Toward a Slow-Release Borate Inhibitor To Control Mild Steel

Subscriber access provided by READING UNIV

Article

Towards a slow-release borate inhibitor to control mild steel corrosion in simulated recirculating water Jun Cui, Yange Yang, Xiuqing Li, Wenjiao Yuan, and Yuansheng Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15507 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Towards a Slow-release Borate Inhibitor to Control Mild Steel Corrosion in Simulated Recirculating Water Jun Cui,† Yange Yang ‡,§, Xiuqing Li,† Wenjiao Yuan,† Yuansheng Pei*,† †

The Key Laboratory of Water and Sediment Sciences, Ministry of Education, School of Environment, Beijing Normal University, Beijing 100875, PR China ‡ Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, China § Navy Coating Analysis and Test Center, Beijing 102442, China

ABSTRACT: Based on their potential passivating characteristics, in this study, borates have been used to synthesise a novel slow-release inhibitor to suppress long-term mild steel corrosion in simulated recirculating water. The passivating performance was characterised by various electrochemical measurements and the passivating mechanism was interpreted by the point defect model. The experimental results indicated that the slow-release inhibitor exhibited a passivating efficiency of over 98% after 30 days of immersion, due to the formation of a passive film that was predominant by a Fe-O-B structure on the mild steel surface. This study provides a novel controlled-release concept and a slow-release borate inhibitor to control long-term corrosion. KEYWORDS: Corrosion inhibitor, Controlled-release, Borate, Electrochemistry, Point defect model Graphical abstract goes here

1. INTRODUCTION Pipeline corrosion, which is one of the most common problems in open recirculating water system (ORWS), has attracted the attention of many researchers.1-3 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Water evaporation is a continuous process in ORWS, leading to a constant increase in the concentrations of salt ions (Cl–, SO42–, HCO3–, etc.), which in turn accelerates the corrosion rate of metal pipelines.4-6 In general, aqueous phosphate and/or metal salt inhibitors are used to control the corrosion of metal pipelines in ORWS.7 However, the inhibitor concentration decreases owing to the discharge of salt-concentrated water from the ORWS. Therefore, aqueous inhibitors need to be added regularly into the ORWS to maintain an acceptable passivating efficiency in the long term. Over the past few decades, the structure of borates has been widely investigated. Previous studies report that the basic structures of borate are [BO3]3– triangle planar and [BO4]4– tetrahedron and these B-O anions are connected by the same atom or edge to form different complicated polyanion clusters.8,9 Meanwhile, these B-O anions and polyanion clusters can combine with metal ions, including Al, Fe, Sc, and In, to form Metal-O-B structures.10-13 Based on these findings, we infer that the Fe derivatives from the mild steel pipeline can react with B-O anions to form a passive film on the pipeline surface and suppress corrosion. Borate solution (0.05 M H3BO3 + 0.075 M Na2B4O7·10H2O, total boron concentration over 3000 mg L–1 in theory) is widely used as a pH buffer in metal corrosion protection research at the lab scale.14-16 Such a high boron concentration in the effluent, however, may increase the treatment cost of plants and environmental pollution in the effluent-receiving region. A relatively low concentration borate solution as a buffer also exhibits resistance to variations in the water quality and can potentially reduce corrosion in pipelines.17 However, the passivating effect and mechanism of low-boron solutions are not very well understood yet. Given the above mentioned disadvantages, in this study, a controlled-release concept for inhibitors is proposed to suppress corrosion in ORWS. The inhibitor is a soluble glass material composed of borates and silicates. The solubility and dissolution rate of the material can be designed in accordance with the raw material composition and the synthesis conditions. Further, the dissolution process may take place with a little fluctuation, depending on the chemical equilibrium between the solution and the inhibitor. The inhibitor continuously dissolves in the ORWS and ACS Paragon Plus Environment

Page 2 of 50

Page 3 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


releases effective inhibitive components, resulting in a good long-term passivating performance at a low boron concentration (less than 300 mg L–1). The present investigation is part of a larger research that aims at developing a novel inorganic agent to control corrosion, scale, and biofouling in ORWS. In the present investigation, we report the synthesis of a novel slow-release borate inhibitor to suppress mild steel corrosion in simulated recirculating water (SRW). The synthesised inhibitor and its passivating performance are discussed in detail and the passivating mechanism is explained based on the obtained results.

2. METHOD AND MATERIALS 2.1 Preparation and characterisation of the slow-release inhibitor. All the inorganic reagents (≥99% purity) used in this study were obtained from Sinopharm Chemical Reagent Beijing Co. Ltd. The synthesis process is shown in Fig. 1, together with the mass-transport system between controlled-release inhibitor and SRW. Moreover, all the inhibitor particles possessed a similar surface area that affected the mass-transport capability by using the same mould. Figure 1 goes here

To analyse the crystalline nature of the inhibitor components, X-ray diffraction (XRD) testing was carried out at a scan rate of 4°/min using a Rigaku D/Max-B diffractometer operated at 40 kV and 40 mA. In addition, X-ray photoelectron spectroscopy (XPS; Axis Ultra Dld spectrometer, Shimadzu, Japan) using a monochromatic Al-Kα X-ray source (hν = 1486.6 eV) and an X-ray beam of around 1 mm was used for the internal elemental analysis of the inhibitor. The high-resolution scan was performed with a 0.1 eV step over the following regions of interest – O 1s, B 1s, and Si 2p. A controlled-release analysis was carried out in SRW (500 mL) with 2.4 g inhibitor circulating at 100 r/min and 40 °C. According to the recirculating water quality of the power plant (Kaifeng, China), the chemical composition and the corresponding

ACS Paragon Plus Environment


Page 4 of 50

concentration of SRW were adjusted (Table 1). The controlled-release experiment was conducted for 30 days and deionized water (Milli-Q, USA) was added every day to compensate for the loss of water by evaporation. To measure the total boron concentration, an aliquot (1 mL) of the water sample was extracted from the solution every day and analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman, Profile). Five samples were picked every day for the ICP-AES analysis, in order to reduce experimental errors. Prior to ICP-AES analysis, all the samples were diluted to 5 mL and passed through a 0.22 µm membrane to ensure that the ion concentration was in the appropriate range for ICP-AES detection. Table 1. Chemical Composition of the SRW (500 mL) Anions

Ca2+

Mg2+

NO3-

HCO3-

Na+

Cl-

Analytic grade reagent Mass / g Concentration / (mg L-1)

CaCl2 0.2775 200

MgCl2•6H2O 0.5083 120

NaNO3 0.0686 100

NaHCO3 0.0689 100

--200

--640

Note: Na+ and Cl- were provided by the CaCl2, MgCl2•6H2O, NaNO3, and NaHCO3.

2.2. Electrochemical measurements. In this work, a mild steel cylinder strip (composed of 0.15 wt.% C, 0.46 wt.% Mn, 0.28 wt.% Si, × ? × 5 × @1 )

(8)

9 = @ × expD 3 + @1

(9)

where iss is the steady-state passive current density derived from the potentiodynamic polarization curve, R is the universal gas constant, F represents Faraday’s constant, and k is determined to be approximately 1.02 × 106 V/cm for the passivating film grown on mild steel. ω1, ω2, and a are all constants determined from the Mott-Schottky experimental data using Eq. (9). Fig. 16 illustrates the variations in the Mott-Schottky plots of the inhibited sample after 1, 5, 13, 21, and 30 days of immersion. It can be seen that all the passivating films exhibited clear n-type semiconductor characteristics.50 According to the point defect model (PDM) theory, in an n-type semiconductor, the oxygen vacancy and metal interstitial serve as the electron donors in the space charge region. In contrast, a cation vacancy is the electron acceptor in the space charge region of a p-type passivating film.51,52 The range of the test potential was -762~-20 mV, -690~60 mV, -595~300 mV, -549~240 mV, and -486~210 mV for the samples after 1, 5, 13, 21, and 33 days of immersion, respectively, which was determined by the passivation region of the potentiodynamic polarization curve. The flat band potential (Efb) gradually increased with increasing immersion time in the following order: 1 d (–1.08 V) < 5 d (–1.02 V) < 13 d (–0.98 V) < 21 d (–0.86 V) < 30 d (–0.82 V). The Nd and D values of the passivating film are calculated and shown in Fig. 17a. The Nd as well as the D values gradually decreased by over two orders of magnitudes after 30 days of immersion and tended to be stable as the immersion time increased. According to many researchers,53,54 the Nd and D values are closely related to the stability of passivating film. Higher Nd and D values imply a weak passivating performance. The Efb and Nd results were consistent with the PDM theoretical predictions, suggesting that a higher passivation potential is helpful in reducing Nd, and promotes the formation of a highly ordered and less defective passivating layer. The Efb, D, and Nd ACS Paragon Plus Environment


Page 18 of 50

results indicated that the protective performance of the passivating film increased with increasing immersion time. According to the electronic energy-band theory, the Fermi level is close to the bottom of the conduction band in n-type semiconductors owing to the high concentrations of oxygen vacancies and metal interstitials. Efb can be approximately regarded as the conduction band edge.55,56 It is generally accepted that a higher Efb represents a lower reducibility. In the current investigation, we observed that the value of Efb gradually increased with increasing immersion time, which indicated that the reducibility of the passivating film gradually decreased with increasing immersion time. In other words, the work function of the electron in the passivating film increased, resulting in a reduced electron transfer capability. Moreover, it can be observed that all the Mott-Schottky curves display a distinct shoulder. This nonlinear phenomenon is attributed to the existence of a broad range of energies for Nd. Figure 16 goes here Figuer 17 goes here

The thickness of the passivating film formed on the mild steel surface was calculated using Eq. (3) and the results are illustrated in Fig. 17b. The thickness of the passive

film

was

only

qualitatively

described

owing

to

the

effect

of

adsorption-desorption layer. The thickness of the passivating film was in the nanometre range and gradually increased with increasing immersion time. Combining the thickness of the passivating film with all the other obtained results, we believed that an increase in the thickness of the passivating film represented a better passivation effect. The Mott-Schottky results indicated that the passivating film possessed n-type semiconductor characteristics in the presence of the inhibitor and the thickness of the passivating film increased with increasing immersion time. Meanwhile, the reducibility of the passivating film decreased after 30 days of immersion, suggesting that the electric work function of the passivating film increased and that electron transfer was retarded, resulting in an enhanced passivating


Page 19 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance. 3.4. Passivating mechanism analysis 3.4.1. Electrochemical noise measurement EN signals were measured to determine the corrosion behaviour of the mild steel surface. Fig. 18 shows the changes in potential and current noise during galvanic corrosion tests for the first 3600 s. It can be seen that the electrochemical current noise (ECN) and electrochemical potential noise (EPN) signal amplitudes of the inhibitor-free sample were higher than those in the inhibited sample. Meanwhile, the ECN and EPN signal amplitudes gradually decreased with increasing immersion time, indicating that the mild steel surface exhibited a more stable state at longer immersion times.57 In addition, it was well recognized that EPN signal fluctuate reflects the capacitive characteristic variations of the passive film, all the EPN signals showed rapid decrease and recovery, suggesting that the passivating film undergoes a rapid breakdown and healing process. The results indicated that a better passivating performance was obtained at longer immersion times.58 Figure 18 goes here

In the time domain curves, it was difficult to distinguish the type of corrosion occurring on the mild steel surface. Thus, in this work, a wavelet transformation method with 8 levels was used to deconvolute the time domain data of the ECN signals and the results were used to establish the relationship between the ECN signals and the type of corrosion. The two-dimensional diagrams of the wavelet transformation deconvolution results are shown in Fig. 19. Each rectangle represents a wavelet coefficient and a higher energy corresponds to a lighter colour. According to the energy distribution of the inhibitor-free sample, a light colour indicates the inhibitor-free sample in the 1~4 levels in the early stage (0~1200 s); later, the light colour evolved during the 5~8 levels in the middle and late stages (1200~3600 s). This variation indicated that general corrosion dominated the corrosion process in the early stages because the mild steel was directly exposed to the SRW without the protection of the passivating film.59 Later, the intensity of corrosion was decreased in ACS Paragon Plus Environment


Page 20 of 50

the middle and late stages because a loose corrosion or precipitate layer had been formed on the mild steel surface.58 Compared with the inhibitor-free sample, the inhibited sample showed a light colour during the 1~4 levels in the early stage and the colour gradually darkened in the middle and later stages. Moreover, the energy strength gradually decreased with increasing immersion time. The wavelet transformation results indicated that corrosion intensity of the inhibited sample was gradually decreased with increasing immersion time. Figure 19 goes here

The parameters derived from the EN measurements are illustrated in Fig. 20. In this study, the noise resistance (Rn) is calculated using Eq. (10)60 " = GH ⁄GI

(10)

where σV and σI refer to the standard deviations in the fluctuations of potential and current signals, respectively. Another useful method, the short noise theory, was employed to analyse ECN signals by considering them to be packets of charge in the frequency domain. Hence, the charge of each electrochemical event (q) and the event frequency (fn) can be calculated using Eqs. (11) and (12), respectively.59 7 = JK × JKI ⁄L

(11)

= L 1 /K ×

(12)

where, ΨE and ΨI are the low frequency PSD values of the potential and current noises, respectively, B is the Stern-Geary coefficient, and A defines the WE area. q was calculated based on the values of ΨE and ΨI at 0.001 Hz and the average B value (0.026 V). qn and fn relate to the charge of each electrochemical event and the corrosion frequency, respectively. High qn and fn values indicate the serious general corrosion on the electrode surface during the electrochemical event. In contrast, low qn and fn value correspond to passivation performance. It can be seen in Fig. 20a, compared to the inhibited sample, the inhibitor-free sample exhibited higher fn and qn values, indicating that the inhibitor-free sample experienced severe general corrosion. For the inhibited sample, both qn and fn values gradually decreased with increasing


Page 21 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


immersion time. The results demonstrated that the passivation performance increased with the increasing immersion time. In addition, Rn is supposed to show a negative relationship with icorr. In the current study, Rn values were higher in the inhibited sample than in the inhibitor-free sample and they gradually increased with increasing immersion time (Fig. 20b). The increase in Rn represented a better passivating performance.60 The EN results demonstrated that a better passivating performance was obtained at a longer immersion time. Figure 20 goes here

3.4.2. PDM analysis It is generally believed that PDM is a useful method to describe the substance transfer processes in the substrate-film-SRW system, which helps us gain a deep understanding on the breakdown of the passivating film. The PDM analysis results from the electrochemical data are illustrated in Fig. 21. Reaction 1 (R1) occurred at the interface between the substrate and the passive film (S/PF). Cation vacancies (VFe) emerged in the passivating film and these VFes moved towards the substrate under the action of an electric field. When the VFe reached the mild steel surface, it immediately combined with Fe to form FeFe and a vacancy (VM). On the one hand, FeFe moved towards the interface between the passivating film and the SRW (PF/SRW). When the FeFe reached the SRW, it immediately reacted with H+, forming Fe2+ and H2 (R2). Subsequently, Fe2+ was oxidized to Fe3+ owing to the extensive amount of NO3– in SRW (R4). On the other hand, VM entered into the substrate and reacted with Fe (R3). This process annihilated the VM in the substrate and the amount of annihilated VM in a unit area was considered as the cation vacancy annihilation flux (JM).61-65 According to the Null pair theory (R5), an electrochemical system generates two oxygen vacancies (VO) in response to the generation of a single VFe. Hence, the VO formed in the passivating film and moved towards the PF/SRW interface, which is opposite to the direction of movement of VFe. Normally, VO combines with oxygen moieties. However, when Cl–, Br–, and I– exist in the electrolyte, VO would combine with these ions instead of oxygen due to the strong binding capability of these ions ACS Paragon Plus Environment


with VO. In the SRW system, VO combined with Cl– to form ClO (R6). Subsequently, ClO reacted with the FeFe (R7), resulting in the formation of VFe, VO, Cl–, and Fe2+. VFe and VO were used to compensate for the loss of VFe and VO in R1 and R7, respectively. The amount of moving VFe in a unit area is defined as the cation vacancy flux (JCV). R7 generated a large number of VFes, resulting in an increase in the JCV (in theory). Normally, if JCV > JM, more number of VFes would accumulate on the S/PF interface. These accumulated VFes are supposed to be the weakest areas of the passivating film, which resulted in its breakdown. If JCV < JM, the VFes can immediately annihilate. Later, there was hardly any accumulation of VFes at the S/PF interface, leading to a better passivation performance.66-68 Based on the above analysis, we theorised that the accumulation of VFes at the S/PF interface could induce the breakdown of the passivating film. However, in this study, the passive film was predominated by the Fe-O-B structure. Boron could provide covalence electron, resulting in reducing the density of VO. This could be used to explain the decrease in Nd and D, deriving from the Mott-Schottky measurement result, after the formation of a passive film with Fe-O-B structure. Then, the reaction rates of R6, R7, and R1 decreased due to the decrease in the density of the VO, resulting in the transfer obstructi on of Fe and Cl- between carbon steel and SRW and a decrease in the corrosion rate. Hence, we believe that the borate inhibitor can effectively suppress the corrosion of mild steel in SRW. Figure 21 goes here

4. CONCLUSIONS (1) A slow-release borate inhibitor was successfully synthesised. The total boron concentration was ~270 mg L–1 after 30 days of release in SRW (500 mL) in the presence of 2.4 g of inhibitor. (2) The passivating performance of the inhibitor was evaluated by various electrochemical analytical techniques. The borate inhibitor exhibited an obvious


Page 22 of 50

Page 23 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mix-type inhibitor behaviour for mild steel in SRW, but its performance was predominated by the anodic reaction. The corrosion rate and icorr of the inhibited sample decreased from 1.7178 mm a–1 to 0.0056 mm a–1 and from 73.67 µA cm–2 to 0.24 µA cm–2, respectively, after 30 days of immersion. An acceptable passivation performacne was obtained when the total boron concentration reached over 100 mg L-1. The passivation efficiency gradually increased with increasing immersion time during the 30 days of experimentation and reached a value of over 98 % after 30 days of immersion. (3) The Mott-Schottky analysis results indicated that the passivating film exhibited n-type semiconductor characteristics and the thickness of the passivating film increased six-fold after 30 days of immersion. (4) The passivating film was mainly composed of FeBO3 FeOOH, Fe3O4 and Fe2O3, but it was predominated by the FeBO3. This structure could effectively suppress corrosion by reducing JCV, resulting in JCV < JM. This condition was advantageous in annihilating the vacancies on the mild steel surface rapidly and thus ensured the passivating performance; furthermore, it reduced the risk of breakdown of the passivating film.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone/fax: +86-10-58801830 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors Yuansheng Pei, Jun Cui, and Wenjiao Yuan received funding from Beijing municipal science and technology plan projects (Z161100001216009). Authors Yuansheng Pei, Jun Cui, and Xiuqing Li received funding from National Natural Science Foundation of China (51579009).



REFERENCES (1) Rochdi, A.; Kassou, O.; Dkhireche, N.; Touir, R.; El Bakri, M.; Ebn Touhami, M.; Sfaira, M.; Mernari, B.; Hammouti, B., Inhibitive properties of 2,5-bis(n-methylphenyl)-1,3,4-oxadiazole and biocide on corrosion, biocorrosion and scaling controls of brass in simulated cooling water. Corros. Sci. 2014, 80, 442-452. (2) Touir, R.; Cenoui, M.; El Bakri, M.; Ebn Touhami, M., Sodium gluconate as corrosion and scale inhibitor of ordinary steel in simulated cooling water. Corros. Sci. 2008, 50 (6), 1530-1537. (3) Chien, S. H.; Chowdhury, I.; Hsieh, M. K.; Li, H.; Dzombak, D. A.; Vidic, R. D., Control of biological growth in recirculating cooling systems using treated secondary effluent as makeup water with monochloramine. Water Res. 2012, 46 (19), 6508-6518. (4) Lu, H.; Zhang, S.; Li, W.; Cui, Y.; Yang, T., Synthesis of Graphene Oxide-Based Sulfonated Oligoanilines Coatings for Synergistically Enhanced Corrosion Protection in 3.5% NaCl Solution. ACS Appl. Mate. Inter. 2017, 9 (4), 4034-4043. (5) Ansari, K. R.; Quraishi, M. A.; Singh, A., Isatin derivatives as a non-toxic corrosion inhibitor for mild steel in 20% H2SO4. Corros. Sci. 2015, 95, 62-70. (6) Xu, L.; Xiao, H.; Shang, W.; Wang, B.; Zhu, J., Passivation of X65 (UNS K03014) carbon steel in NaHCO3 solution in a CO2 environment. Corros. Sci. 2016, 109, 246-256. (7) Zhang, J.; Gong, X. L.; Yu, H. H.; Du, M., The inhibition mechanism of imidazoline phosphate inhibitor for Q235 steel in hydrochloric acid medium. Corros. Sci. 2011, 53 (10), 3324-3330. (8) Guo, S.; Liu, L.; Xia, M.; Kang, L.; Huang, Q.; Li, C.; Wang, X.; Lin, Z.; Chen, C., Be2BO3F: A Phase of Beryllium Fluoride Borate Derived from KBe2BO3F2 with Short UV Absorption Edge. Inorg. Chem. 2016, 55 (13), 6586-91. (9) Parker, T. G.; Chown, A. L.; Beehler, A.; Pubbi, D.; Cross, J. N.; Albrecht-Schmitt, T. E., Ionothermal synthesis of tetranuclear borate clusters containing f- and p-block metals. Inorg. Chem. 2015, 54 (2), 570-5. (10) Ibarra-Palos, A., Darie, C., Proux, O., Hazemann, J. L., Aldon, L., Jumas, J. C., Morcrette, M., Strobel, P., Electrochemical reactions of iron borates with lithium: electrochemical and in Situ mossbauer and X-ray Absorption Studies. Chem. Mater. 2002, 14, 1166-1173. (11) Ibarra Palos, A.; Morcrette, M.; Strobel, P., Reversible lithium intercalation in amorphous iron borate. J. Solid State Electr. 2001, 6 (2), 134-138. (12) Yagupov, S.; Strugatsky, M.; Seleznyova, K.; Mogilenec, Y.; Milyukova, E.; Maksimova, E.; Nauhatsky, I.; Drovosekov, A.; Kreines, N.; Kliava, J., Iron borate films: Synthesis and characterization. J. Magn. Magn. Mater. 2016, 417, 338-343. (13) Santamaría-Pérez, D.; Gomis, O.; Sans, J. A.; Ortiz, H. M.; Vegas, Á.; Errandonea, D.; Ruiz-Fuertes, J.; Martinez-Garcia, D.; Garcia-Domene, B.; Pereira, A. L. J.; Manjón, F. J.; Rodríguez-Hernández, P.; Muñoz, A.; Piccinelli, F.; Bettinelli, M.; Popescu, C., Compressibility Systematics of Calcite-Type Borates: An experimental and theoretical structural study on ABO3 (A = Al, Sc, Fe, and In). J. Phys. Chem. C 2014, 118 (8), 4354-4361. (14) Hamadou, L.; Kadri, A.; Benbrahim, N., Characterisation of passive films formed on low carbon steel in borate buffer solution (pH 9.2) by electrochemical impedance spectroscopy. Appl. Surf. Sci. 2005, 252 (5), 1510-1519. (15) Modiano, S.; Fugivara, C. S.; Benedetti, A. V., Effect of citrate ions on the electrochemical


Page 24 of 50

Page 25 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


behaviour of low-carbon steel in borate buffer solutions. Corros. Sci. 2004, 46 (3), 529-545. (16) Flis, J.; Flis-Kabulska, I.; Zakroczymski, T., Corrosion and passivation of iron and its nitrided layer in borate buffer. Electrochim. Acta 2009, 54 (6), 1810-1819. (17) Jinlong, L.; Tongxiang, L.; Chen, W.; Wenli, G., Investigation of passive films formed on the surface of alloy 690 in borate buffer solution. J. Nucl. Mater. 2015, 465, 418-423. (18) Zohdi, H.; Shahverdi, H. R.; Hadavi, S. M. M., Effect of Nb addition on corrosion behavior of Fe-based metallic glasses in Ringer's solution for biomedical applications. Electrochem. Commun. 2011, 13 (8), 840-843. (19) Kaur, K.; Singh, K. J.; Anand, V., Structural properties of Bi2O3–B2O3–SiO2–Na2O glasses for gamma ray shielding applications. Radiat. Phys. Chem. 2016, 120, 63-72. (20) Moradi, M.; Song, Z.; Tao, X., Introducing a novel bacterium, Vibrio neocaledonicus sp., with the highest corrosion inhibition efficiency. Electrochem. Commun. 2015, 51, 64-68. (21) Yusufali, C.; Kshirsagar, R. J.; Jagannath; Mishra, R. K.; Dutta, R. S.; Dey, G. K., Infrared and X-ray photoelectron spectroscopy studies on sodium borosilicate glass interacted with thermally oxidized aluminides formed on Alloy 690. J. Non-Cryst. Solids 2013, 366, 54-58. (22) Miura, Y.; Kusano, H.; Nanba, T.; Matsumoto, S., X-ray photoelectron spectroscopy of sodium borosilicate glasses. J. Non-Cryst. Solids 2001, 290 (1), 1-14. (23) Liu, H.; Xu, D.; Dao, A. Q.; Zhang, G.; Lv, Y.; Liu, H., Study of corrosion behavior and mechanism of carbon steel in the presence of Chlorella vulgaris. Corros. Sci. 2015, 101, 84-93. (24) Ait Albrimi, Y.; Ait Addi, A.; Douch, J.; Souto, R. M.; Hamdani, M., Inhibition of the pitting corrosion of 304 stainless steel in 0.5 M hydrochloric acid solution by heptamolybdate ions. Corros. Sci. 2015, 90, 522-528. (25) Wang, Z. B.; Hu, H. X.; Liu, C. B.; Zheng, Y. G., The effect of fluoride ions on the corrosion behavior of pure titanium in 0.05 M sulfuric acid. Electrochim. Acta 2014, 135, 526-535. (26) Ramezanzadeh, B.; Haeri, Z.; Ramezanzadeh, M., A facile route of making silica nanoparticles-covered graphene oxide nanohybrids (SiO2-GO); fabrication of SiO2-GO/epoxy composite coating with superior barrier and corrosion protection performance. Chem. Eng. J. 2016, 303, 511-528. (27) Ma, Y.; Han, F.; Li, Z.; Xia, C., Corrosion Behavior of Metallic Materials in Acidic-Functionalized Ionic Liquids. ACS Sustain. Chem. Eng. 2016, 4 (2), 633-639. (28) Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X., Contacts between two- and three-dimensional materials: ohmic, schottky, and p–n heterojunctions. ACS Nano 2016, 10 (5), 4895-4919. (29) Javadian, S.; Yousefi, A.; Neshati, J., Synergistic effect of mixed cationic and anionic surfactants on the corrosion inhibitor behavior of mild steel in 3.5% NaCl. Appl. Surf. Sci. 2013, 285, Part B, 674-681. (30) Migkovic-Stankovic, V.; Jevremovic, I.; Jung, I.; Rhee, K., Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution. Carbon 2014, 75, 335-344. (31) Doğru Mert, B., Corrosion protection of aluminum by electrochemically synthesized composite organic coating. Corros. Sci. 2016, 103, 88-94. (32) Liu, H.; Gu, T.; Asif, M.; Zhang, G.; Liu, H., The corrosion behavior and mechanism of carbon steel induced by extracellular polymeric substances of iron-oxidizing bacteria. Corros. Sci.



2017, 114, 102-111. (33) Mondal, J.; Marques, A.; Aarik, L.; Kozlova, J.; Simoes, A.; Sammelselg, W., Development of a thin ceramic-graphene nanolaminate coating for corrosion protection of stainless steel. Corros. Sci. 2016, 105, 161-169. (34) Li, D. G.; Wang, J. D.; Chen, D. R., Influence of potentiostatic aging, temperature and pH on the diffusivity of a point defect in the passive film on Nb in an HCl solution. Electrochim. Acta 2012, 60, 134-146. (35) Mahdavian, M.; Attar, M. M., Electrochemical behaviour of some transition metal acetylacetonate complexes as corrosion inhibitors for mild steel. Corros. Sci. 2009, 51 (2), 409-414. (36) Sheng, X.; Cai, W.; Zhong, L.; Xie, D.; Zhang, X., Synthesis of functionalized graphene/polyaniline nanocomposites with effective synergistic reinforcement on anticorrosion. Ind. Eng. Chem. Res. 2016, 55 (31), 8576-8585. (37) Husain, E.; Narayanan, T. N.; Taha-Tijerina, J. J.; Vinod, S.; Vajtai, R.; Ajayan, P. M., Marine corrosion protective coatings of hexagonal boron nitride thin films on stainless steel. ACS Appl. Mater. Inter. 2013, 5 (10), 4129-4135. (38) Singh, A.; Lin, Y.; Obot, I. B.; Ebenso, E. E.; Ansari, K. R.; Quraishi, M. A., Corrosion mitigation of J55 steel in 3.5% NaCl solution by a macrocyclic inhibitor. Appl. Surf. Sci. 2015, 356, 341-347. (39) Shubha, H. N.; Venkatesha, T. V.; Vathsala, K.; Pavitra, M. K.; Punith Kumar, M. K., Preparation of self assembled sodium oleate monolayer on mild steel and its corrosion inhibition behavior in saline water. ACS Appl. Mater. Inter. 2013, 5 (21), 10738-10744. (40) Liu, J.; Zhang, T.; Meng, G.; Shao, Y.; Wang, F., Effect of pitting nucleation on critical pitting temperature of 316L stainless steel by nitric acid passivation. Corros. Sci. 2015, 91, 232-244. (41) Gu, L.; Liu, S.; Zhao, H.; Yu, H., Facile Preparation of water-dispersible graphene sheets stabilized by carboxylated oligoanilines and their anticorrosion coatings. ACS Appl. Mater. Inter. 2015, 7 (32), 17641-17648. (42) Bosch, R. W.; Hubrecht, J.; Bogaerts, W. F.; Syrett, B. C., Electrochemical frequency modulation: a new electrochemical technique for online corrosion monitoring. Corrosion 2001, 57 (1), 60-70. (43) Rauf, A.; Bogaerts, W. F., Employing electrochemical frequency modulation for pitting corrosion. Corros. Sci. 2010, 52 (9), 2773-2785. (44) Rauf, A.; Bogaerts, W. F., Monitoring of crevice corrosion with the electrochemical frequency modulation technique. Electrochim. Acta 2009, 54 (28), 7357-7363. (45) Zeng, Q.; Bai, J.; Li, J.; Xia, L.; Huang, K.; Li, X.; Zhou, B., A novel in situ preparation method for nanostructured [small alpha]-Fe2O3 films from electrodeposited Fe films for efficient photoelectrocatalytic water splitting and the degradation of organic pollutants. J. Mater. Chem. A 2015, 3 (8), 4345-4353. (46) Gadala, I. M.; Alfantazi, A., A study of X100 pipeline steel passivation in mildly alkaline bicarbonate solutions using electrochemical impedance spectroscopy under potentiodynamic conditions and Mott–Schottky. Appl. Surf. Sci. 2015, 357, Part A, 356-368. (47) Marcelin, S.; Ter-Ovanessian, B.; Normand, B., Electronic properties of passive films from the multi-frequency Mott–Schottky and power-law coupled approach. Electrochem. Commun.


Page 26 of 50

Page 27 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2016, 66, 62-65. (48) Ahn, S. J.; Kwon, H. S., Effects of solution temperature on electronic properties of passive film formed on Fe in pH 8.5 borate buffer solution. Electrochim. Acta 2004, 49 (20), 3347-3353. (49) BenSalah, M.; Sabot, R.; Triki, E.; Dhouibi, L.; Refait, P.; Jeannin, M., Passivity of Sanicro28 (UNS N-08028) stainless steel in polluted phosphoric acid at different temperatures studied by electrochemical impedance spectroscopy and Mott–Schottky analysis. Corros. Sci. 2014, 86, 61-70. (50) Feng, Z.; Cheng, X.; Dong, C.; Xu, L.; Li, X., Passivity of 316L stainless steel in borate buffer solution studied by Mott–Schottky analysis, atomic absorption spectrometry and X-ray photoelectron spectroscopy. Corros. Sci. 2010, 52 (11), 3646-3653. (51) Pieretti, E. F.; Manhabosco, S. M.; Dick, L. F. P.; Hinder, S.; Costa, I., Localized corrosion evaluation of the ASTM F139 stainless steel marked by laser using scanning vibrating electrode technique, X-ray photoelectron spectroscopy and Mott–Schottky techniques. Electrochim. Acta 2014, 124, 150-155. (52) Sazou, D.; Saltidou, K.; Pagitsas, M., Understanding the effect of bromides on the stability of titanium oxide films based on a point defect model. Electrochim. Acta 2012, 76, 48-61. (53) Sikora, E., Sikora, J., Macdonald, D.D., A new method for estimating the diffusivities of vacancies in passive films. Electrochim. Acta 1996, 41 (6), 783-789. (54) Ahn, S. J.; Kwon, H. S., Effects of solution temperature on electronic properties of passive film formed on Fe in pH 8.5 borate buffer solution. Electrochim. Acta 2004, 49 (20), 3347-3353. (55) Zhang, Z.; Yates, J. T., Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces. Chem. Rev. 2012, 112 (10), 5520-5551. (56) Ramezanzadeh, B.; Arman, S. Y.; Mehdipour, M.; Markhali, B. P., Analysis of electrochemical noise (ECN) data in time and frequency domain for comparison corrosion inhibition of some azole compounds on Cu in 1.0 M H2SO4 solution. Appl. Surf. Sci. 2014, 289, 129-140. (57) Rios, E. C.; Zimer, A. M.; Pereira, E. C.; Mascaro, L. H., Analysis of AISI 1020 steel corrosion in seawater by coupling electrochemical noise and optical microscopy. Electrochim. Acta 2014, 124, 211-217. (58) Sakairi, M.; Sasaki, R.; Kaneko, A.; Seki, Y.; Nagasawa, D., Evaluation of metal cation effects on galvanic corrosion behavior of the A5052 aluminum alloy in low chloride ion containing solutions by electrochemical noise impedance. Electrochim. Acta 2014, 131, 123-129. (59) Jamali, S. S.; Mills, D. J., A critical review of electrochemical noise measurement as a tool for evaluation of organic coatings. Prog. Org. Coat. 2016, 95, 26-37. (60) Guan, L.; Zhang, B.; Wang, J. Q.; Han, E. H.; Ke, W., The reliability of electrochemical noise and current transients characterizing metastable pitting of Al–Mg–Si microelectrodes. Corros. Sci. 2014, 80, 1-6. (61) Geringer, J.; Macdonald, D. D., Modeling fretting-corrosion wear of 316L SS against poly(methyl methacrylate) with the point defect model: fundamental theory, assessment, and outlook. Electrochim. Acta 2012, 79, 17-30. (62) Macdonald, D. D., The history of the Point Defect Model for the passive state: A brief review of film growth aspects. Electrochim. Acta 2011, 56 (4), 1761-1772. (63) Sharifi-Asl, S.; Taylor, M. L.; Lu, Z.; Engelhardt, G. R.; Kursten, B.; Macdonald, D. D., Modeling of the electrochemical impedance spectroscopic behavior of passive iron using a genetic



algorithm approach. Electrochim. Acta 2013, 102, 161-173. (64) Yang, S.; Macdonald, D. D., Theoretical and experimental studies of the pitting of type 316L stainless steel in borate buffer solution containing nitrate ion. Electrochim. Acta 2007, 52 (5), 1871-1879. (65) Zhang, Y.; Urquidi-Macdonald, M.; Engelhardt, G. R.; Macdonald, D. D., Development of localized corrosion damage on low pressure turbine disks and blades. III: application of damage function analysis to the prediction of damage. Electrochim. Acta 2012, 69, 19-29. (66) Fattah-alhosseini, A.; Soltani, F.; Shirsalimi, F.; Ezadi, B.; Attarzadeh, N., The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: A test of the point defect model (PDM). Corros. Sci. 2011, 53 (10), 3186-3192. (67) Pidaparti, R. M.; Patel, R. K., Investigation of a single pit/defect evolution during the corrosion process. Corros. Sci. 2010, 52 (9), 3150-3153. (68) Papadias, D. D.; Ahluwalia, R. K.; Thomson, J. K.; Meyer Iii, H. M.; Brady, M. P.; Wang, H.; Turner, J. A.; Mukundan, R.; Borup, R., Degradation of SS316L bipolar plates in simulated fuel cell environment: Corrosion rate, barrier film formation kinetics and contact resistance. J. Power Sources 2015, 273, 1237-1249.


Page 28 of 50

Page 29 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical abstract 145x113mm (220 x 220 DPI)



Figure 1. Synthesis procedure of the slow-release borate inhibitor 254x72mm (96 x 96 DPI)


Page 30 of 50

Page 31 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Schematic representation of the cell arrangements for EN measurements 163x152mm (96 x 96 DPI)



Figure 3. FESEM-EDS analysis results of the slow-release borate inhibitor. (a) FESEM images; (b) EDS results 144x51mm (220 x 220 DPI)


Page 32 of 50

Page 33 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. XRD pattern of the slow-release borate inhibitor 122x99mm (220 x 220 DPI)



Figure 5. XPS spectra of the slow-release borate inhibitor (a) before the experiment and (b) after 30 days of release in SRW at 40 °C and 100 r/min, including survey spectra and high resolution spectra of B 1s, O 1s, and Si 2p 144x67mm (220 x 220 DPI)


Page 34 of 50

Page 35 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. EOCP variations of the carbon steel samples in the (a) absence and (b) presence of the slowrelease inhibitor 142x49mm (220 x 220 DPI)



Figure 7. Impedance spectra of the samples in the presence and absence of the inhibitor. Nyquist plot of the (a) inhibited and (b) inhibitor-free samples. Phase angle plot of the (c) inhibited and (d) inhibitor-free samples. Impedance magnitude plot of the (e) inhibited and (f) inhibitor-free samples 121x133mm (220 x 220 DPI)


Page 36 of 50

Page 37 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Equivalent circuit models used to fit the experimental impedance data for fitting the sample (a) without and (b) with the passive film 77x54mm (300 x 300 DPI)



Figure 9. Potentiodynamic polarization curves of the carbon steel in the absence and presence of the slowrelease inhibitor 134x100mm (220 x 220 DPI)


Page 38 of 50

Page 39 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Variation in the corrosion rate and passivating efficiency of the inhibitor-free and inhibited samples during 30 days of immersion 233x160mm (96 x 96 DPI)



Figure 11. EFM measurement results. (a) Variation in the corrosion rate with the increasing immersion time. The actual measurement values of causality factors (b) 2 and (c) 3 in the presence of the slow-release inhibitor 145x54mm (220 x 220 DPI)


Page 40 of 50

Page 41 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12. Variation in the total boron concentration during 30 days of dissolution in SRW at 40 °C and 100 r/min. Insert plot: variations in the inhibitor efficiencies derived from the potentiodynamic polarization curve and EIS measurement 124x98mm (220 x 220 DPI)



Figure 13. FESEM images of the samples in the (a) presence and (b) absence of the inhibitor after 30 days of immersion in SRW at 40 °C and 100 r/min 146x54mm (220 x 220 DPI)


Page 42 of 50

Page 43 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14. XPS spectra of the passivating film on the mild steel surface in the presence of the slow-release inhibitor after 30 days of immersion in SRW at 40 °C and 100 r/min. (a) Survey spectra and high resolution spectra of (b) O 1s, (c) B 1s, and (d) Fe 2p 270x208mm (96 x 96 DPI)



Figure 15. Configuration of the FeBO3 structure from (a) a axis, (b) b axis, and (c) c axis. (d) extended structure of FeBO3 from b axis 144x120mm (220 x 220 DPI)


Page 44 of 50

Page 45 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 16. Mott-Schottky plots of the passivating film formed on the carbon steel surface in the presence of the inhibitor after 1, 5, 13, 21, and 30 days of immersion in 500 mL SRW at 40 °C and 100 r/min 139x99mm (220 x 220 DPI)



Figure 17. The parameters derived from EN data. (a) Variation in the Nd and D values of the inhibited sample. (b) Variation in the thickness of the passivating film of the inhibited sample 144x51mm (220 x 220 DPI)


Page 46 of 50

Page 47 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 18. Electrochemical current and potential noise signals of the carbon steel surface in the (a) absence and presence of the inhibitor in SRW during the first 3600 s of measurement after (b) 1, (c) 5, (d) 13, (e) 21, and (f) 30 days of immersion 212x199mm (96 x 96 DPI)



Figure 19. Two-dimensional visual presentation of the discrete time wavelet transformation of the ECN signals of the carbon steel in the (a) absence and presence of the inhibitor in SRW during the first 3600 s of measurement after (b) 1, (c) 5, (d) 13, (e) 21, and (f) 30 days of immersion 145x82mm (220 x 220 DPI)


Page 48 of 50

Page 49 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 20. Parameters extracted from the frequency domain curves of the carbon steel after 1, 5, 13, 21, and 30 days of immersion in SRW at 40 °C and 100 r/min. (a) fn and qn and (b) Rn variations with the increasing immersion time 146x53mm (220 x 220 DPI)



Figure 21. Schematic representation of the transfer of substances in the substrate-film-SRW system, leading to the accumulation of cation vacancies at the interface between the substance and the passivating film 135x57mm (220 x 220 DPI)


Page 50 of 50

Toward a Slow-Release Borate Inhibitor To Control Mild Steel

Recommend Documents