Electro-Wetting Behavior of Sodium Chloride Aqueous Solution on Hydrophobic Surfaces of Stainless Steel and Its Influence on Polarization
2017 Volume 58 Issue 12 Pages 1670-1678
Details
2017 Volume 58 Issue 12 Pages 1670-1678
Hydrophobic polyethylene (PE) and perfluorodecyltriethoxysilane (PFDS) were deposited on stainless steels and the contact angle of aqueous NaCl solution on these surfaces was measured before and after applying different currents through a Pt wire. The polarization curves on some surfaces were measured and their relationship to electro-wetting behaviors was determined. Negative rather than positive current greatly reduced the contact angles on both the plane surface of polished SUS304 steel and the rough surface of sputter-etched SUS316 steel (SUS316SE), which were deposited with PE and PFDS, respectively. On rough surfaces, the negative current-induced reduction in contact angle was greater on SUS316SE-PE than on SUS316SE-PFDS specimens. Prior cathode polarization reduced the pitting potential on SUS316SE-PE specimens, but had no effect on 316SE-PFDS specimens. This difference was likely related to the difference in solution invasion into the root of protrusions, from which chromium was depleted by sputter etching.
Phase contact between solids and liquids causes both wetting and corrosion at the interface. The wetting behavior has been found to be based on the surface and interface energies (or tensions) of solids and liquids1–3). These energies depend on the compositions of both the solid and liquid and the surface structure of the solid. Hydrophobic surfaces have been applied to cloth, paper, glass, wood and concrete, to prevent surface contamination, oxidation, and corrosion, as well as the accumulation of rain and snow4–7). In contrast, the corrosion of metals is an electrochemical process involving the solution and air, with anodic and cathodic reactions. Corrosion reactions may be prevented or retarded by various methods, including barriers that block contact between the corrosive solution and the metal or using a power supply, scarified metal or photoelectric material to alter the electrical charge at the metal/solution interface.
The difference in potential between metal and aqueous solution generally results in the formation of an electrical double layer (EDL) at their interface. This EDL model8,9) describes the diffusive distribution of attracted ions near the surface of the solid, enriched with electrical charges. When the EDL is artificially charged, the increased interface energy generally reduces contact angles (CA), a phenomenon called pseudo electro-wetting10,11). Our previous study assessed the wetting behavior of aqueous NaCl droplets on SUS304 stainless steel before and after applying current flow11). Altering the charge state at the interface should also affect corrosion of the metal. Because a hydrophobic surface that repels solution may protect metals from corrosion, it is important to clarify how the EDL charge affects the wettability of aqueous solutions and the corrosion occurring at their interface. To date, however, this relationship has not been fully investigated.
In this study, stainless steels were treated to make their surfaces hydrophobic, and the wettability of an aqueous solution on these surfaces in response to the application of current was investigated. Furthermore, the polarization behavior of such steels was measured and its relationship to electro-wetting behavior was determined.
Commercially supplied stainless steel plates (solution-treated; thickness 2.0 mm) of SUS304 (chemical composition: C, 0.05 mass%; Si, 0.50%; Mn, 1.11%; P, 0.026%; S, 0.003%; Cr, 18.07%; Ni, 8.04%; Fe, balanced) and SUS316 (chemical composition: C, 0.05%; Si, 0.40%; Mn, 0.82%; P, 0.027%; S, 0.001%; Cr, 17.33%; Ni, 10.24%; Mo, 2.1%; Fe, balanced) were cut into 20 mm × 20 mm × 2 mm pieces. The pieces were polished with emery paper and with alumina powder of mean diameter 0.05 µm, and ultrasonically cleaned in acetone. To obtain a rough surface, some SUS316 pieces were sputter etched in an RF magnetron sputtering apparatus (Sanvac Co.: SP300 (M)) for 3.6 ks at a sputter power of 250 W in the presence of argon gas (purity, 99.999%) at a pressure of 0.67 Pa. SUS316 steel rather than SU304 steel was chosen for sputter etching because the protrusions that formed on the former were much denser and finer than those formed on the latter.
The surfaces of these steel pieces were made hydrophobic by vapor deposition of an organic compound, 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (C16H19F17O3Si; PFDS; boiling point, 482～503 K)12). Briefly, the steel pieces and PFDS liquid were placed in a glass beaker, which was sealed with aluminum foil and heated for 1.8 ks in a furnace at a constant temperature of 423 K. For comparison, and to simulate the adhesion of organic contaminants in air, polyethylene (PE) film was naturally deposited onto steel plates by placing these plates in PE bags for more than 1 week. Polished steel pieces coated with PE and PFDS in the absence of sputter etching were designated 304-PE and 304-PFDS, respectively, whereas polished, sputter-etched steel pieces coated with PE and PFDS were designated 316SE-PE and 316SE-PFDS, respectively.
The wettability of liquid on the specimen surface was investigated using an artificial wetting system equipped with a digital microscope (VT-101, 3R Systems Co.) (Fig. 1). A droplet of 3.5% NaCl aqueous solution, approximately 10 μL in volume, was dropped onto the surface using a pipette with a hole diameter of 0.2 mm. The side-view of the droplet was recorded before, during and after a flow of current through a platinum (Pt) wire of diameter 1.0 mm. The Pt wire was placed 1.0～2.0 mm from the specimen surface. Two types of current were applied, negative current flowing from the power supplier to the Pt wire, and positive current flowing from the power supplier to the specimen. Current, which was controlled using a potentiostat/galvanostat (Hokuto: HAB-151), was applied for 10～15 s. The CA of the droplet was measured after withdrawing the Pt electrode by drawing a line tangential to the three-phases line. In addition, the voltage difference between the specimen and the Pt wire was recorded during current flow of ～60 seconds.
Apparatus used to apply current to steel surfaces through droplets of aqueous NaCl solution and a Pt wire.
Specimens were polarized in a corrosion cell with air-saturated 3.5% NaCl aqueous solution, using the above potentiostat/galvanostat, with a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Because the wettability of solution on a specimen is dependent on the applied current, which may affect polarization behavior, two types of potential sweep were performed. The first was initiated from the cathodic side at −600 mV (vs. SCE) and the other from the open circuit potential (OCP).
All of the specimens, except for SUS316 steel after sputter etching (316SE), had flat morphology. Figure 2 shows the surface morphology of 316SE specimens before and after PFDS deposition. Micro-fine protrusions with nanometer-scaled tips were observed on the surfaces. The major mechanisms underlying the formation of protrusions were described in our previous reports13–18). EDX and XPS analyses found that more chromium was present on the side surfaces of protrusions than on the matrix18). Thus, chromium is depleted inside and around the root of protrusions, which should weaken the corrosion resistance of the steel. The arithmetical mean roughness of a 30 µm × 30 µm area was 306 nm by atomic force microscopy and 231 nm by laser microscopy. No obvious morphologic difference was observed among 316SE surfaces before and after the deposition of PE or PFDS, indicating that these deposits were very thin.
Surface structure of SUS316 steel after sputter etching at 250 W for 3.6 ks without (a), (a') and with (b), (b') PFDS coating. (a), (b) are top views and (a'), (b') are 45° inclined side views.
Figure 3 shows the XPS C 1 s spectra of the sputter-etched specimens of 316-SE with naturally deposited PE (a) and thermally deposited PFDS (b). Functional groups detected on the 316SE-PE surface included C-C, C-O and C=O groups, similar to those observed following the adsorption of organic contaminants in air13,19). The F-C-F functional group was also detected on 316SE-PFDS specimens, indicating the adhesion of PFDS molecules. This finding was similar to the results observed on flat 316-PE and 316-PFDS surfaces12,13). The deposited PFDS is likely a monomolecular layer of ～2 nm thickness12,20), with the deposited PE also being several nanometers thick.
C 1 s spectra of SUS316 steel after sputter etching with naturally deposited PE (a) and thermal vapor-deposited PFDS (b).
Figure 4 shows an example of the CAs of aqueous solution of 3.5% NaCl before and after application of positive currents of 0.5, 0.75 and 1.0 mA to the 304-PE and 304-PFDS plane surfaces. The CA without current ranged from 75～85°. The voltage between the specimen and the Pt wire gradually increased, from 3.0 to 3.3 V, as positive current increased. Gas evolution occurred on either specimen or Pt wire. The CAs of the solution decreased slightly after application of current, indicating that wettability was not very susceptible to positive current. In the absence of current, the CA of the aqueous 3.5% NaCl solution was almost identical to that of a droplet of pure water, indicating that the differential adsorption of Cl- on the solid surface had little effect on wettability. Relative to water, the sodium and chloride ions can enhance current flow in the solution.
Contact angles of aqueous 3.5% NaCl droplets before and after applying positive current (0.5, 0.75, 1.0 mA) to plane surfaces of SUS304 coated with PE or PFDS.
Figures 5 and 6 show examples of the CAs before and after applying negative currents, ranging from 0.001 to 5.0 mA, to the surface of 304-PE and 304-PFDS. In the case of 304-PE, the standard deviations of CAs before and after negative current flow were 5.1 and 2.5～9.0°, respectively. The CAs without current were near 85° and were not affected by application of 0.001 mA. CAs gradually decreased as negative current increased from 0.001 to 0.5 mA. A sharp reduction of CA, to 48°, occurred at 0.5 mA, with the angle remaining stable at 0.5～5.0 mA. In the case of 304-PFDS, the CAs without current are close to 90°. The standard deviation of CAs after current flow ranged from 2.5～10.3°. In contrast to 304-PE (Fig. 5), the CAs on 304-PFDS varied little in response to currents ranging from 0.001 to 0.01 mA. At currents higher than 0.05 mA, however, CAs dropped sharply, by about 38°, lower than on 304-PE surfaces.
Contact angles of aqueous 3.5% NaCl droplets before and after applying negative current to plane surfaces of SUS304 coated with PE. The inset photos show droplets with an inserted Pt wire before and after applying a negative current of 0.5 mA. σ indicates the standard deviation of the contact angles before applying current.
Contact angles of aqueous 3.5% NaCl droplets before and after applying negative current to plane surfaces of SUS304 coated with PFDS. The inset photos show droplets with an inserted Pt wire before and after applying a negative current of 0.5 mA. σ indicates the standard deviation of the contact angles before applying current.
Figure 7 shows changes in voltage over time while applying different magnitudes of negative current to 304-PE specimen, as well as the relationship between voltage and current on 304-PE and 304-PFDS specimens. As shown in Fig. 7(a), the voltage at each applied current increased for the first 10 seconds and remained almost constant for up to 60 seconds. These findings indicate that charging of the EDL for 10 seconds could determine stable voltage during the following ～60 seconds. Voltage on the 304-PE surface increased with increasing current (Fig. 7 (b)). In applying negative currents, cathodic reactions should occur on the specimen surface, resulting in the reduction of passive films (oxide or hydroxide), the consumption of diluted oxygen, and the generation of small amounts of hydrogen gas, with the latter occurring only at high potential regions in neutral solution. During current flow, some PE molecules may leave the surface. Indeed, we showed that PE adhesion to metal surfaces is weak and that PE can be easily removed by ultrasound cleaning, UV irradiation and high temperature12). On the 304-PFDS surfaces (Fig. 7 (c)), voltages were much greater, by about 4.0 V, in response to currents of 0.001 mA and 0.005 mA. Perhaps, at these two current levels, the high stability of PFDS and its strong adhesion made charging difficult. The adhesion of PFDS to metal surfaces is very strong and difficult to remove by ultrasound cleaning, UV irradiation and high temperature12). The relatively large EDLs of integrated PFDS deposits suggest their high condenser capacitance. Voltage dropped sharply, to near 1.8 V, at a current of 0.01 mA, but increased gradually as current was increased to 0.1 mA. The sharp drop in voltage may have been due to partial damage (breakdown) of PFDS deposits. In addition, the exposure of metal after the removal of PFDS should have resulted in high wettability.
Variation of voltage over time when applying different negative currents to SUS304 coated with PE (a) and the relationship between voltage and current on SUS304 coated with PE (b) and with PFDS (c).
Figure 8 shows the specimen surfaces after application of positive or negative current of 1.0 mA for 10～15 seconds to 304-PE and 304-PFDS specimens. Pits or local damage appeared on the surfaces after positive current flow, while substantial wide reduction or damage seemed occur to the passive film after negative current flow. In addition, circle marks resulting from the formation of hydrogen bubbles (Fig. 8(d)) indicates partial damage to PFDS film.
Specimen surfaces after application of 1.0 mA positive (a), (b) or negative (c), (d) current for 10～15 seconds to 304-PE (a), (c) and 304-PFDS (b), (d).
Passive films on stainless steel are only several nanometers thick and consist mostly of chromium oxide and hydroxide and small amounts of iron oxide and hydroxide21,22). These films can act as semiconductors22). Layers of passive film and deposited organic molecules are present at steel/solution interfaces within the EDL condenser. The variations in CA shown in Figs. 4～6 are firstly considered due to increases in interface energy caused by changes in the electric charge of EDLs. The variation of CA can be expressed by the integral version of Lippmann's equation10,11):
| \[ \cos \theta (\Phi) = \cos \theta + (\varepsilon_{0} \varepsilon_{\rm r}A/2{\rm d} \cdot \gamma_{\rm LV}){\rm \Phi}^2 \] | (1) |
Here, $\theta$ (Φ) and $\theta$ denote CAs with and without application of voltage (Φ), respectively; ε0 and εr represent the permittivity of free space and passive film, respectively, with PE or PFDS; and A is the area of the condenser (solid-liquid interface area). Accordingly, the decrease in CA ($\theta$) following the application of voltage (current) can be explained as a relationship between increasing voltage and decreasing CA. It should be noted that the voltage ($\varPhi$) described above is that applied to the condenser (EDL), not that between the specimen and the Pt wire (V), although they generally correlate. After a short charging time, the condenser will maintain stable voltage despite excess current, inducing an electrochemical reaction. Regardless of the effect of voltage, however, the desorption of PE molecules should also affect the CA, altering the capacitance of the condenser and exposing the hydrophilic substrate. The larger decrement of CA by negative than by positive current was likely due to the wide reduction of the passive film by negative current when greater current is applied. The exposed bare metal is generally hydrophilic11,23).
3.3 Wettability on rough surfaces of SUS316 steel with current flowFigure 9 shows an example of CAs before and after applying positive current to the rough surfaces of 316SE-PE and 316SE-PFDS samples. Similar to results observed on plane surfaces, CA on rough surfaces changed slightly in response to positive current. The high CA observed before applying current to rough-surfaced specimens was due to both the rough surface and the deposition of hydrophobic PE or PFDS, suggesting the presence of many air pockets below the solution. Figure 10 shows the specimen surfaces after application of positive current of 1.0 mA for 10～15 seconds. More pits appeared on 316SE-PE than on 316SE-PFDS samples.
Contact angles of aqueous 3.5% NaCl droplets before and after applying positive current (0.5, 0.75, 1.0 mA) to rough surfaces of SUS316SE coated with PE or PFDS.
Specimen surfaces after application of 1.0 mA positive current for 10～15 seconds to 316SE-PE (a) and 316SE-PFDS (b).
Figures 11 and 12 show examples of CAs before and after applying negative current to the rough surfaces of 316SE-PE and 316SE-PFDS. Surface examination after applying current of 1.0 mA found no obvious changes in the protrusions, except for water stains. In case of 316SE-PE (Fig. 11), the standard deviations of CAs after current flow were 1.2～11.9°. As current was increased from 0.001 to 0.2 mA, CA decreased from about 135° to about 10°, with a further increase in current, from 0.2 to 1.0 mA, further reducing CA to near 0°. The largest decrement was 133°. The finding, that CAs after applying current were much smaller with rough than with plane surfaces of 304-PE, suggested that the solution had thoroughly invaded the substrate at the roots of protrusions. Moreover, the surface area of rough 304-PE specimens was much larger than that of plane 304-PE specimens, resulting in a much larger electrical charge on the former. The disappearance of part of the PE surface may have exposed the hydrophilic metal; or PE may not have fully deposited onto the root of protrusions, leaving the substrate bare. The resulting increase in EDL charge may have enabled water to easily invade the roots of protrusions. Moreover, during current flow some oxygen in the air pockets may have been consumed by the cathodic reduction reaction. In the case of 316SE-PFDS (Fig. 12), the standard deviation of CAs after current flow ranged from 1.7～10.5°. The high CA of about 140° in the absence of current indicates that the area of contact between the solution and the protrusions was small, with air pockets at the protrusion roots, similar to results observed with 316SE-PE. As current was increased from 0.001 to 3.0 mA, the CA gradually decreased, to about 80°, a much smaller decrease than observed with 316SE-PE. The CA after applying current was close to that observed on the plane surface of 304-PFDS. These findings suggested that only a fraction of solution had invaded the roots of protrusions.
Contact angles of aqueous 3.5% NaCl droplets before and after applying negative current to rough surfaces of SUS316SE coated with PE. The inset photos show droplets with an inserted Pt wire before and after applying a negative current of 0.5 mA. σ indicates the standard deviation of the contact angles before applying current.
Contact angles of aqueous 3.5% NaCl droplets before and after applying negative current to rough surface of SUS316SE coated with PFDS. The inset photos show droplets with an inserted Pt wire before and after applying a negative current of 0.5 mA. σ indicates the standard deviation of the contact angles before applying current.
The above measurements show the differences in CA on various surfaces after applying a negative current greater than 1.0 mA. The CA decrement was slightly smaller on 304-PFDS than on 304-PE, indicating that homogeneous PFDS deposits are more stable than PE deposits. However, the CA decrement was much larger on 316SE-PE than on 304-PE, a difference likely due to the larger surface area of the former and the less than complete deposition of PE on the roots of protrusions. A comparison of the rough, highly hydrophobic surfaces of 316SE-PE and 316SE-PFDS showed that the CA decrement was much smaller on 316SE-PFDS than on 316SE-PE, a difference likely due to the more complete deposition of PFDS than of PE on the roots of protrusions. These reductions in CA will affect the contact state of corrosive liquid with steel, which should largely influence the corrosion behavior of the latter.
Figure 13 shows the relationship between voltage and negative current on 316SE-PE and 316SE-PFDS surfaces. The voltage levels on 316SE-PE surfaces (Fig. 13 (a)) were similar to those observed with 304-PE, gradually increasing as current increased. However, there was a large voltage increase at 0.02 mA, which corresponded to a significant reduction in CA. This indicates an increase in impedance (resistance), perhaps due to the rearrangement of PE molecules on specimen surfaces accompanied by a reduction of passive film at this current. Voltages sharply decreased at currents of 0.03 and 0.04 mA, corresponding to large CA reductions (Fig. 11), which may be related to the disappearance of PE from the protrusions. On the other hand, on the 316SE-PFDS (Fig. 13 (b)), voltages gradually increased. These findings suggest that PFDS is less compact on protrusions than on plane SUS304, but that some PFDS molecules were deposited on the roots of protrusions. Therefore, the hydrophobic roots obstructed the invasion of aqueous solution, which may be the difference between PFDS and PE deposits.
Relationships between voltage and current on SUS316SE coated with PE (a) and with PFDS (b).
Figure 14 shows several examples of polarization curves of SUS316 and SUS316SE steels with PE and PFDS, measured by sweeping potential from either the open circuit potential (OCP) or a cathodic side potential of −600 mV vs. SCE to the noble side. These measurements were performed to clarify the relationships between polarization behavior and electro-wetting phenomena while current flows in different directions. When polarization was started from the OCP, only positive current was applied, resulting in a small change in CA of water during polarization. In the case of sputter-etched specimens, the solution comes in contact with only the higher part of the protrusions but leaves the roots of protrusions dry with air pockets, according to the Cassie-Baxter principle. In contrast, when polarization was started from −600 mV, negative current flows until the OCP and is accompanied by a reduction in CA. Because this reduction cannot be recovered during subsequent anode polarization, the solution should maintain contact with the entire surface of the protrusions, including their roots, during anode polarization.
Polarization curves of SUS316 and SUS316SE without (a), (b) and with (c), (d) PFDS. A cathode start indicates that the potential was swept from −600 mV (vs. SCE), and an OCP start indicates that the potential was swept from OCP to the noble side.
Assessments of flat 316-PE and 316-PFDS specimens showed little difference when polarization was started from the cathode and from the OCP (Fig. 14(a), (b)). In contrast, the passive current densities were lower on 316-PFDS (Fig. 14 (b)) than on 316-PE (Fig. 14 (a)), indicating that PDFS deposits in solution were more stable and had a greater barrier effect than PE deposits. The pitting potential of 316SE-PE with cathode start was about 200 mV (Fig. 14(c)), much lower than the pitting potential without sputter etching (Fig. 14 (a)) or with OCP start (Fig. 14 (c)). These findings indicated that pitting potentials differed on 316SE-PE specimen at different potential starts. In the case of cathode start, the solution has good wettability to the roots of the protrusions, from which chromium is depleted with low corrosion resistance18). In the case of OCP start, however, it is difficult for the solution to invade the Cr-depleted roots of the protrusions, allowing the current to flow through the higher part of the protrusions, which are enriched in chromium18), resulting in a higher pitting potential. The above difference corresponds well with the CAs after polarization. When the 316SE-PE specimen was taken out of solution after polarization, solution was found to be adhered on the steel surface with CA of about 85.4° (OCP start) or 18.5° (−600 mV start). Following polarization, pits appear at the roots of protrusions, allowing invasion of solution. A comparison of the different potential starts of 316SE-PFDS showed that passive current density and pitting potential were almost the same, without having any effect on negative current (Fig. 14(d)). The 316SE-PFDS specimens after either polarization of OCP start or −600 mV start kept high hydrophobicity with CAs over 140°. In addition, a comparison of polarization curves with PFDS (Fig. 14 (d)) and with PE (Fig. 14 (c)) showed that passive current density was lower with PFDS than with PE and that pitting potential in case of cathode start increased from 200 mV to 450 mV. These findings indicate that 316SE-PFDS has almost the same corrosion resistance as SUS316 steel, due to the high barrier effect of PFDS film. This PFDS barrier effect is important, as it enhances the resistance of the chromium-depleted, weakened roots of protrusions. That is, homogeneous deposits of PFDS can protect steel from corrosion, even in cathodic regions with solution invading the roots of protrusions. Following polarization, each of these specimens showed pits larger than protrusions. On the rough 316SE surfaces (Fig. 14 (c), (d)), the passive currents were not largely affected by different potential starts. This should be attributed to the not so largely varied interface area between solution and solid, considering the higher sharp-parts of the protrusions should have been immerged into the solution before polarization.
Figure 15 schematically depicts these observed variations in CA and the initial corrosion behavior of these rough specimens. Figure 15 (a) shows protrusions coated with hydrophobic PE or PFDS, along with solution and underlying air pockets. When current is applied, the CAs of aqueous solution are determined by several factors, such as the direction of current, the surface area of the liquid/solid interface, the thicknesses of the EDL and passive film, the adhesion state of PE or PFDS and the exposure of substrate. Compared with plane surfaces, the sputter-etched surfaces (316SE-PE, 316SE-PFDS) have much wider areas, increasing the charged energy of EDL and allowing solution to invade the roots of protrusions, resulting in a greater reduction in CA. Deposits of PE and PFDS directly increase the thickness of the EDL, reducing the charged energy and CA. When PE or PFDS is partially or completely placed in the current flow, decreasing EDL thickness and exposure of the hydrophilic substrate, CA decreases more. When negative current is applied (Fig. 15 (b)), the passive film is markedly reduced and may be accompanied by extensive damage to or disappearance of PE or PFDS deposits. If positive current is applied, however, the passive film may be only partially damaged, accompanied by the local disappearance of PE or PFDS, reducing CA slightly. This study found that PFDS adhered better than PE to sputter-etched protrusions, with PFDS-coated steel samples showing smaller reductions in CA and greater resistance to corrosion of chromium-depleted roots of protrusions than PE-coated steel samples (Fig. 15 (c)). PFDS is thought to adhere to metals through –Si–O–Mn+ chemical bonds12,24–27), preventing invasion of metal by water molecules or chloride ions that cause corrosion.
Schematic drawing of the electro-wetting and initial corrosion behavior of rough SUS316SE specimens coated with deposits of PE or PFDS.
In this study, two types of hydrophobic coatings, PE and PFDS, were deposited on stainless steels and the contact angles of 3.5% NaCl solution on these surfaces were measured after the application of electrical current through a Pt wire. In addition, the polarization curves on the hydrophobic surfaces of NaCl solution were measured in response to different potentials, and the relationships between electro-wetting and polarization behavior were determined. The main results of this study were:
We thank Mr. K. Yoshida for his assistance with the experiments.
