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Inhibition of chloride localized corrosion of mild steel by PO43−, CrO42−, MoO42−, and NO2− anions
Applied Surface Science 158 Ž2000. 190–196 www.elsevier.nlrlocaterapsusc
Inhibition of chloride localized corrosion of mild steel by PO43y, CrO42y, MoO42y, and NOy 2 anions S.A.M. Refaey a,) , S.S. Abd El-Rehim b, F. Taha a , M.B. Saleh a , R.A. Ahmed a a
Chemistry Department, Faculty of Science, Minia UniÕersity, 61519 Minia, Egypt Chemistry Department, Faculty of Science, Ain Shams UniÕersity, Cairo, Egypt
b
Received 16 March 1999; accepted 19 December 1999
Abstract . ions on the pitting The effect of phosphate ŽPO43y ., chromate ŽCrO42y ., molybedate ŽMoO42y . and nitrite ŽNOy 2 corrosion of steel in 0.1 M NaCl solution has been studied using potentiodynamic polarization and SEM techniques. The Ž . addition of increasing concentrations of PO43y, CrO42y, MoO42y and NOy 2 anions causes a shift of the pitting potential Epit in the positive direction, indicating the inhibitive effect of the added anions on the pitting attack. The adsorption characteristics of these anions on the steel surface play a significant role in the inhibition processes. The PO43y anion has a strong inhibitive effect of chloride pitting corrosion. The effect of different inorganic anions on the pitting corrosion of steel samples I and II Žwith different composition. was also studied in 0.1 M NaCl solution. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Mild steel; Localized corrosion; Potentiodynamic; Inorganic anions; Inhibition
1. Introduction The effect of Na 2 CrO4 , Na 2 MoO4 , and Na 2WO4 in deareated acid solutions at pH s 1.5 as inhibitors for Cu, Ni, Sn, Al and 18-8 stainless steel was examined w1x by weight loss and electrochemical measurements. Molybedate and tungstate were generally ineffective. Chromate was the most effective, especially for Cu, Al, and 18-8 stainless steel. Pitting corrosion is recognized as an insidious type of attack that results in many unexpected failures of metallic structures. The pitting corrosion of metals and alloys occurs when passivity breaks down at local points on
)
Corresponding author. E-mail address: [email protected] ŽS.A.M. Refaey..
the surfaces exposed to corrosive environments containing aggressive anions w2–5x. At these points, anodic dissolution proceeds while the major part of surface remains passive. The CrO42y, Cr2 O 72y and MoO42y anions were used as good inhibitors for the pitting corrosion of tin w2x. It was found that the pitting corrosion resistance of tin in alkaline and near neutral medium increased with increasing the concentration of CrO42y, Cr2 O 72y and MoO42y anions, indicating the inhibitive effect of the added anions on the pitting attack. The NOy 2 is entirely ineffective as an inhibitor and actually increases pitting corrosion. Cr2 O 72y anion had a strong inhibitive effect at very low concentrations. The mild steel was investigated w5x in the sodium gluconate solutions, it was found that the pitting corrosion of mild steel is inhibited by low concentration of sodium gluconate
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 1 6 - 7
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
in alkaline medium. But the presence of chloride ions in sodium gluconate solutions decreases the inhibition effect of steel by sodium gluconate in alkaline medium. In recent studies, an attempt has been made to study the inhibition of chloride pitting corrosion of mild steel in sodium chloride solution. The purpose of this paper is to establish the role of some inorganic anions in improving the passive film resistance to localized attack caused by chloride ions in neutral medium.
2. Experimental Experiments were carried out in 0.1 M NaCl solution, in absence and presence of different concentrations of sodium salts of phosphate, chromate, molybedate, or nitrite. All solutions were prepared from doubly distilled water and A.R. chemicals, and new polished electrodes were used for each run. All experiments were carried out at room temperature Ž258C.. All solutions were used under purified nitrogen gas. Two vanadium steel samples were used in the present work as working electrodes. These samples were produced in the electric arc furnace of the Delta Steel Mill, Cairo. The composition Žwt.%. of sample ŽI. is 0.16% C, 1.0 Mn, 0.45 S, 0.30 P, 0.20 Si, and 0.08 V. The composition of the second sample ŽII. is 0.24% C, 1.30 Mn, 0.049 S, 0.035 P, 0.53 Si, and 0.11 V. The ingots were hot-rolled Žat 1000–12008C. and then machined in the form of short rods Žhot-rolled electrodes., each 27 mm in length and 8 mm in diameter. Each working electrode was constructed and treated following the procedure described previously w5x. A Pt sheet was used as a counter electrode. The potential was measured against a AgrAgCl electrode. The potentiodynamic measurements were carried out using a potentiostat ŽAMEL model 2094. which was controlled by a PC. The morphology of tin surface after treatment in solutions was examined by a scanning electron microscope ŽCamScan Cambridge Scanning Company..
3. Results and discussion Fig. 1 shows the typical potentiodynamic curves of steel samples I and II in various concentrations of
191
NaCl in the potential range from y2.0 to 2.0 V at scan rate 30 mV sy1 . In NaCl solution, the samples did not exhibit an active passive transition. The lack of an activerpassive transition near the open circuit Žzero current. potential is due to spontaneous passivity. This indicates that the air-formed film is stable at and near the open circuit potential. When the potential reaches a certain critical potential, Epit , the passive current suddenly raises steeply without any sign for oxygen evolution, denoting breakdown of the passive film and initiation and propagation of pitting attack. Initiation of pitting attack could be ascribed to competitive adsorption between Cly ions and the passive species ŽOHy and H 2 O dipoles. w6x. At Epit , Cly anions displace the adsorbed passivating species at some locations and accelerate local anodic dissolution. On the other hand, the initiation of pitting attack could be due to the ability of Cly anions to penetrate the passive film with the assistance of a high electric field across the passive film and to attack the base metal surface w7x. The pitting growth Žpropagation. occurs as a result of an increase in Cly ion concentration resulting from its migration inside pits and hydrolysis of Fe 2q ions produced, since the high level of acidity required for pitting corrosion site growth must be achieved by hydrolysis of Fe 2q inside pits. Inspections of the data of Figs. 1 and 2 reveals that for the two samples, as the Cly ion concentration is increased, the pitting potentials Epit shift to more
Fig. 1. Anodic potentiodynamic curves for steel anode in solutions of various concentrations of NaCl at scan rates 30 mV sy1 . Concentrations: Ž1. 0.1, Ž2. 0.2, Ž3. 0.4, Ž4. 0.6, Ž5. 0.8, Ž6. 1.0 M for steel sample I.
192
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
Fig. 2. The pitting potential Ž Epit . as a function of NaCl concentrations for steel samples I and II.
sion of the two steel samples to an extent depending on the type and concentration of the inhibitor and the composition of the steel samples. Figs. 4–7 shows the dependence of the critical pitting potential Ž Epit . of the two steel electrodes on the concentration of the inorganic salts. The presence of phosphate, chromate, and nitrite causes a marked shift of critical pitting potential into the nobel wpositivex direction. The concentration of the inorganic salts required causing the marked shift in Epit to positive direction Žmarked inhibition. increases in the order: molybedate ) nitrite ) chromate ) phosphate. This order reflects the increased tendency of these compounds to act as pitting inhibitors, i.e., molybedate is less effective while phosphate is highly
negative Žactive. values indicating that increasing of Cly ion concentration would provide most favorable condition for pit initiation. Fig. 2 shows Epit vs. logwNaClx for the two samples, straight lines are obtained according to the fitting of the following equation Epit s a y blog w NaCl x , where a and b are constants which depend upon the composition of the samples. The pitting potential of sample II is higher positive than that for sample I in all concentrations of NaCl used. This result can be interpreted by the difference in the metal composition in steel samples under investigation. The Mn% Ž1.3%. in sample II ) Mn% Ž1.0%. in sample I, where the increase of the Mn content in the steel in the range 0.25% to 1.75% resulted in a significant decrease in pitting resistance w8x. The increase of V content also in the steel, V% Ž0.11% in sample II. ) V% Ž0.08% in sample I., increase the pitting corrosion resistance w5x. The effect of adding different concentrations of phosphate, chromate, nitrite, and molybedate as sodium salts on potentiodynamic anodic polarization curves of the two steel samples in 0.1 M NaCl were studied. Fig. 3a and b represent the polarization curves of sample II under the effect of adding increasing concentrations of phosphate and chromate, respectively, as examples. The same behaviour is observed in the presence of molybedate and nitrite ions. Also, the steel sample I show the same behaviour. The results obtained show that the presence of these inorganic anions inhibits the pitting corro-
Fig. 3. Ža. Potentiodynamic curves for steel anode in 0.1 M NaCl solution in the presence of various concentrations of Na 3 PO4 , Ž1. 0.0, Ž2. 0.02, Ž3. 0.04, Ž4. 0.06, Ž5. 0.08, Ž6. 0.1 M for sample II. At scan rate 30 mV sy1 . Žb. Potentiodynamic curves for steel anode in 0.1 M NaCl solution in the presence of various concentrations of Na 2 CrO4 , Ž1. 0.0, Ž2. 0.02, Ž3. 0.04, Ž4. 0.06, Ž5. 0.08, Ž6. 0.1 M for sample II. At scan rate 30 mV sy1 .
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
193
Fig. 4. The pitting potential Ž Epit . as a function of Na 3 PO4 concentrations for steel samples I and II.
Fig. 6. The pitting potential Ž Epit . as a function of Na 2 MoO4 concentrations for steel samples I and II.
effective as pitting corrosion inhibitor. The inhibiting effect of these salts can be explained on the basis of the competitive adsorption between the inorganic anions and the aggressive Cly ions on the passive electrode surface and thus retards their corresponding destructive action w9x. In this case, the pitting corrosion potential shifts into the positive direction. This mechanism seems to be the most effective way to avoid pitting corrosion. The inhibitive anions may be incorporated into the passive layer on the metal surface, forming an improved stability against the aggressive ions. The inhibition effect of the phosphate ions may be due to specific passivation of steel by the deposition the metal phosphate from the solution, the accumulation on the steel surface of a poorly soluble iron phosphate creates conditions favorable for ordinary
oxide passivation. The protective film formed on steel surface in the presence of Na 3 PO4 using the electron diffraction method showed the film to consist of a mixture of g-Fe 2 O 3 and FePO 3 P 2H 2 O w10x. The inhibition effect of chromate ions may be due to the reduction of the Cr 6q to Cr 3q Žas Cr2 O 3 . during film formation w11x according to
Fig. 5. The pitting potential Ž Epit . as a function of Na 2 CrO4 concentrations for steel samples I and II.
Fig. 7. The pitting potential Ž Epit . as a function of NaNO 2 concentrations for steel samples I and II.
CrO42yq 4H 2 O q 3e
™ CrŽ OH.
y y 4 q 4OH .
According to Pourbaix diagram for Cr w12x, the CrO42y ions becomes more stable in neutral medium at high potential value, this leads to increase the number of the oxidized CrO42y anions on the passive film. The CrO42y species may be plugging the pores in the passive film formed on steel surface and increasing the corrosion resistance of the film to-
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
194
wards the pitting corrosion. The inhibition behavior of chromate may be explained by the difference between the polarizability of the CrO42y anion Ž670 mm2 moly1 . and the chloride ion Ž890 mm2 moly1 . w13x. Therefore, the CrO42y anion has a higher specific adsorption than the aggressive anion ŽCly. , meaning that the CrO42y anion can displace the adsorbed Cly ions and consequently increase the protectiveness of the passive layer. Fig. 7 shows the dependence of Epit for the two samples on logarithmic concentration of NOy 2 . It is clear that the addition of NOy inhibits the pitting 2 corrosion. The inhibition effect of NOy ions in2 creases with increasing the anion concentration. It is probable that the role of NOy 2 in the inhibition of pitting corrosion may be due to its fast reduction to NHq 4 during the steel dissolution reaction,
™ NH q 2H O.
q NOy 2 q 6e q 8H
q 4
2
The residual oxygen on the surface is triggering the oxidation of steel to give Fe 2 O 3 w6x. Moreover, NOy 2 ions can adsorb on the surface oxide film and dislodge Cly ion from the sites through which it prefer-
entially penetrate the passive film and thereby enhance the pitting corrosion resistance. The protection effect of the steel by using molybedate anion may be due to the reduction of the Mo 6q to Mo 4q Žas MoO 2 . during film formation w14x. MoO42yq 4Hqq 2e
´ MoO q 2H O 2
2
The reduction of the molybedate anion could provide additional oxygen anions that interfere with the ability of Cly anion to reach the metalrfilm interface, possibly by blocking sites through which aggressive anions preferentially penetrate the film. The formation of MoO 2 in neutral medium ŽpH ( 7. at low potential value Ž100 mV. is predicted by the Pourbaix diagram for Mo w12x. In addition, the inhibitive nature of the molybedate anion may be due to the formation of a thin film of molybedate. The amount of Mo 4q in solution is limited by its solubility w12x. With increased molybedate solution concentration, the fraction of Mo as Mo 4q decreases due to its solubility. In this case the MoO42y is able to be adsorbed on the surface in sufficient amount and
Fig. 8. Scanning electron micrograph of the surface of steel anode after potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M NaNO 2 . For steel sample I.
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
195
Fig. 9. Scanning electron micrograph of the surface of steel anode after potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M Na 2 Cr2 O 7 . For steel sample I.
form a thin molybedate layer. The restriction of the anion diffusion by the presence of the molybedate in the film would lead to less pitting. This may explain why an increase in molybedate concentration leads to an increase in pitting potential. The mechanism of corrosion inhibition of mild steel by MoO42y has been interpreted w15x. Moreover, MoO42y adsorbs on the oxide surface through hydrogen bonding between the hydrogen atom of the dangling hydroxyl group of the oxide and the oxygen atom of the MoO42y ion and this makes the surface repels aggressive Cly and ensures the protection and stability of the oxide. SEM examination of steel surfaces potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M of different inhibitors Žfor steel samples I and II. was carried out. In presence of phosphate, chromate, molybedate and nitrite inhibitors, the data gave no evidence of pits and formation of thick films on steel surface Žas example, Figs. 8 and 9.. This may be interpreted by the adsorption of these inhibitor anions on the passive film or it was incorporated into the passive film. One may conclude that the inhibitors and anions are adsorbed around pits and
argued that this, coupled with the increase in Epit as a function of inhibitors concentration, indicated competitive adsorption between these inhibitors and the aggressive anions into the passive film.
References w1x S. Matsuda, H.H. Uhlig, J. Electrochem. Soc. 111 Ž1958. 156. w2x S.A.M. Refaey, S.S. Abd El Rehim, Electrochim. Acta 42 Ž4. Ž1996. 667. w3x S.A.M. Refaey, Electrochim. Acta 41 Ž16. Ž1996. . w4x S.A.M. Refaey, J. Appl. Electrochem. 26 Ž1996. 503. w5x S.A.M. Refaey, manuscript submitted for publication in Appl. Surf. Sci., 1999. w6x T.B. Hoar, D. Mears, G. Rothwell, Corros. Sci. 5 Ž1965. 279. w7x D.C.W. Kannagara, B.E. Conway, J. Electrochem. Soc. 134 Ž1987. 894. w8x V. Mitrovic-Scepanovic, R.J. Brigham, Corros. Sci. 52 Ž1. Ž1995. 23. w9x E.W. Abel, in: J.C. Bailar ŽEd.., Comprehensive Inorganic Chemistry 2 Pergamon, Oxford, 1965, p. 124. w10x J.P. Hancock, J.E. Mayne et al., J. Appl. Chem. 9 Ž1959. 345.
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w11x E. McCaffery, M.K. Berrett, J.S. Murday, Corros. Sci. 28 Ž6. Ž1988. 559. w12x M. Pourbaix, Atlas of Electrochemical Equilibria, p. 256 ŽCr., p. 272 ŽMo., Pergamon, Oxford Ž1066.. w13x J.M. West, in: Electrodeposition and Corrosion Processes, Van Nostrand-Reinhold, London, 1970, p. 123T.
w14x J. Augustynski, in: R.P. Frankenthal, J. Kruger ŽEds.., Passivity in Metals, Electrochemical Society, Princeton, NJ, 1987, p. 989. w15x C.M. Mustafa, S.M. Shahinoor Islam Dulal, Corrosion 52 Ž1. Ž1996. 16.
Inhibition of chloride localized corrosion of mild steel by PO43y, CrO42y, MoO42y, and NOy 2 anions S.A.M. Refaey a,) , S.S. Abd El-Rehim b, F. Taha a , M.B. Saleh a , R.A. Ahmed a a
Chemistry Department, Faculty of Science, Minia UniÕersity, 61519 Minia, Egypt Chemistry Department, Faculty of Science, Ain Shams UniÕersity, Cairo, Egypt
b
Received 16 March 1999; accepted 19 December 1999
Abstract . ions on the pitting The effect of phosphate ŽPO43y ., chromate ŽCrO42y ., molybedate ŽMoO42y . and nitrite ŽNOy 2 corrosion of steel in 0.1 M NaCl solution has been studied using potentiodynamic polarization and SEM techniques. The Ž . addition of increasing concentrations of PO43y, CrO42y, MoO42y and NOy 2 anions causes a shift of the pitting potential Epit in the positive direction, indicating the inhibitive effect of the added anions on the pitting attack. The adsorption characteristics of these anions on the steel surface play a significant role in the inhibition processes. The PO43y anion has a strong inhibitive effect of chloride pitting corrosion. The effect of different inorganic anions on the pitting corrosion of steel samples I and II Žwith different composition. was also studied in 0.1 M NaCl solution. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Mild steel; Localized corrosion; Potentiodynamic; Inorganic anions; Inhibition
1. Introduction The effect of Na 2 CrO4 , Na 2 MoO4 , and Na 2WO4 in deareated acid solutions at pH s 1.5 as inhibitors for Cu, Ni, Sn, Al and 18-8 stainless steel was examined w1x by weight loss and electrochemical measurements. Molybedate and tungstate were generally ineffective. Chromate was the most effective, especially for Cu, Al, and 18-8 stainless steel. Pitting corrosion is recognized as an insidious type of attack that results in many unexpected failures of metallic structures. The pitting corrosion of metals and alloys occurs when passivity breaks down at local points on
)
Corresponding author. E-mail address: [email protected] ŽS.A.M. Refaey..
the surfaces exposed to corrosive environments containing aggressive anions w2–5x. At these points, anodic dissolution proceeds while the major part of surface remains passive. The CrO42y, Cr2 O 72y and MoO42y anions were used as good inhibitors for the pitting corrosion of tin w2x. It was found that the pitting corrosion resistance of tin in alkaline and near neutral medium increased with increasing the concentration of CrO42y, Cr2 O 72y and MoO42y anions, indicating the inhibitive effect of the added anions on the pitting attack. The NOy 2 is entirely ineffective as an inhibitor and actually increases pitting corrosion. Cr2 O 72y anion had a strong inhibitive effect at very low concentrations. The mild steel was investigated w5x in the sodium gluconate solutions, it was found that the pitting corrosion of mild steel is inhibited by low concentration of sodium gluconate
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 1 6 - 7
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
in alkaline medium. But the presence of chloride ions in sodium gluconate solutions decreases the inhibition effect of steel by sodium gluconate in alkaline medium. In recent studies, an attempt has been made to study the inhibition of chloride pitting corrosion of mild steel in sodium chloride solution. The purpose of this paper is to establish the role of some inorganic anions in improving the passive film resistance to localized attack caused by chloride ions in neutral medium.
2. Experimental Experiments were carried out in 0.1 M NaCl solution, in absence and presence of different concentrations of sodium salts of phosphate, chromate, molybedate, or nitrite. All solutions were prepared from doubly distilled water and A.R. chemicals, and new polished electrodes were used for each run. All experiments were carried out at room temperature Ž258C.. All solutions were used under purified nitrogen gas. Two vanadium steel samples were used in the present work as working electrodes. These samples were produced in the electric arc furnace of the Delta Steel Mill, Cairo. The composition Žwt.%. of sample ŽI. is 0.16% C, 1.0 Mn, 0.45 S, 0.30 P, 0.20 Si, and 0.08 V. The composition of the second sample ŽII. is 0.24% C, 1.30 Mn, 0.049 S, 0.035 P, 0.53 Si, and 0.11 V. The ingots were hot-rolled Žat 1000–12008C. and then machined in the form of short rods Žhot-rolled electrodes., each 27 mm in length and 8 mm in diameter. Each working electrode was constructed and treated following the procedure described previously w5x. A Pt sheet was used as a counter electrode. The potential was measured against a AgrAgCl electrode. The potentiodynamic measurements were carried out using a potentiostat ŽAMEL model 2094. which was controlled by a PC. The morphology of tin surface after treatment in solutions was examined by a scanning electron microscope ŽCamScan Cambridge Scanning Company..
3. Results and discussion Fig. 1 shows the typical potentiodynamic curves of steel samples I and II in various concentrations of
191
NaCl in the potential range from y2.0 to 2.0 V at scan rate 30 mV sy1 . In NaCl solution, the samples did not exhibit an active passive transition. The lack of an activerpassive transition near the open circuit Žzero current. potential is due to spontaneous passivity. This indicates that the air-formed film is stable at and near the open circuit potential. When the potential reaches a certain critical potential, Epit , the passive current suddenly raises steeply without any sign for oxygen evolution, denoting breakdown of the passive film and initiation and propagation of pitting attack. Initiation of pitting attack could be ascribed to competitive adsorption between Cly ions and the passive species ŽOHy and H 2 O dipoles. w6x. At Epit , Cly anions displace the adsorbed passivating species at some locations and accelerate local anodic dissolution. On the other hand, the initiation of pitting attack could be due to the ability of Cly anions to penetrate the passive film with the assistance of a high electric field across the passive film and to attack the base metal surface w7x. The pitting growth Žpropagation. occurs as a result of an increase in Cly ion concentration resulting from its migration inside pits and hydrolysis of Fe 2q ions produced, since the high level of acidity required for pitting corrosion site growth must be achieved by hydrolysis of Fe 2q inside pits. Inspections of the data of Figs. 1 and 2 reveals that for the two samples, as the Cly ion concentration is increased, the pitting potentials Epit shift to more
Fig. 1. Anodic potentiodynamic curves for steel anode in solutions of various concentrations of NaCl at scan rates 30 mV sy1 . Concentrations: Ž1. 0.1, Ž2. 0.2, Ž3. 0.4, Ž4. 0.6, Ž5. 0.8, Ž6. 1.0 M for steel sample I.
192
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
Fig. 2. The pitting potential Ž Epit . as a function of NaCl concentrations for steel samples I and II.
sion of the two steel samples to an extent depending on the type and concentration of the inhibitor and the composition of the steel samples. Figs. 4–7 shows the dependence of the critical pitting potential Ž Epit . of the two steel electrodes on the concentration of the inorganic salts. The presence of phosphate, chromate, and nitrite causes a marked shift of critical pitting potential into the nobel wpositivex direction. The concentration of the inorganic salts required causing the marked shift in Epit to positive direction Žmarked inhibition. increases in the order: molybedate ) nitrite ) chromate ) phosphate. This order reflects the increased tendency of these compounds to act as pitting inhibitors, i.e., molybedate is less effective while phosphate is highly
negative Žactive. values indicating that increasing of Cly ion concentration would provide most favorable condition for pit initiation. Fig. 2 shows Epit vs. logwNaClx for the two samples, straight lines are obtained according to the fitting of the following equation Epit s a y blog w NaCl x , where a and b are constants which depend upon the composition of the samples. The pitting potential of sample II is higher positive than that for sample I in all concentrations of NaCl used. This result can be interpreted by the difference in the metal composition in steel samples under investigation. The Mn% Ž1.3%. in sample II ) Mn% Ž1.0%. in sample I, where the increase of the Mn content in the steel in the range 0.25% to 1.75% resulted in a significant decrease in pitting resistance w8x. The increase of V content also in the steel, V% Ž0.11% in sample II. ) V% Ž0.08% in sample I., increase the pitting corrosion resistance w5x. The effect of adding different concentrations of phosphate, chromate, nitrite, and molybedate as sodium salts on potentiodynamic anodic polarization curves of the two steel samples in 0.1 M NaCl were studied. Fig. 3a and b represent the polarization curves of sample II under the effect of adding increasing concentrations of phosphate and chromate, respectively, as examples. The same behaviour is observed in the presence of molybedate and nitrite ions. Also, the steel sample I show the same behaviour. The results obtained show that the presence of these inorganic anions inhibits the pitting corro-
Fig. 3. Ža. Potentiodynamic curves for steel anode in 0.1 M NaCl solution in the presence of various concentrations of Na 3 PO4 , Ž1. 0.0, Ž2. 0.02, Ž3. 0.04, Ž4. 0.06, Ž5. 0.08, Ž6. 0.1 M for sample II. At scan rate 30 mV sy1 . Žb. Potentiodynamic curves for steel anode in 0.1 M NaCl solution in the presence of various concentrations of Na 2 CrO4 , Ž1. 0.0, Ž2. 0.02, Ž3. 0.04, Ž4. 0.06, Ž5. 0.08, Ž6. 0.1 M for sample II. At scan rate 30 mV sy1 .
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
193
Fig. 4. The pitting potential Ž Epit . as a function of Na 3 PO4 concentrations for steel samples I and II.
Fig. 6. The pitting potential Ž Epit . as a function of Na 2 MoO4 concentrations for steel samples I and II.
effective as pitting corrosion inhibitor. The inhibiting effect of these salts can be explained on the basis of the competitive adsorption between the inorganic anions and the aggressive Cly ions on the passive electrode surface and thus retards their corresponding destructive action w9x. In this case, the pitting corrosion potential shifts into the positive direction. This mechanism seems to be the most effective way to avoid pitting corrosion. The inhibitive anions may be incorporated into the passive layer on the metal surface, forming an improved stability against the aggressive ions. The inhibition effect of the phosphate ions may be due to specific passivation of steel by the deposition the metal phosphate from the solution, the accumulation on the steel surface of a poorly soluble iron phosphate creates conditions favorable for ordinary
oxide passivation. The protective film formed on steel surface in the presence of Na 3 PO4 using the electron diffraction method showed the film to consist of a mixture of g-Fe 2 O 3 and FePO 3 P 2H 2 O w10x. The inhibition effect of chromate ions may be due to the reduction of the Cr 6q to Cr 3q Žas Cr2 O 3 . during film formation w11x according to
Fig. 5. The pitting potential Ž Epit . as a function of Na 2 CrO4 concentrations for steel samples I and II.
Fig. 7. The pitting potential Ž Epit . as a function of NaNO 2 concentrations for steel samples I and II.
CrO42yq 4H 2 O q 3e
™ CrŽ OH.
y y 4 q 4OH .
According to Pourbaix diagram for Cr w12x, the CrO42y ions becomes more stable in neutral medium at high potential value, this leads to increase the number of the oxidized CrO42y anions on the passive film. The CrO42y species may be plugging the pores in the passive film formed on steel surface and increasing the corrosion resistance of the film to-
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
194
wards the pitting corrosion. The inhibition behavior of chromate may be explained by the difference between the polarizability of the CrO42y anion Ž670 mm2 moly1 . and the chloride ion Ž890 mm2 moly1 . w13x. Therefore, the CrO42y anion has a higher specific adsorption than the aggressive anion ŽCly. , meaning that the CrO42y anion can displace the adsorbed Cly ions and consequently increase the protectiveness of the passive layer. Fig. 7 shows the dependence of Epit for the two samples on logarithmic concentration of NOy 2 . It is clear that the addition of NOy inhibits the pitting 2 corrosion. The inhibition effect of NOy ions in2 creases with increasing the anion concentration. It is probable that the role of NOy 2 in the inhibition of pitting corrosion may be due to its fast reduction to NHq 4 during the steel dissolution reaction,
™ NH q 2H O.
q NOy 2 q 6e q 8H
q 4
2
The residual oxygen on the surface is triggering the oxidation of steel to give Fe 2 O 3 w6x. Moreover, NOy 2 ions can adsorb on the surface oxide film and dislodge Cly ion from the sites through which it prefer-
entially penetrate the passive film and thereby enhance the pitting corrosion resistance. The protection effect of the steel by using molybedate anion may be due to the reduction of the Mo 6q to Mo 4q Žas MoO 2 . during film formation w14x. MoO42yq 4Hqq 2e
´ MoO q 2H O 2
2
The reduction of the molybedate anion could provide additional oxygen anions that interfere with the ability of Cly anion to reach the metalrfilm interface, possibly by blocking sites through which aggressive anions preferentially penetrate the film. The formation of MoO 2 in neutral medium ŽpH ( 7. at low potential value Ž100 mV. is predicted by the Pourbaix diagram for Mo w12x. In addition, the inhibitive nature of the molybedate anion may be due to the formation of a thin film of molybedate. The amount of Mo 4q in solution is limited by its solubility w12x. With increased molybedate solution concentration, the fraction of Mo as Mo 4q decreases due to its solubility. In this case the MoO42y is able to be adsorbed on the surface in sufficient amount and
Fig. 8. Scanning electron micrograph of the surface of steel anode after potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M NaNO 2 . For steel sample I.
S.A.M. Refaey et al.r Applied Surface Science 158 (2000) 190–196
195
Fig. 9. Scanning electron micrograph of the surface of steel anode after potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M Na 2 Cr2 O 7 . For steel sample I.
form a thin molybedate layer. The restriction of the anion diffusion by the presence of the molybedate in the film would lead to less pitting. This may explain why an increase in molybedate concentration leads to an increase in pitting potential. The mechanism of corrosion inhibition of mild steel by MoO42y has been interpreted w15x. Moreover, MoO42y adsorbs on the oxide surface through hydrogen bonding between the hydrogen atom of the dangling hydroxyl group of the oxide and the oxygen atom of the MoO42y ion and this makes the surface repels aggressive Cly and ensures the protection and stability of the oxide. SEM examination of steel surfaces potentiodynamically treated in 0.1 M NaCl solution containing 0.1 M of different inhibitors Žfor steel samples I and II. was carried out. In presence of phosphate, chromate, molybedate and nitrite inhibitors, the data gave no evidence of pits and formation of thick films on steel surface Žas example, Figs. 8 and 9.. This may be interpreted by the adsorption of these inhibitor anions on the passive film or it was incorporated into the passive film. One may conclude that the inhibitors and anions are adsorbed around pits and
argued that this, coupled with the increase in Epit as a function of inhibitors concentration, indicated competitive adsorption between these inhibitors and the aggressive anions into the passive film.
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