Superhydrophobic and oleophobic microtextured aluminum surface with long durability under corrosive environment

Surface morphology and roughness

The chemical etching process produced a micro-step like texture resulting from the reaction between HCl and the surface of crystalline aluminum. The appearance of the surface of the AA1050 after chemical etching is presented in Fig. 1a. The large amount of dislocation defects in the crystalline surface acts as preferential sites for reacting with some specific dislocation etchants such as HCl¹⁰. This reaction begins producing pits that spill over the surface, and by progressively interconnecting them, a homogeneous micro-step like roughness is obtained all over the aluminum surface¹⁹. This micro-step like texture is schematically represented in Fig. 2. A minimum reaction duration is required to achieve homogeneous micro-step structure. Through visual inspection, the optimum reaction duration for a complete surface etching was fixed between 17 and 24 min. In this interval, apparently, the surface was completely and homogenously etched, and a homogeneous mate finish was observed.

The arithmetic average roughness (Ra) of pristine AA1050 was 0.36 ± 0.02 µm and the maximum height of the profile (Rt) was 2.84 ± 0.18 µm (see Table 1). After chemical etching in 3.0 M HCl between 17 and 24 min, mean Ra of the surfaces was between 5 and 7 µm and mean Rt was between 35 and 47 µm, as presented in Fig. 3. From these measurements, it was observed that Ra of the surfaces etched at shorter duration showed higher Ra values, ⁓ 7 µm, and large dispersion, which was attributed to an incomplete surface processing, presenting both pits and un-etched areas. Etching duration longer than 19 min resulted in surfaces with lower Ra values, ⁓ 6 µm, and from 21 min onwards Ra dispersion was lower, which was related to the homogenous surface obtained. Concerning Rt similar average results were observed for the different samples, close to 40 µm, although slightly larger dispersion was observed at shorter etching duration. From 21 min duration onwards Rt dispersion was lower. For the 24 min reaction, Rt dispersion values increased probably due to an initial over-etching process of the alloy. The Fig. 4 shows the profile of AA1050 before and after 22 min of reaction (CE) and Ra and Rt are presented in Table 1.

The surfaces etched during 22 min were subjected to two types of post-processes, (i) anodization and (ii) introduction of polyfluoroalkyl moieties, with the aim to improve durability and amphiphobicity.

Anodizing process produces a thick aluminum oxide layer on the surface of raw aluminum through an electrochemical anodic reaction. The structure of the anodized aluminum oxide layer is nanoporous, formed by a hexagonal array of cells with cylindrical pores with a diameter of 25 nm to 0.3 μm and depth up to 100 μm²⁷. The combination of chemical etching and anodization processes is expected to produce the combination of micro and nanostructures and the attainment of a hierarchical structure, which has also been pursued by several authors^18,19,25. Figure 1b,c shows top view images of the anodic layers grown on the aluminum surfaces previously etched during 22 min (CE-AL5 and CE-AL20). As it can be observed, the micro-step like structure morphology obtained with the chemical etching process was maintained after anodization with two different thicknesses. Anodic layer replicated the microtexturing and the growth of aluminum oxide was conformal. Furthermore, the roughness (Ra and Rt, as presented in Table 1) of the 5 and 20 μm thick anodic layers were similar to the roughness of the surface etched at 22 min. Therefore, a well distributed micro-step like structure similar to the original CE was maintained after the oxide layer growth up to 20 µm.

After attaining micro and micro-nano structures, two main strategies were followed to modify the composition of the surfaces. Both were based on the introduction of polyfluoroalkyl moieties. The first one was based on surface functionalization through FAS17 grafting. FAS17 grafting is a surface modification in which Al–OH terminal groups are replaced by Al–O–Si–(CH₂)₂–(CF₂)₇–CF₃, known as self-assembled monolayer. Consequently, no major modification of the surface was observed, neither from the morphological point of view (see Figs. 1, 2 and 4) nor from the Ra nor Rt measured on the surfaces (see Table 1). The second one consisted in the application of a thin hybrid coating synthesized by sol–gel, with the presence of FAS17 in its formulation. This ⁓ 1.6 μm thick coating was composed of methacrylate, silica and zirconia with polyfluoroalkyl moieties covalently linked to the matrix through Si–C bonds. It has been demonstrated in previous works of authors^28,29, that the application of sol–gel coatings on aluminum surfaces by dip-coating does not follow a conformational growth, since it contributes to the surface levelling as schematically represented in Fig. 2. As observed in Fig. 1d–i and Fig. 4, the relatively low thickness of the FAS17-hybrid coating allowed to maintain the micro-step like texture. However, the roughness of the FAS17-hybrid coatings on CE, CE-AL5 and CE-AL20 surfaces, compiled in Table 1, where Ra and Rt are shown, was slightly reduced in all cases due to its levelling effect.

Analysis of the wettability

It is well known that the wetting of a surface by a liquid is affected by the roughness and morphology of the surface^30,31,32. Indeed, an effective way to enhance the hydrophobic properties of a surface is to increase its surface roughness. In fact, in the case of a material with one of the lower known surface free energy, polytetrafluoroethylene (PTFE), superhydrophobic properties have only been achieved when combined with high roughness³³. In this sense, the micro-step like structure processing on the aluminum surface, is a promising approach to produce amphiphobic surfaces¹⁰.

Surface functionalization through fluorine containing groups is an extended strategy for superhydrophobicity. Surfaces containing -CF₃, -CF₂– and -CF₂-CH₂– are among the materials with lower interfacial energy³⁴ and the length of polyfluoroalkyl groups introduced in a coating has a direct effect on its hydrophobicity³⁵. In this study, FAS17 molecules integration has been considered in two ways, one grafting the molecules on the aluminum surface and the other one, through a hybrid coating containing FAS17 molecules, representing about ⁓7.4% of the dried coating material.

Wettability properties of the aluminum surface with micro-step like structures combined with FAS17-grafting have been studied with the aim to establish the influence of the etching duration on processed surfaces. As FAS17-grafting is a self-assembled monolayer of long-chained polyfluoroalkyl that does not have effect on the surface morphology, the WCA and HCA of FAS17-grafted chemical etched surfaces varied depending on chemical etching duration, as presented in Fig. 5. The surfaces were in all cases superhydrophobic although WCA was not quantitatively measured due instability of the droplet when contact angle was higher than 140° (droplet not holding on to the surface).

HCA of the FAS17-grafted and chemical etched surfaces at different duration was quantitatively analyzed. As observed in Fig. 5, an increase in HCA took place up to 22 min of etching duration, being the major change when increasing from 17 min, with HCA of 48° to 19 min, with HCA of 107º. However, the values up to 21 min showed relatively high dispersion, which can be in agreement with surface roughness measurements. Above 21 min etching, a slight HCA increase was observed, reaching the maximum HCA of 123° at 22 min etching with low HCA dispersion, in agreement with the homogeneous surface morphology observed.

The FAS17-grafted anodized surfaces showed again superhydrophobicity although WCA could not be quantitatively determined, resulting in a demonstration of the SHP behavior. The HCA for these surfaces was also extremely high (oleophobic), since it reached almost 120° for the FAS17-grafted CE-AL5 and CE-AL20, showing that anodizing up to 20 µm thick had a limited impact on sample wettability performance.

All the FAS17-grafted surfaces presumably presented a monolayer of polyfluoroalkyl moiety thanks to the reaction of the -OH groups of the surface with the alkoxy groups of the FAS17 resulting in a surface highly functionalized with covalent stable bonding of the polyfluoroalkyl group to the metal: Al–O–Si–CH₂–CH₂–(CF₂)₇–CF₃.

Table 2 compiles the WCA and HCA values of the FAS17-grafted and FAS17-hybrid coated AA1050 after 22 min of chemical etching (CE), as well as after anodization processes (CE-AL5 and CE-AL20).

In the case of the FAS17-hybrid coated surfaces, the material deposited was composed by a matrix of methacrylate-silica-zirconia, with low presence of polyfluoroalkyl moieties. Even with this low percentage of polyfluoroalkyl groups, although the achieved WCA did not reach superhydrophobicity, it was high, being close to 140º on CE and ⁓130º on CE-AL5 and CE-AL20, as presented in Table 2. However, the HCA was notably lower in comparison to FAS17-grafted, which could be attributed to the lower concentration of polyfluoroalkyl groups in the external surface, and the reduction of the roughness due to the levelling effect of the hybrid coating respectively.

Mechanical properties

Microhardness measurements were carried out to evaluate the effect of the chemical etching and anodizing process on the mechanical properties of the bulk microtextured AA1050 alloy.

On the one side, it is important to assess how immersion in a strong acid solution and consequently created porosity and roughness affect the mechanical properties on both the surface and bulk material of the AA1050. On the other side, anodizing process, apart from providing corrosion protection to aluminum, usually allows to improve the hardness, abrasion and wear resistance of the aluminum surface.

Firstly, the bulk material AA1050 before and after treatments was studied through conventional Vickers microhardness testing by optical evaluation of indentation tracks performed along cross-section. This value is designated as Vickers Hardness HV, as described in the ISO 6507 standard, and only takes into account the plastic deformation. Considering that the roughness of the microtextured samples showed a peak to valley distance between 40 and 50 µm, microhardness measurements starting at 40 µm depth were considered in this study. Figure 6 shows the results on the AA1050, CE and CE-AL20 samples at different distances from the surface. It can be observed that microhardness results are similar for the three materials, all falling between 30 and 40 HV0.01. Consequently, it was concluded that the bulk of the AA1050 was not affected by surface microtexturing.

Then, dynamic indentation testing according to ISO 14,577 standard was conducted on the surface of pristine AA1050, CE and CE-AL20 surfaces. This method takes into account both the elastic and plastic components of the deformation. Figure 7 shows the loading–unloading indentation curves corresponding to bare AA1050, CE and CE-AL20 and Table 3 shows the summary of dynamic microhardness results on tested samples. At maximum load, 1000 mN, indentation depth of pristine AA1050 was 10.7 ± 0.4 µm, while after chemical etching treatment, it grew up to 23.1 ± 1.6 µm. The indentation hardness measured at the maximum load, H_IT, of AA1050 was of 330 ± 23 MPa and it was reduced up to 71 ± 10 MPa after chemical etching. Therefore, it was proved that chemical etching with HCl solution on AA1050 aluminum alloy had a negative impact on surface mechanical properties, which were considerably reduced. The indentation depth measured on the 20 µm-thick anodic layer was 11.4 ± 0.9 µm and the H_IT was 287 ± 47 MPa, in the same range of the pristine AA1050. Consequently, the detrimental effect produced on the surface mechanical properties after chemical etching, was counteracted by the deposition of the anodic layer.

Therefore, it can be concluded that after chemical etching, the affected area below the surface is restricted to the roughened area (approximately 50 µm depth) with no influence on the bulk of the material, according to the results obtained in the cross-section study.

Corrosion resistance

Up to now, different works have studied the corrosion resistance of SHP aluminum surfaces through electrochemical testing^22,23,24, although little information on their wettability properties after long exposure to corrosive environments has been found^21,25,36. Herein, in order to evaluate the durability of processed superhydrophobic and oleophobic surfaces, their corrosion resistance was assessed under NSST, as well as the change of wettability after long exposure of 2016 h.

The Fig. 8 shows the appearance of pristine AA1050, chemical etched AA1050 (CE), FAS-17 grafted CE and FAS17-hybrid coated CE, as well as FAS17-grafted and FAS17-hybrid coated anodized samples after 2016 h of exposure. The pristine and chemical etched AA1050 presented signs of corrosion after 24 h of exposure, in particular, white and grey corrosion products emerged on the aluminum surface. At the end of the test, the chemical etched AA1050 presented highly extended white corrosion products all over the surface. The chemical etching treatment alone, had a negative effect on the durability of AA1050.

Hybrid coatings prepared by sol–gel method have been widely used for corrosion protection of metals. In particular, formulations based on mixtures of silicon and zirconium alkoxides in combination with organo-alkoxides such as MAPTMS have demonstrated corrosion protection of aluminum alloys highly susceptible to corrosion, such as AA2024²⁸. In this case, the hybrid coating has been modified with FAS17 to also improve superhydrophobicity. Both chemical modifications (grafting and coating) with FAS17 on CE samples delayed the emergence of signs of corrosion. The surface of FAS17-grafted CE exhibited color changes during the test while the FAS17-hybrid coated CE presented color change and presence of corrosion products on the surface after 336 h of exposure. It can be concluded that both FAS17-grafting and FAS17-hybrid coating slightly protected the AA1050 after chemical etching.

Anodization treatment provides corrosion protection and hardness to aluminum parts. Although the properties of the anodic layer depend on the aluminum alloy, the electrolyte, and the anodizing parameters, industrial processes usually provide corrosion protection to aluminum surface.

According to MIL-A-8625F, the sealed anodized aluminum alloys must be exposed during 336 h to NSST in accordance with ASTM-B117, except that the surface shall be inclined 6 ± 2° from the vertical. The CE samples covered with anodic layers of two thickness, 5 and 20 μm, and then modified with FAS17-grafting and FAS17-hybrid coating did not present any corrosion failure after 336 h. Therefore, the exposure to the corrosive environment was prolonged up to 2016 h in order to observe differences among the different treatments. It is also important to envisage that the corrosion resistance required for anodized aluminum with a subsequent organic primer applied on top of the conversion coating is 2016 h in NSST, according to specifications MIL-P-85582B and MIL-PRF-23377 J. At the end of the test, the anodized CE-AL5 and CE-AL20 AA1050 both with FAS17-grafting and hybrid coating appeared intact, without signs of white corrosion. Therefore, the anodic layers of 5 and 20 μm demonstrated to protect against corrosion the chemical etched AA1050 surface.

To study the durability of SHP and oleophobic properties, WCA and HCA were measured on the FAS17 modified surfaces after the 2016 h of exposure to NSST and presented in Table 2. Considering the FAS17-grafting treatment, the chemical etched surface presented a considerable reduction of WCA and HCA after NSST exposure, falling up to 88.7 ± 14 and 49.8 ± 10, respectively. FAS17-grafted CE-AL5 showed a slight reduction in the WCA and HCA values, which were 123.9 ± 4.6 and 115.7 ± 3.3, respectively. Finally, outstanding result was obtained for FAS17-grafted CE-AL20 surface, which maintained the SHP and oleophobic properties after corrosion test. In the case of FAS17-hybrid coated surfaces, they presented a considerable reduction of WCA and HCA after NSST exposure. None of them maintained the hydrophobicity, since WCA was < 90° in all cases and the aged surfaces presented high affinity to hexadecane, since HCA was < 10° in all cases, even if no damage was observed visually.

Some works addressing the corrosion resistance of SHP aluminum surfaces suggested³⁶ that even being SHP, the coating permeability may be the reason for the penetration of water into the coating after certain exposure time. This may cause defects on FAS17 treatments and the modification of the chemically microtextured surface due to corrosion products forming under the coating. This could be the reason for the reduction in wettability of FAS17-grafted 5 µm thick anodic layer, while the 20 µm thick anodic layer seemed to help maintaining the integrity of the chemically microtextured surface and the FAS17-graftig layer.

With the aim to assess the integrity of the FAS17 molecules on the surface, the variation of fluorine content on the FAS17-grafted CE-AL20 was studied by EDX analysis before and after NSST exposure together with the bare CE-AL20 for reference. As presented in Table 4, the results of surface composition in atomic percentage confirmed that fluorine concentration on the surface was not modified after the 2016 h exposure in NSST.

Therefore, the combination of the FAS17-grafting and the 20 µm-thick anodic layer on the chemically microtextured aluminum surface protected the surface against corrosion and consequently favored the durability of the wettability properties after 2016 h of exposure to NSST.