Synthesis, Characterization, and Application of a Multifunctional

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Applied Chemistry

Synthesis, Characterization and Application of a Multifunctional Cellulose Derivative as an Environmentally Friendly Corrosion and Scale Inhibitor in Simulated Cooling Water Systems Tao Gan, Yanjuan Zhang, Meini Yang, Huayu Hu, Zuqiang Huang, Zhenfei Feng, Dong Chen, Congjin Chen, and Jing Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02128 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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Industrial & Engineering Chemistry Research

Synthesis, Characterization and Application of a Multifunctional Cellulose Derivative as an Environmentally Friendly Corrosion and Scale Inhibitor in Simulated Cooling Water Systems Tao Gan,† Yanjuan Zhang,*,† Meini Yang,† Huayu Hu,† Zuqiang Huang,*,† Zhenfei Feng,† Dong Chen,‡ Congjin Chen,† Jing Liang†

†

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004,

China ‡

State Key Laboratory of Non-Food Biomass and Enzyme Technology, Guangxi Academy of

Sciences, Nanning 530007, China

1

ACS Paragon Plus Environment

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

ABSTRACT:

A borated aminated cellulose citrate (BACC) inhibitor was synthesized from

bagasse cellulose with citric acid, diethanolamine and boric acid by mechanical activation-assisted solid-phase reaction (MASPR) technology. The BACC was characterized using FTIR, XPS, and solid-state CP/MAS NMR. The effect of the BACC inhibitor on the corrosion inhibition of A3 steel in simulated cooling water was investigated using weight loss and electrochemical measurements. It was found that BACC mainly acted as a mixed-type inhibitor in simulated cooling water and predominantly controlled the anodic reaction; furthermore, the inhibition efficiency increased with increased BACC concentration. BACC also showed excellent scale inhibition in simulated cooling water. XRD and SEM analyses of the scale deposits showed that the surface morphology and size of the scale deposits were changed by BACC. These results indicated the potential of BACC as an effective corrosion and scale inhibitor in cooling water.

KEYWORDS: Cellulose derivative; Solid-phase reaction; Corrosion inhibition; Scale inhibition; Simulated cooling water

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1. INTRODUCTION Recycling cooling water is one of the most effective ways to conserve industrial water. However, during the recycling of cooling water, the salt and suspended particulates in the cooling water are continuously concentrated, and the cooling tower is directly connected with the atmosphere, which causes water pollution to different degrees.1 As a result, three of the main problems in cooling water systems are corrosion, scale phenomena, and microbial propagation. These problems cause a significant reduction in the heat transfer efficiency and working life of the equipment and a corresponding increase in the energy consumption of the equipment.2 The use of organic molecules as corrosion inhibitors in cooling water systems is one of the most popular, efficient, and practical methods applied extensively to solve corrosion and scaling problems.3 Most of the well-known inhibitors are organic compounds containing heteroatoms such as nitrogen, oxygen, phosphorous, and sulfur and/or delocalized π electrons in their molecular structure.4-7 The polar functional group is usually regarded as the reaction center for establishing the adsorption process. Organic compounds containing nitrogen atoms have been reported as efficient inhibitors for different metals in various aggressive media.8,9 In addition, boric acids are widely applied in medicine as antibiotics and inhibitors.10,11 Polymers are preferred over simple organic compounds as inhibitors, as they possess multiple reaction centers, which help in forming complexes with metal ions.12 Generally, the inhibitory capacity of polymers is related to the presence of heteroatoms and cyclic rings. Unfortunately, synthetic polymers are commonly nonbiodegradable and nonrenewable. When considering the costs of the industrial and large-scale use of inhibitors, the toxicity,

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availability, and environmental friendliness of the selected compound are very important. Therefore, studies on inhibitors have focused on the application of biopolymers. Because of their renewable, biodegradable, cheap, and nontoxic characteristics, biopolymers originating from plants have been developed as eco-friendly inhibitors. Biopolymers are rarely used directly because of their relatively unstable performance, the large amount that must be added, their high impurity content, and the facile reproduction of microbes in their presence. However, biopolymers can be modified to overcome these disadvantages. Cellulose, as the most abundant biopolymer, is abundant in nature and can be utilized as an inhibitor. Cellulose derivatives, such as hydroxypropyl methylcellulose (HPMC), carboxymethyl cellulose, and ethyl hydroxyethyl cellulose, have been widely reported to be good corrosion inhibitors in different corrosive media.12,13 These compounds usually form very thin and persistent adsorbed films on metal surfaces, causing a decrease in the corrosion rate by retarding anodic, cathodic, or both reactions.14 However, the dosage of these inhibitors is relatively large, and the inhibitors do not possess antibacterial behavior. In the present study, an effort is made to improve the inhibition and antibacterial efficiency of cellulose by introducing ester, amide, and borate groups to prepare a functional cellulose derivative. At present, using a single water treatment agent has shortcomings and cannot simultaneously solve the corrosion and scaling problems in cooling water systems. Hence, the application of water treatment agents is not a single-component system in actual industrial production but rather a complex system with more than two components. However, there are different interactions between different types of water treatment agents, which may manifest as synergistic effects, additive effects, or antagonistic effects of mutual interference. Therefore,

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multifunctionality is a developing trend in water treatment agents. According to the “molecular design” viewpoint, various functional groups gathered in a macromolecule skeleton can result in multifunctional characteristics with a synergistic effect. In general, the esterification of cellulose is carried out in organic solvents or ionic liquids, such as N,N-dimethylacetamide (DMAc)/LiCl, toluene/triethylamine (TEA), and 1-ethyl-3-methyl-imidazolium acetate ([EMIM]OAc).15-17 Such esterification reactions present the disadvantages of complex procedures, including difficulties in the purification of products and in effectively recovering the large amounts of used organic solvents. In the amination of cellulose, cellulose generally cannot react directly with amines; cellulose is first esterified and then reacted with the amines. It is well known that enamine bonds can be formed by reacting amines with acetyl compounds under mild conditions.18,19 Cellulose borates have been studied predominantly in order to improve the unique application properties of cellulosic materials, for example, antibacterial properties or heat stability. However, the synthesis of cellulose borates requires acid or alkali as a catalyst, tedious steps and a long reaction time.20 Therefore, it is very important to develop a green, simple, and efficient method for the modification of cellulose. In our laboratory, we applied mechanical activation (MA) to enhance the chemical modification of natural polymers by solid-phase reaction (SPR) without the use of organic solvents, and this simple technology is defined as MA-assisted SPR (MASPR).21,22 Consequently, the aim of the present study was to synthesize a multifunctional cellulose derivative, borated aminated cellulose citrate (BACC), by MASPR for use as a novel environmentally friendly inhibitor. Fourier transform infrared spectroscopy (FTIR), X-ray

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photoelectron spectroscopy (XPS), and solid-state CP/MAS NMR were used to characterize the chemical structure of BACC. Weight loss measurements, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and static tests, as well as characterization by XPS, scanning electron microscopy (SEM), and X-ray diffraction (XRD), were carried out to investigate the corrosion inhibition and antiscaling performance of BACC in simulated cooling water media, and the inhibition mechanism of BACC was also discussed. 2. EXPERIMENTAL SECTION 2.1. Materials. The cellulose material used in this study was separated from sugarcane bagasse (cellulose 41.30 wt.%), with a cellulose content of over 98.49 wt.% and a degree of polymerization of 563. Sugarcane bagasse was supplied by a local sugar factory (Nanning, China). The separation and purification of cellulose were performed according to a previously reported method.23 Citric acid, sodium hypophosphite, anhydrous ethanol, and other reagents were of analytical grade, used without further purification and obtained from commercial sources. The chemical composition of the simulated cooling water system is presented in Table S1. The temperature and pH were adjusted to 45 ± 1 °C and 7.3 ± 0.02, respectively. The chemical composition of A3 steel is presented in Table S2. 2.2. Preparation of BACC by MASPR. The preparation of BACC by MASPR technology was performed in a customized stirring ball mill,21 and the schematic diagram is shown in Figure 1. A fixed amount of milling balls (500 mL, 5 mm diameter) was first added into a jacketed stainless steel chamber (1200 mL), and then, 10.00 g of cellulose, 11.85 g (molar ratio of anhydroglucose units (AGUs) of cellulose: citric acid = 1: 1) of citric acid (esterifying agent), and 10 wt.% (weight percent compared to cellulose) sodium

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hypophosphite (catalyst) were added into the chamber. The mixture was subjected to dry milling at a speed of 300 rpm under a constant temperature of 80 °C by circulating thermostatic water in the jacket of the chamber. After milling for 120 min, the balls were removed from the milled sample. The sample was first washed to neutral with distilled water to remove the sodium hypophosphite and unreacted reagents and then washed with absolute alcohol to prevent cellulose hornification in the presence of water. The sample was then vacuum-dried at 55 °C. After drying for 48 h, cellulose citrate was obtained.

Figure 1. Schematic diagram of stirring ball mill.

Next, 10.00 g of cellulose citrate and diethanolamine (molar ratio of diethanolamine: cellulose citrate = 1: 2) were placed in the chamber and subjected to dry milling at a fixed stirring speed of 300 rpm; the reaction temperature was fixed at 120 °C by circulating thermostatic oil in the jacket of the chamber. After the mixture had been milled for 120 min, boric acid (molar ratio of boric acid: cellulose citrate = 1: 1) was added, and the mixture was milled for an additional 120 min. The balls were removed from the resulting sample by a sieve, and the crude product was purified by a repeated washing-filtration process with 7



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anhydrous ethanol to remove unreacted reagents. After vacuum-drying at 50 °C for 24 h, the resulting BACC was obtained and stored in a sealed container for analysis. 2.3. Weight Loss Tests. Weight loss tests for evaluating the corrosion inhibition of BACC were carried out as described in the literature.24 The corrosion rate and corrosion inhibition efficiency were determined according to China National Standard GB/T 18175-2000, “Performance measurement of corrosion inhibitor in water-treatment with a rotating apparatus”. The main steps were as follows: 1600 mL of simulated cooling water was added to a beaker containing different concentrations of BACC to evaluate the corrosion inhibition effect of BACC. The beaker was placed in a water bath to maintain a temperature of 45 °C. A3 steel test coupons with dimensions of 50 mm × 25 mm × 2 mm were sectioned from plates. After being successively polished with emery paper (400–1200 grade) and weighed, the A3 steel test coupons with a surface area of 28 cm2 were then hung in the solution and rotated at 85 rpm for 72 h. The corrosion rate (v) was calculated as follows:

ν=

87600 ×  ( m1 − m0 ) − ∆m 

(1)

S ×T × ρ

where m1 is the mass of the A3 steel slices before testing (g); m0 is the mass of the A3 steel slices after testing (g); ∆m is the mass loss of the A3 steel slices caused by washing in acid solution (g); S is the surface area of the A3 steel slices (28.0 cm2); T is the test time (72 h); ρ is the density of the A3 steel slices (7.85 g cm–3); and v is the annual corrosion rate of the carbon steel test coupon (mm y–1). The corrosion inhibition efficiency (IE) was calculated as follows:

IE =

ν 0 −ν ×100% ν0

(2)

8


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where v0 and v are the corrosion rates of the A3 steel slices in the absence and presence of BACC (mm y–1), respectively. 2.4. Static Tests. Static tests for evaluating the scale inhibition of BACC were conducted according to the Chinese National Standard method.24 The tests were performed for 10 h at a certain temperature with a certain amount of BACC. The remaining Ca2+ was determined by EDTA titration after cooling to room temperature. The scale inhibition efficiency (S) was calculated as follows: S=

ρ1 − ρ 2 × 100% nρ 0 − ρ 2

(3)

where ρ1 is the mass concentration of Ca2+ with BACC after heating (mg L−1); ρ2 is the mass concentration of Ca2+ without BACC after heating (mg L−1); ρ0 is the mass concentration of Ca2+ of the blank solution before heating (mg L−1); and n is a concentration factor for the simulated cooling water.

2.5. Characterization. The samples were characterized by Fourier transform infrared spectroscopy (FTIR) and solid-state CP/MAS (cross-polarization, magic angle spinning) 13

C-NMR, 1H-NMR and 11B-NMR. The operating conditions for these analyses are described

in the supplementary material. The test coupons were immersed in 200 mL of simulated cooling water with a BACC concentration of 200 mg L−1 at 45 °C, washed with distilled water, and then dried in air at 25 °C. Structural and topographical information was obtained using an ESCALAB 250Xi X-ray photoelectron spectroscope (Thermo Fisher Scientific, US). The monochromatized Al-Kα X-ray source was operated in constant analyzer energy mode. The internal calibration was referenced to the C1s energy at 284.8 eV for aliphatic-like species. 9



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The changes in morphology of the A3 steel without BACC and with different BACC concentrations were observed by SEM analysis using an S-3400N scanning electron microscope (Hitachi, Japan). SEM micrographs were obtained to observe the surface morphologies of different samples. Deposited scale crystals were collected from simulated cooling water in the presence of BACC (150 mg L−1) and the absence of BACC after reaction for 10 h at 80 °C. The scale deposits were characterized by SEM (S-3400N, Hitachi, Japan) and XRD (D/MAX 2500 V, Rigaku, Japan).

2.6. Electrochemical Characterization. A Zennium electrochemical workstation was used for electrochemical tests. The working electrode was a square cut from A3 steel coupons with a surface area of 1 cm2. A platinum sheet and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. The working electrode and reference electrode were connected via a salt bridge. To minimize the ohmic voltage error, the salt bridge was as close as possible to the electrode surface. The working electrode was polished mechanically, washed with acetone and distilled water, and dried before measurement. Potentiodynamic polarization measurements were conducted in scans from −1.0 to 0 V relative to the open circuit potential (OCP) at a scan rate of 1.0 mV s−1. The results were plotted as E-logi curves, and Tafel extrapolation was used to determine the corrosion voltage (Ecorr) and corrosion current (icorr) characteristics.25 IE was calculated using the following equation:

IE =

0 icorr − icorr ×100% 0 icorr

(4)

where i0corr and icorr represent the corrosion current densities in the absence and presence of BACC (µA cm–2), respectively. 10


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Electrochemical impedance spectroscopy (EIS) was performed in potentiostatic mode. The frequency was scanned from 100 kHz to 10 mHz using a 5.0 mV (rms) sinusoidal potential perturbation. The charge transfer resistance Rct was obtained from the diameter of the semicircle in the Nyquist representation, which was also used to calculate IE via Eq. (5):26

IE =

Rct − Rct0 ×100% Rct

(5)

0

where Rct and Rct are the charge transfer resistance values in the absence and presence of BACC (Ω cm2), respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized BACC. 3.1.1. FTIR Analysis. FTIR was applied to characterize the cellulose, cellulose citrate, BACC, and BACC adsorbed on A3 steel, and the spectra are presented in Figure 2. The absorption band at 3373 cm−1 is attributed to the O−H bond stretching vibration. The weak peaks at 2924 and 2856 cm−1 are attributed to the aliphatic C−H asymmetric and symmetric stretching vibrations, respectively. The band at 1732 cm−1 relates to the C=O stretching vibration, and that at 1622 cm−1 corresponds to the N−C=O of amide bonds. The stretching modes of C−N and C−O−B are observed at 1398 cm−1. The peak at 1318 cm−1 corresponds to the stretching mode of the B−O bond of borate ester. The peak for the C−O−C bond of cellulose ester is noticed at 1058 cm−1. The characteristic absorption peaks of the C−O bond of borate ester appear at 1064 cm−1. The absorbance at 707 cm−1 is assigned to the stretching mode of the B−O semipolar bond. The FTIR results show that the synthesized product includes −OH, N−C=O, and C−O−B, indicating the successful production of BACC, which contains ester, amide, and borate groups. 11



a b

3500

3000

2500

2000

-1

1068

1398 1318

1500

1058

1732 1632

4000

1400

d

1622

2888

707

2904

c

3435

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

1000

500

Wavenumbers (cm )

Figure 2. FTIR spectra of (a) cellulose, (b) cellulose citrate, (c) BACC, and (d) BACC adsorbed on A3 steel.

3.1.2. CP/MAS NMR Analysis. CP/MAS 13C, 1H, and 11B spectra of the unmodified cellulose and BACC samples are shown in Figure 3. For the spectrum of cellulose shown in Figure 3a-1, the characteristic peaks in the range of 55−110 ppm correspond to the signals from carbon atoms of the carbohydrate moiety. The signals at 105.8, 89.5, 84.8, 75.7, 73.4, 65.8, and 63.6 ppm are related to C-1 and C-4 of crystalline cellulose; C-4 of amorphous cellulose; C-2,3, C-5, and C-6 of crystalline cellulose; and C-6 of amorphous cellulose, respectively.27 As shown in Figure 3a-2, the spectrum of BACC obviously changed: the peaks at 89.5 and 65.9 ppm for crystalline cellulose disappeared, indicating that the crystal structure of cellulose was destroyed by mechanical force and the amorphous region increased, which helped to improve the accessibility and reactivity of the cellulose. The intensity of the signals at 75.7, 73.4, 65.8, and 63.6 ppm weakened, which indicates that the reaction predominantly occurred at those positions. New signals at 180.3, 58.9, 50.1, and 23.5 ppm were attributed to 12


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the presence of C=O and N−C=O carbon atoms in ester and amide groups, C−N carbon atoms in amide groups, C−O−B carbon atoms in borate, and −CH2 carbon atoms in diethanolamine, respectively, confirming that BACC was successfully synthesized.

C-2,3 cryst

(a) C-1 cryst

C-4 C-4 amph cryst

C-6 cryst C-6 amph 74.8

a-1

58.9 50.1

104.1

a-2 200

C-5 cryst

180.3

180

23.5

160

140

120

100

80

60

40

20

0

ppm

(b)

5.13

(c)

1.99

2.37

5.49

1.52

b-1 1.97

4 13.36 2 3

5.25 1

b-2 20

15

10

5

0

-5

-10

-15

-20 40

30

20

ppm

10

0

-10

-20

ppm

Figure 3. CP/MAS (a) 13C NMR and (b) 1H NMR spectra of (1) cellulose and (2) BACC and (c) 11B NMR spectrum of BACC.

CP/MAS 1H NMR analysis of unmodified cellulose and BACC samples was also carried out. As presented in Figure 3b-1, signals appear at 5.13, 1.99 and 1.52 ppm, corresponding to −OH and −CH hydrogen atoms in the structural unit of cellulose. In the case of BACC, new signals appear at 5.25 and 1.97 ppm, which correspond to the presence of B−OH, C−OH,

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B−O−CH2 and −N−CH2 hydrogen atoms in BACC. Since these absorption signals overlap, the results must be further confirmed by CP/MAS 11B NMR analysis, and the spectrum of BACC is presented in Figure 3c. The spectrum can be decomposed into four components centered at 13.8, 8.2, 5.7, and 2.4 ppm. The first peak at 13.8 ppm with lower intensity is mainly attributable to the bond of B−OH in boric acid ester.28 The second peak at 8.2 ppm may be assigned to the B−O−C=O bond in BACC. The third peak at 5.7 ppm may be ascribed to N→B (borate) species. The last peak at 2.4 ppm, exhibiting the largest contribution, may be ascribed to the B−O−C bond. Based on the analyses by CP/MAS 13C, 1H and 11B NMR, it can be confirmed that BACC was successfully produced by MASPR technology. The synthesis route of BACC is shown in Scheme 1.

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Scheme 1 Synthesis route of BACC.

3.2. Corrosion Inhibition Action of BACC. 3.2.1. Weight Loss and Potentiodynamic Polarization Measurements. The corrosion inhibition efficiency (IE) values of BACC were obtained from weight loss and potentiodynamic polarization measurements. The results of the gravimetric determination of A3 steel in the simulated cooling water medium in the absence and presence of BACC are summarized in Table S3, showing that the addition of BACC inhibited the corrosion of A3 steel in simulated cooling water at all the concentrations used in this study. It can be concluded that the corrosion rate of A3 steel gradually decreased with

15



increases in BACC concentration. IE increased upon increasing the concentration of BACC and achieved a maximum value of 88.33% at a very low concentration of 200 mg L–1. IE sharply increased with increasing BACC concentration when the concentration was less than 150 mg L–1 but slowly increased as the BACC concentration increased from 150 to 200 mg L– 1

. This result suggests that BACC is a good corrosion inhibitor for A3 steel in a simulated

cooling water medium. To better understand the corrosion inhibition mechanism of BACC in the simulated cooling water medium, a detailed study was carried out using Tafel polarization and AC impedance studies. Figure 4 shows the potentiokinetic polarization curves and electrochemical impedance spectroscopy (EIS) results for A3 steel, recorded in simulated cooling water in the absence and presence of BACC at concentrations varying from 50 to 200 mg L–1. Figure 4a presents the effect of BACC concentration on the cathodic and anodic polarization curves of A3 steel in simulated cooling water. From Table S3, the decrease in icorr with increasing BACC concentration indicates that BACC demonstrated corrosion inhibition with A3 steel. IE increased with increases in BACC concentration and achieved a maximum value of 87.4% at a very low concentration of 200 mg L–1. It can be observed that the anodic and cathodic reactions were affected by the addition of BACC. The corrosion potential (Ecorr) was positively shifted and current densities decreased with the increase in BACC concentration, which indicates that BACC was a mixed-type inhibitor. In Figure 4a, the anodic slopes of BACC decreased more sharply than the cathodic slopes, showing that the decrease in anodic reaction rates was more obvious than the corresponding cathodic ones. This difference in slopes indicates that BACC mainly acted as a mixed-type inhibitor in simulated cooling water

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and predominantly controlled the anodic reaction.

-3.5

1600

(b)

(a)

-4.0

1200 2

-Zim (Ohm· cm )

-2

Log current density (A·cm )

1400

-4.5 -5.0 -5.5 -6.0

Blank -1 50 mg·L -1 100 mg·L -1 150 mg·L

1000 800

Blank

600

-1

-1

0.095 Hz

0.01 Hz

200

-1

-0.9

-1

-1

50 mg· L 100 mg· L 150 mg· L 200 mg· L

400 93.67 kHz

200 mg·L

-1.0

-0.8

-0.7

-0.6

-0.5

-0.4

0 0

-0.3

0.025 Hz

200

400

600

800

1000

1200

1400

1600

2

ZRe (Ohm· cm )

Potential (V/SCE) 4.0 3.8 (c) 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

-90 Blank -1 50 mg· L -1 100 mg· L -1 150 mg· L -1 200 mg· L

-80 -70 -60 -50 -40 -30

Phase angle (Degree)

2

Log|Z| (Ω· cm )

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


-20 -10 0 0.5

1.0

1.5

2.0

2.5

3.0

Log freguency (Hz)

Figure 4. (a) Polarization curves, (b) Nyquist plots, and (c) Bode plots of A3 steel in simulated cooling water with different concentration of BACC; and (d) Equivalent electrical circuit diagram. (Rs: solution resistance; Rct: charge transfer resistance; and Cdl: double layer capacitance.)

Electrochemical impedance spectroscopy (EIS) is a good technique for investigating corrosion inhibition processes. Nyquist plots and Bode plots of A3 steel recorded in simulated cooling water at different BACC concentrations are given in Figure 4b and c. The Nyquist plots were analyzed by fitting the experimental data to a simple circuit model (Figure 4d). All

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the plots obtained in the absence and presence of inhibitor show only one depressed capacitive loop at the higher frequency range. The size of the semicircular impedance diagram increased upon increasing the BACC concentration, and consequently, the protection efficiency increased, which suggests that a BACC film was formed on the A3 steel surface. The characteristic absorption peaks of BACC are also observed in the FTIR spectrum of BACC adsorbed on A3 steel. Comparing Figure 2c and d, the peaks at 1732, 1632, 1400, and 1068 cm−1 suggest that BACC was adsorbed on the surface of the A3 steel. This observation indicates that BACC may be responsible for corrosion inhibition. The values of associated electrochemical parameters and IE are given in Tables S3 and S4. The values obtained by means of AC impedance tests were somewhat lower than those obtained in polarization experiments, but the trends were the same, indicating that BACC did not alter the electrochemical reactions responsible for corrosion. The quality of the fit to the equivalent circuit was judged by the χ2 value. The obtained χ2 values (0.00041−0.0027) in Table S4 indicate a good fitting to the proposed circuit. The results show that an increase in BACC concentration led to increased Rct and Rp values, but the Cdl values tended to decrease, which is attributed to the decrease in the local dielectric constant and/or the increase in the thickness of the electrical double layer. The decrease in Cdl was due to the adsorption of the inhibitor on the metal surface. The inhibition efficiency obtained from this technique was also in good agreement with those obtained from weight loss and potentiodynamic polarization measurements. To investigate the interaction between the ester, amide, and borate groups in BACC, it is significant to use the weight loss method to evaluate the IE values of different cellulose

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derivatives in the simulated cooling water system under certain reaction conditions. Here, the corrosion inhibition performance of different cellulose derivatives was determined by comparing the IE values, and the results are presented in Table S5. The IE of BACC was 88.33% at a BACC concentration of 200 mg L−1 in the simulated cooling water system. The

IE values for cellulose, cellulose citrate, aminated cellulose citrate, and borated cellulose citrate at the same concentration of 200 mg L−1 were 16.26%, 30.42%, 43.51%, and 31.22%, respectively. Therefore, comparative investigation indicates that the ester, amide, and borate groups in BACC have a synergistic effect, resulting in the good inhibition performance of BACC in the simulated cooling water system.

3.2.2. Adsorption Isotherm. It is generally accepted that the efficiency of a good corrosion inhibitor mainly depends on its adsorption behavior on the metal surface. Therefore, important information about the nature of metal-inhibitor interactions can be derived from the adsorption isotherm. For this purpose, the surface coverage values (θ = IE/100) for different inhibitor concentrations were calculated to present the best adsorption isotherm. Several attempts were made to fit various adsorption isotherms, including the Frumkin, Temkin, Langmuir, and Freundlich isotherms.29 The best fit was determined to be the Frumkin adsorption isotherm (Figure 5). This isotherm can be expressed as Eq. (6):30

C

θ

=

1 +C K ads

(6)

where Kads is the equilibrium constant of adsorption, θ is the surface coverage, and C is the concentration of the inhibitor. Figure 5 shows the plots of C/θ versus C; the correlation coefficient was 0.9866, which suggests that the experimental data conformed well to the Langmuir isotherm. A large adsorption equilibrium constant represents a higher adsorption 19



tendency of the BACC inhibitor on the A3 steel surface. The adsorptive equilibrium constant (K) is related to the standard adsorption free energy (∆G0ads), obtained according to Eq. (7):31

K ads =

0 1 −∆Gads exp( ) RT 55.5

(7)

where R represents the gas constant, T specifies absolute temperature, and 55.5 is the molar concentration of water in solution. Negative values of ∆G0ads usually indicate a spontaneous adsorption process. Generally, ∆G0ads ≤ − 20 kJ mol−1 is linked to physisorption, while ∆G0ads ≥ − 40 kJ mol−1 points to chemisorption.32,33 The calculated ∆G0ads value for the investigated BACC inhibitor was − 40.11 kJ mol−1, indicating that the adsorption mechanism of BACC on the surface of A3 steel in simulated cooling water was chemisorption.

240 220

y=1.032x+22.895 ∧ R 2=0.9866

200 180 -1

C/θ (mg L )

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

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160 140 120 100 80 60 60

80

100

120

140

160

180

200

-1

C (mg L )

Figure 5. Langmuir isotherm model of BACC on A3 steel.

3.2.3. Surface Morphology of the A3 Steel Surface. SEM images of the initial A3 steel and A3 steel immersed in simulated cooling water in the absence and presence of different

20


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concentrations of BACC are shown in Figure 6. It can be seen from Figure 6a that the initial A3 steel sample before immersion seems smooth and shows some abrading scratches on the surface. In Figure 6b, the A3 steel coupon immersed in simulated cooling water without BACC shows a severely porous structure on the surface. The corrosion products appear to have a rather uneven and lepidocrocite-like morphology, and the surface layer is very rough. In the presence of 50 mg L−1 BACC (Figure 6c), a rare corrosive phenomenon is observed in some small black holes on the steel surface. The corrosion on the steel surface was significantly reduced with increased BACC concentration (Figure 6d−f). In the presence of 200 mg L−1 BACC (Figure 6f), crevices and pits were rarely observed on the steel surface due to the formation of a protective layer by adsorbing BACC on the A3 steel surface.

a

b

c

d

e

f

Figure 6. SEM micrographs of the surface of A3 steel electrodes after 72 h immersion in simulated cooling water: (a) initial A3 steel, (b) without BACC, (c) 50 mg L−1 BACC, (d) 100 mg L−1 BACC, (e) 150 mg L−1 BACC, and (f) 200 mg L−1 BACC.

21



3.2.4. XPS Analysis. Further information regarding the A3 steel surface in the presence of BACC was obtained using XPS and compared with the results for pure BACC, and the spectra are presented in Figures 7 and 8. As shown in Figure 7a, the peak in the O1s spectrum of pure BACC is broad and deconvoluted into two peaks centered at 533.3 and 532.1 eV. The first peak is mainly attributed to the C−O−C bond in esters. The second peak is attributed to the C−O bond in the carbon chain structure of cellulose.34 The N1s spectrum of pure BACC is a single major N1s core-level component at approximately 400.0 eV, which is characteristic of amide (Figure 7b). The spectrum can be decomposed into two components centered at 398.5 and 400.2 eV. The first component is mainly attributable to C−N bonds. The second peak, which has the largest contribution, may be assigned to the N−C=O bond in amides.35 The C1s spectrum can be curve-fitted into three peaks with binding energies of approximately 284.8, 286.3, and 288.6 eV. The peak at 284.8 eV is attributed to carbon atoms in the forms of C−C and C−H bonds in BACC molecules, and the peak at 286.3 eV is attributed to carbon atoms in C−O bonds.36 The third peak at 288.6 eV may be ascribed to C=O (ester) and C−O−B (borate) species. The B1s spectrum of pure BACC contains a single peak at 190.6 eV, mainly attributed to the B−O bond. XPS analysis also clearly shows that BACC was successfully prepared by MASPR technology.

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13000

(a)

O1s

12000

N1s

(b)

11000

20000

Counts (a. u.)

Counts (a. u.)

25000

10000

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10000 C=O

9000

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C-N

8000 7000

5000 540

538

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532

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526

406

404

Binding energy (ev) 30000

C1s

(c)

402

400

398

396

394

392


B1s

(d)

2000 25000

1800

20000

Counts (a. u.)

Counts (a. u.)

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


15000 C-C C-H

10000

0 292

B-O

1200 1000

600

C=O C-O-B

290

1400

800

C-O

5000

1600

400

288

286

284

282

280

194

Binding energy (ev)

192

190

188

186

184

Binding energy (ev)

Figure 7. XPS spectra of the (a) O1s, (b) C1s, (c) N1s, and (d) B1s regions of pure BACC.

The Fe2p spectrum (Figure 8a) for the A3 steel surface after immersion depicts a double peak profile located at binding energies of approximately 710.5 (Fe2p3/2) and 725 eV (Fe2p1/2), with an associated feature structure on the high-energy side showing the subsequent oxidation of the surface of A3 steel.35 The Fe2p3/2 XPS spectrum exhibits two peaks. The peaks at lower binding energies of 710.5 and 712.3 eV were assigned to Fe3+ in ferric oxide/hydroxide species such as Fe2O3, Fe3O4, and/or FeOOH. The formation of an oxide layer could improve the corrosion of A3 steel in simulated cooling water by reducing ion diffusion. As shown in Figure 8b, a signal in the O1s spectrum is observed on the steel surface in the presence of the BACC inhibitor. The peak at 531.4 eV is attributed to oxygen atoms bonded to Fe3+ in Fe2O3 and/or FeOOH. The N1s spectrum of the A3 steel surface after immersion in simulated cooling water containing BACC can be fitted into two main peaks at

23



401.8 and 400.4 eV (Figure 8c). The first component is mainly attributed to the coordinate nitrogen in BACC on the A3 steel surface. The second component at 401.8 eV is attributable to the C−N=O bond, in agreement with the spectrum of pure BACC. Indeed, the deconvoluted C1s spectrum of the A3 steel surface shows four main components (Figure 8d). The first peak at 287.5 eV, which has the weakest intensity, is mainly attributed to C=O (ester) and C−O−B (borate) bonds related to the different constituents of BACC. The second component at 286.4 eV may be associated with the C−O bond in BACC. The third peak, located at approximately 284.8 eV, has the largest contribution and is attributed to C−N and C−C aliphatic bonds. The last component, at a lower binding energy (located at approx. 283.4 eV), may be ascribed to the carbon atoms of C−H and N−C−H in the structural unit of BACC.37 The single peak in the B1s spectrum at 192.2 eV may be attributed to the B−O bond in BACC molecules.

24


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Page 25 of 40

Fe2p3/2

(a)

11600

50000 Fe2p1/2

Counts (a. u.)

Counts (a. u.)

11400 11200 11000 10800

O1s

(b)

40000 30000 C-O

20000 10000

10600

735

730

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0 540

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Binding energy (ev)

532

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9000 N1s

(c)

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(d) 25000

Counts (a. u.)

8500

Counts (a. u.)

8000 N-C=O

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C-N

20000 15000

406

C-C C-H

10000 5000

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C=O C-O-B

N-C-H C-O

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0 290

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Binding energy (ev)

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Binding energy (ev)

700 650

(e)

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Counts (a. u.)

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


550

B1s

500 450 400

B-O

350 300 194

192

190

188

Binding energy (ev)

Figure 8. XPS spectra of the (a) Fe2p, (b) O1s, (c) C1s, (d) N1s, and (e) B1s regions of A3 steel surface after immersion in simulated cooling water with BACC at a concentration of 200 mg L−1.

The XPS results provide direct evidence of the chemical interactions between BACC and the A3 steel surface. A chemisorption mechanism was involved: electrons were shared between nitrogen, oxygen and the vacant d-orbital of iron. The BACC adsorbed onto the A3 steel surface to form a protective film, which kept the steel from corroding. This was confirmed by the results of FTIR analysis.

25



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Based on EIS, FTIR, SEM, and XPS analyses, the corrosion inhibition mechanism of BACC in the simulated cooling water system was mainly due to the adsorption of inhibitor at the metal/solution interface. The corrosion of A3 steel in the simulated cooling water medium involves the anodic dissolution of iron, accompanied by the cathodic oxygen reduction reaction.

Fe → Fe2+ + 2e−

(8)

O2 + 2H2O+ 4e− →4OH−

(9)

The corrosion inhibition mechanism of BACC in the simulated cooling water system is shown in Scheme 2. BACC possesses multiple reaction centers, such as nitrogen and oxygen, which enable interactions between the unshared electron pairs of the heteroatoms and the vacant d-orbitals of iron atoms on the surface of A3 steel and form chelates. An increase in alkyl groups in BACC molecules could enhance the investing electronic effect of BACC molecules and increase the electron cloud density of nitrogen atoms in BACC molecules, causing the coordination bonds to become more stable. The formation of an adsorption layer isolated the metal from dissolved oxygen and water, covering the active centers of the metal and effectively inhibiting corrosion on the metal surface.

26


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Scheme 2 Corrosion inhibition mechanism scheme of BACC in the simulated cooling water system.

3.3. Antiscaling Behavior of BACC. 3.3.1. Scale Inhibition Performance of BACC. The effects of BACC concentration and temperature on scale inhibition efficiency were investigated using the simulated cooling water concentrated to two fold of its original concentration, and the results are shown in Figure 9. With the increase in BACC concentration from 25 to 200 mg L−1, the inhibition efficiency of calcium carbonate scale increased. The rate of scale inhibition increased slowly as the BACC concentration increased above 150 mg L−1. When the concentration factor n for the simulated cooling water was two, inhibition efficiencies of 91.57% and 89.14% were obtained at 70 and 80 °C, respectively, with a BACC concentration of 150 mg L−1. It is possible that BACC might decompose at high temperature and cause a reduction in scale inhibition efficiency. The results suggest that 27



BACC had a good performance for scale inhibition, and SEM and XRD analyses provided further confirmation.

100 o

70 C o 80 C

90

Rate of scale inhibition (%)

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

Page 28 of 40

80 70 60

a

b

50 40 30

25

50

75

100

125

150

175

200

－1

Concentration of BACC (mg L )

Figure 9. Effect of BACC concentration and temperature on scale inhibition efficiency and pictures of the scale deposits in simulated cooling water (a) without BACC and (b) with a BACC concentration of 150 mg L−1.

3.3.2. XRD Analysis of Scale Deposits. XRD patterns of the scale deposits are shown in Figure 10a. The diffraction peaks at 23.14 (012), 29.52 (104), 36.06 (110), 39.52 (113), 43.24 (202), 47.58 (018), and 48.58 (118) are characteristic peaks of calcite.38 The results show that the scale deposits formed in the simulated cooling water without and with BACC were mainly calcite. The intensity of the characteristic peaks of calcite is weaker in the presence of the BACC inhibitor (spectrum a-2), which implied a change in surface morphology and particle size. This result indicates that the addition of the BACC inhibitor led to a reduction in the crystal faces of CaCO3 and a decrease in the degree of order from that of complete 28


Page 29 of 40

crystallinity. Comparing Figure 10a-1 and a-2, the changes in peaks illustrate that the growth and distortion of CaCO3 crystals were retarded in the presence of BACC. These changes led to the loosening of CaCO3 crystals. The soft deposits could be easily removed and had poor adhesion to the beaker in the presence of BACC. These phenomena and the analysis results indicate that BACC acted as a scaling inhibitor in simulated cooling water.

Intensity

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


a-1

a-2 20

30

40

50

60

70

2θ (°)

b-1

b-2

Figure 10. (a) XRD patterns and (b) SEM micrographs of the scale deposits formed in the simulated cooling water (1) without BACC and (2) with 150 mg L−1 BACC.

3.3.3. SEM Analysis of Scale Deposits. Scale formation occurs through a series of steps that include the formation of Ca2+ and CO32− ion pairs, microaggregate formation, microcrystal formation, macrocrystal formation, and finally, scale/deposit formation.39 The 29



ability of the BACC inhibitor to inhibit CaCO3 scale was compared with that in the absence of any inhibitor in simulated cooling water in static tests. The CaCO3 scale deposits were observed by SEM. As shown in Figure 10b, most of the CaCO3 scale crystals formed in the simulated cooling water without BACC were regular diamond-shaped with a glossy surface, clear angular boundary and size of 5–10 µm. In contrast, the CaCO3 scale crystals formed in the simulated cooling water containing BACC (150 mg L−1) were irregular, loose, and fine (Figure 10b-2). There were many crystal defects, bumps and wrinkles on the scale surface, and the surface morphology was more complex. The presence of BACC minimized the deposition of large amounts of calcium carbonate, which may have occurred because BACC has a “crystal distortion effect” and “threshold effect”.40 In a way, BACC could be adsorbed on scale surfaces and distort scale crystals by disrupting their lattice structure and normal growth patterns. Based on XRD and SEM analyses, the mechanisms for scale inhibition by BACC (Scheme 3) are associated with active functional groups (C=O, −OH, N−C=O, and B−O). Electrostatic balance is necessary for propagation of the crystal growth. When this balance is interrupted, the rate of scale formation decreases. The hydroxyl groups in BACC can form stable complexes with metal ions, and different Ca(II)-BACC complexes can be formed in solution. Therefore, the active functional groups in BACC can obstruct the formation of scale deposits by threshold inhibition, crystal dispersion, and modification of the growth of microcrystals. During the scale formation process, the BACC inhibitor disordered the crystalline form of the precipitates with intensive surface stresses and inhibited crystal growth by altering the shape of the microcrystals.

30


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Scheme 3 Scale inhibition mechanism scheme of BACC in the simulated cooling water system.

4. CONCLUSIONS Different characterization methods used to analyze the chemical structure of BACC showed that BACC contains polar atoms, such as nitrogen and oxygen, which act as reaction centers for the adsorption process. Weight loss and electrochemical tests confirmed the inhibition effects of BACC, as well as its effect on A3 steel corrosion in a simulated cooling water system. The IE of A3 steel increased with increased BACC concentration and reached a value of 88.33% with 200 mg L−1 BACC in simulated cooling water. Polarization curves revealed that BACC mainly acted as a mixed-type inhibitor in simulated cooling water and predominantly controlled the anodic reaction. The calculated value for the thermodynamic parameter ∆G0ads indicated that the adsorption of BACC on the A3 steel surface obeyed the Langmuir adsorption isotherm and was typical of chemisorption. SEM and XPS analyses 31



further confirmed the formation of a protective film against corrosive attack composed of the adsorbed BACC molecules on the A3 steel surface. BACC also showed a promising scale inhibition effect in simulated cooling water, achieving an inhibition rate of 91.57% with a BACC concentration of 150 mg L−1 at 70 °C. SEM and XRD results implied that the particle size and surface morphology of CaCO3 scale deposits were changed by BACC. The scale inhibition mechanism of BACC could be that BACC obstructed the formation of CaCO3 scale deposits by threshold inhibition and modification of the growth of microcrystals. These results demonstrated that BACC could be considered as an effective inhibitor for both corrosion and scale inhibition.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Supplementary experimental details; chemical composition of the simulated cooling water and A3 steel; weight loss and polarization parameters for the corrosion of A3 steel at different BACC concentrations; impedance parameters for A3 steel in simulated cooling water containing different concentrations of BACC; comparison of the IE of BACC with those of other cellulose derivatives for A3 steel in the simulated cooling water system.

AUTHOR INFORMATION Corresponding authors * Tel./Fax: +86-771-3233718. E-mail: [email protected] (Y. Z.). 32


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* Tel./Fax: +86-771-3233718. E-mail: [email protected] (Z. H.).

ORCID Zuqiang Huang: 0000-0003-0727-8016

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (No. 51463003 and 21666005), Guangxi Natural Science Foundation of China (No. 2017GXNSFEA198001), Guangxi Distinguished Experts Special Foundation of China, and the Scientific Research Foundation of Guangxi University (Grant No. XJPZ160713).

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Abstract Graphic

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