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Corrosion protection of mild steel by electroactive polyaniline coatings
Synthetic Metals 88 (1997) 237–242
Corrosion protection of mild steel by electroactive polyaniline coatings P. Li, T.C. Tan, J.Y. Lee U Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119265, Singapore Revised 11 March 1997; accepted 13 March 1997
Abstract The ability of polyaniline (PANi) to act as a protective coating for mild steel corrosion in saline and acid was investigated by electrochemical impedance spectroscopy. The impedance behaviour is best explained by a mediated redox reaction in which PANi passivates the metal surface and reoxidizes itself by dissolved oxygen. The effectiveness of such a process, which also provides the repassivation of damaged films, is greater in acids. The performance of PANi is further enhanced by the presence of a top coat to increase the diffusional resistance for the corrosion species. Keywords: Polyaniline and derivatives; Corrosion protection; A.c. impedance; Passivation; Anti-corrosion coating
1. Introduction
Organic coatings have long been used to protect metals against corrosion. The primary effect of an organic coating is to act as a physical barrier against aggressive species such as O2 and Hq. However, all organic coatings are not permanently impenetrable and, once there are defects in the coatings, pathways will be created for the corrosive species to reach the substrate, and localized corrosion will occur. Therefore, as a second line of defence, anticorrosive pigments are added to the coating. These pigments may protect by a physicochemical or an electrochemical as well as an ion exchange mechanism. Pigments which protect by physicochemical mechanisms generally have a lamellar, flaky, or plate-like shape, which greatly increases the length of the diffusion pathways for oxygen and water and decreases the permeability of the coating. Examples of such pigments are micaceous iron oxide and aluminium flakes. 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, anodic process or both. Examples of this type of pigment are red lead and zinc chromate. Pigments that protect by an ion exchange mechanism consist of ion exchangers that hinder the transport of Cly and Feqq to the substrate. Kinlen and Silverman [1] reported that annealed perfluorinated cation exchange polyU
Corresponding author. Tel.: q65 772 2186; fax: q65 779 1936; email: [email protected]
mers have unique anion rejection properties which could minimize localized corrosion due to Cly ions. Recently, there have been attempts to use electroactive polymers to control pitting corrosion resulting from the permeation and breakdown of the protective coating. The electroactive polymer can be applied singly or in conjunction with other surface treatments. DeBerry [2] reported that polyaniline (PANi) electrochemically deposited on stainless steel could provide a form of anodic protection that significantly reduces corrosion rates in acid solutions. Ahmad and MacDiarmid [3] noticed that PANi in the emeraldine oxidation state has adequate oxidation power to passivate stainless steel. In the study of Lu et al. [4] on the corrosion protection of mild steel by coatings containing PANi, the surface could be easily repassivated by PANi even when scratches existed in the coatings. Wessling [5] also reported that mild steel, stainless steel and copper could be passivated by repeatedly dipping clean surfaces of the metals in dispersions of doped PANi. Passivation was found to occur by the formation of metal oxide films through the metal contact with PANi. Removal of PANi had exposed a grey metal surface with persistent passivated behaviour [5]. There are two methods to deposit an electroactive polymer on the metal surface, namely, electrochemically or chemically. From the viewpoint of application convenience, electrochemical deposition is cumbersome and virtually impossible on large structures such as ships, bridges and pipelines. In these cases chemical deposition is the only feasible alternative. Electroactive polymer coating is different from conventional coating in that it does not protect simply
0379-6779/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 3 8 6 0 - 5
Journal: SYNMET (Synthetic Metals)
Article: 4994
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P. Li et al. / Synthetic Metals 88 (1997) 237–242
state and to ensure that no blistering occurred during the conditioning period. All experiments were carried out at a laboratory temperature of 25"1 8C. All raw data were replicated three times to ensure their reproducibility and statistical significance.
by offering a physical barrier. Although there are suggestions to attribute the repassivation in the presence of an electroactive polymer to redox reactions between the polymer and metal, the origin of such effects has not been ascertained. These intriguing findings have prompted us to investigate and characterize corrosion protection with coatings containing electroactive polymers by electrochemical impedance spectroscopy (EIS).
3. Results and discussion
2. Experimental
3.1. Impedance characteristics of PANi coatings in saline and HCl
2.1. Sample preparation and corrosion conditions The working electrodes were cylindrical discs cut from a carbon steel rod with the following composition: C: 0.18%, Si: 0.25%, Mn: 0.71%, P: 0.012%, S: 0.013%. Each disc was pressure fitted into a Teflon holder, leaving only 0.5 cm2 of the surface area exposed to the testing environment. The working electrodes were mechanically abraded with a series of emery papers ending with 1200 grade, followed by thorough rinsing in acetone and deionized water, and drying in air. The PANi coating was a proprietary dispersion supplied by Zipperling. It comes in the form of a corrosion repair kit consisting of a primer for rust removal (Correpair I), an active PANi coating (Correpair II), and a protective overcoat (Correpair III). Only Correpair II and Correpair III were used in this work and they were applied either singly or jointly to clean working electrodes. The corrosion resistances of coatings were tested in 1 M HCl and 1 M NaCl. Some PANi coated samples also had their coatings removed by N-methylpyrrolidone (NMP) after 1 day and testing was continued for the exposed surface.
Figs. 1 and 2 show the Nyquist impedance plots of PANicoated mild steel after 6 h either in 1 M HCl or 1 M NaCl, respectively. While two depressed semicircles are obviously present in Fig. 1, only one is found in Fig. 2. The difference in impedance behaviour can largely be attributed to the electroactivity of PANi in different environments. Both the conductivity and electroactivity of PANi have been known to depend on the extent of protonation of the imine nitrogen sites of the polymer [6–9]. The dramatic increase in electrical conductivity of PANi upon protonation was first reported by MacDiarmid et al. [10] and observed by many others [11]. The proton-induced insulator-to-conductor transition, generally known as ‘protonic acid doping’, arises from the formation of polysemiquinone radical cations which contribute collectively to a half-filled polaron conduction band [12]. UV–Vis spectroscopy has shown that protonation is reversible [13]. The electroactivity of the polymer is benefited by the conductivity increase due to protonation [10].
2.2. Electrochemical measurements EIS was used to characterize the corrosion of mild steel in the presence of a PANi coating. The corrosion cell consisted of a three-electrode system. A platinum electrode and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The reference electrode was placed in a separate cell and was connected to the corrosion cell through a salt bridge with a Luggin capillary terminal in the corrosion cell. The corrosion media were 1 M HCl and 1 M NaCl diluted from AR grade chemicals with deionized water. For a.c. impedance measurements, an EG&G model 273 potentiostat/galvanostat was connected to an EG&G model 2310 lock-in amplifier and the ensemble was placed under the control of the EG&G model 388 software system. Impedance measurements were performed in the frequency range of 100 K–0.05 Hz with an a.c. modulation of amplitude "10 mV about the rest potential. The working electrode was stabilized in the test environment for 6 h before the impedance run. This served to put the electrode in a reproducible initial
Fig. 1. Nyquist plot of PANi-coated mild steel in 1 M HCl for 6 h.
Fig. 2. Nyquist plot of PANi-coated mild steel in 1 M NaCl for 6 h.
Journal: SYNMET (Synthetic Metals)
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Fig. 3. General equivalent circuit representation for a polymer-coated metal with blistering.
Fig. 4. Equivalent circuit of an insulating coating without blistering.
239
associated with the thickness of the coating. Rt, Cdl jointly represent the electrochemistry of corrosion at the coating/ metal interface after coating penetration by the corrosion species. Zw is the Warburg impedance characteristic of the transport of corrosion species to the metal surface. For an impervious insulating coating, the charge transfer components Rt, Cdl and the Warburg impedance Zw are non-existent, and the equivalent circuit is reduced to a parallel arrangement of Rc and Cc, which gives a semicircle in the Nyquist plot (Fig. 4). Without additional experimental findings Fig. 2 would appear to fit this description aptly. A Nyquist plot of two semicircles such as that of Fig. 1 is quite common among charge transfer reactions mediated by the presence of a redox polymer on the electrode [15], and can be represented by the equivalent circuit of Fig. 5. Here the high and low frequency semicircles can be assigned to charge transfer reactions at the metal/polymer interface and the polymer/solution interface, respectively. As no blistering is observed experimentally in this case, the high frequency semicircle may correspond to the repassivation of a scratched metal surface by PANi. PANi is reduced in the process and its reoxidation by a suitable cathodic depolarizer in the environment (notably dissolved oxygen) is represented by the low frequency semicircle. The details of the repassivation mechanism will be discussed later. The charge transfer resistances and the double-layer capacitance of these two reactions are represented by Rt1, C1 and Rt2, C2 respectively in the equivalent circuit. C is a capacitance due to the thickness of PANi coating. Although PANi is reasonably conductive in acid solutions, some limitations may still arise from its electrical resistance and contribute towards the observed value of R1. 3.2. The post-treatment effects of PANi on mild steel surface
Fig. 5. Equivalent circuit of an electroactive coating without blistering.
In EIS investigations, the opinion is near unanimous that a polymer-coated metal with blisters can be represented by the simple equivalent circuit of Fig. 3 [14]. In this figure Ru is the uncompensated solution resistance, Rc is the coating resistance that decreases with polymer conductivity and the penetration of coating by an electrolyte. Cc is the capacitance
EIS was also used to examine the activity difference between mild steel surface before and after PANi treatment. To this end three samples were prepared. The first sample (a) was pristine mild steel without PANi coating. The second sample (b) was mild steel coated with PANi which was then exposed to 1 M NaCl for 24 h. The PANi coating was subsequently removed by soaking in NMP. The last sample (c) was mild steel coated with PANi and exposed to 1 M HCl for 24 h, followed by the removal of PANi coating in NMP. The corrosion of these samples in 1 M HCl for various lengths of time was followed by EIS. Fig. 6 shows the Nyquist plots of the three samples after 48 h in 1 M HCl. The charge transfer resistances of samples a, b and c as determined by the intersection of the low frequency end of the semicircle arc with the real axis are 50, 120 and 240 ohm cm2, respectively. The results clearly demonstrate that the sample, coated with PANi and treated in HCl to render the polymer conductive and electroactive, has the greatest corrosion resistance after PANi removal. The steel surface was in a passivated state after being in contact with
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Fe|Feqqq2e
PANi removal by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy. Their results showed the presence of a multi-layered oxide structure on the metal surface consisting of predominantly g-Fe2O3 on the exterior and Fe3O4 in the sub-surface region. Scanning electron microscopy (SEM) examination of the surface after the individual steps of PANi treatment also verified the complex multi-step nature of passivation [6]. The first step of interaction between PANi and iron after coating appears to be an etching process which removes a few microns of iron. Passivating iron oxide is then formed in subsequent oxidation of the fresh iron surface. Elemental analysis by EDAX showed that a pure iron surface contains virtually no oxygen, whereas the freshly etched area is covered with only a very thin oxide layer 10–20 nm in thickness. High oxygen content is only limited to the passivated areas [6]. The PANi-mediated oxidation of steel continues until a stable passive oxide film is formed on the metal surface. In the event of mechanical damage to the film, the PANi remaining on the surface restarts the aforementioned mediated redox process, effectively repassivating the exposed surface and regenerating itself to the emeraldine oxidation state ready for the next remedial action. Lu et al. [4] reported that PANi could repair damage within a diameter of 1.2 mm. In this way the steel surface is continuously protected until PANi is depleted by some irreversible means. The a.c. impedance results presented previously show that the sample coated with PANi and treated in NaCl also passivates the steel surface to some extent, although not as effectively as the sample coated with PANi and treated in HCl. Slow PANi redox in neutral media is largely accountable for the ineffectiveness of the mediated oxidation of metal in this case. Corrosion is also inhibited by the physical barrier in the presence of the polymer on metal surface which blocks the approach of corrosion species. EIS of samples after PANi removal therefore confirms the reactivity of PANi in passivation, although its relative contribution with respect to the physical barrier effects could not be easily resolved from the EIS data.
2Hqq2e°H2
3.3. Anti-corrosion performance of PANi coatings
Fig. 6. Nyquist plot of three mild steel samples in 1 M HCl for 48 h: (a) without PANi coating; (b) coated with PANi and treated in 1 M NaCl for 1 day before PANi removal; (c) coated with PANi and treated in 1 M HCl for 1 day before PANi removal.
PANi. The versatility of PANi redox is largely responsible for such behaviour. Ideal PANi is a linear chain polymer consisting of quinoid and benzenoid repeating units. Different combinations of quinoid and benzenoid sequences result in various oxidation states for the polymer [16]. During oxidation the benzenoid units are progressively converted into quinoid units. The asdeposited polymer is in the emeraldine (EM) oxidation state, which is midway between the fully reduced state of leucoemeraldine and the fully oxidized state of pernigraniline, and consisting of an equal number of alternating benzenoid and quinoid units. Interconversions of these various oxidation states either in the base or protonated forms have been described schematically by MacDiarmid et al. [17]. The interconversion between the fully reduced leucoemeraldine oxidation state and the emeraldine oxidation state is rapid and reversible in aqueous acids. This is the primary redox reaction that drives the repassivation of steel. In acid solutions containing dissolved oxygen, the anodic reaction of metal dissolution is counterbalanced by a hydrogen evolution reaction or dissolved oxygen reduction as protons and dissolved oxygen are the most common cathodic depolarizers in aqueous corrosion:
q
O2q4H q4e°H2O Oxygen is unable to passivate metals under acidic conditions due to the solubility of the passive film in acids. Passivation is however a possibility in the presence of PANi. According to Wessling’s reaction scheme [5], the redox potential of the emeraldine–leucoemeraldine transition is adequate to bootstrap the oxidation of iron to the passivated state. PANi is reduced to leucoemeraldine in due course but can be reoxidized rapidly to emeraldine by dissolved oxygen. The process when examined in its entirety reveals the role of PANi as a redox mediator in the indirect oxidation of mild steel by oxygen. Protons in the environment are bounded to the polymer and hence are not readily available for oxide film dissolution. Lu et al. [4] examined the steel surface after
The Correpair kit contains a top coat (Correpair III) to be applied on top of PANi. PANi without the top coat blistered in 1 M HCl in 2 days, while the top coat alone was able to withstand corrosion in the same medium up to 5 days before any blistering. However, the performance of the top coat dwindled badly when mechanical damage was introduced to the coating. PANi in comparison was able to repair the damage to some extent by the mechanism outlined in previous sections. The top coat therefore acts simply as a physical barrier and protects by substrate isolation. Quite to our surprise, PANi with the top coat has the best corrosion resistance, and blistering only occurred after 20 days. Some synergism between PANi and the top coat was probably at work to produce a result better than the sum of the components.
Journal: SYNMET (Synthetic Metals)
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Fig. 7. Nyquist plots of mild steel coated with (a) PANi, (b) insulating top coat alone, and (c) PANi with the insulating top coat; in 1 M HCl for 6 h.
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impedance attests to the occurrence of redox reactions within the composite film. However, the finite length effect was not observable under the experimental conditions. If PANi were simple overlaid by the top coat, it would have behaved like an insulator layer and the equivalent circuit of Fig. 8 would predict an impedance spectrum different from that observed. It is therefore hypothesized that the presence of top coat does not quarantine the corrosion reaction by isolation, but rather lengthens the diffusion path of the corrosion species to lower the net corrosion rate at the metal/polymer surface. A possible equivalent circuit representation is given in Fig. 9. The situation is comparable, though not identical, to the efforts of adding inert materials or inhibitors to the coating to increase corrosion resistance. The presence of PANi has added the advantage to provide an in situ repair facility for damage in the passivating film through the mediated redox process described earlier. Similar results were also obtained for corrosion in 1 M NaCl. The Nyquist plots of PANi, the top coat, and PANi with top coat are again shown in a composite plot (Fig. 10). The corrosion resistance of PANi, as represented by one depressed semicircle, is virtually invisible in the figure at
Fig. 8. A suggested model of non-interacting layers of electroactive polymer and top coat and its Nyquist plot.
The Nyquist impedance plots of the PANi layer, the top coat, and the PANi layer with top coat after 6 h in 1 M HCl are overlaid in Fig. 7. Porosity in the PANi layer enables the penetration of free protons which may dissolve the passivating layer at a rate faster than PANi is able to repassivate the surface, leading to gradual corrosion after some time. The impedance of the top coat is a near-perfect semicircle typical of an impervious insulating coating. Its equivalent circuit is a simple parallel arrangement of film resistance and capacitance whose values can be determined from the intersection of the semicircle with the real axis at the low frequency end, and the maximum point on the circle respectively. The impedance plot of PANi with the top coat shows the classical behaviour of a charge transfer reaction rate limited by the diffusion of corrosion species at low frequencies. The presence of a straight line of unit slope indicative of the Warburg
Fig. 9. A suggested model of interacting layers of electroactive polymer and top coat and its Nyquist plot.
Fig. 10. Nyquist plots of mild steel coated with (a) PANi coating, (b) insulating top coat, and (c) PANi with the insulating top coat; in 1 M NaCl for 6 h.
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such a scale. Protection by isolation is definitely more aptly done by the top coat, which shows a bigger semicircular arc than the impedance plot of PANi alone. The impedance plot of mild steel coated with PANi and the top coat in sequence shows again the characteristics of a corrosion reaction inhibited by mass transfer limitations at low frequencies. The duplex coating therefore does not behave as if it were a simple overlay of two non-interacting layers (cf. Fig. 8). We believe that mutual interactions have led to an increase in the diffusional resistance for the corrosion species. An equivalent circuit representation is given in Fig. 9. The repassivation facility due to PANi redox should still be available, albeit operating less effectively because the PANi layer was not previously protonated. The corrosion resistance was nevertheless enhanced greater than what is expected from a simple linear sum of individual resistances. The origin of synergism between PANi and the top coat is not clear and should be worthy of further investigation. 4. Conclusions Mild steel coated with PANi shows different corrosion behaviour in 1 M HCl and 1 M NaCl. The Nyquist impedance plots for corrosion in acids are best interpreted by PANi acting as a redox mediator, passivating the metal at the metal/polymer interface and reoxidizing itself by dissolved oxygen at the polymer/solution interface. The tortuosity of the polymer and the affinity of the polymer for protons also reduce the dissolution of the passive film in acids. The electroactivity of PANi is reduced in the neutral medium of NaCl, and protection is primarily by the isolation of metal from the corrosion species. Nevertheless, the mediated redox reaction is still at play. Although PANi layers by themselves are not as impervious as some of the conventional insulating coatings, the
mediated redox reaction enables PANi to repassivate effectively an inadvertently exposed metal surface due to scratches or other mechanical damage in the passive film; a repair facility not provided by conventional coatings. The PANi performance can be further enhanced by the presence of a top coat to increase the diffusional pathway for the corrosion species. There are also experimental indications that synergism may exist between PANi and the top coat.
References [1] P.J. Kinlen and D.C. Silverman, J. Electrochem. Soc., 140 (1993) 3140. [2] D.W. DeBerry, J. Electrochem. Soc., 132 (1985) 1022. [3] N. Ahmad and A.G. MacDiarmid, Synth. Met., 78 (1996) 103. [4] W.K. Lu, R.L. Elsenbaumer and B. Wessling, Synth. Met., 69–71 (1995) 2163. [5] B. Wessling, Adv. Mater., 6 (1994) 226. [6] A.F. Diaz and J.A. Logen, J. Electroanal. Chem., 111 (1980) 111. [7] E.M. Genie`s, A.A. Syed and C. Tsintavis, J. Electroanal., 200 (1986) 127. [8] B. Wang, J. Tang and F. Wang, Synth. Met, 18 (1987) 323. [9] C.D. Batich, H.A. Laitinen and H.C. Zhou, J. Electrochem. Soc., 137 (1990) 883. [10] A.G. MacDiarmid, S.L. Hu and L.D. Nanayakkara, Mol. Cryst. Liq. Cryst., 121 (1985) 187. [11] J.P. Travers, F. Genond, C. Menerdo and M. Nechtschen, Synth. Met., 35 (1990) 159. [12] S.K. Manohar, A.G. MacDiarmid and A.J. Epstein, Synth. Met., 41– 43 (1991) 711. [13] C.D. Batich, H.A. Laitinen and H.C. Zhou, J. Electrochem. Soc., 137 (1990) 883. [14] A. Amirudin and D. Thierry, Br. Corros. J., 30 (1995) 128. [15] M.M. Musiani, Electrochim. Acta, 35 (1990) 1665. [16] A.G. Green and A.E. Woodhead, J. Chem. Soc., 97 (1910) 2399; 101 (1912) 1117. [17] A.G. MacDiarmid, J.C. Chiang, M. Halpern and W.S. Huang, Mol. Cryst. Liq. Cryst., 121 (1985) 173.
Journal: SYNMET (Synthetic Metals)
Article: 4994
Corrosion protection of mild steel by electroactive polyaniline coatings P. Li, T.C. Tan, J.Y. Lee U Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119265, Singapore Revised 11 March 1997; accepted 13 March 1997
Abstract The ability of polyaniline (PANi) to act as a protective coating for mild steel corrosion in saline and acid was investigated by electrochemical impedance spectroscopy. The impedance behaviour is best explained by a mediated redox reaction in which PANi passivates the metal surface and reoxidizes itself by dissolved oxygen. The effectiveness of such a process, which also provides the repassivation of damaged films, is greater in acids. The performance of PANi is further enhanced by the presence of a top coat to increase the diffusional resistance for the corrosion species. Keywords: Polyaniline and derivatives; Corrosion protection; A.c. impedance; Passivation; Anti-corrosion coating
1. Introduction
Organic coatings have long been used to protect metals against corrosion. The primary effect of an organic coating is to act as a physical barrier against aggressive species such as O2 and Hq. However, all organic coatings are not permanently impenetrable and, once there are defects in the coatings, pathways will be created for the corrosive species to reach the substrate, and localized corrosion will occur. Therefore, as a second line of defence, anticorrosive pigments are added to the coating. These pigments may protect by a physicochemical or an electrochemical as well as an ion exchange mechanism. Pigments which protect by physicochemical mechanisms generally have a lamellar, flaky, or plate-like shape, which greatly increases the length of the diffusion pathways for oxygen and water and decreases the permeability of the coating. Examples of such pigments are micaceous iron oxide and aluminium flakes. 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, anodic process or both. Examples of this type of pigment are red lead and zinc chromate. Pigments that protect by an ion exchange mechanism consist of ion exchangers that hinder the transport of Cly and Feqq to the substrate. Kinlen and Silverman [1] reported that annealed perfluorinated cation exchange polyU
Corresponding author. Tel.: q65 772 2186; fax: q65 779 1936; email: [email protected]
mers have unique anion rejection properties which could minimize localized corrosion due to Cly ions. Recently, there have been attempts to use electroactive polymers to control pitting corrosion resulting from the permeation and breakdown of the protective coating. The electroactive polymer can be applied singly or in conjunction with other surface treatments. DeBerry [2] reported that polyaniline (PANi) electrochemically deposited on stainless steel could provide a form of anodic protection that significantly reduces corrosion rates in acid solutions. Ahmad and MacDiarmid [3] noticed that PANi in the emeraldine oxidation state has adequate oxidation power to passivate stainless steel. In the study of Lu et al. [4] on the corrosion protection of mild steel by coatings containing PANi, the surface could be easily repassivated by PANi even when scratches existed in the coatings. Wessling [5] also reported that mild steel, stainless steel and copper could be passivated by repeatedly dipping clean surfaces of the metals in dispersions of doped PANi. Passivation was found to occur by the formation of metal oxide films through the metal contact with PANi. Removal of PANi had exposed a grey metal surface with persistent passivated behaviour [5]. There are two methods to deposit an electroactive polymer on the metal surface, namely, electrochemically or chemically. From the viewpoint of application convenience, electrochemical deposition is cumbersome and virtually impossible on large structures such as ships, bridges and pipelines. In these cases chemical deposition is the only feasible alternative. Electroactive polymer coating is different from conventional coating in that it does not protect simply
0379-6779/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 3 8 6 0 - 5
Journal: SYNMET (Synthetic Metals)
Article: 4994
238
P. Li et al. / Synthetic Metals 88 (1997) 237–242
state and to ensure that no blistering occurred during the conditioning period. All experiments were carried out at a laboratory temperature of 25"1 8C. All raw data were replicated three times to ensure their reproducibility and statistical significance.
by offering a physical barrier. Although there are suggestions to attribute the repassivation in the presence of an electroactive polymer to redox reactions between the polymer and metal, the origin of such effects has not been ascertained. These intriguing findings have prompted us to investigate and characterize corrosion protection with coatings containing electroactive polymers by electrochemical impedance spectroscopy (EIS).
3. Results and discussion
2. Experimental
3.1. Impedance characteristics of PANi coatings in saline and HCl
2.1. Sample preparation and corrosion conditions The working electrodes were cylindrical discs cut from a carbon steel rod with the following composition: C: 0.18%, Si: 0.25%, Mn: 0.71%, P: 0.012%, S: 0.013%. Each disc was pressure fitted into a Teflon holder, leaving only 0.5 cm2 of the surface area exposed to the testing environment. The working electrodes were mechanically abraded with a series of emery papers ending with 1200 grade, followed by thorough rinsing in acetone and deionized water, and drying in air. The PANi coating was a proprietary dispersion supplied by Zipperling. It comes in the form of a corrosion repair kit consisting of a primer for rust removal (Correpair I), an active PANi coating (Correpair II), and a protective overcoat (Correpair III). Only Correpair II and Correpair III were used in this work and they were applied either singly or jointly to clean working electrodes. The corrosion resistances of coatings were tested in 1 M HCl and 1 M NaCl. Some PANi coated samples also had their coatings removed by N-methylpyrrolidone (NMP) after 1 day and testing was continued for the exposed surface.
Figs. 1 and 2 show the Nyquist impedance plots of PANicoated mild steel after 6 h either in 1 M HCl or 1 M NaCl, respectively. While two depressed semicircles are obviously present in Fig. 1, only one is found in Fig. 2. The difference in impedance behaviour can largely be attributed to the electroactivity of PANi in different environments. Both the conductivity and electroactivity of PANi have been known to depend on the extent of protonation of the imine nitrogen sites of the polymer [6–9]. The dramatic increase in electrical conductivity of PANi upon protonation was first reported by MacDiarmid et al. [10] and observed by many others [11]. The proton-induced insulator-to-conductor transition, generally known as ‘protonic acid doping’, arises from the formation of polysemiquinone radical cations which contribute collectively to a half-filled polaron conduction band [12]. UV–Vis spectroscopy has shown that protonation is reversible [13]. The electroactivity of the polymer is benefited by the conductivity increase due to protonation [10].
2.2. Electrochemical measurements EIS was used to characterize the corrosion of mild steel in the presence of a PANi coating. The corrosion cell consisted of a three-electrode system. A platinum electrode and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The reference electrode was placed in a separate cell and was connected to the corrosion cell through a salt bridge with a Luggin capillary terminal in the corrosion cell. The corrosion media were 1 M HCl and 1 M NaCl diluted from AR grade chemicals with deionized water. For a.c. impedance measurements, an EG&G model 273 potentiostat/galvanostat was connected to an EG&G model 2310 lock-in amplifier and the ensemble was placed under the control of the EG&G model 388 software system. Impedance measurements were performed in the frequency range of 100 K–0.05 Hz with an a.c. modulation of amplitude "10 mV about the rest potential. The working electrode was stabilized in the test environment for 6 h before the impedance run. This served to put the electrode in a reproducible initial
Fig. 1. Nyquist plot of PANi-coated mild steel in 1 M HCl for 6 h.
Fig. 2. Nyquist plot of PANi-coated mild steel in 1 M NaCl for 6 h.
Journal: SYNMET (Synthetic Metals)
Article: 4994
P. Li et al. / Synthetic Metals 88 (1997) 237–242
Fig. 3. General equivalent circuit representation for a polymer-coated metal with blistering.
Fig. 4. Equivalent circuit of an insulating coating without blistering.
239
associated with the thickness of the coating. Rt, Cdl jointly represent the electrochemistry of corrosion at the coating/ metal interface after coating penetration by the corrosion species. Zw is the Warburg impedance characteristic of the transport of corrosion species to the metal surface. For an impervious insulating coating, the charge transfer components Rt, Cdl and the Warburg impedance Zw are non-existent, and the equivalent circuit is reduced to a parallel arrangement of Rc and Cc, which gives a semicircle in the Nyquist plot (Fig. 4). Without additional experimental findings Fig. 2 would appear to fit this description aptly. A Nyquist plot of two semicircles such as that of Fig. 1 is quite common among charge transfer reactions mediated by the presence of a redox polymer on the electrode [15], and can be represented by the equivalent circuit of Fig. 5. Here the high and low frequency semicircles can be assigned to charge transfer reactions at the metal/polymer interface and the polymer/solution interface, respectively. As no blistering is observed experimentally in this case, the high frequency semicircle may correspond to the repassivation of a scratched metal surface by PANi. PANi is reduced in the process and its reoxidation by a suitable cathodic depolarizer in the environment (notably dissolved oxygen) is represented by the low frequency semicircle. The details of the repassivation mechanism will be discussed later. The charge transfer resistances and the double-layer capacitance of these two reactions are represented by Rt1, C1 and Rt2, C2 respectively in the equivalent circuit. C is a capacitance due to the thickness of PANi coating. Although PANi is reasonably conductive in acid solutions, some limitations may still arise from its electrical resistance and contribute towards the observed value of R1. 3.2. The post-treatment effects of PANi on mild steel surface
Fig. 5. Equivalent circuit of an electroactive coating without blistering.
In EIS investigations, the opinion is near unanimous that a polymer-coated metal with blisters can be represented by the simple equivalent circuit of Fig. 3 [14]. In this figure Ru is the uncompensated solution resistance, Rc is the coating resistance that decreases with polymer conductivity and the penetration of coating by an electrolyte. Cc is the capacitance
EIS was also used to examine the activity difference between mild steel surface before and after PANi treatment. To this end three samples were prepared. The first sample (a) was pristine mild steel without PANi coating. The second sample (b) was mild steel coated with PANi which was then exposed to 1 M NaCl for 24 h. The PANi coating was subsequently removed by soaking in NMP. The last sample (c) was mild steel coated with PANi and exposed to 1 M HCl for 24 h, followed by the removal of PANi coating in NMP. The corrosion of these samples in 1 M HCl for various lengths of time was followed by EIS. Fig. 6 shows the Nyquist plots of the three samples after 48 h in 1 M HCl. The charge transfer resistances of samples a, b and c as determined by the intersection of the low frequency end of the semicircle arc with the real axis are 50, 120 and 240 ohm cm2, respectively. The results clearly demonstrate that the sample, coated with PANi and treated in HCl to render the polymer conductive and electroactive, has the greatest corrosion resistance after PANi removal. The steel surface was in a passivated state after being in contact with
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Fe|Feqqq2e
PANi removal by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy. Their results showed the presence of a multi-layered oxide structure on the metal surface consisting of predominantly g-Fe2O3 on the exterior and Fe3O4 in the sub-surface region. Scanning electron microscopy (SEM) examination of the surface after the individual steps of PANi treatment also verified the complex multi-step nature of passivation [6]. The first step of interaction between PANi and iron after coating appears to be an etching process which removes a few microns of iron. Passivating iron oxide is then formed in subsequent oxidation of the fresh iron surface. Elemental analysis by EDAX showed that a pure iron surface contains virtually no oxygen, whereas the freshly etched area is covered with only a very thin oxide layer 10–20 nm in thickness. High oxygen content is only limited to the passivated areas [6]. The PANi-mediated oxidation of steel continues until a stable passive oxide film is formed on the metal surface. In the event of mechanical damage to the film, the PANi remaining on the surface restarts the aforementioned mediated redox process, effectively repassivating the exposed surface and regenerating itself to the emeraldine oxidation state ready for the next remedial action. Lu et al. [4] reported that PANi could repair damage within a diameter of 1.2 mm. In this way the steel surface is continuously protected until PANi is depleted by some irreversible means. The a.c. impedance results presented previously show that the sample coated with PANi and treated in NaCl also passivates the steel surface to some extent, although not as effectively as the sample coated with PANi and treated in HCl. Slow PANi redox in neutral media is largely accountable for the ineffectiveness of the mediated oxidation of metal in this case. Corrosion is also inhibited by the physical barrier in the presence of the polymer on metal surface which blocks the approach of corrosion species. EIS of samples after PANi removal therefore confirms the reactivity of PANi in passivation, although its relative contribution with respect to the physical barrier effects could not be easily resolved from the EIS data.
2Hqq2e°H2
3.3. Anti-corrosion performance of PANi coatings
Fig. 6. Nyquist plot of three mild steel samples in 1 M HCl for 48 h: (a) without PANi coating; (b) coated with PANi and treated in 1 M NaCl for 1 day before PANi removal; (c) coated with PANi and treated in 1 M HCl for 1 day before PANi removal.
PANi. The versatility of PANi redox is largely responsible for such behaviour. Ideal PANi is a linear chain polymer consisting of quinoid and benzenoid repeating units. Different combinations of quinoid and benzenoid sequences result in various oxidation states for the polymer [16]. During oxidation the benzenoid units are progressively converted into quinoid units. The asdeposited polymer is in the emeraldine (EM) oxidation state, which is midway between the fully reduced state of leucoemeraldine and the fully oxidized state of pernigraniline, and consisting of an equal number of alternating benzenoid and quinoid units. Interconversions of these various oxidation states either in the base or protonated forms have been described schematically by MacDiarmid et al. [17]. The interconversion between the fully reduced leucoemeraldine oxidation state and the emeraldine oxidation state is rapid and reversible in aqueous acids. This is the primary redox reaction that drives the repassivation of steel. In acid solutions containing dissolved oxygen, the anodic reaction of metal dissolution is counterbalanced by a hydrogen evolution reaction or dissolved oxygen reduction as protons and dissolved oxygen are the most common cathodic depolarizers in aqueous corrosion:
q
O2q4H q4e°H2O Oxygen is unable to passivate metals under acidic conditions due to the solubility of the passive film in acids. Passivation is however a possibility in the presence of PANi. According to Wessling’s reaction scheme [5], the redox potential of the emeraldine–leucoemeraldine transition is adequate to bootstrap the oxidation of iron to the passivated state. PANi is reduced to leucoemeraldine in due course but can be reoxidized rapidly to emeraldine by dissolved oxygen. The process when examined in its entirety reveals the role of PANi as a redox mediator in the indirect oxidation of mild steel by oxygen. Protons in the environment are bounded to the polymer and hence are not readily available for oxide film dissolution. Lu et al. [4] examined the steel surface after
The Correpair kit contains a top coat (Correpair III) to be applied on top of PANi. PANi without the top coat blistered in 1 M HCl in 2 days, while the top coat alone was able to withstand corrosion in the same medium up to 5 days before any blistering. However, the performance of the top coat dwindled badly when mechanical damage was introduced to the coating. PANi in comparison was able to repair the damage to some extent by the mechanism outlined in previous sections. The top coat therefore acts simply as a physical barrier and protects by substrate isolation. Quite to our surprise, PANi with the top coat has the best corrosion resistance, and blistering only occurred after 20 days. Some synergism between PANi and the top coat was probably at work to produce a result better than the sum of the components.
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Fig. 7. Nyquist plots of mild steel coated with (a) PANi, (b) insulating top coat alone, and (c) PANi with the insulating top coat; in 1 M HCl for 6 h.
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impedance attests to the occurrence of redox reactions within the composite film. However, the finite length effect was not observable under the experimental conditions. If PANi were simple overlaid by the top coat, it would have behaved like an insulator layer and the equivalent circuit of Fig. 8 would predict an impedance spectrum different from that observed. It is therefore hypothesized that the presence of top coat does not quarantine the corrosion reaction by isolation, but rather lengthens the diffusion path of the corrosion species to lower the net corrosion rate at the metal/polymer surface. A possible equivalent circuit representation is given in Fig. 9. The situation is comparable, though not identical, to the efforts of adding inert materials or inhibitors to the coating to increase corrosion resistance. The presence of PANi has added the advantage to provide an in situ repair facility for damage in the passivating film through the mediated redox process described earlier. Similar results were also obtained for corrosion in 1 M NaCl. The Nyquist plots of PANi, the top coat, and PANi with top coat are again shown in a composite plot (Fig. 10). The corrosion resistance of PANi, as represented by one depressed semicircle, is virtually invisible in the figure at
Fig. 8. A suggested model of non-interacting layers of electroactive polymer and top coat and its Nyquist plot.
The Nyquist impedance plots of the PANi layer, the top coat, and the PANi layer with top coat after 6 h in 1 M HCl are overlaid in Fig. 7. Porosity in the PANi layer enables the penetration of free protons which may dissolve the passivating layer at a rate faster than PANi is able to repassivate the surface, leading to gradual corrosion after some time. The impedance of the top coat is a near-perfect semicircle typical of an impervious insulating coating. Its equivalent circuit is a simple parallel arrangement of film resistance and capacitance whose values can be determined from the intersection of the semicircle with the real axis at the low frequency end, and the maximum point on the circle respectively. The impedance plot of PANi with the top coat shows the classical behaviour of a charge transfer reaction rate limited by the diffusion of corrosion species at low frequencies. The presence of a straight line of unit slope indicative of the Warburg
Fig. 9. A suggested model of interacting layers of electroactive polymer and top coat and its Nyquist plot.
Fig. 10. Nyquist plots of mild steel coated with (a) PANi coating, (b) insulating top coat, and (c) PANi with the insulating top coat; in 1 M NaCl for 6 h.
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such a scale. Protection by isolation is definitely more aptly done by the top coat, which shows a bigger semicircular arc than the impedance plot of PANi alone. The impedance plot of mild steel coated with PANi and the top coat in sequence shows again the characteristics of a corrosion reaction inhibited by mass transfer limitations at low frequencies. The duplex coating therefore does not behave as if it were a simple overlay of two non-interacting layers (cf. Fig. 8). We believe that mutual interactions have led to an increase in the diffusional resistance for the corrosion species. An equivalent circuit representation is given in Fig. 9. The repassivation facility due to PANi redox should still be available, albeit operating less effectively because the PANi layer was not previously protonated. The corrosion resistance was nevertheless enhanced greater than what is expected from a simple linear sum of individual resistances. The origin of synergism between PANi and the top coat is not clear and should be worthy of further investigation. 4. Conclusions Mild steel coated with PANi shows different corrosion behaviour in 1 M HCl and 1 M NaCl. The Nyquist impedance plots for corrosion in acids are best interpreted by PANi acting as a redox mediator, passivating the metal at the metal/polymer interface and reoxidizing itself by dissolved oxygen at the polymer/solution interface. The tortuosity of the polymer and the affinity of the polymer for protons also reduce the dissolution of the passive film in acids. The electroactivity of PANi is reduced in the neutral medium of NaCl, and protection is primarily by the isolation of metal from the corrosion species. Nevertheless, the mediated redox reaction is still at play. Although PANi layers by themselves are not as impervious as some of the conventional insulating coatings, the
mediated redox reaction enables PANi to repassivate effectively an inadvertently exposed metal surface due to scratches or other mechanical damage in the passive film; a repair facility not provided by conventional coatings. The PANi performance can be further enhanced by the presence of a top coat to increase the diffusional pathway for the corrosion species. There are also experimental indications that synergism may exist between PANi and the top coat.
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Article: 4994