Online corrosion monitoring of photoelectrochemical cathodic protection of carbon steel using particle video microscope

Journal Pre-proof Online corrosion monitoring of photoelectrochemical cathodic protection of carbon steel using particle video microscope Xianqiang Xiong, Liya Fan, Yiming Xu, Qiqiang Zhang, Jiangshan Li, Yong Wang, Guihua Chen, Chenglin Wu, Tianding Li, Shuai Fu, Guolin Zhang

PII:

S0030-4026(19)31990-4

DOI:

https://doi.org/10.1016/j.ijleo.2019.164091

Reference:

IJLEO 164091

To appear in:

Optik

Received Date:

14 November 2019

Accepted Date:

17 December 2019

Please cite this article as: Xiong X, Fan L, Xu Y, Zhang Q, Li J, Wang Y, Chen G, Wu C, Li T, Fu S, Zhang G, Online corrosion monitoring of photoelectrochemical cathodic protection of carbon steel using particle video microscope, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164091

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Online corrosion monitoring of photoelectrochemical cathodic protection of carbon steel using particle video microscope Xianqiang Xiong,a,b,c,‡ Liya Fan,d,‡ Yiming Xu,a Qiqiang Zhang,c Jiangshan Li,b Yong Wang, b Guihua Chen,b Chenglin Wu, b ,* Tianding Li,b Shuai Fu,b Guolin Zhangd,* Department of Chemistry, Zhejiang University, Hangzhou 310027, China

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School of Pharmaceutical and Materials Engineering, Taizhou University, Jiaojiang 318000, China

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Zhejiang erg technology Inc, Sanmen 317000, China

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College of Chemistry, Liaoning University, Shenyang 110136, China

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*Corresponding author: [email protected], [email protected] These authors contributed equally to the work.

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‡

Highlights

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In situ imaging technique based on particle video microscope (PVM) is developed. PVM technique is successfully applied for the in-situ imaging of carbon steel. Pitting corrosion results in the severe damage of carbon steel. Irradiated TiO2 photoanodes can protect the carbon steel from corrosion.

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   

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Abstract. The photo induced open circuit potential and Tafel curves have been widely used to evaluate

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the photoelectrochemical cathodic protection performance, but these conventional measurements cannot real-time monitor the images of the protected metal, and thus we are not able to judge whether

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the metal is protected or not during operation. To address these issues, in situ imaging technique based on particle video microscope (PVM) probe is developed to visually monitor the real-time images of the carbon steel. Anatase and rutile TiO2 in different nanostructures were selected as the model photoelectrode materials, and their photogenerated cathodic protection performance was investigated 1

by conventional electrochemical characterizations and in-situ PVM measurements, respectively. The in-situ imaging tool reveals that pitting corrosion can destroy the carbon steel in the chloride containing environment. However, once coupled with the TiO2 photoanodes, the carbon steel can maintain a longterm stability without being corroded. Therefore, the in-situ imaging technique developed in this study provide us a direct route to detect and evaluate the long-term photocathodic protection performance, which will allow us to better clarify the corrosion mechanism, and finally for the better design of

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photoanode materials.

Keywords: Photocathodic protection; In-situ measurement; Particle video microscope; Titanium

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dioxide; Carbon steel.

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1. Introduction

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Cathodic protection technologies, such as impressed current cathodic protection and sacrificial anode protection, have been widely applied in engineering metal anticorrosion[1]. However, impressed

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current cathodic protection needs high electric energy consumption, and the sacrificial anode protection will results in severe waste of anode materials and cause environmental pollution. As a new,

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green, non-polluting metal cathodic protection method, PEC cathodic protection technology has

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attracted considerable research interests since the first discovery that the copper substrate can be effectively protected by the TiO2 coating under light illumination[2]. In the past two decades, various semiconductor photoanode materials have been developed, such as TiO2[3], SrTiO3[4], ZnO[5], C3N4[6] and BiVO4[7]. Among these semiconductor photoelectrodes, TiO2 is the most widely used,

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due to its low cost, non-toxicity and stable chemical properties[1]. However, the bandgap (3.2 eV) of TiO2 restricts the photoresponse to only the ultraviolet region, and the rapid recombination of photoexcited electron-hole pairs which greatly limit the photocathodic protection performance of TiO2[8]. Thus, many strategies have been conducted on improving the PEC cathodic protection performance of TiO2, including metal and non-metal doping[9], coupling with other semiconductor[10-13], sensitized with quantum dots[14], loading of noble metal or surface modification with cocatalysts[15, 16].

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Traditionally, in order to compare the photocathodic protection performance, open circuit potential (OCP) measurements and Tafel curves are basically measured[1]. In general, the more negative of the

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OCP or the self-corrosion potential, the better of the photocathodic protection performance. However, no corrosion-related information was obtained by these PEC measurements, due to the lack of direct

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observation of the progress of corrosion degradation of the protected metal. Currently, the accurate

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and comprehensive monitoring of the corrosion process of the protected metal is still a technological issue[17]. In general, corrosion damages initiates from the surface of the subjects. The real-time

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monitoring of the surface morphology can obtain a lot of important information, such as corrosion type, corrosion location, corrosion rate and corrosion products. In addition, the visualization and

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analysis of surface morphology can reveal the initial destruction that would be beneficial in initiating

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protective actions and thus contribute to enhancing the life of the protected metal[18]. It thus becomes imperative to develop a visual imaging system to monitor the corrosion behavior of the protected metal during the PEC cathodic protection process. It is known that the scanning electron microscope (SEM) can be used to study the morphology of the metal electrode before and after exposure to the corrosion

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media[19, 20]. However, SEM is off-line measurement, and they are not suitable for in situ observation of corrosion behavior in the long-term photocathodic protection process. The off-line measurement basically require to collect the samples with a certain representativeness and thus limit the measurement into a small-scale[21]. In addition, the samples need to be further treated, which may alter the surface morphology and destroy the corrosion products accumulated on the surface of the protected metal. In contrast, in-line techniques avoids these drawbacks. However, there is no research

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focusing on the development of in-situ technique to monitor the real-time morphology of the protected metals in the field of PEC cathodic protection.

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As a probe-based in situ imaging tool, Particle Video Microscope (PVM) technology has attracted considerable attention, especially in the field of pharmaceutical, food and environmental sectors[21-

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24]. This is because the PVM technique can in-line monitor the manufacturing processes at any given

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instant without the need of sampling and sample preparation. PVM uses six independent laser lights to illuminate a fixed region in front of the probe face. The scattered beams are reflected back toward

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the probe and detected with a charge-coupled device element to produce digital images[24]. Thus, the high-resolution images of particles and droplets were collected and stored in succession, followed by

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analyzing the images online or offline using software to obtain particle numbers, shapes and size

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distributions. Inspired by the PVM technique, it was decided to use this technology to extract the realtime information about the corrosion process of the protected metal, with the aim to better detect and evaluate the PEC cathodic protection performance.

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In this study, the PVM imaging setup was built and used to visually monitor the real-time images of the carbon steel. Anatase and rutile TiO2 in different nanostructures were prepared according to a twostep anodization and hydrothermal methods respectively, and used as the photoelectrodes materials for evaluating the PEC cathodic protection performance. The conventional PEC measurements including OCPs and Tafel curves has also been carried out for comparison with in situ PVM probe method. Both test techniques confirm that titanium dioxide is able to protect carbon steel, but in-situ PVM

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technology provides us with more insight on the corrosion process, allowing us to more intuitively determine photocathodic protection performance in a non-destructive manner. The novel approach

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developed here will contribute to the better understanding of the corrosion mechanism and reduce the

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time spent in the development of photoelectrode materials for cathodic protection.

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2. Experimental section

2.1. Materials. Titanium butoxide (99%), NH4F, HF (40%), CH3OH, NaOH, HCl, HNO3, NaCl

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(99.5%), acetone (99.5%) and ethylene glycol were purchased from Aladdin. Ti sheets were first machined with dimensions of 2.5 × 1 × 0.1 cm, followed by chemically polished in an acid solution

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containing HF, HNO3 and H2O with a volume ratio of 1:4:5, and then sonicated with ultrapure water

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and ethanol. The carbon steel was machined into rectangular block with an area of 2.5 × 1 cm, thickness of 2 mm, and then ground with 1000-grit SiC papers, polished with alumina powder, and finally ultrasonically cleaned in Milli-Q ultrapure water and ethanol for 10 min, respectively. Fluorine-doped tin oxide (FTO) glasses were purchased from Nippon Sheet Glass Co Ltd, and cleaned by sonication

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with ultrapure water, ethanol, and acetone for 15 min, respectively. Finally, they were dried under N2 stream at room temperature. 2.2. Synthesis of TiO2 nanotube films. Anatase TiO2 nanotube arrays were fabricated on Ti sheet through a two-step anodic oxidation method as described in the literature[25]. Briefly, two-electrode system was used with the Ti sheet as the working electrode and the platinum grid as the counter electrode. The Ti sheet was firstly oxidized in the ethylene glycol electrolyte containing 0.5 wt% NH4F

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at an applied bias of 20 V for 1 h. Afterward, the pre-anodized TiO2 layer was ultrasonically peeled off in an ultrapure water solution. The cleaned Ti sheet was used for the secondary anodization at 20 V for

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4 h in the same electrolyte. The as-prepared TiO2 nanotubes were then cleaned with ultrapure water and dried in the air at the room temperature, and finally annealed in a muffle furnace at 500 oC for 2

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h.

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2.3. Synthesis of TiO2 nanorod films. Rutile TiO2 nanorod arrays were grown on FTO glass using a hydrothermal method described elsewhere[26]. Briefly, 0.189 mL of Titanium (IV) butoxide was

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quickly added into the 6 mol/L HCl aqueous solution with a volume of 22.6 ml under continuous stirring. Then, the mixed solution was transferred into a 50 ml Teflon-lined stainless-steel autoclave,

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followed by placing the cleaned FTO substrate into the in-wall of the autoclave and the conductive

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side of FTO glass was faced down. The hydrothermal reaction was carried out in an electric oven at 150 °C for 9 h. After the reaction, the autoclave was naturally cooled down to room temperature. The resulting sample was then rinsed with ultrapure water to remove the residual reactants, and dried in

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the air. Finally, the sample was heat-treated in a muffle furnace at 450 oC for 30 min to obtain the crystalline TiO2. 2.4. Characterization. The crystalline phase of samples was identified by X-ray diffraction (XRD, Bruker D8 advance with Cu Kα radiation, λ=0.15418 nm). UV/Vis absorption spectroscopy was performed using a Hitachi U-4100 UV–VIS-NIR spectrophotometer with BaSO4 as background. The surface morphology of the TiO2 samples was characterized by a Hitachi S-4800 field emission

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scanning electron microscope (Tokyo, Japan).

2.5. Testing methods of the PEC cathodic protection of carbon steel. The conventional PEC test

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setup is shown in Fig. 1a, which contains a photoanode cell and a corrosion cell. The TiO2 photoelectrode was placed in the photoanode cell with 1 M NaOH solution as the supporting electrolyte,

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while the carbon steel electrode was placed in the corrosion cell with 3.5 wt% NaCl solution as the

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corrosion media, and the two cells are connected by a salt bridge. The TiO2 and carbon steel electrode were connected by a Cu wire and served as working electrode, a saturated calomel electrode (SCE) as

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reference electrode and a Pt grid as counter electrode. The photoanode and the protected steel electrode are connected to the electrochemical workstation. The open circuit potential of the carbon steel

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electrode connected with TiO2 films was measured under interrupted illumination of simulated solar

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light, and the Tafel curves were tested with a scan rate of 5 mV/s. A schematic illustration of the Mettler Toledo PVM measurement setup was shown in Fig. 1b. The setup contains a photoanode cell and a corrosion cell, but no electrochemical workstation is used. The PVM probe was positioned in the stand to keep it stable, and the images were collected by placing the

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carbon steel in close contact with the sapphire window mounted on the video microscope probe. The PVM probe uses six independent laser lights to enlighten the surface of the carbon steel. The probe is portable, which can provide images of different areas to be inspected. The PVM microscope offers a 1,320×1,760 µm field of view and a clear resolution of about 5 µm. Uncompressed video was captured over each 5 min period using the ParticleView probe combined with iC PVM 7.0 software. 3. Results and discussion

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Fig. 2a show the XRD pattern of anatase TiO2 films prepared by two-step anodization method. The diffraction peaks at 2θ = 25.3o and 48.1o are indexed to the characteristic peaks of

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anatase TiO2 (JCPDS No.21-1272), whereas the other diffraction peaks are in good agreement with the Ti substrates (JCPDS No.44-1294). The crystal size of (101) crystal planes of the

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anatase TiO2 was estimated to be 30 nm using Scherrer equation. The top-view morphology of

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anatase TiO2 was revealed by SEM images and the tube-like structure of TiO2 was uniformly distributed on the Ti substrate, see Fig. 2c. Interestingly, the magnified view of TiO2 nanotubes

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show the tube wall is spiral (Fig. 2e), with the tube outer diameter at approximately 80 nm and wall thickness of about 15 nm. The helical TiO2 nanotubes structure will provide a large surface

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area and interface area between the electrode and electrolyte, which is very favorable for the

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PEC cathodic protection of metal. Fig. 2b displayed the XRD pattern of another TiO2 films grown on the FTO substrate using a hydrothermal method. The diffraction peaks at 2θ = 27.4o, 36.1o and 54.3o are well matched with rutile TiO2 (JCPDS No.21-1276), while the additional peaks are attributed to the FTO substrate (JCPDS No.46-1088). The high-magnification and

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low-magnification SEM images shown in Fig. 2d and 2f confirm that the TiO 2 nanorods were successfully synthesized. These nanorods have diameter varying from 40 nm to 180 nm, accompanied with a smooth wall and a coarse rod tip. In addition, the optical properties of anatase and rutile TiO2 were evaluated by using UV-vis diffuse reflectance spectrum, and the results were shown in Fig. S1. The anatase and rutile TiO2 exhibited a light absorption edge starting at approximately 387 nm and 410 nm, respectively. This indicates that the energy band

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gap of anatase and rutile TiO2 corresponds to 3.2 and 3.0 eV, respectively. The larger band gap values imply that the both film can mainly absorb the UV light of the solar energy spectrum.

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In order to evaluate the photocathodic protection properties of the two TiO2 films, conventional PEC measurement including open circuit potential and tafel curves were firstly

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measured, see Fig. 3. Fig. 3A is the time profile of OCP variations of carbon steel coupled with

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anatase or rutile TiO2 films under interrupted illumination of simulated solar light. When the light was on, the potentials of coupled electrodes all shifted negatively, which may be attributed

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to the cathodic polarization of carbon steel which results from the excited photoelectrons transferring from TiO2 photoanode to carbon steel. After the light was off, the OCP of carbon

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steel was quickly shifted toward positive value, ascribed to the fast recombination of the

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photogenerated electrons and holes pair or the electron transfer from TiO2 to redox species in the aqueous solution. In general, the more negative of the photogenerated potentials, the better the PEC cathodic protection, due to the increased electron concentration on the carbon steel. Clearly, the rutile TiO2 presents a larger OCP drop from – 0.40 V to – 0.53 V in comparison

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with that of the anatase TiO2 from – 0.42 V to – 0.48 V. However, the photopotential of anatase and rutile TiO2 coupled with carbon steel upon light illumination is both more negative than the self-corrosion potential of carbon steel (-0.44 V vs. SCE). This means that the carbon steel can be protected by both TiO2, whereas the rutile TiO2 have a higher photocathodic protection performance. Moreover, it is noted that the OCP drop shown here is obviously smaller than those reported with stainless steel coupled TiO2 photoelectrode. This is because the self-

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corrosion potential of carbon steel is more negative, compared to those with stainless steel[5]. Fig. 3B shows the Tafel polarization curves of the carbon steel uncoupled and coupled with

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TiO2 films under illumination of simulated solar light (100 mW/cm2). The self-corrosion potential measured with carbon steel was -438 mV (vs. SCE), whereas it was negatively shifted

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to -475 and -531 mV (vs. SCE) when the carbon steels were connected with anatase and rutile

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TiO2, respectively. This is in consistent with the result of the OCP measurement. The significant reduction of the self-corrosion potential illustrated that electron transfer rate from

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irradiated TiO2 to the carbon steel electrode is faster than the electrons consumption rate on the carbon steel electrode. Thus, the photogenerated electrons are accumulated on the carbon steel

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electrode and results in the lower potential, which allows carbon steel to be cathodically

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protected by the TiO2 photoelectrode. Obviously, the carbon steel coupled with rutile TiO2 exhibits a larger shift of the corrosion potential compared to that of anatase TiO2, further indicated that better photogenerated cathodic protection performance can be achieved by rutile TiO2. Clearly, these results indicate that the TiO2 in different crystal structure and

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nanostructures have great effect on the photogenerated cathodic protection performance, which is in good agreement with the result reported in the literature[27, 28]. To better understand the performance difference of anatase and rutile TiO2, the PEC activity of this two photoanodes was examined by linear sweep voltammetry under 1 sun illumination, performed in 1 M NaOH aqueous solution using a three-electrode configuration, see Fig. S2. The rutile TiO2 films displayed an anodic photocurrent that increased as the applied bias was

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positively shifted, with the turn-on potential for water oxidation starting at approximately -0.78 V (vs. SCE). However, the photocurrent density of anatase TiO 2 is far below than that of the

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rutile TiO2, with the turn-on potential locating at -0.71 V (vs. SCE). Specifically, the rutile TiO2 photoanode exhibited a photocurrent density of 0.66 mA/cm 2 at 0.0 V (vs. SCE), which

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is about 4 times higher than that of anatase TiO2 photoanode. These results illustrate that the

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rutile TiO2 has a better oxygen evolution reaction (OER) activity than anatase TiO 2. Furthermore, the rutile TiO2 produces photovoltage as high as 700 mV measured with open-

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circuit techinique (Fig. S2), which is larger than that of anatase TiO2 (556 mV). This means that less bias is needed for rutile TiO2 to drive the water oxidation as compared to anatase TiO2,

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which is in consistent with the observation of cathodic shift of turn-on potential. The enhanced

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OER and increased photovoltage on the rutile TiO2 photoanode will accelerate the interfacial water oxidation activity and enable an effective electron injection from TiO2 to the carbon steel, and thus enhance the photocathodic protection performance.

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Based on above results, we find that traditional PEC characterization can indeed be used to compare the photocathodic protection activity of photoelectrodes. However, the OCP measured with the protected metal connected with the photoelectrode can only reflect the mixed potential of photoelectrode and dark electrode. The real-time potential of the single metal and the individual photoelectrode cannot be simultaneous measured by OCP technique[29]. Thus, errors may produce if we evaluate the photocathode anticorrosion ability by simply comparing the mixed potential, rather

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than the single electrode potential with the self-corrosion potential of the single metal. It is noted that the mixed potential shown in Fig. 3A is completely different from the individual potential of the TiO2

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photoelectrode which is not connected with the carbon steel (Fig. S3). There was a sharper change in the potential transient in Fig. 3A when light is on and off, and it takes a much long time to reach a

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stable OCP. On the contrary, the open circuit potential of the single TiO2 photoelectrode is easily

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balanced when the light is on (Fig. S3). This is because there is a photocurrent flowing when the carbon steel is connected with the irradiated TiO2 photoelectrodes, and thus a longer time is needed to reach

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the equilibrium. However, a fast equilibrium between the generation and recombination of photogenerated electrons will reach for the TiO2 photoelectrode if disconnecting with the carbon steel.

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These results illustrates that it is very difficult to detect the real-time potential of a single electrode if

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current flows from photoelectrode to the protected metal. As for the Tafel curves, the greatest limitation is that they can be only used as the initial assessment of photocathodic protection performance. After that, the metal has been polarized and cannot continue to measure its corrosion potential unless the metal is re-polished. Corrosion is a long-term and gradual process for material,

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and thus the long-term protection efficiency needs to be detected and evaluated. However, we are not able to real-time monitor the long-term photocathodic protection property by traditional electrochemical measurements. Although the traditional evaluation methods consider that TiO2 photoelectrodes can protect carbon steel, there is no direct evidence to prove that it is protected. Thus, in the following part, we will mainly focus on a direct visual method to monitor the in-situ images of the carbon steel using the PVM probe, which aims to test the reliability and long-term

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performance of the photocathodic protection technique. The probe collects digital images of the illuminated area that records the real-time change of the carbon steel. Fig. 4 shows the typical

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photographic images taken at every 30 min for the carbon steel when disconnecting with the TiO2 photoelectrodes. The carbon steel is immersed in the brine solution, it is obvious that local corrosion

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occurs on the surface of the carbon steel as time increases. The corrosion images show that pitting

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corrosion dominates, but the corrosion rate varies greatly in different region. The corrosion in the area containing large hole is obviously faster and spread around to form a circular area, but the corrosion

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in regions without the large holes is relatively slower, and finally resulting in inhomogeneous and porous corrosion layer on the metal surface. There results indicate that the small imperfection on the

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surface of the carbon steel can result in fast corrosion. When the carbon steel is connected to the

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irradiated anatase or rutile TiO2 photoelectrodes, interestingly, the carbon steel is almost free from corrosion within 6 hours even though the surface of carbon steel has cracks or large holes which are easily corroded in the chloride containing medium, see Fig. 5 and Fig. 6. This provide a direct evidence that TiO2 photoelectrodes can actually prevent carbon steel from corrosion during the PEC cathodic

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protection process, which is in consistent with the PEC measurements conducted in OCPs and Tafel curve tests. However, unlike the electrochemical characterization, the rutile TiO2 does not exhibit better photocathodic protection performance than anatase TiO2. On the contrary, the two TiO2 photoanodes act as an equal role in protecting the carbon steel and both can maintain a long-term protection performance. This means that it is not necessary to always pursue the very negative OCP or cathodic polarization potential in the development of photoelectrode materials. Clearly, the metal can

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be well protected as long as the semiconductor photoelectrodes can supply enough photogenerated electrons to the protected metal. Therefore, the long-term stability of the photoelectrodes seems to be

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more important because it determines whether the photogenerated electrons can continuously transfer to the protected metals. However, less researches have focused on study of the long-term

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photocathodic protection by semiconductor photoelectrodes because traditional PEC characterizations

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is difficult to evaluate the long-term corrosion resistance ability. Thus, our in-situ imaging system exhibits obvious advantages compared to conventional electrochemical measurements. More

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importantly, this is the first time that in-situ imaging system has been used to directly monitor the realtime corrosion behavior of the protected metal, and successfully confirms that PEC cathodic protection

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techniques are viable and effective for the long-term protection of metal electrode. With this method,

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we will able to accurately evaluate and detect the photocathodic protection performance caused by the photoelectrode materials. We believe that the new approach developed here will extend to other areas of corrosion research and find a wider application for inspection of materials in the future. 5. Conclusions

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In this work, we demonstrated for the first time that PVM technology were successfully employed as a powerful tool for the online monitoring and visualization of the PEC cathodic protection process. It was observed that the surface of carbon steel was quickly corroded in the chloride containing environment, due to chloride pitting. However, its surface can be continuously protected no matter connected with anatase or rutile TiO2 photoelectrodes, which directly confirmed that the PEC cathodic protection was real efficiency for metal anticorrosion. Clearly, compared to the conventional PEC test

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technique, the in situ video microscope system provide more valuable insight regarding the morphology of corrosion scales, which is simple and effective technique for detecting and evaluating

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the long-term PEC cathodic protection performance and revealing the corresponding corrosion mechanisms. The visual inspection approach developed here will provide direct evidence of metal

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anticorrosion and contribute to the better design of semiconductor photoelectrodes for future PEC

Declaration of interests

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cathodic protection application.

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 The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

This work was supported by National Students’ platform for innovation and entrepreneurship training program (201910350032, 201910350015), School of Research and Cultivation Project (Z2018056) and Zhejiang Provincial Natural Science Foundation of China (LY19E020002, LY19E030001) and Liaoning Provincial Natural Science Foundation of China (20180550947). 15

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one-step electroless deposition of NiFeOOH nanosheets onto photoanodes for highly durable and efficient solar water oxidation. J Mater Chem A 6:20678-20685

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27. Zuo J, Wu H, Chen A, Zhu J, Ye M, Ma J, Qi Z (2018) Shape-dependent photogenerated cathodic protection by hierarchically nanostructured TiO2 films. Appl Surf Sci 462:142-148

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28. Xie X, Liu L, Chen R, Liu G, Li Y, Wang F (2018) Long-term photoelectrochemical cathodic

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protection by Co(OH)2-modified TiO2 on 304 stainless steel in marine environment. J Electrochem Soc 165:H3154-H3163

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29. Ma X, Mai M, Lin H, Zeng L, Zhang J, Zhou H, Chen D (2019) A novel electrochemical method for

simultaneous

measurement

of

real-time

potentials

and

photocurrent

of

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photoelectrochemical systems. J Power Sources 415:99-104

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Figures captions Fig. 1 Schematic illustration of experimental setup used for PEC measurement (A) and PVM measurement (B), respectively. Fig. 2 XRD patterns of anatase (a) and rutile (b) films. (c, d) low-magnification and (e, f) highmagnification SEM images and of anatase and rutile films, respectively. Fig. 3 (A) Variation of the open circuit potential of pure carbon steel connected with anatase

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and rutile TiO2 films; (B) Tafel curves of pristine carbon steel electrode measured in the dark, carbon steel connected with anatase and rutile TiO2 photoelectrodes in 3.5 wt% NaCl solution

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under simulated solar light (100 mW/cm2).

Fig. 4 PVM images taken within 4 h at every 30 min interval for carbon steel soaked in the

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3.5wt% NaCl aqueous solution, when disconnecting with the TiO 2 photoelectrodes.

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Fig. 5 PVM images taken within 6 h at every 2 h interval for carbon steel when connected with the anatase TiO2 photoelectrode.

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Fig. 6 PVM images taken within 6 h at every 2 h interval for carbon steel when connected with

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the rutile TiO2 photoelectrode.

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Fig. 1

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Figures

(b)

(c)

(d)

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(e)

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(a)

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Fig. 2

(f)

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Anatase TiO2

Carbon steel Anatase TiO2

(B)

Rutile TiO2

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log i (A cm-2)

light on

-450 -500 -550 0

200

-4

-5

-6

light off

OCP (mV vs. SCE)

Rutile TiO2

(A)

-300

400

600

800

1000

1200

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-550

-500

Time (s)

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-400

-350

E (mV vs. SCE )

t=0 min

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Fig. 3

t=30 min

100 μm

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100 μm

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100 μm

t=60 min

t=120 min

100 μm

100 μm

100 μm

100 μm

100 μm

t=210 min

t=240 min

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t=180 min

t=150 min

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t=90 min

100 μm

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Fig. 4

t=2 h

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100 μm

t=4 h

t=6 h

100 μm

100 μm

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Fig. 5

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t=0 h

t=0 h

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100 μm

t=6 h

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Fig. 6

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