Inhibition Effect of Environment-Friendly Inhibitors on the Corrosion of

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Inhibition Effect of Environment-Friendly Inhibitors on the Corrosion of Carbon Steel in Recirculating Cooling Water Chengjun He,†,§ Zhipeng Tian,†,§ Bingru Zhang,*,†,‡ Yu Lin,† Xi Chen,† Meijing Wang,† and Fengting Li† †

State Key Lab of Pollution Control & Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Rd, Shanghai 200092, China ‡ Key Laboratory of Yangtze Aquatic Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, 1239 Siping Rd, Shanghai 200092, China S Supporting Information *

ABSTRACT: We studied the corrosion inhibition properties of a combination of polyaspartic acid, polyepoxysuccinic acid, polyamino polyether methylenephosphonate, and sodium gluconate for carbon steel in recirculating cooling water. Characterization employed weight loss measurements, Tafel polarization, electrochemical impedance spectroscopy, X-ray fluorescence (EDX), and atomic force microscopy (AFM). The results show that the composite efficiently inhibited corrosion on carbon steel at relatively low dosages in severely corrosive soft water media. The EDX spectrum determined the nature of the adsorption layer on the steel surfaces. Scanning electron microscopy and AFM further confirmed the formation of a protective film on the carbon steel surface.

1. INTRODUCTION Soft and acidic waters normally appear in open recirculating cooling and water distribution systems. These systems are highly corrosive to metals1 and enable the release of iron corrosion byproducts into potable water.2−4 Carbon steel is a well-known material used in marine applications, chemical processing, petroleum production and refining, construction, and metal processing equipment, but is particularly susceptible to severe metal corrosion in recirculating cooling water.5 Metals used in industrial applications are protected from corrosion in different ways apart from surface treatment such as coating.10 The use of organic molecules as corrosion inhibitors is one of the most popular, efficient, and practical methods applied extensively to protect metals against corrosion damage,6,11,12 especially in aggressive acidic media.11,13 Nonpolymeric and polymeric agents are often used to inhibit the growth of mineral crystals and the corrosion of metals. However, the environmental effects of these chemicals have become an increasingly important global issue. The development of chemical corrosion inhibitors reflects efforts of continuous adaptation to changing environmental legislation and the success of coping with these challenges. Polyaspartic acid (PASP) and polyepoxysuccinic acid (PESA) are representative green scale inhibitors because of their nonphosphorus and biodegradable features.7−9 Hence, they are valuable in terms of environmental acceptability and waste disposal.12−18 Many relevant studies on PASP have focused on their synthesis, properties, and application as a mineral scale inhibitor in water treatment applications.19 PASP can also be used as a dispersing agent in detergents, paints, and papermaking.25,31 PESA has good chelating ability with various metal ions and exhibits a versatile antiscaling performance.29,31,33,34 It is even used in the extraction of Cd from sewage sludge.19−24 Polyamino polyether methylenephosphonate (PAPEMP), a newly discovered inhibitor, has been developed for large-scale © XXXX American Chemical Society

control in water desalination. PAPEMP has a high calcium tolerance in water and good performance at controlling both calcium carbonate and calcium sulfate scales at extremely high supersaturations. PAPEMP is also capable of suppressing calcium phosphonate precipitation.26,36−38 Numerous studies have focused on the mineral scale inhibition efficiency of PASP, PESA, and PAPEMP. However, studies on their corrosion inhibition performances are limited. Previous studies have indicated that the sodium and calcium salts of gluconic acid are effective corrosion inhibitors for mild steel immersed in nearneutral media and chloride solutions.39−42 Sodium gluconate (Glu) can be applied in simulated cooling water as a corrosion and scale inhibitor because of its low toxicity.40 Energy dispersive X-ray spectroscopy measurements (EDX) allowed us to study mechanistic aspects and evaluate relative inhibition efficiency.25−28 Scanning electron microscope (SEM) and atomic force microscopy (AFM) are powerful tools that provide topographical and compositional surface information on carbon steel.5,28−30 Here we study the effects of inhibitors on the metal interface and elucidate the formation of an adsorbed barrier layer.26−28,31−35 We investigated a novel, environmentally friendly inhibitor composite that can efficiently inhibit carbon steel corrosion at relatively low dosages in soft water media. The purpose was to overcome the inhibition challenges in a single component that is highly dependent on the inhibitor. The inhibition effects of a single material (including PASP, PESA, PAPEMP, and Glu) and their multicomponent blends on carbon steel corrosion in soft water media were studied by weight loss experiments. The underlying mechanisms of the inhibitors on carbon steel Received: October 3, 2014 Revised: December 30, 2014 Accepted: January 6, 2015

A

DOI: 10.1021/ie504616z Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Abbreviations, Names, and Structures of the Phosphonate Inhibitors in This Study

≤0.025 Cu with the balance Fe. These components were used for the weight loss and electrochemical experiments. The rectangular specimens used for the weight loss test were 5.0 × 2.5 × 0.2 cm3. For the electrochemical tests, the working electrode (WE) was embedded in an epoxy resin leaving a 1 cm2 geometrical surface area of carbon steel exposed to the electrolyte. Prior to all measurements, the carbon steel specimens were mechanically abraded with different types of emery papers (grade 320−400−600−800−1000−1200), degreased with acetone, and rinsed with distilled water before immersion in the experimental solution. Test solution was from Baosteel Group Corporation, Shanghai, China. The analysis of soft water is shown in Table 2. The conductivity of the test water was 719 μS·cm−1, and the tests were conducted at pH 7.1. The Langelier saturation index (LSI), the Ryznar stability index (RSI), and the Puckorius scale index (PSI) values for the test water were −1.27, 9.63, and 9.70, respectively. The LSI = pH − pHs, where pHs is the pH at which the water is saturated in calcium carbonate. RSI = 2pHs − pH and PSI = 2pHs − pHeq (pHeq = pH of water at equilibrium conditions).31,63 The above analysis suggests that attention must

corrosion were examined by electrochemical measurements. The optimum additive concentrations of the inhibitors involved in the compound formula were determined by the orthogonal test. The EDX spectrum and SEM and AFM images were utilized to further confirm the corrosion attack as well as inhibitor molecule adsorption and film-forming processes.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. All chemicals used in this study were analytical reagent grade and obtained from the Sinopharm Chemical Reagent Shanghai Co., Ltd., China. Double-distilled water was used throughout. The polymer inhibitors were commercial grade, and their structures are shown in Table 1. PESA (average Mw ≈ 3800) was obtained from Meijing Environmental Protection Material Co., Ltd., China. PASP (average Mw ≈ 8000) was purchased from Zibo Leadbond Chemical Products Co., Ltd. PAPEMP was obtained from Jianghai Chemical Group. All polymer inhibitor concentrations were calculated on a dry polymer basis. Corrosion tests were performed on carbon steel samples with the following composition (in wt %): 0.17−0.24 C, 0.17−0.37 Si, 0.35−0.65 Mn, ≤0.035 P, ≤0.035 S, ≤0.025 Ni, ≤0.025 Cr, and B


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Industrial & Engineering Chemistry Research Table 2. Analysis of Test Solution (Electrolyte)

η (%) =

parameter

value

pH conductivity (μs·cm−1) total hardness (mg·L−1) calcium (mg·L−1) TDS (mg·L−1) chloride (mg·L−1) LSI RSI PSI

6.2 719 150 105 367 82 −1.27 9.63 9.70

ν0 − ν × 100 ν0

(2)

where ν0 and ν are the corrosion rates in the absence and presence of inhibitors, respectively. 2.4. Electrochemical Measurements. All electrochemical measurements were carried out using an AUTOLAB PGSTAT30 electrochemistry workstation (Metrohm Corporation). The data were processed using Nova 1.10 Software (Metrohm Corporation). The electrochemical experiments were performed using a three-electrode cell assembly in a water bath at 318 K under deaerated conditions. A platinum foil was used as the counter electrode; a saturated calomel electrode (SCE) was the reference electrode, and carbon steel served as the working electrode (WE). After the WE was prepared, it was first immersed into the test soft water for 2 h to establish the steady-state corrosion potentials (Ecorr). Potentiodynamic polarization curves were obtained by changing the potential automatically versus Ecorr at a scan rate of 1.0 mV/s from a potential of −250 mV to +250 mV versus open circuit potential (OCP) after the working electrode reaches a stable state. The EIS measurements were performed at Ecorr over the frequency range of 100 kHz−0.1 Hz with a perturbation amplitude of 5 mV. The electrochemical experiments were carried out with test water (Table 2) containing various concentrations of inhibitors as an electrolyte under static and naturally aerated conditions. After potentiodynamic polarization measurements, the inhibition efficiency (η) of the inhibitor of the carbon steel corrosion was calculated with the following equation:46,47

be given to corrosion prevention. These values highlight the tendency of the test water to cause severe corrosion. 2.2. Orthogonal Test and Taguchi Method. The additive concentrations of the inhibitors involved in the compound formula were examined by the orthogonal test. This study used an L9 (34) orthogonal array prepared via the Taguchi method, which has nine experiments at four parameters with three levels. The most important stage in the design of an experiment lies in the selection of the control factors. As many factors as possible should be included so that one can identify nonsignificant variables at the earliest opportunity.50 The Taguchi approach creates a standard orthogonal array to accommodate this requirement. Depending on the number of factors, interactions, and levels needed, the choice is left to the user to select either the standard or column-merging method or the idle-column method.62 The Taguchi method is a systematic approach to the design and analysis of experiments. The steps to the Taguchi method are (1) identification of the quality characteristics and selection of the design parameters; (2) determination of the number of factor levels; (3) selection of the appropriate orthogonal array; (4) executing the experiments based on the arrangement of the orthogonal array; (5) evaluating the results using the signal-tonoise (S/N) ratios; (6) analysis of variance (ANOVA); (7) selection of the optimum levels of factors; and (8) verifying the optimum process parameters through the confirmation test. 2.3. Corrosion Weight Loss Measurements. For the weight loss experiments, static beaker corrosion tests were conducted in 2 L beakers under deaerated conditions. The beakers were immersed in a water bath at 318 K. Carbon steel sheets were prepared as described above. After being dried and accurately weighed, the specimens were immersed in beakers with the test soft water in the absence and presence of inhibitors for 72 h. The rotary speed for the tested metal samples was 85 rpm (0.45 m·s−1). After the tests, the carbon steel specimens were removed, thoroughly rinsed with distilled water, sequentially washed with acid and alkali solution, dried, and accurately weighed. Each set of experiments was repeated three times to ensure reproducibility. The analytical balance used here has a precision of 0.1 mg (Shanghai Precision & Scientific Instrument Co., Ltd., China). The corrosion rate (v) was calculated by the following equation: W v= (1) St

η (%) =

0 Icorr − Icorr 0 Icorr

× 100 (3)

I0corr

where Icorr and are the corrosion current values of carbon steel immersed in a test solution with and without inhibitors, respectively. The impedance of the CPE is described in56 ZCPE = Y 0−1(jω)−n

Here, Y0 is the magnitude of the constant-phase element (CPE) and ω is the angular frequency; j2 = −1 is an imaginary number; and n is the CPE exponent. For n = 1, CPE is a pure capacitor, and for n = −1, CPE is an inductor. The double-layer capacitance (Cdl) was simulated using the constant-phase element. The values of Cdl were obtained at a frequency at which the imaginary component of the impedance is at a maximum:57,58 Cdl = Y0(2πfmax )n − 1

Here, f max is the frequency at which the imaginary part of the impedance spectrum is maximal. The impedance parameters obtained from the EIS were also used to calculate the inhibition efficiency via literature and eq 4:2,47 η (%) =

R ct − R cto × 100 R ct

(4)

R0ct

Here, Rct and are charge-transfer resistance values in the presence and absence of inhibitors, respectively. All potentials reported in the paper were referred to the SCE. 2.5. In Situ AFM Measurements. The carbon steel samples were first immersed in the test media with and without the compound inhibitors during the static beaker corrosion tests

where W is the average weight loss of three parallel sheets, S the total area of one specimen, and t the immersion time. Subsequently, the inhibition efficiency (η) of the inhibitor was calculated by C


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Industrial & Engineering Chemistry Research (section 2.3). The specimens were then rinsed with distilled water and dried under nitrogen. To observe any change on the sample surface, the contrastive samples were finally cleaned and dried after pickling and alkali neutralization. The morphologies of the carbon steels in the presence and absence of the composite inhibitors were investigated by AFM using a Dimension 3100 model AFM (Veeco) instrument equipped with a Nanoscope IIId controller (Digital Instruments) under ambient conditions. All AFM images were collected with tapping mode in air at room temperature with a scan rate of 0.5 Hz (512 pixels × 512 pixels for every image). 2.6. Energy Dispersive X-ray Spectroscopy Measurements. We used EDX to investigate the elements present on the steel surface with and without inhibitor. The carbon steel samples were exposed to soft water for 72 h in the absence or presence of various inhibitor concentrations. The samples were washed thoroughly by distilled water and dried in cold air to remove loosely adsorbed ions. The composition and morphology of the corrosion products formed on the steel surface with and without the addition of inhibitors were tested with a Traktor TN-2000 energy dispersive spectrometer.

experiments at each level for each factor as indicated by the I, II, and III values in Table S2 of Supporting Information.43,44 On the basis of the mean values, range values (R) were calculated to determine the influence of each factor. With increased quantities of all inhibitors, the corrosion inhibition performances improved to different degrees within the selected range. A significant synergistic effect of the combination was observed. The optimum quantities of the inhibitors were 12, 12, 4, and 2 mg·L−1 for PASP, PESA, PAPEMP, and Glu, respectively (30 mg·L−1 accumulated in total). The extremely low phosphorus content (about 0.79−0.87 mg·L−1) highlights the environmentally friendly nature of this scheme. Inhibition mechanism studies were then implemented by electrochemical measurements and AFM. Figure 2 shows the inhibitive efficiencies of different concentrations of inhibitors and the composite inhibitor on

3. RESULTS AND DISCUSSION 3.1. Weight Loss Tests. To determine the corrosion inhibition efficiency of each single inhibitor, as well as possible synergistic effects from inhibitor blends, the additive concentrations of the inhibitors involved in the compound formula were examined with the orthogonal test. PESA, PASP, Glu, and PAPEMP were selected as the main components for the dose study. The test project contained four factors, and each factor consisted of three levels (Table S1 of Supporting Information). This study used an L9 (34) orthogonal array settled via the Taguchi method, which has nine experiments for four parameters with three levels. There are eight degrees of freedom because of the four factors. The experimental plan is shown in Table S2 of Supporting Information. The corrosion inhibition efficiency (η) calculated by eq 2 in the weight loss measurements was the primary end point indictor. The response of each factor to its individual level was calculated by averaging the inhibition efficiencies (Figure 1) in all

Figure 2. Corrosion efficiencies of the single inhibitors and the composite by weight loss measurements in test solution at 313 K after 72 h.

carbon steel in the weight loss experiments. The corrosion inhibition efficiency calculated by eq 2 in the weight loss measurement was the primary evaluation metric. We concluded from Table 3 that when PASP, PESA, PAPEMP, or Glu were used alone, the corrosion efficiencies are relatively low unless they have a high dose. This is neither economical nor practical. On the other hand, using them in tandem results in inhibition efficiency as high as 96.2% with low phosphorus levels. This illustrates the remarkable synergistic effect on carbon steel corrosion inhibition. We found that the best formula is 12, 12, 4, and 2 mg·L−1 for PASP, PESA, PAPEMP, and Glu, respectively (30 mg·L−1 accumulated in total). The inhibition mechanism studies were then implemented by electrochemical measurements and AFM. 3.2. Potentiodynamic Polarization. The Tafel polarization curves for carbon steel in the tested soft water media in the absence and presence of various concentrations of inhibitors are shown in panel a of Figures 3−7. Table S3 of Supporting Information shows the electrolyte polarization parameters with and without different inhibitors, which were obtained by using AUTOLAB PGSTAT30 electrochemistry workstation (Metrohm Corporation) and its Nova 1.10 Software (Metrohm Corporation). To eliminate any doubt about the IR drop in the electrolyte due to its dilution, the IR drop compensation was done with

Figure 1. Corrosion efficiencies of the weight loss data by the Taguchi method in test solutions of various concentrations of the composite with different proportions of inhibitors at 318 K for 72 h. D


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Table 3. Corrosion Efficiencies of Single Inhibitors and the Composite by Weight Loss Measurements in Test Solution at 318 K after 72 h inhibitor PASP

PAPEMP

Glu

concentration (mg·L−1)

corrosion rate (mm·a−1)

η (%)

50 100 200 10 20 50 50 100 200

1.02 0.79 0.32 0.56 0.19 0.34 0.83 0.58 0.50

35.3 49.5 79.9 64.7 88.0 78.6 47.6 63.5 68.3

concentration (mg·L−1)

corrosion rate (mm·a−1)

η (%)

composite

50 100 200 30

1.09 0.89 0.52 0.07

30.9 43.5 67.1 96.2

blank

−

1.57

−

inhibitor PESA

Figure 3. Tafel polarization curves (a) and Nyquist plots (b) for carbon steel in the test water at 318 K containing different concentrations of PASP.

Figure 4. Tafel polarization curves (a) and Nyquist plots (b) for carbon steel in the test water at 318 K containing different concentrations of PESA.

Figure 5. Tafel polarization curves (a) and Nyquist plots (b) for carbon steel in the test water at 318 K containing different concentrations of PAPEMP.

by the flow of current through the cell solution resistance is an inescapable consequence of voltammetric measurements. The current interruption method corrects for IR drop by periodically interrupting the current through the electrochemical cell. The

current interruption methods.49 However, because the corrosion inhibitor results were comparative, the difference between polarization curves with and without IR drop compensation was not significant. The Ohmic potential drop (IR drop) caused E


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Figure 6. Tafel polarization curves (a) and Nyquist plots (b) for carbon steel in the test water at 318 K containing different concentrations of Glu.

Figure 7. Tafel polarization curves (a) and Nyquist plots (b) for carbon steel in the test water at 318 K containing different concentrations of composite inhibitors. (Compostie inhibitors: PESA, 12 mg·L−1; PASP, 12 mg·L−1; PAPEMP, 4 mg·L−1; and Glu, 2 mg·L−1.)

composite inhibitors at 318 K. It is observed that the combinations of PESA, PASP, PAPEMP, and Glu produce pronounced effects on the corrosion current density compared to those displayed by an individual inhibitor. According to data of Table S4 of Supporting Information, the corrosion current density decreases substantially, leading to higher inhibition efficiency of the mixture, up to 94.8%. The data shown in Figure 7 and Table S3 of Supporting Information demonstrate that the composite acted as a mixed-type corrosion inhibitor that could retard both metal dissolution and the cathodic process. This indicates the synergistic effects of the inhibitor combination. These results also support the potential application of this blend as an environmentally friendly and economical inhibitor. The inhibition efficiencies obtained from the polarization measurements displayed the same trend as those calculated from the weight loss tests. 3.3. Electrochemical Measurements. We used EIS to measure the corrosion inhibition on carbon steel in the absence and presence of inhibitors at various concentrations (panel b in Figures 3−7). As shown in Figure 3b, all the Nyquist plots for carbon steel with PASP as the inhibitor were a depressed semicircle with a high-frequency capacitive loop. In the Nyquist plots, there was a deviation from the perfect semicircle, i.e., a depressed semicircle in the center under the real axis. This is attributed to the frequency dispersion.6,51,52 This finding is a typical impedance feature of solid metal electrodes in the corrosion process.53 The depression is mostly caused by the roughness and inhomogeneity on the solid electrode surface as well as the geometrical behavior of the current distribution.54 The corrosion of carbon steel characterized by the highfrequency capacitive loop can be attributed to a charge-transfer process.55 The small tail at the low-frequency region of the

working electrode potential is measured during the current interruption; hence, corrections can be made for zero iR drop conditions.64 The inhibition efficiencies of the inhibitors were calculated using eq 3. Figure 3a and Table S3 of Supporting Information show that the polarization curves had a significant shift in the corrosion potential, Ecorr, to more positive values versus the control solution. This indicates that PASP behaved as an anodic inhibitor.44−46 The more pronounced effect of PASP on the anodic slopes than on the cathodic Tafel slopes confirmed that the anodic process of carbon steel corrosion in the tested soft water was effectively retarded by the addition of PASP. The electron acceptor reaction did not obviously change.2,7 Table S3 of Supporting Information also illustrates that the corrosion current density, Icorr, decreased appreciably. These findings suggest retardation of carbon steel corrosion in the inhibited solution.7,9 The corrosion rate also decreased with an inhibition efficiency of 91.7% at 200 mg·L−1. These results revealed that the inhibitor concentration significantly influenced the corrosion. Improved corrosion resistance was achieved in the presence of PASP.47 This corrosion inhibition for carbon steel in test solution can be attributed to the formation of a barrier layer by the adsorbed inhibitor molecules on the carbon steel surface.2,7,9 Figure 4 also demonstrates that the inhibitors PESA, PAPEMP, and Glu had similar effects on the corrosion inhibition process of carbon steel and acted predominantly as anodic inhibitors. Ample inhibition efficiency is a balance between inhibitor dose and environmental limits. The addition of composite inhibitors caused positive shift in Ecorr (vs SCE) in comparison with that obtained without the addition of inhibitors. Figure 7 shows the potentiodynamic polarization curves for carbon steel in test solution containing F


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double layer.54 These changes were attributed to the gradual replacement of water molecules and other ions by the adsorption of inhibitor molecules at the steel/solution interface.8,58 In other words, the decreased Y0 suggested a decreased surface oxide layer thickness. The values of n also increased relative to those in the uninhibited solution. This can be attributed to the adsorption of the inhibitors on the most active sites and improved surface heterogeneity.11,61Panel b of Figures 3−7 and Table S4 of Supporting Information prove that other inhibitors displayed similar performances, which can also be analyzed by the above theories. The blend of inhibitors indeed exhibited a synergistic effect in terms of the corrosion resistance. The corrosion inhibition efficiency reached a maximum of 93.3% at a relatively low total dose (30 mg·L−1) of inhibitors (PESA, 12 mg·L−1; PASP, 12 mg·L−1; PAPEMP, 4 mg·L−1; and Glu, 2 mg·L−1). The results in Table S4 of Supporting Information indicate that the inhibition efficiencies calculated from the impedance measurements had the same trend as those via the weight loss method. The inhibition properties of the inhibitors obtained from the impedance data also corresponded to those observed in the polarization measurements. 3.4. Surface Morphological Observation and Analysis. We present photographs of carbon steel immersed in water media for 72 h in the absence and presence of inhibitors in Figures 9−12 to highlight the corrosion. Figure 9 shows that sample A (the specimen before immersion) had a surface sufficiently smooth to reflect light. In contrast, sample B (the carbon steel after immersion without inhibitor, Figure 10) displayed a severely porous structure as a result of corrosion. In the presence of 30 mg·L−1 (PESA, 12 mg·L−1; PASP, 12 mg·L−1; PAPEMP, 4 mg·L−1; and Glu, 2 mg·L−1) mixed inhibitors (sample C, Figure 11), little corrosion is noted. There is an inhomogeneous material coverage on the steel surface. The sample surface became glossier with increased inhibitor concentration. The adsorbed inhibitor layer can be clearly seen on the specimen surface as a thin yellow layer (sample D: the carbon steel after immersion in the presence of composite inhibitors at 300 mg·L−1). Figures 10 and 11 show images of the carbon steel in the absence and presence of inhibitors, respectively, after the corrosion test with and without the posttreatment detailed in section 2.2. Figure 10 shows that the rust materials covered the porous surface because of the strong corrosive effects. Nevertheless, no corrosive damage was seen on the specimen surface and no yellow film was observed after washing. This finding further confirmed that the formation of the

Nyquist plot from soft water samples without inhibitor is caused by metal dissolution.9,55 The impedance response of the electrode significantly changed after the addition of inhibitors to the testing solution. Versus the control sample, the diameters of the semicircles in the Nyquist plots increased with inhibitor concentration, indicating corrosion inhibition and the adsorption of inhibitor molecules on the metal surface.56 The electrochemical impedance parameters were determined by a semicircle fitting method.54 The data from various impedance profiles were fitted into an equivalent circuit (Figure 8) to model the impedance spectra on the carbon steel/solution

Figure 8. Equivalent circuit proposed for fitting the impedance spectrum obtained on carbon steel surfaces in the tested water solution with and without inhibitors.

interface in the absence and presence of inhibitors using Nova 1.10 Software (Metrohm Corporation). In this equivalent circuit, Rs is the solution resistance and Rct is the charge-transfer resistance. The double layer capacitance (Cdl) was simulated using the constant-phase element (CPE), in which the phase shift (n) represents the degree of surface inhomogeneity.57−59 In the model, Rct is the resistance between the electrode surface and outer Helmholtz plane.16 A CPE was applied to model the depression phenomenon that was mostly related to the frequency dispersion, surface heterogeneity, and formation of adsorbed layers.60 The fitted parameters and corresponding electrochemical results are presented in Table S4 of Supporting Information. The corrosion inhibition efficiency was calculated by eq 4 from the impedance spectra measurements. Table S4 of Supporting Information clearly shows that the value of Rct and the proportional factor Y0 of CPE changed in a regular manner with increased inhibitor concentration. The Rct values increased while the values of Y0 decrease with increasing inhibitor concentration. The decreased Y0 correlates with the decreased dielectric constant and/or increased thickness of the

Figure 9. AFM images of carbon steel sample A (the specimen before immersion) in Figure 10. Panel (a) is the result of 3D model by AFM. Panel (b) is the height profile of the steel surface by the Nanoscope v710 software. G


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Figure 10. AFM images of carbon steel sample B (the carbon steel after immersion for 72 h at 318 K without inhibitor). Panel (a) is the result of 3D model by AFM. Panel (b) is the height profile of the steel surface by the Nanoscope v710 software.

Figure 11. AFM images of carbon steel sample C (the carbon steel after immersion for 72 h at 318 K with the composite inhibitors at 30 mg·L−1). Panel (a) is the result of 3D model by AFM. Panel (b) is the height profile of the steel surface by the Nanoscope v710 software. (Composite inhibitors: PESA, 12 mg·L−1; PASP, 12 mg·L−1; PAPEMP, 4 mg·L−1; and Glu, 2 mg·L−1).

Figure 12. AFM images of carbon steel sample D (the carbon steel after immersion for 72 h at 318 K with the composite inhibitors at 300 mg·L−1). Panel (a) is the result of 3D model by AFM. Panel (b) is the height profile of the steel surface by the Nanoscope v710 software. (Composite inhibitors: PESA, 120 mg·L−1; PASP, 120 mg·L−1; PAPEMP, 40 mg·L−1; and Glu, 20 mg·L−1.)

adsorbed inhibitor layer provided protection against corrosion to give excellent corrosion inhibition. For further evaluation of the corrosion inhibition effect of the mixed inhibitors, the surface morphologies of the specimens in Figure 11 were also evaluated by AFM. The results are presented as three-dimensional AFM images (Figures 9−12). The images in Figure 9 reveal that the steel surface was freshly abraded before immersion. Other parts were flat and relatively uniform, except for pronounced and regular scratch marks on the metal surfaces due to mechanical grinding and abrading. The average roughness of the steel surface was 19.7 nm.

These results can be verified by the height profiles in Figure 9, which shows that the curve of specimen A was relatively flat and had a slight fluctuation within 60 nm. The greatest surface heterogeneity was observed on the surface of the uninhibited carbon steel exposed to the soft water solution (Figure 10). The steel surface cracked considerably and acquired large, deep pits. The roughness also increased to 143 nm. The longitudinal view presents large valleys and peaks more than 400 nm in height. Figure 11 shows little obvious corrosion damage on the surface at a 30 mg·L−1 combination of inhibitors, but many small spikes were distributed on the surface. The roughness was reduced to H


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Industrial & Engineering Chemistry Research 75.2 nm, which can be attributed to the inhibitor molecules being adsorbed on the metal surface in sharp contrast to the uninhibited sample. With increased inhibitor concentration, a thicker adsorbed layer developed. Most apophyses disappeared and were replaced by a smoother and more efficient coverage layer of inhibitors (Figure 12).5,6,48 Consequently, the height curve appeared to be smoother and the dispersive small peak (sample C) became a wide-gathering peak. This reflects the accumulation of inhibitor molecules on the metal surface. These findings further present the gradual adsorption and accumulation of inhibitors in good agreement with the photos in Figure 12. 3.5. Energy Dispersive X-ray Spectroscopy Measurements. The detailed surface analysis shows an estimation of the composition of each phase (Figures 13−16). These data show

Figure 16. SEM (a) and EDX (b) analysis for carbon steel in test solution with composite inhibitor at 300 mg·L−1. (Composite inhibitors: PESA, 120 mg·L−1; PASP, 120 mg·L−1; PAPEMP, 40 mg·L−1; and Glu, 20 mg·L−1.)

nitrogen and phosphorus values increased to 1.53 mol % and 0.24 mol %, respectively, with inhibitors. Figure 16b shows that the intensity of the nitrogen and phosphorus signals increase with inhibitor concentration (N content, 20.29 mol %; and P content, 0.50 mol %). Meanwhile, the spectrum in Figure 15b shows that the iron peaks are considerably suppressed relative to the test samples. The value of iron decreases from 31.57 mol % (Figure 14b) to 26.75 mol % (Figure 15b), and Figure 16b illustrates how this suppression increases with increasing inhibitor concentration (Fe content, 24.77 mol %). This enhancement in the carbon and nitrogen signals and suppression of iron and oxygen signals indicates that a layer containing phosphorus atoms and nitrogen atoms cover the electrode surface. This layer is undoubtedly due to the inhibitor because of the high contribution of the carbon, nitrogen, phosphorus, and oxygen signals seen only in the samples with inhibitors.25,61 These results confirm the AFM measurements, which suggest that a surface film inhibited the metal dissolution and hence retarded hydrogen evolution.

Figure 13. SEM (a) and EDX (b) analysis for pure and ground carbon steel before immersion.

4. CONCLUSIONS The inhibition effects of a single material containing PASP, PESA, PAPEMP, and Glu, as well as their combination on carbon steel corrosion in soft water media were studied by weight loss experiments as well as electrochemical, EDX spectroscopy, and AFM measurements. The purpose was to develop a novel, environmentally friendly composite formula to inhibit corrosion in soft water. The principal findings are summarized as follows: (1) The orthogonal test for weight loss proved the synergistic effect of the combination of inhibitors and provided the optimum inhibitor concentrations. This was 12, 12, 4, and 2 mg·L−1 for PASP, PESA, PAPEMP, and Glu, respectively (30 mg·L−1 in total). (2) By electrochemical measurements in soft water under experimental conditions, PASP, PESA, PAPEMP, and Glu behaved as mixed-type inhibitors. The inhibitors demonstrated synergistic effects when blended. The composite showed mixed-type inhibition behavior that not only retarded metal dissolution and the cathodic process but also formed a barrier layer on the carbon steel surface. (3) The environmentally friendly composite inhibitors offer synergistic effects and remarkable corrosion inhibition. Single inhibitors had limited effect unless added at high concentrations. The corrosion inhibition efficiency of the composite reached 97.32% at a relatively low dosage (30 mg·L−1 in total) with a low phosphorus content (about 0.79−0.87 mg·L−1). (4) The AFM images confirmed the formation of a homogeneous protective film containing the composite

Figure 14. SEM (a) and EDX (b) analysis for carbon steel in test solution without inhibitor.

Figure 15. SEM (a) and EDX (b) analysis for carbon steel in test solution with composite inhibitor at 30 mg·L−1. (Composite inhibitors: PESA, 12 mg·L−1; PASP, 12 mg·L−1; PAPEMP, 4 mg·L−1; and Glu, 2 mg·L−1.)

EDX data recorded for carbon steel samples exposed for 72 h in soft water at 318 K in the absence and presence of various inhibitor concentrations. Figure 13b shows the characteristics peaks of some of the elements constituting carbon steel (C content, 8.57 mol %; O content, 16.26 mol %; and Fe content, 72.17 mol %). Figure 14b shows intensity of oxygen signals (O content, 51.34 mol %) without inhibitors after 72 h immersion time at 318 K, which may be attributed to the formation of FeOOH and/or Fe(OH)3. In contrast, the EDX analysis of carbon steel in the presence of inhibitors showed intensity peaks of the nitrogen and phosphorus signals (Figure 15b.) The I


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inhibitors. This was adsorbed onto the carbon steel surface to prevent corrosion.

ASSOCIATED CONTENT

S Supporting Information *

Tables showing factors and their values of the orthogonal test, Taguchi method, inhibition efficiency of polarization, and impedance parameters; figures showing the results of EIS, AFM, and SEM-EDX analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 21 65980567. Fax: +86 21 65985059. E-mail: [email protected]. Author Contributions §

These two authors contributed equally to this work (C.H. and Z.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank the State Key Laboratory of Pollution Control and Resource Reuse. This work was funded by the “Safe Water Supply and Demonstration of Water Resources Use in Typical African Countries” program (2010DFA92820), “Technical Cooperation on Water Monitoring, Waste Water Treatment and Ecological Conservation with Typical Countries/ Basins in Africa” program (2010DFA92800), and “Application study and demonstration of advanced papermaking wastewater treatment technology” program (2012DFG91870), Ministry of Science and Technology, China.

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REFERENCES

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