Nitrotetrazolium blue chloride as a novel corrosion inhibitor of steel in sulfuric acid solution

Corrosion Science 52 (2010) 3840–3846

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Short Communication

Nitrotetrazolium blue chloride as a novel corrosion inhibitor of steel in sulfuric acid solution Shuduan Deng a,*, Xianghong Li b, Hui Fu b a b

Faculty of Wood Science and Decoration Technology, Southwest Forestry University, Kunming 650224, PR China Department of Fundamental Courses, Southwest Forestry University, Kunming 650224, PR China

a r t i c l e

i n f o

Article history: Received 24 May 2010 Accepted 12 July 2010 Available online 16 July 2010 Keywords: A. Steel B. EIS B. Polarization B. Weight loss C. Acid inhibition

a b s t r a c t The inhibition effect of nitrotetrazolium blue chloride (NTBC) on the corrosion of cold rolled steel (CRS) in 0.5 M H2SO4 was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The results show that NTBC is a good inhibitor, and the adsorption of NTBC on CRS surface obeys Langmuir adsorption isotherm. Polarization curves show that NTBC acts as a mixed-type inhibitor. EIS spectra consist of large capacitive loop at high frequencies followed by a small inductive one at low frequency values, and charge transfer resistance increases with the inhibitor concentration. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The use of inhibitors is one of the most practical methods for protection metal against corrosion, especially in acidic media [1]. N-heterocyclic compounds are considered to be the most effective corrosion inhibitors on steel in acid media [2], such as imidazoline derivatives [3–5], isoxazolidines [6,7], 1,2,3-triazole derivatives [8], 1,2,4-triazole derivatives [9–21], thiadiazole derivatives [22–26], pyrrole [27], pyridine derivatives [28–30], pyrazole derivatives [31–34], bipyrazole derivatives [35–37], pyrimidine derivatives [38], pyridazine derivatives [39], indole derivatives [40–42], benzimidazole derivatives [43–47], quinoline derivatives [48] and purine derivatives [49–51]. It is generally accepted that N-heterocyclic compounds exert their inhibition by adsorption on steel surface through nitrogen heteroatom, as well as those with triple or conjugated double bonds or aromatic rings in their molecular structures. Furthermore, inhibition efficiency of N-heterocyclic organic inhibitor always increases with the number of aromatic systems and the availability of electronegative atoms in the molecule [9]. Tetrazole derivatives whose molecules possess many aromatic systems and electronegative N atoms have been received some attention. In 1996, Kertit and Hammouti [52] studied the inhibition of 1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT) on the corrosion of iron in 1.0 M HCl, and its maximum inhibition efficiency (E)

* Corresponding author. Tel.: +86 871 3863609; fax: +86 871 3863989. E-mail address: [email protected] (S. Deng). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.07.020

was 98% at 2  103 M. Bensajjay et al. [53] extended their work to study the inhibition effect of PMT on steel in 0.5 M H2SO4 and 1/3 M H3PO4. Results obtained showed that E reached an optimum value of 98% at 103 M PMT. Afterwards, Morales-Gil et al. [54] reported the inhibition effect of 5-mercapto-1-tetrazoleacetic sodium salt (MTAc) on steel in 1.0 M H2SO4. Their results showed that E was about 69% at 200 mg l1, while then decreased with the MTAc concentration. Recently, Li et al. [55–57] investigated the inhibition effect of red tetrazolium (RT) on the corrosion of cold rolled steel (CRS) in HCl, H2SO4 and H3PO4 solutions. The results show that maximum E value of 500 mg l1 RT at 20 °C is 94% in 1.0 M HCl [55]; 79%, 1.0 M H2SO4 [56]; and 63%, 3.0 M H3PO4 [57]. Besides steel, copper in acidic [58] and neutral [59] media, brass [60] and zinc [61] in HNO3 solution, aluminum in HCl solution [62] have been protected against corrosion using tetrazole derivatives. Noticeably, these tetrazole compounds studied contain only one tetrazole ring, the literature available to date about the corrosion inhibition of the tetrazole compound which simultaneously possesses two tetrazole rings is very scarce. Nitrotetrazolium blue chloride (NTBC) is the tetrazole compound containing two tetrazole rings, which could be seemed as a good potential inhibitor. However, to the best of our knowledge, NTBC has not been treated as a corrosion inhibitor. In the present work, the inhibition effect of NTBC on cold rolled steel (CRS) in 0.5 M H2SO4 is studied for the first time by weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. A probable inhibitive mechanism is proposed from the viewpoint of adsorption theory.

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The corrosion rate (m) was calculated from the following equation [50]:

2. Experimental method 2.1. Materials

m¼ Tests were performed on a cold rolled steel (CRS) of the following composition (wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and the remainder Fe. 2.2. Inhibitor Nitrotetrazolium blue chloride (NTBC) was obtained from Shanghai Chemical Reagent Company of China. Fig. 1 shows the molecular structure of NTBC. Clearly, it contains two tetrazole rings (eight N heteroatoms), six O atoms in –NO2 and –OCH3 groups, and many aromatic systems. Also, it could be easily protonated in acid solution, and a great deal of lone electrons p-electrons exists in this molecule. Furthermore, the molecule is big enough (molecular weight 817.60 g mol1) and likely to efficiently cover more surface area (due to adsorption) of the mild steel. 2.3. Solutions The aggressive solution 0.5 M H2SO4 was prepared by dilution of AR grade 98% H2SO4 with distilled water. The concentration range of NTBC used was 0.005–0.1 mM (5.0  106–1.0  104 M). 2.4. Weight loss measurements Weight loss tests were conducted under total immersion conditions 250 ml of test solutions (0.5 M H2SO4 without and with different concentrations of NTBC) maintained at 25 °C controlled by water thermostat (±0.1 °C). Three parallel CRS sheets of 2.5 cm  2.0 cm  0.06 cm were abraded by a series of emery paper (grade 320–500–800) and then washed with distilled water and acetone. After weighing accurately by digital balance with sensitivity of ±0.1 mg, the specimens were suspended in a beaker containing test solutions using glass hooks and rods. All tests were made in non-deaerated solutions. The specimens were taken from test solutions after immersion for 6 h, washed with bristle brush under running water in order to remove the corrosion product, dried with a hot air stream, and re-weighed accurately. The mean weight loss of three parallel CRS sheets was obtained and reported.

W St

ð1Þ

where W is the average weight loss of three parallel CRS sheets, S the total area of one CRS specimen, and t is immersion time (6 h). With the calculated corrosion rate, the inhibition efficiency (Ew) was calculated as follows [50]:

Ew % ¼

m0  m  100 m0

ð2Þ

where m0 and m are the values of corrosion rate without and with inhibitor, respectively. 2.5. Electrochemical measurements Electrochemical experiments were carried out in the conventional three-electrode cell with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. To minimize ohmic contribution, the Luggin capillary was close to working electrode (WE) which was in the form of a square CRS embedded in PVC holder using epoxy resin so that the flat surface was only surface in the electrode. The working surface area was 1.0 cm  1.0 cm, abraded with emery paper (grade 320–500–800) on test face, rinsed with distilled water, degreased with acetone, and dried with a cold air stream. Before measurement the electrode was immersed in test solution at open circuit potential (OCP) for 2 h at 25 °C to be sufficient to attain a stable state. All electrochemical measurements were carried out using PARSTAT 2273 advanced electrochemical system (Princeton Applied Research). Each experiment was repeated at least three times to check the reproducibility. The potential of potentiodynamic polarization curves was increased at 0.5 mV s1 and started from a potential of 250 mV to +250 mV vs. free corrosion potential (Ecorr vs. SCE). Inhibition efficiency obtained from polarization curves (Ep%) is defined as:

Ep % ¼

Icorr  IcorrðinhÞ  100 Icorr

where Icorr and Icorr(inh) represent corrosion current density values without and with inhibitor, respectively.

NO2

_

OCH3

Cl N

N

N + N

+ N

N H3CO

ð3Þ

Cl

N

_

N

NO2 Fig. 1. Chemical molecular structure of nitrotetrazolium blue chloride (NTBC).

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Electrochemical impedance spectroscopy (EIS) was carried out at OCP in the frequency range of 0.01 Hz–100 kHz using a 10 mV r.m.s voltage excitation. Inhibition efficiency obtained from EIS (ER%) is calculated by the following equation:

h¼

RtðinhÞ  Rtð0Þ ER % ¼  100 RtðinhÞ

ð4Þ

where Rt(0) and Rt(inh) are charge transfer resistance values for CRS in the absence and presence of inhibitor, respectively.

3. Results and discussion 3.1. Weight loss measurements The weight loss method of monitoring inhibition efficiency is useful because of its simple application and reliability [63,64]. For the present study, the reproducibility of results obtained for both weight loss and percentage inhibition efficiency values for triplicate determination was very precise (±5%). Fig. 2 shows the corrosion rate (m) and inhibition efficiency (Ew) values with different concentrations of NTBC in 0.5 M H2SO4 solution at 25 °C. m decreases noticeably with the increase in NTBC concentration from 20.18 to 0.87 g m2 h1 at 0 and 0.1 mM of NTBC, respectively, i.e. the corrosion inhibition enhances with the inhibitor concentration. This behavior is due to the fact that the adsorption coverage of inhibitor on CRS surface increases with the inhibitor concentration [65]. When the inhibitor concentration reaches about 0.04 mM, the corrosion rate value does not change markedly with the inhibitor concentration. Fig. 2 also show that when the concentration of NTBC is less than 0.04 mM, Ew increases sharply with increase in concentration, while a further increase causes no appreciable change in performance. The maximum Ew is 95.7% at 0.1 mM (1.0  104 M) and the inhibition is estimated to be higher than 71.1% even at 0.01 mM (1.0  105 M), and its protection is >90% at 0.04 mM, which indicates that NTBC is a very good inhibitor for CRS in 0.5 M H2SO4. Assuming the increase of the inhibition is caused by the adsorption of inhibitor on the CRS surface and obeys Langmuir adsorption isothermal equation [50]:

ð5Þ

20

95

18

90

-2

16 85

14

10

Corrosion rate v Inhibition efficiency Ew

8

80 75 70

6 65

4

ð6Þ

where Wm is the smallest average weight loss. A straight line is obtained on plotting c/h vs. c as shown in Fig. 3. The linear correlation coefficient (r) is almost equal to 1 (r = 0.9998) and the slope is very close to 1 (slope = 0.9622), indicating the adsorption of NTBC on steel surface obeys the Langmuir adsorption isotherm. The adsorptive equilibrium constant (K) value calculated is 2.71  105 M1, which is related to the standard free energy of adsorption (DG0) as shown the following equation [66]:

K¼

1 DG0 expð Þ 55:5 RT

ð7Þ

where R is the gas constant (8.314 J K1 mol1), T the absolute temperature (K), the value 55.5 is the concentration of water in solution expressed in M [66]. The DG0 value calculated is 40.97 kJ mol1. The negative values of DG0 indicate that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. Generally, values of DG0 up to 20 kJ mol1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than 40 kJ mol1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a co-ordinate type of bond (chemisorption) [67]. In the present study, the value of DG0 is slightly negative than 40 kJ mol1; probably mean that the adsorption mechanism of the NTBC on steel in 0.5 M H2SO4 solution is mainly the chemisorption. Noticeably, it is generally accepted that physical adsorption is preceding stage of chemisorption of inhibitors on metal surface [68]. Therefore, in the adsorption process of NTBC on steel surface, the adsorption will be further discussed in detail as shown the following parts. 3.2. Polarization studies Potentiodynamic polarization curves for CRS in 0.5 M H2SO4 without and with various concentrations of NTBC at 25 °C are shown in Fig. 4. The presence of NTBC causes a prominent decrease in the corrosion rate i.e. shifts both anodic and cathodic curves to lower values of current densities. Namely, both cathodic and

0.11 0.1

60

2

W0  W W0  Wm

0 55 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

concentration of NTBC c (mM)

0.09 0.08

c/θ (mM)

100

Inhibition efficiency Ew (%)

22

-1

Corrosion rate v (g m h )

c 1 ¼ þc h K

12

where c is the concentration of inhibitor, K the adsorptive equilibrium constant and h is the surface coverage, calculated by the following equation [65]:

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

concentration of NTBC c (mM) Fig. 2. Relationship between corrosion rate (m) and inhibition efficiency (Ew) obtained by weight loss method with the concentration of NTBC (c) in 0.5 M H2SO4 at 25 °C (immersion time is 6 h).

Fig. 3. Langmuir isotherm adsorption mode of NTBC on the CRS surface in 0.5 M H2SO4 at 25 °C from weight loss measurement.

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300

-0.4

250

15

200

blank

10 5 0

2

-Zr (Ω cm )

E (V vs. SCE)

-0.3

20

0.01 mM 0.03 mM 0.05 mM 0.10 mM

2

blank 0.01 mM 0.03 mM 0.05 mM 0.10 mM

-Zr (Ω cm )

-0.2

-0.5

-0.6

0

150

5

10

15

20

Z r (Ω cm ) 2

100 50

-0.7 0 -0.8

-6

-5

-4

-3

-2

0

-1

50

100

150

200

250

300

2

-2

Zr (Ω cm )

log I (A cm ) Fig. 4. Potentiodynamic polarization curves for CRS in 0.5 M H2SO4 without and with different concentrations of NTBC at 25 °C (immersion time is 2 h).

Fig. 5. Nyquist plots of the corrosion of CRS in 0.5 M H2SO4 without and with different concentrations of NTBC at 25 °C (immersion time is 2 h).

anodic reactions of CRS electrode corrosion are drastically retarded by NTBC in 0.5 M H2SO4. Values of corrosion current densities (Icorr), corrosion potential (Ecorr), cathodic Tafel slope (bc), anodic Tafel slope (ba), and inhibition efficiency (Ep) are listed in Table 1. Clearly, Icorr decreases remarkably while Ep increases with the inhibitor concentration, and the maximum Ep is up to 95.7% at 0.10 mM NTBC. There is no definite trend in the shift of Ecorr in the presence of NTBC, therefore, NTBC can be arranged as a mixed-type inhibitor, and the inhibition action is caused by geometric blocking effect [69]. Namely, the inhibition action comes from the reduction of the reaction area on the surface of the corroding metal [69]. Tafel slopes of bc and ba do not change upon addition of NTBC, which indicates that adding NTBC does not change the reaction mechanism.

inhibitor is bigger than that in the absence of inhibitor (blank solution) and increases with the inhibitor concentration. This indicates that the impedance of inhibited substrate increases with NTBC concentration. The EIS results of high frequency capacitive loops are simulated by the equivalent circuit shown in Fig. 6 to pure electric models that could verify or rule out mechanistic models and enable the calculation of numerical values corresponding to the physical and/or chemical properties of the electrochemical system under investigation [74]. The circuit employed allows the identification of both solution resistance (Rs) and charge transfer resistance (Rt). It is worth mentioning that the double layer capacitance (Cdl) value is affected by imperfections of the surface, and that this effect is simulated via a constant phase element (CPE) [75]. The constant phase element is composed of a component Qdl and a coefficient a. The parameter a quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. So the capacitance is deduced from the following relation [76]:

3.3. Electrochemical impedance spectroscopy (EIS) Fig. 5 shows the Nyquist diagrams for CRS in 0.5 M H2SO4 at 25 °C without and with various concentrations of NTBC. These diagrams have similar shape throughout all tested concentrations, indicating that almost no change in the corrosion mechanism occurs due to the inhibitor addition [70]. The impedance spectra consist of large capacitive loop at high frequencies followed by a small inductive one at low frequency values. The high frequency capacitive loop is related to the charge transfer of the corrosion process and double layer behavior. On the other hand, the low frequency inductive loop may be attributed to the relaxation process obtained by adsorption species like FeSO4 [71] or inhibitor species [72] on the electrode surface. It might be also attributed to the re-dissolution of the passivated surface at low frequencies [72]. These high frequency loops are not perfect semicircles which can be attributed to the frequency dispersion as a result of the roughness and inhomogeneous of electrode surface [73]. Furthermore, the diameter of the capacitive loop in the presence of

C dl ¼ Q dl  ð2pfmax Þa1

ð8Þ

where fmax represents the frequency at which imaginary value reaches a maximum on the Nyquist plot. The electrochemical parameters of Rt, Cdl and ER are calculated by ZSimpWin software and presented in Table 2.

CPE Rs Rt Fig. 6. Equivalent circuit used to fit the capacitive loop at high frequencies without and with different concentrations of NTBC.

Table 1 Potentiodynamic polarization parameters for the corrosion of CRS in 0.5 M H2SO4 containing different concentrations of NTBC at 25 °C. c (mM) 0 0.01 0.03 0.05 0.10

Ecorr (mV vs. SCE)

Icorr (lA cm2)

bc (mV dec1)

ba (mV dec1)

Ep (%)

495 481 479 474 470

893.6 247.7 104.9 58.8 38.7

112 116 118 114 112

50 44 42 40 41

– 72.3 88.4 93.4 95.7

Table 2 EIS parameters for the corrosion of CRS in 0.5 M H2SO4 containing NTBC at 25 °C. c (mM)

Rt (X cm2)

Cdl (lF cm2)

ER (%)

0 0.01 0.03 0.05 0.10

18.9 67.0 114.3 258.4 286.3

202.1 99.2 101.3 84.6 65.0

– 71.8 83.5 92.7 93.4

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Inspection of Table 2 reveals that Rt values increases prominently while Cdl reduces with the concentration of NTBC. The decrease in Cdl comparing with that in blank solution (without inhibitor), which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the inhibitor molecules function by adsorption at the metal/solution interface [71]. ER increases with the concentration of NTBC, and the maximum ER reaches up to 93.4%, which further confirm NTBC exhibits very good inhibitive performance for CRS in 0.5 M H2SO4. Inhibition efficiencies obtained from weight loss (Ew), potentiodynamic polarization curves (Ep) and EIS (ER) are in good reasonably agreement. 3.4. Explanation for inhibition The adsorption process is affected by the chemical structure of inhibitor, the nature and charged surface of the metal and the distribution of charge over the whole inhibitor molecule. In general, owing to the complex nature of adsorption and inhibition of a given inhibitor, it is impossible for single adsorption mode between inhibitor and metal surface. In the present system, NTBC has many active sites facilitate the adsorption process. Thus, five types of adsorption may take place in the inhibiting phenomena involving NTBC molecules on steel surface as follows: (i) NTBC can be classified as an electrolyte, namely, the organic part (NTB2+) is the cation and the inorganic part (Cl) is the anion: 

NTBC ! NTB2þ þ 2Cl

ð9Þ

Owing to neutral N atoms in NTB2+, NTB2+ could further be protonated in the acid solution as following:

NTB2þ þxHþ $ ½NTBHx ðxþ2Þþ

ð10Þ

Thus, in aqueous acidic solutions, the NTBC exists as protonated cations of NTB2+ and [NTBHx](x+2)+. The charge of the metal surface can be determined from the value of Ecorr  Eq=0 (zero charge potential) [77]. The value of Ecorr obtained in 1.0 M H2SO4 is 499 mV vs. SCE. The Eq=0 of iron in H2SO4 is 550 mV vs. SCE [78]. Accordingly, chloride ions (inorganic part of BT) and sulfate ions (in H2SO4 solution) are firstly adsorbed on the surface and consequently, the steel surface becomes negatively charged [79]. Due to electrostatic attraction, NTB2+ and [NTBHx](x+2)+ may adsorb on the negatively charged metal surface. (ii) It should be noted that the molecular structure of NTB2+ and [NTBHx](x+2)+ remains unchanged with respect to NTBC, that is, the N atoms on the ring remaining strongly blocked, NTB2+ and [NTBHx](x+2)+ may be adsorbed on the metal surface via the chemisorption mechanism, involving the displacement of water molecules from the metal surface and the sharing electrons between the N atoms and Fe. (iii) NTB2+ and [NTBHx](x+2)+ molecules can be also adsorbed on the metal surface on the basis of donor–acceptor interactions between p-electrons of the heterocycles and vacant d-orbitals of Fe. The simultaneous adsorption of the two tetrazole rings induces greater adsorption of the inhibitor molecules onto the surface of CRS. The larger molecule area plays an important role in retarding the corrosion. (iv) Due to the good chelating action of tetrazole ring, some metal complexes of Fe2+-tetrazole compounds have been widely prepared [80,81]. Thus, NTB2+ or [NTBHx](x+2)+ may combine with freshly generated Fe2+ ions on steel surface forming metal inhibitor complexes:

Fe ! Fe2þ þ 2e NTB

2þ

þ Fe

2þ

$ ½NTB  Fe

ð11Þ 4þ

½NTBHx ðxþ2Þþ þ Fe2þ $ ½NTBHx  Feð4þxÞþ

ð12Þ ð13Þ

These complexes might adsorb onto steel surface by the van der Waals force to form a blocking barrier to keep CRS from corrosion. (v) A combination of the above. 4. Conclusions

(1) NTBC acts as a novel good inhibitor for the corrosion of CRS in 0.5 M H2SO4. Inhibition efficiency values increase with the inhibitor concentration. The adsorption of NTBC obeys Langmuir adsorption isotherm. (2) NTBC is a mixed-type inhibitor in 0.5 M H2SO4, and the inhibition action is caused by geometric blocking effect. EIS spectra exhibit large capacitive loop at high frequencies followed by a small inductive one at low frequency values. The addition of NTBC to 1.0 M HCl solutions enhances Rt values while reduces Cdl values. (3) The adsorption of NTBC involves electrostatic attraction, sharing electrons between the N atoms and Fe, donor–acceptor interactions between p-electrons of two tetrazole rings and vacant d-orbitals of Fe, adsorption of the complexes of [NTB-Fe]4+ and [NTBH-Fe](x+4)+.

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