Anodisation of aluminium alloy AA7075 – Influence of intermetallic particles on anodic oxide growth

Journal Pre-proof Anodisation of Aluminium Alloy AA7075 – Influence of Intermetallic Particles on Anodic Oxide Growth ¨ Fan Zhang, Cem Ornek, Jan-Olov Nilsson, Jinshan Pan

PII:

S0010-938X(19)30729-2

DOI:

https://doi.org/10.1016/j.corsci.2019.108319

Reference:

CS 108319

To appear in:

Corrosion Science

Received Date:

17 April 2019

Revised Date:

25 September 2019

Accepted Date:

30 October 2019

¨ Please cite this article as: Zhang F, Ornek C, Nilsson J-Olov, Pan J, Anodisation of Aluminium Alloy AA7075 – Influence of Intermetallic Particles on Anodic Oxide Growth, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108319

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Anodisation of Aluminium Alloy AA7075 - Influence of Intermetallic Particles on Anodic Oxide Growth Fan Zhanga*, Cem Örneka, Jan-Olov Nilssonb, Jinshan Pana a

KTH Royal Institute of Technology, Department of Chemistry, Division of Surface and Corrosion

Science, Sweden Hydro Extruded Solutions, Surface Treatment & corrosion, SE-612 81 Finspång, Sweden

*

Corresponding author: [email protected]

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Investigation of anodisation process of AA7075-T5 under operando conditions In-situ EC-AFM and operando EIS measurements under anodic potentials Real-time evidence of IMPs’ influence on the formation of AAO and breakdown Comprehensive characterisation of complex microstructure with SEM, XRD and SKPFM

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Highlights

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Abstract

Microstructure and Volta-potential analyses were conducted to characterise intermetallic-particles (IMPs) in AA7075-T5. EIS and AFM were applied under operando-conditions to investigate anodisation

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processes. IMPs have pronounced influence on the growth of anodic aluminium oxide (AAO) films resulting in low charge-transfer resistance. Cu-bearing constituents show cathodic-character, whereas Mg2Si and MgZn2 particles show anodic-character. During anodisation, Al7Cu2Fe remain stable with peripheral-dissolution around boundary. De-alloying of S-phase particles leads to the detachment. Mg2Si undergoes de-alloying at low potential, and re-passivation at high potential. MgZn2 dissolves entirely upon

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anodization. Localised-dissolution in large-IMPs boundaries or nanometre-sized IMPs facilitates bubble evolution, confirming local breakdown of barrier-layer. Key words: intermetallic particles; anodization; SKPFM; in-situ EC-AFM; operando EIS; localised dissolution

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1. Introduction Among commercial Al alloys, 7xxx series is one of the most widely used high strength alloys due to a high strength-to-weight ratio. Grade AA7075 is one of the commonly used high strength materials in military and civil aerospace industry [1]. Their excellent mechanical properties are obtained by the addition of alloying elements and thermomechanical treatments, which lead to the formation of intermetallic phases/particles (IMPs) in an optimal microstructure. Zinc is the principal alloying element in 7xxx alloys, imparting high hardness and yield strength by the formation of fine dispersoids, in particular, η’-phase (MgZn2), dispersed in the matrix [2, 3]. The MgZn2 particles have the size of the order

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of nm and decorate grain boundaries, and usually, undergo preferential dissolution in chloride-containing media [4]. The addition of copper (> 1 wt. %) typically leads to the formation of AlMgCu phases, which

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further contribute to an increased strength due to precipitation hardening. Constituent IMP particles of a relatively large size and irregular shape often show a different electrochemical character as compared to

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the surrounding matrix due to local chemical variations and strain localisation [1]. Dominant coarse constituent IMPs in alloy 7075 are of Al7Cu2Fe and (Al, Cu)6(Fe, Cu) types that often reported as being cathodic with respect to the matrix, and fewer anodic Mg2Si particles are also present in the microstructure

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[5, 6]. The S-phase (Al2CuMg) intermetallic compound may also be present [7, 8]. Aluminium and its alloys generally possess good corrosion resistance under ambient conditions due to the

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spontaneous formation of a protective surface oxide layer [9]. However, this native oxide layer is very thin, and only provides limited corrosion resistance to the metal. The electrochemical anodisation process is widely used in the surface treatment of Al alloys for industrial applications to increase the thickness of the

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oxide layer and thus achieve a high corrosion resistance of the metal. The anodising applicability of an Al alloy is influenced by electrochemical properties of the IMPs since they form heterogeneous sites in the microstructure [10]. Previous investigations of electrochemical properties of anodised aluminium materials showed the lower applicability of 7xxx alloy for the anodising treatment due to the formation of a weaker barrier-type oxide film [11, 12]. Thick anodic Al oxide (AAO) films can be developed on the

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7xxx alloy, however, without the formation of a barrier-type oxide layer giving the resistance of the material against corrosion [11]. This phenomenon is associated with the presence of a large variety of Zn-, Mg- and Cu-containing IMPs in the microstructure, which can readily affect the microstructure of AAOfilms [13]. In principle, IMPs will be oxidised at different rates than the surrounding Al-matrix, resulting in the formation of flaws within the AAO film. The electrochemical nobility and the anodisation propensity of Al alloys are highly dependent on the microstructure as well as environmental conditions, which all can influence the anodisation process. The electrochemical nobility of the IMPs may alter during anodisation due to de-alloying or anodic dissolution, which could result in a flawed AAO film. Therefore, 2

the study of the IMPs regarding their electrochemical behaviour during anodic oxidation at various anodisation potentials is needed to understand and optimise the anodisation process [14, 15]. Usually, Cu-, Fe- and Ti-containing IMPs are nobler than the Al-matrix [14]. For instance, Al7Cu2Fe has been reported to act as a strong cathode and being highly efficient in supporting oxygen reduction reactions, causing periphery dissolution of the surrounding matrix, known as ‘trenching’ [6, 16]. The Sphase (Al2CuMg), a very common IMP, has often been reported as an anode, i.e., more active than the surrounding Al matrix [7, 8]. However, selective Mg dissolution from S-phase can occur, which leads to Cu enrichment on the surface and thus a nobility inversion [17]. De-alloying of the S-phase has been

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observed under anodic polarisation [10, 18], leaving a porous Cu-rich particle remnant at the particle’s location [19]. On the other hand, Zn and Mg usually dissolve at very low anodisation potentials [20].

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Therefore, MgZn2 particles readily dissolve under anodic potentials, ultimately leaving behind surface cavities [14, 21, 22]. In contrast, Mg2Si particles have been reported to dissolve in an incongruent manner

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upon anodising, causing the incorporation of Si or SiOx particles into the AAO film [21, 23]. The anodisation process of AA7075 alloys has been extensively investigated with special focus on the

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influence of second phases on the microstructure and surface properties of AAO-films [14, 16, 21, 24-28]. However, there is a lack of real-time evidence for the influence of IMPs on the formation of the AAO film. The AA7075 alloy has a very complexity microstructure. In this study, SEM, XRD and SKPFM were

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utilised in combination for comprehensive characterisation of the microstructure and assessment of the nobility of IMPs with respect to the matrix. Electrochemical measurements including electrochemical impedance spectroscopy (EIS), and atomic force microscopy (AFM) measurements, were performed to

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investigate the anodisation process of the alloy at various anodic potentials. The anodisation process was monitored in-situ under operando conditions at step-wisely increased potentials, to determine the changes of the surface oxide film due to the anodic polarization, aiming to establish a mechanistic understanding of the process, especially the influence of IMPs on the anodic growth and dissolution of such AAO films. 2. Experimental

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2.1 Material used and sample preparation The sample material was the aluminium alloy AA7075 with following chemical composition (in wt. %): 0.17% Si, 0.19% Fe, 1.6% Cu, 0.04% Mn, 2.3% Mg, 0.18% Cr, 0.01% Ni, 5.5% Zn, 0.02% Ti, 0.01% Pb, and Al (bal.). The material was extruded by Sapa Profiles. Samples in sizes of 20 mm × 20 mm × 4 mm were cut from the extruded bar along its extrusion direction and wet-ground with SiC grinding paper down to 1200-grit size for EIS measurements. Further samples were ground and polished with 3, 1, and 0.25 μm diamond paste for SEM, SKPFM and electrochemical controlled AFM (EC-AFM) measurements. All 3

samples were cleaned ultrasonically, dried with purging nitrogen and kept in the lab air environment for no more than two days before being measured. 2.2 Operando EIS measurements EIS measurements were performed under operando conditions at the open-circuit potential (OCP) and during anodisation at step-wisely increased potentials in a neutral pH 0.2 M Na2SO4 solution at room temperature. EIS spectra were recorded after 30 min of anodisation at 1, 2, 4 and 8 V, successively, with a perturbation amplitude of 10 mV and a frequency range from 104 Hz to 10-2 Hz. At each potential for each

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sample, EIS was conducted two times in sequence to investigate the development of oxide films with time. Three parallel experiments were carried out. All EIS spectra were obtained by an Autolab potentiostat (Metrohm Autolab B.V., Netherlands), using a three-electrode electrochemical cell, with a saturated

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Ag/AgCl reference electrode and a platinum mesh counter electrode. The exposure area of the sample was 1 cm2. The software Nova 1.11 was employed for data acquisition and analysis. All measurements were

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performed at room temperature.

The equivalent electric circuit representing a two-layer structure (a compact inner-layer and a relatively

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porous outer-layer), as shown in Figure 1a, was used to describe the interface structure of the alloy at OCP, where Rs is the solution resistance, R1 and R2 are the resistance of the outer and inner oxide layers

ZCPE (ω) = 1 / Y0 (iω)n

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respectively. CPE is a constant phase element that is given by [29]: (1)

where Yo is a constant, and n (0 ≤ n ≤ 1) is a mathematical factor obtained from the fitting. In this model,

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CPE1/R1 represents the outer part of the interface close to the electrolyte, and CPE2/R2 represents the inner part close to the metal substrate. The equivalent circuit in Figure 1b [30] was used to fit the EIS data under applied anodic potentials (operando conditions), where Rct is the charge transfer resistance, L is the inductance, and RL is the inductive resistance of adsorbed charged species on the surface. The capacitive behaviour of the AAO film was assumed to be equivalent to a parallel-plate capacitor, which enabled the

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calculation of its thickness from fitted EIS results [11, 13]. The capacitance can be determined from the constant Yo and exponential factor n of the CPE according to the equation below [31, 32]: C = (Yo×Rct )(1/n) / Rct

(2)

If the oxide capacitance (Cox) is much smaller than the double-layer capacitance Cdl, the measured C is approximately equal to Cox, then the thickness of AAO film can be calculated from the capacitance values according to the equation: C = ε0εrA / d

(3) 4

where εo is the dielectric permittivity of vacuum, εr is the dielectric constant of the Al oxide when it dominates the capacitive response. A range of εr values have been reported in the literature [33], and we used a value of εr = 10 for the Al oxide for the AAO film thickness calculation. d is the thickness of the AAO film. A is the effective surface area that increases slightly with increasing surface roughness, however, in this study the exposed geometric sample area was used for the calculation. 2.3 EC-AFM measurements EC-AFM measurements were performed at varying anodising potentials in neutral 0.2 M Na2SO4 solution,

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using a Bruker Dimension Icon AFM system equipped with an electrochemical cell. This solution was chosen to avoid fast reactions that may occur in acidic/alkaline solutions and disturb the AFM measurement. The solution was added to the cell until the sample’s surface was covered with a liquid

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layer of about 3 mm in thickness. A silver wire and a platinum wire were used as the reference and counter electrode, respectively. The sample was anodised potentiostatically in steps by applying anodic

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potentials of 1, 2, 4 and 8V vs. Ag/AgCl (sat.) for 30 minutes at each potential. Potential was successively increased from one potential step to the other. AFM mapping was started immediately after applying the

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potential, and the topography was scanned three times at each potential. The topography was measured in contact mode using a silicon AFM-probe from Budget Sensors, having a spring constant of 0.2 N/m. The scan was conducted at a rate of 1 Hz over a scanned area with 512 × 512 pixels, thus each scan will take

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about 10 min. The height data were subjected to a 1st order flattening with Nanoscope Analysis V1.5 software to remove the Z offset and tilt.

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2.4 Microstructure characterisation

Microstructure characterisation consisted of SEM and XRD analyses. SEM analyses were performed using a JEOL JSM-7001F SEM, which was equipped with an XMax EDX Silicon Drift Detector with 80 mm² window size from Oxford Instruments, operated by AZtec V3.3 acquisition software. EDX spectra and point analyses were carried out at 10–15 kV to obtain the chemical composition of large IMPs (>3 µm). EDX maps were recorded at an accelerating voltage between 3.5–10 kV. XRD measurement was

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done for identification of phases in the microstructure using the Bruker Discover D8 diffractometer, which was operated at a tube voltage of 40 kV at a tube current of 40 mA with a Co radiation source (λKα1 = 1.788965 nm). X-ray signals were collected in the 2-theta range of 20-120° with a step size of 0.01° and a radiation dwelling time of 7 seconds to obtain high-resolution diffraction data. The data were smoothed and processed using the OriginLab 2018 software. The XRD database of the International Centre for Diffraction Data was used for comparison of indexed peaks with the peak position and intensity of possible phases.

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2.5 Local Volta potential measurements SKPFM measurement was performed to evaluate the relative nobility of the IMPs in the microstructure with respect to the aluminium-alloy matrix in order to reveal electrochemical properties in local scale. Local Volta potential measurements were done in the air using the Bruker Dimension Icon AFM system in frequency-modulated (FM-KPFM) single-path mode to obtain local nobility information in the microstructure. The probe used was the SCM-PIT probe from Bruker, which is a Pt-coated n-doped Si tip. A DC voltage of 6 V was applied to the tip, and the sample was ground. Nanoscope Analysis V1.5 software was used for the analysis of the data. Measurements were conducted at 23 °C and 55% relative

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humidity. The pixel resolution was 512 × 512 with a map area of 20 µm × 20 µm, yielding a lateral resolution of 39 nm/pixel. The topographic maps were subjected to a 1st order flattening to remove the Z

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offset and tilt, whereas Volta potential images were subjected to a 0th order flatten to remove the Z offset only. The Volta potential maps were inverted using the software in order to conform to the

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electrochemical nobility concept to show noble (cathodic) regions as high and active (anodic) regions as low values [15, 34-38]. Thus, regions with higher Volta potentials indicate higher electrochemical nobilities whereas lower values indicate enhanced propensity to oxidation. In this respect, large Volta

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potential differences would indicate a large driving force for micro-galvanic corrosion.

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3 Results

3.1 Anodic current transients during anodisation

Current density vs. time data obtained during anodisation are summarised in Figure 2. The curves were

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divided into two regimes, as indicated by the vertical line in Figure 2. In regime I, the current density dropped exponentially with time, and in regime II the current density became relatively stable. At 1V, the first step of anodization, the current density dropped quickly in the initial stage, and it was even higher than that of 2 and 4 V. Whereas, from 2 to 8 V, the current density increased with increasing potential. Moreover, a slight rise of current density was observed in regime II at 1 V and 8 V of the anodisation.

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3.2 Electrochemical impedance spectroscopy analysis Figure 3 summarises the EIS spectra collected at OCP and during anodisation. The Nyquist plots show two time constants at OCP (Figure 3a). The spectra fitting were done using the electrical equivalent circuit as shown in Figure 1a, representing the heterogeneous electrode/electrolyte interface. The fitting results obtained at OCP are summarised in Table 1. The capacitance values for CPE1 are at the level of 105

F/cm2 that is close to the reported level of electrolytic double layer capacitance (Cdl) [39]. The resistance

of the second time constant feature decreased at the second run of EIS, indicating the existence of electrochemical processes due to dissolution of the alloy. 6

The spectra obtained under anodic potentials have three time-constant features (Figure 3b), with a capacitive loop at high frequency, an inductive loop at the medium frequency, and a second capacitive loop at low frequency. The high-frequency capacitive behaviour is associated with the charge transfer reactions (faradaic processes) and related to dielectric properties and thickness of AAO films [40]. The appearance of the inductive loop is associated with ionic relaxation effects of the charged species on/near the surface, suggesting the active/local dissolution of the metal and/or oxide film at each applied anodic potential [41]. The origin of the low-frequency capacitive response is controversially debated [42-46], and outside the focus of this work. Therefore, the fitting of the EIS spectra was restricted to the high and

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medium frequency range. As shown in Table 2, the measured Rct and inductance of the AAO-film decreased with increasing

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potential. In this case, the measured CPE represents the overall capacitive response of the entire electrode/electrolyte interface. The parallel resistive path corresponds to the faradaic reactions. As a whole,

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this corresponds to the so-called leakage capacitor allowing the transport of electrons through faradaic reactions. In general, the Cmea consists of contributions from both the electrolytic double layer and oxide layer in series connection. However, when the Cox is much smaller than the Cdl, the Cmea is dominated by

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the Cox, i.e. the contribution of Cdl is negligible [29]. Therefore, the thickness of the oxide film can be calculated with equations (1) and (2). Figure 4 summarises the calculated oxide film thickness for each

the growth factor of the AAO film.

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anodised potential. There is a clear linear relationship with a regression slope line of 0.89 nm/V, which is

3.3 AFM mapping under electrochemical control

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In-situ AFM results, as summarised in Figure 5 - Figure 8, revealed that active anodic dissolution took place on the surface during anodisation in the solution, occurring simultaneously with the growth of AAO film. According to dominant electrochemical activities, we categorised the surface areas into three groups: (i) region with uniform growth of the AAO film, (ii) region with general dissolution, and (iii) region with local dissolution.

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(i) Region with uniform growth of the AAO film - Figure 5 shows the topographic images of a region with uniform growth of the AAO film, where the surface morphology did not alter substantially during anodisation up to 8 V. The results show that some polishing scratches disappeared with increasing anodisation potential, and their enlargement images present the formation of oxide like particles fully covering the surface upon the anodisation (Figure 5b and 5c). A statistical analysis of the scanned area was conducted as given in Table 3. The surface area difference is the percentage increase of the threedimensional area over the two-dimensional area, and Rq is calculated as: 𝑅𝑞 = √

∑(𝑧𝑖 )2 𝑛

, where zi and n are 7

the local height value and number of points, respectively. The small values of the surface area difference and the slight increase of Rq with the increasing potential correspond to a slightly increased surface roughness, and imply that the growth of AAO film was quite uniform on the scanned surface. (ii) Region with general dissolution - Figure 6 shows the observation of a surface region with general dissolution during anodisation. As can be seen in Figure 6a, the heterogeneous alloy surface with many particles seemed to be covered by a native oxide film at OCP. The outmost layer of the oxide film dissolved gradually with the increase of the anodising potential, leaving numerous pits and holes on the surface (Figure 6b – 6f). A crater-like feature with a particle in the centre was observed, initially covered

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by a cap (square marked and enlarged in Figure 6a – 6b), which was most likely hydrous Al oxide. The cap broke up at 2 V and left a deep hole behind. Apparently, dissolution took place beneath the cap and

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resulted in complete removal of the particle (Figure 6d). Moreover, at the OCP, a constituent particle with the size of ca. 4 µm was observed (arrow marked in Figure 6a), which was slightly higher (brighter) than

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the matrix. The top layer of the arrow marked particle was dissolved at 2V (Figure 6c). Within 10 min of anodisation at 4V (Figure 6d), further dissolution of this particle occurred (darker), however, after ca. 20 min of anodisation at 4V, the particle site became in the similar height of the matrix (Figure 6e). After 30

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marked particle (brighter in Figure 6f) [47].

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min of anodisation at 8V, some deposit most likely in form of Al(OH)3 was formed on top of the arrow

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(iii) Region with local dissolution -

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Figure 7 and Figure 8 display the AFM images of the regions with dominant localised dissolution during the anodisation process. The scanned area of Figure 7 contained several constituent particles present as colonies, and localised anodic dissolution of the boundary region surrounding the particles occurred during anodisation at 1 and 2V (Figure 7b and 7c). Figure 8 shows the development of a small pit due to

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the local dissolution of an anodic particle, as evidenced by the profile lines in Figure 8e. The results revealed that small pits (ca. 25nm) formed by the dissolution of nano-sized IMPs started to become visible during anodisation at 1V, and their depth remained constant (ca. 20 nm) with the increase of potential due to the complete dissolution of nanometre-sized IMPs. On the other hand, the dissolution of interphase boundary regions adjacent to the constituent particles started at OCP and grew further during anodisation at 1 and 2V. Moreover, the body of the constituents remained stable until to 2V. The AFM mapping of this region could not be conducted at 4 and 8 V due to evolved gas bubbles, which disturbed the AFM tip.

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3.4 In-situ optical microscope observation The optical images in Figure 9 show that during anodisation at 4 and 8V, gas bubbles were generated at some sites on the surface. Since the bubble evolution only interferes the AFM scan of the region with local dissolution, therefore, bubbles only are generated on localized dissolution dominant sites. In contrast, on the uniform AAO growing region and the general dissolving region, no bubbles were generated during anodisation up to 8V. 3.5 SEM and XRD characterisation of IMPs

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An overview of the microstructure is shown in Figure 10a. Numerous IMPs in sizes between tens of nanometres up to several micrometres were observed decorating the entire microstructure. Most IMPs

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were intra-granular with some particles also seen on grain boundaries. Larger IMPs (>1 µm), also termed as constituents, were often seen in clusters having particles of different kinds, being attached to each other. They show different signal contrast of the SEM image due to different mean atomic numbers from each

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phase (Figure 10b). The smaller particles (<1 µm), also termed as dispersoids, seemed to be homogeneously dispersed in the microstructure (Figure 10c and 10d). EDX analyses showed that most of

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the large IMPs were enriched in Fe, Cu, and Mn (Figure 11). In general, three distinct types of large particles having different chemical composition were observed: i) Cu-Zn-Cr-Fe-Mn-Si-Ti-rich, ii) Fe-Mn-

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Cr-Cu-Si-rich, and iii) Mg-Zn-rich particles.

XRD analysis revealed the presence of various types of IMPs in the microstructure (Figure 12). In total, 36 distinct and clear diffraction peaks were collected, as shown in Figure 12 and summarised in Table 4.

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Most and strongest signals came from Cu and Fe aluminides (Al7Cu2Fe, Al2CuMg (S-phase), Al3Fe), and some minor peaks were detected and indexed as Mg2Si, MnAl6, ZnAl2, MgZn2, and Al4.01MnSi0.74. The peaks of the smaller particles had broad 2-theta widths indicating nanometre-sized particles, possibly those shown in Figure 10d. Some of the detected peaks remain unidentified. Hence, the large particles in Figure 11 were most likely the Cu and Fe aluminides, e.g., Al7Cu2Fe. The

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bright particles in the SEM image in Figure 10b were most-likely Al7Cu2Fe on a coarse S-phase (Al2CuMg), due to more electron signal scattered because of heavier elements in the structure than in the remaining phase [26]. Moreover, most of the dark particles in the SEM images in Figure 10a were Mg2Si or Si particles as confirmed by the EDX analysis (data not shown). 3.6 SKPFM analysis Figure 13a displays the topographic image, and Figure 13b presents the variation of the Volta potential over the scanned area, showing the relative nobility differences between the particles and the matrix. The 11

higher Volta potentials indicate the higher relative nobilities. The scanned surface area contained micrometre-sized IMPs, and their boundaries were slightly dissolved through the polishing processes (Figure 13a). Volta potential data suggest that these particles have an intrinsically cathodic character with respect to the matrix (Figure 13b). Moreover, some smaller particles in sizes of several tens nanometres to hundreds of nanometers can also be observed, which is higher than the matrix (brighter colour in Figure 13a). These small particles showed an anodic character relative to the matrix (Figure 13b).

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4. Discussion 4.1 Intermetallic particle type

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The type of IMPs observed in this study can be identified based on the particle characterisation results and their size, shape, and electrochemical activities.

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It has been reported that the Cu-bearing constituents in AA7xxx alloys are often found in colonies, being comparatively large and irregularly shaped [6, 14], and can lead to peripheral dissolution at the particle-

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matrix interface [6, 48]. In this study, the micrometre-sized IMPs were also found in colonies, and have been identified as Al7Cu2Fe particles (Figure 11), being cathodic with respect to the matrix (Figure 13b). Therefore, the micrometre-sized IMPs observe in Figure 7 and Figure 8 are in the type of Al7Cu2Fe, as

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judged by the size, location and their inert character upon anodization. XRD results revealed the presence of the other type of Cu-bearing constituent, S-phase (Al2CuMg) in the

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microstructure (Figure 12). The Al2CuMg IMP is more active than the surrounding Al matrix and dealloying occurs during anodisation [7, 8]. Therefore, the square marked IMP in Figure 6 is most likely in the Al2CuMg type.

Whereas, the arrow marked particle in Figure 6 shows an incongruent manner upon anodising, i.e., dealloy at low potential and re-passivate at high potential. This particle most likely is Mg2Si that shows a

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dark colour in SEM-BSE (Figure 10a), on which similar anodising processes have been reported [21, 23]. Moreover, judge by the size, the particle in size of hundreds of nanometres observed in our SKPFM map (Figure 13) can be assigned to the Mg2Si particle that is anodic to the alloy matrix. MgZn2 is another type of IMP present in the AA7075 alloy as finely-dispersed dispersoids in the size of several to several tens of nanometres [4]. The XRD results and SEM image (Figure 10d) show that the microstructure of the studied sample contains an appreciable amount of MgZn2 particles, and they show an anodic character with respect to the matrix as shown in the SKPFM result in Figure 13. Moreover, it has been reported that Zn and Mg usually become dissolved at very low anodisation potentials, leaving a pit in 12

the substrate with the same size of the particle [20]. Therefore, the circle marked nanometre-sized IMP in Figure 8 that completely dissolved at 1V are most likely the anodic MgZn2.

4.2 The oxide film formed at open circuit potential At OCP, EIS spectra show two time-constants features, which relates to the electrolytic double layer and an active dissolution processes of the alloy. The high level of the capacitance (Y2) suggests the EIS detectable oxide layer is non-protective, which is proved by the decrease of Rp2 at the second run.

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Moreover, the Y1 and Y2 increased with exposure time at OCP, which likely relates to the increase of surface roughness due to the local dissolution of the alloy. The statement is supported by the AFM results

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showing that the dissolution occurred at OCP along the boundaries of cathodic IMPs (Figure 7a). Localised dissolution at the boundary region, and possibly also at the nanometre-sized IMP sites, increases the porosity of the oxide film and decreases the film resistance. The EIS and AFM results obtained at OCP

of oxide in the neutral aerated condition.

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4.3 The anodic oxide film formed at applied potentials

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indicate that dissolution processes occur on the alloy, and such Zn-enrich alloy barely forms barrier-type

Anodic oxidation and dissolution occur spontaneously on the AA7075 surface during the anodisation. The

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current density decreases almost exponentially with time because of the growth of a barrier-type AAO layer. The growth rate of the barrier-type AAO layer decreases exponentially in the first regime (Figure 2), which determines the maximum thickness of the layer at the set potential [49]. During the second regime

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of anodisation (Figure 2), the porous-type AAO layer grows continuously [13]. However, the steady-state current density indicates that the growth of the porous-type AAO layer does not affect total film resistance. The current density increases with increasing potential (Figure 2), and EIS results also show that the resistance of the AAO film decreases with increasing potential (Table 2). The phenomena can be explained in several aspects. First, the active dissolution of Al2CuMg type IMPs as revealed by EC-AFM

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(Figure 6) results in an increased porosity of the AAO film and thus a decreased resistance. Second, incorporation of the alloying elements and electrolyte ions into the AAO film may also contribute to an increased electrical conductivity and thus the decreased resistance. Third, an increased effective surface area due to the increased porosity of the AAO film may also result in a decreased resistance. The thickness of AAO film increases linearly with increasing potential as calculated from EIS fitting data (Figure 4). The EIS measured film thickness only represents the layer with a relatively compact structure, i.e. the barrier oxide layer. The porous layer filled with electrolyte cannot be determined by EIS. The 13

AAO growth factor of the AA7075 is significantly lower than that for AA6060 [23], which is due to the higher content of Zn and Mg forming more anodic IMPs in the alloy. The appearance of the inductive loop in EIS spectra (Figure 3) indicates that the formed AAO film was not compact enough and thus allowed active dissolution, leading to charged species concentrated in the vicinity of the active dissolution sites and thus giving rise to the inductive response in EIS spectra [41]. The inductive response is associated with the adsorption of charged species on the oxide/electrolyte interface and the ionic transport within the metal/oxide interface [50-52]. At higher anodising potentials, the transport of charged ions is faster both in the electrolyte and at the metal/oxide interface [53], therefore,

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the relaxation time within the AAO film would be shorter, and thus the inductance value decreases with increasing potential (Table 2).

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4.4 Influence of IMPs on the anodisation process

IMPs present a high impact on electrochemical activities of AA7075 during anodisation, due to their

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different electrochemical characters as compared to the Al-matrix. The in-situ AFM measurements confirmed the peripheral dissolution in the closest vicinity of Al7Cu2Fe particles during anodisation

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(Figure 7 and Figure 8). As a contrast, for the Al2CuMg particle, as shown in the inserts of Figure 6, the native oxide film formed on the particle can be dissolved during anodisatoin, and it is less noble than the

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surrounding matrix [7, 8]. The Cu content of the constituent particle can promote enhanced dissolution along the boundary [6, 48, 54], and Al and Mg are de-alloyed under anodic polarisation [1]. The remnant particle decomposes into a porous Cu-rich cluster at the particle’s location [19], which can be easily

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detached from the surface and moved away by the mechanical action of the AFM tip [55]. At low anodic potentials (1 and 2 V) - the current density is exceptionally high in the first regime, and it increases slightly with time in the second regime (Figure 2). This is due to the limited blocking effect of the AAO film as proved by the EIS results. Moreover, under the constant potentials, the EIS measured thickness of the AAO film decreases with time, showing the occurrence of active dissolution of the compact layer. The localised dissolution is mainly related to the boundaries surrounding cathodic IMPs

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and the body of anodic IMPs, as revealed by EC-AFM in Figure 7 and Figure 8. The localised dissolution increases the porosity of the AAO film, and thus decreases the film resistance. Moreover, as shown in Table 2, the standard deviation of EIS data is relatively larger at 1 and 2 V. The higher deviation reflects a higher heterogeneity of the sample, which in this case is associated with a larger number of IMPs present in the surface layer. Since samples were step-wisely anodised, the number of surface IMPs, especially in anodic type, should decrease with increasing potential. Therefore, more IMPs give the rise of more active

14

anodic dissolution at lower potentials, and thus the higher heterogeneous structure of the formed AAO film. At high anodic potentials (4 and 8 V) - the EIS measured AAO film thickness remains stable over time. Moreover, the standard deviation of EIS data is relatively small (Table 2). At a higher potential, fewer anodic IMPs are present on the surface, which results in a lower chemical heterogeneity of the surface layer compared with that at a lower potential. Although the local dissolution is still very active at some surface areas, as revealed by the EC-AFM images in Figure 6, it mainly occurs on the outer porous AAO

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layer and is not detectable by EIS. 4.5 Mechanism of bubbles evolution

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The growth of AAO film on Al alloys may be accompanied by the generation of oxygen through the following reactions [56]: 2𝑂2− → 𝑂2 (𝑔) + 4𝑒 −

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(4)

4𝑂𝐻 − → 𝑂2 (𝑔) + 2𝐻2 𝑂 + 4𝑒 −

(5)

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or

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As has been reported in many previous papers, the bubbles are the released oxygen gas, and only the local sites such as nanometre-sized anodic IMPs and cathodic IMPs grain boundaries can facilitate the release of oxygen [56-59]. On the surface area with uniform oxide as shown in Figure 5, the growth of the AAO layer proceeds with time, but the applied anodic potentials of 4 and 8 V are not sufficient to support the

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generation of the oxygen bubbles without initial defects. Whereas, on the active dissolving area as shown in Figure 6, the anodic dissolution is the dominating anodic reaction other than the electrolytic reaction of water. However, the possibility of hydrogen evolution producing the bubbles cannot be excluded. It can be explained by the complete dissolution of some local sites exposing the Al matrix to the electrolyte [60]. At these sites, the area is very small (in nanometre size), the current density is high which in turn leads to a

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high IR drop and then a high over-potential for the hydrogen evolution. The exact identification of the bubbles is out of the scope of this work, but in both cases, the bubble generation is related to the IMPs in the alloy and contribute to a higher porosity of the formed AAO film. 5. Conclusions In this study, microstructure and Volta potential analyses were carried out to characterise the IMPs in an AA7075-T5 alloy, and EIS and In-situ AFM measurements were performed under operando conditions to investigate the anodisation process up to 8 V vs. Ag/AgCl, focusing on the influence of IMPs. The alloy 15

contains a large number of IMPs in various sizes, having either anodic or cathodic characters relative to the alloy matrix. The results suggest that the AAO films have a low resistance against charge transfer. IMPs in the microstructure have a pronounced influence on the growth of the AAO film. The Cu-bearing constituent particles exhibit a cathodic character, whereas Mg2Si and small MgZn2 particles exhibit an anodic character with respect to the matrix. During anodisation, Al7Cu2Fe type constituent particles remain stable, whereas peripheral dissolution occurs in the closest vicinity of these particles. De-alloying can occur in the S-phase (Al2CuMg) constituent particles leading to detachment from the alloy surface under anodising. Mg2Si particles show de-alloying at low potential and re-passivation at high potential.

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MgZn2 particles may dissolve entirely at low potential leaving pits in the substrate. The localised dissolution of the boundaries around large IMPs and nanometre-sized IMPs facilitate gas bubble evolution at potentials above 2V, which is only observed at the surface sites associated with the localised dissolution

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of IMPs. Gas bubble evolution confirms the local breakdown of the barrier-type AAO film on the alloy at

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high electrochemical potentials.

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Conflicts of interest

5. Acknowledgement

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The authors declare that there are no competing financial, professional or personal interests.

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This work was financially supported by the Foundation for Strategic Research Program (RMA11-0090).

Statement of data availability

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Data will be made available on request.

16

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Figures

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Figure 1: Electric equivalent circuits used to fit EIS data obtained at OCP (a) and anodising potentials (b).

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Figure 2: Plots of current density vs. time recorded during anodisation in 0.2 M Na2SO4 at various anodic

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potentials increased continuously in steps.

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Figure 3: EIS spectra in Nyquist plot and the fitting curves of AA7075 sample obtained in 0.2 M Na2SO4

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solution at OCP (a), and at applied anodising potentials of 2, 4 and 8V (b).

Figure 4: Calculated oxide film thickness as a function of anodisation from EIS data.

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Figure 5: In-situ EC-AFM topography images of an AAO growing region of the AA7075 in 0.2 M Na2SO4 solution at (a) OCP, (b) 4 V and (c) 8 V vs. Ag/AgCl, respectively. In each figure, the image on

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the right side is the corresponding enlargement of the area marked on the left side image.

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Figure 6: In-situ EC-AFM topography images of an actively dissolving region of the AA7075 in 0.2 M Na2SO4 solution at (a) OCP, (b) 1 V, (c) 2 V, (d) 4 V fist scan, (e) 4 V second scan, and (f) 8 V vs.

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Ag/AgCl, respectively. The inset of each image is the enlargement of the area marked with a square.

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Figure 7: In-situ EC-AFM topography images of the region showing the attack on peripheral boundaries adjacent to cathodic IMPs of the AA7075 in 0.2 M Na2SO4 solution at (a) OCP, (b) 1 V, and (c) 2 V vs.

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Ag/AgCl, respectively.

Figure 8: In-situ EC-AFM topography images of the region showing the localized dissolving of anodic IMPs and grain boundaries of cathodic IMPs of the AA7075 in 0.2 M Na2SO4 solution (a) in air, at (b)

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OCP, (c) 1 V, and (d) 2 V vs. Ag/AgCl, respectively. (e) The single height profile extracted from the lines

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marked in (a)-(d)

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of ro -p re lP ur na Jo Figure 9: In-situ optical microscopic image of the surface during the ECAFM scanning at the anodic potential of (a) 2, (b) 4, and (c) 8 V. Note, the white dots on bubbles are the reflection of light.

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Figure 10: SEM-BSE images showing the microstructure of aluminium alloy AA7075 (a). Note the

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nanometre-sized IMPs as shown in (c) and (d), finely dispersed next to micrometre-sized precipitates as

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shown in (b).

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Figure 11: EDX maps of a region containing a cluster of micrometre-sized IMPs.

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Figure 12: XRD results showing the collected signals and indexed phases.

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Figure 13: AFM topography (a) and SKPFM Volta potential (b) maps of the AA7075 surface showing the micrometre-sized and nanometre-sized IMPs with respectively cathodic Volta potentials and anodic Volta

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potential relative to the matrix.

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Tables

Table 1 Fitting results of the EIS spectra obtained in 0.2 M Na2SO4 at OCP. Potential OCP-1 OCP-2

Y1 / Ω-1cm-2sn (1.5±0.4) ×10-5 (2.0±0.7) ×10-5

Rp1/ Ω cm2 (1.1±0.6) ×104 (1.4±0.8) ×104

n1 0.9 0.9

Y2 / Ω-1cm-2sn (1.7±0.6) ×10-4 (2.0±0.6) ×10-4

n2 0.9 0.9

Rp2 / Ω cm2 (1.6±0.5) ×105 (1.0±0.2) ×105

L / H cm2 (9.2±5.5) ×103 (2.1±1.6) ×103 (1.0±0.5) ×103 (7.7±1.6) ×102 (3.5±0.3) ×102 (3.5±0.6) ×102 (1.2±0.2) ×102 (1.2±0.2) ×102

RL / Ω cm2 (1.5±0.7) ×104 (4.8±3.3) ×103 (3.6±1.8) ×103 (2.7±0.6) ×103 (2.0±0.2) ×103 (2.0±0.2) ×103 (1.5±0.1) ×103 (1.5±0.1) ×103

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Rct / Ω cm2 (3.7±2.7) ×104 (2.0±2.1) ×104 (4.5±2.5) ×103 (3.2±0.6) ×103 (2.4±0.2) ×103 (2.5±0.3) ×103 (2.2±0.2) ×103 (2.2±0.2) ×103

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n 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0

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Y / Ω-1cm-2sn (3.5±0.3) ×10-6 (3.8±0.4) ×10-6 (2.9±0.3) ×10-6 (3.1±0.2) ×10-6 (2.0±0.0) ×10-6 (2.0±0.0) ×10-6 (1.1±0.0) ×10-6 (1.2±0.0) ×10-6

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Potential 1V-1st run 1V-2nd run 2V-1st run 2V-2nd run 4V-1st run 4V-2nd run 8V-1st run 8V-2nd run

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Table 2 Fitting data of the EIS spectra obtained in 0.2 M Na2SO4 at anodising potentials.

d/Å 28.8±2.6 26.9±2.9 35.7±3.9 33.2±2.2 52.3±0.9 52.1±0.6 91.0±0.5 89.9±0.6

Table 3 Analysis results of the AFM images shown in Figure 5b - 5e.

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Surface area difference Rq

1V 0.018% 4.32 nm

2V 0.021% 4.45 nm

4V 0.025% 4.48 nm

8V 0.035% 5.11 nm

Table 4: XRD results of all collected and indexed diffraction peak signals.

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Peak No. Peak Position 1 26.53 2 26.60 3 27.96 4 28.00 5 28.30 6 40.93 7 42.64 8 42.90 9 43.60

Phase Al7Cu2Fe Al3Fe Al7Cu2Fe Al3Fe Mg2Si Al2CuMg Al7Cu2Fe Al2CuMg Al7Cu2Fe

Orientation 103 -401 004 400 & -113 111 112 213 130 204 33

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024 & -514 111 220 310 041 023 314 & -333 116 301 200 310 301 206 312 311 333 & -803 241 & 531 216 008 & 313 424 & -335 400 006 220 664 11-2 311 224

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Al3Fe Al-matrix Mg2Si MnAl6 Al2CuMg MnAl6 Al3Fe Al7Cu2Fe Al7Cu2Fe Al-matrix Al7Cu2Fe Al7Cu2Fe Al7Cu2Fe Al7Cu2Fe Mg2Si Al3Fe Al3Fe Al7Cu2Fe Al7Cu2Fe Al3Fe Al7Cu2Fe MnAl6 Al-matrix Al4.01MnSi0.74 ZnAl2 Al-matrix MgZn2

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43.75 44.96 47.08 47.62 47.83 48.32 48.47 48.66 50.73 52.39 53.04 53.72 54.45 55.00 55.76 55.98 56.19 57.09 57.88 59.06 68.86 74.42 77.25 83.11 85.00 94.10 106.02

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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

34