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Study of the inhibition of the acid corrosion of steel by auger electron spectroscopy
Surface and Coatings Technology, 30 (1987) 199 - 205
199
STUDY OF THE INHIBITION OF THE ACID CORROSION OF STEEL BY AUGER ELECTRON SPECTROSCOPY B. A. ABD EL-NABEYa, E. KHAMISa, G. E. THOMPSONb and J. L. DAWSONb
aChemistry Department, Faculty of Science, Alexandria University, Alexandria (Egypt) bCorrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester (U.K.) (Received February 10, 1986)
Summary The Auger spectra for mild steel specimens immersed in deoxygenated uninhibited and inhibited acid solutions gave clear peaks for iron, oxygen and carbon. A comparison between the height of the oxygen and carbon peaks in an acid medium and in an acid medium containiv~ 1-phenyl thiosemicarbazide indicated that the height of the oxygen peak becomes less in the presence of the inhibitor while the carbon peak becomes larger: this behaviour was ati~ibuted to the adsorbed molecules of the inhibitor. From the Auger sputtering depth profile for mild steel immersed in H2SO4 solutions in the absence and presence of the inhibitor, the thicknesses of the oxide film formed were found to be similar in both cases, i.e. the inhibitor does not appear to change the composition of the corrosion product film,
1. Introduction Acid solutions are widely used in industry, the most important fields of application being acid pickling, industrial cleaning, acid descaling and oil well acidizing. Because of the general aggressiveness of the acid solutions the practice of inhibition is commonly used to reduce the corrosive attack on metallic materials [1]. The selection of an appropriate inhibitor mainly depends on the type of acid, on its concentration and of course on the type of metallic material exposed to the action of the acidic solution. It has been reported [2 - 6] that many organic sulphur-containing compounds act as good inhibitors for the acid dissolution of metals. In previous work [7, 8] a series of 11 selected compounds based on substituted thiosemicarbazides and thiosemicarbazones has been studied as corrosion inhibitors of steel in H2SO4 solutions. The hydrogen evolution method, d.c. polarization and a.c. impedance techniques were used to investigate the 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
200
mechanism of inhibition of the acid corrosion of steel by this class of substances. It was found that thiosemicarbazide and its derivatives act as mixed adsorption-type inhibitors, with increased adsorption resulting from increase in the electron density at the ~ C = S centre by suitable substitution of a terminal hydrogen atom in hydrazino and/or thiamino groups of the thiosemicarbazide molecule. The aim of this work was to elucidate the mechanism of the inhibition of the acid corrosion of steel by the thiosemicarbazide derivatives. Auger electron spectroscopy is a very useful technique for the study of the depth and the composition of corrosion products on the surface of the corroded metal [9, 10]. Auger electron spectroscopy was used as a surface technique to examine the steel specimens before and after the attack by H2SO4 in the absence and presence of 1-phenyl thiosemicarbazide as a representative compound of the thiosemicarbazide series.
2. Experimental details The experiments were performed with mild steel rods of cross-section 0.37 cm 2 with the chemical composition given in Table 1. TABLE 1 Chemical composition of the mild steel Element A m o u n t (%)
C 0.27
Mn 0.70
S 0.05
P 0.50
Si 0.35
The mild steel specimens were mechanically polished to a 4000 grit finish prior to ultrasonic cleaning in acetone, rinsing in doubly distilled water and final drying. An Auger electron spectroscopy system consists of an ultrahigh vacuum system, an electron gun for specimen excitation, a means for ion etching the sample and an energy analyser for detection of Auger electron peaks in the total electron distribution. Because the Auger peaks are superimposed on a relatively high continuous background, they are more easily detected by differentiating the energy distribution N(E). Thus the conventional Anger spectrum is the function dN(E)/dE vs. E. The Auger analysis is carried out by necessity in a vacuum of better than 10-6 Pa (10 -s Tort). Argon is introduced to produce a background pressure of 1 × 10 -s Tort (required for effective operation of the ion gun). This background pressure does not affect the operation of the electron gun or energy analyser. When the size of the ion beam is large (greater than 1 mm in diameter) compared with the diameter (0.1 mm) of the electron beam, the sputter etching rate is relatively uniform over the region of analysis. After each sputter sequence with 2.5 keV argon ions at a constant current density of 5/~A an Auger spect~Jm was
201
recorded. The evaluated Auger signals are 703, 651 and 598 eV for pure iron, 512 eV for oxygen and 272 eV for carbon. 1-Phenyl thiosemicarbazide was prepared by condensation of benzaldehyde with thiosemicarbazide in the presence of an acid catalyst. Repeated crystallization gave chromatographically pure samples which were then dried over phosphorus pentoxide for at least 12 h before use. Stock solutions of 1phenyl thiosemicarbazide were prepared in methanol; the test solutions conrained 10 vol.% methanol in order to ensure that the inhibitor remained completely soluble.
3. Results and discussion
Figure 1 shows the Auger spectrum of the mild steel specimen after etching with argon for 2 rain (incident ion beam diameter, about 0.25 mm). This Auger spectrum shows clear peaks for iron (703, 651 and 598 eV) and oxygen (512 eV). The relation between the Auger peak-to-peak height with the sputtering time for the mild steel specimen over a period of 400 is illustrated in Fig. 2. It is seen that the oxygen peak has a maximum height at the surface of the specimen and then decreases sharply to reach a background level after 80 s; the iron peak height remains approximately constant after 80 s. These data are replotted in Fig. 3, which represents the Auger sputtering depth profile for the mild steel specimen. From this figure, if the metal-film interface corresponds to the depth of sputtering at one-half of the initial oxygen peak
r
..............
........ " .
. . . . . .
"
dE
I
0
200
I
i
400 600 Electron energy (eV)
|
800
1000
Fig. 1. Auger electron spectrum of a mild steel specimen after etching for 2 rain with argon (Ep = 2.5 keV).
202
Fe
Fe (400 s)
(320 s)
Fe (240 s)
(80 s)
Fe (160 s)
0 (0 s)
Fig. 2. Relation between the Auger peak-to-peak height with the sputtering time for the mild steel specimen.
8
•
w
6
4
2
0
I
}
I
I
i00
200
30(3)
400
Time, seconds
Fig. 3. Auger sputtering depth profile for the mild steel specimen. height [10], the mi|d steel substmte is revealed after sputtering for 30 s. Since the iron peak increases gradually while the oxygen peak decreases sharply, the oxygen peak which appeared in the Auger spectrum o f the mild steel specimen is due to the formation of a very thin oxide film on the metal surface. Figure 4 presents the Auger spectrum for the mild steel immersed in a de-oxygenated test solution of 0.1 M H2SO4 and 10% methyl alcohol for 30 min, after etching for 2 min with argon under t h e same conditions described in Fig. 1. The evaluated Auger peaks are 703, 651 and 598 eV for iron, 512 eV for oxygen, 272 eV for carbon and 152 eV for sulphur. The appearance of the carbon and sulphur peaks even after etching m a y be attributed either
203
dN
703
dE
651
512
400
200
600 Electron
enersy,
800
1000
eV
Fig. 4 . Auger electron spectrum of mild steel immersed in 0.1 M ~2SO4 after etching for 2 rain with argon (Ep ffi 2.5 keY).
•
•
•
&
•
6
4
2 • ._------
i
i
i
i
i
i
200
400
600
800
1000
1200
Time,
seconds
Fig. 5. Auger sputtering depth profile for the mild steel immersed in 0.1 M H2SO4.
to the adsorption of both the carbon and the sulphur contaminant from the air or to the dissolution of the metal itself, revealing carbon~ontaining alloy phases.
Figure 5 shows the Auger depth profile for the mild steel immersed in 0.1 M H2SO4 plus 10% methyl alcohol. It is obvious that with the increase in
204
the sputtering time t h e height of the oxygen peak starts to decrease gradually, takin~ about 600 s to level off, indicating the corrosion product layer, iron oxide, on the metal surface. Also a comparison between the height of the oxygen peaks from mild steel and from mild steel in methylated acid showed that the height of the former is less than that of the latter specimen. Moreover, the sputtering time to reveal the metal which is an indication of the surface "film" thickness, if present, was found to be about 100 s. Figure 6 represents the Auger spectrum for the mild steel immersed in a deoxygenated test solution of 0.1 M H2SO4 containing 0.0005 M 1-phenyl thiosemicarbazide for 30 min after etching for 2 min with argon under the same conditions described in Fig. 1. The illustrated Auger spectrum shows the presence of peaks for iron, oxygen and carbon. Since the level of carbon contamination varies from one specimen to another, then the apparent enhancement of the carbon peak (272 eV) in this Auger spectrum is not necessarily attributed to the presence of 1-phenyl thiosemicarbazide on the surface of the specimen. The Auger sputtering depth profile for the mild steel immersed in 0.1 M H~SO4 containing 0.0005 M of the inhibitor is illustrated in Fig. 7. Increase in the sputtering time led the height of oxygen peak to decrease gradually, i.e. the formation of an oxygen-containing film on the metal surface is possible. The mild steel interface appeared after 100 s, similar to that of the mild steel in acid solutions. A comparison between the height of oxygen and carbon peaks in the acid medium and in the acid medium containing the inhibitor shows that the height of oxygen peak becomes less in the presence of the inhibitor
dE
l
512
I 200
I I 400 600 Electronenergy,eV
I 800
1000
Fig. 6. Auger electron spectrum of mild steel in 0.1 M H2SO 4 containing 0.0005 M 1-phenyl thiosemicarbazide after etching (Ep = 2.5 keV).
205
J 8
6
4
2
0
i
J
I
i
200
400
600
800
i
i000
J
1200
Time. seconds
Fig. 7. Auger sputtering depth profile for the mild steel in 0.1 M H2SO4 containing 0.0005 M lophenyl thiosemie~bazide.
while the carbon peak becomes larger; this behaviour may be due to the adsorbed molecules of the inhibitor. However, the thicknesses of the film in both cases were found to be simile, i.e. the inhibitor does not appear to change the composition of the corrosion product film. Acknowledgments
The authors wish to thank Dr. B. Bethune for kind assistance throughout the duration of this work and Dr. R. P. M. Procter for the provision o f laboratory facilities. References 1 2 3 4 5 6 7 8 9 10
G. Schmitt, Br. Corros. J., 19 (1984) 165. G. Trabanelli and F. Zucchi, Rev. Coat. Corros., 1 (1973) 97. M. Kaminmki and Z. SzklArska-Simialowska,Corros. Sci., 13 (1973) 557. H. Brandt, M. Fischer and K. Schwabe, Corros. Sci., 10 (1970) 631. B.A. Abd EI-Nabey, N. Khalil and A. Mohamed, Surf. Technol., 24 (1985) 383. B. A. Abd EI-Nabey, A. EI-Toukhy, M. E1-Gamal and F. Mahgoub, Surf. Technol., in the press. B. A. Abd EI-Nabey, N. Khalil, M. Shaban, M. N u t , E. Khamk and G. Thompson, Proc. 6th Eur. Syrup. on Corrosion Inhibitors, Ferrara, 1985, p. 205. B. A. Abd E1-Nabey, E. Khamis and M. Shaban, G. E. Thompson and J. L. Dawson, Surf. Technol., in the press. E. K. Oshe, V. P. Persiantseva, V. E. Zorina and V. N. Alekseev, Proc. 6th Eur. Symp. on Corrosion Inhibitors, Ferrara, 1985, p. 1181. E. B. Bas, X. P. Pan, K. J. Ruegg and F. Stueki, Surf. Sci., 138 (1984) 172.
199
STUDY OF THE INHIBITION OF THE ACID CORROSION OF STEEL BY AUGER ELECTRON SPECTROSCOPY B. A. ABD EL-NABEYa, E. KHAMISa, G. E. THOMPSONb and J. L. DAWSONb
aChemistry Department, Faculty of Science, Alexandria University, Alexandria (Egypt) bCorrosion and Protection Centre, University of Manchester Institute of Science and Technology, Manchester (U.K.) (Received February 10, 1986)
Summary The Auger spectra for mild steel specimens immersed in deoxygenated uninhibited and inhibited acid solutions gave clear peaks for iron, oxygen and carbon. A comparison between the height of the oxygen and carbon peaks in an acid medium and in an acid medium containiv~ 1-phenyl thiosemicarbazide indicated that the height of the oxygen peak becomes less in the presence of the inhibitor while the carbon peak becomes larger: this behaviour was ati~ibuted to the adsorbed molecules of the inhibitor. From the Auger sputtering depth profile for mild steel immersed in H2SO4 solutions in the absence and presence of the inhibitor, the thicknesses of the oxide film formed were found to be similar in both cases, i.e. the inhibitor does not appear to change the composition of the corrosion product film,
1. Introduction Acid solutions are widely used in industry, the most important fields of application being acid pickling, industrial cleaning, acid descaling and oil well acidizing. Because of the general aggressiveness of the acid solutions the practice of inhibition is commonly used to reduce the corrosive attack on metallic materials [1]. The selection of an appropriate inhibitor mainly depends on the type of acid, on its concentration and of course on the type of metallic material exposed to the action of the acidic solution. It has been reported [2 - 6] that many organic sulphur-containing compounds act as good inhibitors for the acid dissolution of metals. In previous work [7, 8] a series of 11 selected compounds based on substituted thiosemicarbazides and thiosemicarbazones has been studied as corrosion inhibitors of steel in H2SO4 solutions. The hydrogen evolution method, d.c. polarization and a.c. impedance techniques were used to investigate the 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
200
mechanism of inhibition of the acid corrosion of steel by this class of substances. It was found that thiosemicarbazide and its derivatives act as mixed adsorption-type inhibitors, with increased adsorption resulting from increase in the electron density at the ~ C = S centre by suitable substitution of a terminal hydrogen atom in hydrazino and/or thiamino groups of the thiosemicarbazide molecule. The aim of this work was to elucidate the mechanism of the inhibition of the acid corrosion of steel by the thiosemicarbazide derivatives. Auger electron spectroscopy is a very useful technique for the study of the depth and the composition of corrosion products on the surface of the corroded metal [9, 10]. Auger electron spectroscopy was used as a surface technique to examine the steel specimens before and after the attack by H2SO4 in the absence and presence of 1-phenyl thiosemicarbazide as a representative compound of the thiosemicarbazide series.
2. Experimental details The experiments were performed with mild steel rods of cross-section 0.37 cm 2 with the chemical composition given in Table 1. TABLE 1 Chemical composition of the mild steel Element A m o u n t (%)
C 0.27
Mn 0.70
S 0.05
P 0.50
Si 0.35
The mild steel specimens were mechanically polished to a 4000 grit finish prior to ultrasonic cleaning in acetone, rinsing in doubly distilled water and final drying. An Auger electron spectroscopy system consists of an ultrahigh vacuum system, an electron gun for specimen excitation, a means for ion etching the sample and an energy analyser for detection of Auger electron peaks in the total electron distribution. Because the Auger peaks are superimposed on a relatively high continuous background, they are more easily detected by differentiating the energy distribution N(E). Thus the conventional Anger spectrum is the function dN(E)/dE vs. E. The Auger analysis is carried out by necessity in a vacuum of better than 10-6 Pa (10 -s Tort). Argon is introduced to produce a background pressure of 1 × 10 -s Tort (required for effective operation of the ion gun). This background pressure does not affect the operation of the electron gun or energy analyser. When the size of the ion beam is large (greater than 1 mm in diameter) compared with the diameter (0.1 mm) of the electron beam, the sputter etching rate is relatively uniform over the region of analysis. After each sputter sequence with 2.5 keV argon ions at a constant current density of 5/~A an Auger spect~Jm was
201
recorded. The evaluated Auger signals are 703, 651 and 598 eV for pure iron, 512 eV for oxygen and 272 eV for carbon. 1-Phenyl thiosemicarbazide was prepared by condensation of benzaldehyde with thiosemicarbazide in the presence of an acid catalyst. Repeated crystallization gave chromatographically pure samples which were then dried over phosphorus pentoxide for at least 12 h before use. Stock solutions of 1phenyl thiosemicarbazide were prepared in methanol; the test solutions conrained 10 vol.% methanol in order to ensure that the inhibitor remained completely soluble.
3. Results and discussion
Figure 1 shows the Auger spectrum of the mild steel specimen after etching with argon for 2 rain (incident ion beam diameter, about 0.25 mm). This Auger spectrum shows clear peaks for iron (703, 651 and 598 eV) and oxygen (512 eV). The relation between the Auger peak-to-peak height with the sputtering time for the mild steel specimen over a period of 400 is illustrated in Fig. 2. It is seen that the oxygen peak has a maximum height at the surface of the specimen and then decreases sharply to reach a background level after 80 s; the iron peak height remains approximately constant after 80 s. These data are replotted in Fig. 3, which represents the Auger sputtering depth profile for the mild steel specimen. From this figure, if the metal-film interface corresponds to the depth of sputtering at one-half of the initial oxygen peak
r
..............
........ " .
. . . . . .
"
dE
I
0
200
I
i
400 600 Electron energy (eV)
|
800
1000
Fig. 1. Auger electron spectrum of a mild steel specimen after etching for 2 rain with argon (Ep = 2.5 keV).
202
Fe
Fe (400 s)
(320 s)
Fe (240 s)
(80 s)
Fe (160 s)
0 (0 s)
Fig. 2. Relation between the Auger peak-to-peak height with the sputtering time for the mild steel specimen.
8
•
w
6
4
2
0
I
}
I
I
i00
200
30(3)
400
Time, seconds
Fig. 3. Auger sputtering depth profile for the mild steel specimen. height [10], the mi|d steel substmte is revealed after sputtering for 30 s. Since the iron peak increases gradually while the oxygen peak decreases sharply, the oxygen peak which appeared in the Auger spectrum o f the mild steel specimen is due to the formation of a very thin oxide film on the metal surface. Figure 4 presents the Auger spectrum for the mild steel immersed in a de-oxygenated test solution of 0.1 M H2SO4 and 10% methyl alcohol for 30 min, after etching for 2 min with argon under t h e same conditions described in Fig. 1. The evaluated Auger peaks are 703, 651 and 598 eV for iron, 512 eV for oxygen, 272 eV for carbon and 152 eV for sulphur. The appearance of the carbon and sulphur peaks even after etching m a y be attributed either
203
dN
703
dE
651
512
400
200
600 Electron
enersy,
800
1000
eV
Fig. 4 . Auger electron spectrum of mild steel immersed in 0.1 M ~2SO4 after etching for 2 rain with argon (Ep ffi 2.5 keY).
•
•
•
&
•
6
4
2 • ._------
i
i
i
i
i
i
200
400
600
800
1000
1200
Time,
seconds
Fig. 5. Auger sputtering depth profile for the mild steel immersed in 0.1 M H2SO4.
to the adsorption of both the carbon and the sulphur contaminant from the air or to the dissolution of the metal itself, revealing carbon~ontaining alloy phases.
Figure 5 shows the Auger depth profile for the mild steel immersed in 0.1 M H2SO4 plus 10% methyl alcohol. It is obvious that with the increase in
204
the sputtering time t h e height of the oxygen peak starts to decrease gradually, takin~ about 600 s to level off, indicating the corrosion product layer, iron oxide, on the metal surface. Also a comparison between the height of the oxygen peaks from mild steel and from mild steel in methylated acid showed that the height of the former is less than that of the latter specimen. Moreover, the sputtering time to reveal the metal which is an indication of the surface "film" thickness, if present, was found to be about 100 s. Figure 6 represents the Auger spectrum for the mild steel immersed in a deoxygenated test solution of 0.1 M H2SO4 containing 0.0005 M 1-phenyl thiosemicarbazide for 30 min after etching for 2 min with argon under the same conditions described in Fig. 1. The illustrated Auger spectrum shows the presence of peaks for iron, oxygen and carbon. Since the level of carbon contamination varies from one specimen to another, then the apparent enhancement of the carbon peak (272 eV) in this Auger spectrum is not necessarily attributed to the presence of 1-phenyl thiosemicarbazide on the surface of the specimen. The Auger sputtering depth profile for the mild steel immersed in 0.1 M H~SO4 containing 0.0005 M of the inhibitor is illustrated in Fig. 7. Increase in the sputtering time led the height of oxygen peak to decrease gradually, i.e. the formation of an oxygen-containing film on the metal surface is possible. The mild steel interface appeared after 100 s, similar to that of the mild steel in acid solutions. A comparison between the height of oxygen and carbon peaks in the acid medium and in the acid medium containing the inhibitor shows that the height of oxygen peak becomes less in the presence of the inhibitor
dE
l
512
I 200
I I 400 600 Electronenergy,eV
I 800
1000
Fig. 6. Auger electron spectrum of mild steel in 0.1 M H2SO 4 containing 0.0005 M 1-phenyl thiosemicarbazide after etching (Ep = 2.5 keV).
205
J 8
6
4
2
0
i
J
I
i
200
400
600
800
i
i000
J
1200
Time. seconds
Fig. 7. Auger sputtering depth profile for the mild steel in 0.1 M H2SO4 containing 0.0005 M lophenyl thiosemie~bazide.
while the carbon peak becomes larger; this behaviour may be due to the adsorbed molecules of the inhibitor. However, the thicknesses of the film in both cases were found to be simile, i.e. the inhibitor does not appear to change the composition of the corrosion product film. Acknowledgments
The authors wish to thank Dr. B. Bethune for kind assistance throughout the duration of this work and Dr. R. P. M. Procter for the provision o f laboratory facilities. References 1 2 3 4 5 6 7 8 9 10
G. Schmitt, Br. Corros. J., 19 (1984) 165. G. Trabanelli and F. Zucchi, Rev. Coat. Corros., 1 (1973) 97. M. Kaminmki and Z. SzklArska-Simialowska,Corros. Sci., 13 (1973) 557. H. Brandt, M. Fischer and K. Schwabe, Corros. Sci., 10 (1970) 631. B.A. Abd EI-Nabey, N. Khalil and A. Mohamed, Surf. Technol., 24 (1985) 383. B. A. Abd EI-Nabey, A. EI-Toukhy, M. E1-Gamal and F. Mahgoub, Surf. Technol., in the press. B. A. Abd EI-Nabey, N. Khalil, M. Shaban, M. N u t , E. Khamk and G. Thompson, Proc. 6th Eur. Syrup. on Corrosion Inhibitors, Ferrara, 1985, p. 205. B. A. Abd E1-Nabey, E. Khamis and M. Shaban, G. E. Thompson and J. L. Dawson, Surf. Technol., in the press. E. K. Oshe, V. P. Persiantseva, V. E. Zorina and V. N. Alekseev, Proc. 6th Eur. Symp. on Corrosion Inhibitors, Ferrara, 1985, p. 1181. E. B. Bas, X. P. Pan, K. J. Ruegg and F. Stueki, Surf. Sci., 138 (1984) 172.