Tryptamine as a green iron corrosion inhibitor in 0.5 M deaerated sulphuric acid

Corrosion Science 46 (2004) 387–403 www.elsevier.com/locate/corsci

Tryptamine as a green iron corrosion inhibitor in 0.5 M deaerated sulphuric acid G. Moretti *, F. Guidi, G. Grion Department of Chemistry, University of Venice, D.D.S. Marta 2137, Venice I-30123, Italy Received 30 May 2002; accepted 25 June 2003

Abstract The inhibition effects of tryptamine (TA) on the corrosion behaviour of ARMCO iron in 0.5 M deaerated H2 SO4 (in the 25–55
C temperature range) was studied in both short and long time tests (1, 24 and 72 h) by means of potentiodynamic curves (PCM) and electrochemical impedance spectroscopy (EIS). TA was found to be an effective ARMCO iron inhibitor, even at 55
C and 72 h, but only at 10 mM. At this concentration the inhibition percentages (IP%), calculated by PCM and EIS, ranged from 90% to 99% and did not diminish over time and as the temperature increased. TA adsorption followed Bockris–Swinkels
isotherm (x ¼ 1). The thermodynamic data indicated that, in the more concentrated solutions, TA also chemisorbed on the iron surface.  2003 Elsevier Ltd. All rights reserved. Keywords: Tryptamine; Corrosion green inhibitors; ARMCO iron; Deaerated sulphuric acid; Adsorption isotherm; Electrochemical impedance spectroscopy

1. Introduction Among the numerous organic compounds that have been tested and are applied industrially as corrosion inhibitors, those that are non-toxic are now far more strategic than in the recent past. In the past two decades, the research in the field of ‘‘green’’ corrosion inhibitors has been addressed toward the goal of using cheap, effective molecules at low or ‘‘zero’’ environmental impact.

*

Corresponding author. Tel.: +39-41-234-8559/257-8559; fax: +39-41-234-8517/257-8517. E-mail addresses: [email protected] (G. Moretti), [email protected] (F. Guidi).

0010-938X/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00150-1

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These compounds include certain amino acids and derivatives, which have been tested for various metals, such as Ni [1], Co [2], iron and mild steel [3–5] or copper [6] corrosion in H2 SO4 or HCl. Tryptamine (TA), a derivative of the tryptophan, is relatively cheap, non-toxic and easy to produce in purity greater than 99%. In this paper TA was tested as an ARMCO iron corrosion inhibitor, due to the corrosion inhibition efficiency showed by the indole and some of its derivatives [7–11], and by the tryptophan [6]. AC and DC electrochemical techniques were used throughout this investigation. The EIS was used to better monitor the behaviour of TA over time, while the DC technique (PCM) was used to evaluate TA inhibition efficiency and its effect on the cathodic or anodic reactions of the corrosion process. By comparing EIS with the PCM data, it was found that TA inhibits ARMCO iron corrosion in deaerated 0.5 M sulphuric acid at 25 and 55
C in both short and long time tests (72 h). The study of the adsorption mechanism in the 25–55
C indicated that TA also chemisorbed on the iron surface and probably formed complexes with ferrous ions adsorbed on iron surface. 2. Experimental The methodologies used to investigate ARMCO iron corrosion inhibition of TA in deaerated 0.5 M H2 SO4 included PCM and EIS techniques. Thermostated doublewalled (1 l) Pyrex glass cells (Green cells) were used for the electrochemical tests. ARMCO iron disks having a diameter of 11.3 mm were cut off from the same plate, mounted on a Teflon holder (exposed area of 0.5 cm2 ) and acted as the working electrode. Before use, the specimens were abraded with 1200 grade emery paper, washed with distilled water, degreased with acetone and dried with an air jet. The working electrode, a reference saturated calomel electrode (SCE) (inside a Luggin
s capillary probe), and a platinum counter electrode were placed inside the electrochemical cell. Potentials reported in this paper refer to SCE. TA, whose structure is reported below, was dissolved in 0.5 M H2 SO4 (800 ml) at two concentrations chosen after preliminary tests among those more efficient from the corrosion point of view (10
3 and 10
2 M). Before each experiment the solution, with the electrodes inside, was deaerated by bubbling high purity nitrogen for 2 h. CH2

CH2

NH2

N H

2.1. Potentiodynamic curves method (PCM) The potentiodynamic curves were recorded in the absence (‘‘blank’’ test solution) and in the presence of the various inhibitor concentrations after a 1–72 h immersion

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time on different Armco iron specimens with an Electrochemical Interface SOLARTRON Mod. 1287 interfaced with a PC. The experimental part of the potentiodynamic curves method (PCM) has been reported elsewhere [10]. Each test was repeated three times to verify its reproducibility. The average values of the obtained electrochemical data reported in Table 1 include: Cathodic Tafel slope, bC (mV/dec); free corrosion potential, Ecorr (mVsce ); anodic Tafel slope, bA (mV/dec); corrosion current density, icorr (lA/cm2 ); inhibition percentage, IP (%), corresponding to: IP ð%Þ ¼ ½ðiu
ii Þ  100=iu ¼ h  100, where iu and ii are the icorr in the absence and presence of inhibitor, respectively, and h the surface coverage degree. All the icorr values were determined by: (i) extrapolation to Ecorr of the anodic and cathodic curve branch in the PCM, and (ii) by applying the Stern–Geary relationship in the EIS using the bC and bA Tafel slopes obtained via PCM in the same experimental conditions (see below).

Table 1 Electrochemical data obtained from the potentiodynamic curves carried out on ARMCO iron in 25–55
C temperature range, in deaerated 0.5 M H2 SO4 in the presence of TA (1, 24 and 72 h tests) icorr (lA/cm2 )

Time (h)

T (
C)

[TA] (M)

bC (mV/dec)

Ecorr (mVsce )

1

25

0 10
3 10
2

)129 )113 )67

)525 )505 )512

46 42 38

336 273 82

0.188 0.756

0 10
3 10
2

)130 )111 )105

)526 )524 )496

111 96 40

930 872 724

0.062 0.221

0 10
3 10
2

)180 )165 )140

)501 )499 )525

40 50 44

402 225 41

0.440 0.898

0 10
3 10
2

)210 )163 )111

)479 )523 )544

106 99 108

870 574 127

0.340 0.854

0 10
3 3 · 10
3 7 · 10
3 10
2

)108 )105 )110 )145 )164

)512 )522 )521 )518 )525

36 48 44 41 28

656 61 55 23 16

0.907 0.912 0.954 0.976

0 10
3 3 · 10
3 7 · 10
3 10
2

)128 )183 )152 )120 )118

)485 )487 )489 )488 )488

54 60 72 76 80

6241 161 130 80 22

0.974 0.981 0.982 0.996

55

24

25

55

72

25

55

bA (mV/dec)

h

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2.2. EIS measurements EIS measurements were usually performed at the iron electrode corrosion potential in the potentiostatic mode, by applying a 5 mV amplitude sinusoidal wave perturbation to the corroding system. The frequency values usually ranged from 104 to 10
2 Hz, with 10 frequency values per decade. At the end of each EIS measurement the potentiostatic polarisation was interrupted and the iron electrode was left to corrode freely until the next scheduled EIS measurement (the free corrosion potential, Ecorr , was determined in the meantime). Impedance spectra were collected after 1, 4, 8, 24, 48 and 72 h. In addition, EIS measurements were performed at a cathodic (E ¼ Ecorr
100 mV) and anodic (E ¼ Ecorr þ 100 mV) potential in the same conditions, as described above. The spectra were recorded with an Electrochemical Interface SOLARTRON Mod. 1287 and a Frequency Response Analyser SOLARTRON Mod. 1260. Experimental data was interpolated in order to obtain the charge transfer resistance (Rct ), the double-layer capacity (Cdl ), the adsorbed layer capacity (constant phase element, CPE) and the adsorbed layer resistance (RL ), and to evaluate the best equivalent circuit approximating the impedance data.

3. Results and discussion 3.1. PCM measurements Fig. 1 shows some typical potentiodynamic polarisation curves carried out at 25
C after 1 h of immersion time in deaerated 0.5 M H2 SO4 and in the presence and absence of TA. By using PCM data, meaningful parameters were extracted and listed in Table 1.

100000

i (µA/cm2)

10000 1000 100 10

TA 0 M TA 1 mM TA 10 mM

1 -850

-750

-650

-550

-450

-350

E (mVsce) Fig. 1. Typical potentiodynamic curves for ARMCO iron in deaerated 0.5 M H2 SO4 , in the presence of TA at 25
C (1 h).

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Table 2 h corresponding to various TA concentrations obtained with the potentiodynamic method (72 h tests) T (
C) 25 35 45 55

Inhibitor concentration (M) 10
3

3 · 10
3

7 · 10
3

10
2

0.738 0.854 0.612 0.974

0.912 0.901 0.954 0.981

0.954 0.962 0.980 0.982

0.976 0.981 0.995 0.996

3.1.1. Concentration and temperature effects As said before, preliminary tests among those more efficient from the corrosion point of view demonstrated the best inhibition efficiency of the most concentrated solutions. For simplicity, only the data obtained in 10
3 –10
2 M solutions is reported here. In Table 1 all the tests revealed that, as the TA concentration increased, the inhibition efficiency increased. This fact was also confirmed at 72 h of immersion time, even with some intermediate concentrations, as reported below (see h reported in Table 2). In the 25–55
C temperature range, the IP obtained in the 10
3 M are respectively lower than those obtained in 10
2 M solutions. Further, the icorr values were, respectively, of the same magnitude order even as temperature increased: this could support the hypothesis that TA adsorption is more a chemisorption than a physisorption [12]. 3.1.2. Immersion time effect Data reported in Table 1 showed that IP increased over the time, especially at 10
2 M TA concentration. At 25
C, after 1 h of immersion time, TA inhibited the iron corrosion from about 19 until 76%. After 24 h TA was observed to act as a better corrosion inhibitor. In fact, the polarisation curves obtained in the presence of TA were below those obtained in non-inhibited solution and icorr decreased again as the TA concentration increased, while the Ecorr shifted toward more cathodic values. These results, as well as the expected concentration effect, suggest that, in these experimental conditions, the TA affected the cathodic rather than the anodic part of the iron corrosion reaction. Even after 72 h of immersion time IP reached a value of 90–98%. The best results were obtained at the highest immersion times: the maximum IP was observed at both 25 and 55
C after 72 h of immersion (about 98% and 100% respectively). 3.1.3. Corrosion mechanism As far as the corrosion reaction mechanism is concerned, we can note, by analysing the shape of the potentiodynamic curves, that TA seems to affect both the cathodic and the anodic corrosion reaction, especially at the highest concentration, temperature and immersion time, where the inhibitor shows its maximum efficiency

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[13]. To simplify the study of the corrosion reaction, particularly in the long time tests, we chose a well know system (ARMCO iron in deaerated 0.5 M sulphuric acid), which we had already investigated [7,8]. By analysing the data reported in Table 1, one can see that, in the presence of TA, at 25
C, the cathodic Tafel slopes, bC , are, on the average, around 120 mV/dec ()127 to )123 mV/dec for the 10
3 –10
2 M TA solutions) but, at higher temperature, they are different from those reported in the literature, especially at the highest immersion times [13–15]. This can agree with the previous hypothesis that TA, in particular in the best inhibition efficiency conditions, influenced the cathodic part of the iron corrosion reaction. At low temperatures the anodic Tafel slopes, bA , generally show a similar trend: at 25
C bA average value ranges around the literature data of 40 mV/dec (46 and 37 respectively), but when the temperature and the immersion time increased, its average value almost doubled (85 and 76 mV/dec respectively) [13]. By analysing the free corrosion potential behaviour, one can see that, by increasing the immersion time, Ecorr generally shifted toward more cathodic values (Table 1) especially at 25
C, while at highest temperature the trend was not so evident. Based on TA behaviour in the cathodic and anodic potential range, we can conclude that TA affected mainly the cathodic rather than the anodic process. This conclusion is also supported by the EIS data reported below. 3.1.4. Adsorption isotherm Values of the degree of surface coverage (h) corresponding to different TA concentrations in the 25–55
C temperature range after 72 h of immersion were used to determine which isotherm best described the adsorption process at this immersion time (Table 2). In the 25–55
C temperature range, the best correlation between the experimental results and the isotherm functions was obtained using Bockris–Swinkels
isotherm (x ¼ 1) (Fig. 2). Table 3 reports the data obtained from Bockris–Swinkels
isotherm (x ¼ 1) for ARMCO iron in deaerated 0.5 M H2 SO4 obtained in 25–55
C temperature range (72 h tests). This table also contains the values of DG
ads for the adsorption reaction of TA molecules. The isotherm is (1): 

h x ð1
hÞ

½hþxð1
hÞx
1  xx

¼ KC

ð1Þ

that is: f ðh; xÞ ¼ KC. By plotting ln f ðh; xÞ vs ln C one can obtain a straight line whose intercept is ln K and slope is 1. In Eq. (1) h is the surface degree coverage, ‘‘x’’ is the number of water molecules substituted by one molecule of organic absorbate (independent of coverage or charge of the electrode [16]), C is the TA concentration and K is the constant of the adsorption process (2):

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393

ln f(θ,x)

4 3 2 25˚C

1

35˚C 45˚C

0

55˚C

-1 -8

-7

-6

-5

ln (C) Fig. 2. Bockris–Swinkels (x ¼ 1) TA adsorption isotherms on ARMCO iron in deaerated 0.5 M H2 SO4 obtained in the 25–55
C temperature range (72 h tests).

Table 3 Data from Bockris–Swinkels
isotherm (x ¼ 1) for ARMCO iron in deaerated 0.5 M H2 SO4 obtained in the 25–55
C temperature range (72 h tests) T (
C)

Slope

ln K

DG
ads (kJ/mol)

R2

25 35 45 55

1.00 1.03 0.94 1.03

7.662 7.436 7.589 9.060

)29.0 )29.4 )30.7 )35.7

0.999 0.997 0.995 0.998

OrgðsolÞ þ xH2 OðadsÞ $ OrgðadsÞ þ xH2 OðsolÞ

ð2Þ

The adsorption process is correlated to the adsorption free energy DG
ads by (3): K¼

1
ðDG
ads =RT Þ e 55:5

ð3Þ

The correlation coefficient (R2 ) was used to choose the isotherm that best fit the experimental data (Table 3). It is generally accepted that organic molecules inhibit corrosion by adsorption at the metal–solution interface and that the adsorption depends on the molecule
s chemical structure, the nature of metal surface, the solution
s chemical composition, the temperature and the electrochemical potential at the metal–solution interface [16]. The first step of the TA adsorption involved the water molecules adsorbed at the metal surface. Due to the well accepted model that an organic molecule [OrgðsolÞ ] adsorbs on a metal–solution interface by replacing one or more adsorbed water molecules [H2 OðadsÞ ] in the aqueous solution [17] [reaction (2)], after 72 h of immersion time, TA adsorbed on the metal surface by displacing one water molecule.

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Because the ‘‘water molecule area’’ is smaller in comparison to that of TA, one can conclude that TA adsorbs on the iron surface maintaining a certain contact-angle with the metal surface. This, however, is not sufficient to demonstrate the inhibition efficiency TA showed. Due to the fact that IP increased with temperature at 72 h, we can hypothesise that TA, a molecule with a high electronic density, chemisorbs on the iron surface in these experimental conditions [18], seen in the DG
ads values (ranging between )29 and )35 kJ/mol from 25 to 55
C) and the behaviour of other similar organic compounds [10,11]. Values of DG
ads around )20 kJ/mol or lower are consistent with the electrostatic interaction between organic charged molecules and the charged metal (physisorption); those around )40 kJ/mol or higher involve charge sharing or transfer from the organic molecules to the metal surface to form a co-ordinate type of bond (chemisorption) [19,20]. Chemisorption can be favoured by the TA planarity and the positive charge on the metal at the corrosion potential, because of the zero charge potential Ezc of iron in 0.05 M H2 SO4 is )570 mVsce [21]. However, the TA polarity (TA has a l ranging from 9.32 to 10.73 debye [22]), the electrostatic interaction between the electric field due to the metal charge, and the electric moment of TA probably contribute to the adsorption, particularly at lower temperatures. On the other hand, the adsorption phenomenon of an organic molecule is not considered only as a physical or as chemical adsorption phenomenon. A wide spectrum of conditions, ranging from the dominance of chemisorption or electrostatic effects arise from other adsorption experimental data (such as variations in metal charge, electric moment, and the chemical potential of the organic molecules) [9]. In this case, the DG
ads values, in the 25–55
C temperature range, also seem to indicate a chemisorption. As seen in Table 3, the values of DG
ads are all negative in the 25–55
C temperature range, thus indicating the spontaneity of the adsorption reaction of TA on the iron surface. Fig. 3 shows the effect of temperature on TA
DG
ads on pure iron in deaerated 0.5 M H2 SO4 .

∆ G°ads (kJ/mol)

-25

-30

-35

-40 290

300

310

320

330

340

T, °C Fig. 3. Effect of temperature on TA DG
ads on pure iron in deaerated 0.5 M H2 SO4 (72 h tests).

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One can see that the DG
ads value, ranging from about )29 to )31 kJ/mol from 25 to 45
C, increased at 55
C until about )36 kJ/mol. This means that, at this temperature, the reaction (2) is more favoured [18]. The value of the equilibrium constant of the adsorption reaction (2), almost constant in the 25–45
C temperature range, increased at 55
C (from about 2000 to 8600) after 72 h of immersion time. This can support the hypothesis that, at the highest temperatures and TA concentration used, complexes between ferrous ions adsorbed on iron surface (Feþþ ðadsÞ ) and TA molecules are formed, as indicated in Eq. (4). There is a different corrosion mechanism of the iron in the presence of the inhibitor. These complexes should have lowered the Feþþ concentration making the successive oxidation to Fe3þ (5) negligible. þþ Feþþ ðadsÞ þ n  TA $ ½Fe  TAn ðadsÞ

ð4Þ

3þ
Feþþ ðadsÞ $ FeðsolÞ þ 1e

ð5Þ

A different iron corrosion mechanism, a perturbation of reaction (5), and an increase of the observed equilibrium constant followed, as has been observed for copper with other similar compounds [6,11]. Due to the fact that TA interacts with the metal surface to inhibit the corrosion reaction, we can hypothesise that TA preferentially adsorbs with the ammidic extremity. TA could form, for example, a layer chemically bound on the iron surface, such as via [Fe Æ TAn ]þþ ðadsÞ complexes, while another layer, linked through the indole rings, lays outside associated with solvent molecules. It is known, for example, that a TA molecule, chemically immobilised on a gold electrode surface, and another ‘‘free’’ TA molecule are able to link to each other along the plane formed by the respective indole-rings [23]. Even if this cannot simply be transferred to our case, we could hypothesise that TA adsorbed on the metal surface with the amidic molecule part. In this case, even if the indole rings of TA molecules did not interact directly with the iron surface, they could act as a barrier against the solution bulk, given the nonremote possibility of linking another TA molecule through the indole-rings and the solvation of the organic molecules. It is also true that TA in acidic medium is protonated: TA pKa actually ranges from 9.50 [24] to 10.16 [25]. Notwithstanding this, due the high TA electronic density, it is predictable that it behaved like molecules similar to the tryptophan. It is known, in fact, that tryptophan can form complexes with Fe(II), for example, in 3 M-perchlorate solutions at 25
C [26] or, more insoluble, with Cu(I) [11]. On the other hand, both indole [10], even if at lower temperatures, and tryptophan [6] seem to chemisorb on the metal surfaces in the same experimental conditions. 3.2. EIS measurements EIS measurements are particularly useful in long time tests because they do not perturb the system dramatically and it is possible to follow the evolution of the

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G. Moretti et al. / Corrosion Science 46 (2004) 387–403

inhibitor–metal system over time. In the experimental conditions used (deaerated 0.5 M H2 SO4 ), the tests were carried out until 72 h, an immersion time that guarantees sufficient evaluation of TA inhibition efficiency. For the sake of clarity, Table 4 only reports the results obtained for ARMCO iron in the 10
3 and 10
2 M solutions of TA at 25 and 55
C. The icorr , calculated from the EIS measurements, were obtained by applying the well-known Stern–Geary relationship (6), even though its simple application in inhibited systems may be unreliable, due to the doubts concerning the right values of the anodic (bA ) and cathodic (bC ) Tafel slopes and the right B value [27–29].     bA bC 1 1 icorr ¼ ¼B ð6Þ Rp 2:303ðbA þ bC Þ Rp

Table 4 Data obtained from EIS spectra for ARMCO iron with TA (10
3 and 10
2 M) in deaerated 0.5 M H2 SO4 at 25 and 55
C (1–72 h) TA (M)

T (
C)

Time (h)

0

25

1 24 48 72

34 17 21 27

687 571 414 523

1 24 48 72

34 8 7 4

1853 4189 19 878 13 001

55

10
3

25

55a

10
2

25

55

a

RL (X cm2 )

1 24 48 72

69 211 73 89

Time

Rct a

CPE (lF/cm2 )

24 90 151 30

Rct (X cm2 )

187 169 483 42

Cdl (lF/cm2 )

1 74 75 74

CPEa

a

icorr (lA/cm2 )

IP (%)

1636 5120 18 460 21 548

268

83.6

3912

73.8 IP

icorr

a

1 24

28 8a

48 72

a

4 3a

1 24 48 72

120 196 280 333

198 124 208 130

83 30 3255 146

213 162 159 145

1 24 8 72

7 41 35 31

172 88 63 142

34 48 117 64

496 121 52 24

443 1179a

5245

71.6

13 898

35.5

28

98.3

123

97.6

1543

91.6

87

99.6

a

2926 3646a

The data obtained in 10
3 M TA solution at 55
C were fitted with the equivalent circuit of Fig. 5.

G. Moretti et al. / Corrosion Science 46 (2004) 387–403

397

Notwithstanding this restriction, B was calculated from the bA and bC of the corresponding polarisation curves carried out under the same experimental conditions. The analysis of the EIS data obtained in uninhibited sulphuric solutions revealed that the Nyquist plots, in the capacitive quadrant, were similar to a semicircle, in some cases with a small inductive behaviour at low frequencies. This agrees with what has been reported by other authors [30]. By adding TA, the spectrum become more complicated: generally, a second time constant appeared and a depressed semicircle in the high frequency part of the spectrum was formed. The equivalent circuit that best interpreted the interface phenomena in the presence of TA is reported in Fig. 4. In this circuit one can distinguish a high frequency (HF) part––CPE and RL , representing the adsorbed inhibitor film, and a low frequency (LF) part associated with the faradic process occurring on the Fe surface (under adsorbed inhibitor layer) through layer defects and pores. The CPE is used in place of a capacitor to compensate the non-homogeneity of the surface [31]. This model well represented the real situation when 10
2 M TA concentration and low and high temperatures were used; that is, when the iron resulted covered by inhibitor adsorbed on the oxidised metal surface. In less concentrated solutions (TA 10
3 M; 55
C––Table 4), the TA presence was probably not sufficient to form a time-resistant layer. The experimental data in the Nyquist diagram only showed one ‘‘depressed’’ semicircle, which can be better interpreted by the circuit of Fig. 5. Fig. 6 reports an example of the relative fit obtained with the circuit model of Fig. 4 at 24 h. Fig. 7 shows the experimental data in the Bode format obtained for the ARMCO iron in the presence of 10
3 M TA solution (1–72 h; 25
C). One can observe that the proposed model is well representative of the phenomena which may occur in the investigated system, both in the HF and in the LF part of the spectra.

Fig. 4. Electric circuit model used in fitting the experimental data. Note the constant phase element (CPE) that took into account the non-homogeneity of the system inhibitor––ARMCO iron surface (Rs ¼ solution resistance; RL ¼ layer resistance; Cdl ¼ double layer capacity; Rct ¼ charge transfer resistance).

Fig. 5. Electric circuit model used in fitting the experimental data in the 10
3 M TA solution at 55
C (Rs ¼ solution resistance; CPE ¼ constant phase element; Rp ¼ polarisation resistance).

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G. Moretti et al. / Corrosion Science 46 (2004) 387–403

Fig. 6. The experimental data in the Bode format obtained for the ARMCO iron in the presence of 10
3 M TA at 24 h and 25
C, and the relative fit obtained with the circuit model of Fig. 4.

Fig. 7. The experimental data in the Bode format obtained for the ARMCO iron in the presence of 10
3 M TA solution (1–72 h; 25
C).

The data obtained with the 10
3 M TA solutions at 55
C revealed that this concentration did not guarantee inhibition efficiency at the highest immersion time. We can note (Fig. 7) that, if a second layer effectively formed on the iron surface at the beginning, after 48 and, above all, at 72 h, the corrosion reaction was able to cause the failure in the homogeneity of adsorbed TA film. This is also confirmed by the fact that the CPE value increased over time.

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399

In the long time tests (48 h and more) with 10
3 M TA solutions, we generally obtained spectra like those reported, for example, in Fig. 7. These spectra are typical of ‘‘porous’’ systems in which a ‘‘pore’’ impedance––which depends upon frequency––may be defined [32]. They are due to non-homogeneity in the metaladsorbed inhibitor interface or to the effect of the porosity (for example corrosion products, or, as in our case, adsorbed species on a metal surface, etc.) on the impedance locus for a parallel resistive–capacitive system [32]. As this shape appeared only after 48 h or more (until then it looked like a semicircle), we can conclude that some of ‘‘porous’’ was forming from this immersion time forward. This could only be due to what was happening between the TA, the solvent, the species dissolved in the solution and the metal surface. The adsorption of TA, which, as we have seen, increased over time, probably caused the formation of superficial complexes linked with other TA or solvent molecules and iron ions. Thus, if ZðxÞ was in the form of a semicircle before 24–48 h, after, the phase angle h for the pore system varied from 45
at infinite frequency to 0
at zero frequency [32]. In the short time tests at 25
C the inhibitor behaved practically in the same way in both the diluted and more concentrated solutions. Initially, within about 24 h, the RL increased until about 200 X cm2 . After this period of time it decreased to a stable point (Fig. 8a and b). At 55
C RL values went down until about 1/10 for the TA 10
3 M and about 1/3 for the TA 10
2 M solutions, with the difference that RL increased almost continuously over time in the more concentrated solutions. At the same temperature, the IP increased as the inhibitor concentration increased; this result follows the same general trend of the icorr obtained with the PCM technique. -150

-50 0 50

0

100

200 2

-5

(c)

0

10

20 2

Z' (ohm.cm )

-50 0 50

0

100

200

300 2

Z' (ohm.cm ) -50

1h 2h

2

Z'' (ohm.cm )

1h 2h 4h 24 h

2

Z'' (ohm.cm )

-15

5

1h 2h 4h 24 h

-100

(b)

Z' (ohm.cm )

(a)

2

1h 2h 4h 24 h

Z'' (ohm.cm )

2

Z'' (ohm.cm )

-100

30 (d)

-30 -10 10 0

4h 24 h

20

40

60

80 100 2

Z' (ohm.cm )

Fig. 8. EIS spectra of ARMCO iron in deaerated 0.5 M H2 SO4 (until 24 h): (a) at 25
C; 1 mM of TA; (b) at 25
C; 10 mM of TA; (c) at 55
C; 1 mM of TA; (d) at 55
C; 10 mM of TA.

400

G. Moretti et al. / Corrosion Science 46 (2004) 387–403

-120

2

Z'' (ohm.cm )

-200 2

Z'' (ohm.cm )

-150 -100

H2SO4 0.5M TA 1 mM TA 10 mM

-50 0

-40

H2SO4 0.5M TA 1 mM TA 10 mM

0 0

50

100

150

0

200 2

(a)

Z' (ohm.cm )

50

100

150

(b)

200

250

2

Z' (ohm.cm ) -30

-300 2

Z'' (ohm.cm )

2

Z'' (ohm.cm )

-80

-200 H 2 SO4 TA 1 mM TA 10 mM

-100 0

H 2 SO4 TA 1 mM TA 10 mM

-10 0

0 (c)

-20

200

400

600 2

Z' (ohm.cm )

0 (d)

20

40

60

2

80

Z' (ohm.cm )

Fig. 9. EIS spectra of ARMCO iron in deaerated 0.5 M H2 SO4 : (a) at 25
C (24 h); (b) at 55
C (24 h); (c) at 25
C (72 h); (d) at 55
C (72 h).

One can observe that in the short time tests (until 24 h) at the same temperature the more concentrated TA solutions reached the best IP value (about 98 at 25
C and 92 at 55
C). The inductive loops obtained at low frequencies generally depends on the polarisation conditions, e.g. when, for example, the amplitude of the potential perturbation is changed (here always ±5 mV) [13]. By increasing the immersion time (see, for example, Fig. 9, which reports the time behaviour––discussed below), these inductive loops disappeared. The comparison between the EIS data obtained at 24 and 72 h (long time tests–– TA 10
3 and 10
2 M) is reported in Fig. 9. The EIS data, as well, reasonably confirmed what resulted by the previous PCM data and the consequent discussion on the adsorption mechanism. Due to the fact that the icorr , even in the EIS experiments, decreased as well as the temperature and the immersion time (Table 4), it was evident that TA chemisorbed on the iron surface, but only in the case of the most concentrated solutions [17]. The EIS measurements were also useful in determining the anodic, cathodic or mixed character of the TA. This is why some experiments were carried out at Ecorr , (Ecorr þ 100 mV) and (Ecorr
100 mV). After 24 h of immersion tests in 10
3 M TA solutions (at 25
C), the Rct values obtained at Ecorr decreased both in more anodic (Ecorr þ 100 mV) as well as more cathodic conditions (Ecorr
100 mV). This decrease was more noticeable in the anodic range. This trend was evidenced in the more concentrated TA solutions, where the Rct obtained in the cathodic range became about 17 times lower (from about 240

G. Moretti et al. / Corrosion Science 46 (2004) 387–403

401

to 14 mV), while that in the anodic decreased about 60 times (from about 240 to 4 mV) (Table 5). It is clear that, in these experimental conditions, the inhibitor is more efficient cathodically than anodically because the resistance to the charge transfer is higher in the cathodic range. This trend, even if less evident, was confirmed at 55
C. Table 5 Some Rct values obtained for ARMCO iron from some EIS experiments (deaerated 0.5 M H2 SO4 ––10
3 or 10
2 M of TA both at low and high temperature, at 2 or 4 and 24 h) Time (h)

T (
C)

TA (M)

Ecorr
100 mV

Ecorr

2 or 4

25

0 10
3 10
2

9.5 74.2 24.6

49.0 152.7 307.2

6.2 5.4 5.9

55

0 10
3 10
2

27.1 18.3 11.5

32.2 25.7 16.3

8.8 6.5 3.9

25

0 10
3 10
2

32.4 95.6 14.1

39.0 133.1 239.3

12.7 25.6 4.0

55

0 10
3 10
2

4.2 4.3 3.5

4.4 10.0 12.8

3.9 5.2 2.2

24

Ecorr þ 100 mV

I.P., %

100 25˚C

75 50 1 mM

25

10 mM

0

0

24

48

(a)

72

96

time, h

I.P., %

100 55˚C

75 50 1 mM

25 0

10 mM

0 (b)

24

48

72

96

time, h

Fig. 10. The trend of the IP (%) over time obtained from the EIS spectra carried out in deaerated 0.5 M H2 SO4 in the presence of 1 or 10 mM of TA: (a) at 25
C; (b) at 55
C.

402

G. Moretti et al. / Corrosion Science 46 (2004) 387–403

The difference in the behaviour between the two concentrations used is more evident by examining the IP trend in time (Fig. 10). One can note the best IP of the more concentrated solutions both at 25 and 55
C even at 72 h, while the efficiency of the TA 10
3 M solution, especially at 55
C, failed after 24 h. This was also confirmed by observing the sample surface after each test: at 55
C in 10
2 TA solutions the iron surface appeared light grey without apparent signs of incipient corrosion.

4. Conclusions • Tryptamine, a cheap molecule with a very low environmental impact, was found effective in inhibiting ARMCO iron corrosion in deaerated 0.5 M sulphuric acid in the 25–55
C temperature range. • Results obtained from potentiodynamic polarisation and Electrochemical Impedance Spectroscopy indicated that TA in the more concentrated solution and at 55
C, also chemisorbs. • TA adsorbed on iron surface following the Bockris–Swinkels
isotherm (x ¼ 1): the isotherm study indicated that this inhibitor probably forms some iron-complexes on the metal surface. • EIS long-time tests (72 h and more) demonstrated that only the 10
2 M TA solution attained the maximum protection efficiency both 25 and 55
C: IP ranged from about 95% to 98%.

Acknowledgements The authors would like to acknowledge Dr. G. Gajo for his experimental work and Prof. A. Frignani for his help in the discussion. This research has been supported by the Italian Ministry of University and Scientific and Technological Research (MURST) and Veneto Innovazione S.p.A.e.

References [1] [2] [3] [4] [5] [6] [7] [8]

A.A. Aksut, S. Bilgic, Corros. Sci. 33 (3) (1992) 379–387. S. Bilgic, A.A. Aksut, Br. Corros. J. 28 (1) (1993) 59–62. M.S. Abdel-Aal, M.S. Morad, Z.A. Ahmed, Ann. Univ. Ferrara Sez. 5 (Suppl.) (1995) 343–352. A. Oni, Corros. Prevent. Control 39 (1992) 128. V. Hluchan, B.L. Wheeler, N. Hackerman, Werkstoffe und Korrosion 39 (1988) 512–517. G. Moretti, F. Guidi, Corros. Sci. 44/9 (2002) 1995–2011. G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Werkstoffe und Korrosion 45 (12) (1994) 5–11. G. Moretti, G. Quartarone, A. Tassan, A. Zingales, in: M.G.S. Ferreira, A.M.P. Sim~ oes (Eds.), Electrochemical Methods in Corrosion Research V, Materials Science Forum, Part 1, vol. 192–194, Trans Tech Publications, 1995, pp. 363–378. [9] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, British Corros. J. 31 (1) (1996) 49–54.

G. Moretti et al. / Corrosion Science 46 (2004) 387–403

403

[10] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Electrochim. Acta 41 (13) (1996) 1971–1980. [11] G. Quartarone, G. Moretti, G. Capobianco, A. Zingales, Corrosion 54 (8) (1998) 606–618. [12] G. Trabanelli, in: F. Mansfeld (Ed.), Corrosion Inhibitors, Chemical Industries: Corrosion Mechanism, Chapter 3, vol. 28, 1987, pp. 119–163. [13] A. Frignani, M. Tassinari, Proceedings of the 7th European Symposium on Corrosion Inhibitors, Ann. Univ. Ferrara, N.S. Sez. 5 (Suppl. 9) (1990) 895–909. [14] H. Kaesche, N. Hackerman, J. Electrochem. Soc. 105 (1958) 191. [15] F.M. Donahue, A.A. Akiyama, K. Nobe, J. Electrochem. Soc. 114 (1967) 1006. [16] O.L. Riggs Jr., in: C.C. Nathan (Ed.), Corrosion Inhibitors, NACE, Houston TX, 1973, p. 7. [17] J.O
M. Bockris, D.A.J. Swinkels, J. Electrochem. Soc. 111 (1964) 736. [18] E. Blongren, J.O
M. Bockris, J. Phys. Chem. 63 (1959) 1475. [19] F.M. Donahue, K. Nobe, J. Electrochem. Soc. 112 (1965) 886. [20] E. Kamis, F. Bellucci, R.M. Latanision, E.S.H. El-Ashry, Corrosion 47 (1991) 677. [21] R.S. Perkins, T.N. Andersen, in: J.O
M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Chapter 3, vol. 5, London, Butterworths, 1969, p. 255. [22] M. Kumbar, D.V. Siva Sankar, J. Am. Chem. Soc. 97 (1975) 7411–7416. [23] A.K. Gaigalas, V. Reipa, G. Niaura, J. Colloid Interface Sci. 203 (1998) 299–310. [24] R.E. Galian, A.V. Veglia, R.H. de Rossi, Analyst 123 (1998) 1587–1591. [25] K.J. Box, J.E.A. Comer, A. Hill, K.J. Tam, L. Trowbridge, High throughput physicochemical profiling Part 1: rapid ionization constant (pKa ) determination, Technical Report (2000) by SIRIUS Analytical Instruments Ltd. [26] D.R. Williams, J. Chem. Soc. A (1970) 1550–1555. [27] R.L. Le Roy, Corrosion 31 (1975) 173. [28] F. Mansfeld, The polarization resistance technique for measuring corrosion currents, in: Advances in Corrosion Science and Technology, vol. 6, Plenum Press, 1976, p. 163. [29] C. Gabrielli, Identification of Electrochemical Processes by Frequency Response Analysis, Solartron Monograph (1980) 65. [30] A. Frignani, C. Monticelli, G. Trabanelli, Proceedings of the 9th European Symposium on Corrosion Inhibitors (9SEIC), Ann. Univ. Ferrara, N.S., Sez. 5 (Suppl. n.11) (2000) 749–759. [31] S. Havriliak, S.J. Havriliak, Dielectric and mechanical relaxation in materials, Analysis, Interpretation and application to polymers, Hanser publisher, Cincinnati, 1997. [32] D.D. Macdonald, Corrosion 46 (3) (1990) 229–242.