A Model Corrosion Inhibitor for in Situ Spectroscopic Studies

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A Model Corrosion Inhibitor for in Situ Spectroscopic Studies B. Heeg and D. Klenerman* Department of Chemistry, Cambridge University, Lensfield Road, Cambridge CB2 1EW, U.K. Received November 18, 1998. In Final Form: October 29, 1999 We have investigated a variety of model monolayer films suitable for in situ study by second harmonic generation (SHG). We found that methylene blue (MB), chemisorbed via an intermediate sulfur layer onto nickel and carbon steel substrates, gives SHG signals that are dominated by the MB and are hence suitable to follow film removal during cavitation. No changes in the average orientation of MB were observed for a full monolayer up to a shear stress of 25 Pa, as estimated in previous studies. This indicates that high shear stress does not exert sufficient force on the molecules in the film to change the orientation under flow. Furthermore, we found no removal of the film under shear conditions, and a cavitating jet of liquid was required order to observe removal of the MB from the nickel electrode. This appears to be due to the formation of local pits formed by the cavitating bubbles, resulting in the removal of both the MB and the sulfur monolayer.

Introduction Surface-active molecules (surfactants) are widely used to alter surface properties in applications such as detergency, adhesion, lubrication, and corrosion inhibition. To understand how these molecules work at the molecular level, a wide range of experiments have been performed. The type of experiment mainly falls into two main classes. In the first, one makes controlled variations to the structure of the molecule and observes the effect on the property of interest, during or after use to deduce the structure-property relationships. In the second, one determines the structure of the surface-active film under static conditions or after use to deduce the structureproperty relationships. However, there have been very few studies of the molecular structure of the surface-active film under dynamic conditions such as high pressure, shear, or cavitation despite the fact that these are the conditions under which surfactants have been designed to operate. The ability of surfactants to withstand these conditions controls the wear of the material or the persistency of the film and is clearly an important property. Cavitation is the formation of voids in the liquid and is usually achieved experimentally by the use of ultrasound. The implosion of a cavity near a wall because of instability on the surface of the cavity is known to lead to high masstransport rates, measured, for instance, by fast logging of limiting current densities using microelectrodes,1 and can result in the formation of microscopic pits. In particular, cavitation is known to result in severe problems in film stability, for example, in oil pipelines where multiphase mixtures of gas and liquid are passed simultaneously down the pipeline. Little is known about the mechanism of film removal under cavitation or about the key molecular properties in designing a film resistant to cavitation. One aim of this work is to investigate this, using model films. In this work we use second harmonic generation (SHG) at surfaces, combined with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) first to follow the formation of the model films and then to show their potential use as a probe for film removal under conditions of shear and cavitation. Using methylene blue (1) Birkin, P. R.; Silva-Martinez, S. J. Chem. Soc., Chem. Comm. 1995, 17, 1807.

(MB) as the “reporter“ chromophore in the monolayer film under different rates of shear, the average orientation was determined to help elucidate the mechanism of the film removal by cavitation. MB has a high nonlinear hyperpolarizability and is therefore the dominant contribution to the SHG signal and not the surface itself. By working with model films of well-defined structure, we also hope to be able to identify which bond is broken when the adsorbate is removed from the surface and use this information to determine the energy released and force on the monolayer film when a cavitating jet of liquid hits a surface. Surface SHG is a form of nonlinear spectroscopy pioneered by Shen and co-workers2 to study surfaces in situ with submonolayer sensitivity. Briefly, SHG is the frequency doubling of light at the surface due to the breakdown in symmetry, which occurs at any surface. In the electric dipole approximation, the signal is only generated at the surface, making the technique intrinsically surface sensitive, and it can have submonolayer sensitivity particularly in cases when the frequency of the light or doubled light correspond to an electronic absorption of the adsorbate (resonance condition). SHG has been used previously to study commercial corrosion inhibitors under shear and cavitation conditions3 and was shown to yield very specific information, in combination with electrochemical methods, on the formation, persistency, and breakdown of chemisorbed monolayers. The main disadvantage of using commercial formulations is that they consist of a large variety of chemicals including surfactants and wetting agents, making a quantitative study of the dynamic behavior under shear and cavitation complicated. The formation of a MB monolayer on a sulfur-covered Au surface was first reported in 1987,4,5 and later Pt has been used as a substrate.6-8 The strong interaction (2) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (3) Heeg, B.; Klenerman, D. Corr. Sci. 1998, 40, 1313. (4) Zutic, V.; Svetlicic, V.; Clavilier, J.; Chevalet, J. J. Electroanal. Chem. 1987, 219, 183. (5) Svetlicic, V.; Zutic, V.; Clavilier, J.; Chevalet, J. J. Electroanal. Chem. 1987, 233, 199. (6) Svetlicic, V.; Clavilier, J.; Zutic, V.; Chevalet, J. J. Electroanal. Chem. 1991, 312, 205.

10.1021/la981617l CCC: $19.00 © 2000 American Chemical Society Published on Web 12/17/1999

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Figure 1. Representation of a possible configuration of MB on a sulfur-modified substrate. Included are the angles θ, φ, and ψ, which are used to express the average orientation of MB molecules in the monolayer.

between MB and the sulfur monolayer was ascribed to a chemical bond between the MB-sulfur and a sulfur surface adatom. Figure 1 shows a possible configuration of the MB-sulfur monolayer complex. The orientation of the MB with respect to the surface is denoted in terms of the angles θ, φ, and ψ. The mean S-S bond enthalpy is -264 kJ mol-1 and will, as a first approximation, to a large extent determine the chemical adhesion forces of the MB film in multiphase flow. The sulfur monolayer formation for Au and Pt is simple and consists of dipping the electrode in a 0.01 M Na2S solution for several seconds. The MB monolayer is then formed by dipping the sulfurmodified Au or Pt substrate in a 10-4 M MB solution for several minutes. This procedure is attractive in terms of the short preparation time involved, but for multiphase flow experiments, ideally a rotating cylinder needs to be used that can be easily polished. Hence, it was investigated whether a MB monolayer could be made on a sulfurmodified Ni electrode. The formation of sulfur mono- and multilayers on Ni from alkaline Na2S solutions has been studied to some extent,9 and it was found that the anodic formation of R, β, and γ NiS competes with the formation of NiO and Ni(OH)2; however, sulfur mono- and multilayer formation is favored over the oxide layer formation even at high OH-/S2- concentration ratios. Combined SHG and cyclic voltammetry studies have been performed on a nickel electrode in 0.1 M NaOH 10 in the potential range of -1150 to +550 mV (SCE), which showed that if the potential was swept anodically from +500 mV (interpreted as the Ni(II)/Ni(III) equilibrium potential), a passive Ni(OH)2 layer was thought to form that could not be reduced when switching back to a cathodic current. Experimental Section We built a cell with the working electrode consisting of a rotating cylinder electrode which has well characterized flow conditions and for which it is experimentally easy to reach conditions of turbulent flow. A jet of liquid could be fired at this electrode, and it could be made to cavitate by use of an ultrasonic probe. This system has been described previously in detail11 and was developed to simulate multiphase flow in oil pipelines. The electrochemical cell used in the experiments is depicted in Figure 2. A rotating cylinder electrode (RCE) with a surface area of 2.35 cm2 was driven by a variable EG&G motor (1-10 000 rotations/min, rpm). A threeelectrode arrangement was used whereby the rotating electrode was set up as the working electrode, a platinum (7) Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990, 94, 3681. (8) Clavilier, J.; Svetlicic, V, Zutic, V. J. Electroanal. Chem. 1996, 402, 129. (9) Bohe¨, A. E.; Vilche, J. R.; Arvia, A. J. Corros. Sci. 1993, 34, 151. (10) Biwer, B. M.; Pellin, M. J.; Schauer, M. W.; Gruen, D. M. Surf. Interface Anal. 1989, 14, 635. (11) Heeg, B.; Moros, T.; Klenerman, D. Corros. Sci. 1998, 40, 1303.

Figure 2. Schematic diagram of the jet-cylinder electrochemical cell used for persistency studies of an organic corrosion inhibitor film during multiphase flow.

mesh as the counter electrode at 1 cm distance, and a saturated calomel electrode (SCE) as the reference electrode at a distance of 5 cm from the working electrode. Because strong electrolytes were used, the ohmic potential drop in the solution, or iR drop, could be neglected and the reference electrode kept at a distance. The titanium horn of an ultrasonic probe (UP, manufactured by Ultrasonic Engineering, London, U.K.) was fitted with a flange onto the cell, and a cylindrical hole with a diameter of 4 mm and a length of 5 cm was drilled through the horn and the ultrasonic transducer (PZT of 2 mm thickness). Thus, a liquid jet could be produced through the horn onto the rotating cylinder or a jet with ultrasonic cavities by switching on the liquid jet and the 1500W UP power supply (Sonics & Materials model VC1500). The distance between the RCE and the ultrasonic transducer of the UP could be varied but was set at 3 mm. The power supplied to the transducer was kept constant in all experiments and was measured with a wattmeter to be 200 W. Jet flow rates could be changed with a variable micropump to a maximum of 33 cm3 s-1, corresponding to a flow velocity in the nozzle vicinity of around 2.6 ms-1. A second cell was connected to the electrochemical cell at the overflow to produce a flow loop. Further, for SHG experiments the solution was purged with nitrogen at 25 cm3 s-1 through a glass bubbler. Surfactants could be injected into the cell through a self-healing nylon septum. A schematic of the complete experimental arrangement for SHG experiments is given in Figure 3. An actively mode-locked Coherent Antares Nd:YAG laser, producing 2.5 W of frequency-doubled output in the form of 80 ps pulses at 532 nm, was used to synchronously pump a tuneable Coherent 700/702 cavity-dumped Rhodamine 6G dye laser. Plane-polarized light pulses (200 mW) were produced at 580 nm, with a typical pulse width of 10 ps, a repetition rate of 9.5 MHz, and hence ca. 20 nJ/pulse. This pulse energy is sufficiently low to ensure that no surface laser damage occurs. The light was focused into a 100 µm diameter spot at the center of the metal surface by a planoconvex glass lens of 50 mm focal length at the same level as where the liquid jet could be impinged on the surface. An angle of incidence of 45° was used. The reflected light containing the fundamental light at 580 nm and the second harmonic signal at 290 nm was allowed to exit the electrochemical cell through a quartz window

Model Corrosion Inhibitor for Spectroscopic Studies

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Figure 4. (A) Equivalent circuit used in fitting EIS data, based on the inhomogeneous surface model, represented in part B. Table 1. Fitted Values for Impedance Data Depicted in Figure 5, Obtained for a Ni Electrode in 0.1 M NaOH at -1100 mV SCE (Figure 5a, Negative of Corrosion Potential) and -300 mV SCE (Figure 5b, Positive of Corrosion Potential)a

Figure 3. Schematic diagram of experimental arrangement for SHG electrochemistry studies.

and was recollimated (with a 50 mm focal length planoconvex quartz lens). After the 580 nm component was filtered out, the light was focused into a Jobin Yvon HR 640 spectrometer through a 500 µm slit with a 100 mm focal length quartz lens and the SHG photons were detected using an image-intensified diode array (Princeton Instruments). The signal was then measured with a detector controller (ST-100, Princeton Instruments) and analyzed with a PC using POSMA software (Princeton Instruments). Power fluctuations and long-term drift in laser power were corrected for by splitting off a part of the incident laser beam and sending the resulting beam through a frequency-doubling KDP crystal producing 290 nm photons. After the fundamental photons were filtered out, the UV photons were collected with a gated PMT, controlled by a SR250 boxcar averager and gated interrogator and SR235 analogue processor, and sent as an analogue signal into the ST-100 detector controller. DC electrochemical data were collected using an ACM single-channel electrochemical interrogator-potentiostat, controlled, and analyzed with a PC via ACM software. Electrochemical impedance (EI) data were collected using a Solartron 1286 potentiostat and a Solartron 1255 frequency response analyzer, controlled with a PC and ZPlot/ZView (Scribner) software. In the next two main result sections we will first discuss the results on the formation of MB on a sulfur-modified Ni electrode (Ni from Goodfellows, 99.99%) and its behavior under conditions of shear and cavitation. Then we will discuss the possible use of carbon steel as the substrate in a similar procedure. For this purpose EN3B C-steel containing 0.13-0.18% C and 0.7-0.9% Mn (Johnson Mathey) was used. Results 1. Methylene Blue on Nickel. We investigated the formation of an oxide layer on Ni in the presence of 0.1 M NaOH using SHG and EI. We fitted the EI data using the equivalence circuit model shown in Figure 4A. We have found that this model fits impedance data from a variety of different surfaces with and without adsorbates. The circuit in Figure 4A is a modified version of the

V (SCE) Rsol (Ω) C1 (µF) Rb (Ω) Rp (Ω) T (sφ/Ω‚cm2) φ average error (%) rms error (%)

-1.100 180.1 ( 0.4 22.5 ( 1.0 60.4 ( 7.9 394.3 ( 8.8 45.7 ( 1.2 0.87 ( 0.01 3.93 3.72

-0.300 184.9 ( 1.0 4.9 ( 0.2 113.7 ( 7.6 136690 ( 13262 25.8 ( 0.3 0.82 ( 0.003 5.85 5.09

a Fitted values are obtained using the modified inhomogeneous surface model of Figure 3a.

inhomogeneous surface model12,13 depicted in Figure 4B, which has been used to model EI data from porous oxide layers in aerated solutions. Both models in Figure 4 contain a resistance Rsol, which is the sum of the electrolyte resistance and electrical resistance of the connectors. Furthermore, they contain a resistance Rb and a C1, which have been interpreted as the electrolytic resistance within the macroscopic layer of anodic dissolution sites (i.e., those covered by an oxide or a hydroxide layer)14,15 and its capacitance. In the inhomogeneous surface model (Figure 4B), the cathodic (oxygen reduction) sites are modeled by another resistance Rp and a classical capacitor,7,10 which in the modified version in Figure 4A is replaced by a constant phase element, the impedance of which is

Zcpe ) 1/T(ωj)φ where φ is the phase constant which has been interpreted as a measure of surface inhomogeneity, T is a time constant comparable to a capacitance (but with units sφ/(Ω cm2)) and ω is the ac modulation frequency. Impedance spectra were obtained under potentiostatic control while modulating the potential 10 mV in the frequency range of 2000-0.5 Hz. One impedance spectrum takes around 35 s. Figure 5 shows the complex plane and Bode plots of two typical measurements at a cathodic potential (-1100 mV SCE; Figure 5a) and at an anodic potential (Figure 5b) and includes the fitted curves obtained from using the modified inhomogeneous surface model of Figure 4A. Table 1 shows the fitted values corresponding to the graphs in Figure 5. (12) Ju¨ttner, K.; Lorenz, W. J.; Kendig, M. W.; Mansfeld, F. J. Electrochem. Soc. 1988, 135, 332. (13) Ka´lman, E.; Va´rhegyi, B.; Bako´, I.; Felhoi, I.; Ka´rma´n, F. H.; Shaban, A. J. Electrochem. Soc. 1994, 141, 3357. (14) Mansfeld, F. Corrosion 1988, 44, 856. (15) Morsi, M. A.; Elewadi, Y. A.; Lorbeer, P.; W. J. Werkst. Korros. 1980, 31, 108.

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Figure 5. Impedance and Bode plot of a typical impedance measurement of a reduced nickel surface (at -1100 mV SCE, a) and an oxide-covered nickel surface (at -300 mV SCE, b). Fitted curves based on the modified inhomogeneous surface model are included.

Figure 6a shows the SHG intensity and fitted values for T and φ, all recorded simultaneously as the potential is swept in steps of 10 mV/impedance spectrum from -1100 to +200 mV SCE (i.e., negative of the Ni(II)/Ni(III) equilibrium potential). If the scan is repeated in this range, the data are reproducible; however, if the sweep range is extended positive of the Ni(II)/Ni(III) equilibrium potential, then irreversibility sets in. The error bars, obtained from the Scribner fitting software, are a measure of the error in the fitted values and are effectively a percentage error in the data points. To remove the oxide layer, the electrode was held at -1400 mV SCE for 10 min before the start of the sweep. The corrosion potential after pretreatment was measured to be -720 mV SCE in a separate experiment. Because the potential is constant during every impedance measurement, the average sweep

rate is 16.5 mV/min. Furthermore, the SHG signal is integrated over 10 s; hence, every impedance measurement corresponds to slightly more than three SHG data points. As can be seen, the value for T has a maximum and the SHG signal increases at around -700 mV, which is attributed to the formation of an oxide layer because this potential corresponds roughly to the corrosion potential. Figure 6b shows the values for the remaining parameters used in the fitting procedure, Rp, C1, and Rb. It can be seen that C1 is at a maximum at around -700 mV and that Rp is at a local maximum. Furthermore, the Rb values, although quite scattered, fall to a minimum at -700 mV. Based on the inhomogeneous surface model, the decrease in C1 corresponds to the growing of an oxide layer at the inactive (anodic) sites of the surface. The formation of a sulfur layer on Ni in the presence


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Figure 6. (A) SHG, T, and φ as a function of applied potential for Ni RCE (2000 rpm, T ) 293 K, N2) in 0.1 M NaOH. The average potential sweep rate is 30 mV/min. (b) Values for Rp, Rb, and C1 (conditions as in part a).

of 0.1 M NaOH and 0.01 M Na2S was investigated using the same measurement methods and under identical conditions except the anodic sweep started from -1200 mV (SCE). The SHG, T, and φ values are shown in Figure 7a. As can be seen, in a Na2S solution a maximum in the values for T occurs between -600 and -500 mV (SCE), compared to a maximum at -700 mV in the NaOH solution (Figure 6a). Furthermore, a maximum in the SHG intensity occurs at around -700 mV in Na2S, anodic of which (i.e., at more positive potential values) the intensity decreases slightly compared with Figure 6a, which shows a maximum in SHG at around -350 mV for Ni in 0.1 M NaOH. This indicates that the difference in SHG and capacitance behavior, which reflects the difference in

surface composition, is most likely due to the formation of a Ni-S adlayer. Figure 7b shows the values for Rp, C1, and Rb. It can be seen that C1 is at a maximum at around -940 mV and on sweeping to anodic (i.e., positive) potentials passes through a local minimum at -800 mV and a local maximum at -720 mV. Further, Rp and Rb behave much the same as those for the 0.1 M NaOH solution. The parameter that shows the largest difference for the two solutions is T. The differences in T cannot be due to pH since the pH was the same for both solutions and is therefore attributed to the difference in the reactivity of HS- and OH- with the Ni surface. This is supported by the observation that the SHG intensity increases by about a factor of 5 in NaOH when sweeping

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Figure 7. (a) SHG, T, and φ as a function of applied potential for Ni RCE (2000 rpm, T ) 293 K, N2) in 0.1 M NaOH and 10 mM Na2S. The average potential sweep rate is 17 mV/min. (b) Values for Rp, Rb, and C1 (conditions as in part 7a).

from -1100 to +200 mV (SCE) whereas the increase is only a factor of 2 in Na2S. The SHG data contain less information than the electrochemical data because we observe a gradual change in the SHG signal in Figure 7a, which does not show the pronounced changes around -800 and -550 mV observed in Figure 6a for T. The difference in the SHG signals for a Ni surface in the two solutions is due to differences in hyperpolarizability at 585 and/or 292.5 nm wavelength of the two interfaces.2 Because the dependency of the SHG signal on the potential is complicated, quantitative analysis of the signal is difficult and the data can only be used qualitatively. It is not possible to directly compare our data with the SHG data in ref 10, which was recorded using 1064 and 532 nm excitation, due to the complex wavelength dependence of the SHG signal at or close to resonance with electronic bands of the surface. Figure 8 shows three cyclic potential sweeps (at 2000 mV/min sweep rate). One sweep is for a Ni rotating cylinder in 0.1 M NaOH (N2 purged, cathodic pretreatment at -1400 mV (SCE), 2000 rpm), and two sweeps are with 10

mM Na2S added (the first cyclic sweep and the fifth sweep, after which no changes in the voltage-current curve are observed). Based on ref 9, it is concluded that the anodic peak in the first CV at -825 mV (SCE) corresponds to the formation of a monolayer of sulfur and the smaller peak at -550 mV with the formation of a multilayer. Therefore, the formation of an S multilayer is attributed to the sharp decrease in the values for T in Figure 7a at around -550 mV. It also should be noted that after five cyclic sweeps (Figure 8) there is still an anodic peak at around -825 mV (SCE), which suggests that on a reverse (i.e., cathodic) scan some of the adsorbed sulfur is being desorbed and readsorbs when the potential is swept anodically. It is therefore concluded that a sulfur monolayer can be formed if the potential is kept between -800 and -550 mV SCE, and these conditions were used to produce a Ni-S-MB layer in the next section. Ni-S-Methylene Blue Formation and Stability. After electrochemical polarization of Ni-RCE at -600 mV (SCE) in 0.1 M NaOH and 10 mM Na2S for 20 min, the sulfur-modified Ni-RCE was taken out of solution


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Figure 8. Cyclic voltammograms for a Ni RCE in 0.1 M NaOH (2000 rpm, T ) 293 K, N2) (“clean”) and with 10 mM Na2S added to the solution (cycles 1 and 5).

Figure 9. SHG, current density, and fitted values for C1 and T (using the equivalent circuit depicted in Figure 3) as a function of applied potential for a sulfur-modified Ni electrode with MB adsorbed.

and transported through air into a 0.1 mM MB solution in a borate buffer at pH 8 (in aerated conditions). There it was left rotating gently for 10 min. The electrode was then rinsed in water, and the SHG intensity was found to be 20-30 times larger than the intensity for the S-modified electrode without MB. Figure 9 shows the SHG intensity, dc-current density, and fitted values for C1 and T as a function of applied potential for a sulfur-modified Ni electrode with MB adsorbed. The SHG intensity and capacitance data are obtained simultaneously, and the dc current is from a different but identical experiment. As can be seen from Figure 9, as the potential is swept cathodically, the SHG intensity goes through a minimum and then increases again, while the T and C1 values go through a shallow and a steeper minimum, respectively. The observed behavior is very similar to that observed for MB on Pt,7 and the decrease in the SHG intensity at -300 mV is attributed to the reduction of MB into leucomethylene blue, which

has no optical absorption at 290 nm and therefore produces no large resonance-enhanced SHG signal. MB has been found to become electroactive at sulfur-modified Au and Pt surfaces, whereas adsorbed onto bare Pt, MB and most other phenothiazines are electroinactive.4,5 Because reversibly electroactive molecules adsorbed onto surfaces will show anodic and cathodic peaks in cyclic voltammetry, which are proportional to their surface coverage, this can be used to measure the surface coverage of MB.7 This was not attempted here; however, comparison of the values for T at -100 mV (SCE) for the systems involved can give a clue about the coverage of nickel with an oxide or sulfur layer and MB as well. Thus, both Ni in 0.1 M NaOH and Ni in 0.1 M NaOH + 10 mM Na2S at -100 mV (SCE) have T values of around 18 sφ/(Ω cm2), whereas Ni-S-MB has a value around 50 sφ/(Ω cm2) at -100 mV. This should be a large enough difference to help decide in conjunction with the SHG signal whether the MB monolayer has been removed in slug flow conditions or not, although it will not allow discrimination between a sulfur- and an oxidecovered nickel electrode. Determination of Molecular Orientation. Determination of the average orientation of MB has been reported before for a Pt substrate7 and a fused silica substrate.16 The same approach will be used here to determine the average orientation of MB on a Ni-S substrate. In refs 7 and 16 the dominant components of the hyperpolarizability tensor were determined from Pople-Pariser-Parr (PPP) electron calculations. Further, approximate values for the dielectric constants of the adsorbed monolayer were estimated from a KramersKronig analysis of the UV-vis absorption spectra of MB in solution and adsorbed onto a fused-silica surface. Thus, βzxx and βxxz were found to be the dominant components of the hyperpolarizability tensor for MB and were hence the only two components considered. We have made the same assumption. The values for the refractive index, n, and the extinction coefficient, k, for H2O (medium 1, incidence) and Ni (medium 2) for fundamental and SH (16) Higgins, D. A.; Byerly, S. K.; Abrams, M. B.; Corn, R. M. J. Phys. Chem. 1991, 95, 6984.

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Figure 10. p- and s-polarized SHG intensity as a function of input polarization γ. The fitted curve is obtained with θ ) 32.5° and βr ) -0.1. Table 2. Refractive Index, n, and Extinction Coefficient, k, for Ni and H2O at 585 (ω) and 292.5 nm (2ω) nNi (ω) kNi (ω) nNi (2ω) kNi (2ω) nH20 (ω) nH20 (2ω)

1.84 3.45 1.73 2.02 1.33 1.36

wavelengths of 585 and 292.5 nm, as taken from ref 17, are displayed in Table 2. Further, using the complex Snell’s law and with an angle of incidence of 45°, the angle of the reflected SHG beam was calculated to be 43.75°. Figure 10 shows the p- and s-polarized SHG intensities as a function of input polarization (i.e., γ ) 0° and γ ) 90° correspond to p- and s-input polarization, respectively). The fitted curve is obtained from a nonlinear least-squares fitting procedure, where it is assumed that both φ and ψ are random and that βzxx and βxxz are the dominant components (hence excluding other contributions). From the fitting procedure it followed that θ ) 32.5 ( 7.2 and βr ) -0.1 (βr ) βzxx/βxxz). This value of βr is small compared to what was found from data for MB on Pt where βr was found to be -0.37 and probably reflects the difference in environment of MB on the two surfaces. The metal substrate and possibly the potential drop across the interface are thought to affect the polarizability and hyperpolarizability tensors, and this will also depend on the orientation of the molecule. The calculated value of MB on S-modified Pt, assuming full coverage and a random distribution around the surface normal (i.e., φ ) random), was found to be 35°,7 which would suggest that the orientational angle is not very dependent on the substrate. Film Behavior under Shear and Cavitation. The persistency of the MB monolayer on nickel was tested in the jet-cylinder cell. No film removal was observed under shear alone with the rate of rotation of the cylinder electrode increased up to 4000 rpm (corresponding to an estimated shear stress of 16 Pa11). Furthermore, no film removal was observed when a jet of liquid was fired at the rotating electrode (4000 rpm corresponding to an estimated total shear stress of 25 Pa11). Molecular average orientations were calculated from polarization data, as described above, and no significant change in the average angle with respect to the surface normal was found. (17) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press Orlando, FL, 1985.

Figure 11. SHG intensity and T as a function of time for a MB monolayer on a S-modified Ni electrode (2000 rpm, T ) 293 K, N2) in a phosphate buffer at pH 8, during exposure to a slug flow.

Film removal was observed, however, if the jet of liquid was accompanied by cavities by turning on the ultrasonic probe. If the open circuit potential was measured, either using electrochemical noise (using a zero-resistance ammeter), EIS, or using a potentiostat, we found that the SHG signal decreased rapidly. This was attributed to the reduction of MB and the desorption of leucomethylene blue. In other studies it was found that electrochemical noise is very useful in determining the formation and removal of a chemisorbed monolayer.3 However, the use of a zero-resistance ammeter introduces a small countercurrent at the open circuit potential and is therefore responsible for a slow but irreversible reduction of MB. Hence, the interfacial impedance was measured at -100 mV (SCE), where the MB is stable (i.e., not reduced) and where a difference in values for T can be used as an additional tool to monitor the surface coverage of MB, as mentioned earlier. Figure 11 shows the values for T and the SHG intensity after multiple applications of a cavitating jet, under nitrogen, in a borate buffer solution at pH 8. During the application of the cavitating jet, it is not possible to perform SHG measurements because of light scatter, which is the reason the SHG signal decreases to zero at this time. The SHG intensity is found to decrease stepwise and T undergoes a small but detectable change from around 50 to around 40 sφ/(Ω cm2), We also observe a relaxation in the value of T after the slug is ended, which is probably due to the change in the open circuit potential during the slug. After this treatment, the electrode was placed back in a MB solution and it was found that no further increase in the SHG intensity occurred. This provides evidence that the removal of the MB monolayer proceeds via breaking of the Ni-S or Ni-Ni bonds rather than via the breaking of the MB-S bond, because an oxidecovered Ni electrode was found to have no affinity for MB. Hence, it is concluded that during a cavitating jet an oxide layer is slowly replacing the sulfur-MB layer. The average orientational angle was measured at the end of the experiment of Figure 11 and was found to be θ ) 30.4 ( 10.2 and βr ) -0.1. Although no independent measurement of average orientational angle versus coverage was made, as in ref 7 where a dependency on coverage was found, a similar average orientational angle would suggest that the local coverage is similar to that at the start of the experiment. A reasonable explanation might be that the removal of MB is very local, so that the rest of the monolayer is unaffected and stays in an


environment identical with that of maximum coverage, hence the similar average orientational angle. 2. Thiols and Methylene Blue on Iron. Analogous to the formation of thiol monolayers on Au, Ag, Cu, and other materials, the compactness of which could potentially lead to good corrosion inhibition, the formation of thiol monolayers on iron has been reported by Stratmann et al.18-22 Because thiols were shown not to have any reactivity with iron oxide,23 iron surfaces were reportedly coated with thiols by using a two-phase arrangement. To this end, the oxide layer is first removed electrochemically by polarizing the surface cathodically at -0.7 V SHE in a nitrogen-purged 1 M HClO4 solution, after which the surface is pulled into a second (immiscible) phase of liquid decanethiol under potentiostatic control. This method results in a hydrophobic surface consisting of a multilayer of physisorbed thiols. It is not clear whether the first layer is chemisorbed, although X-ray photoelectron spectroscopy (XPS) data show a marked peak at 162.0 eV, assigned to the Fe-S bond, in accordance with the peak found for sulfur segregated on iron surfaces.24 If the oxide film is not removed, two XPS peaks are found which are assigned to disulfide and sulfonate species,21 in accordance with the findings of catalytic oxidation of mercaptans if Fe3+ ions are present.23 These data showed clearly that if an Fe-thiol chemical bond is to be formed the removal of an oxide film is essential. The method of Stratmann and co-workers was used, and CV measurements showed that a physisorbed multilayer dramatically decreased the cathodic and anodic current densities (data not shown). A highly hydrophobic surface with high linear polarization resistance (LPR) values was formed. However, it was found after sonication for only a few seconds that the observed hydrophobicity was lost and LPR values became as low as those for clean iron surfaces. It was therefore concluded that no chemisorbed film had been formed, and hence it was necessary to try an alternative method. The next method used was to form a sulfur monolayer on iron, which could then be covered with MB, similar to a Ni substrate. To obtain the condition whereby [HS-] > [OH-], a 10 mM Na2S solution was prepared with pH 11.25, by diluting the OH- concentration with acetic acid. Under these conditions [HS-] is approximately 10 × [OH-]. Figure 12 shows cyclic sweeps for C-steel in a pH 11.25 solution of OH- only and in pH 11.25 with 10 mM Na2S. The sweep rate was 3000 mV min-1, and the inset in Figure 12 shows the current density difference between the two cyclic sweeps, as a function of time, which yield a total surface charge density of 1.8 µC cm-2. Because the surface area of the electrode was 2.35 cm2, then the total charge charge is 4.25 µC or 2.6 × 1013 elementary charges. If the surface is assumed, for the sake of simplicity, to consist of a hexagonal closed-packed top layer of Fe atoms with an atomic radius of 1.27 Å,25 this means the area per Fe atom is approximately 5.6 Å (being 2a2x3 for this configuration and having 4.1 × 1015 surface atoms on the (18) Stratmann, M. Adv. Mater. 1990, 2, 191. (19) Volmer, M.; Stratmann, M.; Viefhaus, H. Surf. Interface Anal. 1990, 16, 278. (20) Volmer-Uebing, M.; Stratmann, M. Appl. Surf. Sci. 1992, 55, 19. (21) Volmer, M.; Czodrowski, B.; Stratmann, M. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1335. (22) Volmer-Uebing, M.; Reynders, B.; Stratmann, M. Werkst. Korros. 1991, 42, 19. (23) Capozzi, G.; Modena, G. In The Chemistry of the Thiol Group; Patai, S., Ed.; John Wiley and Sons: New York, 1974; Vol. II, p 785. (24) Panzner, G.; Egert, B. Surf. Sci. 1984, 144, 651. (25) Kittel, C. Introduction to Solid State Physics, 6th ed.; Wiley: New York, 1986.

Langmuir, Vol. 16, No. 4, 2000 1791

Figure 12. Cyclic voltammograms for a C-steel RCE in NaOH (at pH 11.25) and with 10 mM Na2S added to the solution (at pH 11.25). Inset: the peak area resulting from the difference between the two cyclic sweeps equals the total charge flowing into the electrode.

electrode). It is not clear how many electrons are involved in the oxidation of sulfide on the iron surface. However, if it is assumed to be the same as that for Au and Pt and involve two electrons per elementary reaction,8 then it can be seen that one cyclic sweep does not result in full coverage, probably because the sweep rate is too fast. This illustrates that the affinity of S with Au and Pt is much larger than that for Fe because it has been reported that at a scan rate of 3000 mV/min one cycle was enough to cover the surface with a chemisorbed layer of sulfur. The preparation of sulfur on C-steel was therefore obtained by electrochemical annealing at -700 mV (SCE) for 20 min, although it could not be determined whether this resulted in a sulfur mono- or multilayer. A large SHG signal was detected if the sulfur-modified C-steel electrode was placed in a solution of 10-4 M MB in a borate buffer at pH 8 for 30 min. It also remained entirely reflective. From polarization experiments we determined that the average angle of orientation was 30 ( 10° with a βr value of -0.1. Discussion In this work we have explored two different model films with the aim of using these to determine how monolayer films behave under shear and cavitation. We have found that it is possible to make a monolayer of MB on nickel and iron, by formation of a sulfur layer, where the SHG signal is dominated by the MB. The chemistry is straightforward and reproducible, and the removal of the film can be followed in situ. The use of an iron substrate is of particular importance given the wide use of this metal. This opens up the possibility of performing optical studies of monolayer films under a range of different flow conditions and particularly under conditions of boundary lubrication where the shear rate and applied pressure are very high. The use of the MB monolayer was demonstrated using a nickel rotating cylinder electrode. In a previous paper, based on the Fluent 3D Reynolds stress model for turbulence,11 shear stress values for the jet cylinder were estimated to be around 16 Pa for a rotating cylinder at 4000 rotations/min and 25 Pa for the rotating cylinder plus a liquid jet impinging at 2 m/s. The calculation of the forces on the monolayer as a function of different rotation speeds is nontrivial, and this work clearly demonstrates

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experimentally that the forces are insufficient to produce a measurable change in orientation. These experiments were performed on a full monolayer, and it would be interesting to see if the same result was obtained at low coverage where the MB molecules are isolated on the surface. Given that no change in orientation is observed, it would not be expected that the forces are large enough to remove the MB from the surface, as is observed experimentally. Our results show that cavitation provides sufficient energy to remove the MB from the surface. It is possible for this to happen by two main mechanisms: removal of less strongly bound MB from the entire surface or local removal of MB. Our results are consistent with local removal of the MB presumably due to cavitation of a void at the surface locally removing the MB, sulfur monolayer, and possibly also some underlying nickel. Because we have determined that either the sulfur-nickel or nickel-nickel bond is broken in this process, it will be difficult to use this method to determine the adsorbate bond strength or determine the force or energy released when a cavitating jet strikes the surface. However, the advantages of the combination of SHG and electrochemical methods are clear in these experiments, because we can clearly separate effects due to enhanced mass transport, which affect the electrochemical measurements, from changes due to the removal of the monolayer, which change both the electrochemical and SHG measurements. This makes interpretation of the data straightforward. In addition, SHG can be used to make measurements in nonaqueous media. The only point of comparison for this work is tribiological experiments performed by Spikes and co-workers26 where the thickness of lubricant films has been measured under shear and experiments using the surface force apparatus.27 These are very different from the work presented here. A much higher shear stress is applied to a small region between the two surfaces, and the results show changes in orientation and film removal under these conditions. In our case a much lower shear stress (hydrodynamic shear) is applied, via a liquid flow, to the whole surface of the cylinder. In addition, our monolayer is chemically attached to the surface while the bulk of the tribiological (26) Spikes, H. A. Langmuir 1996, 12, 4567. (27) Yoshizawa, H.; Israelachvili, J. J. Phys. Chem. 1993, 97, 11300. (28) Gopal, M.; Kaul, A.; Jepson, W. P. Paper No. 105 NACE, 1995.

Heeg and Klenerman

studies are on physisorbed monolayers or real lubricants. The tribiological results would suggest that at under our conditions of lower shear stress film removal is unlikely, as we have observed. However, because the experimental situations are so different, it is important to verify this experimentally and also demonstrate that, in contrast, cavitation can lead to film removal. This is of practical importance because our results clearly show that cavitation is the dominant mechanism of film removal under conditions of shear stress of similar magnitude to that encountered industrially, for instance, in oil pipelines. This conclusion is supported by experiments performed by Jepson and co-workers in multiphase flow lines where enhanced corrosion is observed under conditions where cavitating bubbles impinge on the bottom surface of the pipe and corrosion inhibitor films are observed to be removed much more rapidly than in the absence of cavitating bubbles.28 This finding also has important consequences for providing a protective film under different flow conditions because it will be important to find a film which can either rapidly dissipate the energy released in cavitation or, if this is not possible, readsorb rapidly back onto the surface. Conclusions We have investigated a variety of model films and found that MB on nickel and on iron give SHG signals that are dominated by the MB and are hence suitable to follow changes in orientation and to follow film removal during multiphase flow. We have studied MB on nickel, but MB on iron would serve as a useful model corrosion inhibitor film for future work. We found no changes in orientation of the MB for a full monolayer up to a shear rate of 25 Pa. This indicates that high shear stress does not exert sufficient force on the molecules in the film to change the orientation under flow. We also found no removal of the film under shear and a cavitating jet of liquid was required in order to observe removal of the MB from the nickel electrode. This appears to be due to the formation of local pits formed by the cavitating voids, which appear to result in removal of both the MB and the sulfur monolayer. Acknowledgment. We are grateful to BP Research for a CASE award (B.H.) and for advice and support during this research. LA981617L

A Model Corrosion Inhibitor for in Situ Spectroscopic Studies

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