Corrosion protection of iron in water by activated carbon fiber (ACF)

Carbon 44 (2006) 19–26 www.elsevier.com/locate/carbon

Corrosion protection of iron in water by activated carbon fiber (ACF) Ji Yang, Juan Peng, Zhemin Shen, Jinping Jia *, Feng Zhang School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 8 April 2005; accepted 15 July 2005 Available online 26 August 2005

Abstract Commercial activated carbon fiber (ACF) was modified and employed to prevent iron corrosion in industrial water supply and circulation system. Static and dynamic experiments were carried out under varying conditions, including different pHs, different temperatures, different adsorbent quantities and different adsorbents. The primary objective was to experimentally demonstrate the suitability of ACF in effectively preventing iron corrosion in water under varying operating conditions, and compare its performance vis-a`-vis to that of the other commercially available adsorbents, such as granular activated carbon (GAC) and powdered activated carbon (PAC). Iron sheet static corrosion simulation test as well as dynamic corrosion simulation test was performed to verify the idea. It was found out that ACF could significantly decrease the zeta potential (from 329 mV to 203 mV when 100 mg ACF was added to 200 ml water) and dissolved oxygen concentration (from 9.60 mg/l to around 9.18 mg/l when 200 mg ACF was added to 200 ml water) of the solution, thereby slowing down iron corrosion rate.
2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon fiber (ACF); Carbon fiber; Adsorption

1. Introduction It is well known that corrosion is the main problem for industrial water supply and circulation system. Corrosion causes high maintenance cost and metal corrosion can ruin the equipments, leading to potential safety consideration for the equipments while shortening their lifespan [20,14]. Currently, corrosion inhibiters are added to most water supply and circulation systems. It is generally believed that introduction of chemicals is not environmental-friendly and for most inhibiters, there are certain application ranges (such as pH, hardness, etc.), which make their application inconvenient because operators

*

Corresponding author. Tel./fax: +86 21 54742817. E-mail addresses: [email protected] (J. Yang), [email protected] (J. Jia). 0008-6223/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.07.006

have to determine related water quality indexes constantly. Also, the effect fades as more resultants and decomposition products are formed and concentrated. In recent times, ACF has been considered to be one of the most promising adsorbents. Compared to traditional powdered activated carbon (PAC) and granular activated carbon (GAC), ACF has many advantages due to its unique structure and characteristics, such as greater inward and outward specific surface area, more abundant micro-pore volume, specific surface functional groups, higher adsorption rate and capacity from the gas or liquid phase and so on [22,18,5,17]. Currently, ACF is widely used in different fields, such as gas and liquid adsorption, waste water treatment, biological and medical treatment, and electronic industry [16]. Several different kinds of ACFs are commercially developed, including viscose-based ACF (VACF), pitch-based ACF (PACF), sisal-based ACF (SACF) and PAN-based ACF (PAN-ACF).

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Reviewing literature, it is fair to say that so far most of the studies carried out on the ACFs pertain to the equilibrium conditions, with primary objective of these studies being the development of an appropriate adsorption isotherm for pollutants removal [6,8,26,12,27,23,7]. Iron corrosion prevention by ACF has been observed phenomenally by limited researchers [11,1], however, there is a deficiency in basic research on the preservation properties of ACF. It is in this context that the present study was undertaken with a primary view to ascertaining the suitability of ACF in iron corrosion prevention under various conditions and exploring the mechanism behind the process. The major objectives of this study were as follows: (1) set-up of an experimental test bench to study iron corrosion prevention characteristics by ACF, (2) exploring the mechanism of iron corrosion prevention by ACF, (3) obtaining working curves under varying operating conditions such as temperature, weight of the adsorbent etc., (4) screening of other commercial adsorbents (GAC, PAC) for determining their comparative iron corrosion prevention performance.

2. Experimental 2.1. Modification of ACF and its characterization GAC and PAC were from Morgan Carbon Company (Shanghai). The average particle size of GAC was 3 · 3 mm, PAC 1.5 · 1.5 mm. Commercial VACF (Anshan activated carbon plant, PR China) in cloth form was put into a quartz reactor, and the reactor was contained in a muffic furnace under N2 gas to make sure the carbonization was complete. The commercial ACF was carbonized for 3 h under nitrogen at 850 C to pyrolize the resin binder to produce modified ACF. Following its carbonization, the modified ACF was activated. The activation conditions were: 850 C, steam at a partial pressure of 0.6 atm. and N2 at 0.4 atm. for durations of 1.5 h. Burn off was calculated from the initial and final weights of ACF. The main concern with a higher burn off is the reduction in the strength of the composite, therefore, the final burn off was controlled around 15%. The specific surface area of the adsorbent was estimated by N2 adsorption–desorption porosimetry via BET method. The instrument employed was a Fisons 1900 Sorptomatic system. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface functional groups of ACF with an Axis Ultra spectrometer (Kratos, UK). The ACF, GAC and PAC used were regenerated before use by 100 C hot steam. The follow rate of the hot steam was 100 ml/min and the regeneration time was 6 h.

2.2. Iron sheet corrosion test 2.2.1. Static corrosion test Carbon steel was from Bao Steel Company (Shanghai). Six groups of iron sheets were degreased and sandblasted to white metal according to standard ISO 8501-1:1998. (Preparation of steel substrates before application of paints and related products—visual assessment cleanliness—Part 1: Rust grades and preparation grades of uncoated steel substrates and steel substrates after overall removal of previous coating.) The sheets were weighted, thereafter three of them were immersed into 200 ml distilled water sealed in flask, with pH 4, 7 and 10 respectively. The other three were added to identical three distilled water with pH 4, 7 and 10 respectively, except that each of them contained 100 mg modified ACF. The iron sheets had been kept in the solutions for 7 days before they were rust-removed and weighed respectively. The mass loss of steel samples due to corrosion was measured using the chemical cleaning methods described in ASTM standard G1-90 [2]. Effect of ACF quantity on static corrosion was investigated as well. Five groups of rust-removed and polished iron sheet were put into five identical 200 ml distilled water sealed in flask, which contain 50 mg, 100 mg, 150 mg, 200 mg and 250 mg modified ACF respectively. The iron sheets had been kept in the solutions for 7 days before they were rust-removed and weighed respectively. Effect of GAC and PAC on static corrosion was also studied. Two groups of rust-removed and polished iron sheet were put into two identical 200 ml distilled water sealed in flask, which contain 0 mg and 100 mg GAC and PAC. The sheets had also been kept in the solution for 7 days before they were rust-removed and weighed. 2.2.2. Dynamic corrosion test Schematic diagram of the experiment system was shown in Fig. 1. It consisted of a pump, a bottle full of tap water, a conical flask full of tap water containing iron sheets at the bottom, a filtration tube set between the bottle and the conical flask, and a flow-meter. Water was pumped into the system with a rate of 0.01 ml/s as shown in the diagram. One sheet of rust-removed and polished iron was put into the conical flask for 7 days before it was rustremoved and weighed again. Experiments with 100 mg modified ACF or 100 mg GAC/PAC were performed respectively to compare the results. 2.3. Polarization curve Potentiostat polarization measurement using threeelectrodes was employed to evaluate the iron corrosion behavior in two 0.1 mol/l H2SO4 solutions, one of which contained modified ACF and the other did not. The

J. Yang et al. / Carbon 44 (2006) 19–26

21

was determined by M05669-20 (Cole Parmer) acidimeter. Five beakers of 200 ml sealed water with pH 4, 6, 7, 9 and 10 were stirred and their dissolved oxygen concentrations were measured. Meanwhile, five beakers of 200 ml identical distilled water each containing 100 mg modified ACF whose pH were 4, 6, 7, 9 and 10 were stirred and their dissolved oxygen concentrations were measured. At temperature of 0 C, 200 ml distilled water was agitated until dissolved oxygen had been saturated. The temperature of the water was raised gradually while the dissolved oxygen concentrations were measured in certain intervals from the temperature of 0 C until the temperature reached 30 C. One hundred milligrams modified ACF was added to identical 200 ml distilled water and the above experiment were repeated. 2.5. Zeta potential of the solutions

1. Tap water, 2. Filtration tube, 3. Filtration materials, 4. Water pump, 5. Flowmeter, 6. Iron sheet Fig. 1. Dynamic corrosion experimental set-up.

system consisted of ZF-3 potentiostat, ZF-4 electrode potential scan signal generator, ZF-6 logarithm converter and WX2400 X–Y function recorder. The saturated calomel electrode (Model 213, Shanghai dianguang Instrument Co. Ltd., PR China) was used as reference electrode and Pt electrode (Model 213, Shanghai dianguang Instrument Co. Ltd., PR China) was used as auxiliary electrode, while steel electrode as the working electrode. Surface area of the working electrode was 1 cm2, and it was rust-removed, polished then cleaned with acetone.

The system is schematically represented in Fig. 2. A potentiostat (Model ZF-3, Shanghai leici Instrument Co. Ltd., PR China) was used to determine the zeta potential of the solution. The saturated calomel electrode (Model 213, Shanghai dianguang Instrument Co. Ltd., PR China) was used as reference electrode and Pt electrode (Model 213, Shanghai dianguang Instrument Co. Ltd., PR China) was used as indicator electrode. Five groups of 200 ml distilled water sealed in flask each containing 100 mg PAC, GAC, commercial ACF and modified ACF respectively were stirred and their zeta potentials were measured after adsorption equilibrium had been reached. Three sealed distilled water of 200 ml with pH 4, 7 and 10 were stirred and their zeta potentials were measured. Meanwhile, five 200 ml solutions each containing 100 mg modified ACF with pH 4, 6, 7, 9 and 10 were stirred and their zeta potentials were measured.

2.4. Dissolved oxygen (DO) in the solutions One hundred milligrams GAC and 100 mg modified ACF were added to 200 ml sealed distilled water, respectively. The solutions were stirred and the dissolved oxygen concentrations of the solutions were measured by an oxygen analyzer (Model RSS-5100, Shanghai Leici Instrument Co. Ltd., PR China). The effect of pH and temperature on DO removal was also examined. 0.1 M NaOH and 0.1 M H2SO4 solution were prepared for pH adjustment and all the chemicals used were certified grade or better. The pH

1. Potentiostat, 2. Pt electrode(indicator electrode), 3. Saturated calomel electrode(reference electrode), 4. Salt bridge Fig. 2. Zeta potential experimental set-up.

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At temperature of 0 C, 200 ml distilled water was agitated until the dissolved oxygen was saturated. The temperature of the water was raised gradually while the zeta potential was measured in certain intervals from the temperature of 0 C until the temperature reached 95 C. Add 100 mg modified ACF to identical 200 ml distilled water and repeat the experiment above.

Table 2 Concentration of functional groups Surface functional groups (%)

Unmodified ACF

Modified ACF (after activation)

PAC

GAC

C–C C–O–R(H) C@O HC–C@O O–(C@C)–O

68.9 12.7 4.3 5.6 3.2

66.7 15.1 5.6 6.9 1.6

75.6 10.2 4.1 2.5 2.2

74.7 11.1 4.3 2.9 1.6

3. Results and discussion 3.2. Corrosion test

3.1. Characteristics of ACF Surface characteristics are very important [9], and an adsorbent exhibiting favorable adsorption isotherm as well as fast kinetics is supposed to be a good adsorbent. In order to be a good adsorbent, an adsorbent must have (a) a reasonably large surface area, and (b) a relatively large pore network for the transport of adsorbed molecules to the interior. It is generally believed that deep carbonization followed by proper activation could improve the surface characteristics such as specific surface and pore volume, enabling ACF special redox capabilities which includes iron corrosion inhabitation [11,21]. Therefore, commercial ACF obtained was modified to improve its surface characteristics. Table 1 shows the specific surface area of the commercial and modified ACF. As observed from following table, more carbon was burned off from the commercial ACF and the specific surface increased significantly. Although the commercial ACF has been carbonized and activated when manufactured, the results show that further carbonization and activation as described in the experimental section have a dramatic effect on improving its surface area and micro-pore volume. Similar results were reported by other researchers [13,19]. Table 2 exhibits the surface functional groups of sorbents. It is distinguishable that modified ACF has more surface functional groups such as C–O–R(H) and HC– C@O than GAC and PAC. This difference could be attributed to the raw materials or the manufacturing process.

3.2.1. Static corrosion test Static experiments in seal flask were performed to examine the corrosion inhibition ability of modified ACF. As observed from Table 3, iron corrosion was significantly slowed down by addition of modified ACF into the water between pH 4–10. Notably, as pH goes up, modified ACF becomes more effective, which indicates that modified ACF works better under basic conditions than acidic condition. It could be seen in Table 3 that the effect of ACF is the weakest when pH = 4. Therefore, pH = 4 was selected to study the effect of ACF dosage on the iron corrosion inhabitation. Table 4 describes the effect of modified ACF quantity on the corrosion protection (at pH = 4). It could be learned that when the addition of modified ACF increases from 50 mg to 100 mg, the corrosion of iron is slowed down more. Further addition has no visible effect on corrosion inhibition. Experiments were also performed to compare the effect of modified ACF, GAC and PAC, it could be seen that for the same addition, the effect of modified ACF is more obvious than those of GAC and PAC. Table 3 Static corrosion test results (with or without modified ACF) pH Iron corrosion per g iron (%)

4 With ACF Without ACF

Corrosion decreased by modified ACF (%)

10

0.12 0.15

0.12 0.14

0.09 0.14

17.12

17.36

32.61

Standard deviation within 2.2% for all the data.

Table 1 Specific surface area of ACF

Commercial ACF Modified ACF (before activation) Modified ACF (after activation) GAC PAC

7

SBET (m2/g)

Vtotal (ml/g)

Vmicro (ml/g)

Vmeso (ml/g)

730 860 1050 670 580

0.33 0.39 0.52 0.30 0.27

0.30 0.37 0.50 0.16 0.15

0.03 0.02 0.02 0.14 0.012

SBET—specific surface area determined using BET method; Vtotal— total pore volume; Vmicro—micro-pore volume; Vmeso—meso-pore volume.

Table 4 Static corrosion test results (effect of modified ACF quantity) Quantity of modified ACF (mg)

50

100

150

200

250

Iron corrosion per g iron (%) Quantity of GAC (mg) Iron corrosion per g iron (%) Quantity of PAC (mg) Iron corrosion per g iron (%)

0.14 50 0.16 50 0.15

0.12 100 0.14 100 0.14

0.13

0.12

0.12

Standard deviation within 1.7% for all the data.

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The reaction of dissolved oxygen is the main cathode reactions during metal corrosion.

Table 5 Dynamic and static corrosion (pH around 7.2)

Iron corrosion per g iron (%)

Static Dynamic

23

Blank

GAC

PAC

Modified ACF

0.14 0.14

0.14 0.13

0.14 0.12

0.12 0.05

3.2.2. Dynamic corrosion test To simulate the situations in practical water supply and circulation system, dynamic corrosion test was performed. It is shown in Table 5 that for the same addition (100 mg), modified ACF is much more efficient for slowing down corrosion than GAC and PAC, and the protection by modified ACF, GAC or PAC is better than that under static condition. 3.3. Polarization curve Polarization curve is an important tool to study the metal corrosion behavior. In this work, polarization curve was plotted to compare the iron corrosion in water with and without modified ACF addition. It is described in Fig. 3 that under the same applied voltage, the corrosion current of iron without modified ACF was higher than that of iron with modified ACF, which proves that iron corrosion rate is lower under the effect of modified ACF, suggesting that corrosion resistance of iron with ACF is superior to that without ACF. This is also proved by extrapolating the data to the x-axis, showing that when the applied voltage is zero, the self corrosion current of iron with ACF is lower than that without ACF.

O2 + 4Hþ + 4e ! 2H2 O

ð2Þ

As shown in above equations, if the dissolved oxygen (DO) concentration could be decreased, usually the corrosion of iron will definitely be slowed. Following experiments were carried out to verify the assumption. 3.4.1.1. Performance comparison by GAC, PAC and modified ACF. Table 6 shows the DO in water when adsorbent was added. As observed from above table, GAC, PAC and modified ACF could evidently reduce DO This could be explained as modified ACFÕs specific surface is much larger than that of GAC or PAC, therefore, more oxygen could be absorbed by modified ACF to decrease the DO in sealed water; Or as shown in Table 2, modified ACF has more surface functional groups such as C–O–R(H) and HC–C@O than GAC and PAC, which could be oxidized by O2 [10,24]. Consequently, the DO is reduced. 3.4.1.2. Effect of ACF with different quantities. Fig. 4 describes the relationship between solution DO and modified ACF addition. It could be seen that as more modified ACF was added, the solution went down even further, from original 9.6 mg/l to around 9.18 mg/l when 200 mg modified ACF was added to 200 ml distilled water sealed in flask. As mentioned above, the decreased DO could be due to modified ACF absorption or the reaction between oxygen and modified ACF surface functional groups.

3.4. Mechanism 3.4.1. DO in the solutions Generally speaking, for ordinary steel, iron always acts as anode and following reaction is the key for corrosion [4]: Fe + 2OH
! Fe(OH)2 # + 2e

ð1Þ

Fig. 3. Polarization curve of iron with and without modified ACF.

Table 6 Desolved oxygen of solutions (three replicates)

DO (mg l
1)

Blank

GAC

PAC

Unmodified ACF

Modified ACF

9.59

9.54

9.52

9.48

9.27

Fig. 4. DO of solutions with different modified ACF quantities.

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J. Yang et al. / Carbon 44 (2006) 19–26

Fig. 5. DO of solutions with ACF at different pHs.

Fig. 6. Effect of temperatures on DO decrease by modified ACF.

3.4.1.3. Effect of modified ACF at different pHs and temperatures. Fig. 5 describes the modified ACF performance on DO decrease at different pHs since real water supply and circulation system might have random pH variations within certain range. It could be seen that modified ACF could decrease solution DO within a wide range of pH, although it works more efficiently under acidic circumstance than at basic circumstances as shown in Fig. 5. The superior performance of ACF under acidic circumstance could be explained as follows: With the pH going up, cathode reaction changed from Eq. (2) to

where Bi is proportionality constant (t
1), E is the activation energy (J/mol), R is the gas constant (8.314 J/ mol K), and T is the temperature (K). When the temperature is higher than 29 C, the solubility of oxygen in water is significantly reduced, which also slows the oxidization of surface functional groups.

O2 + 2H2 O + 4e ! 4OH


ð3Þ

Standard electrode potential of this reaction is 0.403 V compared to 1.229 V for Eq. (2). So oxidization of the surface functional redox groups by oxygen is hindered [10], therefore the overall DO level is higher as pH goes up. It has been shown in Table 3 that the corrosion was inhibited most under pH 10, while Fig. 5 shows that the DO reduction is the highest under pH 4 and lowest under pH 10. Therefore, it could be seen that DO is not the only factor that determines the corrosion inhibition and other factors such as zeta potential should be examined before a conclusion should be drawn. Temperature is another important factor that must be considered for ACF application. Experiments (open system) were also performed to examine modified ACFÕs ability to decrease solution DO at different temperatures. As described in Fig. 6, it is shown that within the temperature range of 15–29 C, modified ACF has obvious effect at lowering solution DO. As temperature goes out of that range, adding or not adding modified ACF has no meaningful difference. This could be explained as follows: when the temperature is lower than 15 C, the oxidization of ACF surface functional groups is slowed exponentially according to Arrhenius expression,
E

k i ¼ Bi expRT

ð4Þ

3.4.2. Zeta potential of the solutions Almost all materials in contact with a liquid acquire an electronic charge on their surfaces and zeta potential is an important and useful indicator of this charge, which can be a controlling parameter in processes such as adhesion, surface coating, and corrosion. Potential energy is needed when metal is oxidized. The zeta potential could be described by [3], h i 0:5 u ¼ /0 exp
ðr=ee0 DÞ y s ð5Þ where u is the zeta potential (mV), /0 is potential at metal surface (V), r is conductivity (S/cm), e is dielectric constant (dimensionless), e0 is permittivity of free space (8.85 · 10
14 C/V cm), D is diffusion coefficient (cm2/s), ys is the slip plane distance. Table 7 compares effect of different adsorbents on changing solutionÕs zeta potential. It is evident in Table 7 that the modified ACF could lower the solution zeta potential most efficiently. The original zeta potential of the water without any adsorbent is 329 mv, when GAC is added, it goes down to 322 mv and 240 mv for PAC addition. As the zeta potential of the system decreases, oxidation is more difficult to occur. Therefore iron corrosion is more likely to be slowed. Based on Eq. (5), since /0, ee0, D and ys are relatively constant [3], the increase of r could effectively decrease zeta potential of solution. Therefore, Table 7 Zeta potential (/) of solutions

//mV

Blank

GAC

PAC

Unmodified ACF

Modified ACF

329

322

240

236

203

J. Yang et al. / Carbon 44 (2006) 19–26

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the different performance of modified ACF, GAC and PAC could be explained as modified ACF has more hydrophilic surface functional groups when being oxidized than GAC and PAC (shown in Table 2), which will contribute to the increase of overall conductivity which leads to significant drop of zeta potential. 3.4.2.1. Effect of ACF quantities. Fig. 7 compares the results by adding different quantities of modified ACF into 200 ml water. As it is shown in the figure, increase of ACF dosage could decrease the zeta potential of the solution further. It is also observed that as more modified ACF is added, the curve becomes flatter. This might be caused by the decrease of DO in solution, as shown in Fig. 4, which will slow the oxidization of surface functional groups and as explained above the overall conductivity will become flatter (as a result, the zeta potential decrease will become flatter). Therefore for possible future applications, cost-benefits balance should be considered. For this experiment, it is thought that adding modified ACF around 100 mg into per 200 ml water is more suitable. 3.4.2.2. Effect of ACF at different pHs and temperatures. To examine the performance of modified ACF at different acidities, experiments were performed by adding modified ACF to water while manipulating the solution pH by adding NaOH or H2SO4. The results are shown in Fig. 8. It could be observed from Fig. 8 that the modified ACFÕs effect becomes stronger as pH goes up. Combined with Table 3, it is reasonable to conclude that the reduction of zeta potential is the key for iron corrosion inhibition. Temperature is an important factor that should always be considered [15,25]. As shown in Fig. 9, when the water temperature was lower than 65 C, modified ACF can efficiently reduce the zeta potential of the solution, especially at room temperature conditions (around 25 C). However, as the temperature went higher than 65 C, modified ACF actually increased the zeta potential of the solution, which made the iron more suscepti-

Fig. 8. Effect of ACF at different pHs.

Fig. 9. Effect of ACF at different temperatures.

ble to corrosion. With the temperature going up higher than 65 C, the DO in solution decreases significantly, and the cathode reaction changed from Eq. (2) to [4], 2H2 O + 2e ! 2OH
+ H2 "

ð6Þ

The surface functional groups of ACF such as C@O and HC–C@O could be reduced [10], therefore help to shift the equilibrium of Eq. (6) to the right and speed up the iron corrosion. Consequently, if ACF is used to prevent iron corrosion in water circulating or supplying system, proper application temperature should be determined.

4. Conclusions This paper explores the possibility of using ACF as a safe and effective iron corrosion inhibitor for industrial water circulating and supplying system. The preliminary mechanism of ACFÕs preservation ability was also discussed. The various conclusions that may be drawn from this study are summarized as follows:

Fig. 7. Zeta potential of solutions with different ACF quantities.

1. ACF could more effectively slow down the iron corrosion in both static and dynamic corrosion test than GAC and PAC, which is verified by polarization curve.

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