DEVELOPMENT OF FUNCTIONAL SUPER WATER-REPELLENT STAINLESS STEEL SURFACE TECHNOLOGY FOR IMPROVING CORROSION RESISTANCE

Description

TECHNICAL FIELD

The present disclosure relates to a technology for developing a functional super water-repellent stainless steel surface for improving corrosion resistance.

BACKGROUND

Stainless steel is a metal alloy which is rust-free by the addition of chromium, and has characteristics such as workability, economic efficiency, excellent corrosion resistance, etc. so that it is used in various industrial fields such as marine, machinery, electronic parts, piping, power generation, and nuclear power. However, despite these advantages, stainless steel has a disadvantage in that it has poor corrosion resistance in environments such as harsh environments including gas piping, marine industries, etc.

In order to solve these disadvantages, research on anti-corrosion surface treatment technology to improve corrosion resistance is being actively conducted. Recently, research on implementing super water-repellent surfaces through research using wettability behavior is attracting attention.

The super water-repellent surfaces may utilize various properties such as water repellency, self-cleaning, oil repellency, anti-icing, anti-frost, etc., and may be used in various industries such as cutting-edge displays, optical films, semiconductors, thin film coatings, etc.

The wettability behavior is determined by the surface energy of a material, and the surface energy is reduced, and thus the surface contact angle becomes 1500 or more so that super water-repellency is realized. Such super water-repellent surfaces were developed by looking at various natural materials such as lotus petals, cicada wings, and rice leaves, and various methods such as fabricating micro- and nano-sized structures and thus reducing surface energy to fabricate them are being studied.

However, methods capable of uniformly implementing micro- and nano-sized structures on metal are limited. An anodization method among various surface treatment methods may artificially form a uniform and thick oxide film on metal.

The oxide film made by the anodization method is divided into a barrier-type film and a pore-type film, the barrier-type film refers to a film in which the inside of the oxide film is formed densely without empty spaces such as pores, and the pore-type film is divided into a porous film having a nanostructure in which pore structures are regularly arranged and a nanotubular film in which the empty spaces are present between the pores.

Here, the anodization is one of the most widely known treatment methods among metal surface treatment methods, and when electricity is applied to, as an anode, a metal base material deposited in an electrolyte, it is a treatment method to improve the physical properties of the base material by forming an oxide film while the surface of the base material is oxidized by oxygen generated from the anode.

That is, oxygen ions or hydroxyl ions in the electrolyte penetrate into the oxide film formed on the surface of the base material and combine with metal ions to form an oxide layer, whereby a porous oxide film and a hydroxide film grow near the interface between the base material and the oxide layer to further improve the physical properties of the base material.

In increasing the physical properties of the metal base metal by anodization, it is more important than anything else to properly set various functions such as anodization treatment voltage, time, and purity of the base material metal as the most core variables of the anodization.

Even in stainless steel, there are various types of alloys depending on the component content, and the desired anodization treatment conditions may vary depending on the component content so that the component content of the base material to be treated is said to be very important.

The present inventor found out conditions for forming an anodized film with improved super-hydrophobicity and corrosion inhibition efficiency by optimizing anodization treatment time and voltage using SUS 304 series stainless steel as a base material, and completed the present disclosure.

Prior Art Document

Patent Document

• • (Patent Document 1) Korean Patent No. 10-1832059

SUMMARY

An object of the present disclosure is to provide a method for forming a corrosion resistant oxide film on a stainless steel surface.

Another object of the present disclosure is to provide stainless steel on which a superhydrophobic anodized film manufactured by the above-described method.

Another object of the present disclosure is to provide stainless steel on which a corrosion resistant anodized film manufactured by the above-described method.

Another object of the present disclosure is to provide a medical device, a maritime transportation means, a ground transportation means, and an air transportation means including stainless steel manufactured by the above-described method.

In order to achieve the above-described object,

• • the present disclosure provides a method for forming a corrosion resistant oxide film on a stainless steel surface, the method including the steps of:
  • washing and drying the stainless steel surface (step 1);
  • performing anodization treatment at an applied voltage of 65 to 75 V for 2.5 to 3.5 hours to form an anodized film on the stainless steel surface (step 2);
  • performing plasma treatment to remove organic residues and make the surface of the anodized film hydrophilic (step 3); and
  • coating it with a hydrophobic coating agent capable of performing self-assembled monolayer (SAM) coating (Step 4).

Stainless steel is applicable to all types of various stainless steel, but may be preferably SUS 304 or SUS 304L.

In the anodization treatment of the step 2, it may be possible to use, as an electrolyte, ethylene glycol containing 0.05 to 0.15M NH4F and 0.05 to 0.15M water, preferably ethylene glycol containing 0.08 to 0.12M NH4F and 0.08 to 0.12M water, and more preferably ethylene glycol containing 0.09 to 0.11M NH4F and 0.09 to 0.11M water. As an example in the present disclosure, ethylene glycol containing 0.1M NH4F and 0.1M water was used as the electrolyte, but is not limited thereto.

In the step 2, anodization treatment may be performed on the washed stainless steel at 65 to 75 V for 2.5 to 3.5 hours, preferably at 68 to 72 V for 2.8 to 3.2 hours, and more preferably at 69 to 71 V for 2.9 to 3.1 hours.

In order to realize super-hydrophobicity of a contact angle of 1600 or more and corrosion inhibition efficiency of 90% or more, anodization treatment may be performed at 69 to 71 V for 2.9 to 3.1 hours, and if it is out of these conditions, there may be problems in that super-hydrophobicity is not implemented or corrosion inhibition efficiency is low.

As the hydrophobic coating agent capable of performing SAM coating in the step 4, a perfluoroalkylsilane having a surface energy of 6 mJ/m² to 20 mJ/m² and a fluorocarbon chain number of 1 to 20, an alkylsilane having 1 to 20 carbon atoms, and the like may be used, and for example, 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), trichlorooctylsilane (OTS), octadecyltrichlorosilane (ODTS), and the like may be used.

Furthermore, the present disclosure provides stainless steel on which a superhydrophobic anodized film manufactured by the above-described method is formed.

Furthermore, the present disclosure provides stainless steel on which a corrosion resistant anodized film manufactured by the above-described method is formed.

Furthermore, the present disclosure provides a medical device, a marine transportation means, a ground transportation means, and an air transportation means including stainless steel manufactured by the above-described method.

The method for forming a corrosion resistant oxide film on a stainless steel surface according to the present disclosure can form a uniform porous oxide film even without a conventional pre-patterning process for forming a uniform porous oxide film, and further has effects of remarkably excellent super-hydrophobicity and corrosion resistance even without a pore expansion step after conventional anodization treatment, and thus manufacturing costs can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is EDS measurement results for three samples obtained after performing the steps 1 to 3 of Examples.

FIG. 2 is images, observed with an FE-SEM, of the surface shapes of the oxide films formed on the three sample surfaces obtained after performing the steps 1 to 3 in the Examples.

FIG. 3 is results of measuring the contact angles of samples (SAM coating not performed) subjected to only the steps 1 to 3 in the Examples.

FIG. 4 is results of measuring the contact angles of samples (SAM coating performed) subjected to all the steps 1 to 4 in the Examples.

FIG. 5 is a drawing showing potentiodynamic polarization curves of samples subjected to all the steps 1 to 4 in the Examples.

DETAILED DESCRIPTION

Hereinafter, the present disclosure will be described in more detail by the following Examples. However, the following Examples are only to illustrate the present disclosure, and the content of the present disclosure is not limited by the following Examples.

<Examples 1-1 to 1-3> Anodization Treatment of Stainless Steel (SUS 304)

Step 1: Preparation of Stainless Steel (SUS 304) Substrate

Stainless steel (SUS 304) with a size of 20 mm×30 mm×0.5 mm was used. For the purpose of removal of surface foreign substances and cleaning of the surface, it was immersed in ethanol and acetone to perform ultrasonic washing and washed once more using distilled water, and then dried.

Components of stainless steel such as SUS 304 and the like are shown below. For reference, there is a considerable difference in the optimal anodization treatment conditions for forming a super-hydrophilic oxide film depending on the type of metal and the type of alloy, and the optimum anodization treatment conditions for forming the super-hydrophilic oxide film by focusing on SUS 304 in the present disclosure were found.

Classification	C	Si	Mn	P	S	Ni	Cr	Mo
SUS 304	0.08	1.00	2.00	0.045	0.030	8.00 to	18.00 to	—
	or less	or less	or less	or less	or less	10.50	20.00
SUS 304L	0.030	1.00	2.00	0.045	0.030	9.00 to	18.00 to	—
	or less	or less	or less	or less	or less	13.00	20.00
SUS 316	0.08	1.00	2.00	0.045	0.030	10.00 to	16.00 to	2.00 to
	or less	or less	or less	or less	or less	14.00	18.00	3.00
SUS 316L	0.030	1.00	2.00	0.045	0.030	12.00 to	16.00 to	2.00 to
	or less	or less	or less	or less	or less	15.00	18.00	3.00

Step 2: Anodization Treatment

In the anodization process, stainless steel was used for the anode and platinum was used for the cathode, and the distance between the electrodes was maintained at 5 cm. An electrolyte solution to which 0.1 M NH4F and 0.1 M H2O were added based on an ethylene glycol solution was maintained at a temperature of 0° C. using a double jacketed beaker and a water cooling-type cooler. The anodization treatment was proceeded at applied voltage of 30 V (Example 1-1), 50 V (Example 1-2), and 70 V (Example 1-3) for 3 hours, and after anodization, the specimen was washed with distilled water and dried.

Step 3: Plasma Treatment

After removing organic residues from the surface of the specimen with oxygen plasma for 15 minutes using a plasma device and making it hydrophilic, the hydrophilic specimen was dried at 150° C. for 10 minutes using a heating stirrer in air. The dried specimen was subjected to plasma treatment for 15 minutes under plasma treatment conditions of 200 W, 50 KHz, O₂ 50 sccm, and an RIE mode.

Step 4: Self-Assembled Monolayer (SAM) Coating

In order to impart super water-repellent properties to the plasma treatment-completed anodized sample, self-assembled monolayer (SAM) coating was performed using a 1H, 1H, 2H, 2H-Perfluorodecyltrichlorosilane (FDTS) solution, which is a material with low surface energy.

<Experimental Example 1> Evaluation of Oxide Film Formation Using Energy Dispersive Spectroscopy (EDS)

EDS (model name: X-MAX, manufacturer: OXFORD) measurement was performed on three stainless steel (SUS 304) samples obtained after performing the steps 1 to 3 (SAM coating not performed) in the Example to evaluate whether or not an oxide film was formed, and the results are shown in FIG. 1.

FIG. 1 is EDS measurement results for three samples obtained after performing the steps 1 to 3 of Examples.

As shown in FIG. 1, oxygen and iron are shown as main components after anodization, and chromium, manganese, nickel, etc. other than those were detected, and carbon corresponds to noise due to the influence on a carbon tape for fixing the sample to the stage. Through this result, it can be confirmed that an oxide film is formed on the stainless steel surface.

<Experimental Example 2> Surface Shape Observation Using Field Emission Scanning Electron Microscope (FE-SEM)

In the Examples, the surface shapes of the oxide films formed on the stainless steel (SUS 304) surfaces subjected to the steps 1 to 3 (SAM coating not performed) was observed using an FE-SEM (model name: MIRA 3 LMH In-Beam Detector, manufacturer: TESCAN), and the results are shown in FIG. 2.

Specifically, in order to observe the surface shapes of the samples, the samples were cut and fixed to the stage with the carbon tape, and since the structure made by anodization is a non-conductive oxide, platinum coating was performed for 40 seconds and then observed.

FIG. 2 is images, observed with an FE-SEM, of the surface shapes of the oxide films formed on the three sample surfaces obtained after performing the steps 1 to 3 in the Examples.

As shown in FIG. 2, (a) and (b) are images at applied voltages of 30 V and 50 V, and pores were not observed on the surfaces since a barrier-type oxide film is formed in (a) and (b) of FIG. 2. However, it could be observed in (c) of FIG. 2 that a porous structure is formed differently from the conditions of (a) and (b) of FIG. 2 above.

Table 1 shows the results of measuring diameters D_p of pores generated on the surfaces after anodization, interpore distances D_int, and solid fractions using the FE-SEM images of FIG. 2. The solid fractions were calculated from the solid-liquid ratios by Equation 1 below.

? = ? - 2 ⁢ π 3 ⁢ r 2 ? [ Equation ⁢ 1 ] ? indicates text missing or illegible when filed

• • f_SL: Solid fraction
  • a: Interpore distance
  • r: Pore radius

	TABLE 1
	Dp (nm)	Dint (nm)	Solid fraction

	30 V	None	None	None
	50 V	None	None	None
	70 V	115.79 ± 17.46	137.06 ± 15.31	0.355

As shown in Table 1, the sample ((c) of FIG. 2) at a voltage condition of 70 V in FIG. 2 had a pore diameter on the porous surface of 115.59 nm, an interpore distance of 137.06 nm, and a solid fraction of 0.355. It was observed through this that a densely formed film was formed without the existence of empty spaces such as internal pores in the anodized film under the voltage conditions of 30 V and 50 V, and a porous film with somewhat regular pores was formed under the voltage condition of 70 V. The solid fraction means roughness rate.

<Experimental Example 3> Contact Angle Evaluation

The contact angles were measured to find out the surface wettabilities of the samples subjected to only the steps 1 to 3 and the samples subjected to all the steps 1 to 4 in the Examples, and the results are shown in FIGS. 3 and 4 and Table 2.

Specifically, 3.5 μl of distilled water was used as a standard liquid during the measurement. After a droplet was dropped on the surfaces, the contact angles were measured after a time of 5 seconds, and the measurement was performed 10 times per specimen.

FIG. 3 is results of measuring the contact angles of samples (SAM coating not performed) subjected to only the steps 1 to 3 in the Examples.

FIG. 4 is results of measuring the contact angles of samples (SAM coating performed) subjected to all the steps 1 to 4 in the Examples.

The results of FIGS. 3 and 4 are summarized and shown in Table 2 below.

	TABLE 2
	Before SAM coating (°)	After SAM coating (°)

30 V	59.26 ± 4.45	115.02 ± 2.99
50 V	44.87 ± 4.46	119.69 ± 1.78
70 V	17.04 ± 3.14	161.80 ± 1.00

As shown in Table 2, in the case of the samples on which SAM coating was not performed, it was confirmed that the superhydrophilicity appeared to be 59.26° at an applied voltage of 30 V and the superhydrophilicity appeared to be 44.87° at an applied voltage of 50 V, whereas the superhydrophilicity appeared to be 17.04° at an applied voltage of 70 V. In the case of the samples on which SAM coating was performed with an FDTS solution with low surface energy, it was confirmed that the super water repellency appeared to be 115.020 at an applied voltage of 30 V and the super water repellency appeared to be 119.690 at an applied voltage of 50 V, whereas the super water repellency appeared to be 161.8° at an applied voltage of 70 V.

Since a shape in which air supports water droplets may be formed between pores or solid surfaces due to the coating, a super water-repellent surface is formed on a surface with a porous oxide film.

<Experimental Example 4> Corrosion Resistance Evaluation

The corrosion resistance of the samples subjected to all the steps 1 to 4 in the Examples was evaluated, and the results are shown in FIG. 5 and Table 3.

Specifically, corrosion resistance was conducted in a 3.5 wt % NaCl solution at room temperature by a potentiodynamic polarization (PDP) test, which is an electrochemical method. The measurement was performed after immersing the samples in the 3.5 wt. % NaCl solution at room temperature for 1 hour prior to proceeding the analysis test. In the polarization test, a three-electrode system included the samples as a working electrode, platinum (Pt) as a counter electrode, and a silver/silver chloride (Ag/AgCl) electrode as a reference electrode. Corrosion resistance was evaluated through electrochemical property analysis under the measurement conditions of a voltage range of −500 mV to +14,000 mV (vs. Ag/AgCl) and a scanning rate of 1 mV/sec.

FIG. 5 is a drawing showing potentiodynamic polarization curves of samples subjected to all the steps 1 to 4 in the Examples.

The results of FIG. 5 are summarized and shown in Table 3 below.

	TABLE 3
	Ecorr (mV)	Icorr (A/cm2)	IE (%)

	Bare SUS304	−37.8	1.12 × 10−8	0
	30 V	112	5.04 × 10−8	77.58
	50 V	199	9.97 × 10−8	88.67
	70 V	254	1.19 × 10−9	90.50
	Ecorr: Corrosion potential
	Icorr: Corrosion current density representing loss of mass
	IE: Inhibition efficiency of treated samples of the Examples compared to bare SUS 304

As shown in Table 3, it was confirmed that the corrosion potential E_corr was moved in the positive direction in structures on which the surface modifications of an applied voltage of 30 V (112 mV), an applied voltage of 50 V (199 mV), and an applied voltage of 70 V (199 mV) were performed compared to bare stainless steel (bare SUS304, −37.8 mV). In addition, it could be confirmed that the corrosion current densities I_corr decreased as the applied voltage increased in structures on which the surface modifications of an applied voltage of 30 V (5.04×10⁻⁸ A/cm²), an applied voltage of 50 V (9.97×10⁻⁸ A/cm²), and an applied voltage of 70 V (1.19×10⁻⁹ A/cm²) were performed compared to bare stainless steel (1.12×10⁻⁸ A/cm²). Corrosion current was used to evaluate the corrosion inhibition efficiencies using the I_corr values since the mass loss reaction is directly related to corrosion. The corrosion inhibition efficiencies calculated using the corrosion current densities were calculated by Equation 2 below. The most important indices, corrosion inhibition efficiencies, were shown to be 77.58% at 30 V, 88.67% at 50 V, and 90.50% at 70 V. These are results indicating that corrosion resistance may be remarkably improved when SAM coating is performed after implementing the surface shapes through anodization treatment.

That is, since the porous oxide film has hydrophilicity, the SAM coating was performed with FDTS for the realization of water repellency to trap air in the pores to improve the corrosion prevention efficiency. It is considered that corrosion resistance can be improved since the porous structure and thick oxide film can trap a lot of air in the pores.

IE ⁢ ( % ) = ( i - i 0 i ) × 100 [ Equation ⁢ 2 ]

• • i: Corrosion current density of samples subjected to all the steps 1 to 4 in the Examples
  • i₀: Corrosion current density of bare SUS 304
  • IE: Corrosion inhibition efficiency of treated samples of the Examples compared to bare SUS 304

<Experimental Example 5> Evaluation of Optimal Anodization Conditions (Time and Voltage) for Realization of Super Water Repellency

Through Experimental Examples 1 to 4, it was confirmed that corrosion resistance was the most excellent when the samples were treated under anodization treatment conditions of 3 hours and 70 V. Therefore, the optimal conditions were found out based on the anodization treatment conditions of 3 hours and 70 V in this Experimental Example 5, and the results are shown in Tables 4 and 5. Samples (SAM coating performed) were prepared in the same manner as in the Examples except that the anodization treatment time and voltage were varied.

	TABLE 4
	Time (h)	Voltage (V)	Contact angle (°)	IE (%)

Example 2-1	2.8	70	119.28 ± 2.16	72.97
Example 2-2	2.9		161.26 ± 2.14	90.37
Example 2-3	3.0		161.80 ± 1.00	90.50
(=Example 1-3)
Example 2-4	3.1		160.87 ± 1.58	90.41
Example 2-5	3.2		120.69 ± 0.89	75.81

	TABLE 5
	Time (h)	Voltage (V)	Contact angle (°)	IE (%)

Example 3-1	3.0	68	120.12 ± 2.47	77.28
Example 3-2		69	160.29 ± 1.25	90.41
Example 3-3		70	161.80 ± 1.00	90.50
(=Example 1-3)
Example 3-4		71	160.11 ± 1.36	90.48
Example 3-5		72	119.47 ± 1.89	74.29

As shown in Tables 4 and 5, the most excellent results could be confirmed at an anodization treatment time of 2.9 to 3.1 hours and an applied voltage of 69 to 71 V in terms of super water repellency and corrosion resistance.

So far, the present disclosure has been looked at with respect to its preferred embodiments. Those skilled in the art to which the present disclosure pertains will be able to understand that the present disclosure can be implemented in a modified form without departing from the essential characteristics of the present disclosure. Therefore, the disclosed embodiments should be considered from an explanatory point of view of view rather than a limiting point of view. The scope of the present disclosure is shown in the claims rather than the foregoing description, and all differences within the scope equivalent thereto will be construed as being included in the present disclosure.