Ind. Eng. Chem. Prod. Res.
Dev. 1986, 25, 133-140
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Corrosion-Inhibitor Orientation on Steel George A. Salensky,' Mlchael 0. Cobb,+ and Dennls S. Everhart* Research and Development Department, Specialty Chemicals Division, Union Carbide Corporation, Bound Brook, New Jersey 08805
Optimum use of organic corrosion inhibitors in coatings requires a better understanding of their protective mechanisms, including their orientation on steel substrates. The molecular orientation of N-oleoylsarcosineon steel was determined by a combination of specular reflectance Fourier transform infrared spectroscopy, ellipsometry, and electron spectroscopy. Studies suggest that it functions as a corrosion inhibitor by forming a chelate with the steel surface.
Introduction Optimum use of organic corrosion inhibitors in coatings requires a better understanding of their protective mechanisms, including their orientation on steel substrates. N-Oleoylsarcosine, reported by Oakes in 1960 to be an effective corrosion inhibitor in aqueous systems, was studied because it contained both carbonyl and nitrogen functional groups which in combination can form complexes with metals such as iron. Furthermore, it has a large aliphatic chain that confers hydrophobic properties to the molecule. The chemical structure of N-oleoylsarcosine is shown in the lower part of Figure 1. On the basis of its chelate-forming properties and molecular model studies, the surface chelate shown in the upper part of Figure 1 is suggested as a possible product of the interaction of Noleoylsarcosine and iron. It is the objective of this paper to give evidence of the above interaction through the use of surface analysis techniques such as infrared spectroscopy, ellipsometry, and electron spectroscopy for chemical analysis (ESCA). Corrosion Tests To demonstrate that N-oleoylsarcosine is effective in a coatings environment as well as in aqueous systems, its corrosion-inhibitor efficiency was demonstrated by an accelerated test method which simulates salt-spray conditions of a coating but uses a model liquid in place of the solid coating (see Table I). This method gives better reproducibility than conventional salt-spray tests on coatings when corrosion inhibitors are studied. Therefore, it allows one to make correlations of corrosion-inhibitor efficiency with the chemical structure of known compounds. However, it does not provide information on the specific type of chemical interaction with the surface. Table I shows that N-oleoylsarcosine remains an effective inhibitor even when its concentration in the organic model compound drops to 0.018%. The oleoylsarcosine was dissolved in diphenyl oxide to simulate an aromatic ether coatipg and its corrosion-inhibitor efficiency measured. Complete protection is indicated by an efficiency of 1.0, whereas no improvement over the control results in zero efficiency. The test method has been described previously (Salensky, 1980). Basically, a steel sample is exposed to fluidized droplets of a mixture of salt water and an organic
* Address correspondence t o Consultant, RD3 Scrabbletown Rd, Whitehouse Station, N J 08889. Accuserv Labs, Atlanta, GA. Kimberly-Clark, Roswell, GA 30077. 0196-432118611225-0133$01.50/0
Table I. Corrosion-Inhibitor Efficiencya of N-Oleoylsarcosine concentration, % efficiency 1.750 0.95 0.95 0.175 0.018 0.88 a
Dissolved in diphenyl oxide.
Table 11. Salt-Spray Resistance of Phenoxy Coatings corrosion treatment exDosure time, h control diD coateda 100 7 8 500 5 7 700 4 6 1%oleoylsarcosine in heptane.
model compound containing a corrosion inhibitor. The corrosion rate of the steel sample is determined by weight-loss measurements. Further evidence of the effectiveness of N-oleoylsarcosine is seen in Table 11,where a cold-rolled steel test panel was dip coated with a 1% heptane solution of the organic compound. A control panel and the treated panel were then coated with unpigmented poly(hydroxy ether) (phenoxy) polymer solution. The dried panels were then subjected to salt spray in accordance with ASTM Method B117. The panels were rated for corrosion according to ASTM Method D-610, where 10 is no change and 0 is complete failure.
Surface Characterization Numerous attempts have been made to characterize adsorbed films of organic molecules on metals. Radio labeling or ellipsometry can be used to determine adsorption isotherms. Sharpe (1961) used multiple internal reflection infrared spectroscopy to determine the structure of films of ethyloctadecanoate transferred to the surface of a germanium internal reflection element by a Langmuir-Blodgett method. Boerio employed specular reflection adsorption infrared spectroscopy and ellipsometry to determine the structure of films formed by the adsorption of polymers, silanes, and model compounds from solutions onto the surface of metal mirrors (Boerio et al., 1978, 1980, 1982). Allara (1980) discussed the use of Fourier transform infrared reflection spectroscopy (FT-IR) for studying organic monolayers. It is recognized that FT-IR offers significant advantages over grating spectrometers by providing a higher signal-to-noise ratio for similar data acquisition times, or a lower data acquisition time for an equivalent signal-to-noise ratio. 0 1986 American Chemical Society
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THIN COATING om MAL SURFACE
, POWIZED
C17H33-C-
ii
PAMLLEL
TO h€ OF kFLECTlDU
S l U I F I CAN1
AISORPTION
FH3 N-Cli2COOH
Figure 1. N-Oleoylsarcosine. C O A T 0 STEEL SAMPLE PUNE POLARIZED
INCIDENT RADIATION
R A D ! A l ION
/
COAlED SlEEL SMPLE
‘\
IIRROR
Figure 2. Multiple reflectance cell.
Infrared Spectroscopy Our initial laboratory work with a semi-mirror-finish steel panel dip coated with 1?& oleoylsarcosine in heptane resulted in distinct absorption bands when a 60’ angle of incidence was used. However, when the steel was polished to a mirror fiiish in order to minimize surface pockets and thereby obtain a thinner organic layer, the poor sensitivity of the system produced no significant absorption bands. However, Boerio has shown that thin organic films on metals can be detected by using grazing angles of incidence and polarized infrared radiation (Boerio et al., 1977, 1978, 1980). Experiments were conducted with F. J. Boerio at the University of Cincinnati, which greatly facilitated our progress. Dr. Boerio’s Perkin-Elmer 180 dispersive infrared spectrometer is equipped with Harrick external reflectance accessories where the coated steel samples are used as reflector elements (Figure 2). The optics were adjusted to provide two reflections from coated samples with a 78O angle of incidence. A wire-grid polarizer placed in the infrared-beam path provides incident radiation polarized parallel to the plane of reflection. Polarized infrared radiation results in signal enhancement by the following mechanism. When light is reflected from a metal surface, the incident and reflected waves combine to form a standing wave with a node near the surface of the metal. No absorption occurs since there is almost no electric field near the surface of the metal. Francis and Ellison (1959) and Greenler (1966) have shown that at grazing angles of incidence the incident and reflected waves combine to form a strong standing wave at the surface when parallel polarized radiation is used. The presence of an optical node at the metal surface prevents the absorption of radiation polarized normal to the plane of incidence (s polarization) (Vasicek, 1960). However, such an optical node does not strictly exist for radiation polarized parallel to the plane of incidence (p polarization), and absorption of this radiation by thin coatings is allowed. These restrictions are depicted graphically in Figure 3. Ellipsometry Ellipsometry is an optical method that provides information on the thickness and refractive index of thin films
/ T H I N COATING ON h l A L SURFACE
Figure 3. Absorption of polarized radiation by thin coatings on highly polished steel surfaces.
on substrates. It is based on the fact that a thin film changes the polarization of an elliptically polarized light beam reflected by the substrate. It has been discussed previously by McCracken et al. (1960). Experimental measurements of thin films on metals allow one to calculate the thickness and optical constants of the surface coating. This information, in addition to molecular-model measurements and spectroscopic studies, permits a prediction of the molecular orientation. ESCA The utility of electron spectroscopy for chemical analysis (ESCA) in providing semiquantitative chemical-state information on the outermost 50-75 A (5-7.5 nm) of a surface is well established. The ESCA experiment involves irradiating a sample in high vacuum (S10-7 torr) with nearly monochromatic X-ray photons and measuring the kinetic energy distribution of photoelectrons ionized from the sample. From the measured kinetic energies and known excitation energy, the binding energies of the photoelectrons are determined. ESCA spectra were recorded on a Hewlett-Packard 5950 B spectrometer (AIK a excitation) equipped with a Surface Science Laboratories (SSL) Model 229 lens aperture, a SSL Model 239 resistive anode detector, and a SSL 926 data system. Binding energy calibration is adjusted weekly such that Au 4f7/2= 83.93 eV, Cu 2p3/2= 932.47 eV, Cu 3s = 122.4 eV, and Cu KLL Auger = 567.8 eV, measured for sputter-cleaned Au or Cu foil.
Experimental Methods Sample Preparation. Steel mirrors were prepared by lapping Society of Automotive Engineers (SAE) 1010 steel specimens with progressively finer abrasive powders ranging from 6- and 1-pm diamond to 0.05-pm y-alumina. Since our previous work has shown that the optical constants of polished steel can vary significantly over the face of a panel, we prepared polished-steel samples with coordinate lines so that the optical constants of the coating could be measured over the exact spot where the reference values for steel were obtained (see Figure 4). Ellipsometry. A pair of steel mirror samples were given a “cleanup” polish with 0.05-pm y-alumina to remove any adsorbed contaminants. Alumina was removed with a water-lubricated clean microcloth. The sample was dried under a stream of filtered dry nitrogen. It was immediately tested at the two spots shown in Figure 4 with the ellip-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
r0'811 +-
135
a ) B e f o r e Heptbne Wash
110X E x p a n s l a n o f Y-Axls! o Coating Thickness 5 60 A
3
Figure 4. Polished-steel mirrors with scribed coordinates. 2920
Table 111. Ellipsometry of Steel Mirrorsa coating thickness spot no. 7 A nm Prior to Organic Coating-Iron Oxide 2 2.808 12 1.2 3 2.755 13 1.3 5 2.760 14 1.4 6 2.757 13 1.3
I 3100
I 3000
1
I
I
2900
7800
2100
FREPL'EIICY l c m - l )
50
40
2 3 5 6 5 6 a
1% N-Oleoylsarcosineb/Heptane 1.525 56.0 1.625 44.9 1.625 53.0 1.587 63.7
Heptane Wash of Coating 1.95 16.4 2.25 16.3
0.05 pm y-alumina finished.
5.6 4.5 5.3 6.4
3100
3000
2900
2800
2700
FREQUEljCY ( c r - l )
1.6 1.6
Figure 5. University of Cincinnati infrared reflectance spectra of organic coating on steel (CH stretching region).
Bulk q, 1.473. COATED STEEL SPnPLE
someter. This permitted calculation of the iron oxide layer, shown to be 13 A (1.3 nm) thick (see Table 111). The two steel samples were then dipped in a 1% oleoylsarcosine solution in heptane for 5 min, machine withdrawn at the rate of 10 cm/min, and air-dried for 10 min.
Results The two reference locations on each panel were then measured again on the ellipsometer. The refractive index of the coating and its thickness over the oxide layer were calculated and found to be approximately 50 A (5 nm) (Table 111). The two coated-steel panels were then placed in the infrared spectrometer using the apparatus shown in Figure 2. Spectra were obtained in the aliphatic and carboxylate spectral regions. These results are discussed below. The pair of samples were then washed in heptane to remove any unbound sarcosine, and the procedure of ellipsometry and infrared analysis was repeated. Table I11 shows that a 16-A (1.6-nm) coating is strongly chemisorbed onto the metal oxide. Infrared Dispersive Spectrometer Results. The above coated samples analyzed at the University of Cincinnati yielded the following spectra, expressed in the percent reflectance mode. Figure 5 shows the specular reflectance spectra in the aliphatic stretching region for the coated and subsequently heptane-washed samples. Strong hydrocarbon-methylene stretching bands are observed at 2850 and 2920 cm-l for the unwashed sample (Figure 5a). However, only extremely weak bands are observed at identical wavelengths in the spectrum of the same sample after it is washed with heptane solvent (Figure 5b). Ellipsometric data indicate that the coating thickness of the unwashed sample is approximately 50 A (5 nm), while after the heptane washing the coating thickness drops to only 16 A (1.6 nm). Although 50-Acoatings of oleoylsarcosine provide good spectra in the aliphatic region, thin films of 16 A give unsatisfactory spectrum with dispersive infrared instrumentation. Carboxylate bands were
REFLECTED
PQLARIZED
MD l AT ION
INCIDENT
TO DETECTOR
T
COATED STEEL SAMPLE
Figure 6. Multiple reflectance cell for FT-IR.
not observed in the spectrum of the heptane-washed sample. Fourier Transform Infrared (FT-IR)Spectroscopy. In order to obtain enhanced sensitivity over the dispersive infrared instrument, a FT-IR spectrometer located at Union Carbide, Bound Brook, NJ, was evaluated. Reflectance FT-IR spectra were obtained with a Digilab FTS-15 spectrometer using a modified attenuated total reflectance sampling accessory and polarized radiation. Spectra were obtained in the percent reflectance mode. Details of the instrument arrangement may be found in the Appendix and Figure 6. Figure 7 shows a transmission FT-IR spectrum of 1% oleoylsarcosine in heptane from 1800 to 1100 cm-'. Bands at 1735, 1610,1465,1405, and 1205 cm-' are assigned to un-ionized amino acid c----O stretching mode, asymmetric zwitterion carboxyl stretching, hydrocarbon CH, deformation, symmetric zwitterion carboxyl stretching, and un-ionized acid C-0 stretching, respectively. The presence of these bands indicates that both zwitterion and un-ionized amino acid exist in 1% oleoylsarcosine in heptane. Upon complexation with a steel surface, the coatingmetal interaction might be expected to form carboxylate species at the interface with asymmetric and symmetric stretching bands near to, but different from, those at 1610 and 1405 cm-l for the free zwitterion. The anodic nature of the metal surface would suggest shifts to higher frequencies for carboxylate stretching bands in complexed oleoylsarcosine. A schematic representation of possible carboxylate forms in free or in complexed oleoylsarcosine
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iwsa \
I I
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95.46
-
"
1735
1610
I 1800
1905
I
I
1 1600
I 1lCM
I
I
Moo
im
loo0
1200
FREWENCY (CM.~)
FREOUENCY (cn-1)
Figure 7. FT-IR transmission spectrum of 1% organic coating in heptane (carboxylate region).
Figure 9. FT-IR reflectance spectrum of organic coating on steel (carboxylate region). I
b 2000
Figure 8. Carboxyl functionalities for free and complexed organic coating.
is presented in Figure 8. One would expect to observe a greater relative abundance of complexed species and less free species with progressively thinner coatings due to coating-metal interaction. Studies of such complexes indicate that C - 0 and C - 0 stretching bands are found near the respective frequencies for asymmetric and symmetric carboxylate bands (Ohsawa and Sutaka, 1979). Figure 9 shows a specular reflectance FT-IR spectrum from 2000 to 1000 cm-* for 6 Fm diamond polished steel that was hand dipped into a solution of 1% oleoylsarcosine in heptane. The coating on this sample is expected from ellipsometry data to greatly exceed a thickness of 100 A. Therefore, prominent bands should be seen for free oleoylsarcosine rather than for the complexed form near the interface. Indeed, strong bands a t 1735, 1610, 1465, 1405, and 1205 cm-', due to free amino acid and zwitterion, are observed in this spectrum.
1Mo
1w)
FREWENCf (cw-l)
Figure 10. FT-IR reflectance spectrum of organic coating on steel (carboxylate region).
In Figure 10 is presented the FT-IR spectrum of 0.05 pm y-alumina polished steel dipped for 5 min in 1% oleoylsarcosine in heptane and machine withdrawn at 4 in./min. The thickness of the resultant coating was measured by ellipsometry to be approximately 60 A (6 nm), considerably thinner than the sample depicted in Figure 9. Relative intensity changes are observed in the spectrum of this thinner sample at 1650,1620, and 1435 cm-l. These bands are believed to arise from complexation of oleoylsarcosine with the metal surface. The presence of multiple bands may suggest differing degrees of complex formation near the coating-metal interface. Bands due to free amino acid are either weak or unobserved in this spectrum. An FT-IR spectrum was obtained for the same sample after it was washed with heptane (Figure 11). The thickness of this coating was measured by ellipsometry to
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 137 I
1W,W
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I
I
1655
#
I
w
92.2
THICKNESS
1
2000
1500
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Figure 11. FT-IR reflectance spectrum of organic coating on steel (carboxylate region). 0
Moo
2500
m
1500
1m
FEWENCY (crr-l)
Figure 13. FT-IR transmission spectrum of 1%organic coating in heptane,
101 31 I
C
\ 0
C
40 -
Figure 12. Effect of surface orientation upon absorbance by carboxylate species.
! i t Y
B
97 29 I
be near 16 A (1.6 nm) and thus approximates a monolayer coverage. Although the signal-to-noise ratio of this spectrum is poor, the presence of a band around 1655 cm-' suggests complex formation at the coating-metal interface. This spectrum was obtained by using an accumulation of only 400 scans. Increased signal averaging to as many as several thousand scans is expected to vastly improve the quality of the spectrum and to afford the ability to examine monolayers of other coatings thinner than 16 A (1.6 nm). For film thicknesses of this order, the need for a complementary surface spectroscopy is apparent, and ESCA proved to be extremely useful. Interpretation of Infrared Results. Figure 12 shows several possible orientations of the carboxylate species of the subject material at the coating-metal interface. The blocked arrows in this figure depict the direction of the dipole moment vectors for the symmetric and asymmetric carboxylate stretching vibrations for the orientations shown. For near-grazing angles one expects only those vibrations with dipole moments normal to the metal surface to significantly absorb incident radiation. Thus, one may predict the orientations of complexed carboxylate species relative to the metal substrate by studying the relative intensities of asymmetric and symmetric bands as progressively thinner coatings are studied. Examination of Figures 9 and 10, with substantially increased intensity
r
2925
m
290
FRFPUEltY ( C d +
Figure 14. FT-IR spectrum of organic coating on steel (CH stretching region).
of the asymmetric stretching band as progressively thinner films are studied, suggests an intermediate carboxylate orientation at the interface. The second orientation in Figure 12 is consistent with these results. These results suggest that the molecule interacts with the steel in the orientation depicted in Figure 1. One may apply the same principles to study the interaction and orientation of the long aliphatic chain of oleoylsarcosine at the metal interface. Figure 13 shows the transmission FT-IR spectrum of 1% oleoylsarcosine in heptane placed between KBr windows. Asymmetric and symmetric CH2 stretching bands at 2925 and 2855 cm-l, respectively, are noted (Sharpe, 1961). Figure 14 shows the aliphatic stretching region of the specular reflectance FT-IR spectrum of 6 km diamond polished steel, hand dipped in 1% oleoylsarcosine in heptane. As noted earlier, this coating is estimated by
138
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COATING THICKNESS ’€08 103.22
Y
E 2Y
. (
Figure 17. Effect of aliphatic chain orientation upon infrared spectrum.
--
Figure 15. FT-IR reflectance spectrum of organic coating on steel (CH stretching region).
.
-1
u Figure 16. Effect of surface orientation upon absorbance by aliphatic species.
ellipsometry to be thicker than 150 A (15 nm). Bands corresponding to free oleoylsarcosine are observed in this spectrum. In Figure 15 is presented the aliphatic region for 0.05 pm y-alumina polished steel dipped for 5 min in a solution of 1% oleoylsarcosine in heptane and machine withdrawn at 10 cm/min. The thickness of this coating, measured by ellipsometry, is approximately 60 A (6 nm), considerably thinner than the sample depicted in Figure 14. One may apply Greenler’s principles, discussed above, to predict the orientation of the aliphatic chain of complexed oleoylsarcosine relative to the metal surface. In Figure 16 are depicted two possible orientations of CH2 groups relative to the steel substrate and the dipole moment vectors corresponding to the respective symmetric and asymmetric CH, stretching vibrations. The relative intensities of these two bands for progressively thinner films may be used to predict the orientation of the ali-
Table IV. N 1s Binding Energies for Oleoylsarcosine Complexed to Fe20, Fe7On,coated Fe70n,coatedlrinsed 399.4 eV 399.6 eV N/N+ = 2.8 N/N+ = 1.2 401.2 eV 401.2 eV “Referenced to C Is for CH2 at 284.6 eV: 0 Is (oxide) = 530.0 eV; Fe 2p3p (oxide) = 710.9 eV; Fe 2p3j2(metal) = 707.0 eV. Table V. Surface Composition-ESCA Results samale % C %O % N control Fep03 40.3 50.0 ND Fez03, coated 61.5 30.6 2.8 Fe203, coated/rinsed 42.1 44.2 2.5
%Fe 9.7 5.1 11.1
phatic chain at the interface. Thus, as shown in Figure 17, a “vertical” aliphatic chain orientation with respect to the steel surface should result in a strong band at 2925 cm-I and little or no absorbance at 2855 cm-I for near-grazing angles of incidence. On the other hand, a “horizontal” orientation in which the aliphatic chain would lie relatively flat against the metal surface would result in strong absorbance at 2855 cm-l and little or no absorbance at 2925 cm-l. Comparison of Figures 14 and 15, with emphasis upon the increase in intensity of the asymmetric stretching band for progressively thinner films, suggests an intermediate aliphatic chain orientation weighted somewhat toward a vertical arrangement, depicted on the left in Figure 17. ESCA Studies. The ESCA spectra (survey and high resolution) for the polished control (Tables IV and V) are consistent with a thin layer of surface oxide on metallic iron, as suggested by ellipsometry measurements. Fe 2p,/, intensities for hydrated iron oxide (BE = 710.9 eV) and iron metal (BE = 707.0 eV) are detected. Although a hydrocarbon contaminant is present, C 1s intensity typical for carboxylate (BE = 288.0 eV) and N 1s intensity are not detected on the control. The 0 1s spectrum shows two signals, which can be assigned to the metal oxide (530.0 eV) and hydroxide (531.5 eV). Several features appear via ESCA that are consistent with adsorption of oleoylsarcosine onto Fe203(Figure 18). C 1s intensity at 288.0 eV, 0 1s intensity at 532.4 eV, and a N 1s doublet (BE = 399.4 eV and 401.2 eV) are detected after oleoylsarcosine exposure. A C 1s BE of 288.0 eV is characteristic of carboxylate. The N 1s doublet is typical of unprotonated (BE = 399.4 eV) and protonated (BE = 401.2 eV) nitrogen. The absence of ESCA intensity for a conjugate acid anion (i.e., C1-, S042-,etc.) precludes assignment at 401.2 eV to a protonated ammonium salt. The zwitterion, however, would produce CO; and N +H species, consistent with ESCA results. Washing in heptane reduced the absolute amount of oleoylsarcosine detected and also resulted in a change in the N/N+ intensity ratio from
Ind. Eng. Chem. Rod. Res. De".. Vol. 25. No. 2. 1986 139
......
l,.6-----710.9
Figure 19. Molecular model orientation.
I
1
2811.0
1M.6
Table VI. Olmylsarcosine Molecular Orientation on Iron height above substrate omitinn vertical horizontal probable angle
h
"rn
28 5 15
2.8 .5
1.5
Molecular Models. Figure 19 illustrates space-filling molecular models of N-oleoylsarcosine in probable vertical and angular orientations. The vertical orientation would be favored if the nitrogen in the molecule is inactive and surface reactivity is through the carboxylate. However, the angular position is probable when the nitrogen participates in the formation of a metal chelate with the surface. Table VI shows the thickness of a monomolecular layer of this material if it were oriented in the shown positions. The probable angle was determined by the access of the nitrogen and oxygen of the sarcosine to the surface iron and the probable position of the methylene portion which contains a central double bond (see Figure 1). Our ellip someter value of 16 A (1.6 nm) agrees very closely to the 15 A (1.5 nm) predicted by the molecular model (Figure 18, right side).
I **I Figure IS. ESCA spectra N-olmylsamamine on steel 2p; (b) C 1s; (e) 0 1s; (d) N 1s.
(a) Fe
an initial value of 2.8 to 1.2 after washing. These results are interpreted aa preferential removal of the nonchelated oleoylsarcosine on the iron oxide surface. It is also noteworthy that ESCA readily detected oleoylsarcosine after heptane washing, although IR provided only marginal intensity at these coating thicknesses. These differences are expected in view of the relative sampling depths of these techniques.
Conclusions A combination of infrared spectroscopy, ellipsometry. ESCA,and molecular models has been used to define the molecular orientation of N-oleoylsarcosine on a polishedsteel surface. Our studies suggest that surface chelation occurs with the amino acid portion of the molecule. Infrared studies indicate that the carboxylate species orients itself on the steel surface almost vertically (--7OO). This position is suggested by the molecular model, assuming that surface iron is chelated in the form of a five-member ring depicted in Figure 1. The proposed configuration of N-oleoylsarcosine contrasts the structure proposed by Mera (1985) for saturated fatty acids absorbed onto aluminum oxide. These differences are easily understood. Without the amide nitrogen, the aliphatic chain of the fatty acid is almost fully extended and tilted approximately loo from the surface
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
normal. For N-oleoylsarcosinethe amide and olefin groups interact with the metal surface to produce the configuration proposed in Figure 1. Infrared studies and ellipsometry measurement both indicate that a significant portion of the aliphatic hydrocarbon chain of N-oleoylsarcosine is oriented diagonally from the steel surface. This agrees with the molecular model's orientation prediction, Figure 18, right model. ESCA provides direct evidence that monolayer coverages of oleoylsarcosine are chemisorbed onto iron oxide. Our studies suggest that N-oleoylsarcosine functions as a corrosion inhibitor by forming a chelate with the steel surface. The hydrophobic long-chain portion of the molecule then orients itself to repel the ingression of corrosion media.
pling accessory was mounted in a Harrick attenuated total reflectance optical beam condenser. The sampling optics were adjusted for angles of incidence of 80'435' from the normal to the metal surface. The optics were adjusted to accommodate one reflection each off a coated sample and a highly efficient mirror, respectively, mounted parallel to each other. A schematic representation of the sampling optics is presented in Figure 6. Sample spectra were referenced against a spectrum of uncoated identically polished steel obtained in the same manner as above. A Perkin-Elmer wire-grid polarizer placed in the incident beam path, in some cases, provided radiation polarized parallel to the plane of reflection. Spectra were obtained in the percent reflectance mode. Registry No. SAE 1010, 12725-33-6;N-oleoylsarcosine, 11025-8.
Acknowledgment We thank Dr. F. J. Boerio, who was engaged as a consultant, for his helpful discussions and the use of his instrumentation at the University of Cincinnati. Appendix: Modified ATR Accessory For Reflectance FT-IR Reflectance FT-IR spectra were obtained with a Digilab FTS-15 spectrometer, operating at a resolution of 4 cm-'. Data were collected by using boxcar apodization and double precision in the single-beam mode. The spectrometer sample chamber was equipped with a modified Harrick attenuated total reflectance optical sampling accessory, in which the ATR crystal was removed and samples were placed where the crystal sidefaces normally would be positioned thereby creating an open channel allowing the passage of radiation. This redesigned sam-
Literature Cited Allera, D. L. Vlbratkmal Spectroscoples for Adsorbed Species; Bell, A. T., Hair, M. L., Eds.; ACS Symposium Series 137; American Chemical Society: Washington, DC, 1980; Chapter 3. Aiiera, D. L. Abst. Sth Annu. Meet. Adhes. SOC.1985, 14a. Boerio, F. J.; Chen, S. L. J. Colloid Interface Scl. 1980, 7 3 , 176. Boerio, F. J.; Schoenleln, L. H.; Grievenkamp, J. E. J. Appl. Polym. Sci. 1978, 22, 203-213. Boerio, F. J.; Gosselin, C. A.; Liu, H. W. J. Adhes. 1982, 73, 159. Francis, S. P.; Ellison, A. H. J . Opt. SOC.Am. 1959, 49, 131. Greenler, R. G. J . Chem. Phys. 1986, 44, 310. McCracken, F. L.; Passagiia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand. Sect. A : 1980, 67A, 363. Oakes, B. D. US. Patent 2931 700, 1960. Ohsawa, M.; Sutaka, W. Corros. Sci. 1979, 2 9 , 709-722. Salensky, G., presented at the National Association of Corrosion Engineers International Corrosion Conference, 1980, Bethlehem, PA. Sharpe. L. H. Proc. Chem. Soc., London 1981, 265, 461. Vasicek. A. Optics of Thin Nlms North Holland: Amsterdam, 1960; Chapter 2.
Received for review January 9, 1985 Accepted December 2, 1985
