Electrochemical and quantum chemical studies on the formation of protective films by alkynols on iron

Corrosion Science 45 (2003) 1685–1702 www.elsevier.com/locate/corsci

Electrochemical and quantum chemical studies on the formation of protective films by alkynols on iron }rik a,*, G Gabriella Lendvay-Gyo abor Mesz aros a, Bela Lengyel a, Gy€ orgy Lendvay b a

Research Laboratory for Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525, POB 17, Budapest, Hungary b Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525, POB 17, Budapest, Hungary Received 4 March 2002; accepted 17 January 2003

Abstract In order to find out the effect of cathodic and anodic polarisation on the formation of inhibitor films we measured the electrochemical impedance of an iron electrode in sulphuric acid and in hydrochloric acid solutions in the presence of oct-1-yn-3-ol or 2-h2-(1,1-dimethylprop-2-ynyloxy)-ethoxyi-ethanol inhibitors. We found that the anodic polarisation helped the initial binding of the inhibitor to the iron surface. Cathodic polarisation does not influence the initial formation but, if applied at a later stage, promotes the growth of the inhibitor film. The results support the assumption that film formation on iron surfaces from alkynols takes place in a two-step process: first chemisorption, then polymerisation. The binding of alkynols to iron was studied in model electronic structure calculations which indicate that a dative bond is formed between Fe and the triple bond. The calculated vibrational frequencies of the alkynol bound to iron can be used to successfully interpret the IR spectra found in the literature. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Inhibitor; Impedance; B. Cyclic voltammetry; Corrosion

*

Corresponding author. Tel.: +36-1-325-6992; fax: +36-1-325-7892. E-mail address: [email protected] (G. Lendvay-Gy} orik).

0010-938X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00019-2

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1. Introduction The corrosion protection properties of acetylenic alcohols as inhibitors, which form a polymeric film on an iron surface, have been widely investigated in the last few decades. Various authors assign the high corrosion protective properties of acetylenic alcohols on iron in an acidic environment to one of three different physical phenomena: (1) adsorption of the inhibitor by the interaction of the CC p bond and the d orbitals of the iron [1–7]; (2) formation of oligomers or a polymer film on the surface acting as a physical barrier between the acidic solution and the iron surface [8–14]; (3) formation of chelate complexes by acetylenic alcohol molecules with iron [15]. It turns out, however, that each of these assumptions can only explain certain experimental findings but in order to rationalise all available information on alkynol-based films on iron, a combination of these theories is needed [16–25]. For example, Bockris et al. performed in situ ellipsometry measurements of octynol and other alkynols on iron in acid solutions [20–24] and studied the time, concentration and potential dependence of the adsorption, the subsequent reorientation transition and polymer growth of these compounds and found that formation of the protective film takes place in two steps. According to Krist of and Salamon [25], the temporary protective behaviour of propargyl alcohol at early stages is associated with the chemisorption of inhibitor species on the metallic surface. Their results show, however, that from the point of view of protection, multilayer formation associated with polymerisation reactions is much more important. Zucchi proposed that the first chemisorbed layer is formed by protonated octynol, on which an olygomer film grows by dehydration and polymerisation [17]. Balezin et al. suggested an adsorption–polymerisation model for acetylenic compounds [19]. The mechanism of ethynylcyclohexanol polymerisation on iron was studied by Duwell et al. [18]. In their opinion the electron rich acetylenic linkage is necessary for the initial adsorption step and the subsequent formation of a hydrocarbon layer contributes to the effectiveness of the inhibitor. Frenier et al. investigated octynol film formation on carbon steel by IR analysis and found that on the surface a protective iron complex is formed and a polymer film is superimposed on it [16]. Based on the results of previous work, one can assume that the protective film made by alkynols on iron is formed in a two-step process: first a monolayer is formed by adsorption, which is followed by film thickening by polymerisation. The formation of a film on a metal surface can be studied by electrochemical methods, but no electrochemical study addressing both steps of the formation of alkynol-based films has been found in the literature. In the present work we planned to gain information on the two stages of film formation using electrochemical techniques. In a metal–electrolyte system both stages of film formation may be influenced by polarisation of the metal, but the influence is expected to be different for the chemisorption step and for the polymerisation step. Accordingly, we applied anodic or cathodic polarisation to an iron sample either in the early or a later stage of film formation. Film formation was investigated in sulphuric acid and in hydrochloric acid solution containing either oct-1-yn-3-ol (OCT) or 2-h2-(1,1-dimethyl-prop-2-ynyloxy)-ethoxyi-ethanol (ETH) inhibitors. We tested the corrosion behaviour of the films formed on the iron surface by three methods: first, from impedance spectra we

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obtained corrosion exchange current density through polarisation resistance and adsorbate coverage through double layer capacity; second and third, the current response of cyclic voltammetry scans and open circuit potential fluctuation measurements gave information on the stability of the film. In order to understand how film formation begins, we performed electronic structure calculations with the purpose of finding out what the preferred binding of the inhibitor molecule to the iron surface is. We used quantum chemical methods and calculated the binding energy of both the dative iron––triple bond and the iron––alcoholic oxygen bond. We also calculated the vibrational frequencies characteristic of the iron-bonded alkynol molecules for comparison with experimental IR spectra.

2. Experimental The electrochemical studies were performed on a rod shaped spectral grade iron electrode which was pre-treated prior to the experiments by grinding with emery paper of 600 grit, then cleaned in ultrasonic bath with ethanol and finally rinsed with doubly distilled water. The solutions were 0.5 M H2 SO4 and 1 M HCl containing 1 mM OCT or 1 mM ETH. A standard glass three-electrode cell was used throughout the experiments; the iron working electrode was coaxially surrounded by a cylindrical Pt net counter electrode in order to ensure uniform current distribution; a Hg2 SO4 or 1 M calomel electrode served as reference in the respective solution. The solutions were deaerated by bubbling nitrogen before and throughout the measurements. The effect of polarisation was studied according to the following programs illustrated in Fig. 1: Scheme A: (reference measurement) No polarisation; impedance measurement at 1 and 4 h after immersion Scheme B: a 2-min cathodic polarisation immediately at immersion, impedance measurement at 1 and 4 h after immersion Scheme C: a 2-min anodic polarisation immediately at immersion, impedance measurement at 1 and 4 h after immersion Scheme D: a 5-min cathodic polarisation at 1 h after immersion, impedance measurement at 2 and 4 h after immersion Scheme E: a 5-min anodic polarisation at 1 h after immersion, impedance measurement at 2 and 4 h after immersion Impedance spectra were measured at open circuit potential under potentiostatic conditions by a set-up consisting of a Solartron 1286 potentiostat and a Solartron 1250 frequency response analyser using 1 mV (RMS) perturbation and 4 or 8 points per decade in the frequency range 56 kHz–10 mHz. Cyclic voltammetry measurements on the iron––immersed in the inhibitor containing acidic solutions (OCT/H2 SO4 , ETH/H2 SO4 , OCT/HCl, ETH/HCl) prior to the measurements for four hours––were carried out with the same potentiostat as the

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Fig. 1. Potential–time courses. The Rp values refer to the Fe/HCl/OCT system.

impedance measurements. The current was recorded while potential was scanned with a sweep rate of 1 mV/s between the limits of 50 mV (or smaller) around the open circuit potential. The potential fluctuations were measured by using an ONO SOKKI FFT analyser coupled to a PC between two identical iron electrodes immersed into the inhibitorcontaining acidic solution. Each noise record consisted of 1024 samples of 20 s. 3. Quantum chemical calculations The purpose of the theoretical model calculations was to gain information about the possible mechanism of the chemisorption step at the electrode surface. To this

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end we explored the possible structures formed between an alkynol and iron using quantum chemical calculations. The interaction of iron and octynol was modelled by calculating the molecular geometries formed between a free iron atom and the simplest analogue of OCT, namely, a propargyl alcohol (prop-1-yn-3-ol, PA) molecule. The idea was to see what kind of complexes Fe and PA can form, and what are the properties––binding energies, vibrational frequencies––of the complexes. The quantum chemical calculations were performed with the Gaussian 94 package of programs [27]. The isomers of the complex of an iron atom with one PA molecule were studied. The geometries of the complexes were obtained by full optimisation at the B3LYP/3-21G* [28,29] level, a method of density functional theory that enables one to estimate electron correlation needed to describe such systems. Vibrational frequencies and IR intensities were calculated at the same level. In the analysis of the structures of the iron-PA adducts we calculated the MayerÕs bond order indices [30,31] using the STO-3G basis set [32] and the two-atomic energy contributions to the molecular energy [33] from the ab initio wavefunctions using Hamza and MayerÕs code APOST. Out of the possible spin states of atomic iron we were able to study the complexes between triplet and singlet state iron and PA, in the hope that if we find very similar behaviour for the two––as it does happen––then the conclusions can be expected to characterise the general behaviour of iron towards acetylenic alcohols. It is clear that this model of the surface–molecule interaction is very simple, but we observed so well expressed differences even at the relatively low level of calculations (B3LYP/3-21G*), that it seems to be safe to state that the energetic ordering of the binding site on PA would not change even at higher quantum chemical levels. Some calculations performed with a larger basis set at a different level (HF/6-311G**) support the goodness of the results we discuss below.

4. Results 4.1. Electrochemical measurements In solutions containing alkynol inhibitors, surface film is formed on iron which is either visible to the naked eye (in the case of OCT), or it is invisible (with ETH). The corrosion inhibition efficiency of the films was studied by impedance measurements; their stability was tested by cyclic voltammetry and by potential fluctuation measurements. For characterising the influence of the inhibitor on the electrochemical properties of the tested electrodes, impedance spectra were recorded according to polarisation schemes A–E illustrated in Fig. 1. Figs. 2 and 3 show the impedance diagrams measured at the open circuit potential according to polarisation schemes A–E in 0.5 M H2 SO4 (Fig. 2) and in 1 M HCl (Fig. 3) solutions with OCT (panels A) and ETH (panels B). All measured impedance spectra can be interpreted by a simple parallel R–C equivalent circuit in which the R element corresponds to the polarisation resistance, Rp , while the C element to the double layer capacity, Cdl . From the spectra

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(A)

(B) Fig. 2. Impedance spectra measured at the open circuit potential in 0.5 M H2 SO4 containing OCT (A) or ETH (B). a: (––) without inhibitor, test at 1 h after immersion, b: (– –) with inhibitor, test at 1 h after immersion, c: (–D–) with inhibitor, 2 min anodic polarisation at immersion, test at 1 h after immersion, d: (–r–) with inhibitor, 2 min cathodic polarisation at immersion, test at 1 h after immersion, e: (–}–) with inhibitor, test at 4 h after immersion, f: (–þ–) with inhibitor, 5 min anodic polarisation at 1 h after immersion, test at 2 h after immersion, g: (– –) with inhibitor, 5 min cathodic polarisation at 1 h after immersion, test at 4 h after immersion. Full and open symbols are magnitudes and phase angles, respectively.



polarisation resistance, Rp , and double layer capacity, Cdl were evaluated. Rp , which corresponds to the low frequency limit of the magnitude of the impedance, is inversely proportional to the corrosion rate. From the high frequency part one obtains the double layer capacity (Cdl ), which is proportional to the surface area not covered by inhibitor i.e. from the double layer capacity the inhibitor efficiency can be calculated.

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(A)

(B) Fig. 3. Impedance spectra measured at the open circuit potential in 1 M HCl containing OCT (A) or ETH (B). a: (––) without inhibitor, test at 1 h after immersion, b: (– –) with inhibitor, test at 1 h after immersion, c: (–D–) with inhibitor, 2 min anodic polarisation at immersion, test at 1 h after immersion, d: (–r–) with inhibitor, 2 min cathodic polarisation at immersion, test at 1 h after immersion, e: (–}–) with inhibitor, test at 4 h after immersion, f: (–þ–) with inhibitor, 5 min anodic polarisation at 1 h after immersion, test at 2 h after immersion, g: (– –) with inhibitor, 5 min cathodic polarisation at 1 h after immersion, test at 4 h after immersion. Full and open symbols are magnitudes and phase angles, respectively.



From the measurements according to scheme A, two effects are clearly seen: first, impedance magnitudes in solutions without inhibitor (curves a, squares) are smaller than in solutions containing inhibitors (curves b, circles and e, diamonds), and second, if inhibitor is present, impedance magnitudes increase with time in each system. The latter effect can be quantitatively characterised by the increase of the

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polarisation resistance 1 (Rp ). For example, for OCT in hydrochloric acid solution Rp increases from 7 kX cm2 (1 h after immersion, Fig. 3A, curve b) to about 32 kX cm2 (4 h after immersion, Fig. 3A, curve e). The comparison of curve c (‘‘triangle up’’ symbols) obtained with polarisation scheme B and curve b (circles) obtained with scheme A shows that a short, 2 min anodic polarisation of +100 mV just upon immersion increased the magnitudes of impedances in each case. The most pronounced effect of the anodic polarisation was observed with OCT in hydrochloric acid where Rp measured at 1 h after immersion changed from about 7 kX cm2 (Fig. 3A, curve b) to about 22 kX cm2 (Fig. 3A, curve c). One can also observe that anodic polarisation is effective in increasing the magnitude of impedance if applied just after immersion. This conclusion is based on the inspection of curves f (plus symbols, scheme D, polarisation at 1 h after immersion, test 1 h later), c (‘‘triangle up’’ symbols, scheme B, anodic polarisation at immersion, test 1 h later) and b (circles, scheme A, no polarisation, test at 1 h after immersion): anodic polarisation is apparently not effective if an inhibitor film already exists then when applied at the very beginning of film formation. Application of a short 2 cathodic polarisation immediately after the immersion did not change impedance magnitudes significantly (compare curves d, ‘‘triangle down’’ symbols, scheme C, cathodic polarisation at immersion, test 1 h later to curves b, circles, scheme A, no polarisation, test at 1 h after immersion). In contrast, if the short cathodic polarisation was applied 1 h after immersion, the magnitudes of impedances measured three further hours later were slightly higher (compare curves g, crosses, scheme E, cathodic polarisation at 1 h after immersion, test at 4 h after immersion to curves e, diamond, scheme A, no polarisation, test at 4 h after immersion). We characterised the corrosion protecting ability of the two studied inhibitors from data of the impedance measurements. Since corrosion is assumed to proceed on the uncovered parts of the surface only, the coverage by adsorbate gives us an estimate for the corrosion efficiency. The coverage can be calculated from the double layer capacity, (Cdl ) obtained from the high frequency part of the impedance diagrams. Accordingly, inhibitor efficiency (g expressed in %) was calculated from the double layer capacity Cdl 0 ¼ 40 lF cm2 measured in electrolyte without inhibitor and Cdl i measured in inhibitor containing electrolyte with the following expression:   Cdl i g ¼ 100 1  0 : Cdl

1

The Rp results in the case of different potential programs belonging to the Fe/HCl/OCT system are summarised in Fig. 1. 2 We note that just after immersion 2-min polarisations were sufficient in producing a large effect on the impedance spectra. At later stages of immersion longer cathodic or anodic polarisations (5 min) were necessary to cause measurable effects, probably because the protective layer formed during the first hour had to be re-formed.

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A

B

Fig. 4. The double layer capacity (A) and the inhibitor efficiency (B) as a function of time after immersion.

Cdl is found to decrease with the time of the inhibitor treatment in all systems (Fig. 4A), indicating that inhibitor efficiency increases with time. Fig. 4B shows that in the case of OCT the inhibitor efficiency was over 80% already after 1 h and reached 95% after 5 h in both acids, while ETH showed a much slower improvement. The results of the cyclic voltammetry experiments are shown in Fig. 5. The stable, hysteresis-free polarisation curve measured in HCl/OCT solution in a 50 mV polarisation range indicates that this film is stable, especially if compared to that of H2 SO4 /OCT. We interpret the hysteresis shown by three of four systems, that the anodic polarisation in some way destroys or alters the film. Based on these curves we set a stability order as HCl/OCT > HCl/ETH > H2 SO4 /ETH > H2 SO4 /OCT. This stability order applies for varying potentials, whereas at constant potentials (at the free potential of the sample) a different stability order can be defined based on potential fluctuation records shown in Fig. 6: the smoother the record, the more stable is the film. 4.2. Quantum chemical calculations We found several minima on the potential energy surface of uncharged Fe atoms with prop-1-yn-3-ol in both the singlet and triplet electronic state, indicating that Fe and alkynols form several complexes. We determined the structures and relative energies of all complexes. The Fe atom can be bound either at the O atom or at the triple bond. In addition, two cyclic isomers were detected in which the iron atom is embedded into a ring. The binding energy of the latter two is comparable to that of the Fe–triple bond interaction. However, these are not chelate complexes that could be imagined, instead, the Fe atom is surrounded by C and O atoms of the alkynol

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Fig. 5. Cyclic voltammetry diagrams measured on inhibitor-covered iron surfaces in HCl and H2 SO4 solutions containing OCT or ETH inhibitors.

Fig. 6. Potential fluctuations of inhibitor-covered iron electrodes in HCl and H2 SO4 solutions containing OCT or ETH inhibitors.

and is connected to them by sigma bonds. As the Fe atom is embedded in a ring, these complexes, due to steric constraints, could hardly represent the binding between an iron surface and a PA molecule, so their properties will not be discussed in what follows. In the open-chain isomer represented in Fig. 7A the Fe atom is attached to the O atom as an extension of the chain in the molecule. This structure is a model of chemisorption corresponding to binding of alkynol molecules to the surface at their hydroxyl groups. In the other important isomer of the complex (Fig. 7B) the Fe atom interacts with the p bond of the triple C–C bond. The relative energies of the complexes are shown in Fig. 8 both for the singlet and the triplet states. The energy of all structures is referred to that of the separated triplet Fe atom and

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(A)

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(B)

Fig. 7. The structure of two Fe–propargyl alcohol complexes: the oxygen-bonded (A) and the p-bonded ). (B) isomers (bond lengths in A

Fig. 8. The relative energies measured from the separated fragments of the open-chain Fe–propargyl alcohol complexes in the singlet and the triplet state.

prop-1-yn-3-ol. Note that the singlet potential energy surface lies at higher energies than that for the triplet state.

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The calculations also revealed that Fe2þ ions also interact with PA, by forming complexes, which were found to be either analogues of the open-chain structure in Fig. 7A or a cyclic molecule. The latter, instead of being a clathrate type complex between Fe and a bidentate PA ligand, contains a four-membered ring involving the Fe, O, the saturated C atom and the substituted C atom of the triple bond. The latter C atom, interestingly, is covalently bound to the Fe atom, instead of a dative bond between the p bond and the ferrous ion. As the calculations indicate that these are stable complex ions, one can assume that the Fe2þ ions dissolved from the iron surface due to corrosion in the early stages of inhibitor treatment can be trapped in the protective layer in the form of such open-chain or cyclic complexes.

5. Discussion 5.1. The properties of the inhibitor films The results of impedance measurements performed without inhibitor and at 1 and 4 h after the immersion of the iron sample into the inhibitor-containing solution (Figs. 2 and 3, curves a, b and e) show that the corrosion rate is reduced if inhibitor is added and decreases with the time of the treatment. This can be explained by the formation of an inhibitor layer whose protection against corrosion increases with time. If we compare the impedance diagrams in Figs. 2 and 3 we can see that both inhibitors showed better performance in HCl than in H2 SO4 . This is probably associated to the presence of Cl ions, as it is known form the literature: for example, Growcock et al. found [14] that octynol was not effective in H2 SO4 solution, but adding some HCl to the solution increased the efficiency significantly. According to Murakawa and Hackerman, chloride ions promote the adsorption of organic molecules [5]. Anodic polarisation applied at the beginning of film formation (curves c) is seen to promote the increase of the impedance values as compared to that when no polarisation was applied (curves b). From this observation we can conclude that at the first stage of the film formation the anodic polarisation promotes the adsorption of the inhibitor by adding positive charge to the iron. This is in agreement with the result of Funkhouser, who postulated that the affinity of the triple bond to the d-electrons of the metal leads to chemisorption preferentially at anodic sites [2]. Application of anodic polarisation 1 h after immersion has a completely different effect on the behaviour of the layer. The impedance diagrams measured an hour later (Figs. 2 and 3, curves f) compared to the ones taken 1 h after immersion, without polarisation (curves b) show similar Rp and somewhat lower Cdl values. This indicates that when a polymer film already exists, anodic polarisation results in the destruction of the film, and formation of a new film begins after the polarisation is turned off. The influence of cathodic polarisation differs from that of anodic in two aspects. First, cathodic polarisation has practically no effect on the impedance diagrams if

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applied immediately after immersion (curves d compared to curves b). Second, cathodic polarisation has a favourable effect when applied to the existing film in contrast to the destruction caused by anodic polarisation. If cathodic polarisation is turned on for 5 min at 1 h after immersion, small increase of the impedances compared to the unpolarised system are observed three additional hours later (curves g compared to curves e). In the case of OCT in hydrochloric acid, Rp changed from about 32 kX cm2 (Fig. 3A, e) to about 36 kX cm2 (Fig. 3A, g). We think that the increase in impedance due to cathodic polarisation shows that the cathodic polarisation promotes the second step of the formation of the protective layer, namely, the growth of the polymer film. In conclusion, polarisation has a different effect if applied to the system immediately after immersion of the metal into the inhibitor-containing solution than if polarisation occurs after a delay of 1 h. This clearly indicates that there exist two well distinguishable stages of the formation of the protective film. According to the assumptions found in the literature, the initial step is the adsorption of the inhibitor and gradual formation of a monolayer [1–7], while the second is deposition of additional inhibitor molecules and polymerisation. Our experiments provide information on the time scale of formation of the two kinds of layers. In the following we summarise and discuss the information available on the two steps. 5.2. The first step of the formation of the inhibitor film: experimental and quantum chemical characterisation and interpretation of IR spectra The ellipsometry studies of Bockris and co-workers [20–24] suggest that in the first 20 min after the immersion of the electrode a chemisorbed monolayer is formed, followed by a linear growth of the film. The film formed in the second step is probably a polymer in agreement with the previously mentioned IR results from the literature [16]. We followed the formation of the inhibitor film by impedance measurements. However, during the first approximately 20 min after immersion (assumedly corresponding to the chemisorption step) the open-circuit potential continuously changes, so the stable potential required when taking impedance spectra is not established. Our impedance measurements show the increase of the protective capability of the film at a longer time scale. To support the fast chemisorption step we applied positive polarisation, i.e. positive charge to the iron surface immediately after immersion. We found that it promotes the adsorption of the alkynol molecules. We think that the beneficial influence of additional positive charge on the surface that is observed in the impedance measurements comes from the fact that the charge added to the electrode assists in the deposition of the first layer by attracting the alkynol molecule. A possible additional effect is that the binding energy of the dative bond between a charged iron surface and the inhibitor may be larger. According to certain assumptions [1–7], the chemisorption step occurs via the interaction of the d orbitals of the metal with the p orbital of the acetylene bond. Our quantum chemical calculations support this assumption: their results indicate that the binding of the alkynol molecule at an iron surface is preferred at the acetylenic

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bond (Fig. 7B) rather than at hydroxyl group. Since the binding energy of the former bond is significantly larger than that of an O–Fe bond. In principle, the oxygen atom could be a better binding site at a positively charged iron surface, because the center of the negative charge of an alkynol molecule, which could be better attracted toward the charged surface, is at the alcoholic oxygen. The OH group of the alkynol, however, is blocked from a direct interaction with the surface by a solvate layer. Our calculations addressing this question show that, as expected, a significant fraction of the energy released by the formation of an O–Fe bond is spent on the destruction of the solvate layer around the OH group. In contrast, formation of a donor–acceptor complex––which is more favourable energetically anyway––leaves intact the solvate layer around the OH group. The analysis of the structures of the ‘‘O-bonded’’ and p-bonded isomers shows that the acetylenic carbon–carbon bond remains a triple bond in the former, while it is degraded to an essentially double bond in the latter. To show this, we calculated the bond order from the wavefunction by MayerÕs method [30–32] for the multiple C–C bond which is 2.91 in the ‘‘Fe–O-bonded’’ isomer, 1.81 in the Fe–p-bonded isomer, 2.94 in free PA (reference triple bond) and 1.99 in free 1-propen-4-ol (reference double bond). We also calculated the so-called carbon–carbon diatomic energy contribution [33], which characterises the strength of the interaction of the two atoms in a molecule. The comparison of their numerical values in these compounds, namely, )1.51, )0.99, )1.52 and )1.02 EH , respectively, agrees very well with the change of the bond orders in these molecules. Based on this we can conclude that the acetylenic bond in the ‘‘O-bonded’’ complex is a triple, that in the p-bonded isomer is closer to a double bond. Experimentally, the two forms can be distinguished by IR spectroscopy, if one finds in the measured spectra IR lines corresponding to the triple carbon–carbon bond (identifying the ‘‘O-bonded’’ complex) or the double C@C bond (identifying the p-bonded form). As the nature of the bond is significantly different, one expects that the difference will be manifested in the characteristic frequency of the carbon–carbon bond. We calculated the carbon–carbon vibrational frequencies for the four compounds listed above. The quantum chemical calculations do not predict accurately these frequencies, but it is known from experience that the calculated frequencies are proportional to the measured frequencies. One can calculate the proportionality constant by comparing reliable and well-assigned measured frequencies to the calculated ones; then the ‘‘prediction’’ of the unknown frequency becomes possible. We derived the proportionality constant based on the comparison of the calculated and measured frequencies corresponding to the carbon–carbon in question in PA and in 1-propen-4-ol. This frequency in free PA is 2256 cm1 according to the B3LYP/3-21G* calculations, 2120 cm1 according to the experiments [34]. From this we can see that the calculations overestimate the frequency corresponding to the triple bond by a factor of about 1.064. The frequency of the double bond in 1-propen-4-ol is 1726 cm1 from the calculations and about 1620 cm1 from the experiments [34]; here the proportionality constant is about 1.065. The two factors are in good agreement, so, as usual in quantum chemical studies, one can justly assume that the frequency is overestimated by the same factor in the p-bonded complex of PA and an iron atom. The calculated frequency for the

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‘‘acetylenic’’ C–C bond in the complex is 1786 cm1 , which, using the correction factor obtained for the free reference molecules translates to 1675–1685 cm1 as an estimate of the real frequency. This is almost as low as that of a C@C double bond (1610–1640 cm1 ). At the same time, the IR intensity of this band increases from 3.4 km mol1 (in PA) to 16.9 km mol1 (in the p-bonded complex), making the band easier to detect. If one reviews this region of the measured reflection IR spectra of the film formed by OCT on iron, one can see that peaks near 1677 cm1 appear in those measured by Poling [10] and by Frenier et al. [16] (1715 cm1 ), by Bartos et al. [4] (1680 and 1655 cm1 ), by Fiaud et al. [11] (1640 cm1 ). The first two spectra were taken after the layer was exposed to oxygen so these workers assign the 1715 cm1 peak to the C@O bond formed by oxidation from the unsaturated bond. Bartos et al. measured the reflection IR spectrum immediately after immersion of a deaerated PA solution and claim that the weak absorption they observed at 1680 cm1 is due to a C@C double bond, while the peak at 1655 cm1 developed when oxygen is bubbled through the solution is due to a carboxylic C@O bond. We think that the former peak corresponds to a special C@C bond, namely, that participating in a dative bond to the iron surface. This is also supported by the fact that its frequency is higher than that of the regular C@C and C@C–C@C bonds observed by Fiaud et al. at 1640 and 1610 cm1 . The appearance of this peak in the early phase of the formation of the film indicates the formation of a dative bond between the surface and the alkynol. 5.3. The second step of the formation of the inhibitor film Concerning the second step of the formation of the protective layer, the occurrence of polymerisation is supported by the observations made by many other authors. In their studies of the mechanism of ethynylcyclohexanol polymerisation on iron by IR analysis, Duwell at al. [18] concluded that the inhibitor is reduced on the iron, is dehydrated afterwards, and subsequently forms a hydrocarbon polymer. To explain their observation made with octynol, Frenier et al. [16] also suggested a mechanism for film formation involving reduction and dehydration on the surface. They found the alkynol to remain unchanged in solution, but if steel is present, octynol is adsorbed onto it and a polymeric product is formed which they identified as an unsaturated oligomer of reduced octynol, in agreement with other authors [4,9]. According to Poling [10], nascent hydrogen atoms, formed by the cathodic part of the corrosion reaction from Hþ of the acid, participate in the polymerisation of propargyl alcohol, of acetylene, and of ethynylcyclohexanol. This assumption was supported by the observation that no polymer film is formed on metals where the Hþ ! H cathodic reaction does not take place (Cu, Ag, Pt). Our experiments provide a further support to this mechanism of the growth of the polymer layer. Namely, we see that cathodic polarisation, applied when the inhibitor layer is already deposited, promotes the formation of a more protective layer. At this stage the formation of the monolayer is complete, and additional alkynol molecules are deposited at the surface. Being unsaturated, they can chemically interact with the existing monolayer. Cathodic polarisation at this stage promotes the growth of the film by producing extra nascent hydrogen atoms due to the dominance of the cathodic

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process of the corrosion reaction, a fraction of which initiate polymerisation and may take part in saturation of the triple carbon–carbon bond. 5.4. The stability of the inhibitor film We performed cyclic voltammetry measurements in order to characterise the stability of the inhibitor films. Fig. 5 shows that for OCT in hydrochloric acid no hysteresis can be seen in the CV diagram. Even with a relatively large polarisation of 50 mV around the corrosion potential, the properties of the film remained unchanged. This indicates, in agreement with the impedance results, that the most stable inhibitor film is formed in this system. For OCT in sulphuric acid where the CV diagrams show that the hysteresis loop increases during the measurement, indicating the gradual destruction of the film. 20 mV around the open circuit potential increased the current rapidly mainly on the anodic side. For systems where increasing hysteresis is seen in the CV curves, nonzero inductive values appear in the low frequency part of the Bode plot. We think that both observations reflect the same property, namely, that these inhibitor films are easy to destroy, which is in agreement with the conclusions by other authors [26]. Similarly to octynol, the other inhibitor, ETH, showed also better performance in hydrochloric acid than in sulphuric acid from stability point of view. A technical advantage of CV measurements is that they can provide helpful information prior to taking impedance spectra. Based on the CV diagrams we can decide how sensitive the studied system is to the potential shift during the impedance measurement and how large amplitude can be used without damaging the system. Potential fluctuation measurements, in accord with the CV results, showed (Fig. 6) that the iron/OCT/sulphuric acid system with its several hundred lV potential peaks is the most sensitive to the changes of the local environment. In the case of an OCT film in HCl solution the potential fluctuation curve is rough compared to the curves obtained with ETH inhibitor. We tentatively attribute the large amplitude of the fluctuation to the observation that the layer formed by OCT is thicker than that by ETH.

6. Conclusions We studied the iron corrosion in acidic solutions containing alkynol inhibitors by electrochemical and quantum chemical methods. Based on impedance measurements we found that anodic polarisation assists in the initial binding of the inhibitor to the iron surface, while it destroys the film if polarisation is applied to an existing inhibitor layer. Cathodic polarisation, in contrast, does not influence the initial layer formation, while it promotes the growth of the inhibitor film. These observations support the assumption that alkynol-type inhibitors form a corrosion protecting layer in two steps: first a monolayer is bound to the iron surface by chemisorption, and second polymerisation proceeds when further inhibitor molecules are deposited over the monolayer. The early anodic polarisation supports the adsorption of the

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first inhibitor layer. According to our quantum chemical calculations, which allow the interpretation of the IR spectra of the chemisorbed layer, a dative bond is formed between the iron and the carbon–carbon triple bond. Cathodic polarisation applied at a later stage assists in the polymerisation of the deposited monomers it provides extra hydrogen atoms because the cathodic step of the corrosion reaction dominates. These H atoms can initiate polymerisation and may take part in partial saturation of the triple bond.

Acknowledgements The authors would like to thank Dr. T. Pajkossy for critical reading of the manuscript. Financial support of the Hungarian Scientific Research Fund (Grant no. T 029727) is acknowledged.

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