Materials Transactions, Vol. 60, No. 4 (2019) pp. 531 to 537 ©2019 The Japan Institute of Metals and Materials

Measurement of pH in a Thin Electrolyte Droplet Using the Kelvin Probe Technique

Saya Ajito+, Eiji Tada, Azusa Ooi and Atsushi Nishikata

Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo 152-8552, Japan

In this study, pH measurement was performed in a thin electrolyte droplet with a thickness <1000 µm by the measurement of the equilibrium electrode potential of an Sb/SbxOy electrode used as a pH sensor. The equilibrium potential of the Sb/SbxOy electrode was evaluated by using the Kelvin probe (KP) technique. To investigate the potential response of the Sb electrode in a thin electrolyte droplet, the dependency of the Volta potential difference between the Sb and a gold wire as a KP on electrolyte droplet thickness was measured. The Volta potential difference had a linear response with respect to the buffer solution pH, independent of the droplet thickness. This result indicates that the KP technique, combined with an Sb electrode, is sensitive to the pH of a thin electrolyte droplet of thickness ²50 µm. This pH measurement technique was also applied to measure pH in a corrosion model of steel. The corrosion model consisted of two steel plates in the same plane as the anode and cathode, with a constant current between them. During the corrosion process, the pH value decreased from 6 to 5 near the anode and increased from 6 to 12 at the cathode. The changes in pH measured in the thin electrolyte droplet were in good agreement with the color changes of the solution containing pH indicators. [doi:10.2320/matertrans.M2018385]

(Received December 12, 2018; Accepted January 28, 2019; Published March 25, 2019) Keywords: metal/metal oxide electrode, atmospheric corrosion, steel, Volta potential difference, Nernst equation

1. Introduction Some metal/metal oxide electrodes, such as antimony (Sb)10­14) tungsten (W),8,15­17) and iridium (Ir),9,18) show When steel is used in atmospheric environments, a equilibrium electrode potential that depends on the pH of the water film forms on its surface due to rain and/or dew aqueous solution. Therefore, these metals can be used as pH condensation, and then a corrosion reaction occurs. If the sensors; the corresponding mechanism is described by the condensation droplets have a neutral pH, the steel dissolves Nernst equation. In addition, the electrode potential of some producing Fe2+ ions, and the dissolved oxygen and water metallic electrodes can be expected to respond to pH even molecules are reduced resulting in OH¹ ion formation. when the electrolyte layer is thin. The Kelvin probe (KP) Additionally, the dissolved Fe2+ ions are oxidized to Fe3+ technique uses a contactless reference electrode,19­21) this ions in the droplet containing dissolved oxygen, and the technique should be useful for the pH measurement in a thin Fe3+ ions interact with the OH¹ ions or water molecules. electrolyte layer because it does not change the thickness of Consequently, these reactions may lead to localized changes the electrolyte layer nor the solution chemistry. Therefore, in the pH on the surface of the corroding steel.1) we hypothesized that the KP technique could be applied to Whitman et al. investigated the effect of pH on the pH measurements of thin electrolyte solutions during steel corrosion rate of steel;2) they clarified that a decrease in pH corrosion by measuring the Volta potential difference accelerates the corrosion rate of the steel, whereas an increase between a metal/metal oxide electrode and a KP. in pH moderates the corrosion process. In atmospheric In this study, we investigated whether the KP technique corrosive environments, one can expect significant changes could be combined with an Sb electrode for pH measure- in pH during corrosion, as the electrolyte solution volume ments of a thin electrolyte droplet. Subsequently, this system is extremely small. Therefore, it is important to know the pH was used to estimate the pH near a simulated anode and in a thin electrolyte solution during atmospheric corrosion of cathode during the steel corrosion process in a thin electrolyte metallic materials such as steel. droplet. With regard to pH measurements during the corrosion process of metallic materials, numerous studies have been 2. Experimental Procedure performed on bulk electrolyte solutions with glass-type pH electrodes. In contrast, few studies have examined pH 2.1 Measurement of the Volta potential difference under changes in thin electrolyte solutions. This may be related to an aqueous droplet the difficulty on the setup of the glass-type pH electrodes into The material used as the pH-sensing electrode was a the thin electrolyte solutions. Other probes or techniques such 5mm© 5 mm Sb plate, with a thickness of 1 mm (99.999%, as fluorescence probes,3,4) scanning chemical microscopy Kojundo Chemical Laboratory Co., Ltd., Japan). Figure 1 (SCHEM),5­7) and metal/metal oxide electrodes8,9) have been shows a drawing of the sample of the Sb electrode, which applied to pH measurements of thin electrolyte solutions. was embedded in an epoxy resin after being connected to a However, these techniques are unable to resolve the difficult lead wire. The sample was ground up to P1200 grit with on the pH measurement in the thin electrolyte solutions. waterproof abrasive papers and then degreased ultrasoni- Thus, a different approach is required to measure the pH of cally in ethanol. The surface area exposed to the aqueous these solutions. solution was held constant at 0.20 cm2, and the sample surface, except for the exposed area, was covered with +Graduate Student, Tokyo Institute of Technology. Corresponding author, Teflon tape (NITOFLON No. 903UL., Nitto Denko E-mail: [email protected] Corporation, Japan). 532 S. Ajito, E. Tada, A. Ooi and A. Nishikata

5 mm In this study, the Volta potential difference between the Sb electrode and the KP was measured using a parasitic Epoxy resin 22,23) Exposed area capacitance method, which is an improved null method. The AC current flowing in the circuit between the Sb Antimony plate electrode and the KP was measured with a low-current Tape amplifier (T-IVA001H, Turtle Industry Co., Ltd., Japan). The amplitude of the AC current was calculated against the direct current (DC) bias voltage between the sample and the KP. The DC bias voltages were applied to the circuit with a data 5 mm acquisition (DAQ) device (USB-6215, National Instruments Corporation, USA). A personal computer with a custom- designed program written in LabVIEW (National Instruments Corporation, USA) was used to control the motion of a Wire z-stage (MM-60Z, Chuo Precision Industrial Co., Ltd., Japan), to operate the DAQ device and record the AC signal 25 mm outputs from the lock-in amplifier. Figure 2(b) shows a schematic diagram of the exper- Fig. 1 Schematic drawing of the sample used for pH measurements in a imental setup to measure the height of the solution (i.e., thin electrolyte droplet. droplet thickness) on the sample. This setup was similar to that reported by Stratmann24) and Nishikata:25) the thickness (a) GPIB of the droplet was determined by measuring the potential Computer difference between a 0.5-mm-diamater nickel (Ni) needle % GPIB Lock-in amp. (99.99 , Nilaco Corporation, Japan) and the sample. The height of the Ni needle was controlled with a manual z-stage, XYZ Stage and the potential difference was monitored with a digital Piezo Piezodriver voltmeter (2000 Multimeter, Keithley Instruments, Inc., KP USA). When the Ni needle was positioned above the thin Op-amp. electrolyte droplet on the sample and gradually lowered, a ff Droplet Tape certain potential di erence was measured when the tip of the Bias needle touched the surface of the droplet. When the needle Sb was lowered further and the tip of the needle touched the sample surface, the potential difference had a value of zero. Therefore, the thickness of the thin electrolyte droplet was ff (b) Digital voltmeter measured from the di erence in the position of the Ni needle; Ni wire the measurement accuracy of this procedure was <10 µm.25) The sample was placed in contact with commercial buffer solutions: potassium hydrogen phthalate (pH 4.01); a mixture Droplet Tape of potassium dihydrogen phosphate and disodium hydrogen phosphate (pH 6.86); sodium tetraborate (pH 9.18); and a Sb mixture of sodium hydrogen carbonate and sodium carbonate (pH 10.01) (Kanto Chemical Co., Inc., Japan). The volume Micro z-stage of the solutions was 15 µL, which corresponds to an electrolyte droplet thickness of 1000 µm. The Volta potential Fig. 2 Schematic drawing of the experimental setup for (a) the potential difference measurement of the Sb electrode and the measurement of an Sb electrode and (b) the measurement of electrolyte determination of the droplet thickness on the Sb electrode, thickness. as mentioned above, were carried out during the drying of the electrolyte droplet of various pH solutions. All measurements Figure 2(a) shows a schematic drawing of the potential were conducted under an ambient laboratory atmosphere of measurement system of the Sb electrode. The sample was 298 K and a relative humidity of 40­60%. set horizontally below a KP. The KP was a 1-mm-diameter gold (Au) wire (99.5%, Nilaco Corporation, Japan) that 2.2 Measurement of pH during steel corrosion was vibrated, along with a bimorph-type piezoactuator 2.2.1 Sample preparation (LPD3713X, Nihon Ceratec Co., Ltd., Japan). In this study, Carbon steel sheet, whose chemical composition is shown a small alternating current (AC) voltage signal from a lock-in in Table 1, was used as the corrosion material. The steel sheet amplifier (5610B, NF Corporation, Japan) was amplified by was cut into small coupons: (10 mmL © 10 mmW © 5mmT) a piezodriver (As-904-150B, NF Corporation, Japan). The and (10 mmL © 5mmW © 5mmT); a 1-mm-diameter hole amplified AC voltage signal was input to the piezoactuator to was drilled in the center of each coupon. The coupon was vibrate the KP. The vibration frequency was 227 Hz, and the then coated with electrodeposited paint. The paint thickness vibration amplitude was estimated to be 50 µm, based on the was about 20 µm. Sb wire was prepared by melting and characteristics of the piezoactuator. extending Sb shot (1­3mmº, 6N, NewMet Koch, UK) in a Measurement of pH in a Thin Electrolyte Droplet Using the Kelvin Probe Technique 533

Table 1 Chemical composition of steel.

10 mm 5 mm GPIB Computer GPIB Lock-in amp. XYZ Stage Piezo Piezodriver KP Op-amp. Sb Droplet Steel(anode) Steel(cathode) Tape 10 mm Epoxy resin Bias

Tape

Potentio/galvano-stat Steel(cathode) Steel(anode) GPIB Sb 0.8 mm Epoxy resin Computer

Fig. 3 Schematic drawing of the sample used as a simulated anode and Fig. 4 Schematic drawing of the experimental setup for pH measurements cathode system of steel corrosion by a thin electrolyte droplet. during the galvanic corrosion test for simulated steel corrosion. heated borosilicate glass tube. The diameter of the Sb wire and the KP was measured using the same system as described extended was ca. 0.8 mm. in Section 2.1. For these measurements, the Au wire vibrated As shown in Fig. 3, a pair of steel plates and a pair of Sb at a frequency of 237 Hz and an amplitude of 25 µm. The wires were embedded in an epoxy resin together. The gap potential difference between the anodic and cathodic steel size between the steel plates was about 0.3 mm. The Sb wires plates was measured simultaneously. The pH values of the were placed at the center of the drilled-hole machined in the electrolyte solution at the anode and cathode were calculated steels. The Sb wires were electrically insulated from the using the Volta potential difference of the Sb wire. A multi- steels. Samples were ground up to P1200 grit with waterproof channel potentiostat/galvanostat (PS-08, Toho Technical SiC papers and then degreased in ethanol before the Research Co., Ltd., Japan) was used to apply galvanostatic experiments. The exposed surface area was fixed at polarization and to measure the potential difference between 3.8 cm2, and the other surface was covered with a tape. The the electrodes. The analog signal outputs from the ratio of the surface area of steels was ca. 1:2. potentiostat/galvanostat were measured with a digital 2.2.2 Measurement of pH during corrosion voltmeter and recorded by a personal computer controlled Figure 4 shows the experimental setup for the pH by a custom-designed LabVIEW program. measurement of steel corrosion. The as-prepared sample To correlate Volta potential difference measured by the KP was placed just below the KP and a 380-µL droplet of with the corrosion potential of the steel, the Volta potential 0.1-mol/dm3 NaCl solution, which was prepared with difference and the corrosion potential for various metals 3 analytical grade chemicals (Kanto Chemical Co., Inc., Japan) under a droplet of 1.0-mol/dm Na2SO4 solution (thickness: and Milli-Q water (18 M³ cm), was put on the sample 1 mm) were measured with the KP and a commercial Ag/ surface. The initial droplet thickness was ³1000 µm. AgCl electrode in a saturated KCl solution (SSE; ESSE = After droplet formation, a constant current was applied +0.197 V vs. SHE at 25°C), respectively.19) The Volta between the steel plates, such that the steel shown on the right potential difference measured against the KP was then side of Fig. 3 worked as an anode and that on the left as a converted to the corrosion potential measured with the SSE. cathode. The actual values of the current densities applied are listed in Table 2. For the initial 30 min after droplet 3. Results formation, current was not applied to the sample, providing free-corrosion conditions for the steel surfaces. After the 3.1 pH measurement in a thin electrolyte droplet initial 30-min period, a constant current was applied and Figure 5 shows the Volta potential difference of the Sb increased step-wise every 15 min. electrode measured in a droplet of various pH standard During the free corrosion and polarization conditions, the solutions as a function of the droplet thickness. The Volta Volta potential difference between the steel of the anodic side potential difference in each pH standard solution was 534 S. Ajito, E. Tada, A. Ooi and A. Nishikata

Table 2 List of the applied current densities on the anode and cathode sides of the steel during the galvanostatic polarization test.

0 P pH 1.68 K 0

.

s

-0.1 pH 4.01 v

pH 6.86 V -0.1

/ -0.2 pH 10.01 E -0.2

,

e

-0.3 c E = -0.031pH-0.219 n -0.3

e

r

-0.4 e

f

f

i -0.4

d

-0.5 l

a

i

t -0.5

n

-0.6 e

t

o -0.6

p

-0.7 a

t 0 200 400 600800 1000 l -0.7

o

Droplet thickness, X / µm V 024681012 pH Fig. 5 Volta potential difference of the Sb electrode against the KP in various pH buffer solutions for different droplet thicknesses. Fig. 6 Relationship between the Volta potential difference of the Sb electrode and droplet pH. ­ independent of the droplet thickness in the range of 50 RT 1000 µm. Additionally, the Volta potential difference of the E ¼ E : ð Þ 2 303 F pH 3 Sb electrode against the KP decreases as the droplet’spH increased. where E° is the formal potential for the equilibrium reaction, The average value of the Volta potential difference from R is the gas constant [8.314 J/(mol K)], F is Faraday’s each pH standard solution was obtained. The Volta potential constant (96480 C/mol), and T is the absolute temperature. difference was independent of the droplet thickness in each The expected slope of the electrode potential with respect to pH standard solution. Figure 6 shows the average value of the pH value is ¹59 mV/pH at 25°C. the Volta potential difference of the Sb electrode as a function As described in eq. (1), the slope of the Volta potential of the pH of the standard solutions. As shown in the figure, difference was ¹31 mV/pH in this study. The slope value the average Volta potential difference has a linear relationship was smaller than that for the theoretical value obtained using with respect to the pH. The relationship between the Volta the Nernst’s equation. However, the Volta potential difference potential difference, E, and pH can be expressed using a least for the Sb electrode used in this study indicated good squares approximation as follows: linearity as a function of pH, i.e., a sub-Nernstian response. Other researchers have reported that some pH-sensing metal/ E=Vvs: KP ¼0:031pH 0:219 ð1Þ metal oxide electrodes show sub-Nernstian responses with Assuming the following equilibrium electrochemical reaction smaller slopes, e.g. ¹35 to ¹45 mV/pH for W,8) ¹42 10) 14) for the Sb/SbxOy electrode given in eq. (2), the electrode mV/pH for Sb, and ¹55 mV/pH for Ir. Some have potential, E, can be expressed by the Nernst equation of attempted to explain this effect in terms of the interference by eq. (3): the reduction reaction of dissolved oxygen,13­15) and the effect 16,17) þ of the surface area. Although there was a slight difference xSb þ yH O  SbxOy þ 2ye þ 2yH ð2Þ 2 in the slope between the obtained value and the theoretical Measurement of pH in a Thin Electrolyte Droplet Using the Kelvin Probe Technique 535

Corrosion Galvanostatic polarization of the applied current density. The pH near the anode

(a) 2 - 100 gradually decreased to ca. 5 after 150 min of immersion.

m

c Anode 50 In contrast, as the applied current increased, the cathode

A

/ 0 potential shifted gradually in the negative direction. When

i -50 Cathode the applied current density exceeded ¹20 µA/cm2, the cathode potential shifted significantly in the negative (b) direction from ca. ¹0.6 V. The negative potential shifts from -0.4 Anode ¹

E 0.6 V is related to the polarization behavior on the cathode

S

S -0.6 (the cathodic polarization curve will appear later in Fig. 9).

.

s ff

v The di usion-limiting current of the reduction reaction of -0.8

V

Cathode

/ dissolved oxygen (ORR) was observed in the potential region

E -1.0 less noble than ¹0.6 V; in the diffusion-limiting current region of ORR the cathodic potential can shift significantly (c) in the negative direction even with a small increase in the 15 Cathode applied current. Regarding the pH near the steel of the cathode, when the current was applied to the cathode, the pH 10

H began to increase and continued to increase with increasing p the applied current. The pH reached a constant value at ca. 12 5 2 Anode when the applied current density reached ca. ¹20 µA/cm ; ff 0 this current was almost the same value as the di usion- 0 30 60 90 120 150 limiting current density of the ORR. The pH did not change Time, t / min very much with increasing the applied current; this is because ff Fig. 7 Changes in (a) the applied current density between the steels, (b) the the ORR occurs under the di usion-limiting condition in the potentials of the anode and cathode sides of the steel, and (c) the pH near potential region less noble than ca. ¹0.9 V. the steel. 4. Discussion value, as shown in Fig. 3, the Sb electrode demonstrated good 4.1 Validation of pH measured with an Sb electrode sensitivity as a pH sensor in solutions with thicknesses during steel corrosion ²50 µm. Furthermore, it can be concluded that solution pH Our results showed that the pH measurement decreased to can be obtained using the Sb electrode/KP configuration. ca. 5 near the steel of the anode and increased to ca. 12 near the steel at the cathode during the steel corrosion process. 3.2 pH estimation on a corroding steel in an NaCl To validate the pH measurement, a galvanostatic polarization droplet test was performed in a droplet of 0.1-mol/dm3 NaCl The KP technique with an Sb electrode as a pH sensor was containing a pH indicator. The pH was estimated from the applied to measure local pH on a corroding steel surface in a change in color of the solution during the galvanostatic thin electrolyte layer. As shown in Fig. 4, current was applied polarization. The pH indicator used in this test was between two steels to mimic anodic and cathodic regions. Takemura’s pH indicator (Takemura Denki Seisakusho, Figure 7 shows changes in the current density, the potential Japan); this indicator can be used to resolve pH in the range of each steel, and pH evaluated from the Volta potential of 4 to 10. difference of the Sb electrode during the simulated steel Figure 8 shows the change in the applied current density, corrosion. As shown in the figure, a 380-µL droplet of surface images of the steel, and solution color. The steel 0.1-mol/dm3 NaCl solution was placed on the sample’s shown on the right acted as the anode and on the left as the surface at 0 min. For the initial 30 min, both the anode and cathode. In the initial 30 min, the pH changes that occurred cathode were under the free corrosion conditions, meaning during this time period were caused by free corrosion of the that no current was applied between the anode and cathode. steel, as current was not applied. Figure 8(a) shows the The corrosion potentials of the anode and cathode were surface image of the steel plates taken at 30 min after droplet monitored during the steel corrosion under a thin electrolyte formation; pits were evident in the steel surfaces, as indicated droplet. Simultaneously, the pH values near each steel plate by the arrows. This suggests that localized corrosion occurred were measured with the Sb electrode. As shown in the figure, on the steel plates in the droplet, with the pits acting as anodic the corrosion potentials for the anode and cathode were sites and the other surfaces as cathodic sites.26) Additionally, ¹0.4 V vs. SSE when a droplet was placed on the sample. the pH distribution in the droplet vicinity of the pits showed a The corrosion potential for both steels gradually decreased pH of 5.5 to 7. At some distance away from the pits, the pH to ca. ¹0.5 V vs. SSE. At this time, the solution pH near the was over 10. Thus, the pH distribution was attributed to Sb electrodes was estimated to be in the neutral region and localized corrosion on the steel. This result is consistent with remained nearly constant over the immersion time. the findings of Hirohata et al., who measured the local pH After 30 min of immersion, a constant current was applied changes from steel corrosion under a modified droplet.27) between the anode and cathode. The current increased step- According to the results of pH measurement in their study, wise every 15 min. The potential of the steel at the anode the pH during the steel corrosion was estimated to be in the reached a constant reading of ¹0.5 V vs. SSE, independent range of 6 to 13. 536 S. Ajito, E. Tada, A. Ooi and A. Nishikata

Corrosion Galvanostatic polarization 10-3

2

- 100

m

c Anode 50

A

/ 0 (c) (d)

i Cathode -50 (a) (b) -2 10-4 /Acm (a) 30min (b) 60min i Steel Sb Steel pH y, (Cathode) (Anode) 5.5 6.0

7.0 10-5 8.0 Current densit (c) 120min (d) 150min 9.0 10.0

10-6 5 mm -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 Potential, E / V vs. SSE Fig. 8 Applied current density on the anode and cathode sides of steel during galvanostatic polarization and photographs of the samples in the Fig. 9 Plots of the applied current densities on the anode and cathode sides solution taken after (a) 30, (b) 60, (c) 120, and (d) 150 min. of steel as a function of the measured potential during the galvanostatic polarization test in 0.1-mol/dm3 NaCl. The galvanostatic polarization was conducted by applying a constant current density between the steels, such that the anode and cathode could be separated. As shown in 4.2 pH change during steel corrosion under an NaCl Figs. 8(b)­(d), the low pH region expanded over time on droplet the anode. The pH distribution around the pits (Figs. 8(a) and The corrosion reaction of steel in a neutral NaCl solution is (b)) disappeared with increasing the current density since a combination of the anodic dissolution reaction of iron (Fe) anodic reaction was enhanced on the whole surface of the and the cathodic reduction reaction of dissolved oxygen anode. In the final stage of galvanostatic polarization, the pH (ORR) as follows: over the whole surface of the anode was estimated as 5.5 at þ Fe ! Fe2 þ 2e ð4Þ point (d) in Fig. 8 based on the change in the solution color, þ þ ! ð Þ as the applied current density approached 105.7 µA/cm2.In O2 2H2O 4e 4OH 5 contrast, the pH near the anode, which was evaluated by the If steel is polarized at a very negative potential, the cathodic measurement of the Volta potential difference of the Sb reduction reaction of water molecules, which is hydrogen electrode, decreased to ca. 5 as the anode was polarized at the evolution reaction (HER), can evolve, in addition to ORR, as current density of 105.7 µA/cm2. Thus, these pH results are follows: in good agreement. 2H O þ 2e ! H þ 2OH ð6Þ The color distribution at the cathode and the color of the 2 2 solution changed over time. The pH in the vicinity of Figure 9 shows plots of the applied current density as a localized corrosion during the free corrosion period (on the function of the electrode potential of the steels during the cathode side) was estimated to be 5.5, and the pH distant galvanostatic polarization experiment in a 0.1-mol/dm3 NaCl from the localized corrosion region was 10. Additionally, the solution. As shown in the figure, a diffusion-limiting current pH near the Sb electrode on the cathode increased to over 10 region due to ORR was observed from the rest potential to and maintained a constant value based on the solution color ¹0.9 V; the current increase due to HER was also observed results. The pH distribution observed around the pits on the from ca. ¹0.9 V. Therefore, during the galvanostatic polar- cathode during the free corrosion (Fig. 8(a)) disappeared with ization experiment, in the thin electrolyte droplet, ORR was increasing the cathodic polarization since the ORR was the main cathodic reaction even when the applied current enhanced on the whole surface of the cathode. In contrast, the density was less than ca. ¹30 µA/cm2. pH measurement using the KP system showed that the pH As described in eqs. (4) and (5), during the galvanostatic near the steel of the cathode side increased gradually from 7, polarization test for steel, Fe2+ ions and OH¹ ions are finally reaching a stable value of 12. Therefore, the pH generated in the solution. Additionally, dissolved Fe2+ ions change induced during the simulated steel corrosion, as are immediately oxidized to Fe3+ ions by dissolved oxygen in measured by the KP system, agreed with that measured with the electrolyte solution, and Fe3+ ions react with OH¹ ions the pH indicator. Thus, it can be concluded that the pH to form Fe(OH)3 via a hydrolysis reaction depending on the measurement system using the KP technique/Sb electrode solution pH as follows: combination can be applied to pH measurements of thin þ 4Fe2 þ O þ 2H O þ 8OH ! 4FeðOHÞ ð7Þ electrolyte droplets on a corroding steel. 2 2 3 Measurement of pH in a Thin Electrolyte Droplet Using the Kelvin Probe Technique 537

Consequently, concentration distributions of OH¹ and Fe3+ droplet. The KP system is capable of estimating the pH ions form on the corroding steel surfaces through a of electrolyte layers of thickness ²50 µm. combination of separate anodic and cathodic reactions, mass (3) pH changes on simulated anode and cathode sites transport of the generated ions, and successive chemical during steel corrosion in a thin electrolyte droplet were reactions. measured by combining the Sb electrode and KP. When As shown in Fig. 7, when the steel was polarized to a the steel was polarized, the pH decreased to 5 at the potential less noble than ¹0.6 V, the pH over the cathode side anode and increased to 12 at the cathode. of the steel reached to ca. 12. Numerous experimental and theoretical studies have been performed to investigate the pH Acknowledgement on the surface where ORR occurs under the diffusion-limiting condition. For example, Hirohata et al. found that the surface This work was supported by JSPS KAKENHI Grant pH over the cathode site reached ca. 13 during corrosion in a number JP15K14144. thin electrolyte droplet.27) Ogle et al. also showed that the / 3 surface pH rose to 11.2 over the steel surface in 0.5-mol dm REFERENCES NaCl.28) Engell et al. calculated the theoretical pH in the ff solution over the cathode under the di usion-limiting 1) A.T. Kuhn and C.Y. Chan: J. Appl. Electrochem. 13 (1983) 189­207. condition of ORR by accounting for a flux balance of 2) G.W. Whitman, R.P. Russell and V.J. Altieri: Ind. Eng. Chem. 16 reactants and products, and found that the surface pH (1924) 665­670. increased to 10.9.29) These findings are in good agreement 3) A.A. Panova, P. Pantano and D.R. Walt: Anal. Chem. 69 (1997) 1635­ with the pH results obtained by the KP technique in this 1641. 4) M. Büchler, T. Watari and W.H. Smyrl: Corros. 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Lett. the solution over the anode. However, the dissolved Fe2+ 17 (1998) 551­553. ions can be transformed to Fe(OH) in the presence of 10) M.D. Capelato, A.M. dos Santos, O. Fatibello-Filho and R. Gama: 3 Anal. Lett. 29 (1996) 711­724. dissolved oxygen, according to eq. (7). Finally, a steady state 11) G. Edwall: Med. Biol. Eng. Comput. 16 (1978) 661­669. is formed by the dissolution equilibrium reaction of eq. (8), 12) G. Edwall: Electrochim. Acta 24 (1979) 595­603. in which the Fe3+ ion concentration reaches 1.0 © 10¹10 13) G. Edwall: Electrochim. Acta 24 (1979) 605­612. mol/dm3 which is calculated by the solubility product 14) G. Edwall: Electrochim. Acta 24 (1979) 613­621. constant of Fe(OH) at 25°C, 2.8 © 10¹39.31) Using eq. (9), 15) L.T. Dimitrakopoulos, T. Dimitrakopoulos, P.W. Alexander, D. Logic 3 and D.B. Hibbert: Anal. Commun. 35 (1998) 395­398. the equilibrium pH was calculated to be about 4.9. 16) L.B. Kriksunov, D.D. Macdonald and P.J. Millett: J. Electrochem. Soc. 3þ 141 (1994) 3002­3005. FeðOHÞ3  Fe þ 3OH ð8Þ 17) S.E.S.El Wakkad, H.A. Rizk and I.G. Ebaid: J. Phys. Chem. 59 (1955) 1 ­ pH ¼ 1:61 log½Fe3þð9Þ 1004 1008. 3 18) T. Katsube, I. Lauks and J.N. Zemel: Sens. Actuators 2 (1981­1982) 399­410. It can be concluded from the above discussion on the 19) M. Stratmann and H. Streckel: Corros. Sci. 30 (1990) 681­696. experimental results in this study that the KP technique with 20) M. Stratmann and H. Streckel: Corros. Sci. 30 (1990) 697­714. an Sb electrode can be useful to get the pH change over the 21) M. Stratmann, H. Streckel, K.T. Kim and S. Crockett: Corros. Sci. 30 anode and cathode sites on the corroding steel even in a thin (1990) 715­734. electrolyte droplet. 22) T. Tsuru, Y. Yokoyama and J. Wang: Proc. 39th Japan Corrosion Conference, (JSCE, 1992) pp. 47­50. 23) A. Tahara and T. Kodama: Corros. Sci. 42 (2000) 655­673. 5. Conclusions 24) M. Stratmann and H. Streckel: Ber. Bunsenges. Phys. Chem. 92 (1988) 1244­1250. The pH over the surface of corroding steel was 25) A. Nishikata, Y. Ichihara and T. Tsuru: Corros. Sci. 37 (1995) 897­ investigated using the combination of an Sb electrode as 911. 26) A. Araoka, A. Nishikata and T. Tsuru: Zairyo-to-Kankyo 51 (2002) a pH sensor and a KP in a thin electrolyte droplet of ­ / 3 fi 250 255. 0.1-mol dm NaCl. Our ndings are summarized below. 27) Y. Hirohata, K. Nishida, T. Haruna and K. Noda: J. Japan Inst. Met. (1) The Volta potential difference between the Sb electrode Mater. 81 (2017) 495­501. and the KP exhibited a linear relationship against pH 28) K. Ogle, V. Baudu, L. Garrigues and X. Philippe: J. Electrochem. Soc. in a thin electrolyte droplet. This suggests that the Sb 147 (2000) 3654­3660. ­ electrode can be applied as a pH sensor in a thin 29) H.-J. Engell and P. Forchhammer: Corros. Sci. 5 (1965) 479 488. 30) M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous electrolyte droplet. Solutions, (NACE, Houston, TX, 1974) pp. 310. (2) The Volta potential difference of the Sb electrode was 31) D.R. Lide: CRC Handbook of Chemistry and Physics, (CRC Press, successfully measured using the KP in a thin electrolyte New York, 2005).