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Electrochemically etched porous stainless steel for enhanced oil retention
Surface & Coatings Technology 264 (2015) 127–131
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrochemically etched porous stainless steel for enhanced oil retention Chan Lee, Aeree Kim, Joonwon Kim ⁎ Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Pohang, Kyungbuk 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 October 2014 Accepted in revised form 3 January 2015 Available online 10 January 2015 Keywords: Surface modification Stainless steel Electrochemical etching Porous structures Oil-infused surface Hierarchical structures
a b s t r a c t This paper reports an electrochemical etching method for stainless steel and its effect in wetting, anti-corrosion and oil retention properties. Specimens of a 304 stainless steel were electrochemically etched in diluted Aqua Regia to form hierarchically-porous surface structures, while maintaining the steel's corrosion resistance. The surfaces consist of multi-scale hierarchical structure and are highly hydrophobic, but water drops stick to it instead of rolling off because of the presence of microscale bumps. Surface structures can be controlled by changing the voltage applied during the etching process. The etched structures significantly increased the steel's oil retention because of very high roughness and steep asperity. This surface modification method could be valuable to extend the lifetime of lubrication, to improve the effectiveness of protective coatings, and to achieve oilinfused surfaces. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Stainless steels are corrosion-resistant alloys of Fe, typically with Cr content ≥ 10.5 wt.%. Various classifications are available according to various additives, e.g., Ni, Mo, V. Stainless steels are widely used as materials for construction, automobiles, marine vessels, and airplanes due to the steels' appropriate mechanical properties as well as their resistance to corrosion. Surface modification to enhance their performance could further extend their usability. Surface modification is a processing technique that changes the properties of a solid surface to enhance performance and functionality. Heat treatments and coatings are typical methods, but micro/nanoscale structures have been introduced as a result of advancements in microscopy and micro/nanoscale fabrication technologies. A notable result of surface structures is that they change the wettability of the surface; i.e., the likelihood that the liquid will adhere to the solid surface. Two major factors; i.e., micro/nanoscale surface morphology and chemical composition, strongly affect the wettability. In general, the chemical composition determines whether surface treatment increases the surface's hydrophobicity or hydrophilicity, but the degree of change is limited in a purely chemical way to contact angle typically ≤120° on hydrophobic surfaces [1]. Surface structures can promote the surface to be extremely hydrophilic or hydrophobic. Many useful functionalities, e.g., self-cleaning, enhanced evaporation
⁎ Corresponding author. Tel.: +82 54 279 2185; fax: +82 54 279 5899. E-mail addresses: [email protected] (C. Lee), [email protected] (A. Kim), [email protected] (J. Kim).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
rate, and enhanced critical heat flux have been reported as results of surface modification [2–4]. Although surface wettability is usually measured using water for ease of experiments [5], wettability of oil (nonpolar chemicals) is also important in practical applications. Various types of oil-based films such as lubricants, anti-corrosion oils, paints and various polymer coatings are used in industries and in everyday products. Loss of oil films from a surface may degrade the performance and life of an entire system; for example in an air conditioning system, oil loss can cause large pressure drops and decreased heat transfer coefficient [6]. Adhesion force between the surface and coating material is the key parameter for the performance of a coating [7–9]. Pretreatments, i.e., primer coatings are used to improve the metal–oil adhesion, but require extra time and cost. Theoretically, a high roughness or porosity of the solid surface can increase the contact area between the oil and the surface, and thereby increases the stability of adhesion and the life of the coatings. In principle, on a slippery liquid-infused porous surface lubrication oil covers the solid surface to provide both non-wetting and selfhealing properties [10,11]. Appropriate oil retention characteristics for base solid materials will be required for practical application of this kind of surface. Currently, practical applications of surface modification using micro/ nanostructures are difficult to find despite its many reported special functionalities. This lack of applications is partly due to problems in materials. Usually high-purity materials have been used in laboratories to ensure exact properties and repeatability. However, pure materials are usually not economically practical. Also, results on pure materials do not assure the same results on alloys. Therefore surface modification
128
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
Fig. 1. Surface morphology of the etched surfaces with applied voltage, with constant duration of etching (10 min). Pore size tended to increase as the applied voltage was increased until 15 V, but decreased after a certain point.
techniques for widely-used alloys like stainless steels can be technically valuable and useful. We report a simple yet useful, electrochemical etching method to achieve surface modification of 304 stainless steel. The method makes the surface highly porous with hierarchical structures, while retaining its corrosion resistance. The surface also shows notably increased oil retention.
2. Materials and methods 2.1. Fabrication Aqueous solutions of 3.6% (1 N) HCl and 1.2% HNO3 were prepared and mixed in a 1:1 ratio (v:v) to make a dilute Aqua Regia solution. The solution was mixed immediately before the experiments because of its rapid self-decomposition. Specimens of a 304 stainless steel were cut into 20 mm × 25 mm × 0.5 mm samples as the target material. The steel was placed parallel to a carbon plate at a separation of 5 cm, then the plates were immersed in the dilute Aqua Regia and a constant electric potential was applied to start electrochemical etching. The steel was subjected to positive polarity and the carbon to negative polarity.
The specimens were rinsed in deionized water immediately after etching and completely dried for the next experiment.
2.2. Characterization methods The morphology and wettability was characterized on all specimens. Surface structure images and energy dispersive spectroscopy (EDS) spectra were obtained using a field emission variable pressure scanning electron microscope (FEVP-SEM, Hitachi SU6600). Surface roughness was measured using a 3D profiler (Veeco Touson Wyko NT1100). The contact angle of water droplets on the surfaces was measured using the sessile drop technique with a drop analysis machine (Femtofab SmartDrop Lab). Anti-corrosion properties were tested using a salt spray test machine (Suga CAP90), according to KS D 8334 standard [19]. Throughout the test period, chamber temperature was set to 35 °C and air saturator temperature was set to 47 °C. For oil retention experiments, control specimens were prepared by polishing with #1000 SiC abrasive paper and sandblasting at 0.7 MPa. A commercial engine oil (Castrol Magnatec 5W40) was applied to the specimens as a lubricant. The lubricated specimens were spun at 1000 RPM 5 s per step using a spin coater (Midas system). The mass of the specimen
Fig. 2. Detailed surface morphology of the etched specimen in 10 V condition. Pores with different scales make the surface hierarchical, which changes surface wettability greatly.
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
129
Fig. 3. Wettability change of electrochemically etched stainless steel. (a) Bare and (b) etched in 10 V condition. The etched specimen is highly hydrophobic in terms of contact angle, but drops do not roll on the surface. Microscale bumps on the surface could be the reason.
was measured using a moisture analyzer (AND MS-70) after each spinning. Oil retention ratio was calculated from the mass difference per step. 3. Results and discussion 3.1. Surface morphology and wettability Etching affected the surface morphology of the steels (Fig. 1). The applied voltage during the etching clearly affects the surface morphology. Pore size tended to increase with voltage until 15 V, but decreased at
voltages N15 V. This result agrees with those of some studies on anodic oxidation, which is similar to electrochemical etching; they reported that pore size increased with applied voltage until a certain point, after which the pore size stabilized or decreased [12,13]. The morphology of a specimen etched at 10 V had nanoscale and microscale pores, and also microscale bumps (Fig. 2); this property makes the surface hierarchical, which causes an extreme change in wettability. Etching affected the wettability of the surface (Fig. 3). The etched surface was highly hydrophobic in terms of contact angle, but the drop did not roll on the surface as it would on well-known hydrophobic
Fig. 4. EDS peak comparison for bare (left) and etched (right) specimens. Both results showed similar peaks, which means similar material compositions.
130
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131 Table 1 Elemental composition of typical a 304 stainless steel. Element
wt.%
C Cr Mn Fe Ni Si P S
≤0.08 18.00–20.00 ≤2.00 Balance 8.00–11.00 ≤1.00 ≤0.040 ≤0.030
Numbers slightly differ among industrial standards.
Table 2 Elemental composition of bare (left) and etched (right) specimen from EDS analysis. Element
wt.%
Element
wt.%
Cr Mn Fe Ni
19.5 1.4 70.3 8.8
Cr Mn Fe Ni
21.3 1.7 70.8 6.2
Fig. 6. Result of rotating disk experiment. Specimens etched at both 10 V and 15 V showed significantly high oil retention properties, whereas sandblasting and etching in 5 V resulted in almost no enhancements.
Cr content was relatively increased while Ni content was decreased, possibly because of etch rate difference.
surfaces; instead it stuck. This phenomenon is sometimes called the “rose petal effect” and is known to be the result of high contact angle hysteresis (CAH) [14]. CAH can be strongly affected by the spacing between surface microstructures, and its value has a local maximum at a certain spacing. The liquid can penetrate between the microstructures when the period is large enough and the pinning phenomenon occurs [15]. The presence of microscale bumps in our surface may be the main reason for this petal effect. Oils, which normally have much less surface tension than water and are thus highly infiltrating, could easily soak into the bumps and strongly reside on the surface. 3.2. EDS analysis and corrosion test Etching affected the EDS spectra of the samples (Fig. 4). Typical 304 stainless steel contains 18–20% Cr and 8–11% Ni and small amounts of other additives (the numbers slightly differ among industrial standards) (Table 1). EDS data of the bare 304 specimen (Table 2, left) fit this standard well, but the etched specimen (Table 2, right) slightly differed in composition. The difference may be due to differences in the etch rates of chemical species. Strictly speaking, the surface of the etched specimen is no longer a 304 stainless steel. However, the etched specimen can be predicted to be a stainless steel with very similar mechanical properties because it still retained high Cr content, which is the most important element for the “stainless” property.
Etching did not affect the corrosion-resistance of the specimens (Fig. 5). Six control and six etched specimens were placed inside the chamber for 3, 5 or 10 days. Bare and etched specimens both showed signs of corrosion after 10 days, usually near the edges. However difference in corrosion between both types of specimen was not significant; this result confirms that the etched specimen is still stainless steel. Therefore, this etching process is a very useful fabrication technique to achieve porous surface structures, while retaining the most advantageous characteristics of stainless steel. 3.3. Oil retention property The etching increased the oil retention property of the etched steels (Fig. 6). Polished and sandblasted specimens were prepared as control groups and three types of etched specimens (etched at 5 V, 10 V, or 15 V conditions) were prepared as the experimental group. After spinning for 50 s, the oil retention rates were ~36% on the polished specimen, ~38% on the sandblasted specimen, ~39% on the specimen etched at 5 V, ~55% on the specimen etched at 10 V, and ~50% on the specimen etched at 15 V. The chemical compositions are similar among the specimens (Fig. 4), therefore the main reason for the increased oil retention on the specimens etched at 10 and 15 V may be the surface structures. Etching increased the arithmetic average roughness Ra of the specimens (Fig. 7). According to a theoretical study, the asymptote of the oil retention ratio increased as Ra increased when liquid surface tension
Fig. 5. Result of corrosion test. Holding in a salt spray chamber for 10 days showed no significant difference for both types of specimen, which means that the etched specimens retain original anti-corrosion properties.
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
131
Fig. 7. 3D profile data for specimens in rotating disk experiment. Measured roughness value for (a) sandblasted (b) etched in 5 V (c) etched in 10 V (d) etched in 15 V and retained oil ratio shows a consistent trend.
was non-negligible [16,17]. Ra is the factor that affects oil retention most strongly, but detailed surface characteristics, e.g., asperity, and the skewness and frequency of the roughness distribution can affect the oil retention on surfaces that have the same Ra [18]. In the present case, the oil retention increased with Ra, but the steep asperity and large number of microscale bumps on specimens etched at 10 V and 15 V may have also helped to increase their oil retention.
(No. 2011-0030075) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2006305). The authors also thank the Pohang Institute of Metal Industry Advancement (POMIA) for supporting both corrosion test and SEM analysis and National Institute for Nanomaterials Technology (NINT) for supporting 3-D surface analysis.
4. Conclusion References A simple, but effective electrochemical etching technique that uses dilute Aqua Regia was presented for a 304 stainless steel. The etched stainless steel surface became highly porous and hydrophobic, but also caused water droplets to stick because of its hierarchical structures with microscale bumps. Pore size can be controlled over a certain range by adjusting the voltage applied during the etching process. The etched surface retained anti-corrosion properties after etching, because its chemical composition was not changed significantly. The specimens etched at 10 V and 15 V had very rough surface morphology, and as a result retained significantly more oil than untreated surfaces, than the surface that had been etched at 5 V. This fabrication method could be very useful in industry, especially to improve lubrication, to provide pretreatments for protective coatings, and to achieve oil-infused surfaces. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)
[1] B. He, N.A. Patankar, J. Lee, Langmuir 19 (2003) 4999–5003. [2] D. Kim, J. Kim, H.C. Park, K. Lee, W. Hwang, J. Micromech. Microeng. 18 (2008) 015019. [3] H. Kim, J. Kim, J. Micromech. Microeng. 20 (2010) 045008. [4] H.S. Ahn, C. Lee, J. Kim, M.H. Kim, Int. J. Heat Mass Transfer 55 (2012) 89–92. [5] D.Y. Kwok, A.W. Neumann, Adv. Colloid Interf. Sci. 81 (1999) 167–249. [6] L. Cremaschi, Y. Hwang, R. Radermacher, Int. J. Refrig. 28 (2005) 1018–1028. [7] C.K. Lin, C.C. Berndt, J. Therm. Spray Technol. 3 (1994) 75–104. [8] R.A. Dickie, Prog. Org. Coat. 25 (1994) 3–22. [9] W. Funke, R.A. Dickie, Prog. Org. Coat. 31 (1997) 5–9. [10] T. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, J. Aizenberg, Nature 477 (2011) 443–447. [11] M. Nosonovsky, Nature 477 (2011) 412–413. [12] A.P. Li, F. Muller, A. Birner, K. Nielsch, U. Gosele, J. Appl. Phys. 84 (1998) 6023–6026. [13] S. Zixue, Z. Wuzong, J. Mater. Chem. 21 (2011) 357–362. [14] L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia, L. Jiang, Langmuir 24 (2008) 4114–4119. [15] B. Bhushan, M. Nosonovsky, Philos. Trans. R. Soc. A 368 (2010) 4713–4728. [16] J.H. Hwang, F. Ma, J. Appl. Phys. 66 (1989) 388–394. [17] F. Ma, J.H. Hwang, J. Appl. Phys. 66 (1989) 5026. [18] J.S. Kim, S. Kim, F. Ma, J. Appl. Phys. 73 (1993) 422. [19] http://www.standard.go.kr/code02/user/0B/03/SerKs_View.asp?ks_no=KSD8334 (in Korean, accessed Dec 2014).
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrochemically etched porous stainless steel for enhanced oil retention Chan Lee, Aeree Kim, Joonwon Kim ⁎ Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Pohang, Kyungbuk 790-784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 October 2014 Accepted in revised form 3 January 2015 Available online 10 January 2015 Keywords: Surface modification Stainless steel Electrochemical etching Porous structures Oil-infused surface Hierarchical structures
a b s t r a c t This paper reports an electrochemical etching method for stainless steel and its effect in wetting, anti-corrosion and oil retention properties. Specimens of a 304 stainless steel were electrochemically etched in diluted Aqua Regia to form hierarchically-porous surface structures, while maintaining the steel's corrosion resistance. The surfaces consist of multi-scale hierarchical structure and are highly hydrophobic, but water drops stick to it instead of rolling off because of the presence of microscale bumps. Surface structures can be controlled by changing the voltage applied during the etching process. The etched structures significantly increased the steel's oil retention because of very high roughness and steep asperity. This surface modification method could be valuable to extend the lifetime of lubrication, to improve the effectiveness of protective coatings, and to achieve oilinfused surfaces. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Stainless steels are corrosion-resistant alloys of Fe, typically with Cr content ≥ 10.5 wt.%. Various classifications are available according to various additives, e.g., Ni, Mo, V. Stainless steels are widely used as materials for construction, automobiles, marine vessels, and airplanes due to the steels' appropriate mechanical properties as well as their resistance to corrosion. Surface modification to enhance their performance could further extend their usability. Surface modification is a processing technique that changes the properties of a solid surface to enhance performance and functionality. Heat treatments and coatings are typical methods, but micro/nanoscale structures have been introduced as a result of advancements in microscopy and micro/nanoscale fabrication technologies. A notable result of surface structures is that they change the wettability of the surface; i.e., the likelihood that the liquid will adhere to the solid surface. Two major factors; i.e., micro/nanoscale surface morphology and chemical composition, strongly affect the wettability. In general, the chemical composition determines whether surface treatment increases the surface's hydrophobicity or hydrophilicity, but the degree of change is limited in a purely chemical way to contact angle typically ≤120° on hydrophobic surfaces [1]. Surface structures can promote the surface to be extremely hydrophilic or hydrophobic. Many useful functionalities, e.g., self-cleaning, enhanced evaporation
⁎ Corresponding author. Tel.: +82 54 279 2185; fax: +82 54 279 5899. E-mail addresses: [email protected] (C. Lee), [email protected] (A. Kim), [email protected] (J. Kim).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.004 0257-8972/© 2015 Elsevier B.V. All rights reserved.
rate, and enhanced critical heat flux have been reported as results of surface modification [2–4]. Although surface wettability is usually measured using water for ease of experiments [5], wettability of oil (nonpolar chemicals) is also important in practical applications. Various types of oil-based films such as lubricants, anti-corrosion oils, paints and various polymer coatings are used in industries and in everyday products. Loss of oil films from a surface may degrade the performance and life of an entire system; for example in an air conditioning system, oil loss can cause large pressure drops and decreased heat transfer coefficient [6]. Adhesion force between the surface and coating material is the key parameter for the performance of a coating [7–9]. Pretreatments, i.e., primer coatings are used to improve the metal–oil adhesion, but require extra time and cost. Theoretically, a high roughness or porosity of the solid surface can increase the contact area between the oil and the surface, and thereby increases the stability of adhesion and the life of the coatings. In principle, on a slippery liquid-infused porous surface lubrication oil covers the solid surface to provide both non-wetting and selfhealing properties [10,11]. Appropriate oil retention characteristics for base solid materials will be required for practical application of this kind of surface. Currently, practical applications of surface modification using micro/ nanostructures are difficult to find despite its many reported special functionalities. This lack of applications is partly due to problems in materials. Usually high-purity materials have been used in laboratories to ensure exact properties and repeatability. However, pure materials are usually not economically practical. Also, results on pure materials do not assure the same results on alloys. Therefore surface modification
128
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
Fig. 1. Surface morphology of the etched surfaces with applied voltage, with constant duration of etching (10 min). Pore size tended to increase as the applied voltage was increased until 15 V, but decreased after a certain point.
techniques for widely-used alloys like stainless steels can be technically valuable and useful. We report a simple yet useful, electrochemical etching method to achieve surface modification of 304 stainless steel. The method makes the surface highly porous with hierarchical structures, while retaining its corrosion resistance. The surface also shows notably increased oil retention.
2. Materials and methods 2.1. Fabrication Aqueous solutions of 3.6% (1 N) HCl and 1.2% HNO3 were prepared and mixed in a 1:1 ratio (v:v) to make a dilute Aqua Regia solution. The solution was mixed immediately before the experiments because of its rapid self-decomposition. Specimens of a 304 stainless steel were cut into 20 mm × 25 mm × 0.5 mm samples as the target material. The steel was placed parallel to a carbon plate at a separation of 5 cm, then the plates were immersed in the dilute Aqua Regia and a constant electric potential was applied to start electrochemical etching. The steel was subjected to positive polarity and the carbon to negative polarity.
The specimens were rinsed in deionized water immediately after etching and completely dried for the next experiment.
2.2. Characterization methods The morphology and wettability was characterized on all specimens. Surface structure images and energy dispersive spectroscopy (EDS) spectra were obtained using a field emission variable pressure scanning electron microscope (FEVP-SEM, Hitachi SU6600). Surface roughness was measured using a 3D profiler (Veeco Touson Wyko NT1100). The contact angle of water droplets on the surfaces was measured using the sessile drop technique with a drop analysis machine (Femtofab SmartDrop Lab). Anti-corrosion properties were tested using a salt spray test machine (Suga CAP90), according to KS D 8334 standard [19]. Throughout the test period, chamber temperature was set to 35 °C and air saturator temperature was set to 47 °C. For oil retention experiments, control specimens were prepared by polishing with #1000 SiC abrasive paper and sandblasting at 0.7 MPa. A commercial engine oil (Castrol Magnatec 5W40) was applied to the specimens as a lubricant. The lubricated specimens were spun at 1000 RPM 5 s per step using a spin coater (Midas system). The mass of the specimen
Fig. 2. Detailed surface morphology of the etched specimen in 10 V condition. Pores with different scales make the surface hierarchical, which changes surface wettability greatly.
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
129
Fig. 3. Wettability change of electrochemically etched stainless steel. (a) Bare and (b) etched in 10 V condition. The etched specimen is highly hydrophobic in terms of contact angle, but drops do not roll on the surface. Microscale bumps on the surface could be the reason.
was measured using a moisture analyzer (AND MS-70) after each spinning. Oil retention ratio was calculated from the mass difference per step. 3. Results and discussion 3.1. Surface morphology and wettability Etching affected the surface morphology of the steels (Fig. 1). The applied voltage during the etching clearly affects the surface morphology. Pore size tended to increase with voltage until 15 V, but decreased at
voltages N15 V. This result agrees with those of some studies on anodic oxidation, which is similar to electrochemical etching; they reported that pore size increased with applied voltage until a certain point, after which the pore size stabilized or decreased [12,13]. The morphology of a specimen etched at 10 V had nanoscale and microscale pores, and also microscale bumps (Fig. 2); this property makes the surface hierarchical, which causes an extreme change in wettability. Etching affected the wettability of the surface (Fig. 3). The etched surface was highly hydrophobic in terms of contact angle, but the drop did not roll on the surface as it would on well-known hydrophobic
Fig. 4. EDS peak comparison for bare (left) and etched (right) specimens. Both results showed similar peaks, which means similar material compositions.
130
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131 Table 1 Elemental composition of typical a 304 stainless steel. Element
wt.%
C Cr Mn Fe Ni Si P S
≤0.08 18.00–20.00 ≤2.00 Balance 8.00–11.00 ≤1.00 ≤0.040 ≤0.030
Numbers slightly differ among industrial standards.
Table 2 Elemental composition of bare (left) and etched (right) specimen from EDS analysis. Element
wt.%
Element
wt.%
Cr Mn Fe Ni
19.5 1.4 70.3 8.8
Cr Mn Fe Ni
21.3 1.7 70.8 6.2
Fig. 6. Result of rotating disk experiment. Specimens etched at both 10 V and 15 V showed significantly high oil retention properties, whereas sandblasting and etching in 5 V resulted in almost no enhancements.
Cr content was relatively increased while Ni content was decreased, possibly because of etch rate difference.
surfaces; instead it stuck. This phenomenon is sometimes called the “rose petal effect” and is known to be the result of high contact angle hysteresis (CAH) [14]. CAH can be strongly affected by the spacing between surface microstructures, and its value has a local maximum at a certain spacing. The liquid can penetrate between the microstructures when the period is large enough and the pinning phenomenon occurs [15]. The presence of microscale bumps in our surface may be the main reason for this petal effect. Oils, which normally have much less surface tension than water and are thus highly infiltrating, could easily soak into the bumps and strongly reside on the surface. 3.2. EDS analysis and corrosion test Etching affected the EDS spectra of the samples (Fig. 4). Typical 304 stainless steel contains 18–20% Cr and 8–11% Ni and small amounts of other additives (the numbers slightly differ among industrial standards) (Table 1). EDS data of the bare 304 specimen (Table 2, left) fit this standard well, but the etched specimen (Table 2, right) slightly differed in composition. The difference may be due to differences in the etch rates of chemical species. Strictly speaking, the surface of the etched specimen is no longer a 304 stainless steel. However, the etched specimen can be predicted to be a stainless steel with very similar mechanical properties because it still retained high Cr content, which is the most important element for the “stainless” property.
Etching did not affect the corrosion-resistance of the specimens (Fig. 5). Six control and six etched specimens were placed inside the chamber for 3, 5 or 10 days. Bare and etched specimens both showed signs of corrosion after 10 days, usually near the edges. However difference in corrosion between both types of specimen was not significant; this result confirms that the etched specimen is still stainless steel. Therefore, this etching process is a very useful fabrication technique to achieve porous surface structures, while retaining the most advantageous characteristics of stainless steel. 3.3. Oil retention property The etching increased the oil retention property of the etched steels (Fig. 6). Polished and sandblasted specimens were prepared as control groups and three types of etched specimens (etched at 5 V, 10 V, or 15 V conditions) were prepared as the experimental group. After spinning for 50 s, the oil retention rates were ~36% on the polished specimen, ~38% on the sandblasted specimen, ~39% on the specimen etched at 5 V, ~55% on the specimen etched at 10 V, and ~50% on the specimen etched at 15 V. The chemical compositions are similar among the specimens (Fig. 4), therefore the main reason for the increased oil retention on the specimens etched at 10 and 15 V may be the surface structures. Etching increased the arithmetic average roughness Ra of the specimens (Fig. 7). According to a theoretical study, the asymptote of the oil retention ratio increased as Ra increased when liquid surface tension
Fig. 5. Result of corrosion test. Holding in a salt spray chamber for 10 days showed no significant difference for both types of specimen, which means that the etched specimens retain original anti-corrosion properties.
C. Lee et al. / Surface & Coatings Technology 264 (2015) 127–131
131
Fig. 7. 3D profile data for specimens in rotating disk experiment. Measured roughness value for (a) sandblasted (b) etched in 5 V (c) etched in 10 V (d) etched in 15 V and retained oil ratio shows a consistent trend.
was non-negligible [16,17]. Ra is the factor that affects oil retention most strongly, but detailed surface characteristics, e.g., asperity, and the skewness and frequency of the roughness distribution can affect the oil retention on surfaces that have the same Ra [18]. In the present case, the oil retention increased with Ra, but the steep asperity and large number of microscale bumps on specimens etched at 10 V and 15 V may have also helped to increase their oil retention.
(No. 2011-0030075) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2006305). The authors also thank the Pohang Institute of Metal Industry Advancement (POMIA) for supporting both corrosion test and SEM analysis and National Institute for Nanomaterials Technology (NINT) for supporting 3-D surface analysis.
4. Conclusion References A simple, but effective electrochemical etching technique that uses dilute Aqua Regia was presented for a 304 stainless steel. The etched stainless steel surface became highly porous and hydrophobic, but also caused water droplets to stick because of its hierarchical structures with microscale bumps. Pore size can be controlled over a certain range by adjusting the voltage applied during the etching process. The etched surface retained anti-corrosion properties after etching, because its chemical composition was not changed significantly. The specimens etched at 10 V and 15 V had very rough surface morphology, and as a result retained significantly more oil than untreated surfaces, than the surface that had been etched at 5 V. This fabrication method could be very useful in industry, especially to improve lubrication, to provide pretreatments for protective coatings, and to achieve oil-infused surfaces. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)
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