Inhibition of galvanic corrosion in Al/Cu coupling model by synergistic combination of 3-Amino-1,2,4-triazole-5-thiol and cerium chloride

1. Introduction

Galvanic corrosion describes induced corrosion of metals in connection with other metals with higher potential [1]. There are variations in the localized corrosion mechanisms of different aluminium alloys due to the nature, concentration and distribution of intermetallic particles (IMPs). For instance, galvanic corrosion in Al alloys depends on the differing degrees of potential of IMPs relative to the Al matrix [[2], [3], [4], [5]]. When portions of Al alloy with more noble IMPs, which have cathodic potential relative to Al matrix, are exposed to corrosive environment, localized corrosion in the region surrounding the IMPs is encouraged [6]. With less noble IMPs, which have more anodic potential relative to Al matrix, anodic reactions of the IMPs can result in dealloying of IMPs [7]. A broad array of research has been devoted to understanding the effects of intermetallic inclusions on the corrosion of Al alloys [[8], [9], [10], [11], [12], [13], [14]]. Among the 2xxx series of aluminium alloys, AA2024-T3 has high strength-to-weight ratio, making it suitable for application in the aerospace sector where the mechanical properties are crucial. However, the matrix of AA2024-T3 contains different kinds of intermetallic particles (IMPs) which offer sites for the propagation of corrosion reactions. The S-phase precipitates are the most predominant intermetallic phase (above 60%) and are composed of Al₂CuMg [15]. The S-phase covers a geometrical surface area of about 3% of AA2024-T3 [16]. Other intermetallic phases present in AA2024-T3 include: A1₆(Cu, Fe, Mn) (second largest), Al₂₀Mn₃Cu₂, Al₂Cu, Al₇Cu₂Fe, and (Al, Cu)₆Mn [16,17]. In terms of initiation and propagation of corrosion activity in local regions of AA2024-T3 bearing S-phase (Al₂CuMg) intermetallic particle, the S-phase is initially less noble than Al matrix and undergoes anodic reaction, resulting in selective dissolution of intermetallic Al and Mg. The remaining Cu-rich intermetallic phase subsequently acts cathodically towards Al matrix [16,18]. This has been attributed to the micro-galvanic couplings resulting from Cu-rich IMPs and the Al matrix [19]. The Cu-rich phase is the main phase for oxygen reduction reactions (ORR) [20].

Modelling of the galvanic coupling between IMPs and Al matrix in aluminium alloy 2024-T3 has received significant attention aiming to understand the corrosion mechanism of AA2024-T3 [9,21,22]. Al/Cu coupling model allows the simulation of a typical local AA2024-T3 environment with Cu-rich IMP and estimation of the effect of inhibitors [23,24]. Such investigations provide information on key areas that offer insights into the behaviour of real systems. For example, Jorcin et al. showed that for an Al/Cu couple, cathodic reactions take place on the Cu, increasing the local pH reaching values higher than 9, and allowing the dissolution of Al [19]. Blanc et al. reported that corrosion of Cu is also possible even though it is polarized cathodically at the potential of Al/Cu couple [25].

To prevent corrosion of aluminium alloys, many studies have investigated environmentally benign alternatives to chromate inhibitors [[26], [27], [28]]. Chromates have been applied as anticorrosive additives in pre-treatment and finishing procedures for aircraft aluminium alloys due to their efficiency/cost ratio [29,30]. The problem with chromates is their high toxicity and carcinogenicity, such that strict imposition of environmental laws has discouraged the use of chromates, and encouraged research on environmentally friendly alternatives [[31], [32], [33], [34]]. For the development of chromate-free inhibition systems for AA2024-T3, an understanding of the corrosion mechanism/inhibition in local intermetallic regions is crucial [21]. Due to the small sizes of IMPs (usually 1-30 μm), the measurement of the corrosion current densities directly from the alloy may be inaccurate [35]. Therefore, with coupling models simulating IMPs, study on particle-induced pitting arising from galvanic couplings can be carried out [8,19,22], with clear estimation of the effect of inhibitors on the anticorrosion protection of aluminium alloy [36]. Moreover, corrosion inhibition synergies was reported to offer increased protection [21]. Utilizing 8-hydroxyquinoline and BTA as combined inhibitor system for AA2024-T3 and Al/Cu couple model, Marcelin and Pébère illustrated their synergistic effect [37]. In two separate works, Coelho et al. demonstrated the synergistic inhibition effect of benzotriazole and cerium chloride on Al/Cu couple [38] and on AA2024-T3 [39]. Worthy of note is that the galvanic corrosion in Al/Cu couple is expected to be more severe than what is obtained between Al matrix and IMPs in AA2024-T3 [38]. This is because the exposed Cu surface in AA2024-T3 compared to Al surface is relatively small. However, the construction of Al/Cu model with Cu surface area higher than its surface area in the real micro-galvanic couple in AA2024-T3 allows the acceleration of galvanic corrosion, to which the mechanism and effectiveness of corrosion inhibition systems can be examined, leading to an understanding of the real system [21,38]. Moreover, investigations on a micro-scale would require in-situ techniques having spatial resolutions suitable to resolve electrochemical reactivity of micro-galvanic cells of AA2024-T3, which is currently not possible with traditional electrochemical methods [8,40].

Herein, an Al/Cu coupling model, consisting of pure aluminium/pure copper wire was investigated. The synergistic inhibition effect of the combination of 3-amino-1,2,4-triazole-5-thiol (ATAT) and cerium chloride on the protection of an Al/Cu couple against galvanic corrosion is demonstrated for the first time. Immersion of samples for 24 h in a 50 mM Cl^- solution with or without inhibitors 3-amino-1,2,4-triazole-5-thiol and/or cerium chloride was followed by in-situ scanning vibrating electrode technique (SVET) assessment to evaluate galvanic corrosion current densities. SVET has proven useful in the evaluation of galvanic corrosion of intermetallic phases [8,9,21,22,36,38]. Complementary ex-situ scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX) was used for surface characterizations after SVET measurements. X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS) were utilized for analysis of film formed and to explain mechanism of inhibition.

2. Experimental

2.1. Materials

The aluminium and copper wires used in this work were locally sourced. The wires, separated by about 0.18 mm gap were embedded in an epoxy mount, with an electrical connection at the rear (Fig. 1). Cerium (III) chloride heptahydrate (CeCl₃·7H₂O, 99% purity) and 3-amino-1,2,4-triazole-5-thiol (ATAT, 95% purity) were purchased from Sigma-Aldrich. Sinopharm Chemical Reagent Co. (China) supplied sodium chloride (NaCl) and other chemical reagents. Analytical grade reagents were used in all cases without further purification.

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Fig. 1.   Schematic diagram and optical micrograph of Al/Cu coupling model.

2.2. Methods

SVET investigations were carried out on Al/Cu model immersed in 50 mM Cl^- solutions to demonstrate the galvanic corrosion protection efficiency of ATAT and CeCl₃. For this purpose, SiC paper was used to abrade the Al/Cu model, followed by polishing with diamond paste to obtain smooth surface. A cleaning procedure with ethanol was carried out before compressed air was used to dry the sample. Applicable Electronics supplied the SVET equipment with a Sciencewares ASET program. SVET measurements involved a vibrating electrode made up of Pt-Ir probe with platinum on a spherical tip of 30 μm diameter. The frequencies of vibration were 272 Hz (X) and 872 Hz (Y). Measurements were taken from a probe distance of 100 μm from the sample surface. A 40 × 40 grid map of ionic current densities was obtained. Scans were made at 0 h, 0.5 h, 2 h and 24 h of immersion. The reference electrolyte was a neutral aerated 50 mM NaCl solution. Samples involving CeCl₃·7H₂O confers additional chloride ions, therefore the concentration of Cl^- in such cases were modified to maintain the solution concentration at 50 mM [38]. The compositions of the electrolytes used are given in Table 1. Visualization of SVET data was aided by QuikGrid software.

The morphologies and elemental composition of sample surface after SVET analyses were assessed by SEM/EDX characterizations. A Philips environmental scanning electron microscope (ESEM, XL30) was employed, with a probe distance of 10 mm and applied accelerating voltage of 25 kV. All samples were spray-coated with gold before SEM examinations.

XPS analysis was performed on both Al and Cu surfaces after 24 h of immersion in Ce_2.5ATAT_2.5, in order to examine the mechanism of inhibition and study the film formed by the combination of inhibitors. For this purpose, an ESCALAB250 system, operated under 5 × 10^-10 mbar, with an AlK_α anode (λ = 1486.6 eV), was used. The spot size of the area examined was 500 μm and the applied pass energy was 50 eV. Spectra were obtained for Al 2p, Cu 2p, Ce 3d, N 1s, and O 1s. Additionally, depth profiles of the elements were obtained. An XPSPEAK41 processing software was used to fit data. Fittings proceeded by fixing carbon peak at 284.6 eV in order to calibrate the binding energies of the elemental components of the film.

To further study the inhibitive film, ToF-SIMS analysis was carried out on Al/Cu couple after immersion in Ce_2.5ATAT_2.5 for 24 h. The mass spectra and profiles of component ions were obtained. The instrument employed was a ToF-SIMS 5 spectrometer (ION TOF, GmbH - Muenster, Germany). The experimental conditions included 1 × 10^-9 mbar pressure for the analysis chamber, 30 keV Bi⁺ ion source, and a current of 1.0 pA. To obtain depth profiles, Cs⁺ sputter beam was utilized under dual mode, for negative ion clusters. Acquisition of data and processing were aided by Ion-Spec commercial software.

3. Results and discussion

3.1. Corrosion activity of uninhibited Al/Cu galvanic couple

The assessment of the corrosion current distribution in the local environment of Al/Cu couple by SVET allows demonstration of the galvanic corrosion and illustrates the effect of inhibitors on the galvanic corrosion process. Fig. 2(a)-(c) depicts the current density maps obtained from the surface of Al/Cu couple after 0 h, 2 h and 24 h, respectively, and an optical photo (Fig. 2(d)) of the sample surface after 24 h of immersion in reference solution. The SVET maps reveal intense anodic corrosion activity over Al (anodic currents in red) and intense cathodic corrosion activity over Cu (cathodic currents in blue) as a result of galvanic corrosion. The anodic and cathodic reactions over Al and Cu, respectively, are shown (Eqs. (1) and (2)). The intense anodic and cathodic currents, even at 0 h, show that chloride ion content in 50 mM of NaCl solution is enough to induce corrosion reaction immediately after immersion. Furthermore, it can be observed that while cathodic current density over Cu is evenly spread over the area of the Cu surface, the anodic currents over Al surface is denser in some local regions within the Al surface. This phenomenon is related to pit formation usually associated with corrosion of Al after the passive film is destroyed by aggressive Cl^- [18,41]. The initiation of pitting can be observed immediately after immersion, which underscores the aggressiveness of the electrolyte. A closer look at the current density map after 2 h of immersion (Fig. 2(b)) indicates propagation of pitting evidenced by more local regions with intense current densities. At 24 h, although the corrosion products passivate some area of the Al surface, the current density map still reveals an area with a clear intensive pit. Additionally, the Cu surface reveals strong cathodic current densities throughout the immersion period. Clearly, corrosion activity persists in the absence of inhibitors.

Al→Al³⁺+3e^-(1)

2H₂O+O₂+4e^-→4OH^-(2)

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Fig. 2.   Distribution of galvanic corrosion current densities on the surface of Al/Cu couple immersed in aerated 50 mM aqueous NaCl reference solution. Scans obtained at (a) 0 h, (b) 2 h, and (c) 24 h of immersion; (d) optical image of sample after 24 h of immersion.

3.2. Corrosion activity of Al/Cu galvanic couple in individual inhibitor solutions

Fig. 3 shows current density maps and photo for Al/Cu couple immersed in 35 mM NaCl + 5 mM CeCl₃ solution. As in the uninhibited system, a strong galvanic corrosion activity indicated by intense anodic and cathodic current density (Fig. 3(a)) is observed at the beginning of immersion. With increase of immersion time, an obvious decrease in current density then sets in (Fig. 3(b) and (c)). The reduction in electrochemical activity with time may be due to precipitation of insoluble cerium oxides/hydroxides preferentially on cathodic areas-Cu surface [36]. Cerium salts are cathodic inhibitors which hinder oxygen reduction reaction by the mechanism of blocking dissolved oxygen from getting to the surface of the electrode. When pH of local environment increases at cathodic regions, a reaction between Ce³⁺ and hydroxyl ions produces Ce(OH)₃ precipitates in these regions [27]. In this case, the Cu surface is first covered by Ce(OH)₃, after which further reaction can also precipitate on Al surface. A typical reaction that describes precipitation of cerium oxides/hydroxides is given (Eq. (3)) [38]:

Ce³⁺+3OH^-→Ce(OH)₃→CeO₂+H₃O⁺+e^-(3)

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Fig. 3.   Distribution of galvanic corrosion current densities on the surface of Al/Cu couple immersed in aerated 35 mM NaCl + 5 mM CeCl₃ aqueous solution obtained at (a) 0 h, (b) 2 h, and (c) 24 h of immersion; (d) optical image of sample after 24 h of immersion.

The initial strong electrochemical activity indicated by intense anodic and cathodic current density in the presence of cerium (Fig. 3(a)) conforms to findings in literature [39,42]. Although the reason is unclear, it may be as a result of the effect of cerium salt on the corrosion redox reaction. Transfer of electrons usually accompanies redox reaction. Initially, in an aerated solution, the presence of salts (or any electrolyte) tends to accelerate the redox reaction because it increases the concentration of ions, and invariably, the conductivity of water. This increase in the concentration of ions results in increase in the rate of oxidation (corrosion) of the metal, possibly giving the initial strong anodic and cathodic current signal. However, as precipitation of cerium oxides/hydroxides begins, corrosion is inhibited.

Fig. 4 shows current density maps and photo for Al/Cu couple immersed in 50 mM NaCl + 5 mM ATAT solution. As can be seen by the reduction in the current density even from beginning of immersion (Fig. 4(a)), there is an indication of inhibition of galvanic corrosion for Al/Cu couple in the presence of ATAT. Earlier investigations have revealed that ATAT is a mixed type inhibitor, with the adsorption of ATAT molecules on the substrate as the mechanism of inhibition [43]. Therefore, ATAT interferes with both anodic activities on Al and cathodic activities on Cu, reducing the current densities dramatically. However, the physical adsorbed film of ATAT molecules on the surface of Al and Cu surfaces seem not strong enough to withstand the ingress of aggressive Cl^- and galvanic corrosion activity. As seen in Fig. 4(b) and (c), after 2 h and 24 h of immersion the Al surface shows a localized region of intense anodic current density. The region is typical of pit formation and suggests that the physically adsorbed ATAT film in that region has been compromised as a result of penetration of Cl^-. From 2 h to 24 h, there seem to be increase in the current density in the pit region, revealing further destruction of ATAT film with time. For the individual inhibitor systems, considering the galvanic corrosion current distribution, ATAT₅ (Fig. 4) seems to show better overall inhibition when compared with Ce₅ (Fig. 3).

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Fig. 4.   Distribution of galvanic corrosion current densities on the surface of Al/Cu couple immersed in aerated 50 mM NaCl + 5 mM ATAT aqueous solution obtained at (a) 0 h, (b) 2 h, and (c) 24 h of immersion; (d) optical image of sample after 24 h of immersion.

3.3. Corrosion activity of Al/Cu galvanic couple in combined inhibitor solutions

The combination of CeCl₃ and ATAT reveals the best inhibition due to synergistic effect of the inhibitors. Fig. 5 shows current density maps and photo for Al/Cu couple immersed in 42.5 mM NaCl + 2.5 mM CeCl₃ + 2.5 mM ATAT solution. The immersion of Al/Cu couple in equal ratio of inhibitor combination (Ce_2.5ATAT_2.5) can be seen to proceed with high current densities just after immersion (Fig. 5(a)), which may be caused by the presence of cerium ions. However, a sharp reduction in the current density is observed after 2 h of immersion (Fig. 5(b)), which reduces further over time to an even lesser current density after 24 h of immersion (Fig. 5(c)). Interestingly, the current density maps after 2 h (Fig. 5(b)) and 24 h (Fig. 5(c)) shows no clear region of pitting. This indicates a cooperative interaction of Ce³⁺ and ATAT in the local attenuation of galvanic corrosion currents. The results suggest inhibition based on Ce-ATAT complex film. At any instance where the film is destroyed, cerium oxides/hydroxides counteract the effect, particularly on the Cu surface, thereby maintaining resistance to ingress of Cl^-. Individually, as ATAT₅ leads to more reduction in galvanic corrosion activity and better protection of Al/Cu couple than Ce₅ (see Section 3.2), a tweak in the concentration ratio of inhibitor combination (Ce_1.5ATAT_3.5) achieves significant inhibition. Fig. 6 demonstrates significant reduction in galvanic corrosion of Al/Cu couple when immersed in 45.5 mM NaCl + 1.5 mM CeCl₃ + 3.5 mM ATAT solution. The significant reduction in galvanic corrosion activity is possibly due to the nature of complex film formed by Ce- and ATAT-based film. With more ATAT molecules, immediate adsorption of Ce-ATAT complex film on the entire surface of both anodic and cathodic region may be possible, which is discussed further in Section 3.6. Therefore, with some Ce³⁺ ions to produce precipitates of cerium oxides/hydroxides upon changes in the local pH, a chemisorbed Ce-based film, together with initially formed physisorbed ATAT film, leads to better attenuation of corrosion activity [38].

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Fig. 5.   Distribution of galvanic corrosion current densities on the surface of Al/Cu couple immersed in aerated 42.5 mM NaCl + 2.5 mM CeCl₃ + 2.5 mM ATAT aqueous solution obtained at (a) 0 h, (b) 2 h, and (c) 24 h of immersion; (d) optical image of sample after 24 h of immersion.

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Fig. 6.   Distribution of galvanic corrosion current densities on the surface of Al/Cu couple immersed in aerated 45.5 mM NaCl + 1.5 mM CeCl₃ + 3.5 mM ATAT aqueous solution obtained at (a) 0 h, (b) 2 h, and (c) 24 h of immersion; (d) optical image of sample after 24 h of immersion.

3.4. SVET integrated current profiles

To obtain quantitative information from SVET data, peak heights of maximum anodic and cathodic current densities, or integrated current densities, which represents the total anodic and cathodic ionic currents, can be used [21]. The integrated current densities j_z demonstrate better suitability in quantitatively estimating corrosion process. The equations for obtaining total anodic currents I_a, total cathodic currents I_c, and the total currents I, respectively, are given (Eqs. (4), (5), (6)). Surfer 15 (Golden software) was used to obtain integrated data [21,38].

I_a=∫₀^Xmax∫₀^Ymax [jz(x,y)]>0] dxdy (4)

I_c=∫₀^Xmax∫₀^Ymax[jz (x,y) <0] dxdy (5)

I=I_a+|I_c| (6)

In an ideal reaction situation, the sum of integrated anodic current I_a and the integrated cathodic current I_c should be zero. However, in many SVET investigations, there is a slight deviation from ideality [21,44]. The reasons include, but not limited to: (I) placement of SVET probe at some distance above the sample surface thereby missing some anodic and cathodic currents, (II) current flows in three directions in space but SVET is limited to measuring one or two of them leading to underestimation of true current, and (III) the movement of the SVET probe triggers transport of cathodic reactant (oxygen) to the surface causing imbalance in the estimated corrosion activity [[45], [46], [47]]. Fig. 7 presents the total anodic and cathodic current densities versus time for Al/Cu couple during SVET investigations. Deviations from ideality are observed in the profiles as the magnitude of I_a is not always equal to that of I_c. The reference sample shows I_a and I_c higher in magnitude than the ones for inhibited samples, clearly giving evidence of higher corrosion activity in the absence of inhibitors. However, from Fig. 7, the presence of inhibitors generally attenuates corrosion activity. After 24 h of immersion, a quantitative summary of galvanic corrosion activity can be drawn: Reference > Ce₅ > ATAT₅ > Ce_2.5ATAT_2.5 > Ce_1.5ATAT_3.5. Therefore, the best inhibitor system is Ce_1.5ATAT_3.5.

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Fig. 7.   Profiles of total anodic and cathodic currents from the surface of Al/Cu couple after immersion in different electrolytes for 24 h.

SVET measurements can be used to express the inhibition efficiency of the inhibitors. From the total currents I obtained from SVET measurements, the inhibition efficiency (η) can be calculated using the relationship in Eq. (7) [21]:

$η=\frac{I_{0}-Iin}{ I_{0}}$(7)

where I₀ and I_inh are the magnitude of the total currents as expressed in Eq. (6) obtained in the absence and presence of inhibitors, respectively. Calculated values are given in Table 2. Clearly, there is improvement in the inhibition efficiency by the combination of inhibitors, with Ce_1.5ATAT_3.5 displaying highest percent inhibition efficiency, 99%.

3.5. SEM characterization of sample surfaces after SVET measurements

Morphological assessments of sample surfaces after SVET measurements were carried out by SEM. Fig. 8 displays SEM images of Al and Cu surfaces, respectively, after 24 h SVET measurements for samples. As shown in Fig. 8(a), formation of pits in different regions on the Al surface for the reference sample demonstrates intense galvanic corrosion activity. Pit regions are indicated with yellow arrows. Clearly, the surface of Cu (Fig. 8(b)) is not as attacked by corrosion as Al surface, due to the difference in galvanic potential of the two metals. In the presence of Ce₅, the Al surface reveals fewer pits (Fig. 8(c)) which is as a result of inhibition of corrosion activity. Moreover, the Cu surface in the presence of Ce₅ displays deposits typical of cerium based films (Fig. 8(d)), suggesting the preferential precipitation of cerium oxides/hydroxides on Cu surface. Microcracks (trenching) characteristic of cerium based films can be seen [39]. More so, EDX analysis (Ce: 86.6 wt%; O: 9.1 wt%; Al: 0.2 wt%; Cu: 4.1 wt%) of the region shown with red box in Fig. 8(d) gives evidence that the deposits on Cu surface are composed of cerium compounds. Furthermore, ATAT₅ inhibits galvanic corrosion of Al/Cu couple as indicated by Fig. 8(e) and (f). Only few pits can be seen on Al surface (Fig. 8(e)). Clearly, the best inhibition is achieved for the combined inhibitor systems. There are no obvious pits for the system with Ce_2.5ATAT_2.5 (Fig. 8(g) and (h)), and clearly, no pit formation in the presence of Ce_1.5ATAT_3.5 (Fig. 8(i) and (j)). Therefore, a relatively significant protection against galvanic corrosion of Al/Cu couple is achieved by the combination of CeCl₃ and ATAT. In terms of the surface morphologies, corrosion activity seems to be in the order: reference > Ce₅ > ATAT₅ > Ce_2.5ATAT_2.5 > Ce_1.5ATAT_3.5, which agrees with the order suggested by SVET current density maps.

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Fig. 8.   SEM images of Al and Cu surfaces of Al/Cu couple after 24 h of immersion in 50 mM NaCl solution for (a, b) reference, (c, d) Ce₅, (e, f) ATAT₅, (g, h) Ce_2.5ATAT_2.5, (i, j) Ce_1.5ATAT_3.5, respectively.

3.6. Inhibition mechanism

3.6.1. XPS and ToF-SIMS investigations

To demonstrate the mechanism of inhibition, evaluation of the film formed by the sample with equal proportion of inhibitor combination, Ce_2.5ATAT_2.5, was carried out. Results of XPS analysis from the Al surface of Al/Cu couple are presented in Fig. 9. An assay on the XPS survey obtained from Al surface after 24 h of immersion and sputtering time of 0 s, 30 s and 60 s (Fig. 9(a)) reveals peaks at binding energies of 77.4, 288, 400, 535, and 888.7 eV, corresponding to Al 2p, C 1s, N 1s, O 1s and Ce 2p, respectively. A closer look at the spectra of Ce 2p and N 1s (Fig. 9(b) and (c)), which represent CeCl₃ and ATAT inhibitors, respectively, offer insights into the nature of the film formed on the Al surface. Firstly, the inhibitive film is a complex film indicating coexistence or complexation of Ce- and ATAT-based film. Secondly, the proportion of Ce- and ATAT-enrichment in the complex film surface (sputtering time = 0 s) and further into the film matrix (up to sputtering time = 60 s) seem to be markedly different for both inhibitors. As observed in Fig. 9(b), there is an increase in Ce-enrichment from 0 s to 60 s, whereas a reverse scenario is clearly seen for ATAT (Fig. 9(c)), as ATAT-enrichment decreases with sputtering time. Although the reason for this observation is unclear, it seems to suggest the preference of cerium-based component of the complex film to settle at the inner film region (i.e. close to the film/metal surface interface), while ATAT-rich component is more available on the outer surface of the film.

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Fig. 9.   XPS survey (a) of the Al surface of Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5 and sputtering time of 0 s, 30 s and 60 s, and corresponding spectra for Ce 2p (b) and N 1s (c).

The results of XPS analysis from the Cu surface of Al/Cu couple are presented in Fig. 10. An assessment of the XPS survey obtained from Cu surface after 24 h of immersion in Ce_2.5ATAT_2.5, and sputtering time of 0 s, 30 s and 60 s (Fig. 10(a)) shows peaks at binding energies of 285, 400, 532, and 886.8 eV assigned to C 1s, N 1s, O 1s and Ce 2p, respectively. A peak for Cu 2p appears at 935 eV. Similar to the film formed on Al surface, the inhibitive film on Cu surface is also composed of Ce- and ATAT-based complex film. As indicated by Fig. 10(b) and (c), Ce-enrichment increases with sputtering time, while ATAT-enrichment decreases with sputtering time. As sputtering time of 60 s is closer to the inhibitive film-Cu surface interface, it is clear that there is the preference of cerium-based component of the complex film to settle at the region close to the film/metal surface interface, while ATAT-rich component is available at the outer surface of the film.

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Fig. 10.   XPS survey (a) of the Cu surface of Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5 and sputtering time of 0 s, 30 s and 60 s, and corresponding spectra for Ce 2p (b) and N 1s (c).

Furthermore, comparing the peak intensities of Ce 2p for Cu surface with that of Al surface, it can be observed that peak intensities for Ce 2p are relatively higher for Cu surface (Fig. 10(b)) than Al surface (Figs. 9(b)). 10 (b) shows Ce 2p counts above 80,000 a.u. for all sputtering time for Cu surface, while that for Al surface in Fig. 9(b) are either 80,000 a.u. or below for all sputtering time. The relative abundance of Ce-based film on Cu surface validates the preference of cerium-based film for Cu surface, and confirms SEM observations.

In Fig. 9(b), it is observed that for Al surface, the Ce 3d peak area between binding energies 870 eV and 930 eV has two main peaks (as indicated by the arrows). However, in Fig. 10(b), the same peak area for Cu surface reveals that the two main peak areas having two peaks each, make a total of four peaks (as indicated by the arrows). Fig. 11(a) presents the fitting results of the N 1s spectra obtained from Al and Cu surfaces after 60 s of sputtering time. The formation of chelates by Ce³⁺ and ATAT (Ce-ATAT complex) may be indicated by peaks with binding energies 397.9 eV and 397.5 eV assigned to Ce-N bonds [48] on Al and Cu surfaces, respectively. It is important to note that Ce³⁺ can form chelates with organic compounds via interactions with N-atom of organic moieties [49]. This is made possible by the empty 5d orbitals of cerium, which can interact with heteroatoms of organic compounds (such as nitrogen). Therefore, the Ce-ATAT complexes formed under these conditions precipitate on both Al and Cu surfaces as inhibitive films preventing corrosion activity. In this regard, an enhancement of inhibition efficiency is obtained by the synergistic interaction between ATAT and cerium ions. The generations of polymeric film, high molecular weight, low solubility in electrolyte, and the stabilization effect of chelating agents, are some of the factors that may be responsible for the effective inhibitive performance of organic compound-metal complex [50]. Appearance of peaks at 398.8 eV on Al surface and 399.2 eV on Cu surface represent the bond formed by N with two neighbouring carbon atoms (C＝N—C), while the peaks at 400.4 eV and 400.3 eV are attributed to N bonded to one carbon atom and also to one nitrogen atom (C＝N—NH—C) [51,52], which proves the presence of ATAT-based film. Fig. 11(b) presents the fitting results of the Ce 3d spectra obtained from Al and Cu surfaces after 60 s of sputtering time. As seen for Al surface, the two main peaks appear at binding energies of 884.9 eV and 904 eV. The peaks can be assigned to Ce-N chelate bonds which may appear on the Al surface from the formation of Ce-ATAT coordination complex film [22]. The complexation of transition metals with organic inhibitors in neutral chloride solutions has been reported to produce chelates that improve corrosion inhibition [38,53,54]. These complexes adsorb on anodic and cathodic regions of a substrate exposed to chloride solutions and inhibit corrosion. Clearly, there is the absence of peaks for Ce⁴⁺ on Al surface (Fig. 11(b)), meaning the cerium signal on Al surface may not be due to precipitation of Ce_xO_y/Ce(OH)_x. This is because such precipitates are favoured in cathodic sites, and if observed on Al surface, occurs only when Cu surface is preferentially covered, and in such cases less intensive peaks can be found on Al surface [38]. Therefore, from this result, it is interesting to infer that the precipitation of Ce_xO_y/Ce(OH)_x is rather limited on the Al surface. Conversely, for Cu surface as shown in Fig. 11(b), peaks for both Ce³⁺ and Ce⁴⁺ are detected. Besides the peaks corresponding to Ce³⁺ at binding energies of 885.3 eV and 903.7 eV, the peaks appearing at binding energies 882 eV and 900 eV are assigned to Ce⁴⁺ [22,55,56]. This reveals that in addition to Ce-ATAT complex film, there is preferential precipitation of Ce_xO_y/Ce(OH)_x in the cathodic region, a possible reason for which the film on the Cu surface is thicker.

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Fig. 11.   Fitting of XPS curve of N 1s (a) and Ce 3d (b) after sputtering time of 60 s obtained from Al and Cu surfaces of Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5.

The precipitation of Ce_xO_y/Ce(OH)_x is apparently due to local increases in pH over Cu surface. It is well documented [57] that, for an Al/Cu couple exposed to 50 mM NaCl solution for 1 h, local alkalization of the solution over Cu surface increases the pH to around 10.3, while local acidification over Al surface decreases the pH to around 4.4, and decreases even further to pH of 3.65 upon further corrosion of the Al surface. Such increase in pH is reported to favor deposition of Ce_xO_y/Ce(OH)_x over Cu surface [21,24,38,39]. It is worthy to note that cerium in neutral solution exist as Ce³⁺ [15], and that for a cerium/organic inhibitor system, not all Ce³⁺ ions form complexes with the organic inhibitor [38]. With local pH changes and persistent Cl^- attack on the adsorbed Ce-ATAT film, parts of the film may be destroyed triggering electrochemical activities on both Al and Cu surfaces. Free Ce³⁺ can then undergo further reactions upon changes in pH. Therefore, the presence of Ce³⁺ and Ce⁴⁺ peaks over Cu surface confirms that the local conditions over Cu surface were sufficient to precipitate cerium oxide/hydroxide film. Worthy of mention is that the cerium oxide/hydroxide film formed in the absence of the organic inhibitor is more clearly seen by SEM (Fig. 8(d)), than it is in the presence of the organic inhibitor (Fig. 8(h) and (j)). This further indicates that cerium oxide/hydroxide precipitates are accommodated in the inner film regions and have higher adhesion to the substrate when formed in the presence of the organic inhibitor. Precipitation of Ce_xO_y/Ce(OH)_x consumes OH^- generated by oxygen reduction reactions over Cu surface. The precipitation of Ce_xO_y/Ce(OH)_x can be clearly summarized by the reactions shown in Eqs. (8), (9), (10), (11), (12), (13) [38,56,58]. This inhibits the electrochemical activity over the Cu surface. When the electrochemical activity on Cu surface is reduced, simultaneously, the induced galvanic dissolution of Al is also reduced, leading to more efficient protection of Al/Cu couple. Therefore, the actions of the adsorbed complex Ce-ATAT film and cerium oxide/hydroxide film work synergistically to improve corrosion protection of Al/Cu couple in chloride solution.

O₂+2H₂O+4e^-→4OH^- (8)

Ce³⁺+3OH^-→Ce(OH)₃↓(9)

2Ce(OH)₃→Ce₂O₃+3H₂O(10)

4Ce³⁺+O₂+4OH^-+2H₂O→4Ce(OH)₂²⁺(11)

Ce(OH)₂²⁺+2OH^-→Ce(OH)₄↓(12)

Ce(OH)₄→CeO₂+2H₂O(13)

To further evaluate the film formed by inhibitor combinations, ToF-SIMS analysis was carried out. Fig. 12 presents ToF-SIMS positive and negative spectra from the Al surface and Cu surface of an Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5, respectively. The appearance of Ce⁺ and CeO⁺ signals on the positive spectra from both Al (Fig. 12(a)) and Cu (Fig. 12(c)) surfaces confirms the presence of cerium-based film. Furthermore, the negative spectra obtained from both Al (Fig. 12(b)) and Cu (Fig. 12(d)) surfaces confirms the presence of ATAT-based film by the appearance of C₂H₄N₄S^- signal (m/z = 116). The results confirm the complex existence of Ce- and ATAT-based film on both Al and Cu surfaces of the Al/Cu couple. Based on the intensities of CeO⁺ and C₂H₄N₄S^- on both Al and Cu surfaces, Cu surface seems to show more ion counts, predicting a thicker film layer when compared with that for Al surface. This is in agreement with XPS findings.

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Fig. 12.   ToF-SIMS positive and negative spectra from the Al surface (a, b) and Cu surface (c, d) of Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5, respectively.

Tracing the film components into the bulk matrix of Al and Cu helps to further elucidate the film formed. Negative ion profiles obtained from Al and Cu surfaces by ToF-SIMS are presented in Fig. 13(a) and (b), respectively. Considering the Al surface, rising profiles, except for Cu^-, from 0 s to about 50 s of sputtering time for Al^-, CN^-, CeO^- and O^2- represents the coexistence of Al-, ATAT-, Ce- and oxide-based components of the film, respectively. Similarly, considering Cu surface, rising profiles, except for Al^-, from 0 s to about 50 s of sputtering time for Cu^-, CN^-, CeO^- and O^2- represents the coexistence of Cu-, ATAT-, Ce- and oxide-based components of the film, respectively. From about 60 s of sputtering time, the profiles for Al^- on Al surface become more stable indicating that the Al matrix has been penetrated. In the case of Cu^- from Cu surface, penetration of Cu matrix seems to be from about 150 s of sputtering time. This may be related to a thicker film formed on Cu surface, which is in agreement with XPS findings. The difference in the profiles seems to be in the O^2- profile. As revealed in Fig. 13(a), a clear rise is observed for O^2- from 0 s to about 50 s on the Al surface, which is not commensurate with its profile for Cu surface (see Fig. 13(b)). This may predict additional oxides from corrosion products on the Al surface. The difference in the profile of O^2- is expected because, as already established by SEM results (see Fig. 8), the galvanic coupling between Al and Cu induces anodic dissolution of Al, making Al surface more prone to corrosion products.

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Fig. 13.   ToF-SIMS negative profiles obtained from the Al surface (a) and Cu surface (b) of Al/Cu couple after 24 h of immersion in Ce_2.5ATAT_2.5.

In an attempt to understand the nature of corrosion products on the Al/Cu model, the activity of different chemical species present during the corrosion of AA2024-T3 vs pH modelled using Hydra/Medusa software [59] is shown in Fig. 14. The diagram was generated based on the following parameters: for the electrolyte concentration of the present work, [Na⁺] = [Cl^-] = 50 mM, and from the Literature [60], due to ambient conditions, [CO₃^2-] = 0.05 mM. Furthermore, the concentration of ions of alloying elements [Al³⁺] = 5 mM, [Cu²⁺] = [Mg²⁺] = 2.5 mM was selected as previously reported [60]. As seen the local pH can inform the stability of corrosion products formed. Clearly, it is expected that Al(OH)₃ forms as the most stable species in the pH range of ~4-11 for the concentrations here considered. Therefore, Al(OH)₃ formed on the surface of Al can cause the increase in oxide signals observed in the ToF-SIMS profiles from Al surface.

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Fig. 14.   Logarithm of the activity of chemical species in solution in relation to pH for AA2024-T3 dissolution. The diagram was constructed using Hydra/Medusa software [59].

3.6.2. The individual inhibiting mechanism of Ce³⁺ or ATAT

The inhibiting mechanism of Ce³⁺ as a single inhibitor for Al/Cu couple model is mainly by the interruption of oxygen reduction reactions occurring over Cu surface. As shown by the XPS and ToF-SIMS investigations, and supported by SEM assessment, there is a clear preferential chemical deposition of cerium oxides/hydroxides over Cu surface, apparently due to local increase in pH. The inhibition of oxygen reduction reactions by deposits of cerium oxide/hydroxides at the cathode isolates a part of the electrochemical circuit, hindering the overall corrosion process.

ATAT interrupts both anodic and cathodic processes. The formation of a passivating film that is physically adsorbed on both Al and Cu surfaces results in protection from direct attack of aggressive chloride ions. Prevention of ingress of these chloride ions cushions the activation of corrosion reactions from the galvanic couple, leading to an inhibiting effect.

3.6.3. The combined inhibiting mechanism of CeCl₃ and ATAT

Obviously, a complex film formation during CeCl₃ and ATAT combination as shown by XPS and ToF-SIMS results fosters a synergistic inhibition effect and significantly retards the galvanic corrosion activity on both Al and Cu surfaces. ATAT, a mixed type inhibitor, forms coordination complexes with cerium and adsorbs on both surfaces. However, only physically adsorbed film is formed with Ce-ATAT complex, and soon gives way upon ingress of corrosive Cl^-. Upon corrosion onset, the accompanying local pH changes encourages free Ce³⁺ to form cerium-based compounds due to the precipitation of cerium hydroxides on the Cu surface which cushions the effect of Ce-ATAT complex film damage. Overall, such synergistic reinforcements results in strong resistance against galvanic corrosion.

4. Conclusions

An investigation of the inhibitive effect of cerium chloride and 3-amino-1,2,4-triazole-5-thiol on an Al/Cu couple in NaCl solution was performed using SVET and complementary SEM/EDX, XPS and ToF-SIMS observations. Analysis of the results led to the following conclusions:

(1) Cerium chloride and 3-amino-1,2,4-triazole-5-thiol individually showed effectiveness in inhibiting galvanic corrosion reactions of Al/Cu couple through precipitation of Ce-based compounds and physical adsorption mechanisms of ATAT-based films, respectively.

(2) A significant inhibition of corrosion reaction was achieved by the synergistic effect brought by the combination of cerium chloride and 3-amino-1,2,4-triazole-5-thiol.

(3) Improvement of inhibition for the combined inhibitor system results from formation of a complex film composed of Ce- and ATAT-based film and preferential precipitation of cerium oxides/hydroxides on Cu surface as evidenced by XPS and ToF-SIMS analyses.

(4) For the combined inhibitor system, the film formed on Cu surface is thicker than that obtained for Al surface as demonstrated by XPS and ToF-SIMS. The Ce-ATAT complex film adsorbs on both surfaces, however, an additional preferential precipitation of cerium oxide/hydroxides on Cu surface seems to promote more film growth on the Cu surface.

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (Nos. 51571202 and 51001109).

Reference

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