Origin of Aragonite Scale Deposition on Carbon Steel at Ambient

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Origin of Aragonite Scale Deposition on Carbon Steel at Ambient Circumstances Tianzhen Zhu,† Lida Wang,† Wen Sun,† Meng Wang,† Zhengqing Yang,† Tingnian Ji,† Suilin Wang,‡ Yaowei Wang,§ Liang Xia,§ and Guichang Liu*,† †

Department of Chemical Engineering, Dalian University of Technology, No. 2, Linggong Road, Dalian 116024, China School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, No. 1, Exhibition Hall Road, Beijing 100044, China § Shandong Chambroad Petrochemical Co., Ltd., Economic Development Zone, Boxing county, Binzhou 256500, China ‡

S Supporting Information *

ABSTRACT: The deposition of calcium carbonate scale on a metal surface is undesirable in industrial processes. Electrochemical corrosion processes at the metal−water interface can induce the deposition of calcium carbonate scale. In this paper, galvanic deposition and potentiostatic deposition methods were used to investigate the influence of electrochemical corrosion behavior of carbon steel on the deposition behavior of calcium carbonate. Results reveal that aragonite tends to deposit on carbon steel due to the disturbance of ferrous ions released from a micro galvanic corrosion cell. Once anodically released ferrous ions diffuse to the adjacent cathodic regions; they not only can decrease the interfacial oxygen concentration by preferentially consuming local dissolved oxygen but also can lower the local pH by forming iron hydroxides. Such processes build a relative low CaCO3 supersaturation environment at the metal−water interface, favoring the deposition of aragonite.

1. INTRODUCTION The deposition of calcium carbonate scale on a metal surface is an extremely common and important problem in industrial processes where mineral water is widely used.1 Such deposits often lead to pipe blockage, decrease of heat transfer efficiency, localized corrosion attack, and unscheduled equipment shutdown.2,3 Generally, calcium carbonate crystallizes naturally in three polymorphs: calcite, aragonite, and few vaterite.4 Calcite (hexagonal crystal shape, specific gravity: 2.71) is easily removable with weak HCl and less adherent than aragonite (dendritic crystal shape, specific gravity: 2.94).5 Besides, aragonite scale is expected to have a higher thermal resistance and more troublesome than calcite scale because it forms a harder and denser deposit than calcite on the metal surface.5,6 Underdeposit corrosion occurring beneath calcite scale (304 stainless steel) has been reported by Brennenstuhl et al. and resulted in the shutdown of Ontario Hydro’s Canadian deuterium nuclear reactors.7 Therefore, it is of great significance to investigate the deposition behavior of calcium carbonate. Factors influencing the deposition behavior of calcium carbonate on a metal surface includes substrates,8 suspended solids,9 temperature,10 hydrodynamic conditions,11 magnetic field,12 electric field,13 foreign ions,3,14,15 organic additives,16,17 pH value,18 supersaturation,19 microorganism,7 and electrochemical corrosion behavior at the metal−water interface.20 © XXXX American Chemical Society

Among these factors, electrochemical corrosion of metal in aqueous media is an extremely widespread phenomenon.21,22 The processes can be readily understood by a short-circuited battery where cathodic reduction of oxidants (such as O2) into hydroxide ions occurs with the release of metal cations from anodic dissolution.23 The produced hydroxide ions may change the local inorganic carbonic equilibrium and increase the deposition tendency of calcium carbonate to some extent. In practice, calcium carbonate deposition on the metal surface with poor corrosion resistance, especially for carbon steel, is a very common phenomenon,24−27 where calcium carbonate is frequently detected as a major constituent of the scale deposits. Troup and Richardson have studied the interaction between metal corrosion and calcium carbonate nucleation.20 Under identical conditions, the nucleation and growth rate of calcium carbonate decrease in the order mild steel > aluminum > copper, which is consistent with their corrosion tendency. However, since then no further work has been carried out to study the deposition behavior of calcium carbonate scale during electrochemical corrosion processes. More specifically, beside the inductive effects of the cathodic reduction reaction, the Received: Revised: Accepted: Published: A

August 16, 2017 December 2, 2017 December 8, 2017 December 8, 2017 DOI: 10.1021/acs.iecr.7b03399 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2.3. Galvanic Deposition. In order to illustrate how the electrochemical corrosion influences the deposition behavior of calcium carbonate on 20CS, galvanic deposition of calcium carbonate was carried out. AISI 304 stainless steel (304SS) and 20CS were selected as cathodic and anodic materials, respectively. Two types of galvanic couples (GC I and II) were designed and are shown schematically in Figure 1. For GC I three set ups with different cathode-anode area ratios (1:3, 1:1, and 6:1) were used and the cathode was kept at a constant distance of 10 mm from the anode. For GC II (cathode-anode area ratio: 6:1), the cathode and anode were coplanar and adjacent but insulated from each other (ca. 100 μm) by epoxy resin (E51, HangzhouWuhuigang Adhesive Co., Ltd., Hangzhou, China), which was more analogous to the micro galvanic corrosion cell formed between microstructural phases on the carbon steel surface compared with GC I. Each electrode specimen was connected to a copper wire at one end and sealed using epoxy resin with the other end exposed as the working surface. Before each immersion test, electrode specimens were first ground with SiC paper (grade 400, 600, 1200, and 1500), then polished with diamond paste (diamond particles of 2.5 μm), washed with deionized water, and finally ultrasonically cleaned in isopropyl alcohol for 30 min. The galvanic potential and current density of GC I were recorded by a CS310 electrochemical workstation (CorrTest, China) with a saturated calomel electrode (SCE) as reference electrode. A blank test was carried out by immersing a single 304SS electrode in synthetic water. Each as-deposited electrode was rinsed with deionized water and alcohol and then dried using compressed air. 2.4. Potentiostatic Deposition. In order to further investigate the influence of cathodic current density on the deposition behavior during corrosion processes, potentiostatic deposition of calcium carbonate on 304SS and 20CS was carried out on the CS310 electrochemical workstation with a three-electrode system. A platinum net, SCE, and steel specimen were used as counter electrode, reference electrode, and working electrode, respectively. Deposition tests were carried out in quiescent synthetic water for 2 h. The treatment procedures of the electrodes before and after tests were similar to that of galvanic deposition. 2.5. Fe2+ Ion Effects. The effect of Fe2+ ions on the electrodeposition behavior of calcium carbonate was also investigated by potentiostatic deposition method. Fe2+ ions

disturbance of the anodically released cations should not be neglected, because previous studies28−30 have shown that metal cations like Fe2+, Mg2+, and Cu2+ would have some influences on the nucleation and growth of calcium carbonate. In this work, a galvanic deposition method was used to illustrate how the electrochemical corrosion behavior influenced the deposition behavior of calcium carbonate on carbon steel. Then, the influences of cathodic current density and anodically produced ferrous ions on the deposition behavior were investigated by potentiostatic deposition method. Finally, a mechanism explaining the deposition behavior of calcium carbonate on carbon steel is described.

2. EXPERIMENTAL SECTION 2.1. Solution Preparation. The synthetic water was prepared by mixing two aerated solutions, 250 mL of brine A containing calcium ions and 250 mL of brine B containing bicarbonate ions. Before mixing, the two solutions were filtered using a 0.45 μm filter. The specific composition is shown in Table 1. In all experiments, synthetic water was maintained at Table 1. Chemical Compositions of the Synthetic Water species synthetic water brine A brine B

Cl− (mg L−1)

Na+ (mg L−1)

Ca2+ (mg L−1)

HCO3− (mg L−1)

11710

7080

720

930

12940 10480

6730 7430

1440 1860

26 ± 1 °C and in contact with atmosphere. Initial pH was adjusted to 6.9 ± 0.1 using concentrated HCl. All chemicals were analytical grade (Tianjin Kemiou Chemical Reagent Co., Ltd.) and all solutions were prepared using double distilled water. 2.2. CaCO3 Deposition on Carbon Steel. AISI 1020 low carbon steel (20CS), one of the most common materials used in industrial processes, was selected for deposition tests. Deposition of calcium carbonate was carried out by immersing a carbon steel specimen in synthetic water for 24 h. The preparation method of the electrode specimen is presented in the next section in detail. The as-deposited specimen was rinsed with deionized water and alcohol and then dried using compressed air.

Figure 1. Schematic diagrams of two types of galvanic couples. B

DOI: 10.1021/acs.iecr.7b03399 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) FE-SEM image of 20CS after 24 h of immersion in quiescent synthetic water at 26 ± 1 °C; (b, c) corresponding X-ray diffraction data and FT-IR spectra of the deposits. “▲” represents aragonite.

S1). All crystals identified were counted. The proportion of each polymorph (Px) by each observer was calculated using the formula.

were introduced into synthetic water by pipetting appropriate volumes of concentrated aqueous stock solutions prepared from ferrous chloride (FeCl2.4H2O) in double distilled water. Potentiostatic deposition of 304SS and 20CS were carried out at −0.85 V vs SCE for 2 h. The treatment procedures of the electrodes before and after tests were similar to that of galvanic deposition. 2.6. Characterization. The morphology and composition of the deposits were characterized by field emission-scanning electron microscope (FE-SEM, ZEISS Ultra 55, Germany) equipped with an energy dispersive X-ray spectroscopy (EDS). Polymorphs with different crystal shapes were confirmed by micro-Raman spectra (RS, Thermo Fisher Scientific, America). Then, adequate amounts of scale deposit were scraped from asdeposited electrode specimens if available. Crystalline structures of the deposit were confirmed by X-ray diffraction (XRD, EMPYREAN with Cu radiation target) with the 2θ scan from 20 to 80°. Fourier-transform IR (FT-IR) spectra were also recorded using a Nicolet 6700 (Thermo Scientific, America) in the infrared domain 400−1900 cm−1 with a KBr matrix. The crystals deposited on the cathode of GC II were observed along the straight line perpendicular to the borderline (Figure 1b) by means of a metallurgical microscope (BX51M, Olympus, Japan).31−33 Due to the difficulty in collecting the scale deposit at each micro region for XRD or FT-IR, the polymorphs of calcium carbonate in this case were identified according to the relationship between polymorphs and crystal shapes built by Raman analysis. At each equidistance area (Figure 1b), as it was too large to be observed as a whole with a sufficient resolution, six pictures (922 × 583 μm2 per image) were taken along the equidistance line (Figure 1b) parallel to the borderline by making observations on contiguous microscopic fields (Figure

Px =

Nx × 100% NT

(1)

where the number of crystals per mm2 electrode area was defined as number density, N. Nx was the number density of polymorph x, and NT was the number density of all polymorphs.

3. RESULTS AND DISCUSSION 3.1. Deposition Behavior on Carbon Steel under SelfCorrosion Condition. Figure 2a shows the FE-SEM images of 20CS. XRD data (JCPDS: 41-1475, Figure 2b) and infrared spectra (Figure 2c, characteristic peaks at 1787, 1083, 857, and 698 cm−1)34 indicate that aragonite crystals deposit on 20CS as a main polymorph of calcium carbonate. Corrosion of 20CS can be easily observed from the digital image while the corrosion products cannot be detected by XRD due to its low content. The weak peak at approximately at 1629 cm−1 corresponds to the −OH stretching vibration, also indicating the existence of small amounts of corrosion products such as iron hydroxides.35 These phenomena are consistent with the observation by Troup et al.20 However, no clear explanation for the deposition behavior has been presented. During corrosion processes, factors affecting the calcium carbonate crystallization may include anodically produced Fe2+ ions,28−30 substrate,8,36 and electrochemical parameters (such as cathodic current density).36 The three effects occur simultaneously in a micro region (cathodic regions) and are difficult to make a C


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Figure 3. Evolution of open-circuit potential (a), galvanic potential (b), and anodic (c) and cathodic (d) current density with immersion time in quiescent synthetic water at 26 ± 1 °C.

Figure 4. (a−g) FE-SEM images of 304SS and 20CS for GC I with different cathode-anode area ratios after 24 h of immersion in quiescent synthetic water at 26 ± 1 °C: (a, d) 1:3, (b, e) 1:1, (c, f) 6:1, (g) blank test for 304SS. Inset table shows EDS results of the area marked with dashed box.

comparison. Therefore, macro galvanic deposition of calcium carbonate was carried out to simplify the deposition processes of calcium carbonate. 3.2. Deposition Behavior for GC I. Figure 3 presents the evolution of electrochemical parameters versus time for

different specimens. As seen, the open circuit potentials of 304SS and 20CS (blank tests) rapidly reach steady-state values at ca. −0.18 and −0.77 V vs SCE, respectively (Figure 3a). The galvanic potential (Figure 3b) and anodic current density (Figure 3c) increase with the increase of cathode-anode area D


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Figure 5. Raman spectra of calcium carbonate crystals with different shapes.

Figure 6. X-ray diffraction data and FT-IR spectra of the deposit formed on 304SS (a, c) and 20CS (b, d) for GC I with different cathode−anode area ratios. “■” and “▲” represent calcite and aragonite, respectively. E


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Industrial & Engineering Chemistry Research ratio (−0.77 to −0.70 V vs SCE, 5−120 μA cm−2), whereas the cathodic current density (Figure 3d) varies within a small range (10−22 μA cm−2) and decreases in the order 1:3 > 6:1 > 1:1. It should be noted that, unlike the other galvanic couples (1:3 and 1:1), the cathodic current density at 6:1 cathode-anode area ratio decreases sharply during the first 4 h and then gradually approaches that of 1:1 cathode-anode area ratio (Figure 3d). Figure 4a−g shows the FE-SEM images of GC I. Different from the deposition behavior for 20CS (Figure 2), five different crystal morphologies are observed and shown in Figure 5b. RS results (Figure 5a,c,d) indicate these particles are all crystals of calcium carbonate. Three polymorphs are identified: calcite (Cal), aragonite (Ara), and vaterite (Vat). Crystals with a hexahedron shape (i) are confirmed to be calcite. Crystals with rosette37 (ii), twin-rosette37 (iii), and unique hubbard squashlike38 (iv) shapes are aragonite. Crystals with half a pricot kernel shape (v) are vaterite.39 The characteristic Raman peaks for calcite, vaterite and aragonite are consistent with those reported in the literature.40,41 The characteristic symmetric stretching bands around 1080−1090 cm−1 indicate the clear crystalline polymorphs of CaCO3.40 Bands at 1074 and 1090 cm−1 are assigned to vaterite, whereas the band at 1085 cm−1 is assigned to both calcite and aragonite.41 In-plane bending bands at 711 cm−1, at 705 and 716 cm−1, and at 750 cm−1 are assigned to calcite, aragonite, and vaterite, respectively.41 Lattice modes below 400 cm−1 also can be used to distinguish the three polymorphs, for aragonite at 152, 179, and 205 cm−1, calcite at 155 cm−1 with overlapping band and 282 cm−1, vaterite at 105, 115, 209, 267, and 300 cm−1.40,41 Based on this information, the polymorphs of CaCO3 crystals formed under other electrochemical conditions can be rapidly confirmed through observing their morphologies if specimens are not available for XRD or FT-IR measurements. It can be seen from the FE-SEM images that three polymorphs deposit on 304SS and no evident corrosion deposit can be observed after galvanic deposition (Figure 4a− c). Apparently, calcite is the major polymorph, followed by aragonite and very few vaterite crystals. In contrast, only calcite and aragonite crystals deposit on 304SS (Figure 4g, blank test) and 20CS (Figure 4d−f, after galvanic deposition), respectively. These results are further confirmed by XRD (calcite: JCPDS: 01-0628; aragonite: JCPDS: 41-1475) and FT-IR shown in Figure 6. No signal of vaterite is observed due to its extremely low content. Under ambient conditions, vaterite is the most unstable and least common polymorph and can easily transform into calcite or aragonite in aqueous solution.42,43 In the case of 20CS anode, weak peaks of corrosion products at 1629 cm−1 are also observed in the FT-IR spectra (Figure 6d), whereas the corresponding XRD signal (Figure 6b) cannot be effectively detected due to their low crystallinity.44 It should be noted that almost no CaCO3 is detected by EDS and many small holes appear on 20CS at 6:1 cathode−anode ratio (Figure 4f). This can be attributed to the fact that the local cathodic current density is too low to induce the deposition of calcium carbonate under a stronger anodic polarization (Figure 3c).23 These holes enlarge the anode area and lower the cathode− anode ratio, resulting in a sharp decrease in the current density (Figure 3c,d) during the first 4 h. 3.3. Deposition Behavior for GC II. Figure 7 shows the distribution of crystal number density and polymorphs proportion for the cathode of GC II, and the FE-SEM images of three representative cathodic regions are shown in Figure 8. As shown, three polymorphs deposit in each cathodic region,

Figure 7. Evolution of total crystal number density and polymorphs proportion of CaCO3 with the distance from the border.

which is similar to GC I. The total crystal number density reaches a maximum value at ca. 1 mm first and then decreases with the increase of the distance from the border (Figure 7). Among these crystals, aragonite is the predominant polymorph near the border (Figure 8a,d), and its proportion decreases with the increase of the distance and then reaches a relatively stable value (Figure 7), whereas the variational trend of calcite is opposite to that of aragonite (Figure 7). Very few vaterite crystals deposit in all regions (Figure 7), which is consistent with GC I. In the region near the border (such as region A, Figure 8), beside calcium carbonate crystals, corrosion deposits (Figure 8d) also can be detected by EDS(Figure S2b). RS results (Figure S2b in the Supporting Information) show that these deposits are γ-FeOOH,45 which is probably formed by the oxidation of Fe(OH)2 at room temperature.46 This indicates that Fe2+ ions from anodic dissolution diffuse to this area (electromigration can be neglected due to high conductivity of the synthetic water47) and react with OH− produced by oxygen reduction reaction (ORR). However, no γFeOOH deposits can be observed in farther regions (such as region B and C, Figure 7). This phenomenon can be attributed to the fact that Fe2+ ions are rapidly oxidized by dissolved oxygen in neutral aqueous media and precipitated as hydroxides (such as Fe(OH)3) in bulk solution before they reach these regions.48,49 In fact, a yellow solution is observed after immersion test further indicating the occurrence of these processes. 3.4. Effects of Cathodic Current Density. The results of galvanic deposition indicate that cathodic current density and Fe2+ ions play important roles in the deposition behavior of calcium carbonate. Potentiostatic deposition on both 304SS and 20CS (avoiding the influence of substrate) was carried out to investigate the influence of cathodic current density. Before deposition tests, cathodic polarization curve measurements (Figure 9a) were performed in quiescent synthetic water without calcium ions (avoiding deposition of calcium carbonate on active surface) using a three-electrode system. The potential sweep started at each open-circuit potential and was scanned cathodically at a scan rate of 1 mV/s. For 20CS, two reduction processes are observed: process I between the open-circuit potential and −1.0 V vs SCE and process II at a more negative potential. In a previous characterization of this system,50 F


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Figure 8. FE-SEM images of the representative cathodic regions of GC II after 24 h of immersion in quiescent synthetic water at 26 ± 1 °C. (a, d) Region A; (b, e) region B; (c, f) region C.

Figure 9. (a) Cathodic polarization curves of 304SS and 20CS measured in quiescent synthetic water without calcium ion at 26 ± 1 °C; (b, c) chronoamperometric curves of 304SS and 20CS in quiescent synthetic water at 26 ± 1 °C.

observed with decreasing applied potential between −0.50 and −0.80 V vs SCE. This is related to the low ORR activity of the naturally passivated electrode surface.52 On the contrary, when polarization is greater than −0.80 V vs SCE, the current density increases apparently and follows a similar trend with 20CS.

process I and process II correspond to ORR and hydrogen evolution reaction, respectively. No limiting current plateau for ORR is observed, indicating the reaction is controlled mainly by electron transfers.51 Polarization behavior of 304SS is relatively more complex. A slight increase in cathodic current density is G


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Industrial & Engineering Chemistry Research This indicates that the passive film is mostly reduced and the as-reduced surface favors the four electron pathway for ORR.52 Based on the cathodic polarization curves, potential values near the galvanic potential of GC I (Figure 3b) were selected for potentiostatic deposition. Figure 9b,c shows the chronoamperometric curves traced during potentiostatic deposition processes. It can been seen that the curves for 304SS (under −0.85, −0.77, and −0.68 V vs SCE) and 20CS (under −0.85, −0.81, and −0.78 V vs SCE) show the same trend, and the current density increases remarkably with the enhancement of the polarization. In addition, the current density under individual potential decreases with the increase of immersion time, indicating the progressive covering of the electrode by calcium carbonate. Especially, the current densities at −0.85 V vs SCE for both materials increase sharply in the first 20 min. This can be attributed to an activation of the electrode following the release of active sites necessary to the reduction of dissolved oxygen.53 FE-SEM images of 304SS and 20CS are presented in Figures 10 and 11, respectively. XRD and FT-IR in Figure 12 further

Figure 11. FE-SEM images of 20CS after deposition under a fixed potential of −0.85 (a), −0.81 (b), and −0.78 (c) V vs SCE and selfcorrosion condition (d) for 2 h in quiescent synthetic water at 26 ± 1 °C.

the galvanic couple.58 Nonetheless, similar to GC I, the cathodic potential of GC II is thermodynamically unlikely to fall below the open circuit potential of 20CS (−0.77 V vs SCE, Figure 3a). Such a relatively weak cathodic polarization eventually results in the simultaneous deposition of three polymorphs on 304SS for GC I (Figure 4a−c) and GC II (Figure 8b,c,e,f). Potentiostatic deposition under −0.85 V vs SCE was also carried out using GC I (cathode−anode area ratio: 1:1) with 304SS and 20CS as working electrode and counter electrode, respectively. As shown in Figure S3 in the Supporting Information, results of XRD (calcite: JCPDS: 010628) and FT-IR indicate that calcite is still the predominant polymorph and no iron compound occurs on 304SS. The results also show that a gap of 10 mm between 304SS and 20CS is too far for Fe2+ ions (released from counter electrode 20CS) to disturb the deposition behavior of calcium carbonate. However, when Fe2+ ions diffuse to cathode surface, such as the region A of GC II (Figure 8a,d) and 20CS during potentiostatic deposition processes (Figures 11 and S4 in the Supporting Information), aragonite crystals can be observed apparently near corrosion products. Such a phenomenon indicates that Fe2+ ions probably exert some influences on the crystallization behavior of calcium carbonate. 3.5. Effects of Fe2+ Ions. In order to further elucidate how 2+ Fe ions influence the deposition behavior of calcium carbonate, different amounts of Fe2+ ions were added into synthetic water before potentiostatic deposition (applied potential: −0.85 V vs SCE, both 304SS and 20CS). Chronoamperometric curves are shown in Figure 13. As shown, the curves for 304SS and 20CS show the same trend, and current densities decrease with the increase of Fe2+ concentration. This can be attributed to the partial consumption of dissolved oxygen by the oxidation of Fe2+ ions (Figure S5a in the Supporting Information).59 Compared with blank tests (curve 1), a relatively stable current density is obtained in the presence of Fe2+ ions for both materials (curves 2 and 3), which indicates a low coverage of the electrode by calcium carbonate. FE-SEM images of 304SS and 20CS are shown in Figure 14. Corresponding XRD and FT-IR are shown in Figure 15. As shown, aragonite gradually become the

Figure 10. FE-SEM images of 304SS after deposition under a fixed potential of −0.85 (a), −0.77 (b), and −0.68 (c, d) V vs SCE for 2 h in quiescent synthetic water at 26 ± 1 °C.

indicate that, for both materials, calcite is the predominant polymorph (calcite: JCPDS: 01-0628) when the applied potential is −0.85 V vs SCE, whereas lower polarization potentials apparently favor the deposition of aragonite (aragonite: JCPDS: 41-1475). This indicates that the polymorphs are mostly dependent on cathodic current density or electromigration effect, whereas the influence of substrate can be neglected. In fact, electromigration has only a minor influence on the concentrations of scale species (CO32− and Ca2+) due to their low contents in synthetic water (Table 1).18 Especially, almost no CO32− exists in bulk solution at pH 6.9 ± 0.1 according to the equilibrium diagram of carbonate species.54 As a rule, cathodic current density is in direct relation to the interfacial pH, which further influences the supersaturation or nucleation rate of CaCO3.55 Previous studies showed that a relatively low pH or nucleation rate favored the crystallization of unstable polymorphs, especially aragonite, whereas a high one favored the formation of calcite.36,37,39,56,57 For GC II, the cathodic current density is difficult to measure due to its uneven distribution, which depends on the geometric shape of H


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Figure 12. X-ray diffraction data and FT-IR spectroscopy of the deposits formed on 304SS (a, c) and 20CS (b, d) under different polarization potentials. “■” and “▲” represent calcite and aragonite, respectively.

Figure 13. Chronoamperometric curves of 304SS (a) and 20CS (b) under −0.85 V vs SCE in quiescent synthetic water containing different concentrations of Fe2+ ions at 26 ± 1 °C.

dominant polymorph with increasing Fe2+ concentration, whereas calcite crystallization is apparently inhibited (calcite: JCPDS: 01-0628; aragonite: JCPDS: 41-1475). Representative EDS, RS (Figure S6 in the Supporting Information), and FT-IR (Figure 15c,d, peaks at approximately at 1629 cm−1) results show that large amounts of γ-FeOOH (transformation from Fe(OH)2) deposit on the electrode surface (Figure 14c−f), which is similar to the region A of GC II (Figure 8d). In comparison with CaCO3 (Ksp = 8.54) and FeCO3 (Ksp = 10.5), Fe(OH)2 has a stronger deposition tendency because its higher solubility product constant (Ksp = 15.1).60 Besides, according to the studies by Gutjahr and Meyer et al.,29,30 Fe2+ ions can inhibit the growth of calcite by the reversible adsorption of Fe2+

ions at kinks sites, which is further borne out through molecular dynamics simulations.61 3.6. Mechanism. Deposition behavior of calcium carbonate during galvanic corrosion processes is the combined contribution of cathodic and anodic processes (such as GC II), which can be divided into three phases as presented schematically in Figure 16. In phase I, anodic dissolution (R1) and cathodic reduction reaction (mainly ORR, R2) occur simultaneously. In phase II, in the cathodic region near the anode, anodically produced Fe2+ ions diffuse to this region, consume a good part of local dissolved oxygen (Figure S5 in the Supporting Information), and react with cathodically produced OH− to form iron hydroxides (R3). Thus, a relatively low interfacial pH (Figure S5 in the Supporting Information) or supersaturation I


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Figure 16. Schematic illustration of the mechanism for the deposiotion behavior of calcium carbonate during galvanic corrosion processes.

are obtained and further contribute to the deposition of aragonite as a predominate polymorph (area I, phase III).36,37,39,56,57 For the farther cathodic region (area II), it is difficult for Fe2+ ions to reach this region due to its chemical instability48,49 and the produced OH− ions are mainly consumed by HCO3− (R4, phase II). Even so, three polymorphs simultaneously deposit in area II as a result of the low cathodic current density in this region (phase III). Besides, owing to the discrepancy of solution resistance, the

Figure 14. FE-SEM images of 304SS (left column) and 20CS (right column) after 2 h of potentiostatic deposition at −0.85 V vs SCE in quiescent synthetic water containing different concentrations of Fe2+ ions at 26 ± 1 °C. (a, b) 0 mg L−1; (c, d) 18 mg L−1; and (e, f) 36 mg L−1.

Figure 15. X-ray diffraction data and FT-IR spectroscopy of the deposits formed on 304SS (a, c) and 20CS (b, d) in quiescent synthetic water containing different concentrations of Fe2+ ions. “■” and “▲” represent calcite and aragonite, respectively. J


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Province (No. 2015020178), and Fundamental Research Funds for the Central Universities (No. DUT16RC(3)106).

cathodic current density decreases with the increase of the distance from the anode,58 and thus the crystal number density decreases with increasing distance as well. For 20CS, a typical micro galvanic corrosion cell is formed between microstructural phases,62 where ferrous ions released from micro anodic phases can easily reach the adjacent micro cathodic phases. Therefore, only aragonite deposited on 20CS can be ascribed to the disturbance of ferrous ions. The present work suggests that anodically produced metal ions during corrosion processes exert a significant influence on the scaling processes of calcium carbonate on metal wall with poor corrosion resistance (such as carbon steel). Unfortunately, such interfacial effect has been neglected in previous studies. It was found that the metal ions in mineral water, especially ferrous ions, could affect the performance of scale inhibitor.63 Therefore, a further investigation of the influence of interfacial corrosion electrochemical behavior on the performance of various scale inhibitors will be more practical. It is believed that these studies can provide a better understanding of the scaling behavior of calcium carbonate and contribute to the improvement of the existing counter measurements for scaling problems.

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4. CONCLUSION Electrochemical corrosion processes on 20CS surface tend to induce the deposition of aragonite scale. Such deposition behavior was explained using galvanic deposition and potentiostatic deposition methods. A higher cathodic current density can induce the deposition of calcite, whereas a lower one favors the formation of aragonite and vaterite, especially the former polymorph. A higher concentration of ferrous ions not only can reduce the cathodic current density by consuming a good deal of dissolved oxygen but also can lower the interfacial pH by preferentially forming iron hydroxides. These processes build a relative low CaCO3 supersaturation environment at the metal−water interface, favoring the deposition of aragonite. During the immersion processes of 20CS in synthetic water, ferrous ions released from micro galvanic corrosion cell can easily diffuse to the adjacent cathodic regions and induce the deposition of aragonite scale.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03399. Complementary figures and experimental details. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tianzhen Zhu: 0000-0002-1403-053X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of National Key R&D Program of China (No. 2016YFB0601100), the National Natural Science Foundation of China (Nos. 51671047 and 21403030), Natural Science Foundation of Liaoning K


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Origin of Aragonite Scale Deposition on Carbon Steel at Ambient

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