Electrochemical Impedance of Rust Film Fabricated by Deposition from Fe(III) in Solution

75，No. 12（2007） 945

― Article― Electrochemical Impedance of Rust Film Fabricated by Deposition from Fe(III) in Solution

Masayuki ITAGAKI,a Harumi ARAKI,a Isao SHITANDA,a Kunihiro WATANABE,a Hideki KATAYAMA,b and Kazuhiko NODAc

aDepartment of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science (Noda, Chiba 278-8510, Japan) bCorrosion Research Group, National Institute for Materials Science (Sengen, Tsukuba, Ibaraki 305-0047, Japan) cDepartment of Materials Science and Technology, Faculty of Engineering, Shibaura Institute of Technology (Tokyo 108-8548, Japan)

Received July 4, 2007 ; Accepted September 27, 2007

The physical and chemical properties of the rust film are quite significant for the corrosion resistance of weathering steel. The investigations of the ion-selective permeability and the ion-permeation resistance (film resistance) are necessary to characterize the rust film. In the present paper, the membrane potential and the electrochemical impedance of the rust film, which was fabricated by the precipitation of Fe(III) from the solution, were measured. The rust film showed the anion-selective permeability in the neutral solution, indicating that the pore wall in the rust film has a positive fixed charge. The measurements of electrochemical impedance of the rust film membrane were performed by four-electrode method. The working and reference electrodes were platinum wire and silver/silver chloride electrode, respectively. The current between two working electrodes was measured by controlling the alternative voltage between two reference electrodes. The electrochemical impedance showed an apparent capacitive loop on the complex plane, and the diameter of the capacitive loop corresponds to the film resistance. The ion-selective permeability and the film resistance were changed depending on the adsorption of metallic ions on the rust film.

Key Words : Rust Film, Ion-selective Permeability; Ion-permeation Resistance (film resistance), Electrochemical Impedance, Adsorption of Metallic Ions

1 Introduction generated from thin sheet of the low alloy steels by In atmospheric environment, steel is oxidized easily wet/dry cycles and determined their ion-selective per- and forms the rust layer as corrosion product on the sur- meabilities. They5,6）found that the rust films of Fe-Co face. The weathering steel has high corrosion resistance and Fe-Al low alloy steels have the cation-selective per- due to this corrosion product. The weathering steel is meability, and revealed the importance of the ion-selec- low-alloy steel doped with small amount of Cu, Cr, Ni ele- tive permeability for the corrosion resistance. Itagaki et ments and so on. The weathering steel has realized the al.7）measured the impedance of the rust film made from practical use without painting on the surface, and shows the low alloy steel sheet by the complete oxidation at the higher resistance against the atmospheric corrosion wet/dry cycles, and evaluated the film resistance related than the usual steel.1）Furthermore development of new to the ion migration inside the rust film. It is considered weathering steel is expected because high corrosion that the alloy element has two factors to increase the resistance of the weathering steel decreases in sea-shore film resistance, namely, the modifications of structure environment. Moreover, the role of alloying element on and surface. In the present paper, the influence of the corrosion resistance of the weathering steel has not adsorption of metallic ions on the rust film is investigat- been clarified yet. A lot of physical analyses like struc- ed to discuss the contribution of alloy element to modifi- ture analyses of the rust formed on the weathering steel cation of the rust film surface. The ion-selective perme- surface have been carried out.1-3）Characterizations of ability and the film resistance are measured to evaluate chemical property of the rust film were performed since the chemical properties of the rust film surface with the the corrosion reaction at metal surface is influenced by adsorption of various metallic ions. the mass transfer of ions inside the rust film. Sakashita et al.4）measured the membrane potential of an artificial 2 Experimental Methods rust film to evaluate the mass transfer of ions inside the 2. 1 Fabrication of the rust film rust film. They4）reported that the hydrous Fe(III) oxide The rust film was fabricated by the following proce- had the anion-selective permeability and the hydrous dure.4）The cellulose membranes were hold between a 2− Fe(III) oxide with the adsorption of MoO4 showed the pair of plastic cells which have hole of 30 mm diameter. cation-selective permeability. On the other hand, Noda et One cell contained 0.1 mol/dm3 NaOH aqueous solution 5,6） 3 al. measured the membrane potential of the rust films and another cell contained 0.1 mol/dm Fe(NO3) aqueous 946 Electrochemistry

Fig. 1 Scheme of electrochemical cell to measure the membrane potential. Fig. 2 Scheme of experimental set-up to measure the electrochemical impedance of the rust film. WE1 and WE2 solution. The cellulose membranes were fixed between are working electrodes. RE1 and RE2 are reference two cells and immersed between two solutions for 3 electrodes. days, and the rust film was precipitated on the cellulose membrane of Fe(NO3) solution side. The cells were exchanged, and the rust film was precipitated on another measured by controlling the potential between two refer- side of membrane for 5 days. The rust film was rinsed ence electrodes. The amplitude of the potential modula- by distilled water and was retained in distilled water for tion of the working electrode was 10 mV. Therefore, the one day. When the influence of the adsorption of metallic measured impedance involved the solution resistance in ions was investigated, the rust film was immersed in the two cells and film resistance. The frequency range of the solution containing various metallic ions of 0.1 mol/dm3 impedance was from 10 mHz to 10 kHz. for one hour. The metallic ions investigated in the present experiment were prepared by the dissolution of 3 Results and Discussion

ZnCl2, CrCl3, Na2CrO4, Na2WO4, Na2MoO4 or Na4P2O7. The 3. 1 Membrane potential rust film was rinsed by distilled water after the immer- The rust film was ground to powder with agate mor- sion in the solution containing metallic ions. The thick- tar and the composition of the rust film was analyzed by ness of rust film was measured with a micrometer after powder X-ray diffraction (PHILIPS X’Pert Pro). As the drying. All processes in this fabrication were carried out result, it was found that main component of the rust film in the room temperature (25℃). was α-FeOOH. 2. 2 Measurement of membrane potential Figures 3 and 4 show the plots of the membrane Figure 1 shows scheme of electrochemical cell to mea- potential of the rust film with and without the adsorption sure the membrane potential. The thin rust film was of metallic ions and the concentration ratio of NaCl in sandwiched between two acrylic sheets which have the two cells. The membrane potential Vm is expressed by hole of 11 mm diameter. These acrylic sheets fixed Eq. (1), between the two cells containing NaCl solutions. The NaCl concentration in one cell was 0.01 mol/dm3, and RT Vm ＝ (t+−t−) 1n (c2/c1) (1) that in another cell was from 0.001 mol/dm3 to 0.1 F mol/dm3. The membrane potential was measured as potential difference between two reference electrodes where R is the gas constant, T is the absolute tempera- which were saturated KCl/AgCl/Ag electrode (SSE) ture, F is the Faraday constant, t+ is the transport num- with electrometer (Hokuto, HA1010mM1A). The solu- ber of the cation, t− is the transport number of the anion tions in two cells were stirred, and the membrane poten- and c2/c1 is the concentration ratio of NaCl in two cells. tial was measured after obtaining the steady state. In Fig. 3, the dotted lines that have positive (60 2. 3 Measurement of electrochemical impedance mV/decade) and negative (−60 mV/decade) slopes mean Figure 2 shows scheme of set-up to measure the elec- the perfect cation and anion-selective permeabilities, trochemical impedance of the rust film. The rust film respectively. The perfect cation and anion-selective per- was fixed between two cells by the same way to the meabilities correspond to t+ ＝1 and t−＝1, respectively in measurement of membrane potential. The both elec- Eq. (1). In Fig. 3, the plots of the rust film that are denot- trolyte solutions in two cells were 0.1 mol/dm3 NaCl. The ed by solid circle show negative slope, indicating that the LCR meter (Hioki, 3522-50) was used to measure electro- rust film has the anion-selective permeability. Moreover, chemical impedance by four-electrode method. The the plots represented by open circle also show negative working electrodes were platinum wires and the refer- slope in the case of the rust film with Zn2+ or Cr3+ ence electrodes were Ag wire covered with AgCl. The adsorption. Suzuki et al.8）stated in the measurement of current flowing between two working electrodes was membrane potential that artificial rust film has anion- 75，No. 12（2007） 947

Fig. 5 Schematic diagrams of pore wall surface of Fe(III) iron hydroxide contacted with solution.

Fig. 3 Plots of membrane potential and the ratio of concentrations of NaCl in two cells. Open and solid circles denote the plots for the rust film with and without metallic ions adsorption, respectively. Dotted lines of the perfect cation-selective (60 mV/decade) and anion-selective (−60 mV/decade) permeability, respectively. ●: No adsorption, △: Adsorption of Zn(II) and ○: Adsorption of Cr(III). Fig. 6 Schemes of steel covered with two kinds of rust films of ion-selective permeability.

fixed charge of the surface. Since oxo acid ions that have a negative charge adsorb on the pore wall surface in the 2− rust film, the rust film with the adsorption of MoO4 , 2− 2− WO4 or CrO4 shows the cation-selective permeability. 2− The amount of adsorption of MoO4 was determined by using inductively coupled plasma atomic emission spec- trometry (ICP) by dissolving the rust film in acid solution. It was found that the molecular of adsorption per unit area of cellulose membrane was 4.1×10−6 mol/cm2. Figure 6 shows the conceptual illustration to explain the influence of the ion-selective permeability of the rust film on steel. It is assumed that there are anodic and cathodic reaction sites during the corrosion. When the anodic reaction progresses under the rust film, the current flows from the steel to the rust film surface. This current corresponds to the ion migration through the Fig. 4 Plots of membrane potential and the ratio of rust film. The ion migration is controlled by the ion-selec- concentrations of NaCl in two cells. The dotted lines of the tive permeability of the rust film. When the steel is cov- perfect cation-selective (60 mV/decade) and anion-selective ered with the rust film of the anion-selective permeabili- (−60 mV/decade) permeability, respectively. □: Adsorption ty, anions migrate from the solution to the rust of Mo(VI), △: Adsorption of Cr(VI) and ○: Adsorption of film/metal interface with the progress of corrosion. If the W(VI). anion is a chloride ion in this case, the concentration of chloride ions at the rust film/metal interface accelerates selective permeability. The results in Fig. 3 agree with the anodic reaction. Besides, when the steel is covered Suzuki’s report.8）On the other hand, all plots show posi- with the rust film of the cation-selective permeability, tive slope in Fig. 4, indicating that the rust film with the the anions do not concentrate at the rust film/metal adsorption of oxo acid ions has the cation-selective per- interface. And the ion migration rate is small because meability. the ions dissolved from the steel must be carried Figure 5 shows schematic diagrams of the pore wall through the film. surface of Fe(III) oxyhydroxide contacted with solution.9） 3. 2 Electrochemical impedance Since the fixed charge at pore wall surface of α-FeOOH Figure 7 shows the equivalent circuit of the corroding has a positive fixed charge,9）the rust film shows the steel covered with the rust film. In general, the quality of anion-selective permeability. Moreover, the immersion in the rust film formed on the steel has an important role the solution containing Zn2+ or Cr3+ does not change the for the corrosion resistance of steel in atmospheric corro- 948 Electrochemistry

Fig. 7 The equivalent circuit of the corroding steel covered with the rust film. Fig. 9 Nyquist plots of the electrochemical impedance of the rust film in 0.1 mol dm−3 NaCl solution. Open and solid circles denote the plots for rust film with and without 2− MoO4 adsorption, respectively.

Fig. 8 Nyquist plots of the electrochemical impedance of the rust films of different thickness in 0.1 mol dm−3 NaCl solution.

sion. Thus, the estimation of the film resistance is quite Fig. 10 The plots of Rf of the rust film without and with significant to discuss the corrosion behavior of steel cov- metallic ions adsorption. The dotted line indicates the ered with the rust film. In this paper, the electrochemical relation of Rf of the rust film without the adsorption against impedance of the rust film itself was measured by four- the film thickness. electrode method. Since the rust film has the time constant of the ion permeation resistance (film resistance) Rf and the film capacitance Cf as shown in Fig. 7, it is that the capacitance was related to the space charge in expected that the Nyquist plots of the electrochemical the oxide film contacted with electrolyte. Though there impedance show the capacitive behavior. The diameter are two possibilities to explain the origin of this capaci- of capacitive loop corresponds to Rf, and Cf and Rsol were tance, we cannot decide it due to lack of sufficient infor- also obtained by the curve-fitting. mation in this moment. Two results of the solution resis-

Figure 8 shows the Nyquist plots of the electrochemi- tance Rsol in Fig. 8 were identical because the electro- cal impedance of the rust films of different thickness. chemical impedance was measured by using the same The electrolytes in two cells were 0.1 mol/dm3 NaCl cell and experimental set-up. Itagaki et al.11）reported in solution. The impedance showed the capacitive semicir- the measurements of the electrochemical impedance of cle on the Nyquist plane and the diameter of the capaci- an artificial rust film that the Rf decreases with increase tive semicircle increased with the increase of the film of NaCl concentration, and that ions in the rust film take thickness. This result means that the ion migration in a charge as electric carrier. Figure 9 shows the Nyquist the rust film is controlled by the film thickness. Some plots of the electrochemical impedance of the rust film 2− parameters of these rust films were obtained by the with and without MoO4 adsorption. The electrolyte was curve-fitting to the electrochemical impedance in Fig. 8. 0.1 mol/dm3 NaCl solution. The thicknesses of the rust 2− The film capacitance of the rust film of 0.70 mm thick- film with and without MoO4 adsorption were 0.80 mm −4 2 2− ness was 9.6×10 F/cm . The calculated relative per- and 0.86 mm, respectively. It was found that the MoO4 mittivity is an order of 109. The value of this relative per- adsorption increased the diameter of the capacitance mittivity is remarkably larger than the relative permit- semicircle on the Nyquist plane. The Rf increased 2− tivity of general dielectric material: for example that of remarkably by the adsorption of MoO4 , indicating that 3 2− barium titanate is a digit of 10 . This huge value may the presence of MoO4 gave the low migration rate of originate from the pseudo-capacitance related to the ions in the rust film. redox reaction of the bivalent/trivalent oxide in the rust The measurements of the electrochemical impedance film. Fujimoto et al.10）found the large capacitance of of the rust film with the various metallic ions adsorption 2− porous oxide film formed on Fe-Cr alloy, and reported were performed. The various metallic ions were WO4 , 75，No. 12（2007） 949

2− 4− 2+ 3+ CrO4 , P2O7 , Zn and Cr . The Rf obtained by curve- 4 Conclusion fitting are plotted against the film thickness in Fig. 10. The rust film was manufactured from NaOH aqueous

The Rf of the rust film without the adsorption, that is solution and Fe(NO3) aqueous solution by the precipita- denoted by solid circles in Fig. 10, is in proportional to tion reaction. The following knowledges were obtained the film thickness. The Rf’s with the adsorptions of by the measurements of the membrane potential and the 2− 2− WO4 and CrO4 as oxo acid ions were larger than the electrochemical impedance of the rust film. 2− Rf without the adsorption, as well as the Rf with MoO4 It is found that the rust film, which is composed of α- adsorption. As above-mentioned, the oxo acid ions includ- FeOOH, has the anion-selective permeability by the 2− ing the MoO4 change the anion-selective permeability of membrane potential measurement in NaCl solution and 2− 2− 2− the rust film into the cation-selective permeability. These the rust films with MoO4 , CrO4 and WO4 adsorptions results indicate that the cation-selective permeability and have the cation-selective permeability. The impedance high film resistance should give the high corrosion resis- spectra of the rust film were measured. The immersion tance in the presence of oxo acids ions. In order to verify of the rust film in Na2MoO4, Na2CrO4 and Na2WO4 solu- the role of oxo acids ions, the electrochemical impedance tion increased the film resistance, since oxo acid ions 4− of the rust film with P2O7 adsorption was measured. adsorb on the pore wall surface of the rust film and 4− Though P is not metal, the adsorption of P2O7 enlarges reduce the ion permeation rate through the rust film. the Rf, indicating that the negative charge of oxo acids ion has an important role to modify the quality of the References 3+ rust film. The effect of Cr on the Rf was examined in 1）S. Misawa, M. Yamashita, and H. Nagano, Materia 2− order to compare with the adsorption effect of CrO4 . As Japan, 35, 783 (1996). 2− above-mentioned, the Rf with CrO4 absorption was larg- 2）N. Masuko and Y. Hisamatu, Boushoku Gijutsu, 17, 37 er than the Rf without the adsorption. However, the Rf (1968). with the absorption of Cr3+ shows the similar value to 3）T. Ishikawa and T. Nakayama, Zairyo-to-Kankyo, 52, 140 (2003). the Rf without the adsorption. There are two possibilities 4）M. Sakashita, Y. Yomura, and N. Sato, Denki Kagaku for the reason why the immersion in CrCl3 solution 3+ (Electrochemistry), 45, 165 (1977). doesn’t influence the Rf ; i.e. , no adsorption of Cr and 5）K. Noda, T. Nishimura, H. Masuda, and T. Kodama, J. no action of Cr3+. Then, the amount of adsorption of Cr3+ Japan Inst. Met., 63, 1133 (1999) . was determined by using ICP. It was found that the mol- 6） J. −6 2 K. Noda, T. Nishimura, H. Masuda, and T. Kodama, ecular density of adsorption was 3.4×10 mol/cm . Japan Inst. Met., 64, 767 (2000). 3+ Therefore, it was confirmed that Cr adsorbed on the 7）M. Itagaki, R. Nozue, K. Watanabe, H. Katayama, and 3+ rust film and Cr does not effect the ion-selective perme- K. Noda, Corros. Sci., 46, 1301 (2004). 2+ ability and the Rf . Secondly, The effect of Zn on the Rf 8）I. Suzuki, N. Masuko, and Y. Hisamatsu, Boushoku 2+ was examined. The Rf with Zn adsorption was larger Gijutsu, 20, 319 (1971). than the Rf without adsorption and it was found that the 9）N. Sato, Denkyoku Kagaku, Nittetsu Gijutsu Joho Center, presence of Zn2+ gave the low migration rate of ions p.374 (1994). through the rust film. Though the reason why Zn2+ 10）S. Fujimoto, S. Kawachi, T. Nishio, and T. Shibata, J. enlarges Rf is not clarified in this moment, this result Electroanal. Chem., 473, 265 (1999). may become the fundamental knowledge of corrosion 11）M. Itagaki, H. Araki, K. Watanabe, H. Katayama, and resistance of galvanized steel. K. Noda, Passivation of Metals and Semiconductors and Properties of Thin Oxide Layers, Elsevier, p.317 (2005).