- Email: [email protected]
Polypyrrole layers for steel protection
Applied Surface Science 172 (2001) 276±284
Polypyrrole layers for steel protection P. Herrasti*, P. OcoÂn Departamento de QuõÂmica-FõÂsica Aplicada, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, 28049 Madrid, Spain Received 6 August 2000; accepted 21 October 2000
Abstract This report describes the behaviour of polypyrrole and polypyrrole/carboximethylcellulose (Ppy/CMC) prepared by different methods in order to prevent the corrosion of steel. Potentiodynamic polarization curves and open circuit potential were used to evaluate the capacity of these materials to protect the surface. The results obtained allowed to assume that Ppy/ CMC is better material to prevent corrosion than polypyrrole (Ppy) and potentiostatic method better than galvanostatic to obtain the deposit. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Polypyrrole/carboxymethylcellulose; Corrosion; Morphology; Stainless steel; Open circuit potential
1. Introduction The corrosion of steel has long been an important problem, causing losses in excess of 100 billion dollars annually world wide [1]. Pigments which protect by physicochemical mechanisms generally have a lamellar, ¯aky, or plate-like shape, which greatly increases the length of the diffusion pathways for oxygen and water and decreases the permeability of the coating. Pigments that protect by electrochemical mechanisms consist of inhibitors which are dissolved by the electrolyte entering from the environment to retard corrosion due to cathodic process or both. Pigments that protect by an ion exchange mechanism consist of ion exchangers that hinder the transport of Clÿ and Fe2 to the substrate. Conductive polymers have been candidates for metallic protection against corrosion since the work
*
Corresponding author. Tel.: 34-91-397-4831; fax: 34-91-397-4785. E-mail address: [email protected] (P. Herrasti).
of DeBerry [2] who observed effective passivation of iron by polyaniline (Pani) layer. The ef®ciency of an organic compound to act as a corrosion inhibitor is strongly dependent on its adsorption on the metal surface. To be an ideal protective cover against atmospheric and/or electrochemical corrosion a polymer must present a structure without free ions, water or oxygen. Also, the mobility of the ions must be equal to zero as well as the oxygen and water diffusion coef®cients. In the last year, many research groups have studied anti-corrosion activity of electrically conductive polymer (Pani) and polypyrrole (Ppy) [3±11]. Corrosion protective properties of those Pani and Ppy ®lms depend on the conditions of synthesis such as electrolyte, preparation method, temperature, stirring, etc. [12]. The purpose of this study is (a) to evaluate the effectiveness of thin ®lms of Ppy and a composite polypyrrole/carboximethylcellulose (Ppy/CMC) as corrosion controlling layer for stainless steel and (b) investigate if such ®lms can proved adequate protection under conditions in which conventional
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 6 6 - 7
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
inhibitors fail, such as under high applied potential and elevated temperatures. 2. Experimental The working electrodes stainless steel plates were polished with grit emery paper followed by thorough rinsing in acetone and deionized water, and drying in air. No notable differences were observed in electrochemical experiments due to different surface preparations. The area of working electrode was about 1 cm 1 cm. Polypyrrole (Ppy) and Ppy/CMC were obtained in 0.25 M pyrrole, 0.1 M LiClO4 and 0.25 M Pyrrole, 0.1 M CMC in aqueous solutions, respectively. Electrochemical corrosion measurements were performed in 3% NaCl solution at room temperature. The electrochemical methods included open circuit potential and potentiodynamic polarization experiments were carried out with a potentiostat model VersaStat (EG&G PAR) controlled by M352/252 corrosion software (EG&G PAR). Before polarization the samples were immersed into the solution and the open circuit potential was monitored until a constant value was reached. Potentiodynamic polarization measurements were always started from the open circuit
277
potential (corrosion potential) at the scan rate of 2 mV/s. Surface morphology of the ®lms before an after corrosion were characterized with scanning electron microscopy (SEM). Some analysis of the surface were carried out with EDAX. 3. Results and discussion We have studied the behaviour of different deposits on stainless steel substrates. So, we have deposited polypyrrole like a composite with carboximethylcellulose and polypyrrole, both in aqueous solution. Ppy and Ppy/CMC ®lms of similar thickness were grown (by adjusting the electrolysis time in order to have the same electrodeposition charge Q 1C/cm2 for all ®lms) at various potential and with various monomer concentration. When the charge is lesser or higher than the previous value the ®lms had not been a good quality. Our interest was the deposition of thick, homogeneous and adherent ®lms on the surface. The best results producing good quality ®lms were obtained at 0.8 V versus SCE and the monomer and electrolyte concentrations indicated in experimental section.
Fig. 1. Potentiodynamic polarization curves of: (Ð) stainless steel, ( ) Ppy and (- - -) Ppy/CMC. Curves were obtained in 3% NaCl at scan rate of 2 mV/s.
278
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
The potentiodynamic polarization curves for three different samples in 3% NaCl aqueous solution are shown in Fig. 1. It is possible to observe that the corrosion potential presents a displacement to the positive direction when the surface is covered by Ppy until 90 mV and this displacement is high when the surface is covered by the composite Ppy/CMC until 250 mV. These shifted in the corrosion potential indicates the best protection of the metal surface when the composite is deposited. A quantitative description of the corrosion protection offered by the Ppy/CMC is dif®cult due to the interactions of the variables involved and the complexity of the passivation process itself. But a qualitative description can be given through the structure of the material. Polypyrrole (Ppy) and Ppy/CMC can be represented by a globular structure (Fig. 2). The differences in both deposits are the interstices between globules, lesser in the case of the composite, in this case there are a decreasing of the effective exposed areas. A major complication is that the properties of the passive ®lm formed are different. The anodic passive current density is found to decrease from steel to Ppy/CMC at a given potential, presumably due at ®rst to an increase in thickness of the passive metal oxide ®lm and then to repair of
defects in its structure. This corrosion protection tendency was also observed for the open circuit potential (Fig. 3) where the potential is stable with a lighter decrease in the case of stainless steel covered with Ppy and Ppy/CMC compared with stainless steel where the open circuit potential presented a decreasing. The changes in the corrosion potential have been explained by many authors, indicating that part of the anticorrosion properties of conductive polymers are due to the fact that such material, when they are in their conductive states, might be capable of displacing the electroactive interface from its usual localization (metal/solution interface). The idea is that most of the electrical potential drop occurs at the polymer/solution interface. There will be almost no potential drop at the metal/polymer interface and there will be no driving force for metal oxidation. On the other hand better inhibition of corrosion has been observed on using a polymer which is rich in availability of P-electrons. These electrons are able to move easier in a composite like Ppy/CMC. As well the increase in coverage of the metal surface by the polymer may give rise to various types of interactions between neighboring to in¯uence the inhibition ef®ciencies.
Fig. 2. Scheme of a deposit Ppy/CMC.
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
279
Fig. 3. Open circuit potential vs. exposure time of: (^) Ppy/CMC; (*) Ppy and (&) steel in a solution containing 3% NaCl.
Fig. 4. SEM surface micrographs of (a) Ppy obtained on stainless steel at V 0:8 V vs. SCE in a 0.25 M pyrrole, 0.1 M LiClO4 aqueous solution and (b) Ppy/CMC obtained at V 0:8 V vs. SCE in a 0.25 M pyrrole, 0.1 M CMC aqueous solution.
280
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
Fig. 5. SEM surface micrographs after corrosion at 1.5 V in 3% NaCl of the same sample of (a) Fig. 4a and (b) Fig. 4b.
The coverage of the metallic surface is one of the most important role to prevent the corrosion like we explained previously. In this sense the coverage of the stainless steel by Ppy and Ppy/CMC is very similar. Fig. 4 shows typical morphology called globular in both coverage Ppy and Ppy/CMC. When these samples are introduced in 3% NaCl during long time, the morphology changed. In the case of Ppy, this change is drastic, appearing hole on the polymer surface. When a high potential (1.5 V) is applied, these holes can be observed very clear (Fig. 5a). With the same treatment for Ppy/CMC samples without potential, the morphology did not change. High constant potential applied to the electrode in this solution changed lightly the morphology like is showed in Fig. 5b. Previous studies of electrogeneration of Ppy/CMC onto platinum and tin oxide electrodes [13] have demonstrated that the
electrodeposition occurs ®rst with the adsorption, physical adsorption, of CMC on the substrate. This layer presents lesser conductivity than the rest of the interface and it produces a block layer for the corrosion. On the other hand, probably the differences observed in the surface topologies are not re¯ected in the packing structure bulk. This is an example where the nature of the anion (small anion ClO4ÿ or polyelectrolyte CMC) important for those properties of the ®lm which involve charge transport like redox reaction of the ®lm and the electrical conductivity. We can observe in the polarization curves that the corrosion current density increase from steel to Ppy/ CMC. It means that the polymeric ®lm changes to a more conductive form and also takes place reactions in the polymeric matrix increasing the current density of the process. The corrosion rate expressed in mdd (mg per day per dm2) was measured monitoring the loss of weight for different samples. The samples covered with polymers have a corrosion rate very low, the weigh remained unchanged; while stainless steel has a corrosion rate around 10 mdd. Another kind of experiments was performed using the steel covered with polymer after thermal treatment in air. The open circuit potential curves with these samples are shown in Fig. 6. From which one can observe a increase in the corrosion potential. In samples treated at 1508C, it is observed that after a stabilization an abrupt decrease of corrosion potential takes place. The morphology of these samples shown a small agglomerates. The EDAX analysis indicated a content of Fe, probably with formation of oxides what indicates that the attack has arrived to the substrate as can be seen in Fig. 7a and b. This behaviour may be explained as well like a decompose irreversibly of the ®lm; at this temperature. To demonstrate the differences in morphology and corrosion with the preparation method samples were grown to different current densities. From an industrial point of view, galvanostatic deposition at high current density, in a one-step process, is a key criterion for most electrochemical deposition processes (especially onto oxidizable metals). Fig. 8 shown the open circuit potential curves for two samples obtained at two different current densities. The unusual oscillation in the open circuit voltage occurs along the time, this behaviour has been observed by DeBerry [2]. These results can be
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
281
Fig. 6. Open circuit potential vs. exposure time of steel/Ppy interface at (&) 100 and (*) 1508C in air, in a solution of 3% NaCl.
interpreted in part considering the localized corrosion (pitting) of the electrode. It appears that the passive ®lm is periodically undergoing partial breakdown and is then being reformed with the aid of the polypyrrole. Literature reports from different laboratories describe the polymer deposit as a ®lm formed by granules, ®bbers or smooth structures [14]. The var-
iation in the topologies of the surface, are due to the complexity of the electropolymerization process producing regions in the ®lms with different grow rates. Scanning electron microscopy was employed in an effort to examine the morphologies of Ppy ®lm grown at different conditions. Two phases seen to predominate: a globular phase prevalent in the ®lms grown and a cauli¯ower phase in the edge, in this area the growth
Fig. 7. (a) SEM micrograph for a sample treated thermally at 1508C after corrosion during 16 days in 3% NaCl and (b) EDAX analysis of the square in Fig. 7a.
282
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
Fig. 7. (Continued ).
rate is very high and growth is in open structures as can be seen in Fig. 9a. When the current density is higher, 5 mA, these structures are closed (Fig. 9b). If this last sample is performed to corrosion process in 3% NaCl, the morphology changed. The surface has
holes and the cauli¯ower structures have been opened (Fig. 9c). No important improve has been observed when current density has been used. The morphology was irregular and that means preferential attacks where the corrosion takes place in a quick way.
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
283
Fig. 8. Open circuit potential vs. exposure time of Ppy samples growth at (&) 2.5 and (*) 5 mA/cm2, in solution containing 3% NaCl. The samples have been obtained in solution containing 0.25 M pyrrol 0:1 M LiClO4.
Fig. 9. SEM surface micrographs of Ppy ®lm obtained in 0.25 M pyrrole, 0.1 M LiClO4 in aqueous solution. Galvanostatic: (a) 2.5 mA/cm2; (b) 5 mA/cm2 and (c) sample of Fig. 9b after corrosion in 3% NaCl solution.
284
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
4. Conclusions
References
The results of the present work con®rmed the corrosion protective effect of Ppy on stainless steel by producing a shift of corrosion potential to positive direction and reducing the oxidation current. The best results were found under potentiostatic condition, indicating that the metal/polypyrrole interface remain stable to long exposure time in 3% NaCl solutions. At low current density the ®lms are more uniform and with good stability against corrosion. When high current density are used the ®lms are not able to prevent corrosion. On the contrary, ®lms deposited potentiostatically are more stable against corrosion. Thermal treatment of the samples improve these at temperatures less than 1508C. Higher temperatures produced a damage in the morphology, decreasing the stability and increasing corrosion. The composite Ppy/CMC is more stable than Ppy due to a more compact structure. Further work is underway in our laboratory to develop a mechanism to fully characterization of the Ppy/CMC on stainless steel. It is also evident that the levels of protection provided by this blend depends on the form of the composite and the nature of corrosion environment.
[1] N. Ahmad, A.G. MacDiarmid, Synth. Met. 78 (1996) 103. [2] N.D.W. DeBerry, J. Electrochem. Soc. 132 (1985) 1022. [3] A. Ferreira, S. Aeiyach, J.J. Aaron, P.C. Lacaze, Electrochim. Acta 41 (1996) 1801. [4] J.R. Santos Jr., L.H.C. Mattoso, A.J. Motheo, Electrochim. Acta 43 (1998) 309. [5] J.C. Lacroix, J.L. Camalet, S. Aeiyach, K.I. Chane-Ching, J. Petit jean, E. Chauveau, P.C. Lacaze, J. Electroanal. Chem. 481 (2000) 76. [6] G. Troch-Nagels, R. Winand, A. Weymeersch, L. Renard, J. Appl. Electrochem. 22 (1992) 756. [7] B. Wessling, Adv. Mater. 6 (1994) 226. [8] W.K. Lu, R.L. Elsenbaumer, B. Wessling, Synth. Met. 71 (1995) 2163. [9] V. Brusic, M. Angelopoulos, T. Graham, Proc. Electrochem. Soc. 144 (1997) 436. [10] N.V. KrstajicÂ, B.N. Grgur, S.M. JovanovicÂ, M.V. VojnovicÂ, Electrochim. Acta 42 (1997) 1685. [11] P. Li, T.C. Tan, J.Y. Lee, Synthetic. Met. 88 (1997) 237. È pik, O. ForseÂn, K. Idla, Accepted to the [12] J. Reut, A. O EUROCORR'98 in Utrecht, The Netherlands, 28 September± 1 October 1998 (extended abstract). [13] T.F. Otero, P. Herrasti, P. OcoÂn, C.R. Alves, Electrochim. Acta 43 (1998) 1089. [14] F. Beck, R. Michaelis, J. Coating Technol. 64 (1992) 808.
Polypyrrole layers for steel protection P. Herrasti*, P. OcoÂn Departamento de QuõÂmica-FõÂsica Aplicada, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, 28049 Madrid, Spain Received 6 August 2000; accepted 21 October 2000
Abstract This report describes the behaviour of polypyrrole and polypyrrole/carboximethylcellulose (Ppy/CMC) prepared by different methods in order to prevent the corrosion of steel. Potentiodynamic polarization curves and open circuit potential were used to evaluate the capacity of these materials to protect the surface. The results obtained allowed to assume that Ppy/ CMC is better material to prevent corrosion than polypyrrole (Ppy) and potentiostatic method better than galvanostatic to obtain the deposit. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; Polypyrrole/carboxymethylcellulose; Corrosion; Morphology; Stainless steel; Open circuit potential
1. Introduction The corrosion of steel has long been an important problem, causing losses in excess of 100 billion dollars annually world wide [1]. Pigments which protect by physicochemical mechanisms generally have a lamellar, ¯aky, or plate-like shape, which greatly increases the length of the diffusion pathways for oxygen and water and decreases the permeability of the coating. Pigments that protect by electrochemical mechanisms consist of inhibitors which are dissolved by the electrolyte entering from the environment to retard corrosion due to cathodic process or both. Pigments that protect by an ion exchange mechanism consist of ion exchangers that hinder the transport of Clÿ and Fe2 to the substrate. Conductive polymers have been candidates for metallic protection against corrosion since the work
*
Corresponding author. Tel.: 34-91-397-4831; fax: 34-91-397-4785. E-mail address: [email protected] (P. Herrasti).
of DeBerry [2] who observed effective passivation of iron by polyaniline (Pani) layer. The ef®ciency of an organic compound to act as a corrosion inhibitor is strongly dependent on its adsorption on the metal surface. To be an ideal protective cover against atmospheric and/or electrochemical corrosion a polymer must present a structure without free ions, water or oxygen. Also, the mobility of the ions must be equal to zero as well as the oxygen and water diffusion coef®cients. In the last year, many research groups have studied anti-corrosion activity of electrically conductive polymer (Pani) and polypyrrole (Ppy) [3±11]. Corrosion protective properties of those Pani and Ppy ®lms depend on the conditions of synthesis such as electrolyte, preparation method, temperature, stirring, etc. [12]. The purpose of this study is (a) to evaluate the effectiveness of thin ®lms of Ppy and a composite polypyrrole/carboximethylcellulose (Ppy/CMC) as corrosion controlling layer for stainless steel and (b) investigate if such ®lms can proved adequate protection under conditions in which conventional
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 6 6 - 7
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
inhibitors fail, such as under high applied potential and elevated temperatures. 2. Experimental The working electrodes stainless steel plates were polished with grit emery paper followed by thorough rinsing in acetone and deionized water, and drying in air. No notable differences were observed in electrochemical experiments due to different surface preparations. The area of working electrode was about 1 cm 1 cm. Polypyrrole (Ppy) and Ppy/CMC were obtained in 0.25 M pyrrole, 0.1 M LiClO4 and 0.25 M Pyrrole, 0.1 M CMC in aqueous solutions, respectively. Electrochemical corrosion measurements were performed in 3% NaCl solution at room temperature. The electrochemical methods included open circuit potential and potentiodynamic polarization experiments were carried out with a potentiostat model VersaStat (EG&G PAR) controlled by M352/252 corrosion software (EG&G PAR). Before polarization the samples were immersed into the solution and the open circuit potential was monitored until a constant value was reached. Potentiodynamic polarization measurements were always started from the open circuit
277
potential (corrosion potential) at the scan rate of 2 mV/s. Surface morphology of the ®lms before an after corrosion were characterized with scanning electron microscopy (SEM). Some analysis of the surface were carried out with EDAX. 3. Results and discussion We have studied the behaviour of different deposits on stainless steel substrates. So, we have deposited polypyrrole like a composite with carboximethylcellulose and polypyrrole, both in aqueous solution. Ppy and Ppy/CMC ®lms of similar thickness were grown (by adjusting the electrolysis time in order to have the same electrodeposition charge Q 1C/cm2 for all ®lms) at various potential and with various monomer concentration. When the charge is lesser or higher than the previous value the ®lms had not been a good quality. Our interest was the deposition of thick, homogeneous and adherent ®lms on the surface. The best results producing good quality ®lms were obtained at 0.8 V versus SCE and the monomer and electrolyte concentrations indicated in experimental section.
Fig. 1. Potentiodynamic polarization curves of: (Ð) stainless steel, ( ) Ppy and (- - -) Ppy/CMC. Curves were obtained in 3% NaCl at scan rate of 2 mV/s.
278
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
The potentiodynamic polarization curves for three different samples in 3% NaCl aqueous solution are shown in Fig. 1. It is possible to observe that the corrosion potential presents a displacement to the positive direction when the surface is covered by Ppy until 90 mV and this displacement is high when the surface is covered by the composite Ppy/CMC until 250 mV. These shifted in the corrosion potential indicates the best protection of the metal surface when the composite is deposited. A quantitative description of the corrosion protection offered by the Ppy/CMC is dif®cult due to the interactions of the variables involved and the complexity of the passivation process itself. But a qualitative description can be given through the structure of the material. Polypyrrole (Ppy) and Ppy/CMC can be represented by a globular structure (Fig. 2). The differences in both deposits are the interstices between globules, lesser in the case of the composite, in this case there are a decreasing of the effective exposed areas. A major complication is that the properties of the passive ®lm formed are different. The anodic passive current density is found to decrease from steel to Ppy/CMC at a given potential, presumably due at ®rst to an increase in thickness of the passive metal oxide ®lm and then to repair of
defects in its structure. This corrosion protection tendency was also observed for the open circuit potential (Fig. 3) where the potential is stable with a lighter decrease in the case of stainless steel covered with Ppy and Ppy/CMC compared with stainless steel where the open circuit potential presented a decreasing. The changes in the corrosion potential have been explained by many authors, indicating that part of the anticorrosion properties of conductive polymers are due to the fact that such material, when they are in their conductive states, might be capable of displacing the electroactive interface from its usual localization (metal/solution interface). The idea is that most of the electrical potential drop occurs at the polymer/solution interface. There will be almost no potential drop at the metal/polymer interface and there will be no driving force for metal oxidation. On the other hand better inhibition of corrosion has been observed on using a polymer which is rich in availability of P-electrons. These electrons are able to move easier in a composite like Ppy/CMC. As well the increase in coverage of the metal surface by the polymer may give rise to various types of interactions between neighboring to in¯uence the inhibition ef®ciencies.
Fig. 2. Scheme of a deposit Ppy/CMC.
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
279
Fig. 3. Open circuit potential vs. exposure time of: (^) Ppy/CMC; (*) Ppy and (&) steel in a solution containing 3% NaCl.
Fig. 4. SEM surface micrographs of (a) Ppy obtained on stainless steel at V 0:8 V vs. SCE in a 0.25 M pyrrole, 0.1 M LiClO4 aqueous solution and (b) Ppy/CMC obtained at V 0:8 V vs. SCE in a 0.25 M pyrrole, 0.1 M CMC aqueous solution.
280
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
Fig. 5. SEM surface micrographs after corrosion at 1.5 V in 3% NaCl of the same sample of (a) Fig. 4a and (b) Fig. 4b.
The coverage of the metallic surface is one of the most important role to prevent the corrosion like we explained previously. In this sense the coverage of the stainless steel by Ppy and Ppy/CMC is very similar. Fig. 4 shows typical morphology called globular in both coverage Ppy and Ppy/CMC. When these samples are introduced in 3% NaCl during long time, the morphology changed. In the case of Ppy, this change is drastic, appearing hole on the polymer surface. When a high potential (1.5 V) is applied, these holes can be observed very clear (Fig. 5a). With the same treatment for Ppy/CMC samples without potential, the morphology did not change. High constant potential applied to the electrode in this solution changed lightly the morphology like is showed in Fig. 5b. Previous studies of electrogeneration of Ppy/CMC onto platinum and tin oxide electrodes [13] have demonstrated that the
electrodeposition occurs ®rst with the adsorption, physical adsorption, of CMC on the substrate. This layer presents lesser conductivity than the rest of the interface and it produces a block layer for the corrosion. On the other hand, probably the differences observed in the surface topologies are not re¯ected in the packing structure bulk. This is an example where the nature of the anion (small anion ClO4ÿ or polyelectrolyte CMC) important for those properties of the ®lm which involve charge transport like redox reaction of the ®lm and the electrical conductivity. We can observe in the polarization curves that the corrosion current density increase from steel to Ppy/ CMC. It means that the polymeric ®lm changes to a more conductive form and also takes place reactions in the polymeric matrix increasing the current density of the process. The corrosion rate expressed in mdd (mg per day per dm2) was measured monitoring the loss of weight for different samples. The samples covered with polymers have a corrosion rate very low, the weigh remained unchanged; while stainless steel has a corrosion rate around 10 mdd. Another kind of experiments was performed using the steel covered with polymer after thermal treatment in air. The open circuit potential curves with these samples are shown in Fig. 6. From which one can observe a increase in the corrosion potential. In samples treated at 1508C, it is observed that after a stabilization an abrupt decrease of corrosion potential takes place. The morphology of these samples shown a small agglomerates. The EDAX analysis indicated a content of Fe, probably with formation of oxides what indicates that the attack has arrived to the substrate as can be seen in Fig. 7a and b. This behaviour may be explained as well like a decompose irreversibly of the ®lm; at this temperature. To demonstrate the differences in morphology and corrosion with the preparation method samples were grown to different current densities. From an industrial point of view, galvanostatic deposition at high current density, in a one-step process, is a key criterion for most electrochemical deposition processes (especially onto oxidizable metals). Fig. 8 shown the open circuit potential curves for two samples obtained at two different current densities. The unusual oscillation in the open circuit voltage occurs along the time, this behaviour has been observed by DeBerry [2]. These results can be
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
281
Fig. 6. Open circuit potential vs. exposure time of steel/Ppy interface at (&) 100 and (*) 1508C in air, in a solution of 3% NaCl.
interpreted in part considering the localized corrosion (pitting) of the electrode. It appears that the passive ®lm is periodically undergoing partial breakdown and is then being reformed with the aid of the polypyrrole. Literature reports from different laboratories describe the polymer deposit as a ®lm formed by granules, ®bbers or smooth structures [14]. The var-
iation in the topologies of the surface, are due to the complexity of the electropolymerization process producing regions in the ®lms with different grow rates. Scanning electron microscopy was employed in an effort to examine the morphologies of Ppy ®lm grown at different conditions. Two phases seen to predominate: a globular phase prevalent in the ®lms grown and a cauli¯ower phase in the edge, in this area the growth
Fig. 7. (a) SEM micrograph for a sample treated thermally at 1508C after corrosion during 16 days in 3% NaCl and (b) EDAX analysis of the square in Fig. 7a.
282
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
Fig. 7. (Continued ).
rate is very high and growth is in open structures as can be seen in Fig. 9a. When the current density is higher, 5 mA, these structures are closed (Fig. 9b). If this last sample is performed to corrosion process in 3% NaCl, the morphology changed. The surface has
holes and the cauli¯ower structures have been opened (Fig. 9c). No important improve has been observed when current density has been used. The morphology was irregular and that means preferential attacks where the corrosion takes place in a quick way.
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
283
Fig. 8. Open circuit potential vs. exposure time of Ppy samples growth at (&) 2.5 and (*) 5 mA/cm2, in solution containing 3% NaCl. The samples have been obtained in solution containing 0.25 M pyrrol 0:1 M LiClO4.
Fig. 9. SEM surface micrographs of Ppy ®lm obtained in 0.25 M pyrrole, 0.1 M LiClO4 in aqueous solution. Galvanostatic: (a) 2.5 mA/cm2; (b) 5 mA/cm2 and (c) sample of Fig. 9b after corrosion in 3% NaCl solution.
284
P. Herrasti, P. OcoÂn / Applied Surface Science 172 (2001) 276±284
4. Conclusions
References
The results of the present work con®rmed the corrosion protective effect of Ppy on stainless steel by producing a shift of corrosion potential to positive direction and reducing the oxidation current. The best results were found under potentiostatic condition, indicating that the metal/polypyrrole interface remain stable to long exposure time in 3% NaCl solutions. At low current density the ®lms are more uniform and with good stability against corrosion. When high current density are used the ®lms are not able to prevent corrosion. On the contrary, ®lms deposited potentiostatically are more stable against corrosion. Thermal treatment of the samples improve these at temperatures less than 1508C. Higher temperatures produced a damage in the morphology, decreasing the stability and increasing corrosion. The composite Ppy/CMC is more stable than Ppy due to a more compact structure. Further work is underway in our laboratory to develop a mechanism to fully characterization of the Ppy/CMC on stainless steel. It is also evident that the levels of protection provided by this blend depends on the form of the composite and the nature of corrosion environment.
[1] N. Ahmad, A.G. MacDiarmid, Synth. Met. 78 (1996) 103. [2] N.D.W. DeBerry, J. Electrochem. Soc. 132 (1985) 1022. [3] A. Ferreira, S. Aeiyach, J.J. Aaron, P.C. Lacaze, Electrochim. Acta 41 (1996) 1801. [4] J.R. Santos Jr., L.H.C. Mattoso, A.J. Motheo, Electrochim. Acta 43 (1998) 309. [5] J.C. Lacroix, J.L. Camalet, S. Aeiyach, K.I. Chane-Ching, J. Petit jean, E. Chauveau, P.C. Lacaze, J. Electroanal. Chem. 481 (2000) 76. [6] G. Troch-Nagels, R. Winand, A. Weymeersch, L. Renard, J. Appl. Electrochem. 22 (1992) 756. [7] B. Wessling, Adv. Mater. 6 (1994) 226. [8] W.K. Lu, R.L. Elsenbaumer, B. Wessling, Synth. Met. 71 (1995) 2163. [9] V. Brusic, M. Angelopoulos, T. Graham, Proc. Electrochem. Soc. 144 (1997) 436. [10] N.V. KrstajicÂ, B.N. Grgur, S.M. JovanovicÂ, M.V. VojnovicÂ, Electrochim. Acta 42 (1997) 1685. [11] P. Li, T.C. Tan, J.Y. Lee, Synthetic. Met. 88 (1997) 237. È pik, O. ForseÂn, K. Idla, Accepted to the [12] J. Reut, A. O EUROCORR'98 in Utrecht, The Netherlands, 28 September± 1 October 1998 (extended abstract). [13] T.F. Otero, P. Herrasti, P. OcoÂn, C.R. Alves, Electrochim. Acta 43 (1998) 1089. [14] F. Beck, R. Michaelis, J. Coating Technol. 64 (1992) 808.