Achieving excellent anti-corrosion and tribological performance by tailoring the surface morphology and chemical composition of aluminum alloys

Abstract

Aluminum alloy surfaces with micro/nano-structures were fabricated via a simple chemical etching (CE) method. After chemical modification with perfluorodecyltriethoxysilane (PFDS), n-octadecyltriethoxysilane (OTS) and aminopropyltriethoxysilane (APS), surfaces with different wettability were obtained. The morphology and chemical elements of the as-prepared surfaces were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). In addition, the influence of the surface morphology and chemical modification on the wetting/dewetting properties was investigated. Finally, the anti-corrosion and tribological properties of the as-prepared self-assembled monolayers (SAMs) were characterized using an electrochemical workstation and UMT-3 tribometer. The influence of surface morphology and SAMs on the anti-corrosion and tribological performances is discussed in detail. The results showed that the optimal preparation conditions consisted of a 40% volume fraction of hydrochloric acid with a CE time of 2 min. The corrosion resistance of the surfaces chemically modified with hydrophobic groups was much better than that of those modified with hydrophilic groups. Also, the combination of micro/nano-structures and suitable SAMs on aluminum alloy surfaces could greatly enhance the friction reduction and wear resistance behavior.

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Introduction

Aluminum and aluminum alloys possess excellent physical and mechanical properties, such as high electrical capacity, low density and high energy density. Thus, they are important materials due to their wide range of industrial applications, especially in the aerospace and household fields.^1,2 However, aluminum alloys are active and prone to corrosion in water or other corrosion environments, which limits their wider application. So it is important to improve the corrosion resistance properties of aluminum alloys. Treating the alloy with chromium is a very effective method to enhance the corrosion resistance properties. However, chromium(VI) is toxic and harmful to the environment. Yue et al.³ successfully employed a method called laser surface melting (LSM) to prepare a laser-melted layer on aluminum alloy AA 7075-T651, and the anti-corrosion properties of the aluminum alloy improved greatly. However, LSM consumes a large amount of energy and as a result of the existence of stress in the junction, it is easy to form cracks. Hu et al.⁴ prepared three types of protective silane films on aluminum alloy AA 2024-T3 by the electrodeposition technique, and the films were superior for corrosion resistance as compared with the conventional dip-coating method, but the difficulty of handling a large area of metal limits their wide application. Also, the sol–gel method was proposed as an alternative to the toxic chromate-based system, but its application is limited due to poor interfacial adhesion and shrinkage.⁵

Micro/nano-structures on a surface change the contact mode and wetting/dewetting ability of the interface, which have a determinable influence on the tribological and anti-corrosion^6,7 behaviour. Meanwhile, molecular thinness, the relatively strongly chemically bonded interface and the simple, convenient and reliable preparation process make SAMs inherently manufacturable, thus attracting interest for constructing effective boundary lubricants and superhydrophobic interfaces.^8–10 Thus tailoring surface topography and surface chemical modification may be two crucial strategies to improve the anti-corrosion and tribological performance of self-assembled samples.^11,12 Ou et al.¹³ prepared superhydrophobic textured copper surfaces by CE/oxidation and surface fluorination, and found that the film showed much better corrosion protection as compared with the control samples. Mo et al.¹⁴ showed that tribological performance could be induced by designing a suitable surface texture combined with modification with a hydrophobic anion on the surface. Zhao et al.¹⁵ prepared Au surfaces with micro/nano-hierarchical structures by replication of micro-patterned silicon surfaces using polydimethylsiloxane (PDMS) and self-assembly of alkanethiol to create hydrophobic micro/nano-hierarchical structures. All of the above studies verified that surface topography is an effective strategy to enhance anti-corrosion and tribological performance. Through a variety of methods for the preparation of textured or superhydrophobic surfaces, such as laser/plasma/chemical etching,¹⁶ chemical vapor deposition,¹⁷ micro-contact printing,¹⁸ and photolithography,¹⁹ various patterned surfaces have been prepared for application in anti-corrosion, lubrication^16,20,21 and other fields.^22,23

Inspired by the above-mentioned correlation between surface structures and chemical modification, and anti-corrosion and tribological behavior, the structures and chemical composition of aluminum alloy surfaces are adjusted via an acid etching and chemical modification method. In this work, a simple and versatile procedure combining micro/nano-hierarchical structures and further modification with low-surface-energy molecules has been proposed to fabricate hydrophobic and hydrophilic films on aluminum alloy AA 2024 substrates. The procedure is very impressive for its universality, simplicity and flexibility; it can be applied to treat a wide range of aluminum alloys. Moreover, the chemical etching agent is HCl, which is cheap and easy to obtain. Electrochemical and tribological measurements as well as surface analysis methods were used in this paper to reveal the corresponding synergetic anti-corrosion, friction reduction and wear resistance mechanisms of micro/nano-structures and chemical modification.

Experimental

Preparation of SAMs

Octadecyltriethoxysilane (>95%) (OTS), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) and 3-aminopropyltriethoxysilane (APS) were obtained from Alfa Aesar and used without further purification. All other reagents were reagent grade. Aluminum alloy AA 2024 (composition: 4.5% Cu, 1.5% Mg, 0.5% Fe, 0.6% Mn, 0.5% Si, 0.5% others and 91.9% Al) with a size of 20 mm × 20 mm × 3 mm was used as the substrate. The substrates were ground by emery paper (no. 400, 800, 1200) gradually, and then ultrasonically cleaned in acetone and deionized water for 10 min, respectively.²⁴ Hydrochloric acid (V_HCl : V_H2O = 1 : 2) was used as the CE solution. The CE time was 30–180 s, followed by rinsing with an excess of ultrapure water. The as-prepared specimens were immersed in 5 mM OTS, APS or PFDS solution in a mixture of ethanol and water (the volume ratio of ethanol to water was 5 : 1). After 12 h, the obtained samples were taken out of the solution and then heated at 80 °C for 30 min.²⁵

Characterization of surfaces

The static water contact angle (WCA) of various samples was measured with a contact angle meter (OCA20, Germany). Water droplets were placed with a volume of 6.0 μL. The data reported here are the average values of at least 5 measurements for each specimen. The morphological micro/nano-structures on the aluminum alloy surfaces were examined with an atomic force microscope (AFM) (AIST-NT, CETR, USA) in contact mode and a field emission scanning electron microscope (SEM, FEI Quanta 250 FEG, US) under a vacuum environment, with an accelerating voltage of 20 kV. The surfaces were also studied using energy dispersive spectroscopy (EDS) (Oxford) and a PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS, Perkin-Elmer, USA) to characterize the chemical composition on the surface. The measurements were performed using monochromatic Al Kα irradiation and the chamber pressure was about 3 × 10⁻⁸ Torr. The binding energy of adventitious carbon (C1s: 284.8 eV) was used as a base reference.

Electrochemical and tribological measurements

Electrochemical corrosion tests were performed on an electrochemistry workstation (Solartron Modulab, Precision Driven) by potentiodynamic polarization in a three-electrode system: the sample was used as the working electrode with a test area of 0.2 cm², a platinum wire counter electrode, and a saturated calomel reference electrode. All electrochemical measurements were performed in 3.5 wt% NaCl aqueous solution at room temperature. Dynamic measurement of polarization curves in a Tafel model was carried out with respect to the open circuit potential (OCP) at a scanning rate of 2 mV s⁻¹ from −0.5 V to 1 V (ref. 26). Electrochemical impedance spectroscopic measurements were conducted in the frequency ranging from 0.1 Hz to 10 kHz with a sinusoidal perturbation of 10 mV.

The tribological tests were conducted on a UMT-3 tribometer (CETR, USA), using a reciprocating ball-on-plate mode. Commercially available steel balls with a diameter of 4 mm were used as the stationary upper counterparts, while the lower Al alloys coated with various SAMs were fixed on the flat base, which reciprocated over a distance of 5 mm. The friction coefficient-versus-time curves were recorded automatically. At least three repeated measurements were performed for each frictional pair. Failure of the SAM film was assumed to occur when the coefficient of friction (COF) rises sharply to a high and stable value similar to that of a clean Al alloy 2024 against the same counterpart (about 0.7).

Results and discussion

Fabrication and characterization of the self-assembled monolayers

As shown in Fig. 1 the fabrication procedure is quite simple. Considering this simple process, the driving force for the self-assembly is not evident at first glance. In order to reveal such a driving force, it is necessary to know the basic chemical nature and reactivity of the components, i.e., OTS, PFDS and APS.

Fig. 1 Schematic illustration of the preparation process of micro/nano-textures and SAMs on an Al alloy substrate.

The driving force for this self-assembly is the in situ formation of polysiloxane, which is connected to the surface hydroxyl groups (–OH) via Si–O–Al bonds.²⁷ The process in which the OTS-SAM with –CH₃ outer groups is covalently anchored onto the Al alloy wafer is shown in Fig. 2. The figure also indicates how the PFDS-SAM with –CF₃ and APS-SAM with –NH₂ outer groups are formed.

Fig. 2 Possible covalent bonding mechanism of OTS, PFDS and APS to the substrates.

To characterize the surface features of various samples, FESEM and three-dimensional (3D) AFM images were obtained as shown in Fig. 3 and 4, respectively. For the bare substrate, abrasive grooves are observed (Fig. 3a); the treated Al alloys with different processing times ((b) 30 s; (c) 60 s; (d) 90 s; (e) 120 s; (f) 180 s) exhibit different rough surface structures. It can be seen from Fig. 3b–d that the surface morphology of the samples is not completely chemically etched. Scratches exist from the pretreatment as well as a few grooves from etching by dilute hydrochloric acid on the surfaces. By contrast, with the increase of etching time, the Al alloy surfaces become rougher. There are irregularly shaped particles on the surfaces. Under higher magnifications (Fig. 3g and h), these petals can be clearly observed, with a height of several tens of nanometers and a length of several micrometers. The disordered arrangement of these petals leads to a rough structure. Roughness values (R_a) can be read from Fig. 4. The R_a values of the various samples etched for different times are 0.432 μm, 0.679 μm, 0.799 μm, 2.55 μm, 3.89 μm, and 3.28 μm (Fig. 4a–f), respectively. The value of R_a increased with the chemical etching time. So, it can be concluded that such a CE process plays an essential role in fabricating micro/nano-structures on Al alloys and in determining the R_a value. When the CE time is more than 2 min, the surface of the Al alloy is completely etched, and the as-prepared surface is much rougher.

Fig. 3 FESEM images for samples etched for different times: (a) 0 s; (b) 30 s; (c) 60 s; (d) 90 s; (e) 120 s; (f) 180 s; (g) and (h) are higher-magnification images of (c) and (e), respectively.

Fig. 4 3D AFM topographic images and roughness parameters for samples etched for different times: (a) 0 s; (b) 30 s; (c) 60 s; (d) 90 s; (e) 120 s; (f) 180 s.

XPS was performed to investigate the surface chemistry of samples treated with OTS, PFDS and APS. Besides the signals originating from the Al alloy substrates, other signals in the survey spectra (such as F1s, Si2p, C1s, N1s and O1s) can be attributed to the chemically adsorbed silane coupling agents. Specifically, the F1s signal (688 eV) (Fig. 5b) is consistent with the results reported by Saleema.²⁸ In both of the two experiments, F element exists as part of an F–C bond of the silane. Moreover, the high resolution C1s core level spectra (Fig. 5b), resolved into four components,²⁹ namely, –CF₃, –CF₂, –CH₂–CF₂, and –CH₂–CH₂–, can be used as evidence for the adsorption of PFDS. The O1s peak (Fig. 5) can be resolved into three components with binding energies of 530.6 eV, 531.8 eV and 533.1 eV, illustrating the formation of Al–O–Al, Si–O–Al and Si–O–Si,³⁰ respectively. This may prove the assumption that the alloy substrate is not only changed by the formation of direct bonds with oxygen but also the chemically adsorbed materials from OTS, PFDS or APS. Fig. 5c also shows the deconvoluted N1s region. The single strong peak at 400.5 eV is attributed to –NH₂ from the APS.

Fig. 5 XPS survey and various core spectra of the (a) OTS-SAM, (b) PFDS-SAM and (c) APS-SAM.

Fig. 6 shows the results of wettability measurements for the as-fabricated samples. As shown in Fig. 3a, the bare Al alloy used here had a smooth surface without micro- or nanostructures. When the substrates were immersed into the HCl solution, a fast chemical etching process occurred, and the surfaces became rougher with the build-up of numerous intercrossed nano-platelets³¹ (Fig. 3b–f). When the etching time was 2 min, some micropits existed on the Al alloy substrate, and it was noted that the nano-platelets still covered the surface and each micro-convexity had numerous nanosteps on its surface, indicating the appearance of dual-scale roughness on the Al alloy substrate (Fig. 3h). The value of the surface roughness was 3.89 μm and the WCA reached a maximum of 149° and 145° (Fig. 6) when the surface was modified by PFDS and OTS, respectively. The reasons for this were that the surface was modified with low-surface-energy materials and the Al alloy surface was composed of air trapped notches and micro/nano-convexities.³² This explanation can be expressed theoretically by the Wenzel equation, which explains how surface roughness increases or decreases wettability for a liquid droplet on a rough solid surface:^33,34

cos θ_r = r cos θ (1)

where θ is the equilibrium WCA on a smooth surface, θ_r is the apparent WCA on a rough surface made of the same material, and r is the roughness factor. This is responsible for the increase in the WCA for a hydrophobic surface (θ > 90°) and the decrease in the WCA for a hydrophilic surface (θ < 90°). When the etching time was 3 min, the nano-convexities decreased and the microstructure increased, which resulted in the R_a value being reduced to 3.28 μm. Clearly, the WCA increased with the enhanced roughness when the Al alloy surfaces were modified with PFDS and OTS. When the surface was modified with a hydrophilic substance such as APS, the WCA became smaller and reached a minimum of 44° after 2 min. The reason for this may be related to the above, and all the results are consistent with the SEM and 3D AFM images.

Fig. 6 Relationship between the etching time and the WCA with different low-surface-energy molecules.

Corrosion test

In order to estimate the corrosion protection of the generated micro/nano-structures and SAMs on Al alloys, Tafel plots were measured following the open circuit potential (OCP) after the samples were exposed to NaCl solution for a certain period. Fig. 7 shows the polarization curves of (a) PFDS- and (b) OTS-modified samples, (c) the bare Al substrate, (d) the APS-modified sample and (e) the CE substrate. Important parameters, such as the corrosion potential (E_corr) and corrosion current density (I_corr) derived from the polarization curves are listed in Table 1. It is generally believed that a lower I_corr value denotes a lower corrosion dynamic rate and a more positive E_corr represents a lower corrosion tendency.

Fig. 7 Polarization curves of (a) PFDS- and (b) OTS-modified samples, (c) the bare Al substrate, (d) the APS-modified sample and (e) the CE substrate.

Table 1 E_corr and I_corr values of the samples

Samples	Ecorr (mV)	Icorr (μA cm^−2)
OTS-modified sample	−546.8	1.321
PFDS-modified sample	−564.4	1.148
APS-modified sample	−1073	6.602
Bare aluminum alloy	−1057	4.303
CE substrate	−1112	15.35

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The highest I_corr and most negative E_corr values of the plots in Fig. 7 suggest that the chemically etched aluminum alloy substrates are the most prone to corrosion. This may be attributed to the fact that after chemical etching, the native oxide is removed, such that a fresh and active surface is obtained and the chemical reactions causing corrosion occur easily. For CE samples modified by OTS and PFDS, a positive shift in E_corr and a reduction in I_corr are observed, mainly because the generated hydrophobic film can serve as a layer to obstruct the diffusion of water and other corrosive species dissolved in water. Because of the polar terminal groups in APS, the APS-modified sample shows hydrophilic properties, as water and other corrosive species can attach to the surface. A negative shift of E_corr and an increase of I_corr are observed.

Fig. 8 presents the impedance (a) and phase angle (b) graphs for different samples. Generally, the impedance values at low frequency was consider as an indicator of anti-corrosion performance. The impedance values at 0.1 Hz of the PFDS-, OTS- and APS-modified samples, the bare Al and the CE substrate are 18.77 kΩ cm², 17.34 kΩ cm², 14.16 kΩ cm², 6.74 kΩ cm² and 1.01 kΩ cm², respectively. All SAMs-modified surfaces have larger impedance values and a quicker increase in the phase angle than the bare and CE surfaces. Also, it can be seen from Fig. 8 that the impedance spectra of the bare Al, APS-SAM and CE surface have a capacitive loop in the low frequency zone. For the specimens treated with PFDS and OTS, the loop shifts to a higher frequency zone. This is because the structure of the aluminum alloy surface has been changed by CE and self-assembly.

Fig. 8 Impedance (a) and phase angle (b) graphs for different samples.

Two methods were used to analyze the EIS data. The former involved reading values directly from the impedance spectra, and the latter involved equivalent circuit modeling. As shown in Fig. 9, the equivalent circuit model (a) is used for the bare aluminum alloy and CE samples, and (b) is for the PFDS-, OTS- and APS-modified samples. In these circuit models, R_ct||Q_dl is assigned to the impedance of the interface reaction between the film and the substrate. The impedance of a constant phase element (CPE) depends on the frequency via the relation Z_CPE = 1/C(jω)ⁿ where C is the capacitance, J is the square root of −1 and ω is the angular frequency. As n approaches 1, the impedance of a CPE reduces to that of a true capacitor.³⁵ R_film||C_film is assigned to the impedance of the interface and R_s represents the resistance of the 3.5 wt% NaCl solution. The values of these parameters are shown in Table 2. The R_ct values derived from Table 2 are of almost the same order of magnitude as the |Z| values from electrochemical impedance spectroscopy at 0.1 Hz. This indicates that the equivalent circuit models are well suited for electrochemical impedance spectroscopy.

Fig. 9 Equivalent circuit models for (a) bare Al and CE samples and (b) OTS-SAM, PFDS-SAM and APS-SAM samples immersed in 3.5 wt% NaCl solution.

Table 2 Electrochemical model impedance parameters from Bode plots of different samples

Electrode	OTS-modified sample	PFDS-modified sample	APS-modified sample	Bare Al	CE sample
Rct/Ω cm^2	1.734 × 10^4	1.877 × 10^4	1.416 × 10^4	6.74 × 10^3	1.007 × 10^3
Qdl/Ω^−1 s^n cm^2	5.092 × 10^−6	1.267 × 10^−6	6.761 × 10^−5	—	—
ndl	1	1	0.6093	—	—
Rs/Ω cm^2	22.21	28.53	33.31	40.78	18.66
Rfilm/Ω cm^2	809.8	2452	399.9	—	—
Cfilm/F cm^−2	3.263 × 10^−7	1.105 × 10^−7	4.834 × 10^−7	8.703 × 10^−6	3.274 × 10^−4
Equivalent circuit	R(C(R(CR)))	R(C(R(CR)))	R(Q(R(CR)))	R(CR)	R(CR)

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Tribological behaviors

In order to evaluate the tribological behavior of self-assembled films on Al alloys, curves of friction coefficient-versus-sliding time and wear rate for different samples obtained by a ball-on-plate tribometer are presented in Fig. 10. Fig. 10b clearly shows that the APS-SAM displays poor tribological properties characterized by high COF under the testing conditions of 0.6 N and 1 Hz. As soon as the counterpart ball started to slide, the films were worn out (Fig. 10a and b). This may be because the APS-SAM film is easily worn out with its short-chain monolayers and is not strong enough to prevent direct contact between asperities on the steel ball counterpart and the Al alloy surface under the applied load.³⁶ The PFDS-SAM, with a relatively low COF, failed under an applied load of 3 N (Fig. 10c). However, after modification with OTS, the wear resistance performance was greatly enhanced. As shown in Fig. 10d, the sample exhibited reduced COFs (below 0.2) under all applied loads, only failing under conditions of 4 N and 5 Hz. Fig. 11a presents the COFs and wear rate of different samples under an applied load of 4 N and a sliding frequency of 1 Hz. Compared with the APS-SAM and bare Al, the OTS-SAM and PFDS-SAM exhibited much smaller COFs (0.15 and 0.18, respectively); meanwhile, similar results are observed on the wear rate bar graph. Fig. 11b shows the various wear rates of different samples treated for different CE times. The OTS-SAM exhibits the lowest wear rate for all etching times. When the CE time is 120 s, compared with the other samples, the wear rate³⁷ (1.4 × 10⁻¹⁴ m³ N m⁻¹) of the OTS-SAM can be ignored. The greatly enhanced lubricating properties may be attributed to the long chain and nonpolar methyl terminal group;³⁸ the longer alkyl chains are much more flexible and can dissipate the mechanical energy from the shearing process more easily than the short chain compounds. An alternative explanation is that the longer chains have stronger chain–chain interactions.³⁹

Fig. 10 Variation of COF with sliding time for various films: (a) bare Al; (b) APS-SAM; (c) PFDS-SAM; (d) OTS-SAM.

Fig. 11 Tribological properties of different samples under an applied load of 4 N and with a sliding frequency of 1 Hz: (a) COF; (b) wear rate for samples with different CE times.

Conclusion

In this work, aluminum alloy sheets with different wettability were successfully fabricated by means of CE and self-assembly of OTS, PFDS and APS. Their anti-corrosion properties as well as their tribological performance were subsequently studied in detail. As revealed by the potentiodynamic polarization tests and EIS measurements, the as-prepared OTS-SAMs and PFDS-SAMs showed a certain anti-corrosion capability compared with the APS-SAMs and bare samples because of their surface hydrophobicity. Tribological tests indicated that the OTS-SAMs exhibited excellent tribological behavior as compared with PFDS-SAMs, APS-SAMs and bare samples due to their long alkyl chains. Meanwhile, samples prepared after 120 s of CE exhibited better behavior in terms of the water contact angle, anti-corrosion and wear resistance performance than other samples. The CE time is the critical factor for the surface morphology fabrication and it is important for the anti-corrosion and tribological performance. The results presented in this paper provide a new and effective protection method for Al alloys, which can prevent future corrosion via simple chemical modification with SAMs, removing the need for replacement of the whole Al alloy. Through a combination of micro/nano-structure and chemical composition design, Al alloys with excellent anti-corrosion and tribological performance were obtained.

Acknowledgements

We express our great thanks to the National Key Basic Research Program of China (973) (2014CB643305), National Natural Science Foundation of China (51202263 & 51335010), Ningbo Key Technology Project on Graphene (2013B6013), Zhejiang Province Public Welfare Program (2014C31154), and the Municipal Nature Science Foundation (2014A610132).

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