Stability of benzotriazole-based films against AA2024 aluminium alloy corrosion process in neutral chloride electrolyte

Accepted Manuscript Stability of benzotriazole-based films against AA2024 aluminium alloy corrosion process in neutral chloride electrolyte Isaline Recloux, Francesco Andreatta, Marie-Eve Druart, Leonardo Bertolucci Coelho, Cinzia Cepek, Damien Cossement, Lorenzo Fedrizzi, Marie-Georges Olivier PII:

S0925-8388(17)34128-2

DOI:

10.1016/j.jallcom.2017.11.346

Reference:

JALCOM 44039

To appear in:

Journal of Alloys and Compounds

Received Date: 30 August 2017 Revised Date:

10 November 2017

Accepted Date: 28 November 2017

Please cite this article as: I. Recloux, F. Andreatta, M.-E. Druart, L.B. Coelho, C. Cepek, D. Cossement, L. Fedrizzi, M.-G. Olivier, Stability of benzotriazole-based films against AA2024 aluminium alloy corrosion process in neutral chloride electrolyte, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2017.11.346. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

1. Introduction The 2024 aluminium alloys are widely used in aeronautical applications due to their appealing

RI PT

mechanical properties obtained at the expense of their corrosion resistance properties. The microstructure of AA2024 is very complex due to the large range of existing intermetallic phases. Several papers have identified and mapped different types of intermetallic particles

SC

(IMPs) in AA2024 [1-3]. It is well accepted that Al-Cu-Mg-based IMPs are the most numerous category while the second largest type is rich in Al-Cu-Fe-Mn. Fe-rich intermetallic particles

M AN U

interact as cathodes with the aluminium matrix [4]. Mg-rich particles initially behave anodically with respect to the matrix but, after dealloying, the remnant phases are electrochemically nobler than the matrix [2]. As a consequence, the dissolution of the contiguous matrix occurs at the interface with cathodic IMPs due the formation of micro-galvanic couplings.

TE D

Hexavalent chromium-based compounds are very efficient to inhibit corrosion processes occurring on AA2024. However, their use is to be banned as from 2017 in the aerospace industry due to environmental regulations [5-8] and a huge effort has thus been carried out in order to

EP

develop new strategies for corrosion protection of AA2024. To date, several environmentally preferable substances have been identified for the replacement of chromate compounds [9-16].

AC C

Specifically, benzotriazole (BT) has shown very promising results to inhibit corrosion reactions on AA2024 in neutral aerated sodium chloride solutions and does not constitute an environmental hazard [16-22]. Benzotriazole was regarded as corrosion inhibitor in 1967 by Cotton et al. for the protection of copper and its alloys [23]. This organic compound is able to chemically adsorb on metallic copper due to its nitrogen atom acting as a reaction centre for adsorption. It was reported that

1

ACCEPTED MANUSCRIPT

depending on the concentration, a more or less dense arrangement of inhibitive molecules can be achieved on the copper surface [24, 25]. The inhibition efficiency of these species was also attributed to their ability to form a water insoluble complex with copper ions leading to the

RI PT

formation of a highly impermeable and resistive physical barrier layer, inhibiting both anodic and cathodic reactions on copper [26]. The exact mechanism of inhibition has not yet been understood in spite of application of various techniques [27-33]. More recently, benzotriazole

SC

has been employed as corrosion inhibitor on AA2024 and favourable results have been found [16-22]. Benzotriazole is able to form an inhibitive layer on the top of the aluminium oxide

M AN U

surface, hindering the corrosion activity [16]. However, contradictory information about its mechanism of action is reported in the literature. On the one hand, a reduction in cathodic current was noticed, which indicates that benzotriazole acts as a cathodic inhibitor by slowing down the oxygen reduction reaction (ORR). Qafsaoui et al. [34] considered that the beneficial effect of BT

TE D

is mainly due to the formation of a Cu-BT polymeric film that prevents the oxygen reduction reaction on the cathodic intermetallic sites, inhibiting the galvanic corrosion. On the other hand, the shift of the open circuit potential towards more cathodic potentials has not been observed and

EP

one paper [16] also revealed a slight decrease in the anodic current. As a consequence, the classification of benzotriazole as cathodic or mixed inhibitor remains unclear to date concerning

AC C

AA2024. Moreover, quite limited information on the mechanisms of the BT layer formation on this substrate (physisorption, chemisorption, precipitation etc.) is available. Some authors refer to an adsorption process [16], while others attribute the inhibition to a reaction between benzotriazole and monovalent Cu+ ions [21]. Although the formation of the inhibitive layer is not well understood yet, benzotriazole is progressively employed as additive in protective coatings for AA2024 with the aims of providing self-healing ability [35-39]. When the protective coating

2

ACCEPTED MANUSCRIPT

fails to protect the metal due to defects or scratches, the inhibitor is locally released and the formed inhibitive film slows down corrosion processes. The self-healing ability of the coating relies on the quantity of corrosion inhibitor released inside the defects, on the formation rate of

RI PT

the protective film and on its long-term stability in the aggressive electrolyte.

The study of the effect of corrosion inhibitors can be performed with different local

SC

electrochemical techniques. The Scanning Electrochemical Microscope (SECM) is a qualitative method used to identify the passivated and active regions on metallic surfaces and the formation

M AN U

of an isolating inhibitive layer [40]. Micro-potentiometric and amperometric measurements by ion-selective microelectrode allow clarifying the electrochemical processes via the modification of acid-base equilibria [41]. Other local techniques such as the scanning Kelvin probe force microscopy (SKPFM) and the Scanning Vibrating Electrode Technique (SVET) show very useful information on inhibition phenomena [42, 43]. The SKPFM has been used for the study of

TE D

different corrosion inhibitors including triazole and thiazole derivatives [16], salicylaldoxime, 8hydroxyquinoline and quinaldic acid [15, 44] and rare earth compounds [42, 45-47].

EP

The use of the electrochemical micro-cell for the study of inhibition processes in aluminium alloys is a rather new approach initially introduced by Ralston et al. and Birbilis et al., who

AC C

investigated the behaviour of IMPs in AA2024-T3 exposed to aqueous vanadate solutions [48, 49]. Paussa et al. recently employed the micro-cell for the in situ investigation of intermetallics in AA2024-T3 after exposure to aqueous solutions containing Ce salts [14] and to highlight the precipitation mechanism of cerium species [13]. Moreover, this technique was recently employed in view of investigating the effect of Ce species on the electrochemical behaviour of Fe-rich intermetallic particles on clad AA2024 [50, 51].

3

ACCEPTED MANUSCRIPT

This work aims at studying some aspects of the benzotriazole inhibition mechanisms on AA2024, such as the influence of the alloy microstructure, the role of chloride ions and the stability of the benzotriazole-based film in the aggressive solution without inhibitor

RI PT

replenishment. Micro and macro electrochemical measurements and surface analyses (XPS and ToF-SIMS) were performed aspiring to acquire new insights on the corrosion protection mechanisms and to open up new ways to improve the design of self-healing coatings. In

SC

particular, electrochemical micro-cell technique was applied to locally characterize the electrochemical behaviour of areas containing nearly single intermetallic phases in the presence

M AN U

of electrolyte. 2. Material and methods 2.1. Surface preparation

TE D

2024-T3 aluminium alloy was supplied by the SONACA company (Belgium). Its composition is reported in table 1. Substrates were degreased firstly in acetone and secondly in an alkaline aqueous solution containing 50 g/L of TURCOTM 4215 (pH 9) for 12 min at 50°C. Next, they

EP

were rinsed with demineralized water. For the characterization by means of micro-cell, substrates

AC C

were ground using SiC paper and subsequently polished using 6 and 1 µm diamond suspensions. Ethanol was used instead of water in order to avoid or limit the fast dissolution of the magnesium from the magnesium-rich IMPs. 2.2. Formation of benzotriazole-based inhibitive films Benzotriazole (99% assay) was obtained from Merck. Substrates were immersed in inhibitor solution for 24 h before analysis in NaCl media (in the absence and in the presence of

4

ACCEPTED MANUSCRIPT

benzotriazole). Two different inhibitor solutions were employed: 0.035 M benzotriazole (BT solution) and 0.05 M + 0.035 M benzotriazole (NaCl+BT solution).

RI PT

2.3. Electrochemical tests All the electrochemical measurements were performed in presence of chloride ions to maintain the same solution conductivity and aggressive species concentration. Measurements were carried

SC

out either in 0.05 M NaCl solution or in NaCl+BT solution (the same inhibitor solution employed for the 24 h immersion step).

M AN U

A conventional three-electrode set-up was used for all macro electrochemical tests. The working electrode was the AA2024 substrate. The exposed surface area was defined by the diameter of the Plexiglas cell (7.07 cm²) used to contain the testing solution. The counter electrode was a platinum wire and all potentials were measured with respect to an Ag/AgCl/KClsat (+197

TE D

mV/SHE) reference electrode. Samples were placed in a Faraday cage to avoid electromagnetic interferences during the measurement. Electrochemical measurements were performed using an AMETEK Parstat 2273 computer controlled with Powersuite® software. In order to check

EP

results reproducibility, at least two measurements were done for each testing condition.

AC C

Large scale potentiodynamic polarization curves were acquired at a scan rate of 0.83 mV s-1 prior and after immersion in the inhibitor solution. Anodic and cathodic branches were separately recorded on different samples from OCP (open circuit potential) to +0.3 V/OCP and to -0.5 V/OCP, respectively. The OCP was followed for 15 min before measurements in order to ensure that stationary conditions have been reached. EIS measurements were performed to evaluate the stability over time of the inhibitive film formed during the immersion step. A 5 mV RMS

5

ACCEPTED MANUSCRIPT

amplitude sinusoidal voltage was superimposed to the OCP. The frequency of this signal was ranged from 100 kHz to 10 mHz.

RI PT

The characterization by electrochemical micro-cell was performed in order to acquire anodic and cathodic potentiodynamic curves at the intermetallic sites prior and after immersion in the inhibitor solution. A glass capillary with 50 µm internal diameter, corresponding to an area of the

SC

working electrode of 1.96 10−5 cm2, was used. The electrochemical micro-cell equipment employed in this work was developed by Suter [52]. Electrochemical measurements were

M AN U

performed using an IPS potentiostat by Elektroniklabor Peter Schrems with current resolution in the order of 10 fA. The micro-cell presented a three-electrode configuration (sample under investigation as working electrode with area defined by the size of the glass micro-capillary; Pt counter-electrode and Ag/AgCl 3 M KCl reference electrode). Anodic and cathodic branches

TE D

were separately recorded from OCP with a scan rate of 1 mV s−1.

The use of the electrochemical micro-cell introduces some limitations. These critical aspects are mainly related to the necessity of measuring very small currents (in the range of pA or fA) and to

EP

the manual preparation of the capillaries [51]. It is also worth noting that a direct correlation between measurements with the micro-cell and conventional large scale measurements is not

AC C

straightforward due to the reduced size of the exposed area that might affect the reproducibility of results obtained with the micro-cell. Conventional large scale measurements are often useful to assist the correct interpretation of results obtained with the micro-cell. Seeking at improving reproducibility, potentiodynamic curves were acquired at multiple intermetallic sites (at least 10 measurements for each type of intermetallic before and after immersion in the testing solution). 2.4. Surface analyses

6

ACCEPTED MANUSCRIPT

X-ray photoemission spectroscopy (XPS) was used to prove the presence of benzotriazole on the samples after immersion in the inhibitor solution. XPS was performed in normal emission geometry at room temperature, using a hemispherical electron energy analyser and a

RI PT

conventional Mg Kα1,2 (1253.6 eV) X-ray source, with an overall energy resolution of 0.8eV, sampling an area of ~20 mm2. The N 1s XPS core levels were acquired after immersion in NaCl+BT and BT solutions to prove the reaction of the inhibitor on the substrates.

SC

ToF-SIMS imaging was carried out using a TOF-SIMS IV from IONTOF GmbH. A pulsed Ga+

M AN U

25 keV ion beam was used and the current was set to 1 pA. The detection was made in the negative ion mode. When necessary, the sputtering of the surface was performed with a 3 keV Ar+ beam at a current of 10 nA. 3. Results and discussion

TE D

3.1. Influence of the AA2024 microstructure on the formation of the inhibitive layer In this work, intermetallic particles were identified by SEM-EDXS (Figure 1) and classified into two categories, according to their chemical composition: Fe-rich and Mg-rich particles; on the

EP

basis of Boag’s work [3]. In particular, Fe-rich particles include the Al-Cu-Fe-Mn (Cu/Fe > 2.5 and Si < 1 at%) and the Al-Cu-Fe-Mn-Si phases (Cu/Fe = 0.5, Si > 2.4 at%). Mg-rich particles

AC C

comprise the Al2CuMg (S phase) particles. OCP stability of substrates previously immersed for 24 h in NaCl+BT solution and then tested in 0.05 M NaCl solution was checked before performing potentiodynamic polarization curves. OCP trends are reported in Figure 2 for two different samples. The OCP values are stable around -0.5 V vs Ag/AgCl [KCl sat].

7

ACCEPTED MANUSCRIPT

Figures 3 A and B display large scale polarization curves recorded either in NaCl or in NaCl+BT solutions after immersion in NaCl+BT or in BT solutions for 24 h. The 24 h immersion step in the NaCl+BT solution induces a decrease in anodic current densities, when compared to the bare

RI PT

AA2024 substrate, in both selected testing solutions (with and without BT – Figure 3A). A decrease in cathodic current densities is also noticed for samples tested in both testing solutions after 24 h of immersion in NaCl+BT (Figure 3B). The effect of the inhibitor is clearly visible

SC

between -0.5 and -0.7 V vs Ag/AgCl [KCl sat], where the cathodic reaction behavior changes from diffusion-controlled to activation-controlled. On the other hand, after the 24 h immersion

M AN U

step, only small differences be seen in the cathodic curves obtained in testing solutions with or without BT. In the presence of inhibitor, the cathodic curves cross that of the bare metal below 0.8V, which indicates the presence of an additional cathodic reaction. The preferential reaction of BTA towards the Cu-rich IMPs is known to inhibit the ORR likely to take place on these

TE D

locations [53]. Therefore, in the cases where a stable CuBTA was obtained, it is possible that, upon further cathodic polarization (below -0.8V, Fig. 3B), the hydrogen evolution might be

EP

induced due to the limited availability of cathodic surfaces supporting ORR.

Results in Fig. 3 also point out that the presence of chloride ions during the 24 h immersion step

AC C

strongly influences the inhibition ability of benzotriazole., A more marked decrease in the anodic current density and a slight passivation tendency are visible when the inhibitor testing solution is used instead of the NaCl inhibitor-free one. The difference is even more marked on cathodic polarization curves.

Polarization curves were also performed by using the electrochemical micro-cell at different locations on AA2024 substrates. Anodic and cathodic branches are reported in Figures 4 and 5

8

ACCEPTED MANUSCRIPT

respectively (matrix (a), Fe-rich intermetallics (b) and Mg-rich intermetallics (c)). After immersion for 24 h in NaCl+BT solution, polarization curves were carried out in 0.05 M NaCl testing solution both with or without BT. For the sake of comparison, measurements were also

RI PT

carried out in 0.05 M NaCl testing solution using as received samples.

Corrosion and breakdown potentials observed for the bare substrate at the different locations are

SC

in line with results reported in literature [13,14]. In particular, in comparison with the matrix, the breakdown potentials are slightly more negative at the Fe-rich sites, and considerably more

behaviour of the AA2024 alloy.

M AN U

negative at the Mg-rich ones, confirming that both intermetallics affect the localized corrosion

Generally speaking, after immersion for 24 h in the inhibitive solution, the anodic polarization curves for all tested locations show a decrease in the cathodic and anodic current densities, a

TE D

widening of the passive potential range and a shift of the breakdown potential (Eb) towards more noble potentials (Table 2). This trend is generally more marked when using the NaCl+BT testing solution than the NaCl one. With the aim of better highlighting the effect of the inhibitor, Table 2

EP

displays the potential shifts of breakdown potentials (∆Eb) measured after 24 h immersion in NaCl+BT solution relative to those recorded in NaCl solution for the bare substrate.

AC C

The Ecorr recorded from the Mg-rich sites has a remarkable shift in the positive direction after 24 h of exposure in both inhibitive solutions, passing from -0.660 to -0.320/0.340 V (Figure 4). On the contrary, shifts in Ecorr are not clearly observed for the two other locations. The ennoblement verified for the Mg-rich IMPs region is possibly also related to a partial dissolution of Mg [54, 55] during the 24 h immersion step in NaCl+BT solution.

9

ACCEPTED MANUSCRIPT

The potential ennoblement is even more pronounced in the case of Eb and it concerns all the tested locations. For the two testing solutions, the aluminium alloy matrix shows a strong shift towards more positive values suggesting a “passivating” effect brought by the inhibitor (∆Eb of

RI PT

about 0.820/0.890 V). After 24 h of immersion in NaCl+BT, the Fe-rich intermetallics display a ∆Eb of about 0.410 V for polarization curves recorded in NaCl+BT solution. These results suggest that an inhibitive layer is also formed on these sites when the inhibitor is present in the

SC

testing solution. The inhibitive effect seems to be slightly weaker on Mg-rich intermetallic particles since the breakdown potentials remain rather active for regions containing this type of

M AN U

particles. Indeed, after the immersion treatments, the passive range of these polarization curves is not expanded over wider potential ranges and the resulting anodic current densities are also higher when compared to those for the other sites. Nevertheless, the observed shifts in the corrosion and breakdown potentials in the positive direction prove a partial deactivation of

TE D

regions containing Mg-rich IMPs, suggesting the formation of an inhibitive film also on Mg-rich intermetallics. Anodic potentiodynamic polarization curves recorded by electrochemical microcell are qualitatively in quite good agreement with large scale measurements. Moreover, it is

EP

worth stressing that the corrosion behaviour improvement is always more evident when the polarization curves are carried out in the testing solution containing BT, suggesting the active

AC C

effect of the inhibitor not only during the 24 h of immersion in the inhibitive solution (formation of a protective film) but also during the electrochemical test (healing of the protective film); which confirms the importance of continuous inhibitor availability. Figure 5 shows cathodic polarization curves recorded at different locations in both testing solutions (0.05 M NaCl with and without BT) and are well representative of the behaviour detected by the electrochemical micro-cell technique.

10

ACCEPTED MANUSCRIPT

Whatever the location on AA2024 surface, these results highlight a clear inhibition of the cathodic currents, with respect to the as received substrate, when curves were recorded in the BT-containing testing electrolyte. In fact, in contrast to large scale measurements, cathodic

when samples are tested in 0.05 M NaCl solution.

RI PT

polarization curves recorded by micro-cell highlight a weak inhibition effect of benzotriazole

SC

As discussed before, the strong shift of Ecorr in the positive direction observed for Mg-rich intermetallics might be due to the dissolution of magnesium during the immersion for 24 h in

M AN U

NaCl+BT. This magnesium dissolution is accompanied by a local pH increase [56, 57] which might be favourable to the formation of a BTA-based protective film (as addressed in the Discussion section).

It is worth noting that oxygen diffusion through the Si-gasket might affect the electrochemical

TE D

processes inside the glass micro-capillary, in particular for the cathodic branch of potentiodynamic polarization curves. Using Ar shielding at the tip of the glass micro-capillary, R. Oltra et al. [58] demonstrated for a Pt electrode in 0.5 M NaCl solution that the oxygen

EP

reduction reaction could be affected by oxygen diffusion even if there is perfect sealing of the glass micro-capillary. In this work, no shielding was used for recording the cathodic curves.

AC C

Therefore, the cathodic branches reported in Fig. 5 can provide only qualitative information about the cathodic processes. Although benzotriazole is mainly known as a cathodic inhibitor, polarization curves recorded with the micro-cell suggest that the inhibitor influences anodic processes, which could be seen through the widening of the passive region and/or the shift in the Eb. In particular, measurements with the micro-cell highlight that improvement of the corrosion properties takes place on the

11

ACCEPTED MANUSCRIPT

aluminium matrix (regardless of testing solution) as well as in regions containing Fe- and Mgrich intermetallics (especially in NaCl+BT solution).

RI PT

Tof-SIMS analyses were performed in order to confirm results obtained with the micro-cell. Surface imaging was recorded to determine whether benzotriazole molecules preferentially adsorb at some specific sites, such as copper-rich intermetallic particles. Figure 6 presents the map obtained on AA2024 immersed for 24 h in NaCl+BT solution and rinsed with

SC

demineralized water. Figure 6(a) is related to C6H4N3- ions (molecular signature of

M AN U

benzotriazole) and Figure 6(b) is related to C2H- ions (surface pollution) for means of comparison. Although the signal relative to C6H4N3- ions is very weak (Figure 6(a)), it can be seen that these are heterogeneously distributed on the alloy surface. Electrochemical micro-cell results indicate that benzotriazole promotes different inhibitive effects depending on the considered microstructural locations. Therefore, it is expected that benzotriazole reacts

TE D

differently on the different considered locations. The image presented in Fig. 6(a) is composed of circular-like features that are impoverished in signal and whose peripheries appear to present

EP

greater contents in benzotriazole.

Considering that these darker regions have dimensions in the order of tens of microns, they

AC C

probably refer to intermetallic particles. Such a preferential distribution may be the reflex of Cu ions generation from localized corrosion processes: the trenching mechanism related to Cu-rich IMPs allows for Cu-enrichment at their peripheries [57, 59]. In this scenario, BT molecules would be able to react with Cu ions, forming the Cu-BT complex at these locations. Furthermore, due to the increased kinetics of ORR (edge effect) at the peripheries of IMPs, a local alkalisation effect would also be able to drive the adsorption of BT- ions onto metallic copper surfaces [60] (in case where trenching is prevented). Anyway, the inhibitor appears continuously distributed in

12

ACCEPTED MANUSCRIPT

the regions between the referred IMPs, which is an evidence for the presence of benzotriazole also on the aluminium matrix. Figure 7 shows the map obtained after argon ion beam sputtering (3 keV) for C6H4N3- ions. The image can be divided in two areas: the upper part within the frame

RI PT

corresponds to non-sputtered surface; the lower part (about three quarters of the mapping) corresponds to the sample surface which was sputtered with argon ion beam. The intensity related to C6H4N3- ions decreases and makes the lower part of the map darker, suggesting that the

SC

benzotriazole protective film was removed during the sputtering. This outcome suggests that the

M AN U

achieved inhibitive layer is very thin and it is only present on the top surface of AA2024. 3.2. Stability of the benzotriazole-based film in absence of inhibitor in the aggressive solution

A possible explanation for the limited inhibition highlighted mainly on anodic polarization

TE D

curves recorded in NaCl solution after 24 h immersion in NaCl+BT by large scale polarization/ electrochemical micro-cell measurements is that the inhibitor found on the aluminium matrix is only physically adsorbed on it, being subjected to desorption when immersed in a solution

EP

without inhibitor. The desorption process might be time depending and after 15 min of immersion, the remnant BT-based layer (which probably comprises chemically adsorbed

AC C

complexes) is still able to partially inhibit oxygen cathodic reduction on IMPs but it is probably not enough covering to protect the matrix from anodic dissolution. This effect is not observed when there is an excess of inhibitor in the testing electrolyte due to a favourable adsorption/desorption equilibrium. EIS measurements also support the finding that the inhibitive effect is stronger in presence of an excess of inhibitor in the testing electrolyte. Figure 8 reports Bode plots recorded either in 0.05

13

ACCEPTED MANUSCRIPT

M NaCl solution or in NaCl+BT solution for the bare substrate and for the substrate after 24 h of immersion in NaCl+BT. The low-frequency modulus of the sample immersed for 24 h in NaCl+BT solution is around 105 ohm cm² when benzotriazole is present in the testing solution.

RI PT

This is one order of magnitude higher than for bare substrate tested in 0.05 M NaCl, demonstrating the presence of an inhibitive film. Moreover, the corrosion protection offered by the benzotriazole-based layer in these conditions is stable over time as attested by EIS

SC

measurements performed after 24 h in NaCl+BT solution. For measurements recorded in 0.05 M NaCl solution after 24 h of immersion in NaCl+BT, the presence of the inhibitive film is also

M AN U

detected after 15 min of exposure in the aggressive electrolyte, demonstrating the occurrence of a inhibitive reaction on the sample surface during the prior immersion step. However, the absence of benzotriazole in the testing solution leads to a poor stability of the protective layer with immersion time. The protection is not visible anymore after 24 h of exposure in 0.05 M NaCl.

TE D

All experimental EIS spectra were fitted in order to obtain quantitative information on the impedance properties of the inhibitive films. Figure 9 presents the electrical equivalent circuit used for fitting the EIS data. In this circuit, Rs corresponds to the solution resistance, Qox is the

EP

constant phase element (CPE) describing the capacitance of the natural aluminium oxide film,

AC C

Rox is the resistance of the natural aluminium oxide layer, Rpolar is the polarization resistance and Qdl is the CPE relative to the double layer capacitance. Indeed, the corrosion rate is not negligible and a respective time constant (comprising Rpolar and Qdl) is located at low frequencies (ionic motion could take place through the oxide/inhibitor layer because this phase does not entirely cover the surface). The associated resistance could not be properly fitted due to the considerable noise systematically detected in the 10-1 - 10-2 Hz range. A time constant (TC) specific to the inhibitive layer could not be distinguished from the time constant related to oxide

14

ACCEPTED MANUSCRIPT

film. The high frequency relaxation process actually comprises contributions from both the oxide layer and the inhibitive layer impedance properties, but it will be here referred to as the oxide film TC. Therefore, the adsorption of benzotriazole directly influences the fitting parameters Rox

RI PT

and Qox that are presented in Table 3. Under the different conditions investigated, the fitting of Rox was able to establish a fair basis of comparison for the resistance of the oxide/inhibitor layers (although it does not express the total resistance of the systems). The χ² fit parameter was

SC

always below 10-2.

M AN U

The immersion for 24 h in NaCl+BT solution leads to an increase in the oxide layer resistance compared to the bare substrate (Rox = 1.6 104 ohm cm²), providing an additional corrosion protection. This resistance is stable over time when the testing electrolyte contains benzotriazole, but it is degraded in the absence of inhibitor. Accordingly, the capacitance of the oxide film increases more quickly as a function of time in absence of benzotriazole in the electrolyte

TE D

probably due to the desorption of the inhibitive layer. XPS analyses were performed in view of determining the endurance of the inhibitive layer after

EP

immersion in 0.05 M NaCl solution. The N 1s core level was used as a signature of the presence of the inhibitor on the sample surface. Figure 10 compares the N 1s XPS spectra after immersion

AC C

for 24 h either in NaCl+BT (bottom) or in BT solution (top) to the spectra obtained after a successive immersion step for 1 h in 0.05 M NaCl solution. All spectra are normalized to the photon flux, so as to reflect their real nitrogen coverage intensity. The XPS results show that the intensity of the N 1s signal of the sample immersed for 24 h in NaCl+BT solution (Figure 10, bottom) is reduced by a factor of ~3.4 after 1 h of immersion in 0.05 M NaCl solution, supporting the previous conclusion that the inhibitor tends to be removed from the surface in

15

ACCEPTED MANUSCRIPT

NaCl-containing electrolytes. The outcomes after immersion in BT solution (Figure 10, top) will be discussed in the following section.

RI PT

3.3. Role of chloride ions in the formation of the protective layer Results shown in Figure 3 highlight that the presence of chloride ions during the 24 h immersion step increases the inhibition provided by BT. The same effect was observed in the EIS

SC

measurements. Figure 11 presents Bode plots in modulus and in phase recorded in 0.05 M NaCl solution for the substrate after immersion for 24 h in NaCl+BT or in BT solution.

M AN U

The low-frequency modulus after 1 h of immersion in 0.05 M NaCl solution is one order of magnitude higher for the substrate immersed in the NaCl+BT (around 105 ohm cm²) than for that immersed in BT solution (around 104 ohm cm²). Once again, the corrosion protection offered by the inhibitive layer formed in presence of chloride ions seems to be higher. However, the

TE D

stability over time of the benzotriazole-based layer, regardless of the presence of chloride ions during its formation, is very poor in 0.05 M NaCl solution as already mentioned. All EIS spectra were fitted with the electrical equivalent circuit shown in Figure 9. Fitting

EP

parameters related to the oxide layer are presented in Table 4. Based on the oxide layer

AC C

resistance values, it is noticeable that the inhibitive layer formed in NaCl+BT solution is more protective after 1 h of immersion than the one formed in BT solution. In the absence of chloride ions in the inhibitor solution, the increase in the oxide layer resistance is almost insignificant compared to the bare substrate (Rox = 1.6 104 ohm cm²). Whatever the inhibitor solution used to form the benzotriazole based layer, the corrosion protection is no more visible after 6 h of immersion in 0.05 M NaCl solution as attested by the fitted parameters.

16

ACCEPTED MANUSCRIPT

XPS analyses were carried out in order to confirm the influence of chloride ions on the formation of the inhibitive layer. As can be seen in Figure 10, the detection of N 1s XPS signal for all samples indicates that the inhibitor reacts on the sample surface after immersion in both solutions

RI PT

(NaCl+ BT and BT solutions). However, the amount of N detected on the surface depends on the immersion conditions. The intensity of the N 1s signal after 24 h immersion in BT solution (green profile with circles not filled, Figure 10, top) is lower than the intensity of the sample

SC

obtained after immersion in NaCl+BT solution (red profile with circles not filled, Figure, 10 bottom). This result shows that the immersion of the substrate for 24 h in NaCl+BT leads to the

M AN U

deposition of a higher amount (almost 4 times) of inhibitor than the immersion in BT solution (2022 and 546 of intensity, respectively) for the same immersion time, confirming the role of chloride ions in the formation of the benzotriazole layer. After a successive 1 h of immersion in 0.05 M NaCl solution, a higher fraction of inhibitor (about 45%) is detected on the surface of the

TE D

sample immersed for 24 h in NaCl+BT (red profile with filled circles, Figure, 10 bottom) than on that immersed for 24 h in BT solution (green profile with filled circles, Figure 10 top). The presence of relatively high amount of residual benzotriazole on the sample surface even after 1 h

EP

of immersion in 0.05 M NaCl might explain the inhibition observed in polarization and EIS measurements carried out in 0.05 M NaCl solution - especially after immersion in NaCl+BT

AC C

solution.

3.3. Discussion

Based on the achieved results, it is possible to conclude that the extent of corrosion inhibition by benzotriazole on AA2024 is rather limited when the inhibitor molecule is not present in the aggressive environment. As a consequence, its efficiency as corrosion inhibitor in self-healing coatings is time-limited as already pointed out in previous papers [22, 61] addressing the same

17

ACCEPTED MANUSCRIPT

copper-rich alloy. As reported, the efficiency of benzotriazole as inhibitor on copper is related to the formation of an insoluble and polymeric film onto the surface due to a reaction between Cu+ ions and the molecule [23, 25, 62]. This Cu-BT layer is stable and provides an efficient physical

RI PT

barrier preventing the corrosion of copper. In the case of AA2024, the formation of this polymeric film implicates supplementary steps for its formation. This stable complex can only be formed if copper ions are present in the medium [60]; which means, if Cu-rich particles are

SC

detached from the matrix (trenching mechanism) due to the localized dissolution of the aluminium matrix [57]. Actually, the presence of chlorides induces the development of localised

M AN U

corrosion on AA2024 (micro-galvanic couplings between Cu-rich precipitates and Al matrix) and is at the origin of this mechanism. The oxidation of the detached Cu-rich particles leads to the generation of Cu ions at the alloy/electrolyte interface and to the formation of the protective Cu(I)BTA complex[34, 57, 59, 60] Moreover, copper ions could also be generated from the

TE D

matrix dissolution, as the alloy contains 0.2-0.5 wt% of Cu in solid solution [63]. Based on the E-pH diagrams constructed for the Cu-BT-H2O and Cu-BT-CI--H2O systems [64] Cu+ ions are expected to be formed upon Cu oxidation. Moreover, from a thermodynamic point of view, the

EP

CuBTA phase would be stable over a quite broad range of pH, in the absence and presence of Cl. However, from a kinetics perspective, the rate of production of Cu ions is greatly increased in

AC C

the presence of Cl-, which might explain why the protective CuBTA film is more effective when formed in a chloride-containing solution. Nevertheless, for this alloy, copper is mainly present in its metallic form and the inhibitive effect is also associated to the formation of a thin adsorption monolayer on the aluminium matrix, as detected by ToF-SIMS analysis. The barrier performance of the resulting oxide was confirmed by EIS measurements in NaCl+BT solution.

18

ACCEPTED MANUSCRIPT

In the neutral NaCl+BT solution here considered, benzotriazole can be present in both its undissociated (BT) and dissociated (BT-) forms (pKa = 8.2 [65]). However, in the first step of immersion, due to the presence of aggressive chloride ions, corrosion processes occur and a local

RI PT

alkalinisation effect is expected in the vicinity of copper-rich particles increasing the ratio BT/BT. These species can interact with metallic copper to form a Cu:BT- adsorbed layer on the intermetallics [28, 66], which might contribute to the anodic and cathodic inhibition processes

SC

observed on intermetallic particles with the electrochemical micro-cell. This beneficial effect of the chloride ions on the complex formation was also reported by Kokalj et al. [67] and it is

M AN U

explained by the ability of BT to form strong N-Cu chemical bonds when they are in the deprotonated form and when chloride ions are solvated by the solvent. In the BT immersion solution without aggressive species, the concentration in BT- around intermetallic particles is probably not observed, limiting the complex formation and reducing the inhibition effect of the

TE D

layer. Moreover, due to the low corrosion in the absence of chloride ions (BT solution), the trenching mechanism is also prevented and the complexation of the protective Cu-BT phase is not likely to occur. Based on this study, it was proved that a protective film due to the physical

EP

adsorption of BT on the aluminium matrix is able to be formed during immersion in BTcontaining solutions. The reversibility of the physical adsorption process and the high solubility

AC C

of benzotriazole in water in these conditions might be at the basis for the poor stability of this benzotriazole-based film, especially when the inhibitor is removed from the testing electrolyte. Moreover, it is reported that BT molecules are able to link on a previously formed Cu:BTadsorption layer via physisorption if they are present in a sufficient amount next to the inhibitive layer/solution interface [61, 68], which supports the higher protection observed when benzotriazole is present in solution, as indicated by electrochemical tests and XPS analyses.

19

ACCEPTED MANUSCRIPT

In conclusion, it appears that the benzotriazole inhibition mechanism on AA2024 is complex and can be associated to the reversible physical adsorption of the inhibitive species on the aluminium matrix, being able to passivate it and to the possible formation of Cu:BT- and/or Cu-BT

RI PT

complexes on Cu-rich intermetallics (especially when local dissolution and trenching processes are not completely prevented). The physical adsorption phenomena is reversible and the corrosion properties of the layer are time-dependent in NaCl solution, being degraded after a

SC

certain immersion time. When the film is formed in presence of chloride ions, its performance is improved due to the ability of BT to form complexes with metallic copper/copper ions that are

M AN U

able to inhibit cathodic processes, preventing the galvanic corrosion for longer time. 5. Conclusions

Results presented in this paper clearly confirm the time-limited inhibitive action offered by

TE D

benzotriazole on AA2024. The combination of electrochemical techniques and surface analyses highlighted the formation of a thin BTA-based layer of benzotriazole at the extreme surface of AA2024. This film is formed regardless of the presence of intermetallic particles and inhibitive

EP

effects are observed at regions containing Fe- and Mg-rich intermetallics as well on the aluminium matrix. In particular, measurements with the electrochemical micro-cell evidenced

AC C

that BT affects anodic processes widening the passive region although this is mainly reported as a cathodic inhibitor. However, the film stability was strongly dependent on the presence of inhibitor in the aggressive solution. In contrast with the considerable corrosion protection offered by benzotriazole on copper, the low content of copper ions in the case of AA2024 might here explain the poor stability of the semi-protective inhibitive layer, especially in the electrolyte without benzotriazole. A possible explanation is the desorption of benzotriazole molecules physically adsorbed on the Al matrix when the alloy is immersed in solution containing no

20

ACCEPTED MANUSCRIPT

inhibitive species, due to the high solubility of the inhibitor. Finally, the role of chloride ions in the formation of the inhibitive layer was also highlighted and it was attributed to the possible

RI PT

formation of Cu:BT-/Cu-BT complexes onto IMPs when localized corrosion processes occur. Acknowledgements

Isaline Recloux wishes to thank the FRIA (Fonds pour la Formation à la Recherche dans

SC

l’Industrie et l’Agriculture) for funding. This study was also done in the framework of the

Opti2mat and FLYCOAT “Programme Excellence” financed by the Walloon Region (Belgium).

M AN U

Authors also would like to thank the Sonaca for providing aluminium. References

AC C

EP

TE D

[1] G.S. Chen, M. Gao, R.P. Wei, Microconstituent-Induced Pitting Corrosion in Aluminum Alloy 2024-T3, Corrosion, 52 (1996) 8-15. [2] R.G. Buchheit, R.P. Grant, P.F. Hlava, B. Mckenzie, G.L. Zender, Local Dissolution Phenomena Associated with S Phase  ( Al2CuMg )  Particles in Aluminum Alloy 2024‐T3, Journal of The Electrochemical Society, 144 (1997) 2621-2628. [3] A. Boag, A.E. Hughes, N.C. Wilson, A. Torpy, C.M. MacRae, A.M. Glenn, T.H. Muster, How complex is the microstructure of AA2024-T3?, Corrosion Science, 51 (2009) 1565-1568. [4] R.G. Buchheit, A Compilation of Corrosion Potentials Reported for Intermetallic Phases in Aluminum Alloys, Journal of The Electrochemical Society, 142 (1995) 3994-3994. [5] A. Caglieri, M. Goldoni, O. Acampa, The effect of inhaled chromium on different exhaled breath condensate biomarkers among chrome-plating workers, Environmental health …, (2006). [6] M. Faisal, S. Hasnain, Hazardous Impact of Chromium on Environment and its appropriate Remediation, J. Pharmacol. Toxicol, (2006). [7] http://www.rohsguide.com/ accessed on February 2014, in. [8] http://echa.europa.eu/fr/candidate-list-table accessed on February 2014, in. [9] T.G. Harvey, S.G. Hardin, A.E. Hughes, T.H. Muster, P.A. White, T.A. Markley, P.A. Corrigan, J. Mardel, S.J. Garcia, J.M.C. Mol, A.M. Glenn, The effect of inhibitor structure on the corrosion of AA2024 and AA7075, Corrosion Science, 53 (2011) 2184-2190. [10] G. Boisier, N. Portail, N. Pébère, Corrosion inhibition of 2024 aluminium alloy by sodium decanoate, Electrochimica Acta, 55 (2010) 6182-6189. [11] W. Badawy, Electrochemical behaviour and corrosion inhibition of Al, Al-6061 and Al–Cu in neutral aqueous solutions, Corrosion Science, 41 (1999) 709-727. [12] M. Bethencourt, F.J. Botana, J.J. Calvino, M. Marcos, M.A. RodrÍguez-Chacón, Lanthanide compounds as environmentally-friendly corrosion inhibitors of aluminium alloys: a review, Corrosion Science, 40 (1998) 1803-1819.

21

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[13] L. Paussa, F. Andreatta, D. De Felicis, E. Bemporad, L. Fedrizzi, Investigation of AA2024T3 surfaces modified by cerium compounds: A localized approach, Corrosion Science, 78 (2014) 215-222. [14] L. Paussa, F. Andreatta, N.C. Rosero Navarro, A. Durán, L. Fedrizzi, Study of the effect of cerium nitrate on AA2024-T3 by means of electrochemical micro-cell technique, Electrochimica Acta, 70 (2012) 25-33. [15] S. Lamaka, M. Zheludkevich, K. Yasakau, M. Montemor, M. Ferreira, High effective organic corrosion inhibitors for 2024 aluminium alloy, Electrochimica Acta, 52 (2007) 72317247. [16] M. Zheludkevich, K. Yasakau, S. Poznyak, M. Ferreira, Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy, Corrosion Science, 47 (2005) 3368-3383. [17] V. Palanivel, Y. Huang, W.J. van Ooij, Effects of addition of corrosion inhibitors to silane films on the performance of AA2024-T3 in a 0.5M NaCl solution, Progress in Organic Coatings, 53 (2005) 153-168. [18] B.W. Samuels, K. Sotoudeh, R.T. Foley, Inhibition and Acceleration of Aluminum Corrosion, Corrosion, 37 (1981) 92-97. [19] H. Yang, W.J. van Ooij, Plasma-treated triazole as a novel organic slow-release paint pigment for corrosion control of AA2024-T3, Progress in Organic Coatings, 50 (2004) 149-161. [20] H.B. Madsen, A preliminary note on the use of benzotriazole for stabilizing bronze objects, Studies in Conservation, (1967). [21] G. Williams, A.J. Coleman, H.N. McMurray, Inhibition of Aluminium Alloy AA2024-T3 pitting corrosion by copper complexing compounds, Electrochimica Acta, 55 (2010) 5947-5958. [22] I. Recloux, M. Mouanga, M.-E. Druart, M.-G. Olivier, Silica mesoporous thin films as containers for benzotriazole for corrosion protection of 2024 aluminium alloys, Applied Surface Science, 346 (2015) 124-133. [23] J.B. Cotton, I.R. Scholes, Benzotriazole and Related Compounds as Corrosion Inhibitors For Copper, (2013). [24] O.M. Magnussen, M.R. Vogt, J. Scherer, R.J. Behm, Double-layer structure, corrosion and corrosion inhibition of copper in aqueous solution, Applied Physics A: Materials Science & Processing, 66 (1998) S447-S451. [25] G.W. Poling, Reflection infra-red studies of films formed by benzotriazole on Cu, Corrosion Science, 10 (1970) 359-370. [26] M. Finšgar, I. Milošev, Inhibition of copper corrosion by 1, 2, 3-benzotriazole: a review, Corrosion Science, (2010). [27] T. Kosec, D.K. Merl, I. Milošev, Impedance and XPS study of benzotriazole films formed on copper, copper–zinc alloys and zinc in chloride solution, Corrosion Science, 50 (2008) 19871997. [28] F. MANSFELD, T. SMITH, E.P. PARRY, Benzotriazole as Corrosion Inhibitor for Copper, Corrosion, 27 (1971) 289-294. [29] T. Aben, D. Tromans, Anodic Polarization Behavior of Copper in Aqueous Bromide and Bromide/Benzotriazole Solutions, Journal of The Electrochemical Society, 142 (1995) 398-404. [30] J. Rubim, I.G.R. Gutz, O. Sala, W.J. Orville-Thomas, Surface enhanced Raman spectra of benzotriazole adsorbed on a copper electrode, Journal of Molecular Structure, 100 (1983) 571583.

22

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[31] R. Youda, H. Nishihara, K. Aramaki, Sers and impedance study of the equilibrium between complex formation and adsorption of benzotriazole and 4-hydroxybenzotriazole on a copper electrode in sulphate solutions, Electrochimica Acta, 35 (1990) 1011-1017. [32] D. Chadwick, T. Hashemi, Adsorbed corrosion inhibitors studied by electron spectroscopy: Benzotriazole on copper and copper alloys, Corrosion Science, 18 (1978) 39-51. [33] V. Brusic, M. Frisch, B. Eldridge, F. Novak, F. Kaufman, B. Rush, G. Frankel, Copper corrosion with and without inhibitors, Journal of the electrochemical society, 138 (1991) 22532259. [34] W. Qafsaoui, F. Huet, H. Takenouti, Analysis of the inhibitive effect of BTAH on localized corrosion of Al 2024 from electrochemical noise measurements, Journal of the Electrochemical Society, 156 (2009) C67-C74. [35] D. Fix, D.V. Andreeva, Y.M. Lvov, D.G. Shchukin, H. Möhwald, Application of InhibitorLoaded Halloysite Nanotubes in Active Anti-Corrosive Coatings, Advanced Functional Materials, 19 (2009) 1720-1727. [36] M.L. Zheludkevich, D.G. Shchukin, Anticorrosion coatings with self-healing effect based on nanocontainers impregnated with corrosion inhibitor, Chemistry of …, (2007). [37] D. Borisova, H. Möhwald, D.G. Shchukin, Mesoporous silica nanoparticles for active corrosion protection, ACS nano, 5 (2011) 1939-1946. [38] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, P. Cecílio, M.G.S. Ferreira, TiOx self-assembled networks prepared by templating approach as nanostructured reservoirs for self-healing anticorrosion pre-treatments, Electrochemistry Communications, 8 (2006) 421-428. [39] S. Lamaka, M. Zheludkevich, K. Yasakau, R. Serra, S. Poznyak, M. Ferreira, Nanoporous titania interlayer as reservoir of corrosion inhibitors for coatings with self-healing ability, Progress in Organic Coatings, 58 (2007) 127-135. [40] J. Izquierdo, J.J. Santana, S. González, R.M. Souto, Scanning microelectrochemical characterization of the anti-corrosion performance of inhibitor films formed by 2mercaptobenzimidazole on copper, Progress in Organic Coatings, 74 (2012) 526-533. [41] M. Taryba, S.V. Lamaka, D. Snihirova, M.G.S. Ferreira, M.F. Montemor, W.K. Wijting, S. Toews, G. Grundmeier, The combined use of scanning vibrating electrode technique and micropotentiometry to assess the self-repair processes in defects on “smart” coatings applied to galvanized steel, Electrochimica Acta, 56 (2011) 4475-4488. [42] K.A. Yasakau, M.L. Zheludkevich, S.V. Lamaka, M.G.S. Ferreira, Mechanism of Corrosion Inhibition of AA2024 by Rare-Earth Compounds, The Journal of Physical Chemistry B, 110 (2006) 5515-5528. [43] S. Kallip, A.C. Bastos, M.L. Zheludkevich, M.G.S. Ferreira, A multi-electrode cell for highthroughput SVET screening of corrosion inhibitors, Corrosion Science, 52 (2010) 3146-3149. [44] K. Yasakau, M.L. Zheludkevich, M.G. Ferreira, Study of the corrosion mechanism and corrosion inhibition of 2024 aluminum alloy by SKPFM technique, in: Materials Science Forum, Trans Tech Publ, 2008, pp. 405-409. [45] T.H. Muster, H. Sullivan, D. Lau, D.L.J. Alexander, N. Sherman, S.J. Garcia, T.G. Harvey, T.A. Markley, A.E. Hughes, P.A. Corrigan, A.M. Glenn, P.A. White, S.G. Hardin, J. Mardel, J.M.C. Mol, A combinatorial matrix of rare earth chloride mixtures as corrosion inhibitors of AA2024-T3: Optimisation using potentiodynamic polarisation and EIS, Electrochimica Acta, 67 (2012) 95-103.

23

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[46] K.A. Yasakau, J. Tedim, M.F. Montemor, A.N. Salak, M.L. Zheludkevich, M.G.S. Ferreira, Mechanisms of Localized Corrosion Inhibition of AA2024 by Cerium Molybdate Nanowires, The Journal of Physical Chemistry C, 117 (2013) 5811-5823. [47] F. Andreatta, M.E. Druart, E. Marin, D. Cossement, M.G. Olivier, L. Fedrizzi, Volta potential of clad AA2024 aluminium after exposure to CeCl3 solution, Corrosion Science, 86 (2014) 189-201. [48] K.D. Ralston, T.L. Young, R.G. Buchheit, Electrochemical evaluation of constituent intermetallics in aluminum alloy 2024-T3 exposed to aqueous vanadate inhibitors, Journal of The Electrochemical Society, 156 (2009) C135-C146. [49] N. Birbilis, R.G. Buchheit, Inhibition of AA2024-T3 on a phase-by-phase basis using an environmentally benign inhibitor, cerium dibutyl phosphate, Electrochemical and solid-state letters, 8 (2005) C180-C183. [50] F. Andreatta, M.E. Druart, A. Lanzutti, M. Lekka, D. Cossement, M.G. Olivier, L. Fedrizzi, Localized corrosion inhibition by cerium species on clad AA2024 aluminium alloy investigated by means of electrochemical micro-cell, Corrosion Science, 65 (2012) 376-386. [51] F. Andreatta, L. Fedrizzi, The use of the electrochemical micro-cell for the investigation of corrosion phenomena, Electrochimica Acta, 203 (2016) 337-349. [52] T. Suter, H. Böhni, A new microelectrochemical method to study pit initiation on stainless steels, Electrochimica Acta, 42 (1997) 3275-3280. [53] Y. Cheng, Z. Zhang, F. Cao, J. Li, J. Zhang, J. Wang, C. Cao, A study of the corrosion of aluminum alloy 2024-T3 under thin electrolyte layers, Corrosion Science, 46 (2004) 1649-1667. [54] L. Lacroix, L. Ressier, C. Blanc, G. Mankowski, Statistical study of the corrosion behavior of Al2CuMg intermetallics in AA2024-T351 by SKPFM, Journal of the electrochemical society, 155 (2008) C8-C15. [55] L. Lacroix, L. Ressier, C. Blanc, G. Mankowski, Combination of AFM, SKPFM, and SIMS to study the corrosion behavior of S-phase particles in AA2024-T351, Journal of the electrochemical society, 155 (2008) C131-C137. [56] J. Rodriguez, L. Chenoy, A. Roobroeck, S. Godet, M.G. Olivier, Effect of the electrolyte pH on the corrosion mechanisms of Zn-Mg coated steel, Corrosion Science, 108 (2016) 47-59. [57] H. Shi, Z. Tian, T. Hu, F. Liu, E.-H. Han, M. Taryba, S. Lamaka, Simulating corrosion of Al 2 CuMg phase by measuring ionic currents, chloride concentration and pH, Corrosion Science, 88 (2014) 178-186. [58] R. Oltra, B. Vuillemin, F. Thebault, F. Rechou, Effect of the surrounding aeration on microcapillary electrochemical cell experiments, Electrochemistry Communications, 10 (2008) 848-850. [59] X. Zhang, T. Hashimoto, J. Lindsay, X. Zhou, Investigation of the de-alloying behaviour of θ-phase (Al 2 Cu) in AA2024-T351 aluminium alloy, Corrosion Science, 108 (2016) 85-93. [60] L. Coelho, M. Mouanga, M.-E. Druart, I. Recloux, D. Cossement, M.-G. Olivier, A SVET study of the inhibitive effects of benzotriazole and cerium chloride solely and combined on an aluminium/copper galvanic coupling model, Corrosion Science, 110 (2016) 143-156. [61] I. Recloux, Y. Gonzalez-Garcia, M.-E. Druart, F. Khelifa, P. Dubois, J. Mol, M.-G. Olivier, Active and passive protection of AA2024-T3 by a hybrid inhibitor doped mesoporous sol–gel and top coating system, Surface and Coatings Technology, 303 (2016) 352-361. [62] W. Qafsaoui, C. Blanc, N. Pébère, H. Takenouti, A. Srhiri, G. Mankowski, Quantitative characterization of protective films grown on copper in the presence of different triazole derivative inhibitors, Electrochimica Acta, 47 (2002) 4339-4346.

24

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

[63] M. Vukmirovic, N. Dimitrov, K. Sieradzki, Dealloying and corrosion of Al alloy 2024 T 3, Journal of the Electrochemical Society, 149 (2002) B428-B439. [64] D. Tromans, Aqueous Potential‐pH Equilibria in Copper‐Benzotriazole Systems, Journal of the Electrochemical Society, 145 (1998) L42-L45. [65] B. Xu, F. Wu, X. Zhao, H. Liao, Benzotriazole removal from water by Zn-Al-O binary metal oxide adsorbent: behavior, kinetics and mechanism, Journal of hazardous materials, 184 (2010) 147-155. [66] M.C. Zonnevylle, R. Hoffmann, The Absorption of Benzotriazole on Copper and Cuprous Oxide, in, DTIC Document, 1988. [67] A. Kokalj, S. Peljhan, M. Finšgar, I. Milošev, What determines the inhibition effectiveness of ATA, BTAH, and BTAOH corrosion inhibitors on copper?, Journal of the American Chemical Society, 132 (2010) 16657-16668. [68] F. Bayoumi, A. Abdullah, B. Attia, Kinetics of corrosion inhibition of benzotriazole to copper in 3.5% NaCl, Materials and corrosion, 59 (2008) 691-696.

Figure captions

Fig. 1 – SEM image of the AA2024-T3 bare substrate with Fe-rich and Mg-rich IMPs indicated. Fig. 2 – Open circuit potential recorded in 0.05 M NaCl solution for two AA2024-T3 substrates

TE D

after immersion for 24 h in NaCl+BT solution.

Fig. 3 – Anodic (a) and cathodic (b) polarization curves recorded either in 0.05 M NaCl or in NaCl+BT solution for the bare substrate and for samples immersed for 24 h either in NaCl+BT

EP

or in BT solution.

AC C

Fig. 4 – Anodic polarization curves acquired by micro-cell technique in 0.05 M NaCl for the bare substrate and in 0.05 M NaCl and in NaCl+BT solution for the substrate after 24 h of immersion in the inhibitive solution (a) on the matrix, (b) on Fe-rich intermetallic particles and (c) on Mgrich intermetallic particles.

Fig. 5 –– Cathodic polarization curves acquired by micro-cell technique in 0.05 M NaCl for the bare substrate and in 0.05 M NaCl and in NaCl+BT solution for the substrate after 24 h of

25

ACCEPTED MANUSCRIPT

immersion in the inhibitive solution (a) on the matrix, (b) on Fe-rich intermetallic particles and (c) on Mg-rich intermetallic particles.

RI PT

Fig. 6 – ToF-SIMS maps of C6H4N3- ions (a) and C2H- ions (b) detected on AA2024 sample after immersion for 24 h in NaCl+BT solution and rinsing in demineralized water. Surface imaging is done without any sputtering by argon ion beam.

SC

Fig. 7 – ToF-SIMS map of C6H4N3- ions detected on AA2024 sample after immersion for 24 h in NaCl+BT solution and rinsing in demineralized water. Areas within the frame correspond to non

M AN U

sputtered surface while the rest of the sample was bombarded with argon ion beam. Fig. 8 – Modulus (a) and phase (b) Bode plots recorded either in 0.05 M NaCl or in NaCl+BT solution for the bare substrate and for the substrates immersed 24 h in NaCl+BT solution.

TE D

Fig. 9 – Equivalent circuit model used for fitting EIS data.

Fig. 10 – N 1s spectra detected by XPS for AA2024 substrates after different immersion conditions. The intensity of the spectra for samples immersed in the BT solution (green spectra)

EP

was multiplied by a factor 2.2.

Fig. 11 – Modulus (a) and phase (b) Bode plots recorded in 0.05 M NaCl for the bare substrate

AC C

and for the substrates immersed 24 h in BT or in NaCl+BT solution. Tables

Table 1 - Composition in weight percent of 2024-T3 alloy

26

ACCEPTED MANUSCRIPT

Table 2 – Breakdown potentials (Eb) and their shifts (∆Eb) relative to those measured for bare AA2024 obtained from polarization curves carried out by micro-cell technique on the matrix and

RI PT

on regions containing Fe-rich and Mg-rich intermetallic particles in AA2024-T3. Table 3 – Fitting parameters highlighting the influence of the presence of benzotriazole in the EIS testing electrolyte.

SC

Table 4 – Fitting parameters highlighting the influence of the presence of chloride ions in the

AC C

EP

TE D

M AN U

inhibitor solution.

27

ACCEPTED MANUSCRIPT Element

Si Fe

Cu

Mn

Concentration (wt.%) 0.5 0.5 3.8-4.9 0.3-0.9

Mg 1.2-1.8

Zn

Cr

0.25

Ti

0.1

0.15

Others

Al

0.15

Min 90.75

AC C

EP

TE D

M AN U

SC

RI PT

Table 1 - Composition in weight percent of 2024-T3 alloy

ACCEPTED MANUSCRIPT

Location

Matrix Fe-rich Mg-rich

Polarization in NaCl -0.240 -0.260 -0.540 -

Eb (V) ∆Eb (V) Eb (V) ∆Eb (V) Eb (V) ∆Eb (V)

Immersion for 24 h in NaCl+BT

Immersion for 24 h in NaCl+BT

Polarization in NaCl

Polarization in NaCl+BT

+0.580 +0.820 -0.190 +0.070 -0.220 +0.320

+0.650 +0.890 +0.150 +0.410 -0.160 +0.380

RI PT

Bare AA2024

SC

Table 2 – Breakdown potentials (Eb) and their shifts (∆Eb) relative to those measured for bare AA2024 obtained from polarization curves carried out by micro-cell technique on the matrix

AC C

EP

TE D

M AN U

and on regions containing Fe-rich and Mg-rich intermetallic particles in AA2024-T3.

ACCEPTED MANUSCRIPT Qox Conditions

[Ω s cm 2 ]

Rox -

nox [Ω cm²]

EIS in NaCl+BT after 15 min

9.3x10-6

0.9 1.5x105

EIS in NaCl+BT after 24 h

1.1x10-5

0.9 1.9x105

EIS in NaCl after 15 min Immersion for 24h in NaCl+BT EIS in NaCl after 24 h

RI PT

Immersion for 24 h in NaCl+BT

-1 n

5.4x10-6

0.9 1.4x105

2.8x10-5

0.8 2.3x104

Table 3 – Fitting parameters highlighting the influence of the presence of benzotriazole in the

AC C

EP

TE D

M AN U

SC

EIS testing electrolyte.

ACCEPTED MANUSCRIPT

Qox Conditions

[Ω-1 sn cm-2]

Immersion for 24 h in NaCl+BT EIS in NaCl after 6 h EIS in NaCl after 1 h Immersion for 24h in BT

3.7x10-6

0.9

1.3x105

4.6x10-6

0.9

3.0x104

8.7x10-6

0.9

1.9x104

1.8x10-5

0.9

1.3x104

SC

EIS in NaCl after 6 h

[Ω cm²]

RI PT

EIS in NaCl after 1 h

Rox nox

Table 4 – Fitting parameters highlighting the influence of the presence of chloride ions in the

AC C

EP

TE D

M AN U

inhibitor solution.

1

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Highlights

Benzotriazole inhibitor effect is investigated on AA2024 at macro and micro scales.




Entire substrate surface (intermetallics and matrix) is protected by benzotriazole.




The presence of the inhibitor is required to assure the protection of AA2024.




The benzotriazole film formation is guaranteed in presence of chloride ions.

AC C

EP

TE D

M AN U

SC

RI PT