ISIJ International, Vol. 59 (2019), No. 2

© 2019 ISIJ345

ISIJ International, Vol. 59 (2019), No. 2, pp. 345–350

* Corresponding author: E-mail: habazaki@eng.hokudai.ac.jpDOI: https://doi.org/10.2355/isijinternational.ISIJINT-2018-569

1. Introduction

Stainless steels are practically important corrosion-resis-tant metallic materials with high corrosion resistance and good mechanical properties, being widely used in various industries, including building construction, food and chemi-cal processing, automobile, medical equipment industries. Although stainless steels are highly resistant to corrosion because of formation of stable passive films, they often suffer pitting corrosion in aggressive chloride-containing aqueous environments and surface contamination of stain-less steels with organic matters reduces the production effi-ciencies in food and chemical process industries.1)

Superhydrophobic and superoleophobic surfaces have attracted recent interest, because of the self-cleaning,2) anti-fouling,3) anti-fogging,4) anti-bacterial,5) water/oil sepa-ration,6) anti-icing, fluid drag reduction,7) and enhanced corrosion protection properties8,9) of the surfaces. Super-hydrophobic surfaces have been prepared artificially by a bio-inspired approach. There are many examples of superhydrophobic surfaces in nature, such as lotus leaf,10) cicada wing,10) water strider leg.11) These surfaces exhibit unique morphologies of hierarchical micro-/nano-pillars,

Fabrication of Superoleophobic Surface on Stainless Steel by Hierarchical Surface Roughening and Organic Coating

Atsushi KASUGA,1) Akira KOYAMA,2) Katsutoshi NAKAYAMA,2) Damian KOWALSKI,2) Chunyu ZHU,2) Yoshitaka AOKI2) and Hiroki HABAZAKI2)*

1) Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628 Japan.2) Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628 Japan.

(Received on August 20, 2018; accepted on October 5, 2018; J-STAGE Advance published date: November 16, 2018)

Stainless steels is practically important corrosion-resistant metallic materials, and additional surface functionalities including self-cleaning, anti-fouling, anti-ice and snow sticking and fluid drag reduction by the introduction of superhydrophobic and superoleophobic surfaces are of recent interest. Here, we report the micro-/nano-hierarchical roughening of type 304 stainless steel surface by chemical and electrochem-ical etching and anodizing. Chemical etching in HCl + FeCl3 aqueous solution containing a surfactant introduces surface roughness of several tens micrometers scale and the electrochemical etching in HCl + HNO3 aqueous solution produces a number of etch pits of ~1 μm in size. Then, a porous anodic layer of the pore size of ~20 nm is formed on the etched surface by anodizing in ethylene glycol electrolyte containing 0.1 mol dm−3 NH4F and 0.1 mol dm−3 H2O. After fluoroalkylsilane (FAS) coating of the hierarchi-cally rough surface to reduce the surface energy, the surface becomes superhydrophobic and superoleo-phobic; the advanced contact angle for hexadecane (surface tension of 27.6 mN m−1) is ~160° and the contact angle hysteresis is less than 10°. Since the FAS-coated flat surface is oleophilic, so that such hierarchically rough surface is of significant importance to achieve the superoleophobicity even for low surface tension liquids.

KEY WORDS: stainless steel; hierarchically rough surface; superhydrophobic; superoleophobic.

nano-pillar array and oriented spindly micro-setae. Thus, the surface morphology is one of the key parameters to control the surface wettability. In fact, various superhydrophobic surfaces have been tailored by micro-/nano-surface structur-ing of metals, alloys, glass and polymers.12–16)

Another important factor controlling the surface wettabil-ity is the surface composition, i.e., surface free energy. To obtain the superhydrophobic surface by surface morphologi-cal control, the surface must be composed of hydrophobic material; the static contact angle to a water droplet is higher than 90° on the flat surface. Organic surface is usually required to satisfy this criterion, although there are some reports indicating that rare earth oxides exhibit hydropho-bicity without any organic coating.17–19)

In contrast to the superhydrophobic surfaces, the fabrica-tion of superoleophobic surfaces is rather difficult because of the lower surface tension of oils. The most organic sur-faces with -CH3, -CH2, -CF2 and -CF3 outermost groups are oleophilic with static contact angles less than 90° for the low surface tension liquids (<30 mN m −1); these organic surfaces are wetted to oils even after surface roughening. In the last decade, it has been found that well-designed sur-faces such as re-entrant,20,21) overhang,22,23) and optimized hierarchically dual-pillar,24) dual-pore morphologies,6,25) are effectively utilized to achieve super-liquid repellency even for low surface tension liquids as low as ~20 mN m −1.

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Because of self-cleaning, anti-fouling, anti-fogging and fluid drag reduction properties, the superoleophobic stain-less steels are of practical interest. However, superoleopho-bic stainless steel surfaces have been still very limited.26–28) In the present study, we report a simple wet chemical and electrochemical method to prepare superoleophobic stainless steel surface with a hierarchically rough surface morphology. The surface shows high liquid repellency for water, ethylene glycol, rapeseed oil and hexadecane with surface tensions of 72.8, 48.4, 35.0 and 27.6 mN m −1, respectively. The importance of multiscale roughness is also discussed in this paper.

2. Experimental

Stainless steel plates of 0.3 mm thickness were used in this study. The stainless steel contained 18.21 wt% Cr, 8.05 wt% Ni, 0.78 wt% Mn, 0.47 wt% Si and 0.006 wt% S. Prior to chemical and electrochemical etching, the stainless steel surface was degreased ultrasonically in acetone for 5 min. Chemical etching of stainless steel was performed in aque-ous solution containing 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and several concentrations of cocamidopropyl betain (APB, Kao, Amphitol 55AB), which is an amphoteric surfactant derived from coconuts oil, at 60°C. After chemi-cal etching, the specimen was de-smutted in concentrated HNO3 and then washed ultrasonically in milli-Q water.

Electrochemical etching was conducted for the chemi-cally etched specimen at 1 kA m −2 in aqueous electrolyte containing 3.6 vol% HCl and 1.2 vol% HNO3 at room temperature. Type 304 stainless steel plate was also used as a counter electrode. The etched specimens were anodized using a two-electrode cell with a Pt counter electrode at 60 V for 60 s. The electrolyte used was ethylene glycol con-taining 0.1 mol dm −3 NH4F and 0.1 mol dm −3 H2O. After anodizing, the specimen was washed in ethylene glycol and then in acetone.

The anodic film formed in the electrolyte contained fluoride, being chemically unstable. Thus, thermal treat-ment was conducted in air at 350°C for 30 min to remove fluoride and improve the chemical stability. The specimen was heated to the prescribed temperature at a rate of 2.5 K min −1. The specimen was coated with a monolayer of 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS). For the coating, the specimen was immersed in 2 wt% FAS in etha-nol for 60 min. The coated specimen was finally thermally treated at 100°C for 60 min.

The surface of the specimens was observed using a ZEISS, Sigma-500 field-emission scanning electron microscope operated at a low accelerating voltage of 1.0 kV. The surface was also observed using a Keyence, VK-9700 color 3D laser scanning microscope and the surface roughness, Ra, was estimated using a Keyence, VK-H1A1 analyzer software.

Dynamic contact angle measurements were performed by an expansion and contraction method using a Kyowa Inter-face Science DM-CE1 contact angle measurement system. The wettability was evaluated for ethylene glycol (surface tension of 48.4 mN m −1), rapeseed oil (35.0 mN m −1), hexadecane (27.6 mN m −1) and octane (21.6 mN m −1) in addition to water (72.8 mN m −1). Contact angle values used in this study were average data of five different points on

each specimen.

3. Results and Discussion

Figure 1 shows the 3D laser scanning microscopy images of Type 304 stainless steel chemically etched in the solu-tions containing several concentrations of APB surfactant for 30 min. Obviously, the surface roughness increases with an increase in the surfactant concentration. The size scale of roughness is apparently 40 μm–100 μm, and the roughness, Ra, increases from 1.2 μm to 4.7 μm by the increase in the surfactant concentration from 0 to 5.0 wt% (Fig. 2(a)). Amphoteric surfactants are known to work as an inhibitor of corrosion of several metallic materials.29–31) The non-uniform inhibition of etching in the APB-containing solutions may enhance the surface roughness.

Such chemical etching is sufficient to produce a super-hydrophobic surface after organic coating of the etched stainless steel. In the coating solution, FAS reacted with surface -OH group of substrate, forming a M–O–Si bond. As a result, a uniform monolayer coating can be achieved in an ideal case.32) Figure 2(b) shows the change in the advanc-ing contact angle, θadv, and contact angle hysteresis (CAH) for water on the chemically etched stainless steel with the FAS monolayer coating with the APB concentration in the etching solution. It is known that a flat surface coated with a fluoroalkyl monolayer exhibit a static contact angle of ~120°.33) In accord with the Wenzel model,34) a static contact angle on rough surface, θrough, can be correlated with that of respective flat surface, θflat, using a following equation:

cos cos� �rough flat� R ......................... (1)

where R is the roughness factor. The Eq. (1) indicates that θrough increases with R when θflat > 90°. The rise of θadv with APB concentration in Fig. 2(b) is in agreement with the surface roughening with APB concentration. The CAH of the specimens etched in the solutions containing up to 0.5 wt% APB is rather high (>75°), but decreases to <10° at and above 1.0 wt% APB. The large reduction of the CAH may be associated with the transition from the Wenzel state to the Cassie-Baxter state. The Wenzel and Cassie-Baxter states are schematically illustrated in Fig. 3. In the Wenzel state (Fig. 3(a)), the rough surface is fully wetted with water and the CAH is usually high because of strong pinning of the contact line in the texture. In contrast, air pockets are present between water droplet and rough surface in the Cassie-Baxter state, and the water droplet contacts with the solid only at the convex regions of the rough surface.35) The contact area between water and rough surface is, therefore, highly limited, resulting in the reduction of water-solid interaction and hence the low CAH. In this study, as a consequence of increased roughness, the transition from the Wenzel to the Cassie-Baxter state occurs with an increase in the APB concentration in the etching solution. The super-hydrophobic surface is defined by a static contact angle greater than 150° and rather small CAH, usually less than 10°. Since θadv is usually close to the static contact angle, the surfaces formed by etching of stainless steel in the solutions containing 1.0 and 5.0 wt% APB is superhydrophobic after the FAS monolayer coating.

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Etching time is another important factor influencing sur-face roughness of stainless steel. Figure 4 shows the scan-ning electron micrographs of stainless steel surfaces after

chemical etching in the solution containing 5 wt% APB for 3 min to 120 min. On the initial etched surface (Fig. 4(a)) several semi-spherical pits, indicated by arrows, are developed and the size of the pits is ~10 μm or less. The surface roughness is highly enhanced by extending the etch-ing time, but excess etching appears to reduce the surface roughness as shown in Fig. 4(f). The surface roughness, Ra, was also evaluated using a laser scanning microscope, and the change in the Ra with etching time was plotted in Fig. 5. The Ra increases steeply with etching time up to 5 min, at which the roughness shows a maximum of 4.7 μm. Then, the roughness decreases gradually with etching time.

As shown in Fig. 2(b), the chemically etched stainless steel is superhydrophobic after organic coating. However,

Fig. 1. 3D laser scanning micrographs of type 304 stainless steel surfaces chemically etched in aqueous solution con-taining 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and (a) 0 wt%, (b) 0.5 wt%, (c) 1.0 wt%, (d) 5.0 wt% APB for 30 min at 60°C. (Online version in color.)

Fig. 2. (a) Changes in surface roughness, Ra, and (b) the θadv and CAH with APB concentration in etching solution. The contact angle was measured after FAS monolayer coating. (Online version in color.)

Fig. 3. Schematic illustration showing (a) Wenzel and (b) Cassie-Baxter states. (Online version in color.)

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the surface was not superoleophobic; the θadv was 127°, being less than 150° and the CAH was as large as 47° for rapeseed oil (surface tension of 35.0 mN m −1). Even for ethylene glycol (surface tension of 48.4 mN m −1) the CAH was as high as 20° though θadv was ~160°. Since it is known that the micro-/nano-hierarchically porous surface is useful

for superoleophobicity,6,25) we tried to introduce nanopores by anodizing of the chemically etched stainless steel. Figure 6 shows scanning electron micrographs of the chemically etched and subsequently anodized stainless steel surface. Anodizing of the stainless steel in ethylene glycol electro-lyte containing NH4F and a small amount of water produces a nanoporous layer, which is visible in the high magnifica-tion image (Fig. 6(c)). The pore size is approximately 20 nm. Even after anodizing, roughness in the order of several tens micrometers is remained (Fig. 6(a)). Thus, the micro-/nano-hierarchically rough surface was successfully prepared by the combination of chemical etching and anodizing.

It was also reported that the introduction of ~1 μm size roughness in addition to nanoscale roughness is of impor-tance for oil repellency.6) We performed electrochemical etching of stainless steel between chemical etching and anodizing processes to introduce micro-pits. Figure 7 shows scanning electron micrographs of stainless steel surface after chemical etching, electrochemical etching and anodizing. The roughness of several ten micrometers (Fig. 7(a)) is mainly introduced by chemical etching, and newly developed ~1 μm roughness (Fig. 7(b)) is formed by electrochemical etching. Although the ~1 μm roughness was not developed uniformly, it was found that the ~1 μm roughness was mainly introduced at the ridge regions of the several tens micrometer roughness. Again, nanoscale rough-ness was clearly seen in Fig. 7(c). Hierarchically rough

Fig. 4. Scanning electron micrographs of type 304 stainless steel surfaces chemically etched in aqueous solution con-taining 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and 5.0 wt% APB for (a) 1 min, (b) 5 min, (c) 20 min, (d) 30 min (e) 60 min and (f) 120 min at 60°C.

Fig. 5. Change in surface roughness, Ra, with etching time in aqueous solution containing 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and 5.0 wt% APB at 60°C. (Online version in color.)

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Fig. 6. Scanning electron micrographs of type 304 stainless steel surfaces chemically etched in aqueous solution con-taining 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and 5.0 wt% APB for 30 min at 60°C and then anodized at 60 V in ethylene glycol electrolyte containing 0.1 mol dm −3 NH4F and 0.1 mol dm −3 H2O for 60 s.

Fig. 7. Scanning electron micrographs of type 304 stainless steel surfaces chemically etched in aqueous solution contain-ing 3.9 wt% HCl, 18 wt% FeCl3, 0.1 wt% MnCl2 and 5.0 wt% APB for 30 min at 60°C, electrochemically etched at 1 kA m −2 in aqueous electrolyte containing 3.6 vol% HCl and 1.2 vol% HNO3 at room temperature and then anodized at 60 V in ethylene glycol electrolyte containing 0.1 mol dm −3 NH4F and 0.1 mol dm −3 H2O for 60 s.

surface with roughness scales of 10 μm, 1 μm and 10 nm is developed successfully by a combination of chemical and electrochemical etching and anodizing.

After FAS monolayer coating, the wettability of the hierarchically porous stainless steel surface was evaluated. Figure 8 shows the change in the θadv and CAH with sur-face tension of liquid. In this Figure the wettability of triply hierarchical surface formed by chemical etching, electro-chemical etching and anodizing was compared with that of doubly hieratical surface formed by chemical etching and anodizing. Both the doubly and triply hierarchical surfaces exhibit superhydrophobicity with the θadv as high as ~170° and CAH lower than 3°. Super-repellency is also obtained for ethylene glycol (surface tension of 48.4 mN m −1). The chemically etched surface without anodizing showed rather high CAH of 20°, but the hierarchical surfaces reduced the CAH to less than 3°. Thus, it was confirmed that the hier-archical surface is of importance to oil repellency. When flat stainless surface was coated with a FAS monolayer, the static contact angle for water was higher than 90°; the surface was hydrophobic. Thus, chemical etching of stain-less steel is sufficient to superhydrophobicity by surface roughening (Fig. 2(b)). In contrast, the static contact angle of the FAS-coated flat stainless steel for ethylene glycol was 72°, being less than 90°. For oleophilic flat surfaces it is known that surface roughening reduces further the contact angle in accord with the Wenzel model (Eq. (1)).34) In order to obtain the superoleophobic surface with a Cassie-Baxter state, a pinning effect must be utilized to avoid the penetra-tion of an oil droplet into the depression of the rough sur-face. For hierarchically rough surface, the presence of finer scale roughness within the large-scale pillar-type roughness (Fig. 9) effectively stop penetrating a liquid droplet into the depression even if the surface is essentially oleophilic.24)

Thus, the Cassie-Baxter state with air pockets is stabilized. The present doubly and triply hierarchical surfaces show sufficiently low CAH for ethylene glycol.

The triply hierarchical surfaces also show the θadv higher than 150° and the CAH less than 10° for rapeseed oil (sur-face tension of 35.0 mN m −1) and hexadecane (surface ten-sion of 27.6 mN m −1). In contrast, the θadv is reduced and the CAH increases to ~50° for rapeseed oil on the doubly hierarchical surface. The better superoleophobicity of the triply hierarchical surface indicates the necessity of optimi-zation of hierarchical roughness.

The advantage of multiscale roughness for superoleo-phobicity was reported on aluminum surface.6) Chemical etching of aluminum foil in HCl + CuCl2 aqueous solution

Fig. 8. Change in θadv and CAH with surface tension of liquid droplet for the doubly and triply rough stainless steel sur-faces with a FAS coating. (Online version in color.)

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and following anodizing in H2SO4 electrolyte formed hier-archical surface with micrometer pits and nanoporous layer. The size of pits was 1–5 μm and the pore size was ~30 nm. After coating with a fluoroalkylphosphonic acid monolayer, the surface was superoleophobic for liquids with surface tension as low as 22 mN m −1. When the same treatment of chemical etching and anodizing was carried out to an aluminum mech with mesh opening of ~150 μm, superoleo-phobicity was extended to liquid with surface tension of 18 mN m −1. The improved superoleophobicity using aluminum mesh is associated with the presence of roughness of 150 μm scale in addition to 1 μm and 10 nm scales. The doubly rough surface obtained by anodizing of aluminum mesh exhibited rather poor superoleophobicity compared with the aluminum foil chemically etched and anodized. The findings suggest the size scale of roughness is of key importance in superoleophobicity; the combination of 1 μm scale and 10 nm scale roughness is better than that of 100 μm scale and 10 nm scale roughness.

The results obtained in this study is consistent with the findings for aluminum. The relatively poor superoleopho-bicity of the doubly hierarchical stainless steel surface may be associated with the unsuitable combination of roughness scale (10 μm scale and 10 nm scale). Thus, the triply hier-archical surface with the roughness scales of 10 μm, 1 μm and 10 nm exhibits the better superoleophobicity. However, the superoleophobicity is limited to hexadecane with surface tension of 27 mN m −1 and further reduction of surface ten-sion of liquid loses the superoleophobicity. This may arise partly from the non-uniformity of etched surface. Although the superoleophobicity of stainless steel was brought about by a combination of simple chemical and electrochemical etching and anodizing, the processes need to be modified to optimize the micro/nano-structure of stainless steel surface for superoleophobicity to liquids with further lower surface tensions.

4. Conclusions

Superoleophobic stainless steel surface is developed in this study by a combination of chemical and electrochemical etching, anodizing and organic monolayer coating. These

Fig. 9. Schematic illustration showing an oil droplet on hierarchi-cally rough surface. (Online version in color.)

wet processes introduce triply hierarchical roughness on type 304 stainless steel. Such multiscale roughness improves the superoleophobicity to liquids with lower surface ten-sions. The triply hierarchical surface shows θadv higher than 150° and the CAH less than 10° for hexadecane with the surface tension of 27 mN m −1. Further extension of the superoleophobicity to lower surface tension liquids, it is necessary to develop wet processes to control precisely the miucro-/nano-structure of stainless steel.

AcknowledgmentThe present study was supported in part by ISIJ Research

Promotion Grant.

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