Development of Smart Corrosion Inhibitors for Reinforced Concrete

Article pubs.acs.org/IECR

Development of Smart Corrosion Inhibitors for Reinforced Concrete Structures Exposed to a Microbial Environment Enrico Volpi,*,† Cristian Foiadelli,† Stefano Trasatti,† and Dessi A. Koleva‡,§ †

Faculty of Sciences and Technologies, Department of Chemistry, University of Milan, Via, Golgi 1920133, Milano, Italy Faculty of Civil Engineering and Geosciences, Department Materials & Environment, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands § Faculty of Science and Engineering, School of Chemical and Petroleum Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia ‡

S Supporting Information *

ABSTRACT: Reinforced concrete deterioration due to acidification of the environment from microbial activity in view of steel performance is seldom reported and still a debate. An initial scrutiny of several inhibitors indicated methylene blue dye and trisodium-phosphate as the most promising candidates for mild steel protection in diluted H2SO4. Such compounds were combined together into two organic/inorganic hybrids composed of hydroxyapatite (HAP) or vaterite porous matrixes impregnated with methylene blue dye. The novel hybrid systems were characterized by means of scanning electron microscopy, X-ray diffraction, and Brunauer−Emmett−Teller analysis. The electrochemical response of steel specimens in a simulated environment containing loaded and empty HAP host was monitored by means of linear polarization resistance and electrochemical impedance spectroscopy. The results confirmed the inhibitive properties of the chosen compounds in acidic medium, pointing out a synergistic effect resulting from the release of the organic compound and the dissolution of the inorganic matrix.

1. INTRODUCTION Despite attracting less attention than the widespread chlorideinduced steel deterioration mechanism, microbial-induced concrete corrosion (MICC) can significantly decrease the service life of various types of reinforced concrete (RC) structures such as sewer pipes,1,2 wastewater treatment plants,3 swimming pools,4 cooling towers,5 bridge piers,6 and animal housing facilities.7−9 Microbial deterioration of reinforced concrete structures is commonly attributed to bacteria involved in the sulfur cycle; aggressive compounds such as H2S and H2SO4 form as a result from the metabolism of sulfate reducing bacteria (SRB) and sulfur oxidizing bacteria (SOB), respectively.10,11 De Belie et al.9,12,13 thoroughly demonstrated the detrimental effects of the biogenic acidity on the cementitious matrix in that reacting with portlandite (Ca(OH)2) and yielding to gypsum lead to a dramatic decrease of the concrete mechanical properties. Much less effort was instead dedicated to the investigation of the eventual effects of microbial activity on the corrosion of the steel reinforcing bars. Previous studies performed in our group pointed out that among the two metabolites, i.e. H2S and H2SO4, the latter resulted to be the more aggressive toward the integrity of both the cement matrix and the metal (embedded steel).14−16 Although slow, the acidification front propagates into the cementitious matrix, and once the pH at the concrete/steel interface is decreased below a certain threshold value (about 9), the steel rebar corrosion initiates. © XXXX American Chemical Society

A widespread protective approach is based on the application of acid resistant coatings and linings; bituminous materials, polyurethane, and epoxy resins have been used for such applications.17 However, a local failure in the coating could lead to acid penetration, and a consequent local neutralization of the concrete alkalinity, eventually yielding to rebar corrosion. Such issues could be overcome by adding a specific and efficient corrosion inhibitor to the concrete matrix. Several reviews investigating and comparing the protection efficiency of different compounds known for their inhibitive properties are available in the open literature.18−21 However, it is worth mentioning that most of the available papers deal with chloride induced corrosion; many fewer studies were performed on corrosion induced by neutralization of concrete alkalinity due to carbonation22 or acid rains;23,24 none studied a prevention system for microbial induced acidification. In this context, several different substances were tested for their suitability, performance, and applicability. Starting from those well-known and employed for concrete applications such as sodium phosphate salts,25 moving forward to those tested for acid rain acidification such as sodium molibdate,23,24 and ending up with some organic compounds such as cysteine26 and Received: January 10, 2017 Revised: March 17, 2017 Accepted: April 28, 2017

A

DOI: 10.1021/acs.iecr.7b00127 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research methylene blue dye,27,28 which had never been tested for concrete applications. One of the main problems to be faced when dealing with corrosion inhibitors, either in concrete or in any other application, is how to avoid a premature leaching-out of the active substances with the consequent loss of their effectiveness. Storing the inhibitors inside nano/microreservoirs can overcome the above-mentioned limitation.29 Tailor-made polymeric nanovesicles (or micelles), empty or modified with an alkalizing agent, were successfully tested for concrete applications30,31 as well as within nanocomposite galvanic coatings for corrosion control of steel.32,33 Double-layered hydroxides, vaterite, and hyroxyapatite microbeads filled-in with different organic and inorganic inhibitors were studied by Montemor et al.29,34,35 The rationale behind the present work is based on the synergism between the inhibitive properties of phosphate and methylene blue dye (MBD) in a single product, thus realizing a hybrid organic/inorganic inhibitor. Furthermore, the immobilization of MBD in a phosphate-based matrix that can dissolve at a given acidic pH could result in a valid and superior alternative to overcome issues due to the premature depletion of the inhibitor.

10 mHz by superimposing an AC excitation perturbation amplitude of 10 mV (rms). 2.3. Synthesis of the Hybrid Inhibitors. The synthesis of vaterite (VAT) and hydroxylapatite (HAP)-based hybrid inhibitors followed a previously reported procedure.35 A 0.2 M Na2CO3 solution for the former or a 0.2 M Na3PO4 in the latter case were thermostated at 37 °C in a glass reactor together with 0.03 M MBD. An equal volume of a 0.3 M CaCl2 solution was added dropwise through a dropping funnel mounted over the reactor. The only difference between the two syntheses was that vaterite, being thermodynamically unstable, required L-aspartic acid (1 mg/mL) as organic template admixed to the CaCl2 solution. During the addition of CaCl2 and for the subsequent 30 min, the system was maintained under agitation with a magnetic stirrer. The obtained precipitate was repetitively rinsed and centrifuged with distilled water to remove reaction byproducts. Ethanol was used for the last washing; the solid product was then left drying overnight and finally ground in an agate mortar. 2.4. Characterization of the Hybrid Inhibitors. X-ray powder diffraction (XRPD) patterns of the as-synthesized powders were recorded on a Philips PW3020 powder diffractometer using Cu Kα radiation (λ = 1.54056 Å) in a 2θ range from 10 to 80° (0.02° steps, 1 s counting). The Brunauer−Emmett− Teller (BET) specific surface area was obtained from the N2 adsorption/desorption isotherms at 77 K using a Micromeritics Tristar 2 apparatus. Before measurements, sample powders were heat-treated at 150 °C for 4 h under a N2 flow to remove adsorbed and undesired species from the sample surface. To investigate the MBD release mechanism, six different solutions with pH ranging from 12.6 to 1.5 were prepared by neutralizing a saturated Ca(OH)2 solution with 1% H2SO4. A defined amount of inhibitor (10 mg) was left under agitation overnight in 10 mL of such solutions; the MBD concentration was then determined through spectrophotometric measurements. The isoelectric point (IEP) was measured by means of the mass titration method according to which different amounts of the inorganic host (0−1.5 g) were immersed into 10 mL of 0.5 M KNO3 solution, and the resulting pH was measured after 24 h of agitation. The morphological appearance of the as-synthesized inhibitors was observed with a 1430 LEO scanning electron microscope (SEM) working at a chamber pressure of 8 × 10−6 Torr and 20 keV of accelerating voltage. Electron microscopy was also employed for visualization of the steel surface appearance after treatment in all relevant medium. The B450C steel samples were examined immediately after extracting them from the solutions using low-vac, BSE mode at 20 keV of ESEM Philips XL30.

2. EXPERIMENTAL SECTION 2.1. Materials and Testing Environment. All compounds tested for their potential inhibitive properties (that is, methylene blue dye (MBD), benzothiazole (BTZ), cysteine (CYS), sodium molibdate (SMD), cerium nitrate (CNT), disodium phosphate (DSP), and trisodium phosphate (TSP)) were purchased from Sigma-Aldrich as ACS reagent grade. A diluted 0.5% H2SO4 solution (pH 1.3) was used as model media, aiming to simulate the concrete acidification due to bacteria metabolites.36 Steel samples for electrochemical measurements were prepared from an 8 mm diameter B450C mild steel reinforcing bars with chemical composition: C 0.22%, Cr 0.13%, Ni 0.15%, Mn 0.78%, Mo 0.03%, N 0.007%, Si 0.24%, Cu 0.56%, S 0.039%, P 0.013%. The rebars were purchased from Feralpi Group (Lonato, Brescia, Italy). Small specimens were produced by cutting samples of 15 mm length that were subsequently embedded into a thermoplastic resin leaving a 0.6 cm2 crosssection as exposed area. Electrical connection was realized with a 3 mm diameter AISI 304 threaded rod by drilling a hole at the opposite end of the specimen. Before electrochemical testing, the metallic surface was polished with emery paper from 320 to 1200 grit and ultrasonically degreased in hexane for 10 min. 2.2. Electrochemical Measurements. Experiments in simulated solutions were carried out at room temperature in a glass electrochemical cell with the classical three-electrode setup: the carbon steel was the working electrode; a platinum wire served as a counter electrode, and a saturated calomel electrode (SCE) was used as a reference electrode. Anodic and cathodic potentiodynamic polarizations (scan rate of 0.5 mV/s) were performed to assess the corrosion protection performance of the different inhibitors. Given the low pH of model media, the carbon steel resulted to immediately depict an active behavior; thus, a short (300 s) equilibration time at OCP was set before the starting of the potentiodynamic polarizations. Moreover, four days of continuous monitoring was carried out in the presence of the novel hybrid inhibitor by means of linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). Linear polarization resistance measurements were carried out at a scan rate of 0.1667 mV/s in a polarization window of ±15 mV vs OCP. The EIS measurements were performed at OCP in the frequency range from 10 kHz to

3. RESULTS AND DISCUSSION 3.1. Inhibitor Screening. 3.1.1. Overall Considerations. Different organic and inorganic compounds, which have been reported in the literature as potential inhibitors for mild steel, were evaluated (Table 1). Next, the steel samples were subject to electrochemical tests (anodic and cathodic polarization) in 0.5% H2SO4 solution (as model medium) and in the presence or absence of the chosen inhibitors; results from these tests are further presented in Figure 1. Concerning MBD, it is worth it to underline that it was recently reported to inhibit corrosion of mild steel in sulfuric acid solutions,27,28 but MBD is commercialized as a salt and bears a chloride counterion that could potentially result in a negative effect on protection performance. However, in view of the main objective of this work, i.e., development of a smart inhibitor of B


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not found to be significantly dependent on the MBD concentration but rather determined by simply the presence of MBD. Notwithstanding the significant MBD induced increase in corrosion resistance, the current densities of about 3 × 10−06 A/cm2 still remain rather high. However, it is worth mentioning that the direct immersion of steel into the diluted sulfuric acid solution is an accelerated simulation of the acid-induced corrosion. Presumably, in reinforced concrete structure affected by microbial attack, the steel rebars experience a far less aggressive environment due to the protective effect of the surrounding alkaline cementitious matrix. From the extrapolated corrosion current densities, the inhibitor efficiency was calculated according to eq 1:

Table 1. Different Organic and Inorganic Inhibitors Used in the Preliminary Screening compound

reference

benzothiazole cysteine methylene blue dye sodium molibdate cerium nitrate disodium phosphate trisodium phosphate

37−39 26, 40 27, 28 23, 24 35 25 25

microbial induced corrosion, MBD has two important advantages: (i) being a dye, the release from a matrix can be easily investigated by means of vis-spectroscopy, and (ii) MBD is also known for its antiseptic properties, being used as antibacterial agent in different medical devices41,42 and therapies. 3.1.2. Potentiodynamic Polarization and Surface Analysis of Steel in Acidic Medium with and without Inhibitors. Anodic and cathodic potentiodynamic polarizations were performed on steel in a diluted sulfuric acid solution containing different amounts of MBD and TSP (0.01, 0.1, 1, and 5 mM). The polarization curves are presented in Figures 1 and 2. Optical microscopy (Figure 3), performed after anodic polarization, supported the findings and conclusions from electrochemical data. It should be noted that polarization curves similar to those depicted in Figures 1 and 2 were also obtained for steel in the presence of the two other organic compounds included in the preliminary screening (that is, BZT and CYS). However, their protective performances proved to be less significant and are not discussed. For completeness, though, a direct comparison of the anodic polarization curves for MBD, BZT, and CYS at the concentration of 1 mM is shown in Figure S1 of the Supporting Information together with anodic and cathodic polarization curves for the rest of the inorganic inhibitors included in the preliminary screening (Figure S2) MBD Addition. As can be observed in Figure 1, within both anodic and cathodic polarization, a clear ennoblement of the corrosion potential, Ecorr, was recorded as a function of the MBD concentration. The corrosion current reduced in the presence of MBD (Table 2); a corrosion current density of about one order of magnitude lower was recorded upon adding the inhibitor compared to that of the control case. This effect, however, was

ξ=

icorr(CC) − icorr(inh) icorr(CC)

× 100 (1)

Where icorr(CC) corresponds to the corrosion current density of the control case, whereas icorr(inh) corresponds to that of the inhibitor containing case. Similar to the corrosion current densities, the inhibition efficiency, displaying values of about 87%, did not significantly vary as a function of the MBD concentration As can be also seen in Figure 1a, a significant effect of the inhibitor was recorded with regard to the anodic currents compared to that in MBD-free cases: increased MBD concentration from 0.01 to 5 mM resulted in a reduction of anodic currents. Judging from the slope of the anodic curves, it can be concluded that MBD altered the kinetics of the electrochemical reaction. This is evident from the response with anodic polarization as follows: for the MBD-free control case, the active areas on the steel surface were gradually blocked from rapidly forming corrosion products (dashed line in Figure 1a), which resulted in a relatively flat anodic portion of the curve between −550 and −420 mV. In contrast, the presence of 0.01 mM MBD, except ennoblement of Ecorr, exerted changes in the kinetics of anodic dissolution so that the overall corrosion current was reduced. This was accompanied by an increased slope of the anodic branch of the curves around −520 to −420 mV (response of steel with 0.01 mM MBD in Figure 1a), although the anodic currents remained ca. 1 order of magnitude lower compared to those of the control case. Similar response holds for the cases with higher MBD concentrations up to 5 mM, where the anodic shift in corrosion potential was already more pronounced, but the corrosion

Figure 1. Anodic (a) and cathodic (b) potentiodynamic polarization curves for B450C carbon steel in a 0.5% diluted solution of sulfuric acid containing different amounts of MBD. The dashed line is the control case, i.e., no inhibitor. C


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Figure 2. Anodic (a) and cathodic (b) potentiodynamic polarization of B450C carbon steel in a 0.5% diluted solution with different amounts of trisodium phosphate (0.01, 0.1, 1, and 10 mM). The dashed line is the control case without any addition.

Figure 3. Optical microscopy pictures of the steel surface after the anodic polarizations in 0.5% H2SO4. Left row, magnification 50×; right row, magnification 500×. (a and b) Control case, (c and d) 1 mM MBD, and (e and f) 1 mM TSP.

current and the anodic currents remained in the same range. Therefore, judging from the anodic polarization curves only, the mechanism related to the MBD action with anodic polarization altered kinetics of the electrochemical reaction and impeded

corrosion process overall rather than blocking the active surface only and/or induced anodic control. Conversely, the MBD effect on the cathodic curves (Figure 1b) was almost negligible with regard to both corrosion currents and D


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The presence of 1 mM MBD significantly affected the surface morphology. Figure 3c shows a higher level of heterogeneity if compared to the control case. Large areas of the surface, characterized by a brighter color, presented a smooth (clean) steel surface, where the polishing lines were still detectable; darker areas, probably due to the presence of corrosion products, were also observed. At the higher magnification (Figure 3d), the presence of a barely attacked surface together with some deep and well-defined pits were observed. The surface appearance of steel after anodic polarization in the presence of TSP (Figures 3e and f) seemed to be similar to that of the control case. This is well in line with the electrochemical response in Figure 2a, although a more uniform distribution and a slightly more refined product layer was observed in the TSP sample (Figure 3e) in comparison to that in the control (Figure 3a). 3.1.3. Open Circuit Potential Response and Surface Analysis of Steel in Acidic Medium with and without MBD and TSP. The performance of MBD and TSP inhibitors in view of positively altered corrosion resistance of steel in acidic solutions was illustrated through potentiodynamic polarization tests (Figures 1 and 2). The outcome was also supported by surface analysis after anodic polarization (Figure 3). To elucidate further the effect of MBD and TSP on the corrosion activity on the steel surface or limitations thereof, steel electrodes were immersed in model solutions (0.5% H2SO4), and their open circuit potential (OCP) was monitored continuously over 120 h. This was performed in view of the next steps on the design of a hybrid smart inhibitor composed of both MBD and TSP. Open circuit potentials of steel samples exposed to both the inhibited (1 mM concentration, designation TSP and MBD) and noninhibited sulfuric acid solutions (designation CC) are reported in Figure 4.

Table 2. Corrosion Potential, Corrosion Current Density, and Inhibitor Efficiency (ξ) for Carbon Steel in 0.5% H2SO4 with Different Amounts of MBD sample control case MBD 0.01 mM MBD 0.1 mM MBD 1.0 mM MBD 5.0 mM

Ecorr vs SCE (V) −0.558 −0.527 −0.513 −0.487 −0.476

icorr (A/cm2) −5

2.44 × 10 4.27 × 10−6 3.10 × 10−6 2.80 × 10−6 3.00 × 10−6

ξ (%) 0.0 82.5 87.3 88.5 87.7

cathodic currents. This was especially pronounced for the lowest MBD concentrations of 0.01 mM, where a slight decrease only of cathodic current was observed. For the highest employed inhibitor concentration, i.e. MBD at the level of 5 mM, an effect on the cathodic reaction was relevant (Figure 1b). In this situation, a diffusion limiting cathodic current was observed: the curve for steel in the presence of 5 mM MBD presented a plateau region related to diffusion controlled reduction reaction, extending for approximately 100 mV immediately after corrosion potential in the cathodic direction. Such a limitation of the cathodic current in the proximity of Ecorr was not observed for the case of 0.01 mM MBD, but relevant, although less pronounced, for the MBD-containing cases of 1 and 0.1 mM. On the basis of the above results, it can be concluded that MBD is well-suited to act as a corrosion inhibitor by affecting both oxidation and reduction processes on the steel surface, especially at the highest tested concentration of 5 mM. The result is an impeded electrochemical reaction, reduced corrosion and anodic currents, but also pronounced concentration polarization of the cathodic reaction with increased MBD concentration, consequently increasing the overall corrosion resistance. Concerning the inorganic inhibitors listed in Table 1, the most interesting case was undoubtedly that of TSP. The presence of TSP had no effects on the anodic curves but significantly affected the cathodic ones, leading to a decrease in the current density of more than 1 order of magnitude with respect to the control case (Figure 2b). Although different types of phosphate salts have been extensively used for corrosion inhibition in RC structures, the inhibition mechanism is still not fully understood. It is believed that the salt hydrolyzes in aqueous media and then phosphate ions react either with metal ions, resulting from the onset of a corrosive process,18 or with other ions such as Ca2+.25,43 In both cases, a protective film precipitating on the steel surface was observed. In full agreement with the experimental evidence, such a layer was claimed to be effective by limiting the access of a depolarizer to the metal surface.44 After anodic polarization tests were performed, the surface of all tested steel samples was examined by optical microscopy. The aim was to observe the morphological changes and characteristic features of the corrosion product layers formed (or not) in the presence of inhibitors. Figure 3 shows the appearance of the steel surface at two different magnifications, 50× and 500×, after anodic polarization in the following conditions: control case, MBD, and TSP at inhibitor concentration of 1 mM (a complete gallery of pictures of all other cases is reported in Figure S3 of the Supporting Information). As expected, the control case (Figures 3a and b) was characterized by a very rough surface due to the generalized (uniform) corrosion attack induced by the sulfuric acid solution. However, some spots are also clearly visible at higher magnification where a localized attack occurred.

Figure 4. Five days of monitoring of the OCP evolution for B450C carbon steel in 0.5% H2SO4 and in the presence of different corrosion inhibitors. The dashed line corresponds to the control case.

The TSP and CC behaved similarly, characterized by very negative (cathodic) starting values of ca. −0.570 V vs SCE. For these cases, a fast shift to more noble OCP values during the first minutes of immersion was also recorded. Such an increase could be attributed to the formation of some product layer on the freshly polished surface of the steel samples. After the initial rise, the OCP remained rather stable, and slightly more anodic values were recorded in the TSP case. Coherently with the results from anodic polarization (Figure 1), the presence of MBD led to a substantial, but only initial, OCP ennoblement (Figure 4).The OCP in the MBD case drastically E


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after anodic polarization (Figure 3). However, the naturally occurring corrosion process (or limitations thereof) during prolonged treatment and without polarization account for the easily observable differences in Figure 5 when compared to those in Figure 3. The noninhibited CC (Figure 5a) showed the already described rough surface, but the areas of localized corrosion coalesced, leading to very deep damage. When MBD was added to the solution (Figure 5b), similar to the surface appearance after anodic polarization (Figure 3b), large areas of the otherwise active surface remained entirely protected (bare and corrosion product-free surface was even present to a larger extent when compared to that after anodic polarization). However, black deposits of corrosion products were still observed, pointing out the limited protection efficiency of MBD alone. The reasoning behind the observed performance and surface features, i.e., the insufficient protection efficiency of MBD, could arise from the fact that every MBD molecule, being positively charged, bears a chloride ion as a counterion. Hence, the protecting action of the adsorbed on the steel surface organic MBD molecule was counteracted by the action of chloride ions as corrosion-initiating species. Consequently, the result was potentially related to a chloride-induced corrosion activity outpacing the MBD inhibitive action. The result was chloride ion penetration through defects in the inhibitor layer, finally yielding localized corrosion damage. In the case of TSP, as shown in Figure 5c, the steel surface appeared to be with a certain roughness; however, a more homogeneous layer with respect to the control case was observed. In this case, the surface was uniformly attacked in the acidic solution (general corrosion was relevant). This, in addition to the obvious inhibitive action of TSP, led to a well-spread and rigid product layer on the steel surface. In conclusion, among the different inhibitors tested, methylene blue dye and trisodium phosphate showed the most promising results and were selected for further experimentation aiming to develop a hybrid and smart inhibitive system. The smartness of the inhibitor is in view of its potential ability to release the active compounds only when triggered by a significant pH decrease, such as the one induced by the biogenic acidity. Thus, MBD was incorporated into two different inorganic reservoirs: porous calcium carbonate microbeads and calcium phosphate powder. The chosen inorganic reservoirs are known to dissolve only at acidic pH and were therefore supposed to deliver the organic molecule when needed. Furthermore, for

decreased in the subsequent hours, stabilizing after 1 day of immersion at more cathodic values (ca. −0.570 V vs SCE) compared to the CC at this stage (ca. −0.540 V vs SCE). From this stage forward and until the end of the immersion period, the MBD-treated steel remained at the most cathodic potentials (around −0.560 V); the TSP-treated sample exhibited the most noble potentials (ca. −0.530 to −0.540 V) and the CC sample stabilized from −0.550 to around −0.530 V after 120 h. Therefore, according to the OCP evolution for the tested samples, MBD would result in impeded corrosion activity at early stages, while TSP can support a corrosion delay within treatment. This outcome, along with the different effect of MBD and TSP on the observed corrosion kinetics, i.e. MBD affecting mainly the oxidation reactions, while TSP affecting mainly the reduction process (Figures 1 and 2), accounts for a potentially good performance of a “hybrid” inhibitor using both MBD and TSP. After 120 h of OCP monitoring, the steel samples were examined via optical microscopy for a morphological investigation of the product layers and general surface appearance. Figure 5 shows the images for steel treated in inhibitor-free

Figure 5. Optical microscopy pictures of the steel surface after 5 days of OCP monitoring in 0.5% H2SO4. Magnification 50×. (a) Control case, (b) 1 mM MBD, and (c) 1 mM TSP.

solution sample CC (a), steel treated in MBD-containing solutions (b), and steel treated in TSP-containing medium (c). The images present features similar to those previously observed

Figure 6. X-ray diffractograms of the unloaded compounds (a) calcium carbonate-based powder and (b) calcium phosphate-based powder. F


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Figure 7. SEM pictures of the unloaded vaterite. Magnification: (a) 600× and (b) 3200×.

homogeneous dimensions and spherical shape of VAT, HAP exhibited a highly irregular shape. Porosity, although not being the only determining factor, definitely plays a key role in the loading capacity of a material; the BET surface area of the unloaded vaterite and hydroxylapatite are reported in Table 3.

the case of calcium phosphate-based reservoir, phosphate ions would be expected to supply additional protection via active corrosion inhibition of the steel reinforcement. 3.2. Characterization of the Hybrid Inhibitive Systems. Both synthesized hybrid compounds (calcium carbonate and calcium phosphate-based powders) were characterized by means of XRD. Analyses were performed on both the MBD loaded and unloaded form of each compound. As the results showed that in either case the presence of the organic molecule did not affect the crystalline structure, Figure 6 reports X-ray diffractograms of the unloaded forms only. The sharp and well-defined peaks in Figure 6a indicated a highly crystalline structure of calcium carbonate-based powder, and according to the diffractograms library. the main signals and pattern corresponded to vaterite. However, some less intensive peaks were also detected and attributed to calcite. As mentioned above, among the calcium carbonate polymorphs, vaterite and aragonite are not thermodynamically stable; thus, it was not at all surprising that during the synthesis, a small amount of calcite, which is the only stable form, was obtained. SEM pictures of the vaterite powder are shown in Figure 7. From such images, it can be clearly seen that the spherical shape of vaterite is absolutely predominant; however, as represented in the inlet of Figure 7b, a few layered and rhombohedral calcite crystals can be found. The X-ray diffractogram of the calcium phosphate-based powder shown in Figure 6b was characterized by lower peak intensities and broader, less-defined signals when compared to those of vaterite; such features are commonly attributed to poorly crystalline structures. The signal identification pointed out the presence of hydroxylapatite and calcium hydrogen phosphate, whose XRD patterns are almost overlapped. SEM pictures of the as-synthesized hydroxylapatite before mechanical grinding are reported in Figure 8. Different from the

Table 3. VAT and HAP BET Surface Area BET surface area (m2/g) vaterite hydroxylapatite

1.90 ± 0.01 84.3 ± 0.6

The HAP surface area was significantly higher when compared to that of VAT. Because of the low surface area of the VAT powder, the pore size distribution determined by the nitrogen desorption isotherm cannot be considered as reliable, while the same elaboration for HAP data is reported in Figure 9. Histograms representing the pore size distribution were centered around 12 nm, which is in the range of the mesoporous materials.

Figure 9. Pore size distribution of the HAP powder.

The loading capacities of the two MBD-loaded hybrid systems were determined by means of spectrophotometric measurements of a strongly acidic solution in which a precise amount of the inhibitor was completely dissolved. The results, reported in Table 4, were in full agreement with the BET data, thus pointing out a significantly higher loading capacity for the more porous Table 4. MBD Loading Capacity of the Hybrid Systems loading capacity (wt %) vaterite vydroxylapatite

Figure 8. SEM pictures of the as-synthesized unloaded hydroxylapatite. G

0.014 6.95 DOI: 10.1021/acs.iecr.7b00127 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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taking place at pH 3. As can be clearly seen by the superimposition of the equilibrium diagram, the lower pH release is consistent with the complete dissolution of the two solid phases, both of them detected through the XRD analysis (Figure 6). A reasonable explanation for the mechanism involved in the initial dye release, occurring at the more alkaline pH, was found by means of the IEP determination. Results from the mass titration method are reported in Figure 12 and clearly showed a stabilization of the response, corresponding

HAP with respect to the less porous VAT. Furthermore, a confirmation of the reliability of such results can be found in literature,35 where the same vaterite microbeads impregnated with salicylaldoxime instead of MBD had a loading capacity of 0.010 wt %. The same procedure as for the determination of the loading capacity, but using solutions of different pH values ranging from 12.6 to 1.3, allowed determination of the profile of the pH dependent release of methylene blue dye. Such data are reported in Figure 10 for the MBD−VAT hybrid system. In the same figure, the equilibrium diagram of calcium carbonate in an aqueous solution, calculated by means of MEDUSA software,45 is also plotted.

Figure 12. Mass titration data of HAP in a 0.5 M KNO3 solution.

with the IEP occurring at pH 8.95; such value resulted to be rather similar to that measured for a differently obtained HAP.46 In the absence of chemisorbed or physisorbed species, particle surfaces in aqueous suspension are generally assumed to be covered with surface hydroxyl species, HAP−OH. When the pH is higher than the IEP, the predominate surface species is HAP−O−, while at pH values below the IEP, HAP−OH2+ species prevail. In the present case, taking into consideration that the MBD molecule is positively charged, at pH more alkaline than 9 (above IEP), electrostatic attractive forces are developed between the organic molecule and the negatively charged HAP surface. As a consequence of acidification leading to a pH lower than 9 (below IEP), the HAP surface will turn from negative to positive, thus transforming the attractive forces into repulsive ones and consequently causing the initial MBD release.

Figure 10. Spectrophotometric determination of the pH dependent MBD release from the VAT hybrid and equilibrium diagram of a calcium carbonate solution. Dashed lines represent solid species, while continuous lines represent aqueous species.

The experimental data related to the MBD release outlined a sigmoidal shape extremely similar to that of Ca2+ ions resulting from the calcium carbonate dissolution, thus confirming that the latter was the main and only responsible mechanism for the MBD delivery. A similar plot related to the HAP-based hybrid is shown in Figure 11; different from the previous case, the inhibitor release followed a double step mechanism with an initial liberation of a small amount of dye occurring around pH 10 and a second one

Figure 11. Spectrophotometric determination of the pH dependent MBD release from the HAP hybrid and equilibrium diagram of a calcium phosphate solution. Dashed lines represent solid species, while continuous lines represent aqueous species. H


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Figure 13. Potentiodynamic polarization curves of mild carbon steel in a 0.5% diluted solution of sulfuric acid containing (a) HAP 1 mg/mL and (b) MBD−HAP 1 mg/mL. The dashed lines represent the response of the control case (the response of steel in the presence of MBD at various concentrations was as depicted in Figure 1).

hydrogen evolution). Following a mixed cathodic polarization (activation around corrosion potential and concentration in the range of more cathodic potentials), the corrosion resistance increased overall, reflected by a decrease in corrosion current density of approximately 1 order of magnitude (Figure 13a). In contrast, for the hybrid MBD−HAP, a synergetic action of MBD and HAP was the most likely effect. The following considerations support this hypothesis: MBD, being positively charged (Section 3.2) and when present alone, would preferentially adsorb on the negative in charge, more active (anodic) locations. This process would occur in the initial stages of treatment, at the very least, potentially followed by the formation of an organic layer on the steel surface. Reduced anodic current would be the result, as in fact observed (Figure 1a), leading to an increased corrosion resistance overall. Barrier effects due to adsorption of this organic compound were most likely not relevant, also in view of the similar slope with anodic polarization in this case (Figure 1a). When the hybrid MBD−HAP is present in the system (Figure 13b), the effect of MBD with anodic polarization would be hindered to the extent of the counteracting action of Cl− induced corrosion (Cl− ions being present in the MBD structure, as already discussed). The result was a faster anodic reaction compared to that in control cases (Figure 13b, anodic branches of the curves). Yet, corrosion resistance was significantly enhanced due to the synergetic action of both HAP and the already released MBD in the initial steps of the process. This is reflected by the even more pronounced (compared to HAP alone) ennoblement of corrosion potentials and lower corrosion current densities (Figure 13b). LPR and EIS further elucidate the performance of steel in the presence of the hybrid inhibitor. As aforementioned, four different cases were tested: the control case without any inhibitor, the unloaded inorganic matrix 1 mg/mL (HAP), the impregnated hybrid 1 mg/mL (MBD−HAP), and also MBD alone at the concentration of 0.07 mg/mL. Such concentration was defined on the basis of the previously determined loading capacity of ≈7%, aiming to confirm the effects of the same amount of organic dye as introduced in the other cases but without the presence of the inorganic matrix. Steel electrodes were monitored over 4 days in these solutions. The polarization resistance values, calculated from LPR measurements, are reported in Figure 14. The two data sets related to

In conclusion, two different inorganic hosting matrices, based on calcium carbonate (VAT) and calcium phosphate (HAP) were successfully synthesized and impregnated with methylene blue dye. The HAP, having an easier synthesis as it did not require any organic template, resulted also to have a higher porosity area and thus a higher storage capacity, if compared to the VAT microbeads. Furthermore, the dissolution of the MBD−HAP hybrid, leads to the release of both the organic molecule and phosphate ions, both of them having inhibitive properties, as demonstrated above. For such advantages, MBD−HAP was chosen for further experimentation aiming to assess the inhibitive properties of the hybrid system in model media. 3.3. Steel Response in Model Medium in the Presence of MBD−HAP. 3.3.1. Electrochemical Response: DC Techniques. To investigate the expected synergetic (combined) action of the hybrid MBD−HAP as a smart inhibitor for corrosion control, steel electrodes were tested in model solutions (0.5% H2SO4). The employed strategy was to compare electrochemical responses when HAP and MBD are used alone, when MBD− HAP is used as a hybrid, and to compare results to the control case, i.e., inhibitor-free system. DC polarization was performed for steel in all of these cases with immersion in the testing medium (MBD only as presented in Figure 1). EIS was used to qualify and quantify the corrosion state of steel in time of immersion and in each testing medium where the test duration was 4 days. The combined protection of the novel organic/inorganic inhibitor was confirmed through potentiodynamic polarization of steel in diluted sulfuric acid solution containing 1 mg/mL of either the impregnated MBD−HAP or the unloaded HAP (Figure 13). In the case of the unloaded HAP (Figure 13a), coherently with the findings of the initial screening, the anodic curve perfectly overlapped that of the control case, thus pointing out that the hydroxyapatite dissolution did not affect the anodic process, while the cathodic curve showed a significant decrease in terms of cathodic current density. Conversely, the MBD loaded HAP hybrid system (Figure 13b) resulted in affecting both the anodic and the cathodic processes, confirming the combined action of the two different components. In other words, HAP affects the reduction reaction on the steel surface (which in this medium would be predominantly I


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fast decrease with longer treatment and reached the level of those recorded for HAP and the control case. The decrease in the polarization resistance, being inversely proportional to the corrosion current, could be attributed to the already mentioned presence of chloride counterions within the MBD molecules. Chloride ions are known to be able to quickly penetrate through an organic protective layer and thus induce localized corrosion. Conversely, the MBD-loaded hybrid MBD−HAP system exhibited higher Rp values when compared to those of all other cases. This was along with a certain stability in the response, at least for the first 3 days, after which the polarization resistance started to decrease, although remaining more than 600 Ohm·cm2 higher than the control case. This suggests that the combination of MBD and HAP gives far better results than those of the two components singularly used. 3.3.2. EIS Response. Figures 15−19 depict the recorded EIS responses together with the strategy within EIS data evaluation (the response of a control specimen is presented). The response was assessed through evaluation of both complex plane (Nyquist) and Bode plots (Figures 15a and b) in both qualitative and quantitative manner. For a quantitative interpretation of the EIS response, an equivalent electrical circuit is composed (Figure 15c), the application of which determines the fit of the experimental response with varying accuracy. The accuracy of the fit is judged through the graphical presentation (Figures 15a and b) along with the resulting error per element and overall for the fit

Figure 14. Polarization resistances resulting from LPR monitoring of mild steel in 0.5% H2SO4 solution.

HAP and the CC were very similar to each other, being both characterized by a continuous decrease in Rp attributed to the onset of a corrosion process. Such behavior pointed out that the inorganic hosting matrix alone did not seem to guarantee efficient protection. When MBD alone was added to the aggressive solution, the electrochemical response resulted in an initial improvement of the steel performance, as indicated by higher polarization resistance of ca. 700 Ohm·cm2. However, the Rp values showed a

Figure 15. EIS experimental response and fit of a control sample (a and b), example of employed equivalent electrical circuits (c), and error reports on the goodness of the fit (d). J


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Figure 16. EIS response (1−4 days) for the control sample (inhibitor-free solution): (a) Nyquist plot and (b) Bode plot.

Figure 17. EIS response (1−4 days) of steel treated in solution containing MBD. (a) Nyquist plot and (b) Bode plot.

constant (RctCPEdl) is related to the charge transfer resistance Rct and the double layer pseudocapacitance CPEdl, while the lower frequency(LF) time constant (Rpr.lCPEpr.l) described the processes related to inhibitor action as adsorption and/or alterations in the (corrosion) product layer on the steel surface. CPE were used instead of pure capacitances as a generally accepted approach to account for surface heterogeneity and/or nonideal capacitive response.37 The EIS response for all tested specimens and conditions is presented in Figures 16−19. Figure 16 depicts the result for the control sample (REF, identical to designation CC from the PDP and LPR tests). As can be observed, with time of conditioning, the global magnitude of impedance |Z| decreased, reflected by a reduction in the diameter of the semicircle in the Nyquist plot (Figure 16a), which became a more obvious depressed semicircle at later stages. Such depressed shape is commonly reported in the literature for similar systems and is attributed to a frequency dispersion effect due to the roughness and inhomogeneity of the steel surface,37,47,48 as actually observed (Section 3.3.3). This was accompanied by a drop in impedance modulus and phase angle (Figure 16b) from 45 to approximately 15°. All of these features pointed enhanced corrosion activity with conditioning. Similar observations hold for the MBD-treated samples (Figure 17) and HAP-treated (Figure 18), where again, the decreasing global impedance, i.e., decrease in corrosion resistance, was reflected in the Nyquist plots together with a reduction in phase angle from 60 to 15° for the MBD case and from

(Figure 15d). As can be observed in Figure 15, the chosen circuit results in a very good fit with minimum error. According to literature, most of the studies dealing with steel corrosion inhibition in sulfuric acid simplified the EIS data interpretation by using a single time constant circuit,37,48 eventually nesting an RL (resistance, inductance) circuit to more appropriately fit the lower frequency portion of EIS spectra.47,49 Inductive response, however, was not observed in our tests. For other types of acidic solutions (e.g., hydrochloric acid), a two time constant circuit, characterized by two nested RC constants, is commonly used for the fitting of experimental data.50−53 Of course, the number of time constants (i.e., the combination of resistive and capacitive components) is not determined by the experimental medium. Generally, an increase in the number of constants results in a better fit. Additionally, one and the same system can be presented by various combinations of elements in a circuit. For example, Figure 15c) depicts two equivalent circuits, where the first time constant contains either pure capacitance (C) or a pseudocapacitance (CPE). In both cases, the goodness of the fit is acceptable, and both circuits provide similar results. Therefore, the best equivalent circuit would be the one with a clear physical meaning assigned to all elements. This was the approach followed in this work. The circuit used to fit the experimental data (Figure 15c, top) consisted of two time constants in series with the electrolyte resistance. The physical meaning of the elements is as follows: Rs represents the electrolyte resistance, the high frequency (HF) time K


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EIS response itself, especially evident for the phase change: a gradual drop in phase angle was recorded at initial stages, while a more significant reduction, broadening, and shift to LF were observed at the end of the test (Figure 16b). For the MBD-treated steel, a significantly high Rct was observed in the initial period (comparable with that for the MBD−HAP treated sample, Figure 20a). In fact, the HF response of MBD at day 1 (Figure 17a, inset), was capacitive-like, accounting for a well-protected steel surface (significantly minimized charge transfer reaction respectively). This is in line with the LPR results (Figure 14) and related to the already discussed mechanism of the MBD action, i.e., selective adsorption (on anodic locations) in the beginning of treatment. The high Rct together with Rpr.l higher than that of the control specimens and very low (lower than all cases) CPEpr.l (Figure 20b) at the initial stage account for increased corrosion resistance overall. After 2 days of treatment, Rct reduced, CPEdl slightly increased, while Rpr.l and CPEpr.l remained constant. These, together with an abrupt reduction of impedance magnitude (Figure 17a) and phase angle (Figure 17b) at 2 days justify the occurrence of an additional corrosion-enhancing mechanism. The response at 3 and 4 days confirmed this hypothesis; the CPEdl increased further which, independently from the low but stable Rct values, means increase of the surface area for the corrosion reaction. The double layer capacitance is generally attributed to the local dielectric constant and/or the integrity of the adsorbed inhibitor layer on the metal surface.48 Consequently, the observed timedependent increase in CPEdl would also be attributed to the

45 to 25° for the HAP case. These changes reflect the initially higher corrosion resistance for MBD (compare Figures 16 and 17) but similarly low corrosion resistance at the end of the test when compared to that in the control case. The highest magnitude of impedance throughout the test was recorded in the presence of the hybrid MBD−HAP (Figure 19) along with almost constant phase angle (Figure 19b), which remained in the range of 60° throughout the test. The qualitative interpretation of EIS accounts for the protective ability of the hybrid inhibitor MBD−HAP, which apparently merges the positive influence of both MBD and HAP in a single action. The quantitative interpretation of the EIS results provides a more in-depth information on the observed behavior. The bestfit parameters, derived by using the previously discussed equivalent circuit, are summarized in Figure 20. For the control sample, the lowest Rct was recorded at all time intervals. A significant drop of Rct was observed at the end of the test (Figure 20a), accompanied by Rpr.l and CPEpr.l values higher compared to those of MBD and comparable to those of HAP at the end of the test (Figure 20b). All of these account for enhanced steel corrosion in the REF specimens. The reduction of Rpr.l over time, accompanied by a sharp increase in CPEpr.l at 4 days (Figure 20b) together with a constant CPEdl at 3 and 4 days and reduced Rct, especially significant at 4 days (Figure 20a), reflect a nonprotective layer on the steel surface. Consequently, an increasing yet more or less stable activation control in the initial stages and a more pronounced mixed control on later stages was relevant. This mechanism was reflected by the L


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Figure 20. Best fit parameters derived from fit and simulation of the EIS response from 1 to 4 days of continuous treatment: (a) evolution of Rct and CPEdl and (b) evolution of Rpr.l and CPEpr.l..

deterioration of an originally protective organic layer, leading to enhanced oxidation on the steel surface. In addition, CPEpr.l increased significantly at 3 and 4 days, while the change in Rpr.l was marginal. This is a typical response with Cl− induced corrosion, where the localized attack accounts for enhanced corrosion activity and degradation in depth of the steel substrate. This sequence of changes was well-reflected by the EIS response, the Bode plot (Figure 17b) clearly showing that the phase not only reduced significantly but also entirely shifted to the LF range. For the HAP-treated steel, Rct was significantly lower at early stages when compared to that of MBD but higher compared to that of the control steel (Figure 20a). This was accompanied by the highest recorded CPEdl. The relatively constant Rct together with the (although independently judged) highest values for CPEdl indicate “blocking” of the charge transfer reaction and/or corrosion activity, otherwise spread over a large surface, hence globally enhanced corrosion resistance. The results are in line with the recorded performance of HAP alone on inhibiting the

Figure 21. Evolution of OCP values and the corresponding Rp values derived from EIS. M


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Figure 22. SEM pictures of the steel surface after a 4 days of conditioning in diluted sulfuric acid solution. (a and b) Control case, (c and d) 1 mg/mL HAP, (e and f) 0.07 mg/m MBDL, and (g and h) 1 mg/mL MBD−HAP.

alteration) so that, although corrosion damage was relevant, it was spread over a large surface area (as confirmed by SEM observations; see Figure 22), resulting in overall limitation of the corrosion process. For the MBD−HAP-treated steel, Rct was the highest of all studied cases, accompanied by a very low CPEdl throughout the test. This behavior, linked to the first time constant, clearly reflects the high corrosion resistance of steel in the presence of the hybrid inhibitor, i.e., high resistance to charge transfercontrolled corrosion process and activation polarization,

reduction reaction (Section 3.3), consequently leading to limited corrosion activity. The HAP-treated sample, however, exhibited low Rct values throughout the test, although these were higher when compared to those of the control case. Therefore, the almost constant Rct but reduced Rpr.l and significantly increased CPEdl and CPEprl account for competitive mechanisms of enhanced corrosion activity in depth but limited overall charge transfer due to a uniformly attacked steel surface. In other words, the inhibitive action of HAP was counterbalanced by a uniform steel surface modification (in fact, possibly HAP-denoted surface N


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able to protect the steel from the generalized corrosion induced by sulfuric acid, leaving a very clean surface where the polishing lines were still detectable. However, chlorides introduced into the solution together with MBD easily passed through the inhibitor barrier, leading to an extensive localized corrosion. Finally, in full agreement with all electrochemical measurements, the steel sample conditioned in the MBD−HAP acidic solution exhibited a generally smooth and protected surface. Nevertheless, some defects of considerable dimensions were found, as clearly shown in Figures 22g and h; the morphology of these corroded areas was visually somewhat between the control and the HAP-only sample. In conclusion, the results indicate that beneficial effects were obtained by the combination of the organic inhibitor with the inorganic one, resulting in a hybrid characterized by a mixed protective effect. Conversely to the literature findings,27,28 MBD alone is not a valid inhibitor as it displaces the corrosion mechanism from the less severe generalized corrosion to a more insidious localized attack. However, phosphate ions, released as a consequence of the inorganic matrix dissolution, significantly limited such a drawback.

respectively. A gradual (and not as significant as with all other samples) reduction in Rpr.l was recorded together with an increase of CPEpr.l pronounced at the latest stage of 4 days. The relatively “clean” steel surface with only isolated locations of nonuniform and thick product layer (as actually observed; see Figure 22) prove the dependence of resistive and capacitive components of the second time constant and their evolution in time. These account for restructuring and nonuniform thickening of the product layer on the steel surface at the end of the test. Clearly, the hybrid MBD−HAP systems lead to an increase in the corrosion resistance of steel in sulfuric acid, the effects of the hybrid being denoted to the synergy of actions of both MBD and HAP. The evolution of Rct values (Figure 20a) are in line with the recorded Rp values from LPR (Figure 14). This is as expected because Rct, derived from EIS, would be similar or equal to Rp as derived from LPR when the reaction mechanism is mainly activation polarization-controlled. For the later stages of the test, when a product layer developed gradually on the steel surface and has contribution to the overall electrochemical response, the Rct would differ from global Rp. The former is denoted to the faradaic process only, whereas the latter is composed from Rct and resistances due to diffusion or mass transport control reactions. Hence, the global Rp, derived from EIS (Figure 21), is a sum of Rct and Rpr.l and in line with the Rp values, as derived from LPR (Figure 14). Figure 21 also depicts the OCP evolution of all specimens during the test. The most anodic OCP (ca. −475 mV) was recorded for the MBD-treated steel at the beginning of treatment, corresponding to a high Rp of approximately 700 Ohm·cm2. The Rp value derived for the MBD−HAP-treated sample at this stage was significantly higher, approximately 1 kOhm·cm2, corresponding to a significantly more cathodic potential of approximately −520 mV. The Rp values for MBDand HAP treated samples were equally low and comparable to the values derived for the control case. At the end of the test, the highest Rp (highest corrosion resistance) was recorded for steel in the presence of the hybrid (MBD−HAP), while the lowest was recorded for the control case. In conclusion, the EIS response and Rp values for the MBD− HAP hybrid are significantly different from all other tested cases and coherent with the LPR results (Figure 14), proving a rather stable response of the system in the first 3 days and only a slight change at the end of the test. Such behavior justifies the synergetic effect of MBD and HAP together with the ability of HAP to provide a slow release of the organic dye (MBD) and good steel protection. Such a synergic effect of MBD and HAP could be attributed to an initial role played by the hydroxylapatite, leading to a more homogeneous surface, as shown in Figure 22c, thus leading to more ordered and effective packing of the organic molecules and resulting in more efficient protection. 3.3.3. Surface Analysis. At the end of the conditioning period, the surface morphology was examined by SEM; micrographs of the steel surface at two different magnifications are reported in Figure 22. Similar to the results from optical microscopy previously described in Figure 5, SEM micrographs for the control case (Figures 22a and b) show a rough surface typical of a generalized corrosion attack. When HAP was added to the solution, a similar pattern was obtained but characterized by a significantly increased homogeneity (Figures 22c and d). As expected, a completely different corrosion product and surface morphology were detected in the case of the MBD alone (Figures 22e and f). In this case, the adsorbed organic layer was

4. CONCLUSIONS This work reported the development of an efficient smart inhibitor able to increase the corrosion resistance of steel in an environment where acidification is present due to SOB existence and metabolism. The initial screening of different inhibitors in abiotic acidic media simulating the SOB metabolites resulted in the selection of trisodium sulfate because of its cathodic action and because of an already proven efficiency for corrosion protection in reinforced concrete structures. Furthermore, MBD was selected as well because of its inhibitive effects on the anodic process. The effect of MBD alone, however, was counterbalanced by the presence of chloride ions as counterions within the MBD synthesis. MBD was immobilized into a porous calcium phosphate matrix (HAP); in such a way, together with the slow release of the organic molecule resulting from the HAP matrix dissolution, a further beneficial effect was obtained by phosphate ions liberated from the HAP matrix itself. Electrochemical tests performed in simulated acidic solutions pointed out a significant improvement in the corrosion resistance due to the synergistic effect of the MBD−HAP system, while the components (MBD and HAP) did not result in a satisfactory protection if separately used.

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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.7b00127. Comparison of the anodic polarization curves in the presence of three different organic corrosion inhibitors, full set of anodic and cathodic polarization curves with different inorganic inhibitors, and optical microscopy pictures of the steel surface after the anodic polarizations in the aggressive model media in the presence of the different corrosion inhibitors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. O


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Industrial & Engineering Chemistry Research ORCID

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Enrico Volpi: 0000-0001-6852-2006 Dessi A. Koleva: 0000-0002-4704-8824 Notes

The authors declare no competing financial interest.

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