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Acid corrosion of steel in water-dimethyl formamide solutions
Surface and Coatings Technology, 31 (1987)89
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101
89
ACID CORROSION OF STEEL IN WATER-DIMETHYL FORMAMIDE SOLUTIONS B. A. ABD EL-NABEY, M. EL-GAMAL, E. KHAMIS and F. MAHGOUB Department of Material Science, The University ofAlexandria Research Centre, Alexandria (Egypt) (Received January 13, 1986)
Summary The corrosion of steel in 2.5 N H2S04 in water—dimethyl formamide (DMF) solutions with various compositions (from 0.0 to 70 vol.% DMF) was studied. The rate of dissolution of steel was measured chemically using gasometry and mass-loss technique and electrochemically using d.c. polarization and a.c. impedance techniques. The results obtained showed that the acid corrosion of steel was inhibited by dimethyl formamide in the medium. Capacitance measurements indicated that the dimethyl formamide molecules were specifically adsorbed at the steel—solution interface. The effect of variation in the composition of the water—dimethyl formanüde solution on the protection efficiency of 4-amino-3-H-5-mercapto-1,2,4-triazoline was also studied. The results showed that the protection efficiency of this inhibitor generally decreased as the concentration of DMF in the medium increased. It is suggested that there is competitive adsorption between the dimethyl formamide and the inhibitor molecules. An increase in the concentration of DMF in the solution leads to the displacement of adsorbed inhibitor molecules from the double layer at the steel—solution interface.
1. Introduction The acid corrosion of steel in aqueous solutions and in non-aqueous media is of great interest in both basic and applied research. Acid solutions are widely used in industry where the most important fields of application are acid pickling, acid cleaning, acid descaling and oil well acidizing. Because of the general aggressiveness of acid solutions inhibitors are commonly used to reduce the corrosive attack on metallic materials. A good acid pickling inhibitor must meet a number of requirements [1J, such as effective inhibition of metal dissolution, effectiveness at low concentration, effectiveness also at high temperature, good surfactant and good foaming characteristics. From the long list of properties that a good acid pickling inhibitor should have, it is obvious that all such requirements cannot be met by a single 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
90
substance. Therefore all commercial inhibitors contain more than one substance in the so-called inhibitor package. The formulated inhibitor package generally comprises an active inhibitor substance, a surface-active compound, a detergent, a foaming agent, a solvent and, if necessary, a cosolvent. Studies have previously been carried out in this laboratory on the influence of variations in the composition of the solvent on the pitting corrosion of stainless steel [2, 3] and on the alkaline [4] and acid corrosion [5] of aluminium. The results of these studies indicated that some organic solvents inhibit the above corrosion processes. In this work, hydrogen evolution, mass-loss, potentiodynamic polarization and a.c. impedance measurements were made on steel specimens in 2.5 N sulphuric acid in water—dimethyl formamide (DMF) mixtures with various compositions (0.0 70 vol.% DMF) to investigate the effect that variations in the physicochemical properties of the solvent have on the acid dissolution of steel. Another set of experiments was performed in the presence of 1O~M 4-amino-3-H-5-mercapto-1,2,4-triazoline, which was examined in this laboratory [6] and found to be a good inhibitor for the acid dissolution of steel. The aim was to elucidate the effect of the composition of the solvent on the protection efficiency of this inhibitor. -
2. Experimental details The specimens used in the chemical investigations were in the form of rods with surface areas of 9 cm2. Before measurements the samples were polished with emery paper, washed thoroughly with distilled water and dried with acetone. The steel specimens were placed in a vessel of the type described by Mylius [7]. It could be used to measure the volume of hydrogen evolved as a function of time as well as the mass loss. Polarization measurements were carried out using aWenking potentiostat (model POS 73) with a potential scanning rate of 0.02 V min’. Electrochemical impedance determinations were carried out using a Solartron frequency response analyzer (FRA) type 1172 controlled by a Hewlett— Packard 85 microcomputer which was also used to analyse the data by means of a standard software programme [8]. Complex plane graphs of real impedance—imaginary impedance (Z’—Z”) and other standard graphs were obtained using a Hewlett—Packard 7225 A plotter. The electrode assembly comprised a cylindrical piece of steel with a surface area of 0.37 cm2 mounted in a Teflon holder and exposed to the solution. The electrodes were mechanically polished to a 4000 grit finish followed by ultrasonic cleaning in acetone; finally the specimen was rinsed in doubly distilled water and dried. The chemical and electrochemical experiments were carried out at 30 ±0.1 °Con mild steel specimens whose chemical composition is given in Table 1.
91
TABLE 1 Chemical composition of the mild steel
Element
C
Mn
S
P
Si
Amount(%)
0.27
0.07
0.05
0.50
0.35
3. Results and discussion 3.1. Influence of variation in the composition of water—dimethyl formamide solutions on the acid dissolution of steel 3.1.1. Hydrogen evolution and mass-loss measurements The corrosion of mild steel in 2.5 N H2S04 solutions containing various amounts of dimethyl formamide was studied chemically using the hydrogen evolution method and the mass-loss technique. Figure 1 represents the varia-
1.8
O
~
Time, mm
Fig. 1. Effect of dimethyl formamide concentration in water—dimethyl formamide solutions on the corrosion rate of steel in 2.5 N H2S04: curve 1, 0 vol.%; curve 2, 5 vol.%; curve 3, 10 vol.%; curve 4, 30 vol.%; curve 5, 40 vol.%; curve 6, 60 vol.%.
92
tion in the volume of the hydrogen evolved as a function of time when the steel test pieces were allowed to react with 2.5 N H2S04 in water—dimethyl formamide solutions of various compositions (from 0.0 to 70 vol.% DMF). On increasing the percentage of DMF in the medium the rate of evolution of hydrogen (the slope of the plot) decreased. This indicates that the presence of DMF in the medium retarded the corrosion of steel by sulphuric acid and that the extent of corrosion inhibition depended on the amount of DMF present. Figure 2 shows the variation of the inhibition efficiency of dimethyl formamide with its concentration. The results obtained from the hydrogen
90
80 S
. 70 S
60
. C 0 4J
~
50
-
C 0) 00
40 C 0) U 0)
30
20
10
0 0
10
20
I 30
40
I 50
60
I 70
Vol % dimethyl formamide
Fig. 2. Relation between the percentage inhibition of corrosion of steel in 2.5 N H2S04 in water—dimethyl formamide solutions and the concentration of dimethyl formamide: 0, hydrogen evolution measurements;•, mass-loss measurements.
evolution method and the mass-loss technique showed good agreement, as can be seen in the figure. It appears that the inhibition efficiency of dimethyl formamide increases non-linearly with increasing concentration. Addition of small quantities of dimethyl formamide to the pure aqueous solution produced a dramatic increase in the inhibition efficiency. However, in the presence of a high concentration of dimethyl formamide the composition of the medium has little effect on the inhibition efficiency. Demo [9] determined the corrosion rate of various alloys in 0.5 M HC1 in water—dimethyl formamide solution. A plot of the corrosion rate vs. the mole fraction of dimethyl formamide showed a characteristic minimum and this behaviour was explained on the basis that the metal surface was preferentially solvated by organic molecules [10, 11]. At the solvent composition corresponding to the minimum corrosion rate the solvation of the metal surface was a maximum and consequently the metal ion requires maximum energy to cross the energy barrier during its transition through the double layer at the electrode surface. Work was recently carried out in this laboratory on the pitting corrosion of stainless steel [2, 3] and on the alkaline corrosion of aluminium [4] in some water-organic solvent mixtures and showed that variation in the physicochemical properties of the medium with its composition has an important role in controlling the corrosion process. It was found that the pitting corrosion of stainless steel and the alkaline corrosion of aluminium were inhibited by the organic components in the medium and the percentage inhibition was found to increase with increasing concentration of the organic solvent. These results were interpreted on the basis that these types of corrosion processes are controlled by diffusion of the corrosion products from the metal surface to the bulk solution. An increase in the viscosity of the medium the percentage composition is varied leads to a marked decrease in the diffusion coefficient of the corrosion products. The corrosion behaviour of aluminium metal in 0.1 M HC1 in water— ethanol, in water—isopropanol and in water-ethylene glycol solutions of varying composition (0.0 60 vol.% alcohol) has also been investigated in this laboratory [5]. The results indicated that the addition of a small amount of alcohol (up to 20 vol.%) to the aqueous medium produced a marked decrease in the acid corrosion of aluminium. Above that level, for alcohol concentrations between 20 and 60 vol.%, the composition of the medium had little effect on the corrosion of aluminium. This has been attributed to a change in the state of solvation of the hydrogen ions [12 14]. On the addition of small quantities of alcohol to water the acidity decreases markedly owing to the breakdown of the open tetrahedral structure of water [15]. This concept accounted for the marked decrease in the corrosion rate of aluminium on the addition of small amounts of alcohol (up to 20 vol.%) to the purely aqueous medium. However, further additions of alcohol would not influence the tetrahedral structure of water to the same extent. -
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94
In this study the dependence of the acid corrosion rate of steel on the specific adsorption of dimethyl formamide is considered. 3.1.2. A.c. impedance measurements Electrochemical impedance is an appropriate method for use in corrosion studies, particularly in corrosion rate determination [16 18], for mechanistic studies and for the investigation of inhibited systems [19 22]. In previous work this technique has been employed (1) to investigate the mechanism of the inhibition of acid corrosion of steel by thiosemicarbazide derivatives [23] and (2) to determine the temperature coefficient of the reaction between inhibited sulphuric acid and steel and to measure the capacitance of the steel interface at various temperatures [24]. In the present work the impedance technique was employed to investigate the specific adsorption of dimethyl formamide at the steel—solution interface. Figure 3 shows the impedance diagrams for steel in 2.5 N H2SO4 in the absence and in the presence of various concentrations of dimethyl formamide. An increase in the concentration of dimethyl formamide caused an increase in the value of the charge transfer resistance R~,i.e. a decrease in the corrosion rate of steel. Also, the impedance plots were semicircular in shape for all solutions examined, indicating that the corrosion of steel was mainly controlled by a charge transfer process and that the presence of dimethyl formamide did not alter the mechanism of the dissolution of the steel. Figure 4 shows the variation in the value of 1/Re, the reciprocal of the charge transfer resistance, with the concentration of dimethyl formamide. Electrochemical theory shows that l/ll~is proportional to the corrosion rate [16] and is analogous to polarization resistance in the Stern—Geary equation [25]. In the present work it was evident that the value of 1/Ri decreased in an exponential-like manner as the concentration of dimethyl formamide increased. The inhibition efficiency P of dimethyl formamide was calculated from the impedance data using the following relationship [24]: -
-
P= 1—
1/Re 100 (1/R~)0
where (1/R~)oand 1/Re are the reciprocals of the charge transfer resistance in the absence and in the presence of dimethyl formamide respectively. Figure 5 shows the variation in the inhibition efficiency with the concentration of dimethyl formamide. The characteristics are quite similar to those of an adsorption isotherm, and it appears that there is good agreement between the results obtained using the impedance technique and those obtained using mass-loss techniques (Fig. 2). This behaviour can be explained on the basis of the specific adsorption of dimethyl formamide molecules at the steel—solution interface. To confirm this assertion, the capacitance of the
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Fig. 6. Variation in the double-layer capacitance at the corrosion potential of steel in 2.5 N H 2S04 with concentration of dimethyl formamide in the solution.
steel—solution interface was measured in 2.5 N H2SO4 in the absence and in the presence of various concentrations of DMF (Fig. 6). It is clear that the capacitance of the steel—solution interface generally decreased with increasing percentage of dimethyl formainide in the medium. The addition of a small amount of dimethyl formamide (up to 10 vol.%) to the aqueous medium produced a marked decrease in the capacitance of the interface. Above that level, from 10 to 60 vol.% dimethyl formamide, the composition of the medium had little effect on the capacitance of the interface. These results indicated that dimethyl formamide was specifically adsorbed at the steel—solution interface and that an adsorbed layer of dimethyl formamide molecules was complete at a dimethyl formamide concentration of about 10 vol.%.
98
3..L3. D.c. polarization measurements Figure 7 shows the polarization curves for steel in 2.5 N sulphuric acid in the absence and in the presence of various concentrations of dimethyl formamide. The curves show that the inhibitor causes cathodic and anodic overvoltage and that the magnitude of the displacement of the Tafel plots is proportional to the concentration of dimethyl formamide. This result indicates that the dimethyl formamide affects both the cathodic and the anodic processes. The electrochemical parameters of mild steel in 2.5 N sulphuric acid in the absence and in the presence of various concentrations of dimethyl formamide are given in Table 2. The data show that the slopes of the cathodic and anodic Tafel lines (j3~and 13a) remain almost unchanged on increasing the concentration of dimethyl formamide (I3~= 0.14 V decade’, /~a= 0.10 13.0 V decade’). This behaviour indicates that the -
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Fig. 7. Polarization curves for mild steel in 2.5 N H
2S04 in the presence of various concentrations of dimethyl formamide: curve 1, 0 vol.%; curve 2, 10 vol.%; curve 3, 40 vol.%; curve 4, 50 vol.%; curve 5, 60 vol.%.
99 TABLE 2 Electrochemical parameters of steel in 2.5 N H
2504 in water—dimethyl formamide
solutions IDMFJ (vol.%) 0 10 40 50 60
Eeq
(V (SCE)) 0.50 0.48 0.48 0.48 0.48
2) icon (mA cm 4.6 2.8 2.2 1.6 1.3
(V decade’) 13a
(V decade~)
0.12 0.10 0.11 0.13 0.13
0.14 0.14 0.14 0.14 0.18
(%) Inhibition —
39.7 52.1 65.3 72.4
SCE, saturated calomel electrode.
adsorbed molecules of dimethyl formamide mechanically screen the coated part of the electrode and therefore protect it from the action of the corrosive medium. These molecules have no effect on the mechanism of dissolution of steel but cause only inactivation of a part of the surface with respect to the corrosive medium. 3.2. Influence of variation in the composition of water—dimethyl formamide solutions on the protection efficiency of 4-amino-3-H-5-mercap to-i, 2,4triazoline It is quite interesting to investigate the effect of varying the composition of the solvent on the protection efficiency of one of the good inhibitors of the acid corrosion of steel. 4-Amino-3-H-5-mercapto-1,2,4-triazoline has been examined previously [6] as an inhibitor of the corrosion of steel in 2.5 N sulphuric acid. In pure aqueous solution a iO~M solution of this inhibitor produced 82.5% inhibition of the acid corrosion of steel. Figure 8 shows the influence of the composition of the solvent on each of the following: (a) the protection efficiency of the inhibitor, (b) the total protection efficiency of the inhibitor and dimethyl formamide and (c) the protection efficiency of dimethyl formamide. The data show that the total protection efficiency of the inhibitor and dimethyl formamide increases slightly with increasing percentage of dimethyl formamide in the solution. The protection efficiency of dimethyl formamide increases nonlinearly with increasing concentration and this behaviour was discussed in the previous section. However, the protection efficiency of the inhibitor decreases rapidly with increasing percentage of dimethyl formamide. This behaviour can be discussed on the basis of competition between dimethyl formamide and the inhibitor. An increase of the concentration of dimethyl formamide in the solution leads to the displacement of adsorbed inhibitor molecules from the double layer at the steel—solution interface.
100
N 0
70
~
60
U
C 4) .0
S
U ‘.0
‘~
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.2
(c)
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30
20
•
10
0 0
10
I
I
20
30
I
I
40
50
I
60
Vol % dirnethy fornamide
Fig. 8. Effect of variation in the composition of water—dimethyl formamide solutions containing 2.5 N H 2S04 on the percentage inhibition of the corrosion of steel by dimethyl formamide (curve a), the total percentage inhibition of the corrosion of steel by dimethyl formamide and 4-amino-3-H-5-mercapto-1 ,2,4-triazoline (curve b) and the protection efficiency of 4-amino-3-H-5-mercapto-1 , 2,4-triazoline, (curve c).
Acknowledgment The authors would like to thank Dr. G. Thompson, Corrosion and Protection Centre, UMIST, Manchester University, Manchester, England, for providing the necessary facilities used in impedance measurements. References 1 G. Schmitt, Br. Corros. J., 19 (1984) 165. 2 B. A. Abd El-Nabey, N. Khalil, M. M. Eisa and H. Sadek, Surf. Technol., 20 (1983) 209.
101 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25
B. A. Abd El-Nabey, N. Khalil and M. M. Eisa, Surf. Technol., 22 (1984) 9. B. A. Abd El-Nabey, N. KhaIil and E. Khamis, Surf. Technol., 22 (1984) 367. B. A. Abd El-Nabey, N. Khalil and E. Khamis, Corros. Sci., 25 (1985) 225. B. A. Abd El-Nabey, A. El-Toukhy, M. El-Gamal and F. Mahgoub, Surf. Coat. Technol., 27 (1986) 325. F. Mylius, Z. Metallkd., 14 (1922) 233. K. Hladky, Ph.D. Thesis, University of Manchester, England, 1978. J. J. Demo, Chem. Eng. World, 7 (1972) 115. W. Jaenicke and P. H. Schweitzer, Z. Phys. Chem. (N.F.), 52 (1967) 104. E. Heitz, in M. G. Fontana and R. W. Staehle (eds.), Advances in Corrosion Science and Technology, Vol. 4, Plenum, New York, 1975, pp. 149 - 243. F. Franks and D. J. G. Ives, Quart. Rev., 20(1966) 1. F. Franks, in F. Franks (ed.), Physicochemical Processes in Mixed Solvents, American Elsevier, New York, 1967, p. 50. H. Sadek, Z. Phys. Chem. (Leipzig), 266 (1985) 740. E. A. Braude and E. S. Stern, J. Chem. Soc., (1976) 1948. K. Hladky, L. M. Callow and J. L. Dawson, Br. Corros. J., 15 (1980) 20. L. F. G. Williams and R. J. Taylor, Corrosion, 38 (1980) 425. W. J. Lorenz and F. Mansfeld, Corros. Sci., 21 (1981) 647. H. Schweickert, W. J. Lorenz and H. Friendburg, J. Electrochem. Soc., 127 (1980) 1693. M. Keddam, 0. R. Mattos and H. Takenouti, J. Electrochem. Soc., 128 (1981) 257. M. Keddam, 0. R. Mattos and H. Takenouti, J. Electrochem. Soc., 128 (1981) 266. C. Gabrielli, M. Keddam and H. Takenouti, Alternating-current impedance measurements applied to corrosion studies and corrosion-rate measurements. In F. Mansfeld and U. Bertocci (eds.), Electrochemical Corrosion Testing, ASTM STP 727, American Society for Testing and Materials, 1981, pp. 150 - 166. B. A. Abd El-Nabey, E. Khamis, M. Shaban, G. E. Thompson and J. L. Dawson, Surf. Coat. Technol., in the press. B. A. Abd El-Nabey, E. Khamis, G. E. Thompson and J. L. Dawson, Surf. Coat. Technol., 28 (1986) 83. M. Stern and A. L. Geary, J. Electrochem. Soc., 104 (1957) 56.
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101
89
ACID CORROSION OF STEEL IN WATER-DIMETHYL FORMAMIDE SOLUTIONS B. A. ABD EL-NABEY, M. EL-GAMAL, E. KHAMIS and F. MAHGOUB Department of Material Science, The University ofAlexandria Research Centre, Alexandria (Egypt) (Received January 13, 1986)
Summary The corrosion of steel in 2.5 N H2S04 in water—dimethyl formamide (DMF) solutions with various compositions (from 0.0 to 70 vol.% DMF) was studied. The rate of dissolution of steel was measured chemically using gasometry and mass-loss technique and electrochemically using d.c. polarization and a.c. impedance techniques. The results obtained showed that the acid corrosion of steel was inhibited by dimethyl formamide in the medium. Capacitance measurements indicated that the dimethyl formamide molecules were specifically adsorbed at the steel—solution interface. The effect of variation in the composition of the water—dimethyl formanüde solution on the protection efficiency of 4-amino-3-H-5-mercapto-1,2,4-triazoline was also studied. The results showed that the protection efficiency of this inhibitor generally decreased as the concentration of DMF in the medium increased. It is suggested that there is competitive adsorption between the dimethyl formamide and the inhibitor molecules. An increase in the concentration of DMF in the solution leads to the displacement of adsorbed inhibitor molecules from the double layer at the steel—solution interface.
1. Introduction The acid corrosion of steel in aqueous solutions and in non-aqueous media is of great interest in both basic and applied research. Acid solutions are widely used in industry where the most important fields of application are acid pickling, acid cleaning, acid descaling and oil well acidizing. Because of the general aggressiveness of acid solutions inhibitors are commonly used to reduce the corrosive attack on metallic materials. A good acid pickling inhibitor must meet a number of requirements [1J, such as effective inhibition of metal dissolution, effectiveness at low concentration, effectiveness also at high temperature, good surfactant and good foaming characteristics. From the long list of properties that a good acid pickling inhibitor should have, it is obvious that all such requirements cannot be met by a single 0257-8972/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
90
substance. Therefore all commercial inhibitors contain more than one substance in the so-called inhibitor package. The formulated inhibitor package generally comprises an active inhibitor substance, a surface-active compound, a detergent, a foaming agent, a solvent and, if necessary, a cosolvent. Studies have previously been carried out in this laboratory on the influence of variations in the composition of the solvent on the pitting corrosion of stainless steel [2, 3] and on the alkaline [4] and acid corrosion [5] of aluminium. The results of these studies indicated that some organic solvents inhibit the above corrosion processes. In this work, hydrogen evolution, mass-loss, potentiodynamic polarization and a.c. impedance measurements were made on steel specimens in 2.5 N sulphuric acid in water—dimethyl formamide (DMF) mixtures with various compositions (0.0 70 vol.% DMF) to investigate the effect that variations in the physicochemical properties of the solvent have on the acid dissolution of steel. Another set of experiments was performed in the presence of 1O~M 4-amino-3-H-5-mercapto-1,2,4-triazoline, which was examined in this laboratory [6] and found to be a good inhibitor for the acid dissolution of steel. The aim was to elucidate the effect of the composition of the solvent on the protection efficiency of this inhibitor. -
2. Experimental details The specimens used in the chemical investigations were in the form of rods with surface areas of 9 cm2. Before measurements the samples were polished with emery paper, washed thoroughly with distilled water and dried with acetone. The steel specimens were placed in a vessel of the type described by Mylius [7]. It could be used to measure the volume of hydrogen evolved as a function of time as well as the mass loss. Polarization measurements were carried out using aWenking potentiostat (model POS 73) with a potential scanning rate of 0.02 V min’. Electrochemical impedance determinations were carried out using a Solartron frequency response analyzer (FRA) type 1172 controlled by a Hewlett— Packard 85 microcomputer which was also used to analyse the data by means of a standard software programme [8]. Complex plane graphs of real impedance—imaginary impedance (Z’—Z”) and other standard graphs were obtained using a Hewlett—Packard 7225 A plotter. The electrode assembly comprised a cylindrical piece of steel with a surface area of 0.37 cm2 mounted in a Teflon holder and exposed to the solution. The electrodes were mechanically polished to a 4000 grit finish followed by ultrasonic cleaning in acetone; finally the specimen was rinsed in doubly distilled water and dried. The chemical and electrochemical experiments were carried out at 30 ±0.1 °Con mild steel specimens whose chemical composition is given in Table 1.
91
TABLE 1 Chemical composition of the mild steel
Element
C
Mn
S
P
Si
Amount(%)
0.27
0.07
0.05
0.50
0.35
3. Results and discussion 3.1. Influence of variation in the composition of water—dimethyl formamide solutions on the acid dissolution of steel 3.1.1. Hydrogen evolution and mass-loss measurements The corrosion of mild steel in 2.5 N H2S04 solutions containing various amounts of dimethyl formamide was studied chemically using the hydrogen evolution method and the mass-loss technique. Figure 1 represents the varia-
1.8
O
~
Time, mm
Fig. 1. Effect of dimethyl formamide concentration in water—dimethyl formamide solutions on the corrosion rate of steel in 2.5 N H2S04: curve 1, 0 vol.%; curve 2, 5 vol.%; curve 3, 10 vol.%; curve 4, 30 vol.%; curve 5, 40 vol.%; curve 6, 60 vol.%.
92
tion in the volume of the hydrogen evolved as a function of time when the steel test pieces were allowed to react with 2.5 N H2S04 in water—dimethyl formamide solutions of various compositions (from 0.0 to 70 vol.% DMF). On increasing the percentage of DMF in the medium the rate of evolution of hydrogen (the slope of the plot) decreased. This indicates that the presence of DMF in the medium retarded the corrosion of steel by sulphuric acid and that the extent of corrosion inhibition depended on the amount of DMF present. Figure 2 shows the variation of the inhibition efficiency of dimethyl formamide with its concentration. The results obtained from the hydrogen
90
80 S
. 70 S
60
. C 0 4J
~
50
-
C 0) 00
40 C 0) U 0)
30
20
10
0 0
10
20
I 30
40
I 50
60
I 70
Vol % dimethyl formamide
Fig. 2. Relation between the percentage inhibition of corrosion of steel in 2.5 N H2S04 in water—dimethyl formamide solutions and the concentration of dimethyl formamide: 0, hydrogen evolution measurements;•, mass-loss measurements.
evolution method and the mass-loss technique showed good agreement, as can be seen in the figure. It appears that the inhibition efficiency of dimethyl formamide increases non-linearly with increasing concentration. Addition of small quantities of dimethyl formamide to the pure aqueous solution produced a dramatic increase in the inhibition efficiency. However, in the presence of a high concentration of dimethyl formamide the composition of the medium has little effect on the inhibition efficiency. Demo [9] determined the corrosion rate of various alloys in 0.5 M HC1 in water—dimethyl formamide solution. A plot of the corrosion rate vs. the mole fraction of dimethyl formamide showed a characteristic minimum and this behaviour was explained on the basis that the metal surface was preferentially solvated by organic molecules [10, 11]. At the solvent composition corresponding to the minimum corrosion rate the solvation of the metal surface was a maximum and consequently the metal ion requires maximum energy to cross the energy barrier during its transition through the double layer at the electrode surface. Work was recently carried out in this laboratory on the pitting corrosion of stainless steel [2, 3] and on the alkaline corrosion of aluminium [4] in some water-organic solvent mixtures and showed that variation in the physicochemical properties of the medium with its composition has an important role in controlling the corrosion process. It was found that the pitting corrosion of stainless steel and the alkaline corrosion of aluminium were inhibited by the organic components in the medium and the percentage inhibition was found to increase with increasing concentration of the organic solvent. These results were interpreted on the basis that these types of corrosion processes are controlled by diffusion of the corrosion products from the metal surface to the bulk solution. An increase in the viscosity of the medium the percentage composition is varied leads to a marked decrease in the diffusion coefficient of the corrosion products. The corrosion behaviour of aluminium metal in 0.1 M HC1 in water— ethanol, in water—isopropanol and in water-ethylene glycol solutions of varying composition (0.0 60 vol.% alcohol) has also been investigated in this laboratory [5]. The results indicated that the addition of a small amount of alcohol (up to 20 vol.%) to the aqueous medium produced a marked decrease in the acid corrosion of aluminium. Above that level, for alcohol concentrations between 20 and 60 vol.%, the composition of the medium had little effect on the corrosion of aluminium. This has been attributed to a change in the state of solvation of the hydrogen ions [12 14]. On the addition of small quantities of alcohol to water the acidity decreases markedly owing to the breakdown of the open tetrahedral structure of water [15]. This concept accounted for the marked decrease in the corrosion rate of aluminium on the addition of small amounts of alcohol (up to 20 vol.%) to the purely aqueous medium. However, further additions of alcohol would not influence the tetrahedral structure of water to the same extent. -
-
94
In this study the dependence of the acid corrosion rate of steel on the specific adsorption of dimethyl formamide is considered. 3.1.2. A.c. impedance measurements Electrochemical impedance is an appropriate method for use in corrosion studies, particularly in corrosion rate determination [16 18], for mechanistic studies and for the investigation of inhibited systems [19 22]. In previous work this technique has been employed (1) to investigate the mechanism of the inhibition of acid corrosion of steel by thiosemicarbazide derivatives [23] and (2) to determine the temperature coefficient of the reaction between inhibited sulphuric acid and steel and to measure the capacitance of the steel interface at various temperatures [24]. In the present work the impedance technique was employed to investigate the specific adsorption of dimethyl formamide at the steel—solution interface. Figure 3 shows the impedance diagrams for steel in 2.5 N H2SO4 in the absence and in the presence of various concentrations of dimethyl formamide. An increase in the concentration of dimethyl formamide caused an increase in the value of the charge transfer resistance R~,i.e. a decrease in the corrosion rate of steel. Also, the impedance plots were semicircular in shape for all solutions examined, indicating that the corrosion of steel was mainly controlled by a charge transfer process and that the presence of dimethyl formamide did not alter the mechanism of the dissolution of the steel. Figure 4 shows the variation in the value of 1/Re, the reciprocal of the charge transfer resistance, with the concentration of dimethyl formamide. Electrochemical theory shows that l/ll~is proportional to the corrosion rate [16] and is analogous to polarization resistance in the Stern—Geary equation [25]. In the present work it was evident that the value of 1/Ri decreased in an exponential-like manner as the concentration of dimethyl formamide increased. The inhibition efficiency P of dimethyl formamide was calculated from the impedance data using the following relationship [24]: -
-
P= 1—
1/Re 100 (1/R~)0
where (1/R~)oand 1/Re are the reciprocals of the charge transfer resistance in the absence and in the presence of dimethyl formamide respectively. Figure 5 shows the variation in the inhibition efficiency with the concentration of dimethyl formamide. The characteristics are quite similar to those of an adsorption isotherm, and it appears that there is good agreement between the results obtained using the impedance technique and those obtained using mass-loss techniques (Fig. 2). This behaviour can be explained on the basis of the specific adsorption of dimethyl formamide molecules at the steel—solution interface. To confirm this assertion, the capacitance of the
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steel—solution interface was measured in 2.5 N H2SO4 in the absence and in the presence of various concentrations of DMF (Fig. 6). It is clear that the capacitance of the steel—solution interface generally decreased with increasing percentage of dimethyl formainide in the medium. The addition of a small amount of dimethyl formamide (up to 10 vol.%) to the aqueous medium produced a marked decrease in the capacitance of the interface. Above that level, from 10 to 60 vol.% dimethyl formamide, the composition of the medium had little effect on the capacitance of the interface. These results indicated that dimethyl formamide was specifically adsorbed at the steel—solution interface and that an adsorbed layer of dimethyl formamide molecules was complete at a dimethyl formamide concentration of about 10 vol.%.
98
3..L3. D.c. polarization measurements Figure 7 shows the polarization curves for steel in 2.5 N sulphuric acid in the absence and in the presence of various concentrations of dimethyl formamide. The curves show that the inhibitor causes cathodic and anodic overvoltage and that the magnitude of the displacement of the Tafel plots is proportional to the concentration of dimethyl formamide. This result indicates that the dimethyl formamide affects both the cathodic and the anodic processes. The electrochemical parameters of mild steel in 2.5 N sulphuric acid in the absence and in the presence of various concentrations of dimethyl formamide are given in Table 2. The data show that the slopes of the cathodic and anodic Tafel lines (j3~and 13a) remain almost unchanged on increasing the concentration of dimethyl formamide (I3~= 0.14 V decade’, /~a= 0.10 13.0 V decade’). This behaviour indicates that the -
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99 TABLE 2 Electrochemical parameters of steel in 2.5 N H
2504 in water—dimethyl formamide
solutions IDMFJ (vol.%) 0 10 40 50 60
Eeq
(V (SCE)) 0.50 0.48 0.48 0.48 0.48
2) icon (mA cm 4.6 2.8 2.2 1.6 1.3
(V decade’) 13a
(V decade~)
0.12 0.10 0.11 0.13 0.13
0.14 0.14 0.14 0.14 0.18
(%) Inhibition —
39.7 52.1 65.3 72.4
SCE, saturated calomel electrode.
adsorbed molecules of dimethyl formamide mechanically screen the coated part of the electrode and therefore protect it from the action of the corrosive medium. These molecules have no effect on the mechanism of dissolution of steel but cause only inactivation of a part of the surface with respect to the corrosive medium. 3.2. Influence of variation in the composition of water—dimethyl formamide solutions on the protection efficiency of 4-amino-3-H-5-mercap to-i, 2,4triazoline It is quite interesting to investigate the effect of varying the composition of the solvent on the protection efficiency of one of the good inhibitors of the acid corrosion of steel. 4-Amino-3-H-5-mercapto-1,2,4-triazoline has been examined previously [6] as an inhibitor of the corrosion of steel in 2.5 N sulphuric acid. In pure aqueous solution a iO~M solution of this inhibitor produced 82.5% inhibition of the acid corrosion of steel. Figure 8 shows the influence of the composition of the solvent on each of the following: (a) the protection efficiency of the inhibitor, (b) the total protection efficiency of the inhibitor and dimethyl formamide and (c) the protection efficiency of dimethyl formamide. The data show that the total protection efficiency of the inhibitor and dimethyl formamide increases slightly with increasing percentage of dimethyl formamide in the solution. The protection efficiency of dimethyl formamide increases nonlinearly with increasing concentration and this behaviour was discussed in the previous section. However, the protection efficiency of the inhibitor decreases rapidly with increasing percentage of dimethyl formamide. This behaviour can be discussed on the basis of competition between dimethyl formamide and the inhibitor. An increase of the concentration of dimethyl formamide in the solution leads to the displacement of adsorbed inhibitor molecules from the double layer at the steel—solution interface.
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30
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Fig. 8. Effect of variation in the composition of water—dimethyl formamide solutions containing 2.5 N H 2S04 on the percentage inhibition of the corrosion of steel by dimethyl formamide (curve a), the total percentage inhibition of the corrosion of steel by dimethyl formamide and 4-amino-3-H-5-mercapto-1 ,2,4-triazoline (curve b) and the protection efficiency of 4-amino-3-H-5-mercapto-1 , 2,4-triazoline, (curve c).
Acknowledgment The authors would like to thank Dr. G. Thompson, Corrosion and Protection Centre, UMIST, Manchester University, Manchester, England, for providing the necessary facilities used in impedance measurements. References 1 G. Schmitt, Br. Corros. J., 19 (1984) 165. 2 B. A. Abd El-Nabey, N. Khalil, M. M. Eisa and H. Sadek, Surf. Technol., 20 (1983) 209.
101 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25
B. A. Abd El-Nabey, N. Khalil and M. M. Eisa, Surf. Technol., 22 (1984) 9. B. A. Abd El-Nabey, N. KhaIil and E. Khamis, Surf. Technol., 22 (1984) 367. B. A. Abd El-Nabey, N. Khalil and E. Khamis, Corros. Sci., 25 (1985) 225. B. A. Abd El-Nabey, A. El-Toukhy, M. El-Gamal and F. Mahgoub, Surf. Coat. Technol., 27 (1986) 325. F. Mylius, Z. Metallkd., 14 (1922) 233. K. Hladky, Ph.D. Thesis, University of Manchester, England, 1978. J. J. Demo, Chem. Eng. World, 7 (1972) 115. W. Jaenicke and P. H. Schweitzer, Z. Phys. Chem. (N.F.), 52 (1967) 104. E. Heitz, in M. G. Fontana and R. W. Staehle (eds.), Advances in Corrosion Science and Technology, Vol. 4, Plenum, New York, 1975, pp. 149 - 243. F. Franks and D. J. G. Ives, Quart. Rev., 20(1966) 1. F. Franks, in F. Franks (ed.), Physicochemical Processes in Mixed Solvents, American Elsevier, New York, 1967, p. 50. H. Sadek, Z. Phys. Chem. (Leipzig), 266 (1985) 740. E. A. Braude and E. S. Stern, J. Chem. Soc., (1976) 1948. K. Hladky, L. M. Callow and J. L. Dawson, Br. Corros. J., 15 (1980) 20. L. F. G. Williams and R. J. Taylor, Corrosion, 38 (1980) 425. W. J. Lorenz and F. Mansfeld, Corros. Sci., 21 (1981) 647. H. Schweickert, W. J. Lorenz and H. Friendburg, J. Electrochem. Soc., 127 (1980) 1693. M. Keddam, 0. R. Mattos and H. Takenouti, J. Electrochem. Soc., 128 (1981) 257. M. Keddam, 0. R. Mattos and H. Takenouti, J. Electrochem. Soc., 128 (1981) 266. C. Gabrielli, M. Keddam and H. Takenouti, Alternating-current impedance measurements applied to corrosion studies and corrosion-rate measurements. In F. Mansfeld and U. Bertocci (eds.), Electrochemical Corrosion Testing, ASTM STP 727, American Society for Testing and Materials, 1981, pp. 150 - 166. B. A. Abd El-Nabey, E. Khamis, M. Shaban, G. E. Thompson and J. L. Dawson, Surf. Coat. Technol., in the press. B. A. Abd El-Nabey, E. Khamis, G. E. Thompson and J. L. Dawson, Surf. Coat. Technol., 28 (1986) 83. M. Stern and A. L. Geary, J. Electrochem. Soc., 104 (1957) 56.