Development of water-repellent stainless steel (SUS 316l) surface by stepwise anodizing technique

Abstract

The present disclosure relates to a technology for developing a water-repellent stainless steel (SUS 316L) surface for improving a corrosion inhibition (corrosion resistance) efficiency. According to the method for forming a hydrophobic and corrosion resistant oxide film on a stainless steel surface according to the present disclosure, uniform porous oxide film may be formed without a pre-patterning process for forming uniform porous oxide films of the related art to achieve excellent hydrophobicity and corrosion resistance, thereby being applied to medical devices, marine transportation vehicles, land transportation vehicles, air transportation vehicles, water and sewage pipes, water and sewage filters, and the like, including stainless steel and also be useful as a machine learning database for developing stainless steel surface treatment technology.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a technology for developing a water-repellent stainless steel (SUS 316L) surface for improving a corrosion inhibition (corrosion resistance) efficiency to be applied to medical devices, marine transportation vehicles, land transportation vehicles, air transportation vehicles, water and sewage pipes, and water and sewage filters, including stainless steel, and also be useful as a machine learning database for developing stainless steel surface treatment technology.BACKGROUND

[0002] Stainless steel is a metal alloy that does not rust by adding chromium, and is used in many industrial fields such as marine, machinery, electronics, plumbing, power generation, and nuclear power with its machinability, economy, excellent corrosion resistance, and the like. However, despite these advantages, stainless steel suffers from poor corrosion resistance in severe environments such as gas piping and the marine industry.

[0003] In order to solve this disadvantage, an anti-corrosion surface treatment technology for improving the corrosion resistance is being actively studied. Recently, researches on implementing superhydrophobic surfaces through the research using wettability behavior have been attracting attention.

[0004] The superhydrophobic surface may utilize various characteristics such as water-repellency, self-cleaning, oil-repellency, anti-icing, and anti-frost and also be used in various industries such as advanced displays, optical films, semiconductors, or thin-film coating.

[0005] The wettability behavior is determined by a surface energy of a material and the surface contact angle is 150° or larger by reducing the surface energy to implement the superhydrophobicity. These superhydrophobic surfaces have been developed with reference to various natural materials, including lotus petals, cicada wings, rice leaves, and the like, and are being studied in various ways, including fabricating micro-and nanoscale structures to reduce a surface energy, and the like.

[0006] However, there are limited ways to uniformly implement the micro or nanoscale structures in the metal. Among various surface treatment methods, an anodizing method may artificially form a uniform and thick oxide film on the metal.

[0007] The oxide film formed by the anodizing method is divided into a barrier type surface and a hollow type surface. The barrier type surface refers to a surface in which the inside of the oxide film is densely formed without having an empty space such as pores. The hollow type surface is divided into a porous film having a nano structure in which porous structures are regularly disposed and a nano tube type film in which empty spaces are provided between pores.

[0008] Here, the anodization is one of the most widely known metal surface treatment methods and according to the anodization, when a metal base material deposited into an electrolytic solution is used as an anode to be electrified, an oxide film is formed while oxidizing a surface of the base material by oxygen generated from the anode to improve a physical property of the base material.

[0009] That is, oxygen ions and hydroxide ions in the electrolyte penetrate into the oxide film which is formed on the surface of the base material to be bonded to metal ions to form an oxide layer. Therefore, porous oxide film and hydroxide film grow in the vicinity of the interface of the base material and the oxide layer, thereby further improving the properties of the base material.

[0010] When the physical property of the metal base material is enhanced by the anodization, as the most important parameter of the anodization, it is most important to appropriately set various functions, such as an anodizing voltage, time, and a purity of the base material metal.

[0011] There are various types of alloys of the stainless steel depending on a component content and an intended anodization condition may vary according to the component content so that the component content of the base material to be processed is very important.

[0012] The present inventor completed the invention by forming an oxide film with a nano structure by optimizing the anodizing time and voltage with stainless steel (SUS 316L) as a base material and then coating with a hydrophobic self-assembled monolayer (SAM) coating agent to confirm that the hydrophobicity and corrosion resistance (corrosion inhibition efficiency) are significantly improved.RELATED ART DOCUMENTPatent Document

[0013] Patent Document 1: Korean Registered Patent No. 10-1832059SUMMARY

[0014] The present disclosure has been made in an effort to provide a method for forming a hydrophobic and corrosion resistant oxide film on a stainless steel surface.

[0015] An exemplary embodiment of the present disclosure is to provide a stainless steel on which a hydrophobic anodized film manufactured by the above-mentioned method is formed.

[0016] Another exemplary embodiment of the present disclosure is to provide a stainless steel on which a corrosion resistant anodized film manufactured by the above-mentioned method is formed.

[0017] Still another exemplary embodiment of the present disclosure is to provide medical devices, marine transportation vehicles, land transportation vehicles, air transportation vehicles, water and sewage pipes, and water and sewage filters including a stainless steel manufactured by the above-mentioned method.

[0018] In order to achieve the above-described objects, an exemplary embodiment of the present disclosure provides a method for forming a hydrophobic and corrosion resistant oxide film on a stainless steel surface including: washing and drying a stainless steel surface (Step 1); anodizing at an applied voltage of 80 to 100 V for 2.5 to 3.5 hours to form an anodized film on the stainless steel surface (Step 2); immersing the anodized stainless steel of Step 2 into 0.05 to 1.0 M a phosphoric acid solution to widen pores (Step 3); conducting plasma treatment to remove organic residues and making the anodized film surface hydrophilic (Step 4); and coating with a self-assembled monolayer (SAM) coatable hydrophobic coating agent (Step 5).

[0019] The stainless steel may be applied to various stainless steels and desirably, may be SUS 316 or SUS 316L, and more desirably, may be SUS 316L.

[0020] During the anodizing of Step 2, as an electrolyte, ethylene glycol containing 0.05 to 0.15 M NH4F and 0.05 to 0.15 M water may be used, desirably, ethylene glycol containing 0.08 to 0.12 M NH4F and 0.08 to 0.12 M water may be used, and more desirably, ethylene glycol containing 0.09 to 0.11 M NH4F and 0.09 to 0.11 M water may be used. In the present disclosure, as an example, ethylene glycol containing 0.1 M NH4F and 0.1 M water is used as an electrolyte, but is not limited thereto.

[0021] In Step 2, the washed stainless steel may be anodized at 80 to 100 V for 2.5 to 3.5 hours, and desirably, at 85 to 95 V for 2.8 to 3.2 hours, and more desirably, at 88 to 92 V for 2.9 to 3.1 hours.

[0022] In order to implement hydrophobicity and 90% or higher of corrosion inhibition efficiency, the anodization is desirably performed at 88 to 92 V for 2.9 to 3.1 hours and there may be a problem in that the hydrophobicity and the corrosion inhibition efficiency are degraded out of this condition.

[0023] The pore expansion of Step 3 may be conducted by immersion into 0.05 to 1.0 M phosphoric acid solution for 1 to 60 minutes.

[0024] As a SAM coatable hydrophobic coating agent of Step 5, perfluoroalkylsilane having 1 to 20 fluorocarbon chains with a surface energy of 6 mJ / m2 to 20 mJ / m2, alkylsilane with 1 to 20 carbon atoms, or the like may be used and for examples, 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), trichloroctylsilane (OTS), octadecyltrichlorosilane (ODTS), and the like may be used.

[0025] Further, another exemplary embodiment of the present disclosure provides a stainless steel in which a hydrophobic anodized film manufactured by the above-described method is formed.

[0026] Further, another exemplary embodiment of the present disclosure provides a stainless steel in which a corrosion resistant anodized film manufactured by the above-described method is formed.

[0027] Further, another exemplary embodiment of the present disclosure provides a medical device, a marine transportation vehicle, a land transportation vehicle, an air transportation vehicle, a water and sewage pipe, and a water and sewage filter, including a stainless steel manufactured by the above-described method.

[0028] According to the method for forming a hydrophobic and corrosion resistance oxide film on a stainless steel surface according to the present disclosure, uniform porous oxide film may be formed without a pre-patterning process for forming uniform porous oxide films of the related art, thereby achieving excellent hydrophobicity and corrosion resistance.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is an EDS measurement result for a sample obtained after carrying out only Steps 1 to 4 of Example 1-4;

[0030] FIG. 2 is an image obtained by observing a surface shape of an oxide film formed on surfaces of four samples obtained after carrying out only Steps 1 to 4 in Example with FE-SEM;

[0031] FIG. 3 is a result of measuring a contact angle of a sample (SAM coating is not performed) on which only Steps 1 to 4 are conducted in Example;

[0032] FIG. 4 is a result of measuring a contact angle of a sample (SAM coating is performed) on which all the Steps 1 to 5 are conducted in Example; and

[0033] FIG. 5 is a view illustrating a potentio-dynamic polarization curve of a sample on which all Steps 1 to 5 are conducted in Example.DETAILED DESCRIPTION

[0034] In the following detailed description, reference is made to the accompanying drawing, which forms a part hereof. The illustrative embodiments described in the detailed description, drawing, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.<EXAMPLES 1-1 TO 1-4> ANODIZATION OF STAINLESS STEEL (SUS 316L)Step 1: Prepare Stainless Steel (SUS 316L) Substrate

[0035] Stainless steel (SUS 316L) with dimensions of 3 cm×3 cm×0.05 cm was used. In order to remove foreign materials on the surface and clean the surface, the substrate was subjected to ultrasonic washing while being immersed into ethanol and acetone, washed again using distilled water, and then dried.

[0036] Compositions of the stainless steel such as SUS 316L were represented below. For reference, there is considerable variation in the optimal anodizing conditions for the formation of superhydrophilic oxide films depending on the types of metals and alloys. According to the present disclosure, an optimal anodizing condition for forming a superhydrophilic oxide film was found by focusing on SUS 316L.ClassificationCSiMnPSNiCrMoSUS 3040.08 or1.00 or2.00 or0.0450.0308.00 to18.00—lesslesslessor lessor less10.50to20.00SUS 304L0.0301.00 or2.00 or0.0450.0309.00 to18.00—or lesslesslessor lessor less13.00to20.00SUS 3160.08 or1.00 or2.00 or0.0450.03010.0016.002.00 tolesslesslessor lessor lesstoto3.0014.0018.00SUS 316L0.0301.00 or2.00 or0.0450.03012.0016.002.00 toor lesslesslessor lessor lesstoto3.0015.0018.00Step 2: Anodization

[0037] During an anodizing process, stainless steel was used as an anode and platinum (2.5 cm×4 cm×0.05 cm) was used as a cathode and an interval between electrodes was maintained to be 5 cm. Electrolyte solution with addition of 0.1 M NH4F and 0.1 M H2O based on the ethylene glycol solution was maintained at a temperature of 0° C. in a double-jacketed beaker and a water-cooled chiller. The experiment was performed for three hours with applied voltages of 30 V (Example 1-1), 50 V (Example 1-2), 70 V (Example 1-3), and 90 V (Example 1-4), and the specimen was washed with distilled water and dried after anodization.Step 3: Pore Widening

[0038] In order to widen pores of a nano structure fabricated by the anodization in Step 2, the specimen was immersed into 0.1 M phosphoric acid (Junsei) for 10 minutes to widen pores, washed with distilled water, and then dried.Step 4: Plasma Treatment

[0039] Organic residues were removed from the surface with oxygen plasma for 15minutes using a plasma device to have hydrophilicity and then dried at 150° C. for 10 minutes using a heated stirrer in the air. The plasma treatment condition was 200 W, 50 kHz, 50 sccm of O2, and RIE mode for 15 minutes.Step 5: Self-Assembled Monolayer (SAM) Coating

[0040] In order to impart a water-repellent property to plasma-treated anodized sample, self-assembled monolayer (SAM) coating was performed using 1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) solution with a low surface energy.<EXPERIMENTAL EXAMPLE 1> EVALUATION OF FORMATION OF OXIDE FILM USING ENERGY DISPERSIVE SPECTROSCOPY (EDS)

[0041] EDS (model name was X-MAX, manufactured by OXFORD) measurement was performed on a stainless steel (SUS 316L) sample obtained by performing steps 1 to 4 (without SAM coating) in Examples 1 to 4 (anodization on 90 V, for three hours) to evaluate whether an oxide film was formed and a result was represented in FIG. 1.

[0042] FIG. 1 is an EDS measurement result for a sample obtained after carrying out only steps 1 to 4 of Examples 1 to 4.

[0043] As illustrated in FIG. 1, oxygen and iron were represented as main components after anodization and in addition, chromium, magnesium, nickel, and the like were detected. Further, carbon is caused by the influence on the carbon tape used to fix the sample to the stage and corresponded to a noise. This result shows that the oxide film is formed on a stainless steel surface.<EXPERIMENTAL EXAMPLE 2> OBSERVATION OF SURFACE MORPHOLOGY USING FIELD EMISSION SCANNING ELECTRON MICROSCOPE (FE-SEM)

[0044] A surface morphology of an oxide film formed on the stainless steel (SUS 316L) surface on which steps 1 to 4 (without SAM coating) were conducted in Example was observed using FE-SEM (model name was MIRA 3 LMH In-Beam Detector and manufactured by TESCAN) and a result was represented in FIG. 2.

[0045] Specifically, in order to observe a surface morphology of the sample, the sample was cut to be fixed to a stage with a carbon tape and an anodized structure which is a non-conductive oxide was subjected to platinum coating for 40 seconds and then observed.

[0046] FIG. 2 is an image obtained by observing a surface morphology of an oxide film formed on surfaces of four samples obtained after carrying out only steps 1 to 4 in Example with FE-SEM.

[0047] As illustrated in FIG. 2, unlike surfaces of (c) and (d), a barrier-type oxide film was formed on the surface in (a) and (b) so that it was confirmed that pores were not formed on the surface. Further, unlike the two specimens mentioned above, it was confirmed that in (c) and (d), porous structures were formed. The reason why the porous structure is formed is that partial dissolution occurring on the surface of the film due to the contact of the oxide film with the acidic solution increases the surface roughness of the oxide film and the electric field is concentrated on a thinner part of the rough oxide film. The concentration of the electric field promotes the dissolution reaction of the oxide film to form a thinner oxide film, and the continuous behavior of these reactions leads to the formation of pores by localized oxidation.

[0048] In Table 1, a result of measuring a pore diameter (Dp), an interpore distance Dint, and a solid fraction of a pore formed on a surface after anodization using the FE-SEM image of FIG. 2 was represented. The pore diameter and the interpore distance were mean values and the solid fraction was calculated by the following Equation 1.fSL=1-2⁢π3⁢r2a2[Equation⁢ l]fSL: Solid fraction

[0050] a: Interpore Distance

[0051] r: Pore radiusTABLE 1Dp (nm)Dint (nm)Solid Fraction30 VNoneNoneNone50 VNoneNoneNone70 V68.62 ± 8.07 89.78 ± 8.300.455990 V89.17 ± 8.25106.45 ± 7.220.3466

[0052] As represented in Table 1, no pore was found from an oxide film of the specimen in applied voltages of 30 V and 50 V which was formed as a barrier-type film so that Dp, Dint, and the solid fraction were not obtained and on 70 V and 90 V, porous pores were generated so that Dp, Dint, and the solid fraction were obtained. Dp, Dint, and the solid fraction on 70 V were 68.62±8.07 nm, 89.78±8.30 nm, and 0.4559, respectively, and values on 90 V were 89.17±8.25 nm, 106.45±7.22 nm, and 0.3466, respectively. It was observed that a porous film having regular pores was formed in 70 V and 90 V voltage conditions. The solid fraction means a roughness fraction.<EXPERIMENTAL EXAMPLE 3> EVALUATION OF CONTACT ANGLE

[0053] In order to find out surface wettability of a sample on which only steps 1 to 4 were conducted and a sample on which all the steps 1 to 5 were conducted in Example, a contact angle was measured and the result was represented in FIGS. 3 and 4 and Table 2.

[0054] Specifically, 3.5 μl of distilled water was used as the reference liquid for the measurement. The contact angle was measured after 5 seconds after dropping liquid droplets on the surface with 10 measurements per specimen.

[0055] FIG. 3 is a result of measuring a contact angle of a sample (without SAM coating) on which only steps 1 to 4 are conducted in Example.

[0056] FIG. 4 is a result of measuring a contact angle of a sample (SAM coating is performed) on which all steps 1 to 5 are conducted in Example;

[0057] The results of FIGS. 3 and 4 were summarized to be represented in the following Table 2.TABLE 2Before SAM coating (°)After SAM coating (°)30 V21.4 ± 0.91115.5 ± 2.8550 V18.5 ± 1.59123.4 ± 1.6370 V16.5 ± 0.98130.3 ± 1.8290 V8.56 ± 0.68140.3 ± 1.46

[0058] As represented in Table 2, it is understood that after SAM coating, the higher the applied voltage in the sample, the larger the contact angle. According to Cassie-Baxter theory, the coating on the porous oxide film on the surface of the specimen causes the air to push out water droplets on the pores or a solid surface so that it is determined that the contact angle is large on the specimen coated on the surface forming the porous oxide film.<EXPERIMENTAL EXAMPLE 4> EVALUATION OF CORROSION RESISTANCE

[0059] A corrosion resistance of a sample on which all the steps 1 to 5 were conducted in Example was evaluated and the result was represented in FIG. 5 and Table 3.

[0060] Specifically, the corrosion resistance was measured by potentio-dynamic polarization test (PDP) which is an electrochemical method in a 3.5 wt. % NaCl solution at room temperature. The sample was immersed into a 3.5 wt. % NaCl solution at room temperature for 1 hour prior to the analysis test. The polarization test used a three-electrode system in which a sample was used as a working electrode, platinum (Pt) was used as a counter electrode, and silver / silver chloride (Ag / AgCl) electrode as a reference electrode. Under the measurement condition ranging from −500 mV to +14000 mV (vs. Ag / AgCl) at a scanning rate of 1 mV / sec, the corrosion resistance was evaluated by electrochemical characteristic analysis.

[0061] FIG. 5 is a view illustrating a potentio-dynamic polarization curve of a sample on which all steps 1 to 5 are conducted in Example.

[0062] The result of FIG. 5 was summarized to be represented in the following Table 3.TABLE 3Ecorr (mV)Icorr (A / cm2)IE (%)Unprocessed SUS 316L−4042.36 × 10−7030 V−2321.67 × 10−729.2350 V−1922.96 × 10−887.4670 V−1442.49 × 10−889.4490 V−389.02 × 10−996.18Ecorr: Corrosion Potential

[0064] Icorr: Corrosion current density representing loss of mass

[0065] IE: Corrosion inhibition efficiency of processed sample of Example compared to unprocessed SUS 304

[0066] The corrosion potential is a numerical value that shows the rate at which corrosion occurs. The lower the value, the greater the oxidizing tendency so that the corrosion tends to occur faster.

[0067] As a corrosion current density, the higher the current density, the more the current flows so that the more corrosion tends to occur.

[0068] The corrosion inhibition efficiency is expressed using a corrosion current density in which i is a current density of an anodized and coated specimen and i0 is a current density of an unprocessed specimen and was calculated by the following Equation 2.IE⁢ %=(1-iio) × 100[Equation⁢ 2]i: Corrosion current density of sample on which all steps 1 to 5 are conducted in Example

[0070] i0: Corrosion current density of unprocessed SUS 316L

[0071] IE: Corrosion inhibition efficiency of processed sample of Example compared to unprocessed SUS 316L

[0072] As represented in Table 3, it is confirmed that the higher the applied voltage, the more excellent the corrosion resistance. Further, the corrosion inhibition efficiency tends to increase as the applied voltage increases as follows: 29.23% on 30 V, 87.46% on 50 V, 89.44% on 70 V, and 96.18% on 90 V. This is related to the change in the interfacial structure of a material which is in contact with a corrosive material (Cl−) and the wettability of the water-repellent porous oxide film. In the case of the change in the interfacial structure of the material which is in contact with the corrosive material, a coating material with a low surface energy does not have a polarity so that it is difficult for the water molecules to be adhered onto the surface. Therefore, corrosive ions in the water molecules are difficult to react with the coated metal. In addition, even though the structure and the corrosive material are in contact with each other according to the morphology of the Cassie-Baxter during the water-repellent treatment of the porous oxide film, it is determined that a situation in which the corrosive material hardly penetrates into the pores is implemented due to the air filled in the pores so that the corrosion inhibition efficiency is increased.<EXPERIMENTAL EXAMPLE 5> EVALUATION OF OPTIMAL ANODIZATION CONDITION (TIME AND VOLTAGE) FOR CORROSION RESISTANCE

[0073] It was confirmed by Experimental Examples 1 to 4 that the corrosion resistance was the most excellent under the anodization condition on 90 V for 3 hours. Therefore, in Experimental Example 5, an optimal condition was found based on the anodization condition of 3 hours and 90 V and the results were represented in Tables 4 and 5. Samples (SAM coating was conducted) were manufactured in the same way as Example except for varying the anodizing time and voltage.TABLE 4Time (h)Voltage (V)IE (%)Example 2-12.89082.23Example 2-22.995.87Example 2-3 (=Example 1-4)3.096.18Example 2-43.195.48Example 2-53.285.47TABLE 5Time (h)Voltage (V)IE (%)Example 3-13.08689.99Example 3-28895.31Example 3-3 (=Example 1-4)9096.18Example 3-49295.74Example 3-59487.49As represented in Tables 4 and 5, it was confirmed that the best result was obtained with the anodizing time of 2.9 to 3.1 hours and an applied voltage of 88 to 92 V in terms of the corrosion resistance.From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A method for forming a hydrophobic and corrosion resistant oxide film on a stainless steel surface, comprising:washing and drying the stainless steel surface (Step 1);anodizing at an applied voltage of 80 to 100 V for 2.5 to 3.5 hours to form an anodized film on the stainless steel surface (Step 2);immersing the anodized stainless steel of Step 2 into 0.05 to 1.0 M a phosphoric acid solution to widen pores (Step 3);conducting plasma treatment to remove organic residues and making the anodized film surface hydrophilic (Step 4); andcoating with a self-assembled monolayer (SAM) coatable hydrophobic coating agent (Step 5).

2. The method of claim 1, wherein the stainless steel is SUS 316 or SUS 316L.

3. The method of claim 1, wherein during the anodizing of Step 2, a mixture of NH4F, water, and ethylene glycol is used as an electrolyte.

4. The method of claim 1, wherein the anodizing of Step 2 is performed at an applied voltage of 85 to 95 V for 2.8 to 3.2 hours.

5. The method of claim 4, wherein the anodizing of Step 2 is performed at an applied voltage of 88 to 92 V for 2.9 to 3.1 hours.

6. The method of claim 1, wherein the SAM coatable hydrophobic coating agent is any one selected from the group consisting of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), trichloroctylsilane (OTS), and octadecyltrichlorosilane (ODTS).

7. The method of claim 1, wherein the anodized film with a contact angle of 160° or larger is formed on the stainless steel surface.

8. The method of claim 1, wherein the anodized film with a corrosion inhibition efficiency of 90% or higher is formed on the stainless steel surface.

9. A stainless steel in which a hydrophobic anodized film manufactured by the method of claim 1 is formed.

10. A stainless steel in which a corrosion resistant anodized film manufactured by the method of claim 1 is formed.

11. A medical device including a stainless steel manufactured by the method of claim 1.

12. A transportation vehicle including a stainless steel manufactured by the method of claim 1.

13. The transportation vehicle of claim 12, wherein the transportation vehicle is a marine transportation vehicle, a land transportation vehicle, or an air transportation vehicle.

14. A water and sewage pipe including the stainless steel manufactured by the method of claim 1.

15. A water and sewage filter including the stainless steel manufactured by the method of claim 1.

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