution will be repulsed. Therefore, chloride ions causing pitting of passive film formed in air and in chloride-free solution come from the test solution, while chloride ions resulting in pitting of passive film formed in chloride-containing solution are mainly those existing in the film during film-forming stage. It has been demonstrated [20,37,38] that passive film with a higher donor density is always associated with a lower resistance
Passive film formed on 3003 Al alloy in air and in Na2 SO4 solution without and with NaCl addition show n-type semiconductor in nature. Passive film formed in chloride-free solution is most stable, and that formed in chloride-containing solution is most unstable, with the film formed in air in between. Passive film formed in air is associated with a non-uniform structure/composition and the highest donor density in electronic structure, resulting in a reduced stability than those formed in aqueous solution. However, incorporation of chloride ions in passive film would decrease significantly the resistance of the film to pitting when it is formed in a chloridecontaining solution. Pitting of 3003 Al alloy passivated in air and in aqueous solutions is inevitable in the presence of chloride ions in the test solution. There is the strongest capability for the air-passivated 3003 Al alloy to repassivate, and the weakest repassivating capability for Al alloy passivated in chloride-containing solution. The resistance of the passivated 3003 Al alloy to pitting is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). The positive potential differences between Ecorr and potential of zero charge for Al passive films formed under various conditions suggest that the electrode surfaces are positively charged at Ecorr . Consequently, chloride ions are expected to adsorb on electrodes. Passive film formed in air has the strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Chloride ions causing pitting of passive film formed in air and in chloride-free solution come from the test solution, while those resulting in pitting of passive film formed in chloridecontaining solution exist in the film during film-forming stage. Acknowledgements This work was supported by Canada Research Chairs Program, Natural Science and Engineering Research Council of Canada (NSERC) and Dana Canada Corporation. References [1] G. Davies, Materials for Automobile Bodies, Butterworth-Heinemann, Oxford, UK, 2003. [2] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A 280 (2000) 37. [3] Institution of Mechanical Engineers, Corrosion of Motor Vehicles, Mechanical Engineering Publications Limited, London, UK, 1976. [4] R. Baboian, Automotive Corrosion by Deicing Salts, National Association of Corrosion Engineers, Houston, USA, 1981. [5] M.G. Fontana, Corrosion Engineering, McGraw-Hill, CA, 1990. [6] Z. Szklarska-Smialowska, Corros. Sci. 41 (1999) 1743. [7] G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2186. [8] F. King, Aluminum and Its Alloys, Ellis Horwood, Chichester, England, 1987. [9] C. Vargel, Corrosion of Aluminum, Elsevier Science, San Diego, USA, 2004. [10] S. Menezes, R. Haak, G. Hagen, M. Kendig, J. Electrochem. Soc. 136 (1989) 1884.
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163 [11] B.A. Shaw, G.D. Davis, T.L. Ritz, B.J. Rees, W.C. Moshier, J. Electrochem. Soc. 138 (1991) 3288. [12] P.M. Natishan, G.K. McCafferty, E. Hubler, Electrochim. Acta 40 (1995) 323. [13] F.J. Martin, G.T. Cheek, W.E. O’Grady, P.M. Natishan, Corros. Sci. 47 (2005) 3187. [14] C.Y. Chao, L.F. Lin, D.D. Macdonald, J. Electrochem. Soc. 128 (1981) 1187. [15] L.F. Lin, C.Y. Chao, D.D. Macdonald, Electrochem. Soc. 128 (1981) 1194. [16] C.Y. Chao, L.F. Lin, D.D. Macdonald, Electrochem. Soc. 129 (1982) 1874. [17] J.O’M. Bockris, Y. Kang, J. Solid State Electrochem. 1 (1997) 17. [18] J.C.S. Fernandes, R. Picciochi, M. Da Cunha Belo, T.M. Silva, M.G.S. Ferreira, I.T.E. Fonseca, Electrochim. Acta 49 (2004) 4701. [19] K.L. Levinea, D.E. Tallman, G.P. Bierwagen, J. Mater. Process. Technol. 199 (2008) 321. [20] L. Kobotiatis, N. Kioupis, P.G. Koutsoukos, Corrosion 53 (1997) 562. [21] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, New York, 1980. [22] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1985. [23] M.H. Dean, U. Stimming, Corros. Sci. 29 (1989) 199. [24] M.H. Dean, U. Stimming, J. Electroanal. Chem. 228 (1987) 135.
4163
[25] E. Barsoukov, J.R. Macdonald, Impedance Spectroscopy Theory, Experiment, and Applications, A John Wiley & Sons, Inc., New Jersey, 2005. [26] W.E. O’Grady, J.O’M. Bockris, J. Electrochem. Soc. 127 (1980) 555. [27] T.E. Pou, O.J. Murphy, V. Young, J.O’M. Bockris, L.L. Tongson, J. Electrochem. Soc. 131 (1984) 1243. [28] F. Thebault, B. Vuillemin, R. Oltra, K. Ogle, C. Allely, Electrochim. Acta 53 (2008) 5226. [29] A. Davoodi, J. Pan, C. Leygraf, S. Norgren, J. Electrochem. Soc. 155 (2008) C211. [30] M.C. Li, Y.F. Cheng, Electrochim. Acta 53 (2008) 2831. [31] G.Z. Meng, C. Zhang, Y.F. Cheng, Corros. Sci. 50 (2008) 3116. [32] P. Gimenez, J.J. Rameau, M.C. Rebioul, Corrosion 37 (1981) 673. [33] L. Niu, Y.F. Cheng, Wear 265 (2008) 367. [34] T. Hagyar, J. Williams, Trans. Faraday Soc. 57 (1961) 2288. [35] D.D. Macdonald, J. Electrochem. Soc. 139 (1992) 3434. [36] J.O’M. Bockris, A.K.N. Reddy, M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Acad, 2000. [37] Y.F. Cheng, J.L. Luo, Electrochim. Acta 44 (1999) 2947. [38] H. Tsuchiya, S. Fujimoto, O. Chihara, T. Shibata, Electrochim. Acta 47 (2002) 4357. [39] C.J. Lin, R.G. Du, T. Nguyen, Corrosion 56 (2000) 41.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electronic structure and pitting behavior of 3003 aluminum alloy passivated under various conditions Y. Liu a , G.Z. Meng a,b , Y.F. Cheng a,∗ a b
Dept. of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 December 2008 Received in revised form 19 February 2009 Accepted 20 February 2009 Available online 3 March 2009 Keywords: Aluminum alloy Passivity Pitting corrosion Chloride ions Electrochemical measurements
a b s t r a c t Passivity of aluminum (Al) alloy 3003 in air and in aqueous solutions without and with chloride ions was characterized by electrochemical measurements, including cyclic polarization, electrochemical impedance spectroscopy (EIS), localized EIS and potential of zero charge, Mott–Schottky analysis and secondary ion mass spectroscopy (SIMS) technique. Stability, pitting susceptibility and repassivation ability of Al alloy 3003 under various film-forming conditions were determined. Results demonstrated that passive films formed on 3003 Al alloy in air and in Na2 SO4 solution without and with NaCl addition show an n-type semiconductor in nature. The passive film formed in chloride-free solution is most stable, and that formed in chloride-containing solution is most unstable, with the film formed in air in between. Pitting of Al alloy 3003 passivated both in air and in aqueous solutions is inevitable in the presence of chloride ions. There is the strongest capability for the air-passivated Al alloy 3003 to repassivate, and the weakest repassivating capability for Al alloy 3003 passivated in chloride-containing solution. The resistance of the passivated Al alloy 3003 to pitting corrosion is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). According to the differences between corrosion potential and potential of zero charge, passive film formed in air has the strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Chloride ions causing pitting of passivated Al alloy 3003 in air and in chloride-free solution come from the test solution, while those resulting in pitting of passivated Al alloy 3003 in chloride-containing solution mainly exist in the film during film-forming stage. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Aluminum (Al) alloys of 3xxx series, due to their favorable strength-to-weight property, high thermal conductivity, excellent formability, as well as good corrosion resistance, have been widely used in automobile heat exchange systems, replacing more traditional materials like stainless steels and copper alloys [1,2]. However, Al alloys are prone to experience pitting corrosion during service in cooling system [3–5]. It has been acknowledged [6,7] that corrosion resistance of aluminum (Al) alloy depends on formation of a layer of passive film on its surface. However, halide ions, especially chloride ions (Cl− ), show a strong attack to passive film, resulting in pitting corrosion of Al alloy. It was reported [8,9] that 3xxx series Al alloys containing 1–1.5% manganese (Mn) and Al/Mn intermatellic compounds might undergo the attack of chloride ions at vulnerable defect sites. The role of Cl− in pitting processes and its interaction with passive film have been studied extensively, and
∗ Corresponding author. Tel.: +1 403 220 3693; fax: +1 403 282 8406. E-mail address: [email protected] (Y.F. Cheng). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.058
models have been developed to illustrate pitting corrosion [10–13]. In particular, point defect model (PDM) is a relatively mature model to describe the growth mechanism and kinetics of passive film as well as pit initiation and growth in the presence of Cl− [14–16]. Passive films formed on Al alloy under various conditions are associated with different structures. For example, a thin layer of Al oxide film formed immediately in air is observed to be amorphous, while the passive film formed in aqueous solution is usually dense, coherent and compact [6]. It is expected that there are significant effects of the structure of passive film on its electrochemical and semiconducting properties, and thus the pitting corrosion resistance. To date, there has been limited work to investigate and compare mechanistically the electrochemical and semiconducting properties and pitting susceptibilities of passive films formed under the various conditions [17–20]. For example, Bockris and Kang [17] measured Mott–Schottky plots of the passive-film-covered pure Al and its alloys to categorize the passive film on Al and Al alloys are n-type semiconductors. Fernandes et al. [18] investigated the electronic properties of oxide film formed on 99.5% Al and 2024-T3 Al alloy in a sulphuric-boric bath. The results indicated that the film shows an n-type semiconductive behavior, with bandgap energies
4156
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
and semiconducting characteristics depending on the environmental conditions. Levinea et al. [19] employed Mott–Schottky analysis to study the properties of oxide film of Al alloy 2024-T3, and determined that it is a p-type semiconductor, with non-stoichiometric defects or substitutions existing in the ultra-thin layer. Kobotiatis et al. [20] studied the electronic properties of passive layer grown anodically on Al 70775 in chromate and oxalate solutions using electrochemical impedance spectroscopy. It was found that the oxide developed in the presence of chromate (good inhibitor) exhibits a less-noble flat-band potential and a lower average density of state. It has been acknowledged [14–16] that the electronic structure and properties of passive films were responsible for the film breakdown and the initiation of pitting. A high pit density occurring on the metal surface was generally associated with an n-type oxide. However, the actual correlation between the semiconductive behavior of passive films and the pitting susceptibility was nonexistent. Furthermore, passive films formed under different conditions are expected to show different semiconducting properties and have distinct electronic structures, which would result in different pitting susceptibilities. In this work, passive films formed on Al alloy 3003 either in air or in aqueous solutions without and with Cl− were characterized by various electrochemical techniques, including cyclic polarization, electrochemical impedance spectroscopy (EIS), localized EIS (LEIS) and potential of zero charge (PZC), Mott–Schottky analysis, and secondary ion mass spectroscopy (SIMS). Electrochemical corrosion behavior of the passivated 3003 Al alloy electrode was determined, and the composition and electronic structure of the film was studied. The adsorption, penetration and distribution of Cl− in passive film and the role of Cl− in pitting of Al alloy 3003 were discussed. It is anticipated that this research provides an essential insight into the mechanistic understanding of passive film formation and breakdown as well as pitting corrosion of Al alloy 3003 under various conditions. 2. Experimental 2.1. Electrodes and solutions Specimens for electrochemical tests were cut from a round bar of 3003 Al alloy supplied by DANA Canada Corporation, with the chemical composition (wt%): Cu 0.20, Fe 0.70, Si 0.60, Mn 1.50, Mg 0.05, Cr 0.05, Zn 0.10, Ti 0.05 and Al balance. Specimens were machined and embedded in epoxy resin manufactured by LECO, leaving a circular working area of 0.4 cm2 . The working surface was ground with emery papers up to 1200 grit, cleaned by deionized water and degreased in acetone. 2.2. Formation of passive films on 3003 Al alloy Three types of passive film were formed on Al alloy 3003 under controlled conditions. The first type was formed in air naturally when 3003 Al alloy electrode was exposed in air. The second and third types were formed in 0.25 M Na2 SO4 solution, without and with 0.5 M NaCl, respectively. Preparation of oxide film in solution was not simply to immerse the air-exposed 3003 Al alloy electrode to the aqueous solution. The electrode surface was ground first with a 1200-grit emery paper that is installed inside the film-forming solution in order to remove completely the air-formed film before the new film was generated in the solution. The ground electrode continued to stay in solution for 2 h, and there was external potential applied at this stage. After then the film-covered electrode was transferred rapidly to the test solution for electrochemical characterization.
Fig. 1. Cyclic polarization curves of the passivated 3003 Al alloy electrode in 0.25 M Na2 SO4 + 0.5 M NaCl solution (potential scanning rate: 0.333 mV/s).
All solutions were made up from analytical grade reagents and ultra-pure deionized water (18 M cm in resistivity). 2.3. Electrochemical measurements Electrochemical measurements were performed through a Gamry Reference 600 electrochemical system by using a threeelectrode cell, with 3003 Al alloy as working electrode (WE), a saturated calomel electrode (SCE) as reference electrode (RE) and a Pt wire as counter electrode (CE). All electrochemical tests were conducted in 0.25 M Na2 SO4 + 0.5 M NaCl solution. Prior to cyclic polarization measurement, 3003 Al alloy WE was immersed in test solution at least 1 h until corrosion potential (Ecorr ) reached a steady-state value. Anodic polarization scan was performed at a potential sweep rate of 0.333 mV/s, with a reverse in scan direction when anodic current density reached 0.1 mA/cm2 . Pitting potential (Epit ) was determined when anodic current density deviated abruptly from the stable passive current density, as indicated in Fig. 1. The conventional EIS measurements were conducted on the macroscopic Al alloy 3003 WE at Ecorr or Epit , with the measuring frequency ranging from 20 kHz to 0.001 Hz and an applied AC disturbance signal of 10 mV. LEIS measurements were performed on WE through a PAR Model 370 scanning electrochemical workstation, which was comprised of a scanning Pt microprobe with a 10 m tip, a 370 scanning control unit, an M236A potentiostat, an M5210 lock-in amplifier and a video camera system. For LEIS mapping, the Pt microprobe, which was set above the electrode surface at 50 m, was stepped over a designated area of the electrode. The probe scanning took the form of a raster in x–y plane. The step size during LEIS scanning was controlled to obtain a plot of 32 lines × 24 lines with a scanning area of 1000 m × 750 m. An AC disturbance signal of 10 mV was applied to WE that was at Ecorr . The measurement frequency was fixed at 10 Hz. In measurements of PZC of 3003 Al alloy electrode, a frequency of 18 Hz and an AC disturbance signal of 10 mV were applied. Doublecharge layer capacitance was obtained from the measured EIS. All the tests were performed at ambient temperature (∼22 ◦ C) and open to air. 2.4. SIMS characterization Negative and positive SIMS characterizations were performed through a ToF-SIMS IV instrument manufactured by IonTOF GmbH.
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
A chopped 15 keV Ga+ ion beam of about 200 nm in diameter was used to generate secondary ions, which were then separated by the time-of-flight mass analyzer. In the imaging mode, maps of the lateral distribution of elements across the target surface were collected from an area of 40 m × 80 m. For the depth profiling, a dual ion beam technique was used, where the Ga analytical ion gun was scanned over an area of 30 m × 30 m near the centre of a crater of 200 m × 200 m, created by another, sputter ion gun. The sputter guns used either Cs+ or O− ion beams of 80 nA at 1 keV initial energy for negative and positive secondary ion profiles correspondingly. In order to characterize the permeation and diffusion of chloride ions in passive films formed under various conditions, i.e., in air and in aqueous solutions without and with chloride ions, the films were polarized at an anodic potential of −0.65 V (SCE) in 0.25 M Na2 SO4 + 0.5 M NaCl solution for 3 h, and then characterized by SIMS. For comparison, a blank specimen that was filmed in 0.25 M Na2 SO4 solution without further anodic polarization in chloride-containing solution was also under SIMS characterization. 3. Results 3.1. Cyclic polarization measurements Fig. 1 shows the cyclic polarization curves measured on Al alloy 3003 with passive films formed in air and aqueous solutions without and with chloride ions in 0.25 M Na2 SO4 + 0.5 M NaCl solution, where the solid arrows indicated the potential scan direction. It is seen that all passivated electrodes showed a stable passive region where the low passive current density was independent of potential. Current density then increased abruptly at Epit , followed with a positive hysteresis loop during reverse potential scanning. The values of Epit were −0.60 V, −0.53 V and −0.50 V (SCE) for electrodes with passive films formed in air, in Na2 SO4 solution, and in chloridecontaining Na2 SO4 solution, respectively. There was similar Erp of about −0.70 V (SCE) for all electrodes. Furthermore, although the current density was set at 0.1 mA/cm2 for scan reversion, it did not decrease immediately after the potential was reversely scanned, but continued to increase. The different current densities resulted in different sizes of the hysteresis loop for the three types of passive film, which were correspondent with different repassivation abilities of the film. There was the biggest loop for passive film formed in chloride-containing solution, while the smallest loop for the film formed in air. 3.2. Conventional EIS measurements on the macroscopic electrode Fig. 2 shows the Nyquist diagrams measured on Al alloy 3003 electrodes with passive films formed in air and in solutions without and Cl− and solution, respectively, in 0.25 M Na2 SO4 + 0.5 M NaCl solution (at Ecorr ). There was a common characteristic for all curves, i.e., a capacitive semicircle in the high-frequency range and a diffusive tail in the low-frequency range. There was the biggest semicircle for passive film formed in solution without chloride ions, and the smallest semicircle for the film formed in chloridecontaining solution. Fig. 3 shows the Nyquist diagrams of the passivated 3003 Al alloy electrodes at Epit . It is seen that, at Epit , there was a significant decrease of the semicircle size. Moreover, an inductive loop was observed in low-frequency range in all diagrams. Furthermore, there was the biggest diameter of the semicircle for passive film formed in chloride-free solution (∼1300 ), and the smallest one for passive film formed in chloride-containing solution (∼12 only). Observation of electrode morphologies after LEIS
4157
Fig. 2. Nyquist diagrams measured on 3003 Al alloy with passive films formed in air (a), chloride-free solution (b) and chloride-containing solution (c) in 0.25 M Na2 SO4 + 0.5 M NaCl solution at individual corrosion potential.
measurements in Fig. 4 showed that the electrode passivated in chloride-containing solution suffered severe pitting corrosion with deep pits (Fig. 4c), while the passivated electrode in air and in chloride-free solution had slight pitting with small and shallow pits (Fig. 4a and b). The EIS measurements were performed in aerated solutions and thus described the sum of cathodic and anodic processes proceeding on heterogenous surface of oxide and pits. However, at Epit , anodic reaction including pitting corrosion dominated the electrode behavior, and the cathodic response was too small to be ignored. 3.3. LEIS measurements Fig. 5 shows the LEIS maps measured over 3003 Al alloy electrodes with passive films formed in air and the solutions without and with chloride ions, respectively, at Ecorr in 0.25 M Na2 SO4 + 0.5 M NaCl solution. In the x–y–z three-dimensional space, |Z| represents the measured impedance amplitude, which usually refers to the resistance of electrode to localized corrosion at individual measuring point. Thus, the fluctuating plane in the 3D figure represents the distribution of local impedance over the scanned surface of the electrode. The 3D impedance distribution was also projected on x–y plane, where the impedance amplitude of individual point was represented with different colors. It is seen that there were frequent fluctuations of impedance value measured on passive film formed in air (Fig. 5a). The impedance distribution was the most uniform on electrode passivated in chloride-free solution (Fig. 5b). 3.4. PZC measurements Fig. 6 shows the double-charge layer capacitance of passive films formed in air, 0.25 M Na2 SO4 , and 0.25 M Na2 SO4 + 0.5 M NaCl solutions, respectively, as a function of applied potential. It is seen that there is a common feature for the three curves, i.e., a minimum of double-charge layer capacitance that is considered as PZC of the electrode was observed. In addition, steady-state corrosion potential (Ecorr ) of the passivated 3003 Al alloy electrode was also included in each diagram. Generally, PZC ware more negative than Ecorr for all passivated electrodes. The differences between Ecorr and PZC (E = Ecorr – PZC) for passive films formed in air, Na2 SO4 solution and Na2 SO4 + NaCl solution were 0.119 V, 0.093 V and 0.036 V,
4158
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 4. Surface morphology of 3003 Al alloy with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Epit .
Fig. 3. Nyquist diagrams of 3003 Al alloy with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Epit .
respectively. Thus, there was a smaller potential difference, E, for passive film formed in aqueous solutions than that formed in air, and further, passive film formed in chloride-containing solution had a smaller potential difference than that formed in chloride-free solution.
3.5. Capacitance measurements and Mott–Schottky analysis Potential dependence of the capacitance of space-charge layer (Csc ) is expressed by Mott–Schottky relationship [21]: for n-type semiconductor 1 2 CSC
=
2 eεr ε0 ND
E − ϕfb −
T e
(1)
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 5. LEIS maps measured on 3003 Al alloy electrodes with passive films formed in air (a), chloride-free solution (b), and chloride-containing solution (c) at individual Ecorr in 0.25 M Na2 SO4 + 0.5 M NaCl solution.
for p-type semiconductor 1 2 CSC
=−
2 eεr ε0 NA
E − ϕfb −
T e
4159
Fig. 6. Relationship between double-charge layer capacitance vs. applied potential for 3003 Al alloy with passive films formed in air (a), chloride-free solution (b) and chloride-containing solution (c).
(2)
where e is electron charge (1.6 × 10−19 C), εr is dielectric constant of Al oxide, taken as 10 [22], ε0 is the vacuum permittivity (8.85 × 10−14 F cm−1 ), ND is the donor density, NA is the acceptor
density, E is the applied potential, ϕfb is flat-band potential, is Boltzmann constant (1.38 × 10−23 J K−1 ) and T is absolute temperature. ND and NA can be determined from the slope of the linear −2 relationship between CSC and E, while ϕfb is obtained from the −2 = 0. extrapolation to CSC
4160
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
Fig. 7. Mott–Schottky curves for three types of passive film formed on 3003 Al alloy measured at 1000 Hz in 0.25 M Na2 SO4 + 0.5 M NaCl solution.
Fig. 7 shows the Mott–Schottky curves for the three types of passive film measured at 1000 Hz in 0.25 M Na2 SO4 + 0.5 M NaCl solution. To demonstrate the consistence of capacitances measured by EIS and Mott–Schottky at Ecorr , the film formed in chloridecontaining solution was used as an example. The capacitances were 0.36 F m−2 and 0.5 F m−2 , respectively, indicating that the measured capacitance corresponded to the capacitance of passive film. It is seen Fig. 7 that all passive films behaved like an n-type semicon−2 ductor, with a positive slope of the linear CSC ∼ E. The curved form of the lines indicated the highly disordered nature of passive film where highly localized states existed between the valence and the conduction bands [23,24]. The fitted values of flat-band potential and donor density for three passive films are shown in Table 1. It is clear that passive film formed in air had a more negative ϕfb and a higher ND , and there were similar values of ϕfb and ND for passive films formed in aqueous solutions. 3.6. SIMS characterization Fig. 8 shows the chloride concentration profiles of the three passive films formed under various conditions and a blank 3003 Al alloy specimen measured by SIMS. It is seen that the concentration of chloride ions decreased continuously with the sputter depth in the electrode. As expected, there was the lowest or even zero chloride concentration for passive film formed in Na2 SO4 solution without a further anodic polarization in chloride-containing solution. The permeation depth of chloride ions into passive film followed the order: film formed in chloride-free solution < film formed in air < film formed in chloride-containing solution. Despite the slight irregularity of chloride concentration determined by SIMS for passive film formed in chloride-free solution, generally, the concentration of permeated chloride ions was ranked as: film formed in air < film formed in chloride-free solution < film formed in chloridecontaining solution.
Fig. 8. Depth profiles of chloride ions on passive films measured by SIMS.
4. Discussion 4.1. Passive films formed on 3003 Al alloy in air and in aqueous solutions The present work shows clearly (Fig. 2) that there are quite different stabilities of passive films formed on 3003 Al alloy electrode under various conditions. The measured EIS plots at corrosion potential, i.e., a high-frequency capacitive semicircle and a low-frequency diffusive tail, are fitted with an electrochemical equivalent circuit shown in Fig. 9a [25], where Rs is solution resistance, CPE is constant phase element, Rf is charge-transfer resistance of passivated 3003 Al alloy electrode, and W is Warburg diffusive impedance. The high-frequency capacitive semicircle represents the charge-transfer reaction of passivated 3003 Al alloy, while the low-frequency diffusive impedance is associated with the oxygen diffusion. Under stable passivation, the film formation achieves an equilibrium state. Thus, the film formation rate is equal to the dissolution rate of 3003 Al alloy. Electrochemical parameters fitted from EIS data are listed in Table 2. Apparently, there is the largest resistance (thus the most stable) for passive film formed in
Table 1 Flat-band potential, ϕfb , and donor density, ND , for passive films formed under various conditions. Film formation medium
ϕfb vs. SCE (V)
ND (×1027 m−3 )
In-air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
−1.509 −0.833 −0.815
32.39 7.73 7.48
Fig. 9. Electrochemical equivalent circuits used for fitting EIS data measured at individual Ecorr (a) and at Epit (b).
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
4161
Table 2 Electrochemical parameters fitted from EIS data measured at individual Ecorr . Film-forming condition
Rs ()
CPE-Y0 (S sn )
n
Rf (×105 )
W (S s0.5 )
In air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
7.4 9.6 9
7.14 7.48 1.58
0.93 0.80 0.91
1.91 5.71 1.03
226 24.9 247
chloride-free solution, the smallest resistance (most unstable) for passive film formed in chloride-containing solution, and the passive film formed in air in between. In general, passive film formed in aqueous solution is usually associated with a compact, uniform structure because of hydration process occurring on 3003 Al alloy electrode. It is acknowledged [26,27] that the hydrated passive film always shows a higher stability than that without hydration. For passive film formed in air, the film structure is usually non-uniform, with different thickness and compositional distribution. The LEIS mapping on air-formed passive film shows significant fluctuations of local impedance on the film (Fig. 5a), demonstrating the structural non-uniformity. As a comparison, the LEIS mapping on passive films formed in aqueous solutions (Fig. 5b and c) is quite uniform. LEIS has been demonstrated as a unique alternative to characterize the localized corrosion behavior of metal at a microscopic scale [28–31]. While the conventional EIS reflects an averaged impedance response of a macroscopic electrode, LEIS provides information specific to the individual microscopic site. Therefore, a LEIS mapping is capable of detect local active spots where a low impedance is usually identified. It is thus concluded from Fig. 5 that passive films formed in aqueous solutions are much more uniform, with fewer local defects, than that formed in air. For passive film formed in chloride-containing solution, it is expected that chloride ions get involved in the film formation process, as demonstrated by SIMS characterization results (Fig. 8) that there is the deepest chloride sputter depth and the highest chloride concentration at individual depth. It is generally acknowledged [6,7] that Cl− plays an important role in initiation and propagation of pitting corrosion. The high concentration of Cl− existing in the passive film formed in chloride-containing solution results in the difficulty of film to be repassivated, as seen in cyclic polarization measurement in Fig. 1. Upon initiation of the corrosion pit, Cl− also contributes to the rapid propagation of pitting.
4.2. Pitting susceptibility of 3003 Al alloy electrodes passivated in air and in aqueous solutions Electrochemical cyclic polarization measurement is capable of predicting the susceptibility of passivated metal to pitting corrosion [6,32]. Generally, if the reverse anodic curve is shifted to lower currents, i.e., negative hysteresis, or if the reverse curve essentially retraces the ascending curve, i.e., neutral hysteresis, no pitting corrosion will occur on the target metal or alloy. In contrast, if the reverse anodic curve is shifted to higher currents than the forward curve, i.e., positive hysteresis, pitting corrosion will occur. It is apparent from Fig. 1 that positive hysteresis loops are measured on 3003 Al alloy passivated under various conditions, suggesting that pitting of 3003 Al alloy passivated in air and in aqueous solutions is inevitable in the test system. The values of Epit show that
it is earliest for 3003 Al allow passivated in air to occur pitting, while it is relatively most difficult for 3003 Al alloy passivated in chloride-containing solution to initiate pitting, with that passivated in sulfate solution in between. Thus, in accordance with the measured Epit , The pitting susceptibility of passivated 3003 Al alloy is ranked as: in air > in chloride-free solution > in chloride solution. Furthermore, the area of the measured positive hysteresis loop indicates the repassivity capability of the metal or alloy, and a smaller area indicates a stronger ability for metal or alloy to repassivate. Therefore, there is the strongest capability for the air-passivated 3003 Al alloy to repassivate, and the weakest repassivating capability for 3003 Al alloy passivated in chloride-containing solution. The resistance of a passivated metal or alloy to pitting is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). Although the airformed passive film is easy to initiate pitting in chloride solution (the lowest Epit ), it has the strongest capability to repassivate, i.e., to self-repair after pitting initiation. Thus, the overall ability of passivated 3003 Al alloy to pitting is in moderate state. The passive film formed in chloride-containing solution has the relatively most positive Epit , but the weakest repassive ability. Consequently, it shows the most active state. The passive film formed in chloride-free solution is the most stable, which is attributed to the moderate Epit and repassivating ability. The relative stability of passive films formed under various conditions is demonstrated by EIS measurements on passivated 3003 Al alloy electrodes at their individual Epit (Fig. 3). Upon pitting, the roughness of the electrode surface increases, and the electrode state thus becomes more non-uniform. As a consequence, an inductive loop is observed in the low-frequency range, which is one of the typical features indicating pitting corrosion or electrode roughening [33,34]. The EIS feature is fitted with the electrochemical equivalent circuit in Fig. 9b, where L is inductance and RL is inductive resistance. The fitted electrochemical parameters are shown in Table 3. It is seen that there is the highest charge-transfer resistance for passive film formed in chloride-free solution (Fig. 3a), and the lowest charge-transfer resistance for the film formed in chloride-containing solution (Fig. 3b).
4.3. Pitting mechanism of passivated 3003 Al alloy electrodes The present work demonstrates that passive films formed in air and in aqueous solutions behave like an n-type semiconductor, as indicated by a positive slope of Mott–Schottky relationship in Fig. 6. According to point defect model [35], the main electron donors in an n-type semiconductor are oxygen vacancies. Chloride ions would occupy the positions of oxygen vacancies to generate cation vacancies at solution/film interface, which transport towards the film/metal interface to produce cation vacancy condensate, resulting in local depart of passive film and thus pitting. A complete
Table 3 Electrochemical parameters fitted from EIS data measured at Epit . Passivation condition
Rs ()
CPE-Y0 (×10−5 S sn )
n
Rf ()
RL ()
L (H)
In air In 0.25 M Na2 SO4 solution In 0.25 M Na2 SO4 + 0.5 M NaCl solution
39 35 2.9
126 1.93 1.64
0.80 0.71 1
970.1 1573 10.15
687 821 0.97
106 123 K 2.22
4162
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163
to pitting corrosion. The present work shows that there is the highest donor density in passive film formed in air, as seen in Table 1, providing potential sites for chloride ions to occupy. Moreover, it is determined that there is the strongest capability for chloride ion adsorption on passive film formed in air, it is expected that the passive film formed in air has the lowest resistance to pitting, as demonstrated by a lowest Epit . It is realized that the size, shape and distribution of second phase intermetallic particles influence the pitting corrosion behavior. For example, it was found [39] that the adsorption of Cl− in passive film prefers at or around inclusions and second phase particles due to weaker oxide film on these sites. This relevant subject will be explored in more detail in the further work. 5. Conclusions
Fig. 10. Schematic diagrams of electric field distributions at the electrode/solution interface when electrode is at Ecorr (a) and PZC (b).
description about the interfacial electrode reactions and masstransport processes are proposed by Macdonald based on PDM [35]. Therefore, adsorption and permeation of chloride ions into passive film is usually the first step to cause pitting. To understand fundamentally the sources of chloride ions to result in pitting in passive film under different forming conditions, the potential of zero charge is measured and shown in Fig. 6. The potential of zero charge at which the excess charge at the electrode/electrolyte interface could be eliminated usually acts as a reference in determining the type and amount of ions adsorbed on the electrode surface [36]. If an electrode under its open-circuit potential is positively charged and thus adsorbed with anions, the PZC is a more negative potential applied to counteract the excess charges at the interface, as schematically represented in Fig. 10. The potential differences between Ecorr and PZC, E, for Al passive films formed under various conditions show positive values, suggesting that the electrode surfaces are positively charged at Ecorr for all types of passive films. Consequently, chloride ions are expected to adsorb on electrode surface. Furthermore, from the value of E, it is deduced that passive film formed in air has the largest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Thus, passive film formed in aqueous solution, especially the chloride-containing solution, has a weak capability in anions adsorption, which is attributed to the mutual repulsion among anions. It is expected that a high concentration of chloride ions exists in passive film formed in chloride-containing solution, and a further adsorption of chloride ions from the solution will be repulsed. Therefore, chloride ions causing pitting of passive film formed in air and in chloride-free solution come from the test solution, while chloride ions resulting in pitting of passive film formed in chloride-containing solution are mainly those existing in the film during film-forming stage. It has been demonstrated [20,37,38] that passive film with a higher donor density is always associated with a lower resistance
Passive film formed on 3003 Al alloy in air and in Na2 SO4 solution without and with NaCl addition show n-type semiconductor in nature. Passive film formed in chloride-free solution is most stable, and that formed in chloride-containing solution is most unstable, with the film formed in air in between. Passive film formed in air is associated with a non-uniform structure/composition and the highest donor density in electronic structure, resulting in a reduced stability than those formed in aqueous solution. However, incorporation of chloride ions in passive film would decrease significantly the resistance of the film to pitting when it is formed in a chloridecontaining solution. Pitting of 3003 Al alloy passivated in air and in aqueous solutions is inevitable in the presence of chloride ions in the test solution. There is the strongest capability for the air-passivated 3003 Al alloy to repassivate, and the weakest repassivating capability for Al alloy passivated in chloride-containing solution. The resistance of the passivated 3003 Al alloy to pitting is dependent on the competitive effects of pitting (breakdown of passive film) and repassivation (repair of passive film). The positive potential differences between Ecorr and potential of zero charge for Al passive films formed under various conditions suggest that the electrode surfaces are positively charged at Ecorr . Consequently, chloride ions are expected to adsorb on electrodes. Passive film formed in air has the strongest capability to adsorb chloride ions, while the film formed in chloride-containing solution the least. Chloride ions causing pitting of passive film formed in air and in chloride-free solution come from the test solution, while those resulting in pitting of passive film formed in chloridecontaining solution exist in the film during film-forming stage. Acknowledgements This work was supported by Canada Research Chairs Program, Natural Science and Engineering Research Council of Canada (NSERC) and Dana Canada Corporation. References [1] G. Davies, Materials for Automobile Bodies, Butterworth-Heinemann, Oxford, UK, 2003. [2] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A 280 (2000) 37. [3] Institution of Mechanical Engineers, Corrosion of Motor Vehicles, Mechanical Engineering Publications Limited, London, UK, 1976. [4] R. Baboian, Automotive Corrosion by Deicing Salts, National Association of Corrosion Engineers, Houston, USA, 1981. [5] M.G. Fontana, Corrosion Engineering, McGraw-Hill, CA, 1990. [6] Z. Szklarska-Smialowska, Corros. Sci. 41 (1999) 1743. [7] G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2186. [8] F. King, Aluminum and Its Alloys, Ellis Horwood, Chichester, England, 1987. [9] C. Vargel, Corrosion of Aluminum, Elsevier Science, San Diego, USA, 2004. [10] S. Menezes, R. Haak, G. Hagen, M. Kendig, J. Electrochem. Soc. 136 (1989) 1884.
Y. Liu et al. / Electrochimica Acta 54 (2009) 4155–4163 [11] B.A. Shaw, G.D. Davis, T.L. Ritz, B.J. Rees, W.C. Moshier, J. Electrochem. Soc. 138 (1991) 3288. [12] P.M. Natishan, G.K. McCafferty, E. Hubler, Electrochim. Acta 40 (1995) 323. [13] F.J. Martin, G.T. Cheek, W.E. O’Grady, P.M. Natishan, Corros. Sci. 47 (2005) 3187. [14] C.Y. Chao, L.F. Lin, D.D. Macdonald, J. Electrochem. Soc. 128 (1981) 1187. [15] L.F. Lin, C.Y. Chao, D.D. Macdonald, Electrochem. Soc. 128 (1981) 1194. [16] C.Y. Chao, L.F. Lin, D.D. Macdonald, Electrochem. Soc. 129 (1982) 1874. [17] J.O’M. Bockris, Y. Kang, J. Solid State Electrochem. 1 (1997) 17. [18] J.C.S. Fernandes, R. Picciochi, M. Da Cunha Belo, T.M. Silva, M.G.S. Ferreira, I.T.E. Fonseca, Electrochim. Acta 49 (2004) 4701. [19] K.L. Levinea, D.E. Tallman, G.P. Bierwagen, J. Mater. Process. Technol. 199 (2008) 321. [20] L. Kobotiatis, N. Kioupis, P.G. Koutsoukos, Corrosion 53 (1997) 562. [21] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, New York, 1980. [22] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1985. [23] M.H. Dean, U. Stimming, Corros. Sci. 29 (1989) 199. [24] M.H. Dean, U. Stimming, J. Electroanal. Chem. 228 (1987) 135.
4163
[25] E. Barsoukov, J.R. Macdonald, Impedance Spectroscopy Theory, Experiment, and Applications, A John Wiley & Sons, Inc., New Jersey, 2005. [26] W.E. O’Grady, J.O’M. Bockris, J. Electrochem. Soc. 127 (1980) 555. [27] T.E. Pou, O.J. Murphy, V. Young, J.O’M. Bockris, L.L. Tongson, J. Electrochem. Soc. 131 (1984) 1243. [28] F. Thebault, B. Vuillemin, R. Oltra, K. Ogle, C. Allely, Electrochim. Acta 53 (2008) 5226. [29] A. Davoodi, J. Pan, C. Leygraf, S. Norgren, J. Electrochem. Soc. 155 (2008) C211. [30] M.C. Li, Y.F. Cheng, Electrochim. Acta 53 (2008) 2831. [31] G.Z. Meng, C. Zhang, Y.F. Cheng, Corros. Sci. 50 (2008) 3116. [32] P. Gimenez, J.J. Rameau, M.C. Rebioul, Corrosion 37 (1981) 673. [33] L. Niu, Y.F. Cheng, Wear 265 (2008) 367. [34] T. Hagyar, J. Williams, Trans. Faraday Soc. 57 (1961) 2288. [35] D.D. Macdonald, J. Electrochem. Soc. 139 (1992) 3434. [36] J.O’M. Bockris, A.K.N. Reddy, M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Acad, 2000. [37] Y.F. Cheng, J.L. Luo, Electrochim. Acta 44 (1999) 2947. [38] H. Tsuchiya, S. Fujimoto, O. Chihara, T. Shibata, Electrochim. Acta 47 (2002) 4357. [39] C.J. Lin, R.G. Du, T. Nguyen, Corrosion 56 (2000) 41.