The inhibition of the corrosion of Armco iron in HCl solutions in the presence of surfactants of the type of N-alkyl quaternary ammonium salts

Materials Chemistry and Physics 65 (2000) 288–297

The inhibition of the corrosion of Armco iron in HCl solutions in the presence of surfactants of the type of N-alkyl quaternary ammonium salts V. Branzoi a , F. Branzoi b , M. Baibarac c,∗ b

a University Polytechnic, str. Polizu 1, Bucharest, Romania Department of Colloids, Institute of Physical Chemistry, 202 Spl.Independentei, Bucharest 77208, Romania c Laboratory 160, National Institute of Materials Physics, P.O. Box MG-7, Bucharest R-76900, Romania

Received 11 February 1999; received in revised form 28 December 1999; accepted 16 February 2000

Abstract The polarization behaviour of Armco iron in aqueous solutions of HCl with and without inhibitors has been studied by the potentiostatic method. At lower overvoltage values, the dissolution process is controlled by activation, while at higher overvoltage values, the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using four surfactants. Under the critical micelle concentration (CMC), the inhibition of these four surfactants is negligible. At a concentration higher than the CMC, the inhibiting action of tetradecyl trimethyl ammonium iodide (TTAI), tetradecyl trimethyl ammonium bromide (TTAB), hexadecyl trimethyl ammonium bromide (HTAB) and dodecyl trimethyl ammonium bromide (DTAB) increases rapidly. The process of inhibition was attributed to the formation of adsorbed film on the metal surface, that protects the metal against corrosive agents. Protection efficiency improved when the inhibitor concentration and the length of the alkyl chain were increased. The four quaternary ammonium salts tested were adsorbed on the Armco iron surface according to a Langmuir isotherm. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Armco iron; Langmuir isotherm; Surfactants

1. Introduction Less active metals are less accessible and more expensive, so more common metals, which are more disposed to corrosion, have to be used. The protection of these metals is brought about in multiple ways among which treatment of the corrosive medium is one of the most important ones. Treatment of the corrosive medium for the protection of metals can be realized through elimination of the corrosive species, or through the use of inhibitors. Among all inhibitors, the most important are the organic ones, also called adsorption inhibitors. They control corrosion, acting over the anodic or the cathodic surface or both. As they usually affect the whole metal surface, when they are present in sufficient concentration, they cannot be called anodic or cathodic inhibitors [1]. It was shown [2,3] that, in solutions of sulphuric acid or perchloric acid, the rate of dissolution of iron is a function of potential and pH, but does not depend on the concentration of the anions of these acids or the concentration of cation of the base metals. In this case, the inhibition is a result of the adsorption of the organic com∗

Corresponding author.

pound on the metal surface, forming an invisible film a few molecules thick. Thus, quaternary ammonium salts have been used as inhibitors against the acid corrosion of iron and steel [4,5]. Halides are the most effective derivatives as they increase the inhibiting tendency of the positive quaternary ammonium ion by the well known synergistic effect [1]. In general, an increase in temperature reduces an inhibitor’s efficiency due to its desorption from the metal surface. Thus, finding an inhibitor with high efficiency at low and high temperatures is of substantial economic significance [6]. In this paper, we studied the influences of inhibitor concentration and temperature on the corrosion rate in the presence of n-alkyl trimethyl ammonium bromide and iodide containing between 12 and 16 carbon atoms in aqueous 0.5 M hydrochloric acid solutions using Armco iron as the working electrode.

2. Experimental The inhibiting action was studied through plotting of the polarization curves obtained using the potentiostatic

0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 2 6 0 - 1

V. Branzoi et al. / Materials Chemistry and Physics 65 (2000) 288–297

method, finding the kinetic parameters of corrosion (especially the density of corrosion current) and their comparison with the kinetic parameters from the solution without inhibitor. The working electrode made of Armco iron (99.93% Fe, 0.01% C, 0.03% Mn, 0.001% P, <0.001% Si, 0.01% S) had a surface area of 1 cm2 . In all experiments, electrochemical polarization was started about 30 min after the working electrode was immersed in solution, to allow stabilization of the stationary potential. The working electrode potential was always measured with reference to the saturated calomel electrode (SCE) and plotted versus the current in external circuit, obtaining the anodic or cathodic curves according to the variation of the working electrode potential. Before each measurement, the working electrode was polished with fine emery paper of varied granulation up to mirror-lustre; then, it was immersed in boiling benzene for 5 min to remove the remaining fat traces and the abrasive dust on the electrode surface after polishing. After electropolishing, the working electrode was washed with distilled water and inserted into the polarization cell, which was the usual three-electrode type. On plotting the polarization curves, account was kept of the fact that prolonged anodic polarization might give rise to changes at the level of the surface roughness that would imply parallel translation of the Tafel slopes. This effect can be eliminated by first tracing the cathodic polarization curves and then the anodic ones, the method used in the present paper. Electrochemical measurements are carried out using a potentiostat/galvanostat type Princeton Applied Research (PAR) Model 173 equipped with a digital coulometer. The inhibitors used were: dodecyl trimethyl ammonium bromide (CH3 (CH2 )11 N+ (CH3 )3 Br− abbreviated to DTAB), tetradecyl trimethyl ammonium bromide (CH3 (CH2 )13 N+ (CH3 )3 Br− abbreviated to TTAB), hexadecyl trimethyl ammonium bromide (CH3 (CH2 )15 N+ (CH3 )3 Br− abbreviated to HTAB) and tetradecyl trimethyl ammonium iodide (CH3 (CH2 )13 N+ (CH3 )3 I− abbreviated to TTAI), for which the critical micelle concentration (CMC) was determined by the surface tension method and by the equivalent electric conductivity method. The tensioactive properties of these four organic substances were pointed out by the study of their behaviour in aqueous solutions at different concentrations of the organic substance. The solutions were prepared with double-distilled water and kept in hermetically closed glass balloons for about 8–10 h for reaching the superficial equilibrium. The variation of specific conductivity with concentration of the tensioactive substance was determined at a constant temperature of 20◦ C by the conductometric method. The determination of surface tension was made by the method of taking the ring off the surface of the solution, using a torsion balance having the ring attached to its arm. All these determinations were made with the purpose of characterizing the physical and chemical properties of the previous surfactants in aqueous environments.

289

3. Results and discussion 3.1. Influence of the concentration of n-alkyl trimethyl ammonium halides The amphiphilic nature of the surfactants studied, due to the presence in their molecule of groups with different affinities toward the solvent (water), irrespective of the hydrocarbonate hydrophobic groups and hydrophilic ionic and nonionic groups, gives them tensioactive and colloidal properties [7–10]. It is well known that the CMC represents the range of the concentration in which the surfactants, in solution, change their initial molecular solvated state (nonaggregate) into the aggregate dipersive state. The aggregates or colloidal micelles are formed from a variable number of molecules, ranging from tens to hundreds of molecules. The formation of colloidal micelles is dependent on the equilibrium between the force on hydrophobic attraction of the carbon chain, accompanied by a reorganization of solvent molecules, and on the repulsion of polar heads [11]. The CMC is influenced by a series of factors that are dependent on the nature of surfactant, the nature of the aqueous environment of dispersion, and the method of determination used. In this paper, the CMC is determined by measurement of the surface tension (γ ). These values for the CMC of n-alkyl trimethyl ammonium halides are: 1.56×10−2 M for DTAB, 3.65×10−3 M for TTAB, 9.1×10−4 M for HTAB and 7×10−4 M for TTAI. The main importance of the determination of the CMC consists of the fact that, at this concentration, most of the physical and chemical properties of the surfactant solutions present an abrupt variation. Fig. 1 shows the variations of surface tension with the n-alkyl trimethyl ammonium halide concentration. For all four halides, we observed that the surface tension remains practically constant at a concentration greater than the CMC, while for smaller concentrations, the surface tension increases proportionally with decrease in the concentration. A similar variation in the CMC range is also observed for equivalent conductivity. Fig. 2 shows the variation of equivalent conductivity with the n-alkyl trimethyl ammonium halide concentration. We observed that, below the CMC, the equivalent conductivity increases and all these four substances behave as mono-monovalent electrolytes, strongly dissociated, while at concentrations greater than the CMC, they form colloidal micelles that, with increase in the concentration, form micelle aggregates. The inhibitory action of quaternary ammonium salts for the corrosion of Armco iron in a 0.5 M HCl solution depends on the CMC. In Fig. 3, only a few anodic and cathodic polarization curves of Armco iron in an aqueous 0.5 M HCl solution in the presence and the absence of surfactants of the type of n-alkyl quaternary ammonium salts are shown. Analysis of the polarization curves shows that no passive–active transition up to high currents was observed. This could be

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Fig. 1. The variation of surface tension with the concentration of (a) DTAB; (b) TTAB; (c) HTAB and (d) TTAI at 293 K.

explained on the basis of the presence of hydrochloric acid that prevents the formation of passive film and accelerates the process of anodic dissolution, which is directly proportional to the concentration of chloride ion. It is assumed that halogen ions participate by competitive adsorption in the reaction at the metal–electrolyte interface which forms chemisorbed films that satisfy the surface affinity for oxygen atoms or water molecules. The specific adsorption of these anions reduces the potential energy barrier necessary for transition from the metal phase in solution, and consequently, the process of anodic dissolution is accelerated. Also, in Fig. 3, we observe that, at lower overvoltage values, the Tafel relation is met with linear dependence, indicating that both anodic and cathodic reactions are controlled by activation. At high overvoltage, a diffusion-limited current appears on the anodic and cathodic polarization curves, suggesting that, at higher current densities, the transport of Cl− ions on the electrode surface gradually becomes rate-determinant (concentration polarization). In Tables 1–4, all parameters of corrosion obtained from the polarization curves are shown: corrosion current density (icorr , in ␮A cm−2 ), corrosion potential (εcorr , in mV), inhibitor efficiency (E, in %), anodic and cathodic Tafel slopes (ba and bc , respectively, in mV) and corrosion rate (Rmpy , in mil per year; P, in mm per year; and Kg , in g m−2 h−1 ). The

inhibition efficiency is calculated employing the formula E (%) =

(icorr )HCl − (icorr )Inh × 100 (icorr )HCl

(1)

where (icorr )HCl and (icorr )Inh are the corrosion current densities in the absence and in the presence of the inhibitor, respectively. The corrosion current density is converted to a corrosion rate by using Rmpy = 0.13icorr

e ρ

(2)

where Rmpy is the corrosion rate in mil per year, icorr the corrosion current density in ␮A cm−2 , e the equivalent weight (chemical) of metal in g, and ρ the density of metal in g cm−3 . The addition of surfactants in the amounts shown in Tables 1–4 leads in all cases to the inhibition of the corrosion process. The action of the inhibition of these substances has been studied through plotting of the polarization curves, finding the kinetic parameters of corrosion and their comparison with the kinetic parameters from the solution without inhibitor. Also, we found the efficiency of different inhibitors, so we could tell with precision as to which one gives the best result in a given medium and at what concentration. Thus, the results of Tables 1–4 show that the

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Fig. 2. The variation of equivalent conductivity with the concentration of (a) DTAB; (b) TTAB; (c) HTAB and (d) TTAI at 293 K.

order of decreased inhibition efficiency of the quaternary ammonium salts is EHTAB > ETTAI > ETTAB > EDTAB

(3)

The higher inhibitor efficiency for n-tetradecyl trimethyl ammonium iodide than for n-tetradecyl trimethyl ammonium bromide can be explained by the following:

1. The action of inhibition of the corrosion of Armco iron in an aqueous solution of HCl starts in all four cases at greater concentrations that the CMC. Above the value of CMC, the inhibitory action rapidly increased with increase in the surfactant concentration up to a limiting value. 2. Increased synergistic effect in the case of I− ions. 3. The higher volume of I− ion leads to a good adsorption of the inhibitor on the iron surface, and reduces the chemical attack of HCl solution on Armco iron. The higher inhibitor efficiency of n-alkyl trimethyl ammonium bromide when the alkyl chain length increased, at the same inhibitor concentration, is a consequence of the adsorption process. To quantify the effect of inhibitor concentration on the corrosion rate, it is common practice to fit the rate data to equilibrium adsorption expressions, such as the Langmuir equation [12]: θ = Kc, 1−θ

(4)

where θ is the fraction of surface coverage by the inhibitor and K the equilibrium constant for the adsorption reaction. θ is given by Fig. 3. Polarization curves of Armco iron in the solution with and without inhibitors: (a) 0.5 M HCl; (b) 500 ppm DTAB; (c) 500 ppm TTAB; (d) 500 ppm HTAB and (e) 500 ppm TTAI.

θ=

R0 − R , R

(5)

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Table 1 Kinetic parameters of corrosion of Armco iron in a solution of 0.5 M HCl and x×10−3 M DTAB at 20◦ C No.

cInh (M×103 or ppm)

icorr (␮A cm−2 )

Rmpy (mil per year)

P (mm per year)

Kg (g m−2 h−1 )

E (%)

ε corr (mV)

ba (mV)

bc (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Without 0.06 0.16 0.32 0.49 0.65 0.97 1.3 1.62 2.43 3.24 4.05 4.86 6.49

106 100 90 80 75 70 63 52 45 32 29 26 19 15

28.933 46.66 41.99 37.33 34.99 32.66 29.39 24.27 20.99 14.93 13.53 12.13 8.87 6.99

0.7343 1.1844 1.0659 0.9475 0.8883 0.8291 0.7462 0.6159 0.5329 0.3790 0.3435 0.3079 0.225 0.1777

0.6531 1.053 0.9481 0.8427 0.7900 0.7374 0.6636 0.5478 0.4740 0.3371 0.3055 0.2739 0.2001 0.1580

5.66 15.09 24.53 29.24 33.96 40.57 50.94 57.55 69.81 72.64 75.47 82.07 85.85

−320 −340 −335 −330 −330 −320 −300 −292 −290 −280 −280 −275 −270 −270

70 50 56.7 60 62 68 77 80 83 85 87 90 92 94

−117 −117 −115 −118 −119 −120 −128 −127 −130 −130 −133 −140 −139 −140

Without 20 50 100 150 200 300 400 500 750 1000 1250 1500 2000

Table 2 Kinetic parameters of corrosion of Armco iron in a solution of 0.5 M HCl and x×10−3 M TTAB at 20◦ C No.

cInh (M×103 or ppm)

icorr (␮A cm−2 )

Rmpy (mil per year)

P (mm per year)

Kg (g m−2 h−1 )

E (%)

ε corr (mV)

ba (mV)

bc (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Without 0.06 0.15 0.29 0.44 0.59 0.89 1.19 1.49 2.23 2.97 3.72 4.46 5.94

106 88 80 72 66 59 53 49 43 40 35 26 17 10

28.933 41.07 37.33 33.59 30.79 27.53 24.73 22.87 20.07 18.66 16.33 12.13 7.93 4.66

0.7343 1.042 0.9475 0.8528 0.7917 0.6988 0.6277 0.5804 0.5093 0.4738 0.4145 0.3079 0.2013 0.1184

0.6531 0.9270 0.8427 0.7585 0.6952 0.6215 0.5583 0.5162 0.4529 0.4214 0.3687 0.2739 0.1791 0.1053

16.98 24.53 32.07 37.73 44.34 50 53.77 59.43 62.26 66.98 75.47 83.96 90.56

−320 −330 −326 −319 −310 −295 −280 −270 −250 −250 −255 −250 −245 −245

70 60.5 64 70 72 81 86 87 97 102 97 100 105 107

−117 −117 −118 −120 −122 −125 −130 −132 −135 −140 −135 −135 −140 −141

Without 20 50 100 150 200 300 400 500 750 1000 1250 1500 2000

Table 3 Kinetic parameters of corrosion of Armco iron in a solution of 0.5 M HCl and x×10−3 M HTAB at 20◦ C No.

cInh (M×103 or ppm)

icorr (␮A cm−2 )

Rmpy (mil per year)

P (mm per year)

Kg (g m−2 h−1 )

E (%)

ε corr (mV)

ba (mV)

bc (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Without 0.05 0.13 0.26 0.39 0.52 0.78 1.04 1.30 1.96 2.61 3.26 3.91 5.22

106 90 80 70 65 60 56 44 34 27 25 18 9 4

28.933 41.99 37.33 32.66 30.33 27.99 26.13 20.53 15.87 12.59 11.66 8.39 4.19 1.86

0.7343 1.0659 0.9475 0.8291 0.7699 0.7106 0.6633 0.5216 0.4027 0.3198 0.2961 0.2132 0.1066 0.0474

0.6531 0.9481 0.8427 0.7374 0.6847 0.6320 0.5899 0.4635 0.3582 0.2844 0.2633 0.1896 0.095 0.042

15.09 24.53 33.96 38.68 43.39 47.17 58.49 67.92 74.53 76.41 83.02 91.51 96.23

−320 −360 −355 −350 −346 −340 −338 −330 −316 −315 −310 −310 −316 −320

70 57 60 65 68 72 80 83 87 95 100 95 95 90

−117 −104 −110 −110 −115 −119 −128 −131 −134 −134 −140 −135 −134 −135

Without 20 50 100 150 200 300 400 500 750 1000 1250 1500 2000

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293

Table 4 Kinetic parameters of corrosion of Armco iron in a solution of 0.5 M HCl and x×10−3 M TTAI at 20◦ C No.

cInh (M×103 or ppm)

icorr (␮A cm−2 )

Rmpy (mil per year)

P (mm per year)

Kg (g m−2 h−1 )

E (%)

ε corr (mV)

ba (mV)

bc (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Without 0.06 0.13 0.27 0.4 0.52 0.78 1.04 1.30 1.96 2.61 3.26 3.92 5.22

106 70 65 55 50 48 39 36 32 25 20 16 10 6

28.933 32.66 30.33 25.66 23.33 22.39 18.19 16.79 14.93 11.66 9.33 7.46 4.66 2.79

0.7343 0.8291 0.7699 0.6514 0.5922 0.5685 0.4619 0.4264 0.3790 0.2961 0.2369 0.1895 0.1184 0.0711

0.6531 0.7374 0.6847 0.5794 0.5267 0.5056 0.4108 0.3792 0.3371 0.2633 0.2107 0.1685 0.1053 0.063

33.96 33.68 48.11 52.83 54.72 63.21 66.04 69.81 76.41 81.13 84.90 90.57 94.34

−320 −345 −343 −340 −338 −334 −330 −327 −325 −321 −319 −317 −326 −330

70 62 70 72 76 82 94 102 116 115 105 102 98 100

−117 −117 −121 −125 −127 −132 −135 −134 −137 −134 −134 −133 −135 −130

Without 20 50 100 150 200 300 400 500 750 1000 1250 1500 2000

where R and R0 are corrosion rates in the 0.5 M HCl solution with and without inhibitor. Usage of the Langmuir treatment is often justified with the argument that inhibition must involve adsorption. In this paper, the Langmuir isotherm is rearranged to give 1 c =c+ θ K

(6)

and c/θ is plotted against c, when a linear relationship is

obtained for each inhibitor and a slope of near unity for each compound indicates approximate Langmuirian behaviour (see Fig. 4). Adsorption equilibrium constants (K) for the DTAB, TTAB, TTAI and HTAB compounds have the following values: 1.0×103 ; 1.4×103 ; 2.2×103 and 2.9×103 . Adsorption of surfactants at the solid–liquid interface is governed not only by the solution properties of the surfactant but also by the properties of the solid–liquid interface and interactions among the various dissolved species.

Fig. 4. Langmuir plot for: (a) DTAB; (b) TTAB; (c) HTAB and (d) TTAI.

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Adsorption on the Armco iron surface can be explained by an electrostatic interaction between ammonium group and cathodic sites on the metallic surface [5]. For an n-alkyl trimethyl ammonium ion, any variation in the electronic charge density on the central nitrogen atom will depend on the positive inductive effect of the alkyl group. This is because the steric hindrance contribution is negligible since σ ∗ (polar substituent constants for the four groups attached to the nitrogen atom) for a methyl group is 0 [13]. With increasing size of the alkyl group, the electronic charge density on the nitrogen atom in the quaternary ammonium ion will increase, and the inhibiting properties of the positive head group should decrease as a consequence of a now less tightly held layer of positive ions adjacent to the adsorbed bromide and iodide ions. This opposite behavior observed may be explained by the effect of Van der Waal’s forces of attraction between the alkyl chains of adjacently adsorbed positive head group ions. At high concentration levels, the positive ions could be expected to be adsorbed as closely as possible, i.e. with the trimethyl group oriented onto the surface and with the alkyl chains oriented away from the surface. These chains would interact to form a layer above the head groups [14]. The longer the alkyl chain, the greater will be the forces of attraction and hence inhibition. The model of an electrical double layer with specifically adsorbed n-alkyl trimethyl ammonium bromide on a metallic surface is shown in Fig. 5. In the following portion, we shall try to show that the adsorption process of the inhibitor (Inh) on Armco iron is physical adsorption. The reaction from the first step of the corrosion process of Armco iron in HCl solution with inhibitor is Fe + Inh ↔ Fe(Inh)ads ↔ Fe

n+

−

+ ne + Inh

(7)

At first, when there is not enough Fe(Inh)ads to cover the metal surface, because the inhibitor concentration is low or because the adsorption rate is slow, metal dissolution takes place on sites on the Armco iron surface free of Fe(Inh)ads . With high inhibitor concentration, a compact and coher-

ent inhibitor overlayer is formed on Armco iron that reduces chemical attacks on the metal. The adsorption constant (Kads ) is related to the standard free energy of reaction (1G0ads ) by the equation ln Kads =

−1G0ads RT

(8)

where R is the universal gas constant and T the absolute temperature. The obtained values of 1G0ads characterize the spontaneity of the adsorption process under the experimental conditions used. Generally, values of 1G0ads up to −20 kJ mol−1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption), while those more negative than −40 kJ mol−1 involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate type of bond (chemisorption) [15,16]. The 1G0ads values for DTAB, TTAB, HTAB and TTAI at 293 K are: −16.85, −17.66, −19.45 and −18.76 kJ mol−1 . One can note the fact that the adsorption is of the physical type and the spontaneity of this process decreased in the following order: HTAB > TTAI > TTAB > DTAB

(9)

The free energy of adsorption of surfactants at the solid–liquid interface can be considered as being the sum of a number of contributing factors, such as hydrogen bonding, electrostatic interactions, hydrophobic interactions, and such specific interactions as covalent bonding: 1Gads = 1Gelec. + 1GH + 1Ghydrophobic +1Gspecific + · · ·

(10)

Note that adsorption can occur even if some of the factors oppose it, as long as the net free energy change involved in the adsorption is negative. In this case, we presume that the inhibitors of this type are strongly adsorbed on the metal surface, when a protective film is formed on the metal surface,

Fig. 5. Electrical double layer with specifically adsorbed n-alkyl trimethyl ammonium bromide on the Armco iron surface.

V. Branzoi et al. / Materials Chemistry and Physics 65 (2000) 288–297

295

Table 5 Corrosion rate dependence upon temperature in aqueous solutions of 0.5 M HCl with inhibitor (Inh) and activation energies (Ea ) of the dissolution reaction at various concentrations of inhibitor at 293–333 K No. 1

2

Inhibitor

Concentration of Inh (ppm)a

T (K)

icorr (␮A cm−2 )

E (%)

Ea (kJ mol−1 )

DTAB

2000

303 313 333

32 56 139.6

79.77 75 65.91

44.60

303 313 333

72.3 103 189

54.3 54.06 53.95

27.96

303 313 333

144.6 204 378

8.6 8.12 7.89

27.42

303 313 333

24 54 100

84.83 75.91 75.63

45.61

303 313 333

65 93.4 172

58.91 58.34 58.09

28.00

303 313 333

121 174 320

23.5 22.3 21.93

27.92

303 313 333

12 30 122

92.41 86.62 71.09

68.81

303 313 333

53.7 76.8 138

66.05 65.74 66.37

28.13

303 313 333

119.8 170 312

24.25 24.17 23.98

27.48

303 313 333

27 44 87

82.93 80.37 78.80

53.66

303 313 333

49 70 130

69.02 68.77 68.32

28.25

303 313 333

105 150 277

33.3 33 32.6

27.86

303 313 333

158.2 224.2 410.4

DTAB

3

500

DTAB

4

TTAB

5

50

2000

TTAB

6

500

TTAB

7

HTAB

8

50

2000

HTAB

9

500

HTAB

10

TTAI

11

50

2000

TTAI

12

TTAI

13

HCl

a

500

50

0.5

27.33

Concentration of HCl in M.

and as a result, the reaction between the metal and the corrosive solution would take place only through the diffusion of anions of the aggressive medium through the fines pores of the protective film which has been formed. Adsorption is stronger after the CMC has been reached. In this way, we can explain the strong decrease in the corrosion rate after reaching CMC, and thus, the high efficiency of these four inhibitors. 3.2. Influence of temperature The behaviour of Armco iron in 0.5 M HCl aqueous solution in the absence and in the presence of n-alkyl trimethyl

ammonium halide during polarization was studied in the 293–333 K temperature range. For the study of inhibition of corrosion at different temperatures, three concentrations of the inhibitor, 2000, 500 and 50 ppm, are used. In Tables 1–4, we observe that, at a temperature of 293 K, corrosion rates decrease with increase in the concentration of the inhibitor. Table 5 shows that, at the same concentration of the inhibitor, the corrosion rates decrease with increase in the temperature. The dependence of corrosion rate on temperature [17] can be expressed by the Arrhenius equation   −Ea (11) icorr = A exp RT

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V. Branzoi et al. / Materials Chemistry and Physics 65 (2000) 288–297

Fig. 6. Variation in the Armco iron corrosion rate against T−1 (Arrhenius plot) in 0.5 M HCl inhibited with (a) 500 ppm of DTAB; (b) TTAB; (c) HTAB and (d) TTAI.

where icorr is the reaction rate, A a constant, Ea the activation energy of the Armco iron dissolution reaction, T the absolute temperature and R the universal gas constant. Fig. 6 depicts an Arrhenius plot, corrosion rate against the reciprocal of temperature (1/T) for Armco iron immersed in 0.5 M HCl solution in the presence of 500 ppm DTAB, TTAB, HTAB and TTAI. The four inhibitors studied showed a similar behaviour. The plots obtained are straight lines and the slope of each straight line gives its activation energy. The negative slope of Ea indicates the adsorption of organic compounds on the electrode surface. The apparent activation energy obtained for various concentrations of inhibitor at 293–333 K is given in Table 5. The increase in apparent activation energy with DTAB, TTAB, HTAB or TTAI concentration thereby indicates a more efficient inhibiting effect. The highest values of Ea , obtained at the highest concentrations of HTAB and TTAI, TTAB and DTAB, can be correlated to the thickening of the electrical double layer and the CMC at which these molecules can form aggregate micelles [18–21]. Another interesting result is shown in Table 6 that reports the average values in the 293–333 K temperature range of the adsorption entropy (1S) and enthalpy (1H) of DTAB, TTAB, HTAB and TTAI calculated with the 1G0ads values obtained from Eq. (6). Some data seem to be in contrast to that normally accepted in the adsorption phenomena. In

fact, it is well known that adsorption is an exothermic phenomenon (1H<0) accompanied by a decrease in entropy (1S) [22]. In aqueous solutions, the adsorption of organic molecules is generally accompanied by the desorption of Table 6 Adsorption entropy and enthalpy of Armco iron dissolution in the presence of DTAB, TTAB, HTAB and TTAI, in the 293–333 K temperature range Inhibitor T (K) ln Kads 1G (kJ mol−1 ) 1H (kJ mol−1 ) 1S (kJ mol−1 ) DTAB

293 303 313 333

6.92 6.47 6.45 6.54

−16.85 −16.29 −16.77 −18.10

−7.66

0.03

TTAB

293 303 313 333

7.28 7.30 7.43 7.45

−17.66 −18.4 −19.34 −20.48

3.01

0.07

HTAB

293 303 313 333

7.70 7.54 7.64 8.11

−19.45 −18.99 −19.87 −22.45

2.5

0.08

TTAI

293 303 313 333

7.98 8.11 8.18 8.22

−18.76 −20.42 −21.28 −22.74

10.4

0.1

V. Branzoi et al. / Materials Chemistry and Physics 65 (2000) 288–297

water molecules because of the adsorption of an organic adsorbate at the metal–solution interface considered a ‘substitutional adsorption phenomenon’ [23].Therefore, while the adsorption of organic molecules is generally exothermic with decrease in entropy, the contrary occurs for the desorption of water molecules. The thermodynamic values obtained are the algebraic sum of those of the adsorption of organic molecules and the desorption of water molecules [24]. Therefore, the positive values of 1S and 1H related to substitutional adsorption can be attributed to the increase in the solvent entropy and to a more positive water desorption enthalpy.

4. Conclusions This paper has reported new results on the inhibition of corrosion of Armco iron in aqueous solutions of 0.5 M HCl in the presence of surfactants of the type of n-alkyl quaternary ammonium salts. We have studied the dependence of the corrosion process on the n-alkyl trimethyl ammonium halide concentration and the temperature. The results obtained lead to the following conclusions: 1. In the aqueous solutions of 0.5 M HCl, the process of dissolution is controlled by the activation at low overvoltages, while at high overvoltages, it is controlled by diffusion. 2. Under the CMC, the inhibitory action of these four surfactants is insignificant. At a concentration higher than the CMC, the inhibitory action of these four surfactants increases rapidly. 3. The efficiency of the organic inhibitors used increases with increase in the alkyl chain length in the order EDTAB >ETTAB >EHTAB >ETTAI . 4. The physical adsorption of the four n-alkyl trimethyl ammonium halides on the Armco iron surface according to a Langmuir isotherm leads to the formation of a compact and coherent inhibitor layer, which reduces chemical attack of the HCl solution on the metal. 5. For DTAB, TTAB, HTAB and TTAI, the activation energy of the Armco iron corrosion reaction increases as the inhibitor concentration increases.

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6. n-Alkyl quaternary ammonium salts show an inhibiting effect on the corrosion of Armco iron in aqueous 0.5 M HCl solution, which decreases when the temperature increases from 293 to 333 K. References [1] J.L. Rozenfeld, Corrosion Inhibitors, McGraw-Hill, New York, 1981, p. 109. [2] K.F. Bonhoeffer, K.E. Heusler, Z. Physik. Chem., N.F. 8 (1956) 390. [3] K.F. Bonhoeffer, K.E. Heusler, Z. Electrochem. 61 (1957) 122. [4] A. Frignani, F. Zucchi, C. Monticelli, Br. Corrosion J. 18 (1983) 19. [5] V. Branzoi, F. Branzoi, M. Baibarac, M.V. Popa, Adv. Mater. Processing Technol. 2 (1997) 752. [6] J. de Damborenea, A.J. Vazquez, Rev. Metal. (Madrid) 23 (1987) 103. [7] D.G. Leaist, J. Chem. Soc., Faraday Trans. 86 (1990) 3487. [8] J.M. Richmond, Cationic Surfactants, Surfactants Science Series, Vol. 34, Marcel Dekker, New York, 1990. [9] R. Zana, Surfactant Solutions, Surfactants Science Series, Vol. 22, Marcel Dekker, New York, 1987. [10] S. Quni, A. Hafiane, M. Dhahbi, J. Chim. Phys. 95 (1998) 911. [11] C. Tanford, The Hydrophobic Effect: Formation of Miceles and Biological Membranes, Wiley, New York, 1973. [12] M. Beier, J.W. Schultze, Electrochim. Acta 37 (1992) 2299. [13] K. Kobayaski, V. Ashworth, Bashokeu Gijutsu 32 (1983) 627. [14] R.J. Meakins, M.G. Stevens, R.J. Hunter, J. Phys. Chem. 73 (1969) 112. [15] E. Kamis, F. Bellucci, R.M. Latanision, E.S.H. El-Ashry, Corrosion 47 (1991) 677. [16] F. Donahue, K. Nobe, J. Electrochem. Soc. 112 (1965) 886. [17] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Electrochim. Acta 41 (1996) 1971. [18] G. Tourillon, F. Garnier, J. Electroanal. Chem. 135 (1982) 173. [19] R.J. Waltman, A.F. Diaz, J. Bargon, J. Phys. Chem. 88 (1984) 4343. [20] J. Bukowska, K. Jackowska, J. Electroanal. Chem. 322 (1992) 347. [21] C. Cachet, M. Keddam, V. Mariotte, R. Wiart, Electrochim. Acta 37 (1992) 2377. [22] J.M. Thomas, W.J. Thomas, Introduction to the Principles of Heterogeneous Catalysis, 5th Edition, Academic Press, London, 1981, pp. 14–18. [23] J.O’.M. Bockris, D.A.J. Swinkels, J. Electrochem. Soc. 11 (1964) 736. [24] B.G. Ateya, B.E. El-Anadouli, F.M. El-Nizamy, Corrosion Sci. 24 (1984) 509.