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An electrochemical and SERS investigation of the influence of pH on the effectiveness of some corrosion inhibitors of copper
187
J. Electroanal. Chem., 217 (1987) 187-202 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
AN ELECTROCHEMICAL AND SERS INVESTIGATION OF THEI OF SOME CORROSION INBOX OF pH ON THE EFFE tIXVBWSS I~~ORS OF COPPER
M.M. MUSIANI and G. MRNGOLI
Istituto dt Polarografia ed Elettrochimica Preparativa de1 CNR, Corso Stati Unrti, 4-3.5100 Podova (Italy) M. FLEISCHMANN
and R.B. LOWRY
Chemistq Department, The Universily, Southampton SO9 SNH (Great Britain) (Received 12th May 1986; in revised form 4th August 1986)
ABSTRACT The adsorption on Cu of the corrosion inhibitors benxotriazole (BTA), 2-mercaptobenxothiaxole, 2-mercaptob enzimidazole and 2-rnercaptobenzoxaxole has been characterized in both neutral and acid chloride solutions using electrochemical techniques and surface enhanced Ramau spectroscopy. The undissociated inhibitors and their anions are adsorbed simultaneously, the surface concentration ratio depending on the pH and electrode potential. At low pH, BTA is adsorbed weakly and it is displaced from surface sites by both Cl- and the strongly adsorbed 2-mercaptobenzothiale. These spectroscopic results explain the low corrosion inhibition due to BTA in acid solution.
INTRODUCTION
Organic corrosion inhibitors have been extensively investigated during the last three decades as they provide an effective and inexpensive means of reducing the degradation of metals and alloys in many fields of application [I,2]. The identification of the most active compounds has been carried out empiricaIly using a variety of metal substrates and standard environments. It has been concluded that tailormade syntheses of inhibitors are not possible at this stage as comparisons of extensive series of compounds (over 570 in the case of Cu) have not led to definitive structure-effectiveness correlations [3]. At present, it is agreed that benzotriazole (BTA) [4-91 and mercapto-substituted heterocyclic compounds [lo-121, e.g. 2-mercaptobenzothiale (MET), 2mercaptobenzimidazole (MBI) and 2-mercaptobenzoxazole (MBO), are the most effective inhibitors for Cu, although an even higher protection efficiency has been claimed for 2-~op~~d~e 1131. 0022-0728/87/$03.50
6 1987 Elsevier Sequoia S.A.
188
The nature of the protective layers formed on metal substrates by these very effective inhibitors has been investigated using ex-situ spectroscopies, e.g. reflectance infrared [6,14,15] and XPS [16-U]. In-situ spectroscopic investigations of corrosion inhibitors have also been carried out using surface enhanced Raman scattering (SERS) (for reviews of the use of SERS in the study of adsorption on electrodes see, for example, refs. 19 and 20). Such measurements probe the solution side of the interfacial region and avoid any changes which may be caused by the transfer of the system to a high vacuum environment. Results have been reported for mercaptobenzothiazole on Ag [21], for benzotriazole on Ag [22] and Cu [23-301, as well as for mercaptobenzoxazole on Cu [26]. The aim of our own investigations has been to design SERS experiments which can be correlated with conventional corrosion rate measurements to give a dynamic description of the interface as a function of the key variables: potential, concentrations of inhibitors and salts, and of the exposure time [26-28,301. The competitive adsorption of inhibitors and anions has been monitored [26-28,301. The correlation of the effects of pH on corrosion with changes in the SERS spectra has not so far been carried out. Divergent results have been reported in the literature using conventional electrochemical techniques. Thus the corrosion protection by BTA in acid solutions has been stated to range from good [15], fair [8] and poor [31] to corrosion activation [32]. A change in thickness and morphology of surface films was detected at pH 3.5 [33,34]. In this paper we report SERS measurements as a function of pH on the inhibitors benxotriazole and 2-mercaptobenzothiazole (the latter chosen as the most effective corrosion inhibitor for Cu from a set of mercapto compounds) and relate these to corrosion rate measurements made according to well-established electrochemical procedures [35,36]. The Raman spectroscopic investigation was extended to cover Ag in addition to Cu in order to determine the extent to which the adsorption behaviour is governed by electrostatic or specific chemical effects. The pH dependence of the adsorption is of interest also in the well-known use of these compounds as antistaining agents for Ag. EXPERIMENTAL
AnalaR grade KC1 and reagent-grade inhibitors (BTA 99%, MBT and MB1 98%, and MB0 95%) were used as supplied by BDH Chemicals PLC, Aldrich Chemical Co. and Carlo Erba. Solutions were prepared using triply distilled water; KOH was added initially in order to facilitate dissolution of mercapto compounds and the pH was then adjusted using either HCl or KOH solutions. Cu disc electrodes made from Koch Light 99.999% copper had areas ranging from 0.03 to 0.32 r&. Ag electrodes of area 0.25 cm2 were prepared from Johnson-Matthey specpure grade silver. Before each experiment the electrodes were polished using alumina powder (0 = 0.1-0.01 pm). A three-compartment cell with a Pt counter-electrode and a saturated calomel electrode (SCE) as the reference electrode was used for all the voltammetric measurements. A Cu electrode was rotated (1OOOrev min-‘) at open circuit in 1 A4
189
KC1 solutions of the appropriate pH, either in the absence or in the presence of the chosen inhibitor. Current-potential curves were then recorded, starting from the corrosion potential, both in the positive and negative directions, using a scan rate of 0.5 mV s-l. Cyclic voltammograms over a 10 mV range, centred at the corrosion potential, were determined after intervals of variable duration at open circuit (U-60 min) with a sweep rate of 0.1 mV s-l_ The polarization resistance (R,) was then measured as the slope of the tangent to the current-potential curve at the corrosion potential. The electrode was left at open circuit (1000 rev m&l) between successive R, dete~ations, each of which took less than 4 min. _ The cell employed in the sp~tr~l~tr~he~c~ experiments has been described elsewhere [ZO].Electrochemical roughening, necessary for achieving strong surface enhancement, was carried out, in the same cell, either in the inhibitor solution or in 1 M KCl. The latter procedure was always adopted for Cu in order to avoid anodic formation of Cu-inhibitor layers and for MBT solutions in order to prevent inhibitor oxidation. The potential was stepped from -0.2 to i-O.2 V (vs. SCE) for Ag and from -0.5 to +0.15 V for Cu; the length of the pulse was 5 s. Laser illumination was excluded during roughening. Raman spectra were recorded with a Coderg T800 spectrometer using an Ar laser for silver (514.5 nm, 100-110 mW power at sample) and with an Anaspec 36, equipped with a 1024 element Tracer Northern detector, using a He-Ne laser for copper (638.2 nm, 35-40 mW power at sample). The spectra recorded with the Anaspec 36 spectrograph have not been corrected for the decrease in sensitivity of the array towards the edges and the relative band intensities are therefore distorted. A spectral region extending over - 500 cm-’ is monitored simultaneously when using the Anaspec spectrograph. The range from 700 to 1600 cm-l (comprising most of the relevant Raman bands) was therefore covered with two acquisitions; the spectra are given here after subtraction of the background. RESULTS
Benzotriazole (BTA) and three inhibitors ~nt~g the mercapt~function, i.e. Z-mercaptobenzothiazole (MBT), Z-mercaptobenzimidazole (MBI) and 2mercaptobenzoxazole (MBO), were studied in neutral and acidic (pH 2) 1 M KC1 solutions by recording slow potentiodynamic polarization curves (at 0.5 mV s-i) after rotating a Cu electrode at open circuit for l-3 h at 1000’ rev mm-‘. The results obtained with BTA and MBT are compared in Fig. 1 (A: neutral solutions, B: acidic solutions) in which the polarization curves obtained in plain 1 1M KC1 are also reported. The evaluation of corrosion currents by extrapolation of Tafel lines is based on a number of assumptions one of which, the absence of IR drops across resistive surface fihns, is unlikely to be correct in our experiments 1361. The I_ data in Table 1, extrapolated from the cathodic curves, as well as the protection efficiencies
190
/
/
/
/
/
I I
100
“E
r %
‘\
/ I
\
\
I
10
1
01
E/mV
E/mV
Fig. 1. Cu~nt-potenti~ curves obtained at a rotating (1000 rev min-‘) Cu disc. (- * -) 1 M KCl+O.OOl M BTA; (- - -) 1 M KCl+O.OOl M MBT. Scan rate=O.S Immersion time = 1 h. (A) Neutral solutions; (B) PI-I 2.
( -)
X M KCI; mV s-l.
191 TABLE 1 Corrosion current densitiesof Cu in 1 M KCI and inhibited1 M KC1 solutionsas a function of pH, exposure time and inhibitorconcentration Inhibitor None None None None BTA BTA BTA BTA BTA BTA MBT MBT MBT MBT MBT MB1 MB1 MB0 MB0
c/mM
PH
Time/b
I-/CA
1 1 1 1 10 10 1 1 1 1 0.1 1 1 1 1
I 2 7 2 7 2 7 2 7 2 I 2 7 2 2 7 2 7 2
1 1 3 3 1 1 3 3 1 1 1 1 3 3 1 1 1 1 1
6.8 12.7 5.7 8.9 0.35 5.1 0.17 7.0 0.14 3.15 0.35 0.20 0.20 0.22 11.3 ‘0.35 0.35 0.40 0.35
cm-’
Protection efficiency 95 60 97 21 98 70 95 98 96 97 90 95 97 94 97
calculated as the percentage decrease of the corrosion current caused by the inhibitor with respect to the appropriate blank solution, must be considered approximate values. However, some clear-cut features of the behaviour of the investigated inhibitors emerge from the data in Table 1 and Fig. 1: (i) The four inhibitors are equivalent in neutral solutions providing - 95% corrosion protection. (ii) MBT, MB1 and MI30 keep (and even improve) their effectiveness at pH 2, while the corrosion current is suppressed by only 60% in the presence of BTA. (iii) The anodic curves for MBT, Fig. 1, indicate a much higher current in the neutral medium. This is thought to be due to the easier oxidation of MBT anion with respect to the undissociated molecule [37] rather than a faster Cu dissolution reaction. Indeed, oxidation currents were obtained at a Et electrode in neutral 0.001 M MBT solution at potentials > - 180 mV (vs. SCE), while polarization to > - 60 mV was necessary in acid solutions. (iv) The failure of BTA in the acidic medium is further stressed by prolonging the exposure time to 3 h; the corrosion current increases with time. (v) The MBT concentration cannot be increased beyond 0.001 M, at lower concentrations, MBT still provides good protection (90% at pH 2 for a 0.0001 M solution). Conversely, a ten-fold increase of the more soluble BTA (0.01 Iw) only raises its efficiency from 60 to 70% in acid media.
192
1
Fig. 2. Polarization resistance-time curves obtained at a rotating (1000 rev min-‘) Cu disc in neutral (solid lines) and acidic (pH 2, dotted lines) solutions. @) 1 M KCI; (a) 1 M KCl+O.OOl M BTA; (A) 1 A4 Kc1 + 0.001 hf MBT.
Although the polarization resistance technique suffers from the same limitations as those indicated above for the Tafel-line extrapolation method [36] when resistive surface films are present on the electrode, qualitative information on the time dependence of the corrosion current may be achieved by monitoring R, vs. time. The data relative to BTA, MBT and uninhibited 1 M KC1 are shown in Fig. 2 and confirm the conclusions drawn from the data in Table 1: Cu corrosion at pH 7 is largely suppressed by both MBT and BTA, whereas only MBT provides protection in acidic KC1 solution, BTA using the R, values. The R, vs. time curves diverge over a period of 5 h. After 1 h exposure, the R, valueswbited and blank solutions differ by less than a factor of 10. The order of magnitude of the corrosion currents calculated according to the formula IWrr= B/R,, assuming B = 19 mV (found in the literature for Cu in seawater [38,39]), is in good agreement with the data in Table 1 for the blank solutions (8-15 PA cmm2); the same formula, applied for the R, values measured in inhibited solutions, yields larger corrosion currents than those obtained from Tafel-line extrapolation: 1.5-3.5 PA cm-’ for neutral solutions; about 0.85 PA crne2 for MBT in acidic KCI.
193
The corrosion potential of Cu electrodes was shown to change by less than 30 mV over 5 h for all the solutions of Fig. 2, being either in the range from - 340 to - 285 mV for the blank and acidic BTA solutions or in the range from - 170 to - 115 mV for the MBT ‘and neutral BTA ones.
t (A)
(8)
% 2ow g-1
i
, E
.“, ;,L--/ 5 a”
Fig. 3. SERS spectra for Cu in 1 M KCI+O.Ol
- 1.0 v.
N BTA (pH 7). (A) Open circuit (E,,
= -0.12
v); (B)
194
Raman spectroscopy The electrochemical measurements demonstrate the markedly different behaviour of mercapto-inhibitors, which are active over a wide pH range, and of BTA, which is effective only in nearly neutral solutions. MBT (as a model for mercapto-inhibitors) and BTA were therefore chosen for a spectroelectrochemical investigation. Adsorption on both Ag and Cu was studied, with special attention to Cu at open circuit, i.e. under the actual corrosion conditions.
(A)
I
760
llbo llao V/W’ pw3.6
&0
195
Fig. 4. (A) SERS spectrum of 1 M KCl+O.Ol M BTA (pH 2) at Cu (open circuit; I& = -0.3 v). (B) Effect of pH on the SERS spectrum of BTA at Cu (open circuit). (C) Effect of the electrode potential on the SERS spectrum of BTA at Cu (pH 2.0).
Benzotriazole
SERS spectra for Cu in neutral 1 M KC1 + 0.01 it4 BTA are shown in Fig. 3. The low frequency range (< 700 cm-‘) is not shown. However, the Cu-Cl band at 286 cm-’ was found to be absent for 1 M KC1 solutions containing BTA concentrations as low as 10v5 M, in sharp contrast to the strong Ag-Cl band at 240 cm-‘. Despite the non-linearity of the detector, all bands for BTA adsorbed at Cu are easily identified and the expected intensity-potential dependence was found: the sensitivity for Fig. 3B (- 1.0 V) is half of that for Fig. 3A (OX.). Major changes occur after acidification of the BTA solution, (Fig. 4A) (compare ref. 24): for example, the band at 1196 cm-’ is no longer seen; new bands appear at 1127 (very broad), 1170 and 1600 cm- i. The most notable change is found in the shape of the band system at - 1380 cm-‘, the pH and potential dependences of which are shown in Figs. 4B and 4C, respectively. In nearly neutral solutions, at open circuit, the main band at 1386 cm-’ is accompanied by a shoulder around 1370 cm-‘, the relative intensity of which is seen to increase with decreasing pH until, at pH 1, the two bands are equally intense (Fig. 4B). When the electrode potential is changed from the open circuit value ( - 0.3 V at pH 2) to - 0.6 V, i.e. a value close to the pzc [40], the lower frequency band becomes relatively more intense (Fig. 4C). It must be concluded that the band at 1374 cm-’ belongs to the neutral BTA molecule (possibly involved in a [CuCl(BTA)], complex, as suggested by Rubim et al. [24]), while the band at 1387 cm-’ is due to the BTA anion, the adsorption of the latter being favoured by low acidity and at potentials more positive than the pzc. The rather surprising increase of the 1387 cm-’ band observed when the Cu potential is taken to -0.8 V (Fig. 4C) is probably due to the cathodic current (I-I2 evolution) flowing at this potential, causing a pH increase at the electrode surface. Comparable results are obtained at Ag, which yields additional information on the influence of acidity on Cl adsorption. Table 2 illustrates the variation of the
196 TABLE 2 Relative Raman intensity of Cl and BTA for a 1 M KCl+O.OOl M BTA solution at a Ag electrode as a function of pH and potential PH
Potential/V
7 7 7 2 2 2
-0.2 -0.6 -1.0 -0.2 -0.6 -1.0
(vs. SCE)
a Ratio between the intensity of the Ag-Cl
Cl/BTA
a
1.65 0.45 0.15 4.7 2.8 0.15 band (240 cm-‘)
and a BTA band (786 cm-‘).
intensity ratio between the Ag-Cl band and a selected BTA band (786 cm-’ benzene ring breathing mode) as a function of pH and potential. The higher relative intensity of the Ag-Cl band in acid media (except at -1.0 V, where this band is always very weak *) indicates that undissociated BTA competes less efficiently with chloride ion for surface sites than does the anion [22].
244ercaptobenzothiazole Strong SEBS spectra were obtained at Ag for both neutral and acidic 1 M KC1 + 0.001 M MBT solutions (Fig. 5). These spectra, ascribed to the MBT anion and undissociated MBT respectively, correspond to those published by Oshawa et al. [21] as far as the number and relative intensity of the bands are concerned. However, the band positions measured in this investigation were at systematically lower wavenumbers (8-15 cm-‘) than in the earlier report [21]. Additional bands have been observed at < 900 cm-’ (a region not studied by the Japanese group): 393, 604, 711 and 866 cm-’ at pH 7; 390, 685, 711 and 866 cm-’ at pH 2. The Ag-Cl band is weak in both spectra (although the potential is the most favourable potential for the detection of this band) and, most notably, its intensity does not increase at the lower pH. The band at 1388 cm-’ in Fig. 5B, due to the anion, is no longer detected at pH 1. The spectrum recorded at Cu for an MBT solution at pH 2 (Fig. 6A) is dominated by the bands of the anion and only differs from that obtained in neutral solution by the presence of bands at 1336 and 1488 cm-’ **. By further increasing the acidity (pH 1) the spectrum changes, to that of Fig. 6B: the undissociated molecule is preferentially adsorbed, but without complete displacement of the anion (band at 1385 cm-‘). Comparison of Figs. 5 and 6 indicates that for the same pH, i.e. for the same MBT anion/undissociated MBT ratio in solution, a higher surface concentration of
* Local pH increase is expected to affect this result also. l * A shoulder at 1374 cm-’ (also present in the spectrum recorded at pH 7) is seen here, which was not detected at Ag.
197
(A)
260
1700 1 9
I
cm-1
Fig. 5. SERS spectra of 1 M KC3 + 0.001 M MBT at Ag, - 0.2 V. (A) pH 7; (B) pH 2.
the anion is found on Cu than on Ag, although electrostatic forces would promote the opposite behaviour: since all the spectra in Figs. 5 and 6 were recorded at a potential close to -0.2 V, Ag was more positive than Cu with respect to pzc ( - - 1.0 V for Ag [41] and - 0.4 to - 0.7 V for Cu [40,41]). The stronger adsorption of anions on the latter metal must therefore be due to specific chemical interactions.
198
(A)
7bo
fmlko
‘+k
l&O
Fig. 6. SERS spectra of 1 M KCl+O.OOl M MBT at Cu, open circuit. (A) pH 2 (Em, = -0.17 V); (B) pH-1 (Jr&= -0.20 v).
199
C~~~etitiue a~~~ption of BTA and MBT SERS spectra of neutral solutions containing equimolar amounts of BTA and MBT at both Ag and Cu are found to be identical to those of the latter inhibitor
l&O
rho
I
v cm+ Fig. 7. (A) SERS spectrum of 1 M KCl + 0.01 M BTA + 0.1 m M MBT (PH 7) at Ag. - 0.2 V. (B) SERS spectrum of 1 M KC1 + 0.007 M BTA + 0.35 m M MBT (pH 7) at Cu, open circuit (E,, = - 0.14 V).
(A)
Fig. 8. (A) SEW spectrum of 1 M KCI + 0.01 M BTA + 0.1 m M MBT (pH 2) at Ag, - 0.2 V. (B) SERS spectrum of 1 M KCI -+0.007 M BTA + 0.35 m M MBT (pH 1) at Cu, open circuit ( E,, * - 0.20 V).
alone. S~ul~eous adsorption of both ~bitors (as anions) is demonstrate by Figs. 7A (Ag) and 7B (Cu) for solutions containing 0.01 M BTA + 0.0001 M MBT and 0.007 M BTA + 0.00035 M MBT, respectively. Overlapping of the signals from BTA and MBT occurs for many bands (labelled MBT + BTA in Fig. 7); however, the bands at 1248,711 and 393 cm-r on Ag and 1241 cm-’ on Cu are diagnostic for MBT and those at 1163 and 786 cm-l on Ag and 1195 and 792 cm-’ (the latter not shown in Fig. 7B) are due to BTA only. The change from adsorbed anions to adsorbed undissociated molecules, caused by dilution of the solutions, was found to be accompanied by a variation in the relative surface concentrations in favour of MBT at both metal substrates. Thus the spectrum recorded at Ag in 0.01 M BTA + 0.0001 M MBT at pH 2 (Fig. 8A) cannot be distinguished from that in Fig. 5B (MBT alone). For Cu also, at pH 1 (Fig. 8B) the appearance of the bands typical of undissociated MBT (1336 and 1419 cm-‘), the increase in intensity of the band at 1241 cm-“, and the absence of the
201
band at 786 cm-’ (due to BTA) indicate preferential adsorption of MBT in acid media. The assignment of the bands at 1160 and 1365 cm-l is not straightforward; these bands could be due to photolysiswhich was seen to occur on the Cu electrodes and prevented an accurate determination of the time dependence of the Raman intensities, as described in ref. 26. CONCLUSION
The Raman spectroscopic results clarify the fall in the efficiency of BTA as a corrosion inhibitor of Cu in solutions of increasing acidity, compared with the essentially pH-independent efficiency of MBT. Both inhibitors are adsorbed as anions from neutral solutions, in agreement with previous observations [21,22,24]. A complete replacement by the corresponding undissociated molecules does not take place at Cu surfaces even at very high acidity, although both BTA and MBT are weak acids with pK, values close to 8 [6]. Comparison of the strengths of adsorption indicates that MBT is more strongly adsorbed than BTA at both Ag and Cu over the entire pH range investigated, the difference increasing with increasing acidity. Although a complete correspondence between strength of adsorption and corrosion inhibition efficiency cannot be assumed [26], an increasing MBT/BTA surface concentration ratio at lower pH is in keeping with the observed corrosion behaviour. A weakening of BTA adsorption at low pH is also demonstrated by the experiments carried out with Ag electrodes. An increase in the adsorption of the aggressivechloride ion is found with increase of the acidity of BTA solutions, but this is not observed for solutions containing MBT. A &-Cl band has been observed in the spectrum of an acid 0.1 M KC1 + 0.005 M BTA solution [24] but no such band was found in the present investigation, Our suggestion that effective protection is indicated in Raman spectroscopic experiments by the absence of the bands due to adsorbed aggressive anions in solutions containing the inhibitor [25] is confirmed by the results in this work. The present study also emphasizes the usefulness of experiments on the competitive adsorption of species and shows that potential-dependent surface acid-base equilibria (which differ markedly from the solution equilibria) are also of key importance in explaining the relative efficiency of inhibitors.
The authors thank NATO (grant S.A. S-2-05 RG n 815/83) and the United States Office of Naval Research for financial support and Mr. F. Furlanetto of CNR for valuable experimental work. REFERENCES 1 G. Trabanelli and V. Carassitiin M.G. Fontana and R.W. StaehIe(Eds.), Advances in Corrosion Science and Technology, Vol. 1, Plenum,New York, 1970, p. 147.
202 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
C.C. Nathan (Ed.), Corrosion Inhibitors, NACE, Houston, TX, 1973. L. Homer and E. Phefke, We&t. Korros., 33 (1982) 98. I. Dugdale and J.B. Cotton, Corros. Sci., 3 (1963) 69. J.B. Cotton and I.R. Scholes, Br. Corros. J., 2 (1%7) 1. G.W. Poling, Corms. Sci., 10 (1970) 359. F. Mansfeld, T. Smith and E.P. Parry, Corrosion, 27 (1971) 289. R. Walker, Corrosion, 29 (1971) 290. G. Trabanelli, F. Zucchi, G. Brunoro and V. Cam&i, We&t. Korros., 24 (1973) 602. G.B. Hatch, Corrosion, 11 (1955) 461t. G.B. Hatch in ref. 2, p. 142. S. lbibault and J. Talbot, CR. Acad. Sci., 278 (1974) C-503. L. Homer and E. Phefke, Werkst. Korros., 33 (1982) 189. S. Tbibault and J. Talbot, Met. Corros. Ind., 50 (1975) 51. S. Thibault, Corros. Sci., 17 (1977) 701. RF. Roberts, J. Electron Spectrosc., 4 (1974) 273. D. Chadwick and T. Hasemi, Corros. Sci., 18 (1978) 39. D. Chadwick and T. Hasemi, Surf. Sci., 89 (1979) 649. R.K. Chang and T.W. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenm, New York, 1982. M. Fleischnxum and I.R. Hill in R.E. White, J.O’M. Bockris, B.E. Conway and E. Yeager (Fds.), Comprehensive Treatise of Electrochemistry, Vol. 8, Plenum, New York, 1984, p. 373. M. Oshawa, H. Matsuda and W. Suetaka, Chem. Phys. Lett., 84 (1981) 163. J.C. Rubim, I.G.R. Gutz and 0. Sala, J. Mol. Struct., 101 (1983) 1. J.J. Kester, T.E. Furtak and A.J. Bevolo, J. EIectrochem. Sot., 129 (1982) 1716. J.C. Rubim, I.G.R. Gutz, 0. Sala and W.J. Orville-Thomas, J. Mol Struct., 100 (1983) 571. D. Tbierry and C. Leygraf, I. Electrochem. Sot., 132 (1985). M. Fleischmann, I.R. Hill, G. MengoIi, M.M. Musiani and J. Akhavan, Electrochim. Acta, 30 (1985) 879. M. Fleischmann, G. Mengoli, M.M. Musiani and C. Pagura, Electrochim. Acta, 30 (1985) 1591. M. Fleischmamr, G. Mengoli and M.M. Musiani, Proceedings of the 6th SEX, Ferrara, Italy, Sept. 1985, Ann. Univ. Ferrara, N.S., Sez. 5, Suppl. N. 8, (1985) 757. D. Thierry and C. Leygraf in ref. 28, p. 779. M.M. Musiani, G. Mengoli and M. Fleischmann, 36th ISE Meeting, Salamanca, Spain, Sept. 1985, Ext. Abstr. 06360. R. Walker, Corrosion, 31 (1975) 97, T.K. Ross and M.R. Berry, Corros. Sci., 11 (1971) 273. F. Mansfeld and T. Smith, Corrosion, 29 (1973) 105. I.C.G. Ongle and G.W. Poling, Can. Metall. Q., 14 (1975) 37. W.H. Smyrl in J.G’M. Bock&, B.E. Conway, E. Yeager and R.E. White @is.), Comprehensive Treatise of ~~~~he~s~, Vol. 4, Plemun, New York, 1981, p. 97. F. Mansfeld in M.G. Fontana and R.W. Staehle (Eds.), Advances in Corrosion Science and Technology, Vol. 8, Plenum, New York, 1978, p. 163. J.Q. Chambers in A.J. Bard and H. Lund (Eds.), Encyclopedia of Electrochemistry of the Elements, Vol. 12, Marcel Dekker, New York, 1978, p. 329. H. Grubitsch, F. HiIbert and R. Sammer, Werkst. Korros., 17 (1966) 7’60. C. Loss and E. Heitz, Werkst. Korros., 24 (1973) 38. J.O’M. Bock.&, M. Green and D.A.J. Swinkels, J. Electrochem. Sot., 111 (1964) 743. R.S. Perkins, R.C. Livingston, T.N. Andersen and H. Eyring, J. Phys. Chem., 69 (1965) 3329.
J. Electroanal. Chem., 217 (1987) 187-202 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
AN ELECTROCHEMICAL AND SERS INVESTIGATION OF THEI OF SOME CORROSION INBOX OF pH ON THE EFFE tIXVBWSS I~~ORS OF COPPER
M.M. MUSIANI and G. MRNGOLI
Istituto dt Polarografia ed Elettrochimica Preparativa de1 CNR, Corso Stati Unrti, 4-3.5100 Podova (Italy) M. FLEISCHMANN
and R.B. LOWRY
Chemistq Department, The Universily, Southampton SO9 SNH (Great Britain) (Received 12th May 1986; in revised form 4th August 1986)
ABSTRACT The adsorption on Cu of the corrosion inhibitors benxotriazole (BTA), 2-mercaptobenxothiaxole, 2-mercaptob enzimidazole and 2-rnercaptobenzoxaxole has been characterized in both neutral and acid chloride solutions using electrochemical techniques and surface enhanced Ramau spectroscopy. The undissociated inhibitors and their anions are adsorbed simultaneously, the surface concentration ratio depending on the pH and electrode potential. At low pH, BTA is adsorbed weakly and it is displaced from surface sites by both Cl- and the strongly adsorbed 2-mercaptobenzothiale. These spectroscopic results explain the low corrosion inhibition due to BTA in acid solution.
INTRODUCTION
Organic corrosion inhibitors have been extensively investigated during the last three decades as they provide an effective and inexpensive means of reducing the degradation of metals and alloys in many fields of application [I,2]. The identification of the most active compounds has been carried out empiricaIly using a variety of metal substrates and standard environments. It has been concluded that tailormade syntheses of inhibitors are not possible at this stage as comparisons of extensive series of compounds (over 570 in the case of Cu) have not led to definitive structure-effectiveness correlations [3]. At present, it is agreed that benzotriazole (BTA) [4-91 and mercapto-substituted heterocyclic compounds [lo-121, e.g. 2-mercaptobenzothiale (MET), 2mercaptobenzimidazole (MBI) and 2-mercaptobenzoxazole (MBO), are the most effective inhibitors for Cu, although an even higher protection efficiency has been claimed for 2-~op~~d~e 1131. 0022-0728/87/$03.50
6 1987 Elsevier Sequoia S.A.
188
The nature of the protective layers formed on metal substrates by these very effective inhibitors has been investigated using ex-situ spectroscopies, e.g. reflectance infrared [6,14,15] and XPS [16-U]. In-situ spectroscopic investigations of corrosion inhibitors have also been carried out using surface enhanced Raman scattering (SERS) (for reviews of the use of SERS in the study of adsorption on electrodes see, for example, refs. 19 and 20). Such measurements probe the solution side of the interfacial region and avoid any changes which may be caused by the transfer of the system to a high vacuum environment. Results have been reported for mercaptobenzothiazole on Ag [21], for benzotriazole on Ag [22] and Cu [23-301, as well as for mercaptobenzoxazole on Cu [26]. The aim of our own investigations has been to design SERS experiments which can be correlated with conventional corrosion rate measurements to give a dynamic description of the interface as a function of the key variables: potential, concentrations of inhibitors and salts, and of the exposure time [26-28,301. The competitive adsorption of inhibitors and anions has been monitored [26-28,301. The correlation of the effects of pH on corrosion with changes in the SERS spectra has not so far been carried out. Divergent results have been reported in the literature using conventional electrochemical techniques. Thus the corrosion protection by BTA in acid solutions has been stated to range from good [15], fair [8] and poor [31] to corrosion activation [32]. A change in thickness and morphology of surface films was detected at pH 3.5 [33,34]. In this paper we report SERS measurements as a function of pH on the inhibitors benxotriazole and 2-mercaptobenzothiazole (the latter chosen as the most effective corrosion inhibitor for Cu from a set of mercapto compounds) and relate these to corrosion rate measurements made according to well-established electrochemical procedures [35,36]. The Raman spectroscopic investigation was extended to cover Ag in addition to Cu in order to determine the extent to which the adsorption behaviour is governed by electrostatic or specific chemical effects. The pH dependence of the adsorption is of interest also in the well-known use of these compounds as antistaining agents for Ag. EXPERIMENTAL
AnalaR grade KC1 and reagent-grade inhibitors (BTA 99%, MBT and MB1 98%, and MB0 95%) were used as supplied by BDH Chemicals PLC, Aldrich Chemical Co. and Carlo Erba. Solutions were prepared using triply distilled water; KOH was added initially in order to facilitate dissolution of mercapto compounds and the pH was then adjusted using either HCl or KOH solutions. Cu disc electrodes made from Koch Light 99.999% copper had areas ranging from 0.03 to 0.32 r&. Ag electrodes of area 0.25 cm2 were prepared from Johnson-Matthey specpure grade silver. Before each experiment the electrodes were polished using alumina powder (0 = 0.1-0.01 pm). A three-compartment cell with a Pt counter-electrode and a saturated calomel electrode (SCE) as the reference electrode was used for all the voltammetric measurements. A Cu electrode was rotated (1OOOrev min-‘) at open circuit in 1 A4
189
KC1 solutions of the appropriate pH, either in the absence or in the presence of the chosen inhibitor. Current-potential curves were then recorded, starting from the corrosion potential, both in the positive and negative directions, using a scan rate of 0.5 mV s-l. Cyclic voltammograms over a 10 mV range, centred at the corrosion potential, were determined after intervals of variable duration at open circuit (U-60 min) with a sweep rate of 0.1 mV s-l_ The polarization resistance (R,) was then measured as the slope of the tangent to the current-potential curve at the corrosion potential. The electrode was left at open circuit (1000 rev m&l) between successive R, dete~ations, each of which took less than 4 min. _ The cell employed in the sp~tr~l~tr~he~c~ experiments has been described elsewhere [ZO].Electrochemical roughening, necessary for achieving strong surface enhancement, was carried out, in the same cell, either in the inhibitor solution or in 1 M KCl. The latter procedure was always adopted for Cu in order to avoid anodic formation of Cu-inhibitor layers and for MBT solutions in order to prevent inhibitor oxidation. The potential was stepped from -0.2 to i-O.2 V (vs. SCE) for Ag and from -0.5 to +0.15 V for Cu; the length of the pulse was 5 s. Laser illumination was excluded during roughening. Raman spectra were recorded with a Coderg T800 spectrometer using an Ar laser for silver (514.5 nm, 100-110 mW power at sample) and with an Anaspec 36, equipped with a 1024 element Tracer Northern detector, using a He-Ne laser for copper (638.2 nm, 35-40 mW power at sample). The spectra recorded with the Anaspec 36 spectrograph have not been corrected for the decrease in sensitivity of the array towards the edges and the relative band intensities are therefore distorted. A spectral region extending over - 500 cm-’ is monitored simultaneously when using the Anaspec spectrograph. The range from 700 to 1600 cm-l (comprising most of the relevant Raman bands) was therefore covered with two acquisitions; the spectra are given here after subtraction of the background. RESULTS
Benzotriazole (BTA) and three inhibitors ~nt~g the mercapt~function, i.e. Z-mercaptobenzothiazole (MBT), Z-mercaptobenzimidazole (MBI) and 2mercaptobenzoxazole (MBO), were studied in neutral and acidic (pH 2) 1 M KC1 solutions by recording slow potentiodynamic polarization curves (at 0.5 mV s-i) after rotating a Cu electrode at open circuit for l-3 h at 1000’ rev mm-‘. The results obtained with BTA and MBT are compared in Fig. 1 (A: neutral solutions, B: acidic solutions) in which the polarization curves obtained in plain 1 1M KC1 are also reported. The evaluation of corrosion currents by extrapolation of Tafel lines is based on a number of assumptions one of which, the absence of IR drops across resistive surface fihns, is unlikely to be correct in our experiments 1361. The I_ data in Table 1, extrapolated from the cathodic curves, as well as the protection efficiencies
190
/
/
/
/
/
I I
100
“E
r %
‘\
/ I
\
\
I
10
1
01
E/mV
E/mV
Fig. 1. Cu~nt-potenti~ curves obtained at a rotating (1000 rev min-‘) Cu disc. (- * -) 1 M KCl+O.OOl M BTA; (- - -) 1 M KCl+O.OOl M MBT. Scan rate=O.S Immersion time = 1 h. (A) Neutral solutions; (B) PI-I 2.
( -)
X M KCI; mV s-l.
191 TABLE 1 Corrosion current densitiesof Cu in 1 M KCI and inhibited1 M KC1 solutionsas a function of pH, exposure time and inhibitorconcentration Inhibitor None None None None BTA BTA BTA BTA BTA BTA MBT MBT MBT MBT MBT MB1 MB1 MB0 MB0
c/mM
PH
Time/b
I-/CA
1 1 1 1 10 10 1 1 1 1 0.1 1 1 1 1
I 2 7 2 7 2 7 2 7 2 I 2 7 2 2 7 2 7 2
1 1 3 3 1 1 3 3 1 1 1 1 3 3 1 1 1 1 1
6.8 12.7 5.7 8.9 0.35 5.1 0.17 7.0 0.14 3.15 0.35 0.20 0.20 0.22 11.3 ‘0.35 0.35 0.40 0.35
cm-’
Protection efficiency 95 60 97 21 98 70 95 98 96 97 90 95 97 94 97
calculated as the percentage decrease of the corrosion current caused by the inhibitor with respect to the appropriate blank solution, must be considered approximate values. However, some clear-cut features of the behaviour of the investigated inhibitors emerge from the data in Table 1 and Fig. 1: (i) The four inhibitors are equivalent in neutral solutions providing - 95% corrosion protection. (ii) MBT, MB1 and MI30 keep (and even improve) their effectiveness at pH 2, while the corrosion current is suppressed by only 60% in the presence of BTA. (iii) The anodic curves for MBT, Fig. 1, indicate a much higher current in the neutral medium. This is thought to be due to the easier oxidation of MBT anion with respect to the undissociated molecule [37] rather than a faster Cu dissolution reaction. Indeed, oxidation currents were obtained at a Et electrode in neutral 0.001 M MBT solution at potentials > - 180 mV (vs. SCE), while polarization to > - 60 mV was necessary in acid solutions. (iv) The failure of BTA in the acidic medium is further stressed by prolonging the exposure time to 3 h; the corrosion current increases with time. (v) The MBT concentration cannot be increased beyond 0.001 M, at lower concentrations, MBT still provides good protection (90% at pH 2 for a 0.0001 M solution). Conversely, a ten-fold increase of the more soluble BTA (0.01 Iw) only raises its efficiency from 60 to 70% in acid media.
192
1
Fig. 2. Polarization resistance-time curves obtained at a rotating (1000 rev min-‘) Cu disc in neutral (solid lines) and acidic (pH 2, dotted lines) solutions. @) 1 M KCI; (a) 1 M KCl+O.OOl M BTA; (A) 1 A4 Kc1 + 0.001 hf MBT.
Although the polarization resistance technique suffers from the same limitations as those indicated above for the Tafel-line extrapolation method [36] when resistive surface films are present on the electrode, qualitative information on the time dependence of the corrosion current may be achieved by monitoring R, vs. time. The data relative to BTA, MBT and uninhibited 1 M KC1 are shown in Fig. 2 and confirm the conclusions drawn from the data in Table 1: Cu corrosion at pH 7 is largely suppressed by both MBT and BTA, whereas only MBT provides protection in acidic KC1 solution, BTA using the R, values. The R, vs. time curves diverge over a period of 5 h. After 1 h exposure, the R, valueswbited and blank solutions differ by less than a factor of 10. The order of magnitude of the corrosion currents calculated according to the formula IWrr= B/R,, assuming B = 19 mV (found in the literature for Cu in seawater [38,39]), is in good agreement with the data in Table 1 for the blank solutions (8-15 PA cmm2); the same formula, applied for the R, values measured in inhibited solutions, yields larger corrosion currents than those obtained from Tafel-line extrapolation: 1.5-3.5 PA cm-’ for neutral solutions; about 0.85 PA crne2 for MBT in acidic KCI.
193
The corrosion potential of Cu electrodes was shown to change by less than 30 mV over 5 h for all the solutions of Fig. 2, being either in the range from - 340 to - 285 mV for the blank and acidic BTA solutions or in the range from - 170 to - 115 mV for the MBT ‘and neutral BTA ones.
t (A)
(8)
% 2ow g-1
i
, E
.“, ;,L--/ 5 a”
Fig. 3. SERS spectra for Cu in 1 M KCI+O.Ol
- 1.0 v.
N BTA (pH 7). (A) Open circuit (E,,
= -0.12
v); (B)
194
Raman spectroscopy The electrochemical measurements demonstrate the markedly different behaviour of mercapto-inhibitors, which are active over a wide pH range, and of BTA, which is effective only in nearly neutral solutions. MBT (as a model for mercapto-inhibitors) and BTA were therefore chosen for a spectroelectrochemical investigation. Adsorption on both Ag and Cu was studied, with special attention to Cu at open circuit, i.e. under the actual corrosion conditions.
(A)
I
760
llbo llao V/W’ pw3.6
&0
195
Fig. 4. (A) SERS spectrum of 1 M KCl+O.Ol M BTA (pH 2) at Cu (open circuit; I& = -0.3 v). (B) Effect of pH on the SERS spectrum of BTA at Cu (open circuit). (C) Effect of the electrode potential on the SERS spectrum of BTA at Cu (pH 2.0).
Benzotriazole
SERS spectra for Cu in neutral 1 M KC1 + 0.01 it4 BTA are shown in Fig. 3. The low frequency range (< 700 cm-‘) is not shown. However, the Cu-Cl band at 286 cm-’ was found to be absent for 1 M KC1 solutions containing BTA concentrations as low as 10v5 M, in sharp contrast to the strong Ag-Cl band at 240 cm-‘. Despite the non-linearity of the detector, all bands for BTA adsorbed at Cu are easily identified and the expected intensity-potential dependence was found: the sensitivity for Fig. 3B (- 1.0 V) is half of that for Fig. 3A (OX.). Major changes occur after acidification of the BTA solution, (Fig. 4A) (compare ref. 24): for example, the band at 1196 cm-’ is no longer seen; new bands appear at 1127 (very broad), 1170 and 1600 cm- i. The most notable change is found in the shape of the band system at - 1380 cm-‘, the pH and potential dependences of which are shown in Figs. 4B and 4C, respectively. In nearly neutral solutions, at open circuit, the main band at 1386 cm-’ is accompanied by a shoulder around 1370 cm-‘, the relative intensity of which is seen to increase with decreasing pH until, at pH 1, the two bands are equally intense (Fig. 4B). When the electrode potential is changed from the open circuit value ( - 0.3 V at pH 2) to - 0.6 V, i.e. a value close to the pzc [40], the lower frequency band becomes relatively more intense (Fig. 4C). It must be concluded that the band at 1374 cm-’ belongs to the neutral BTA molecule (possibly involved in a [CuCl(BTA)], complex, as suggested by Rubim et al. [24]), while the band at 1387 cm-’ is due to the BTA anion, the adsorption of the latter being favoured by low acidity and at potentials more positive than the pzc. The rather surprising increase of the 1387 cm-’ band observed when the Cu potential is taken to -0.8 V (Fig. 4C) is probably due to the cathodic current (I-I2 evolution) flowing at this potential, causing a pH increase at the electrode surface. Comparable results are obtained at Ag, which yields additional information on the influence of acidity on Cl adsorption. Table 2 illustrates the variation of the
196 TABLE 2 Relative Raman intensity of Cl and BTA for a 1 M KCl+O.OOl M BTA solution at a Ag electrode as a function of pH and potential PH
Potential/V
7 7 7 2 2 2
-0.2 -0.6 -1.0 -0.2 -0.6 -1.0
(vs. SCE)
a Ratio between the intensity of the Ag-Cl
Cl/BTA
a
1.65 0.45 0.15 4.7 2.8 0.15 band (240 cm-‘)
and a BTA band (786 cm-‘).
intensity ratio between the Ag-Cl band and a selected BTA band (786 cm-’ benzene ring breathing mode) as a function of pH and potential. The higher relative intensity of the Ag-Cl band in acid media (except at -1.0 V, where this band is always very weak *) indicates that undissociated BTA competes less efficiently with chloride ion for surface sites than does the anion [22].
244ercaptobenzothiazole Strong SEBS spectra were obtained at Ag for both neutral and acidic 1 M KC1 + 0.001 M MBT solutions (Fig. 5). These spectra, ascribed to the MBT anion and undissociated MBT respectively, correspond to those published by Oshawa et al. [21] as far as the number and relative intensity of the bands are concerned. However, the band positions measured in this investigation were at systematically lower wavenumbers (8-15 cm-‘) than in the earlier report [21]. Additional bands have been observed at < 900 cm-’ (a region not studied by the Japanese group): 393, 604, 711 and 866 cm-’ at pH 7; 390, 685, 711 and 866 cm-’ at pH 2. The Ag-Cl band is weak in both spectra (although the potential is the most favourable potential for the detection of this band) and, most notably, its intensity does not increase at the lower pH. The band at 1388 cm-’ in Fig. 5B, due to the anion, is no longer detected at pH 1. The spectrum recorded at Cu for an MBT solution at pH 2 (Fig. 6A) is dominated by the bands of the anion and only differs from that obtained in neutral solution by the presence of bands at 1336 and 1488 cm-’ **. By further increasing the acidity (pH 1) the spectrum changes, to that of Fig. 6B: the undissociated molecule is preferentially adsorbed, but without complete displacement of the anion (band at 1385 cm-‘). Comparison of Figs. 5 and 6 indicates that for the same pH, i.e. for the same MBT anion/undissociated MBT ratio in solution, a higher surface concentration of
* Local pH increase is expected to affect this result also. l * A shoulder at 1374 cm-’ (also present in the spectrum recorded at pH 7) is seen here, which was not detected at Ag.
197
(A)
260
1700 1 9
I
cm-1
Fig. 5. SERS spectra of 1 M KC3 + 0.001 M MBT at Ag, - 0.2 V. (A) pH 7; (B) pH 2.
the anion is found on Cu than on Ag, although electrostatic forces would promote the opposite behaviour: since all the spectra in Figs. 5 and 6 were recorded at a potential close to -0.2 V, Ag was more positive than Cu with respect to pzc ( - - 1.0 V for Ag [41] and - 0.4 to - 0.7 V for Cu [40,41]). The stronger adsorption of anions on the latter metal must therefore be due to specific chemical interactions.
198
(A)
7bo
fmlko
‘+k
l&O
Fig. 6. SERS spectra of 1 M KCl+O.OOl M MBT at Cu, open circuit. (A) pH 2 (Em, = -0.17 V); (B) pH-1 (Jr&= -0.20 v).
199
C~~~etitiue a~~~ption of BTA and MBT SERS spectra of neutral solutions containing equimolar amounts of BTA and MBT at both Ag and Cu are found to be identical to those of the latter inhibitor
l&O
rho
I
v cm+ Fig. 7. (A) SERS spectrum of 1 M KCl + 0.01 M BTA + 0.1 m M MBT (PH 7) at Ag. - 0.2 V. (B) SERS spectrum of 1 M KC1 + 0.007 M BTA + 0.35 m M MBT (pH 7) at Cu, open circuit (E,, = - 0.14 V).
(A)
Fig. 8. (A) SEW spectrum of 1 M KCI + 0.01 M BTA + 0.1 m M MBT (pH 2) at Ag, - 0.2 V. (B) SERS spectrum of 1 M KCI -+0.007 M BTA + 0.35 m M MBT (pH 1) at Cu, open circuit ( E,, * - 0.20 V).
alone. S~ul~eous adsorption of both ~bitors (as anions) is demonstrate by Figs. 7A (Ag) and 7B (Cu) for solutions containing 0.01 M BTA + 0.0001 M MBT and 0.007 M BTA + 0.00035 M MBT, respectively. Overlapping of the signals from BTA and MBT occurs for many bands (labelled MBT + BTA in Fig. 7); however, the bands at 1248,711 and 393 cm-r on Ag and 1241 cm-’ on Cu are diagnostic for MBT and those at 1163 and 786 cm-l on Ag and 1195 and 792 cm-’ (the latter not shown in Fig. 7B) are due to BTA only. The change from adsorbed anions to adsorbed undissociated molecules, caused by dilution of the solutions, was found to be accompanied by a variation in the relative surface concentrations in favour of MBT at both metal substrates. Thus the spectrum recorded at Ag in 0.01 M BTA + 0.0001 M MBT at pH 2 (Fig. 8A) cannot be distinguished from that in Fig. 5B (MBT alone). For Cu also, at pH 1 (Fig. 8B) the appearance of the bands typical of undissociated MBT (1336 and 1419 cm-‘), the increase in intensity of the band at 1241 cm-“, and the absence of the
201
band at 786 cm-’ (due to BTA) indicate preferential adsorption of MBT in acid media. The assignment of the bands at 1160 and 1365 cm-l is not straightforward; these bands could be due to photolysiswhich was seen to occur on the Cu electrodes and prevented an accurate determination of the time dependence of the Raman intensities, as described in ref. 26. CONCLUSION
The Raman spectroscopic results clarify the fall in the efficiency of BTA as a corrosion inhibitor of Cu in solutions of increasing acidity, compared with the essentially pH-independent efficiency of MBT. Both inhibitors are adsorbed as anions from neutral solutions, in agreement with previous observations [21,22,24]. A complete replacement by the corresponding undissociated molecules does not take place at Cu surfaces even at very high acidity, although both BTA and MBT are weak acids with pK, values close to 8 [6]. Comparison of the strengths of adsorption indicates that MBT is more strongly adsorbed than BTA at both Ag and Cu over the entire pH range investigated, the difference increasing with increasing acidity. Although a complete correspondence between strength of adsorption and corrosion inhibition efficiency cannot be assumed [26], an increasing MBT/BTA surface concentration ratio at lower pH is in keeping with the observed corrosion behaviour. A weakening of BTA adsorption at low pH is also demonstrated by the experiments carried out with Ag electrodes. An increase in the adsorption of the aggressivechloride ion is found with increase of the acidity of BTA solutions, but this is not observed for solutions containing MBT. A &-Cl band has been observed in the spectrum of an acid 0.1 M KC1 + 0.005 M BTA solution [24] but no such band was found in the present investigation, Our suggestion that effective protection is indicated in Raman spectroscopic experiments by the absence of the bands due to adsorbed aggressive anions in solutions containing the inhibitor [25] is confirmed by the results in this work. The present study also emphasizes the usefulness of experiments on the competitive adsorption of species and shows that potential-dependent surface acid-base equilibria (which differ markedly from the solution equilibria) are also of key importance in explaining the relative efficiency of inhibitors.
The authors thank NATO (grant S.A. S-2-05 RG n 815/83) and the United States Office of Naval Research for financial support and Mr. F. Furlanetto of CNR for valuable experimental work. REFERENCES 1 G. Trabanelli and V. Carassitiin M.G. Fontana and R.W. StaehIe(Eds.), Advances in Corrosion Science and Technology, Vol. 1, Plenum,New York, 1970, p. 147.
202 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
C.C. Nathan (Ed.), Corrosion Inhibitors, NACE, Houston, TX, 1973. L. Homer and E. Phefke, We&t. Korros., 33 (1982) 98. I. Dugdale and J.B. Cotton, Corros. Sci., 3 (1963) 69. J.B. Cotton and I.R. Scholes, Br. Corros. J., 2 (1%7) 1. G.W. Poling, Corms. Sci., 10 (1970) 359. F. Mansfeld, T. Smith and E.P. Parry, Corrosion, 27 (1971) 289. R. Walker, Corrosion, 29 (1971) 290. G. Trabanelli, F. Zucchi, G. Brunoro and V. Cam&i, We&t. Korros., 24 (1973) 602. G.B. Hatch, Corrosion, 11 (1955) 461t. G.B. Hatch in ref. 2, p. 142. S. lbibault and J. Talbot, CR. Acad. Sci., 278 (1974) C-503. L. Homer and E. Phefke, Werkst. Korros., 33 (1982) 189. S. Tbibault and J. Talbot, Met. Corros. Ind., 50 (1975) 51. S. Thibault, Corros. Sci., 17 (1977) 701. RF. Roberts, J. Electron Spectrosc., 4 (1974) 273. D. Chadwick and T. Hasemi, Corros. Sci., 18 (1978) 39. D. Chadwick and T. Hasemi, Surf. Sci., 89 (1979) 649. R.K. Chang and T.W. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenm, New York, 1982. M. Fleischnxum and I.R. Hill in R.E. White, J.O’M. Bockris, B.E. Conway and E. Yeager (Fds.), Comprehensive Treatise of Electrochemistry, Vol. 8, Plenum, New York, 1984, p. 373. M. Oshawa, H. Matsuda and W. Suetaka, Chem. Phys. Lett., 84 (1981) 163. J.C. Rubim, I.G.R. Gutz and 0. Sala, J. Mol. Struct., 101 (1983) 1. J.J. Kester, T.E. Furtak and A.J. Bevolo, J. EIectrochem. Sot., 129 (1982) 1716. J.C. Rubim, I.G.R. Gutz, 0. Sala and W.J. Orville-Thomas, J. Mol Struct., 100 (1983) 571. D. Tbierry and C. Leygraf, I. Electrochem. Sot., 132 (1985). M. Fleischmann, I.R. Hill, G. MengoIi, M.M. Musiani and J. Akhavan, Electrochim. Acta, 30 (1985) 879. M. Fleischmann, G. Mengoli, M.M. Musiani and C. Pagura, Electrochim. Acta, 30 (1985) 1591. M. Fleischmamr, G. Mengoli and M.M. Musiani, Proceedings of the 6th SEX, Ferrara, Italy, Sept. 1985, Ann. Univ. Ferrara, N.S., Sez. 5, Suppl. N. 8, (1985) 757. D. Thierry and C. Leygraf in ref. 28, p. 779. M.M. Musiani, G. Mengoli and M. Fleischmann, 36th ISE Meeting, Salamanca, Spain, Sept. 1985, Ext. Abstr. 06360. R. Walker, Corrosion, 31 (1975) 97, T.K. Ross and M.R. Berry, Corros. Sci., 11 (1971) 273. F. Mansfeld and T. Smith, Corrosion, 29 (1973) 105. I.C.G. Ongle and G.W. Poling, Can. Metall. Q., 14 (1975) 37. W.H. Smyrl in J.G’M. Bock&, B.E. Conway, E. Yeager and R.E. White @is.), Comprehensive Treatise of ~~~~he~s~, Vol. 4, Plemun, New York, 1981, p. 97. F. Mansfeld in M.G. Fontana and R.W. Staehle (Eds.), Advances in Corrosion Science and Technology, Vol. 8, Plenum, New York, 1978, p. 163. J.Q. Chambers in A.J. Bard and H. Lund (Eds.), Encyclopedia of Electrochemistry of the Elements, Vol. 12, Marcel Dekker, New York, 1978, p. 329. H. Grubitsch, F. HiIbert and R. Sammer, Werkst. Korros., 17 (1966) 7’60. C. Loss and E. Heitz, Werkst. Korros., 24 (1973) 38. J.O’M. Bock.&, M. Green and D.A.J. Swinkels, J. Electrochem. Sot., 111 (1964) 743. R.S. Perkins, R.C. Livingston, T.N. Andersen and H. Eyring, J. Phys. Chem., 69 (1965) 3329.