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Carbonyl compounds as corrosion inhibitors for mild steel in HCl solutions
Corrosion Science, Vol. 24, No. 8, pp. 649-660, 1984 Printed in Great Britain
0010-938X/84 $3.00 + 0.00 (~ 1984 Pergamon Press Ltd.
CARBONYL COMPOUNDS AS CORROSION INHIBITORS F O R M I L D S T E E L IN HC1 S O L U T I O N S M. N. DESA! and M. B. DESAI Chemistry D e p a r t m e n t , University School of Sciences, Gujarat University, A h m e d a b a d 380009, India
A b s t r a c t - - T h e paper gives an account o f the performance of a n u m b e r of carbonyl c o m p o u n d s as corrosion inhibitors of mild steel in 1-6 N solutions of hydrochloric acid. Furfuraldehyde seems to be the best inhibitor of all the carbonyl c o m p o u n d s investigated. Activation energies in the presence and absence of inhibitors have been evaluated. In cathodic protection studies furfuraldehyde reduces the protective current density considerably. T h e difference effect is positive in the absence and presence of inhibitors. Galvanostatic polarization data indicate that all these c o m p o u n d s are predominantly cathodic inhibitors. These substances are m o r e effective in preventing the corrosion of aluminium alloys in hydrochloric acid solutions than the corrosion of mild steel. INTRODUCTION
the carbon oxygen double bond in the molecule, carbonyl compounds are interesting as corrosion inhibitors for metals in diverse aggressive media. In this laboratory, in earlier work, anisaldehyde, furfuraldehyde, vanillin, acetyl acetone, cyclohexanone and diacetone alcohol have been investigated as inhibitors for the corrosion of aluminium alloys 3 S , 1"2 5 6 S , 3 5 7 S 4'5 in hydrochloric acid solutions. However, it is well known that results obtained with one metal cannot easily be applied to other metals and hence detailed investigations were undertaken to study the behaviour of these compounds as inhibitors for the corrosion of mild steel in hydrochloric acid. D U E TO
EXPERIMENTAL METHOD Galvanostatic and gravimetric m e t h o d s have been followed. T h e preparation of specimens and corrosion tests were carried out as described below. Mild steel, ISS-1079, grade-1 having the following composition was used for the study. Mg 0.5% ; C, 0.5%; S, 0.05% ; P, 0.05%; remainder iron. Rectangular specimens of area 6 x 3 cm (thickness 28 SWG) with a small hole of about 2 m m dia just near the one e n d (3 cm side end) of the specimen for suspension have been used. Specimens were prepared for corrosion tests as described in our earlier publication. J T h e volume of corrosive liquid in gravimetric experiments was 230 ml. For polarization studies, the metal coupon was of a circular design, diameter 2.802 cm with a handle 3 cm long and 0.5 cm wide. T h e handle as well as the back of the coupon were covered with Perspex leaving only a circular portion of apparent surface area 6.156 cm 2 exposed. T h e other electrode was platinum of the same dimension. For these m e a s u r e m e n t s , H type Pyrex glass cell with porous partition to separate t h e two c o m p a r t m e n t s was used. It also contained a capillary to m a k e connection to the reference saturated calomel electrode. In each c o m p a r t m e n t , the volume of the corrosive m e d i u m was 80 ml.
Manuscript received 24 October 1980; in a m e n d e d form 29 June 1981. 649
TABLE 1.
EFFECTOF TIME (h) AND HCI CONCENTRATIONON THE CORROSIONLOSS (mg dm -2) OF MILD STEEL
Inhibitor concentration
Normality of HCI
1h
3h
6h
9h
12 h
24 h
Nil
1.0 2.0 3.0 4.0
32 64 90 124
72 134 425 1029
171 250 900 2642
328 701 1439 3658
592 1160 2222 4806
1229 1668 4729 7066
Anisaldehyde (0.017 m o l l - ' )
1.0
8 (75)* 38 (41) 41 (54) 111 (10)
17 (76) 98 (27) 194 (54) 354 (66)
22 27 (87) (92) 160 . . 210 (36) (70) 385 432 (57) (70) 657 1126 (75) (69)
40 (93) 274 (76) 722 (68) 1748 (64)
153 (88) 313 (81) 1116 (76) 2860 (60)
16 (50) 19 (70) 20 (78) 22 (82)
30 (58) 32 (76) 38 (91) 40 (96)
46 (75) 51 (80) 55 (94) 67 (97)
58 (82) 71 (90) 75 (95) 91 (98)
68 (89) 90 (92) 98 (96) 115 (98)
129 (90) 149 (91) 162 (97) 154 (98)
24 (25) 35 (45) 55 (39) 89 (28)
37 (49) 58 (57) 127 (70) 279 (73)
58 (66) 58 (77) 200 (78) 507 (81)
71 (78) 71 (90) 320 (78) 733 (80)
93 (84) 93 (92) 463 (79) 1069 (78)
127 (90) 269 (84) 838 (82) 1632 (77)
27 (16) 42 (34) 81 (10) 135 (-9)
47 (35) 93 (31) 184 (57) 370 (64)
83 (57) 118 (53) 300 (67) 645 (76)
124 (62) 140 (80) 427 (70) 876 (76)
132 (78) 170 (85) 564 (75) 1203 (75)
164 (87) 227 (86) 976 (79) 1868 (74)
13 (59) 19 (71) 27 (70) 46 (63) 19 (41) 24 (63) 45 (50) 80 (35)
21 (71) 36 (73) 54 (87) 85 (92) 30 (58) 48 (64) 191 (55) 138 (87)
27 (84) 63 (75) 121 (87) 106 (96) 50 (71) 143 (43) 285 (68) 591 (78)
38 (88) 70 (90) 145 (90) 157 (96) 79 (76) 174 (75) 489 (66) 740 (80)
48 (92) 79 (93) 190 (91) 260 (95) 110 (81) 202 (83) 693 (69) 791 (84)
96 (92) 108 (94) 830 (82) 851 (88) 215 (83) 700 (58) 1573 (67) 1614 (77)
2.0 3.0 4.0 Furfuraldehyde (0.42 tool I-')
1.0 2.0 3.0 4.0
Vanillin (0.065mol I-l)
1.0 2.0 3.0 4.0
Acetyl acetone (0.42mol I-t)
1.0 2.0 3.0 4.0
Cyciohexanone (0.40mol 1-')
1.0 2.0 3.0 4.0
Diacetone alcohol (0.39mol 1-~)
1.0 2.0 3.0 4.0
* Values in brackets show % efficiency. Temperature = 35 + 0.5°C.
Carbonyl compounds as corrosion inhibitors 600
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Detailed investigations on the inhibition of the corrosion of iron in HCI solutions was undertaken with a view to studying the effect of variables on corrosion rates, inhibitor efficiencies and both cathodic and anodic polarisation. The optimum conditions for satisfactory retardation of corrosion have been established. Combined cathodic protection of iron in 6.0 N HCI has also been investigated.
EXPERIMENTAL RESULTS
The corrosion of mild steel in HCI solutions increases with time as well as with HCI concentration. Polarisation measurements in 1.0 and 6.0 N HCI indicate that the corrosion process is cathodicaUy controlled. The influence of time and HCI concentration on the efficiency of inhibitors is given in Table I and Fig. 1. The anodic and cathodic galvanostatic polarisation curves of mild steel in 1.0 N HCi in the presence and absence of inhibitors are given in Fig. 2.
Anisaldehyde Even at as low a concentration as 0.0034 mol 1-1 anisaldehyde gives >40% protection to mild steel in HCI and at 0.017 mol 1-1 concentration, its protectivity is >60%. In 1-3 N solutions of HCI, the efficiency of anisaldehyde significantly improves with time but it falls with an increase in HCI concentration (Table 1). In 1 N HCI, addition of 0.0034 mol 1-1 anisaldehyde shifts the corrosion potential in positive direction by +20 mV, and at 0.017 mol 1-1 concentration, this shift is +40 mV. In 6 N HCI, addition of 0.0034 mol 1-1 anisaldehyde shifts the corrosion potential in positive direction by +40 mV, and at 0.017 mol 1-1 concentration the shift is +35 mV. In 1 N HCI, anisaldehyde is a mixed inhibitor with predominant cathodic action and this effect is particularly significant in 6 N HCI which is evident from the current density 4.872 x 10 -3 A cm -2.
652
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[ ] - - [] c y c l o h e x a n o n e • -- • cyclohexanone • - • uninhibited
0 . 0 4 0 tool 1-1 0.40 m o l 1-~ 1.0 N H C I
654
M.N. DESAXand M. B. DESAI
Furfuraldehyde At 0.042 mol 1-I concentration, furfuraldehyde gives quite good protection to mild steel in HCI solutions (81-63%) and at 0.42 mol 1-1 concentration the inhibitor efficiency is more than 90% protection. Its efficiency improves with an increase in HC1 concentration and with time as shown in Table 1. The efficiency of furfuraldehyde shows a significant improvement with an increase in HC1 concentration (Table 1). Addition of furfuraldehyde to 1 N HCI shifts the corrosion potential in the positive direction and this effect is more evident at increased inhibitor concentration. In 6 N HCI, however, at 0.168 mol 1-1 concentration, the shift in corrosion potential is +20 mV, whereas at 0.42 mol 1-1 concentration this shift is + 10 mV. At both concentrations of HCI, addition of 0.42 mol 1-1 furfuraldehyde induces a very significant increase in the cathode polarisation but its effect on anode polarisation is comparatively less. In 6 N HCI, the electrode potential at the cathode current density 9.74 x 10 -4 A c m -2 is -1140 mV, whereas in uninhibited HCI it is only -515 mV.
Vanillin The efficiency of vanillin improves with an increase in its concentration and with the immersion time but it falls with an increase in HCI concentration (Table 1). Addition of 0.065 mol 1-1 vanillin to 1 N HCI shifts the corrosion potential in positive direction by 20 mV, but in 6 N HCI, the shift is +40 mV. Addition of 0.065 mol 1-1 vanillin to 1 or 6 N HCI solutions induces a significant increase in cathode polarisation. In 1 N HC1 vanillin is a mixed inhibitor with predominant effect on the cathodic reaction and in 6 N HCI it is a strictly cathodic inhibitor.
Acetyl acetone At 0.042 mol 1-1 acetyl acetone gives >50% protection to mild steel and at 0.42 mol 1-1 its efficiency is >74%, but the protectivity of acetyl acetone falls with an increase in HCI concentration. In 1--4 N HC1, there is a significant increase in the efficiency of acetyl acetone with the immersion of time (Table 1). In 4 N HCI, for 1 h duration, initially there is an acceleration of the corrosion of mild steel in the presence of acetyl acetone but with an increase in immersion time, its inhibitive action comes into play (Table 1). The influence of HCI concentration on the protectivity of acetyl acetone is dependent on immersion period. Initially (1 h duration) it falls with an increase in HCI concentration but from 3 to 12 h durations, in general, it improves with an increase in HCI concentration. At 0.168 mol 1-1 concentration, in 1 N HCI, acetyl acetone shifts the corrosion potential direction by 30 mV, but at 0.42 mol 1-1 concentration, this shift is only +10 mV. In 6 N HCI a similar tendency is evident, the shift in corrosion potential at 0.168 mol 1-1 and 0.42 mol 1-1 concentrations being +35 and +30 mV respectively. In 1 and 6 N HCI, acetyl acetone is a mixed inhibitor with predominant cathodic action.
Cyclohexanone At 0.040 mol 1-1 cycohexanone affords 59-72% protection to mild steel in HCI
Carbonyl compounds as corrosion inhibitors
655
solutions and a 10-fold increase in its concentration increases the efficiency to 83-94%. The efficiency of cyclohexanone improves with the immersion time and with an increase in HC1 concentration up to 9 h of test but this behaviour is not kept in tests of 24 h as shown in Table 1. Addition of 0.040 mol 1-~ cyclohexanone to 1.0 N HCI shifts the corrosion potential in positive direction by +40 mV, and at 0.4 mol 1-~ concentration, this shift is +60 mV. In 6 N HCI, the shifts in corrosion potential at 0.16 and 0.4 mol 1-1 concentrations of the inhibitor are + 50 and + 30 mV respectively. Addition of cyclohexanone induces an increase in both the cathode and anode polarisations. In 1 N HCI and particularly in 6 N HCI cyclohexanone is a mixed inhibitor with predominant cathodic action.
Diacetone alcohol At low inhibitor concentration, diacetone alcohol is effective only in 1 N HCI, but at higher inhibitor concentration, it is an effective inhibitor for mild steel in 1-6 N HCI~ The efficiency of diacetone alcohol improves with immersion time and HCI concentration (Table 1). In 1 N HCI, addition of 0.039 mol 1-x diacetone alcohol shifts the corrosion potential in the positive direction by +30 mV and at 0.39 mol 1-1 concentration this shift is +50 mV. In 6.0 N HCI, at both the concentrations of diacetone alcohol, the corrosion potential is shifted in the negative direction by 10 mV. In 1 and 6 N HCI diacetone alcohol is a mixed inhibitor, with a predominant action on the cathodic reaction. Plots of log surface coverage against log inhibitor concentration show that the Freundlich adsorption isotherm is obeyed in the effective concentration range.
Galvanostatic data The Tafel slopes and inhibitor efficiencies calculated from polarisation curves in i and 6 N HCI are given in Table 2. In general there is a good correlation between the percentage efficiencies calculated from polarisation data and from weight loss data.
Influence of temperature Activation energies of mild steel in 1 N HCI in the presence and absence of inhibitors were calculated from Arrhenius plots (Fig. 3). It was observed that in uninhibited HC1, the activation energy of mild steel dissolution process is 18.0 kCal mo1-1. The following activation energies in inhibited HCI are found:
Acetyl acetone Cyclohexanone Diacetone alcohol Anisaldehyde Vanillin Furfuraidehyde
tool 1-1
kCal tool-1
0.42 0.40 0.39 0.017 0.065 0.42
20.0 27.0 26.0 12.2 13.4 12.2
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Thus the activation energies in the presence of ketones are higher than those in uninhibited HCI showing that their efficiency falls with an increase in temperature. The activation energies in the presence of aldehydes are lower than those in uninhibited HCI showing that there is an improvement in their efficiency with an increase in temperature.
Cathodic protection The cathodic protection of mild steel in 6 N HCI is possible, and the corresponding results are given in Table 3. In uninhibited 6 N HCI 1.30 A dm -2 current is required to give complete protection to mild steel. In the present tests furfuraldehyde is the most effective inhibitor which considerably reduces the protective current density. A comparison of results obtained in the presence o f anisaldehyde and vanillin show that the introduction of the hydroxyl group in the molecule of alkoxy aldehyde is beneficial. The most efficient ketone is cyclohexanone whereas ketones having enolic or alcoholic - O H groups are less efficient inhibitors.
658
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Carbonyl compounds as corrosion inhibitors
659
Influence of anodic current The influence of external anodic current on the corrosion of mild steel in 6 N HCI in the presence of inhibitors and in uninhibited HCI and the results were used to evaluate the difference effect. It was observed that the difference effect is positive in the presence and absence of inhibitors. DISCUSSION One of the aims of present work was to compare the protective behaviour of the investigated substances towards mild steel and aluminium. Earlier work had shown that for aluminium alloys in HCI furfuraldehyde gave maximum protection at an optimum concentration, but further increase in its concentration induced a decrease in its efficiency. Similar observations were made earlier by Kemkhadze and Balezin 7 for the corrosion of iron in HC1 solutions. In the present work we have found that the efficiency of furfuraldehyde improves with an increase in inhibitor concentration which is in contrast with our earlier observations with aluminium and the observations of Kemkhadze. In general it could be said that furfuraldehyde has been found to be a better inhibitor for the corrosion of mild steel in HCI solutions than for aluminium in HC1 solutions. For both the metals it is a predominantly cathodic inhibitor. Vanillin has been previously used in this laboratory for aluminium alloys 2S, 3S, 57S, 65S in HCI solutions. At low concentrations it accelerates the corrosion of some aluminium alloys, but with an increase in inhibitor concentration its inhibitive action comes into play for all aluminium alloys. With mild steel its protective action is observed even at low concentrations, and further increase brings about 90-95% protection to mild steel. Foroulis s has patented the use of anisaldehyde for the inhibition of corrosion of mild steel in HCI solutions. In the present work p-anisaldehyde has been observed to be a satisfactory inhibitor for the corrosion of mild steel in HCI solutions. Earlier work for aluminium alloys also indicated its excellent inhibitive action towards the corrosion of aluminium alloys in HCI solutions. Acetylacetone is a very well known chelating agent. King and Hillner 9 studied the corrosion of iron in HCI in the presence of complexing agent. Acetyl acetone chelates with bi- and trivalent metal ions. When added in concentrations of 0.05 and 0.1 M to the dichromate containing corroding solution it reduced the dissolution to about 2 mg in 5 rain in all three cases. Visible films formed. The metal chelates are apparently not very soluble. In the earlier work on aluminium alloys in HCI acetyl acetone has been found to be a satisfactory inhibitor. At 43.5 ml 1-1 concentration it gives excellent protection to aluminium alloys. In general, acetyl acetone is a better inhibitor for aluminium alloys than for mild steel. Cyclohexanone has been found to be an excellent inhibitor which gives more than 90% protection to mild steel and to aluminium alloys in HC1 solutions. Diacetone alcohol is an interesting molecule having two functional groups, a carbonyl group and an alcoholic - O H group. For aluminium alloys it gives a very good protection in hydrochloric acid solution but it is not so good in protecting mild steel. In general, it is a better inhibitor for aluminium than for mild steel.
660
M.N. DESAI and M. B. DESA!
REFERENCES 1. R. R. PAI'EL, Corrosion inhibitors of aluminium-3S. Ph.D. Thesis, Gujarat University, India (1972). 2. R. R. SHAH, Studies in corrosion inhibition. Ph.D. Thesis, Gujarat University, India (1974). 3. H. G. DESAI, Studies of various parameters on corrosion inhibitors. Ph.D. Thesis, Gujarat University, India (1977). 4. C.B. SHAH, Studies in corrosion of aluminium alloys. Ph.D. Thesis, Guj arat University, India (1968). 5. G. J. SHAH, Studies in corrosion and corrosion inhibition. Ph.D. Thesis, Gujarat University, India (1971). 6. M. N. DESAI and S. M. DESAI, Second Symposium on Corrosion Inhibitors, p. 863. Ferrara (1966). 7. V. S. KEMKHADZEand S. A. BALEZIN. Corrosion Inhibitors (ed. J. I. Bergman), p. 192. MacMillans, New York (1963). 8. Z. A. FOROULIS, U.S. 3,537,974. Nov. (1970). 9. C. V. KING and E. HILLNER, J. electrochem. Soc. 79 (1954).
0010-938X/84 $3.00 + 0.00 (~ 1984 Pergamon Press Ltd.
CARBONYL COMPOUNDS AS CORROSION INHIBITORS F O R M I L D S T E E L IN HC1 S O L U T I O N S M. N. DESA! and M. B. DESAI Chemistry D e p a r t m e n t , University School of Sciences, Gujarat University, A h m e d a b a d 380009, India
A b s t r a c t - - T h e paper gives an account o f the performance of a n u m b e r of carbonyl c o m p o u n d s as corrosion inhibitors of mild steel in 1-6 N solutions of hydrochloric acid. Furfuraldehyde seems to be the best inhibitor of all the carbonyl c o m p o u n d s investigated. Activation energies in the presence and absence of inhibitors have been evaluated. In cathodic protection studies furfuraldehyde reduces the protective current density considerably. T h e difference effect is positive in the absence and presence of inhibitors. Galvanostatic polarization data indicate that all these c o m p o u n d s are predominantly cathodic inhibitors. These substances are m o r e effective in preventing the corrosion of aluminium alloys in hydrochloric acid solutions than the corrosion of mild steel. INTRODUCTION
the carbon oxygen double bond in the molecule, carbonyl compounds are interesting as corrosion inhibitors for metals in diverse aggressive media. In this laboratory, in earlier work, anisaldehyde, furfuraldehyde, vanillin, acetyl acetone, cyclohexanone and diacetone alcohol have been investigated as inhibitors for the corrosion of aluminium alloys 3 S , 1"2 5 6 S , 3 5 7 S 4'5 in hydrochloric acid solutions. However, it is well known that results obtained with one metal cannot easily be applied to other metals and hence detailed investigations were undertaken to study the behaviour of these compounds as inhibitors for the corrosion of mild steel in hydrochloric acid. D U E TO
EXPERIMENTAL METHOD Galvanostatic and gravimetric m e t h o d s have been followed. T h e preparation of specimens and corrosion tests were carried out as described below. Mild steel, ISS-1079, grade-1 having the following composition was used for the study. Mg 0.5% ; C, 0.5%; S, 0.05% ; P, 0.05%; remainder iron. Rectangular specimens of area 6 x 3 cm (thickness 28 SWG) with a small hole of about 2 m m dia just near the one e n d (3 cm side end) of the specimen for suspension have been used. Specimens were prepared for corrosion tests as described in our earlier publication. J T h e volume of corrosive liquid in gravimetric experiments was 230 ml. For polarization studies, the metal coupon was of a circular design, diameter 2.802 cm with a handle 3 cm long and 0.5 cm wide. T h e handle as well as the back of the coupon were covered with Perspex leaving only a circular portion of apparent surface area 6.156 cm 2 exposed. T h e other electrode was platinum of the same dimension. For these m e a s u r e m e n t s , H type Pyrex glass cell with porous partition to separate t h e two c o m p a r t m e n t s was used. It also contained a capillary to m a k e connection to the reference saturated calomel electrode. In each c o m p a r t m e n t , the volume of the corrosive m e d i u m was 80 ml.
Manuscript received 24 October 1980; in a m e n d e d form 29 June 1981. 649
TABLE 1.
EFFECTOF TIME (h) AND HCI CONCENTRATIONON THE CORROSIONLOSS (mg dm -2) OF MILD STEEL
Inhibitor concentration
Normality of HCI
1h
3h
6h
9h
12 h
24 h
Nil
1.0 2.0 3.0 4.0
32 64 90 124
72 134 425 1029
171 250 900 2642
328 701 1439 3658
592 1160 2222 4806
1229 1668 4729 7066
Anisaldehyde (0.017 m o l l - ' )
1.0
8 (75)* 38 (41) 41 (54) 111 (10)
17 (76) 98 (27) 194 (54) 354 (66)
22 27 (87) (92) 160 . . 210 (36) (70) 385 432 (57) (70) 657 1126 (75) (69)
40 (93) 274 (76) 722 (68) 1748 (64)
153 (88) 313 (81) 1116 (76) 2860 (60)
16 (50) 19 (70) 20 (78) 22 (82)
30 (58) 32 (76) 38 (91) 40 (96)
46 (75) 51 (80) 55 (94) 67 (97)
58 (82) 71 (90) 75 (95) 91 (98)
68 (89) 90 (92) 98 (96) 115 (98)
129 (90) 149 (91) 162 (97) 154 (98)
24 (25) 35 (45) 55 (39) 89 (28)
37 (49) 58 (57) 127 (70) 279 (73)
58 (66) 58 (77) 200 (78) 507 (81)
71 (78) 71 (90) 320 (78) 733 (80)
93 (84) 93 (92) 463 (79) 1069 (78)
127 (90) 269 (84) 838 (82) 1632 (77)
27 (16) 42 (34) 81 (10) 135 (-9)
47 (35) 93 (31) 184 (57) 370 (64)
83 (57) 118 (53) 300 (67) 645 (76)
124 (62) 140 (80) 427 (70) 876 (76)
132 (78) 170 (85) 564 (75) 1203 (75)
164 (87) 227 (86) 976 (79) 1868 (74)
13 (59) 19 (71) 27 (70) 46 (63) 19 (41) 24 (63) 45 (50) 80 (35)
21 (71) 36 (73) 54 (87) 85 (92) 30 (58) 48 (64) 191 (55) 138 (87)
27 (84) 63 (75) 121 (87) 106 (96) 50 (71) 143 (43) 285 (68) 591 (78)
38 (88) 70 (90) 145 (90) 157 (96) 79 (76) 174 (75) 489 (66) 740 (80)
48 (92) 79 (93) 190 (91) 260 (95) 110 (81) 202 (83) 693 (69) 791 (84)
96 (92) 108 (94) 830 (82) 851 (88) 215 (83) 700 (58) 1573 (67) 1614 (77)
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1.0 2.0 3.0 4.0
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1.0 2.0 3.0 4.0
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1.0 2.0 3.0 4.0
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1.0 2.0 3.0 4.0
Diacetone alcohol (0.39mol 1-~)
1.0 2.0 3.0 4.0
* Values in brackets show % efficiency. Temperature = 35 + 0.5°C.
Carbonyl compounds as corrosion inhibitors 600
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Detailed investigations on the inhibition of the corrosion of iron in HCI solutions was undertaken with a view to studying the effect of variables on corrosion rates, inhibitor efficiencies and both cathodic and anodic polarisation. The optimum conditions for satisfactory retardation of corrosion have been established. Combined cathodic protection of iron in 6.0 N HCI has also been investigated.
EXPERIMENTAL RESULTS
The corrosion of mild steel in HCI solutions increases with time as well as with HCI concentration. Polarisation measurements in 1.0 and 6.0 N HCI indicate that the corrosion process is cathodicaUy controlled. The influence of time and HCI concentration on the efficiency of inhibitors is given in Table I and Fig. 1. The anodic and cathodic galvanostatic polarisation curves of mild steel in 1.0 N HCi in the presence and absence of inhibitors are given in Fig. 2.
Anisaldehyde Even at as low a concentration as 0.0034 mol 1-1 anisaldehyde gives >40% protection to mild steel in HCI and at 0.017 mol 1-1 concentration, its protectivity is >60%. In 1-3 N solutions of HCI, the efficiency of anisaldehyde significantly improves with time but it falls with an increase in HCI concentration (Table 1). In 1 N HCI, addition of 0.0034 mol 1-1 anisaldehyde shifts the corrosion potential in positive direction by +20 mV, and at 0.017 mol 1-1 concentration, this shift is +40 mV. In 6 N HCI, addition of 0.0034 mol 1-1 anisaldehyde shifts the corrosion potential in positive direction by +40 mV, and at 0.017 mol 1-1 concentration the shift is +35 mV. In 1 N HCI, anisaldehyde is a mixed inhibitor with predominant cathodic action and this effect is particularly significant in 6 N HCI which is evident from the current density 4.872 x 10 -3 A cm -2.
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Furfuraldehyde At 0.042 mol 1-I concentration, furfuraldehyde gives quite good protection to mild steel in HCI solutions (81-63%) and at 0.42 mol 1-1 concentration the inhibitor efficiency is more than 90% protection. Its efficiency improves with an increase in HC1 concentration and with time as shown in Table 1. The efficiency of furfuraldehyde shows a significant improvement with an increase in HC1 concentration (Table 1). Addition of furfuraldehyde to 1 N HCI shifts the corrosion potential in the positive direction and this effect is more evident at increased inhibitor concentration. In 6 N HCI, however, at 0.168 mol 1-1 concentration, the shift in corrosion potential is +20 mV, whereas at 0.42 mol 1-1 concentration this shift is + 10 mV. At both concentrations of HCI, addition of 0.42 mol 1-1 furfuraldehyde induces a very significant increase in the cathode polarisation but its effect on anode polarisation is comparatively less. In 6 N HCI, the electrode potential at the cathode current density 9.74 x 10 -4 A c m -2 is -1140 mV, whereas in uninhibited HCI it is only -515 mV.
Vanillin The efficiency of vanillin improves with an increase in its concentration and with the immersion time but it falls with an increase in HCI concentration (Table 1). Addition of 0.065 mol 1-1 vanillin to 1 N HCI shifts the corrosion potential in positive direction by 20 mV, but in 6 N HCI, the shift is +40 mV. Addition of 0.065 mol 1-1 vanillin to 1 or 6 N HCI solutions induces a significant increase in cathode polarisation. In 1 N HC1 vanillin is a mixed inhibitor with predominant effect on the cathodic reaction and in 6 N HCI it is a strictly cathodic inhibitor.
Acetyl acetone At 0.042 mol 1-1 acetyl acetone gives >50% protection to mild steel and at 0.42 mol 1-1 its efficiency is >74%, but the protectivity of acetyl acetone falls with an increase in HCI concentration. In 1--4 N HC1, there is a significant increase in the efficiency of acetyl acetone with the immersion of time (Table 1). In 4 N HCI, for 1 h duration, initially there is an acceleration of the corrosion of mild steel in the presence of acetyl acetone but with an increase in immersion time, its inhibitive action comes into play (Table 1). The influence of HCI concentration on the protectivity of acetyl acetone is dependent on immersion period. Initially (1 h duration) it falls with an increase in HCI concentration but from 3 to 12 h durations, in general, it improves with an increase in HCI concentration. At 0.168 mol 1-1 concentration, in 1 N HCI, acetyl acetone shifts the corrosion potential direction by 30 mV, but at 0.42 mol 1-1 concentration, this shift is only +10 mV. In 6 N HCI a similar tendency is evident, the shift in corrosion potential at 0.168 mol 1-1 and 0.42 mol 1-1 concentrations being +35 and +30 mV respectively. In 1 and 6 N HCI, acetyl acetone is a mixed inhibitor with predominant cathodic action.
Cyclohexanone At 0.040 mol 1-1 cycohexanone affords 59-72% protection to mild steel in HCI
Carbonyl compounds as corrosion inhibitors
655
solutions and a 10-fold increase in its concentration increases the efficiency to 83-94%. The efficiency of cyclohexanone improves with the immersion time and with an increase in HC1 concentration up to 9 h of test but this behaviour is not kept in tests of 24 h as shown in Table 1. Addition of 0.040 mol 1-~ cyclohexanone to 1.0 N HCI shifts the corrosion potential in positive direction by +40 mV, and at 0.4 mol 1-~ concentration, this shift is +60 mV. In 6 N HCI, the shifts in corrosion potential at 0.16 and 0.4 mol 1-1 concentrations of the inhibitor are + 50 and + 30 mV respectively. Addition of cyclohexanone induces an increase in both the cathode and anode polarisations. In 1 N HCI and particularly in 6 N HCI cyclohexanone is a mixed inhibitor with predominant cathodic action.
Diacetone alcohol At low inhibitor concentration, diacetone alcohol is effective only in 1 N HCI, but at higher inhibitor concentration, it is an effective inhibitor for mild steel in 1-6 N HCI~ The efficiency of diacetone alcohol improves with immersion time and HCI concentration (Table 1). In 1 N HCI, addition of 0.039 mol 1-x diacetone alcohol shifts the corrosion potential in the positive direction by +30 mV and at 0.39 mol 1-1 concentration this shift is +50 mV. In 6.0 N HCI, at both the concentrations of diacetone alcohol, the corrosion potential is shifted in the negative direction by 10 mV. In 1 and 6 N HCI diacetone alcohol is a mixed inhibitor, with a predominant action on the cathodic reaction. Plots of log surface coverage against log inhibitor concentration show that the Freundlich adsorption isotherm is obeyed in the effective concentration range.
Galvanostatic data The Tafel slopes and inhibitor efficiencies calculated from polarisation curves in i and 6 N HCI are given in Table 2. In general there is a good correlation between the percentage efficiencies calculated from polarisation data and from weight loss data.
Influence of temperature Activation energies of mild steel in 1 N HCI in the presence and absence of inhibitors were calculated from Arrhenius plots (Fig. 3). It was observed that in uninhibited HC1, the activation energy of mild steel dissolution process is 18.0 kCal mo1-1. The following activation energies in inhibited HCI are found:
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Cathodic protection The cathodic protection of mild steel in 6 N HCI is possible, and the corresponding results are given in Table 3. In uninhibited 6 N HCI 1.30 A dm -2 current is required to give complete protection to mild steel. In the present tests furfuraldehyde is the most effective inhibitor which considerably reduces the protective current density. A comparison of results obtained in the presence o f anisaldehyde and vanillin show that the introduction of the hydroxyl group in the molecule of alkoxy aldehyde is beneficial. The most efficient ketone is cyclohexanone whereas ketones having enolic or alcoholic - O H groups are less efficient inhibitors.
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Influence of anodic current The influence of external anodic current on the corrosion of mild steel in 6 N HCI in the presence of inhibitors and in uninhibited HCI and the results were used to evaluate the difference effect. It was observed that the difference effect is positive in the presence and absence of inhibitors. DISCUSSION One of the aims of present work was to compare the protective behaviour of the investigated substances towards mild steel and aluminium. Earlier work had shown that for aluminium alloys in HCI furfuraldehyde gave maximum protection at an optimum concentration, but further increase in its concentration induced a decrease in its efficiency. Similar observations were made earlier by Kemkhadze and Balezin 7 for the corrosion of iron in HC1 solutions. In the present work we have found that the efficiency of furfuraldehyde improves with an increase in inhibitor concentration which is in contrast with our earlier observations with aluminium and the observations of Kemkhadze. In general it could be said that furfuraldehyde has been found to be a better inhibitor for the corrosion of mild steel in HCI solutions than for aluminium in HC1 solutions. For both the metals it is a predominantly cathodic inhibitor. Vanillin has been previously used in this laboratory for aluminium alloys 2S, 3S, 57S, 65S in HCI solutions. At low concentrations it accelerates the corrosion of some aluminium alloys, but with an increase in inhibitor concentration its inhibitive action comes into play for all aluminium alloys. With mild steel its protective action is observed even at low concentrations, and further increase brings about 90-95% protection to mild steel. Foroulis s has patented the use of anisaldehyde for the inhibition of corrosion of mild steel in HCI solutions. In the present work p-anisaldehyde has been observed to be a satisfactory inhibitor for the corrosion of mild steel in HCI solutions. Earlier work for aluminium alloys also indicated its excellent inhibitive action towards the corrosion of aluminium alloys in HCI solutions. Acetylacetone is a very well known chelating agent. King and Hillner 9 studied the corrosion of iron in HCI in the presence of complexing agent. Acetyl acetone chelates with bi- and trivalent metal ions. When added in concentrations of 0.05 and 0.1 M to the dichromate containing corroding solution it reduced the dissolution to about 2 mg in 5 rain in all three cases. Visible films formed. The metal chelates are apparently not very soluble. In the earlier work on aluminium alloys in HCI acetyl acetone has been found to be a satisfactory inhibitor. At 43.5 ml 1-1 concentration it gives excellent protection to aluminium alloys. In general, acetyl acetone is a better inhibitor for aluminium alloys than for mild steel. Cyclohexanone has been found to be an excellent inhibitor which gives more than 90% protection to mild steel and to aluminium alloys in HC1 solutions. Diacetone alcohol is an interesting molecule having two functional groups, a carbonyl group and an alcoholic - O H group. For aluminium alloys it gives a very good protection in hydrochloric acid solution but it is not so good in protecting mild steel. In general, it is a better inhibitor for aluminium than for mild steel.
660
M.N. DESAI and M. B. DESA!
REFERENCES 1. R. R. PAI'EL, Corrosion inhibitors of aluminium-3S. Ph.D. Thesis, Gujarat University, India (1972). 2. R. R. SHAH, Studies in corrosion inhibition. Ph.D. Thesis, Gujarat University, India (1974). 3. H. G. DESAI, Studies of various parameters on corrosion inhibitors. Ph.D. Thesis, Gujarat University, India (1977). 4. C.B. SHAH, Studies in corrosion of aluminium alloys. Ph.D. Thesis, Guj arat University, India (1968). 5. G. J. SHAH, Studies in corrosion and corrosion inhibition. Ph.D. Thesis, Gujarat University, India (1971). 6. M. N. DESAI and S. M. DESAI, Second Symposium on Corrosion Inhibitors, p. 863. Ferrara (1966). 7. V. S. KEMKHADZEand S. A. BALEZIN. Corrosion Inhibitors (ed. J. I. Bergman), p. 192. MacMillans, New York (1963). 8. Z. A. FOROULIS, U.S. 3,537,974. Nov. (1970). 9. C. V. KING and E. HILLNER, J. electrochem. Soc. 79 (1954).