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TiO2 composite films as corrosion protection of mild steel
Journal of Electroanalytical Chemistry 540 (2003) 35 /44 www.elsevier.com/locate/jelechem
Application of polypyrrole/TiO2 composite films as corrosion protection of mild steel Denise M. Lenz a, Michel Delamar b, Carlos A. Ferreira a, a
Department of Materials, Escola de Engenharia, LAPOL-PPGEM-Universidade Federal do Rio Grande do Sul, Av. Osvaldo Aranha, 99, sala 702, 90035-190 Porto Alegre, RS, Brazil b ITODYS-Universite´ Paris 7 Denis Diderot, 1, Rue Guy de la Brosse, 75005 Paris, France Received 16 November 2001; received in revised form 2 September 2002; accepted 27 September 2002
Abstract This work is a study of the anticorrosion protection of polypyrrole (PPy) on AISI 1010 steel (mild steel) by the incorporation of TiO2 pigment into the PPy matrix during electrochemical synthesis. The influence of parameters such as stirring, concentration, pH and nature of the electrolyte were investigated. The degree of pigment incorporation into the polymeric matrix with respect to time and current densities of the electrodeposition process were also investigated. The influence on the current efficiencies of the process was evaluated. The morphology of the composite film was studied by scanning electron microscopy (SEM) and the distribution of the pigment within the polymeric matrix by X-ray photoelectron spectroscopy (XPS). The PPy/TiO2 composite showed a considerable improvement in anticorrosion properties with respect to PPy films after being submitted to salt spray and weight loss tests. These composite films can be applied as a primary coating replacing the phosphatized layers on mild steel. # 2002 Published by Elsevier Science B.V. Keywords: Polypyrrole; Titanium dioxide; Composite films; Anticorrosion protection
1. Introduction Organic coatings on metals are not perfect physical barriers against corrosion when they are in contact with aggressive species such as O2 and H . These corrosive species can reach the substrate via coating defects. Electrically conductive polymers (ECP), for example polypyrrole (PPy) and polyaniline (PANI), are capable of providing corrosion protection. This protection involves the use of chemical and electrochemical techniques such as chemical inhibitors, cathodic and anodic protections. They are thus materials of considerable practical interest for corrosion inhibition and many papers on this subject have been published [1 /13]. The corrosion inhibition on steels using ECP coatings, mainly as primary coatings, is partly due to the formation of passivating stable oxide films such as Fe3O4, a-Fe2O3 and g-Fe2O3 at the metal j ECP interface [3,9]. The oxide layers are dense and act as a Corresponding author. E-mail address: [email protected] (C.A. Ferreira).
physical barrier that protect the metal surface from further corrosion [3]. Pigments can be added to organic coatings in order to improve their protection. The pigments may protect the metal by physicochemical (barrier mechanism), electrochemical or ion exchange mechanisms [14]. The combined effects of pigments and appropriate binders in such coatings should be studied in order to improve their anticorrosion properties. The incorporation of minerals into a PPy matrix with the aim of changing or improving its properties has been reported [15 /19]. Yoneyama and coworkers [15 /18] were the first to attempt the codeposition of TiO2 and WO3 in a PPy matrix using inert electrodes made of Au or ITO in an electrolytic system. This was done without stirring and using a suspension with a pH value greater than the isoelectric point of the oxides. Oxides are negatively charged in this method and are attracted to the positively charged PPy. An oxide incorporation into the PPy matrix of up to 1.15% by weight was achieved. Beck et al. [19] have shown that strong convection is an important factor for the incorporation of TiO2 into
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PPy. The use of strong convection gives up to 17% TiO2 by weight when using platinum or stainless steel as working electrodes (WE) in a 0.1 M LiClO4 aqueous medium. Previous studies [20] have shown that TiO2 incorporation into a PPy matrix during electrochemical synthesis on AISI 1010 steel under magnetic stirring leads to the formation of PPy/TiO2 composite films containing up to 6.5% TiO2 by weight. These composite films show a slight improvement in the PPy corrosion protection of mild steel. Further studies of TiO2 incorporation onto mild steel during electrochemical synthesis of PPy using a stronger electrolyte convection with looping to increase TiO2 concentration into PPy are shown in this work. The mild steel coated with the PPy/TiO2 composite films were then submitted to corrosion tests.
2. Experimental 2.1. Electrochemical synthesis AISI 1010 steel sheets (100 /20 /1 mm) were used as WEs. They were degreased with an industrial alkaline solution (DUROCLEAN 107) at 80 8C for 5 min followed by a treatment with 10% HNO3 [13]. Pyrrole (Aldrich) was bidistilled under an N2 atmosphere. The electrolytes used were 0.1 M oxalic acid and 0.1 M KNO3. TMDD (2,4,7,9-tetramethyl-5-decyne-4,7 diol, 2 /103 M) was added to the electrolyte medium in order to improve electrode wetting by the electrolyte solution and avoid dendritic growth of the polymer. The average particle size of the TiO2 pigment (rutile, Merck) was 0.76 mm and its solubility in aqueous medium is about 1% by weight. Its specific gravity is 4.2 g cm 3 and PPy average specific gravity is around 1.5 g cm 3 [2,10]. A rectangular 110 /85 /50 mm3 acrylic cell was used for the experiments without electrolyte stirring. Two stainless steel sheets and a saturated calomel electrode (SCE) were also used as counter (CE) and reference electrodes (RE), respectively. The electrodes were positioned symmetrically 20 mm from the WE. For experiments with electrolyte stirring a loop system, similar to the one described by Beck et al. [19] was employed. A stationary sheet anode was fixed between two stainless steel CEs (Fig. 1). An Ag/AgCl (/0.034 V vs. SCE) wire was used as a RE, as explained elsewhere [21], which was positioned exactly behind the sheet anode. The suspension was pumped around a loop with the aid of a PSM suction pump which can produce flow rates from 1 to 15 ml s 1. All syntheses were performed galvanostatically at room temperature onto an exposed area of 12 cm2 on each side of the sheet.
Fig. 1. Representation of the electrolytic cell with looping stirring. B, buffer volume; V, valve; and P, pump.
2.2. TiO2 concentration in the PPy/TiO2 composite film After degradation of the composite films in hot concentrated sulphuric acid, the titanium content was quantitatively analysed by ultra-violet spectroscopy. A JASCO 7800 spectrophotometer was used at 420 nm according to the colorimetric method [22]. Plasma spectroscopy (ICP) using an ARL 3410 spectrophotometer at 323.452 nm was also used. The iron content in the same solution was measured at 510 nm. For the X-ray photoelectron spectroscopy (XPS) experiments an SSI X100 photoelectron spectrometer with an aluminum monochromatic source (Al /Ka radiation 1486.6 eV) was used. XPS was carried out using a spot size of 600 mm and a vacuum of 10 9 Torr (1.33/107 Pa). An analysis angle of about 908 with respect to the sample surface was used. Ar ion bombardment was applied to remove thin layers of the composite film using an AG2 ion gun supplied by Vacuum Generators. The conditions of bombardment were an acceleration voltage of 4.8 kV, an Ar pressure of 5 /106 mbar and 200 mA of ion current. The sputtering rate was approximately 2.5 mm per h, as estimated from calculated and measured thicknesses of the composite films. The composite film thicknesses were measured using a FISCHER model DUALSCOPE† MP 20 coating thickness metre. The SEM experiments were performed with a PHILIPS XL20 scanning electron microscope. 2.3. Anticorrosion protection of the PPy/TiO2 composite films Salt spray corrosion tests (ASTM B 117) were carried out in a test chamber (BASS EQUIPAMENTOS) using 6 /2 cm samples (only one side of the sheet). The weight loss tests were performed by dipping samples of PPy and PPy/TiO2 films into a 3.5% NaCl solution. The exposed electrode area was 4 cm2. The amount of iron released from the samples was measured as a function of
D.M. Lenz et al. / Journal of Electroanalytical Chemistry 540 (2003) 35 /44
immersion time using a JASCO 7800 UV/vis spectrophotometer at 510 nm using the o-phenanthroline technique. All exposed electrode areas were delimited by an acrylic-thermoplastic paint layer of approximately 0.5 mm thick. In this work, all experiments were performed twice, except weight loss tests which were performed three times. An average of the results is given.
3. Results and discussion The electrodeposition of conducting polymers onto oxidizable metals is indeed not easy because thermodynamic data predict that the metal will dissolve before the electropolymerization potential of the monomer is reached. Thus to achieve the deposition of a conducting polymer on mild steel it is necessary to find the electrochemical conditions which lead to a partial passivation of the metal and decrease its dissolution rate without preventing electropolymerization [6,7,12,13]. In addition, if a pigment is to be codeposited with PPy, it must not adversly interfere with the pyrrole polymerization reaction. 3.1. KNO3 medium (pH 7.5 /8) The influence of selected parameters on TiO2 incorporation was investigated using a potassium nitrate medium. 3.1.1. Electrolyte stirring As described in previous experiments carried out with TiO2 [19,20] there was a negligible incorporation of pigment (without stirring of the electrolytic medium) as well as pigment sedimentation. A strong convection achieved with the maximum flow rate setting of the pump (15 ml s 1) was better for pigment incorporation. Table 1 shows an increase in TiO2 concentration in the PPy matrix (cc) when the solution is stirred with small TiO2 electrolyte concentrations (200 mg l1). Table 1 TiO2 concentration in PPy (cc) with and without electrolyte stirring in 0.1 M pyrrole, 0.1 M KNO3, 2.3/10 3 M TMDD, TiO2 electrolyte concentration: 200 mg l 1, polymerization time: 10 min j /mA cm 2
2 5 7 10 15 20
Without strring
With stirring
cc/% TiO2
cc/% TiO2
0.1 0.2 0.4 0.1 0.1 0.1
0.5 / 0.4 / / 0.4
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Caution must be taken with strong convection (above 15 ml s 1) because it could repel TiO2 particles from the anode. Table 1 also shows that the effect of the current density on the incorporation of TiO2 in PPy is more pronounced when no stirring is performed. Current densities in the range of 5 /7 mA cm 2 favour incorporation, however higher current efficiencies were found in the 2/5 mA cm 2 range [13,20]. For this reason 5 mA cm 2 was used for all further experiments using the KNO3 electrolyte. The influence of stirring was also analysed using different experimental conditions. The polymerization time was increased to 20 min and the electrolyte pigment concentration (ce) to 50 and 100 g l 1. This led to the formation of PPy/TiO2 composite films which were approximately 18 mm thick. The results are shown in Table 2. Thus, stirring keeps the TiO2 particles in suspension and causes the particles to be continuously in contact with the anode surface where the composite film is being deposited. The results confirm that this incorporation is greatly facilitated by mechanical stirring. Indeed, a very small quantity of TiO2 was observed in the composite films synthesized in the KNO3 electrolyte without stirring even using pH values higher than the TiO2 isoelectric point */between 6.0 and 6.7 in aqueous medium [23] */compared to the same medium with stirring. The use of looping stirring and the presence of TiO2 did not alter the current efficiency (about 90%) for the composite film synthesis. Similar results without stirring have been obtained in previous experiments [20] and by Schirmeisen and Beck [2] for PPy synthesis in NO 3 aqueous medium with slow stirring. It is important to emphasize that only PPy mass is considered in the current efficiency calculation and TiO2 concentration is neglected. Thus, PPy deposition onto mild steel is governed only by charge transfer mechanisms. 3.1.2. Polymerization time Table 3 shows that for small TiO2 concentrations in the electrolyte there are no significant changes in TiO2 Table 2 TiO2 concentration (cc) in PPy using higher TiO2 electrolyte concentrations (ce) ce/g l 1
50 100
With strring
Without strring
cc/%
cc/%
6 9.2
B/1 3.1
Electrolyte composition: 0.1 M pyrrole, 0.1 M KNO3 and 2.3/ 10 3 M surfactant TMDD, j/5 mA cm 2, polymerization time: 20 min
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Table 3 Influence of polymerization time on TiO2 incorporation with electrolyte stirring j /mA cm 2
5 10 20 5 5 5
Ce/g l 1
0.2 0.2 0.2 0.4 8 50
Polymerization time 10 min
20 min
cc/%TiO2
cc/%TiO2
0.5 0.9 0.4 1.5 0.8 1.5
0.8 1.5 0.7 2.5 6.3 6
Electrolyte composition: 0.1 M pyrrole, 0.1 M KNO3, 2.3/10 3 M TMDD.
incorporation. However, for higher TiO2 quantities in the electrolyte, the change in polymerization time from 10 to 20 min results in a remarkable difference in TiO2 incorporation into the PPy matrix. Composite films synthesized over 10 min using 50 g l 1 of pigment in the electrolyte showed an incorporation of 1.5% by weight, while 6% by weight of TiO2 was found in composite films synthesized over 20 min. TiO2 particles should be distributed in an approximately uniform way within a PPy matrix when small quantities of TiO2 are present in the electrolyte. This is because a twofold increase in polymerization time causes a similar increase in TiO2 concentration in the PPy matrix. However, for higher TiO2 concentrations in the electrolyte (above 1 g l 1) the particles should mainly be in the upper half of the composite film. This is because the TiO2 concentration in the composite in-
creases more than twice when the polymerization time is increased from 10 to 20 min. Thus, TiO2 particles are preferentially incorporated when there is a polymer layer of considerable thickness on the metallic surface. This is due to an increase in polymer surface roughness with a respective increase in polymer thickness as reported by Dahlhaus and Beck [24]. Polymerizations carried out with longer times did not show an increase in TiO2 incorporation. 3.1.3. Electrolyte concentration Fig. 2 shows a typical chronopotentiometric curve in a 0.1 M KNO3 medium. It is remarkable that the potential increases quickly, so polymerization begins instantaneously and the potential stabilizes at 1.2 V. This behaviour is typical and independent of TiO2 electrolyte concentration indicating that the pigment does not interfere with the PPy polymerization mechanism. With low concentration of KNO3 electrolyte (ca. 103 M), the potential rapidly increased to above 5 V (current 5 mA cm 2), leading to polymer overoxidation due to the high electrolyte resistivity [25]. In spite of the high TiO2 incorporation into PPy (around 18% by weight) the composite films are porous. The process has a low current efficiency, i.e. less than 50%. In the absence of TMDD the incorporation decreases to 8%. Thus, TMDD promotes electrode wetting and probably favours TiO2 incorporation by superficial tension diminution at the electrode j solution interface. 3.1.4. TiO2 electrolyte concentration The experiments were carried out in a range of 200 mg l 1 to 100 g l1 of TiO2. The results are shown in Fig. 3.
Fig. 2. Chronopotentiometric curve of composite film synthesis in 0.1 M KNO3 medium using 0.1 M pyrrole and 2.3/10 3 M TMDD, j/5 mA cm 2.
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Fig. 3. Relationship between TiO2 concentration in PPy/TiO2 composite film (cc) and its concentration in the electrolyte (ce). Electrolyte: 0.1 M KNO3, 0.1 M pyrrole and 2.3/10 3 M TMDD, j/ 5 mA cm 2 and t /20 min.
The maximum TiO2 concentration in the composite film from the KNO3 medium containing 100 g l 1 of TiO2 in the electrolyte was around 7%. However, it is also possible to obtain a similar composition using 10 g l 1 of TiO2 in the electrolyte because of its saturation within the PPy matrix, as shown by Beck and Dahlhaus [26]. 3.2. H2C2O4 medium (pH 2) In this medium a polymerization time of 30 min was used with a current density of 5 mA cm2 and a monomer concentration of 0.1 M pyrrole. This was
related to previous work that used magnetic stirring [20]. Using different current densities than 5 mA cm 2 in H2C2O4 medium, the TiO2 concentration in the composite film and the current efficiency will be lower. However, even for 5 mA cm 2, the current efficiency in this medium was lower than in KNO3 (around 66%) due to the presence of an induction time during which only iron corrosion and an iron oxalate layer formation occurred [10]. The induction time is illustrated in Fig. 4. After the first 2 min the potential stabilizes at /0.25 V and iron corrosion takes place. After this period pyrrole polymerization starts and the potential stabilizes at around
Fig. 4. Chronopotentiometric curve of composite film synthesis in 0.1 M oxalic acid medium using 0.1 M pyrrole, j/5 mA cm 2.
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1.0 V. The thickness of the PPy/TiO2 composite film synthesized in H2C2O4 medium under these conditions was around 25 mm. The influence of polymerization parameters on TiO2 incorporation was also investigated. 3.2.1. TiO2 electrolyte concentration Experiments with 0.1 M oxalic acid were carried out in electrolytes that contained from 200 mg l 1 to 50 g l 1 of TiO2 (Fig. 5). It is possible to incorporate around 14 wt.% of TiO2 in a composite with a concentration slightly greater than 10 g l1 of pigment in the electrolyte. Above this concentration TiO2 incorporation no longer increases. The same was observed in the KNO3 medium. 3.2.2. Electrolyte pH When the pH value of the oxalic acid medium was increased to 4 the TiO2 incorporation was found to be similar to that at pH 2 (Fig. 4). There was no marked difference in the current efficiency when the pH was changed. Experiments carried out with 0.1 M sodium oxalate solution (pH around 6) gave a similar percentage of TiO2 incorporation into the PPy matrix as for oxalic acid under the same experimental conditions. However, the current efficiency decreased and as a consequence the films had a lower thickness. The results showed that the use of oxalic acid as an electrolytic medium promotes the formation of a PPy/ TiO2 composite film that is visibly smoother and
brighter. The film also has a greater TiO2 incorporation and a stronger adhesion (Gr0 Grade, as DIN 53151) than PPy/TiO2 composite films synthesized in a KNO3 medium. The low roughness and high adhesion of the composite films obtained in this medium, as compared to those obtained in a KNO3 medium, could be due to the formation of a thin layer of iron oxalate during the induction time. Iron oxalate may provide favourable nucleation sites for electrocrystallization of PPy. PPy grows as microhemispheres and TiO2 deposits in the valleys or summits leading to smooth films [10]. Thus, the organized growth of PPy in a H2C2O4 medium is more significant than polymeric roughness found in PPy synthesized in a KNO3 medium for TiO2 incorporation.
3.3. Surface analysis by XPS and SEM XPS and SEM analysis were carried out on the PPy/ TiO2 composite films synthesized in the oxalic acid medium which contained 14% by weight of TiO2 particles. Samples were obtained by cutting the electrode into three (2 /2 cm) parts. The signal for titanium was observed at the surface of the PPy/TiO2 composite film (459 eV for Ti2p3/2 and 465 eV for Ti2p1/2) and each the three parts displayed a constant Ti/C ratio. Because the TiO2 is uniformly distributed throughout the sample, any fraction of the sample will be representative of the sample as a whole.
Fig. 5. Relationship between TiO2 concentration in PPy/TiO2 composite film (cc) and its concentration in the electrolyte (ce) Electrolyte: 0.1 M H2C2O4 pH 2.0 and 0.1 M pyrrole, j/5 mA cm2 and t /30 min.
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Fig. 6. SEM image of the PPy/TiO2 composite film.
After approximately 2 h of Ar ion bombardment, corresponding to 5 mm of composite film removal, the amount of TiO2 was found to be slightly higher than it was at the surface of the composite. However, the TiO2 concentration was observed to decrease further below the surface of the film, as was observed after further exposure to Ar ion bombardment [27]. SEM micrographs confirmed the homogeneous distribution of TiO2 at the surface of the PPy/TiO2 composite film (see Fig. 6, which shows the pigment particles as white dots). 3.4. Corrosion tests All samples used for these tests were synthesized in 0.1 M oxalic acid (pH 2.0) and contain 14% by weight of
titanium dioxide in the PPy/TiO2 composite film. The dedoping process */polymer transition from oxidized to reduced state [5] */was performed by sample immersion after polymerization in 0.1 M NaOH applying /0.7 V (SCE) for 15 min. 3.4.1. Weight loss tests The weight loss test determined the quantity of iron released from mild steel during immersion in a 3.5% NaCl solution. Fig. 7 shows the behaviour of the steel after three different treatments: (1) as degreased, (2) coated by a dedoped PPy film and (3) coated by a dedoped PPy/TiO2 composite film. With degreased steel iron oxidation takes place quickly when in contact with NaCl solution. This results in the formation of a thin layer of iron oxides and
Fig. 7. Iron concentration (mg Fe l1) as a function of immersion time in 3.5% NaCl solution. j degreased 1010 steel; m PPy without TiO2; and ' PPy with TiO2.
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hydroxides. But this protection is limited because the thin layer is easily damaged and the sample then undergoes a severe corrosion process. For the steel coated by dedoped PPy/TiO2 composite films there is at first an acceleration of the oxidation process, probably due to initial iron oxidation through the pores and/or defects of the dedoped polymer film. In this case, the dedoped process did not yield a homogeneous passive layer throughout the sample. After approximately 96 h no further iron is released from the steel and the corrosion process stabilizes at 12.5 mg of iron per litre of NaCl solution. For the steel coated with dedoped PPy film, the initial oxidation rate, attributed to iron oxidation, is slightly higher than for steel coated with PPy/TiO2 composite film. Thereafter the corrosion rate is slowed down but no stabilization is observed. After 7 days the samples released 25 mg of iron per litre of NaCl solution. This behaviour can be explained by the low porosity of PPy/TiO2 composite films due to the filling of polymer pores by TiO2 particles as was observed by SEM. This may contribute to the better performance of PPy/TiO2 composite films in these tests. Some experiments were performed with doped PPy and PPy/TiO2 composite films. The weight losses were slightly greater than those for the dedoped samples; however as before the presence of TiO2 in the polymer film contributed to a decrease in the amount of the corrosion. There are several explanations for the better performance of the dedoped ECP films. Ahmad and MacDiarmid [4] have concluded that during the reduction of a chemically synthesized PANI, iron corrosion inhibition occurs by stable oxide layer formation at the metal j PANI interface. Schauer et al. [9] have reported that the polyaniline transition from the conductor to the insulator form (dedoping process) also improves the barrier properties of the conductive film. This transition also enhances the pore resistance of the coating due to the high resistance and sealing effect of the dedoped polymer.
3.4.2. Salt spray tests (ASTM B117) In spite of giving only qualitative information about the behaviour of the metal/coating system when exposed to an aggressive medium (5% NaCl at 35 8C), the salt spray test is widely used for comparing the performance of coatings. Samples of doped films were submitted to 5 and 24 h of exposure to salt spray. Some of the samples received an X-cut in order to evaluate the underlayer corrosion. After 5 h of exposure to salt spray (Fig. 8a and b), the samples showed evidence of corrosion and the attack was more severe in the samples which had the X-cut. The corroded area with an intense red colour was always
larger for the samples that did not contain TiO2 particles. For films without an X-cut, the red corrosion within the total exposed area was about 10% in electrodes coated by the PPy films (Fig. 8a) and only 0.5% for those coated by the PPy/TiO2 composite films (Fig. 8b). Fig. 8c and d show the results obtained for samples exposed for 24 h. The same trends as for 5 h were observed. As expected the PPy and PPy/TiO2 samples with the X-cut showed intense red corrosion. Samples without the X-cut showed a corroded area that was approximately 70% of the total exposed area for the PPy/TiO2 samples and 100% for the PPy samples. No blistering was found on the coating in the zone outside the region of corrosion in all samples tested. This indicates an excellent adhesion of both the PPy and PPy/TiO2 films, to the substrate.
4. Conclusion In this work we showed that it is possible to synthesize PPy/TiO2 composite films that show good characteristics for the corrosion protection of mild steel. Some parameters greatly influence the synthesis and favour pigment incorporation e.g. looping stirring (there is no significant incorporation due to electrophoresis phenomena), pigment concentration in electrolyte, electrolyte nature (oxalic acid is better than potassium nitrate medium) and the applied current density (around 5 mA cm 2). XPS experiments have shown that the pigment is located throughout the sample thickness but mainly at the superior half part of the composite film. SEM images confirm that the TiO2 particles are homogeneously distributed at the composite film surface with a probable decrease in the number of polymer pores as compared to PPy films alone. Corrosion tests such as weight loss and salt spray tests show that TiO2 significantly improves the anticorrosion properties of PPy. The quantity of iron released from the electrodes coated by PPy/TiO2 composite film showed a stabilization of the corrosion process after 96 h of immersion in 3.5% NaCl solution. The electrodes coated by PPy films showed a continuous increase in the quantity of iron released. The dedoped PPy/TiO2 samples showed a 50% reduction of the iron quantity released as compared to the dedoped PPy samples after 7 days of immersion in 3.5% NaCl solution. Salt spray tests show that the PPy/TiO2 samples without the X-cut displayed a reduction of about 95 and 30% in the corroded area as compared to the PPy samples exposed for 5 and 24 h, respectively. PPy/TiO2 composite film coatings are a feasible method for corrosion protection and can be applied as a primary coating that can replace the conventional
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Fig. 8. Salt spray corrosion test: (A) after 5 h of exposure for PPy films; (B) after 5 h of exposure for PPy/TiO2 composite film. Left, sample after exposition; Right, X-cut sample after exposition. (C) After 24 h of exposure for PPy films; and (D) after 24 h of exposure of PPy/TiO2 composite film. Left, sample before exposition; Centre, sample after exposition; Right, X-cut sample after exposition.
system of phosphatized layers on mild steel. These layers satisfy the demand for an environmentally acceptable solution.
Acknowledgements The authors are grateful to CNPq and CAPESCOFECUB (project 184/96) for their financial support.
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Application of polypyrrole/TiO2 composite films as corrosion protection of mild steel Denise M. Lenz a, Michel Delamar b, Carlos A. Ferreira a, a
Department of Materials, Escola de Engenharia, LAPOL-PPGEM-Universidade Federal do Rio Grande do Sul, Av. Osvaldo Aranha, 99, sala 702, 90035-190 Porto Alegre, RS, Brazil b ITODYS-Universite´ Paris 7 Denis Diderot, 1, Rue Guy de la Brosse, 75005 Paris, France Received 16 November 2001; received in revised form 2 September 2002; accepted 27 September 2002
Abstract This work is a study of the anticorrosion protection of polypyrrole (PPy) on AISI 1010 steel (mild steel) by the incorporation of TiO2 pigment into the PPy matrix during electrochemical synthesis. The influence of parameters such as stirring, concentration, pH and nature of the electrolyte were investigated. The degree of pigment incorporation into the polymeric matrix with respect to time and current densities of the electrodeposition process were also investigated. The influence on the current efficiencies of the process was evaluated. The morphology of the composite film was studied by scanning electron microscopy (SEM) and the distribution of the pigment within the polymeric matrix by X-ray photoelectron spectroscopy (XPS). The PPy/TiO2 composite showed a considerable improvement in anticorrosion properties with respect to PPy films after being submitted to salt spray and weight loss tests. These composite films can be applied as a primary coating replacing the phosphatized layers on mild steel. # 2002 Published by Elsevier Science B.V. Keywords: Polypyrrole; Titanium dioxide; Composite films; Anticorrosion protection
1. Introduction Organic coatings on metals are not perfect physical barriers against corrosion when they are in contact with aggressive species such as O2 and H . These corrosive species can reach the substrate via coating defects. Electrically conductive polymers (ECP), for example polypyrrole (PPy) and polyaniline (PANI), are capable of providing corrosion protection. This protection involves the use of chemical and electrochemical techniques such as chemical inhibitors, cathodic and anodic protections. They are thus materials of considerable practical interest for corrosion inhibition and many papers on this subject have been published [1 /13]. The corrosion inhibition on steels using ECP coatings, mainly as primary coatings, is partly due to the formation of passivating stable oxide films such as Fe3O4, a-Fe2O3 and g-Fe2O3 at the metal j ECP interface [3,9]. The oxide layers are dense and act as a Corresponding author. E-mail address: [email protected] (C.A. Ferreira).
physical barrier that protect the metal surface from further corrosion [3]. Pigments can be added to organic coatings in order to improve their protection. The pigments may protect the metal by physicochemical (barrier mechanism), electrochemical or ion exchange mechanisms [14]. The combined effects of pigments and appropriate binders in such coatings should be studied in order to improve their anticorrosion properties. The incorporation of minerals into a PPy matrix with the aim of changing or improving its properties has been reported [15 /19]. Yoneyama and coworkers [15 /18] were the first to attempt the codeposition of TiO2 and WO3 in a PPy matrix using inert electrodes made of Au or ITO in an electrolytic system. This was done without stirring and using a suspension with a pH value greater than the isoelectric point of the oxides. Oxides are negatively charged in this method and are attracted to the positively charged PPy. An oxide incorporation into the PPy matrix of up to 1.15% by weight was achieved. Beck et al. [19] have shown that strong convection is an important factor for the incorporation of TiO2 into
0022-0728/02/$ - see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 2 7 2 - X
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PPy. The use of strong convection gives up to 17% TiO2 by weight when using platinum or stainless steel as working electrodes (WE) in a 0.1 M LiClO4 aqueous medium. Previous studies [20] have shown that TiO2 incorporation into a PPy matrix during electrochemical synthesis on AISI 1010 steel under magnetic stirring leads to the formation of PPy/TiO2 composite films containing up to 6.5% TiO2 by weight. These composite films show a slight improvement in the PPy corrosion protection of mild steel. Further studies of TiO2 incorporation onto mild steel during electrochemical synthesis of PPy using a stronger electrolyte convection with looping to increase TiO2 concentration into PPy are shown in this work. The mild steel coated with the PPy/TiO2 composite films were then submitted to corrosion tests.
2. Experimental 2.1. Electrochemical synthesis AISI 1010 steel sheets (100 /20 /1 mm) were used as WEs. They were degreased with an industrial alkaline solution (DUROCLEAN 107) at 80 8C for 5 min followed by a treatment with 10% HNO3 [13]. Pyrrole (Aldrich) was bidistilled under an N2 atmosphere. The electrolytes used were 0.1 M oxalic acid and 0.1 M KNO3. TMDD (2,4,7,9-tetramethyl-5-decyne-4,7 diol, 2 /103 M) was added to the electrolyte medium in order to improve electrode wetting by the electrolyte solution and avoid dendritic growth of the polymer. The average particle size of the TiO2 pigment (rutile, Merck) was 0.76 mm and its solubility in aqueous medium is about 1% by weight. Its specific gravity is 4.2 g cm 3 and PPy average specific gravity is around 1.5 g cm 3 [2,10]. A rectangular 110 /85 /50 mm3 acrylic cell was used for the experiments without electrolyte stirring. Two stainless steel sheets and a saturated calomel electrode (SCE) were also used as counter (CE) and reference electrodes (RE), respectively. The electrodes were positioned symmetrically 20 mm from the WE. For experiments with electrolyte stirring a loop system, similar to the one described by Beck et al. [19] was employed. A stationary sheet anode was fixed between two stainless steel CEs (Fig. 1). An Ag/AgCl (/0.034 V vs. SCE) wire was used as a RE, as explained elsewhere [21], which was positioned exactly behind the sheet anode. The suspension was pumped around a loop with the aid of a PSM suction pump which can produce flow rates from 1 to 15 ml s 1. All syntheses were performed galvanostatically at room temperature onto an exposed area of 12 cm2 on each side of the sheet.
Fig. 1. Representation of the electrolytic cell with looping stirring. B, buffer volume; V, valve; and P, pump.
2.2. TiO2 concentration in the PPy/TiO2 composite film After degradation of the composite films in hot concentrated sulphuric acid, the titanium content was quantitatively analysed by ultra-violet spectroscopy. A JASCO 7800 spectrophotometer was used at 420 nm according to the colorimetric method [22]. Plasma spectroscopy (ICP) using an ARL 3410 spectrophotometer at 323.452 nm was also used. The iron content in the same solution was measured at 510 nm. For the X-ray photoelectron spectroscopy (XPS) experiments an SSI X100 photoelectron spectrometer with an aluminum monochromatic source (Al /Ka radiation 1486.6 eV) was used. XPS was carried out using a spot size of 600 mm and a vacuum of 10 9 Torr (1.33/107 Pa). An analysis angle of about 908 with respect to the sample surface was used. Ar ion bombardment was applied to remove thin layers of the composite film using an AG2 ion gun supplied by Vacuum Generators. The conditions of bombardment were an acceleration voltage of 4.8 kV, an Ar pressure of 5 /106 mbar and 200 mA of ion current. The sputtering rate was approximately 2.5 mm per h, as estimated from calculated and measured thicknesses of the composite films. The composite film thicknesses were measured using a FISCHER model DUALSCOPE† MP 20 coating thickness metre. The SEM experiments were performed with a PHILIPS XL20 scanning electron microscope. 2.3. Anticorrosion protection of the PPy/TiO2 composite films Salt spray corrosion tests (ASTM B 117) were carried out in a test chamber (BASS EQUIPAMENTOS) using 6 /2 cm samples (only one side of the sheet). The weight loss tests were performed by dipping samples of PPy and PPy/TiO2 films into a 3.5% NaCl solution. The exposed electrode area was 4 cm2. The amount of iron released from the samples was measured as a function of
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immersion time using a JASCO 7800 UV/vis spectrophotometer at 510 nm using the o-phenanthroline technique. All exposed electrode areas were delimited by an acrylic-thermoplastic paint layer of approximately 0.5 mm thick. In this work, all experiments were performed twice, except weight loss tests which were performed three times. An average of the results is given.
3. Results and discussion The electrodeposition of conducting polymers onto oxidizable metals is indeed not easy because thermodynamic data predict that the metal will dissolve before the electropolymerization potential of the monomer is reached. Thus to achieve the deposition of a conducting polymer on mild steel it is necessary to find the electrochemical conditions which lead to a partial passivation of the metal and decrease its dissolution rate without preventing electropolymerization [6,7,12,13]. In addition, if a pigment is to be codeposited with PPy, it must not adversly interfere with the pyrrole polymerization reaction. 3.1. KNO3 medium (pH 7.5 /8) The influence of selected parameters on TiO2 incorporation was investigated using a potassium nitrate medium. 3.1.1. Electrolyte stirring As described in previous experiments carried out with TiO2 [19,20] there was a negligible incorporation of pigment (without stirring of the electrolytic medium) as well as pigment sedimentation. A strong convection achieved with the maximum flow rate setting of the pump (15 ml s 1) was better for pigment incorporation. Table 1 shows an increase in TiO2 concentration in the PPy matrix (cc) when the solution is stirred with small TiO2 electrolyte concentrations (200 mg l1). Table 1 TiO2 concentration in PPy (cc) with and without electrolyte stirring in 0.1 M pyrrole, 0.1 M KNO3, 2.3/10 3 M TMDD, TiO2 electrolyte concentration: 200 mg l 1, polymerization time: 10 min j /mA cm 2
2 5 7 10 15 20
Without strring
With stirring
cc/% TiO2
cc/% TiO2
0.1 0.2 0.4 0.1 0.1 0.1
0.5 / 0.4 / / 0.4
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Caution must be taken with strong convection (above 15 ml s 1) because it could repel TiO2 particles from the anode. Table 1 also shows that the effect of the current density on the incorporation of TiO2 in PPy is more pronounced when no stirring is performed. Current densities in the range of 5 /7 mA cm 2 favour incorporation, however higher current efficiencies were found in the 2/5 mA cm 2 range [13,20]. For this reason 5 mA cm 2 was used for all further experiments using the KNO3 electrolyte. The influence of stirring was also analysed using different experimental conditions. The polymerization time was increased to 20 min and the electrolyte pigment concentration (ce) to 50 and 100 g l 1. This led to the formation of PPy/TiO2 composite films which were approximately 18 mm thick. The results are shown in Table 2. Thus, stirring keeps the TiO2 particles in suspension and causes the particles to be continuously in contact with the anode surface where the composite film is being deposited. The results confirm that this incorporation is greatly facilitated by mechanical stirring. Indeed, a very small quantity of TiO2 was observed in the composite films synthesized in the KNO3 electrolyte without stirring even using pH values higher than the TiO2 isoelectric point */between 6.0 and 6.7 in aqueous medium [23] */compared to the same medium with stirring. The use of looping stirring and the presence of TiO2 did not alter the current efficiency (about 90%) for the composite film synthesis. Similar results without stirring have been obtained in previous experiments [20] and by Schirmeisen and Beck [2] for PPy synthesis in NO 3 aqueous medium with slow stirring. It is important to emphasize that only PPy mass is considered in the current efficiency calculation and TiO2 concentration is neglected. Thus, PPy deposition onto mild steel is governed only by charge transfer mechanisms. 3.1.2. Polymerization time Table 3 shows that for small TiO2 concentrations in the electrolyte there are no significant changes in TiO2 Table 2 TiO2 concentration (cc) in PPy using higher TiO2 electrolyte concentrations (ce) ce/g l 1
50 100
With strring
Without strring
cc/%
cc/%
6 9.2
B/1 3.1
Electrolyte composition: 0.1 M pyrrole, 0.1 M KNO3 and 2.3/ 10 3 M surfactant TMDD, j/5 mA cm 2, polymerization time: 20 min
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Table 3 Influence of polymerization time on TiO2 incorporation with electrolyte stirring j /mA cm 2
5 10 20 5 5 5
Ce/g l 1
0.2 0.2 0.2 0.4 8 50
Polymerization time 10 min
20 min
cc/%TiO2
cc/%TiO2
0.5 0.9 0.4 1.5 0.8 1.5
0.8 1.5 0.7 2.5 6.3 6
Electrolyte composition: 0.1 M pyrrole, 0.1 M KNO3, 2.3/10 3 M TMDD.
incorporation. However, for higher TiO2 quantities in the electrolyte, the change in polymerization time from 10 to 20 min results in a remarkable difference in TiO2 incorporation into the PPy matrix. Composite films synthesized over 10 min using 50 g l 1 of pigment in the electrolyte showed an incorporation of 1.5% by weight, while 6% by weight of TiO2 was found in composite films synthesized over 20 min. TiO2 particles should be distributed in an approximately uniform way within a PPy matrix when small quantities of TiO2 are present in the electrolyte. This is because a twofold increase in polymerization time causes a similar increase in TiO2 concentration in the PPy matrix. However, for higher TiO2 concentrations in the electrolyte (above 1 g l 1) the particles should mainly be in the upper half of the composite film. This is because the TiO2 concentration in the composite in-
creases more than twice when the polymerization time is increased from 10 to 20 min. Thus, TiO2 particles are preferentially incorporated when there is a polymer layer of considerable thickness on the metallic surface. This is due to an increase in polymer surface roughness with a respective increase in polymer thickness as reported by Dahlhaus and Beck [24]. Polymerizations carried out with longer times did not show an increase in TiO2 incorporation. 3.1.3. Electrolyte concentration Fig. 2 shows a typical chronopotentiometric curve in a 0.1 M KNO3 medium. It is remarkable that the potential increases quickly, so polymerization begins instantaneously and the potential stabilizes at 1.2 V. This behaviour is typical and independent of TiO2 electrolyte concentration indicating that the pigment does not interfere with the PPy polymerization mechanism. With low concentration of KNO3 electrolyte (ca. 103 M), the potential rapidly increased to above 5 V (current 5 mA cm 2), leading to polymer overoxidation due to the high electrolyte resistivity [25]. In spite of the high TiO2 incorporation into PPy (around 18% by weight) the composite films are porous. The process has a low current efficiency, i.e. less than 50%. In the absence of TMDD the incorporation decreases to 8%. Thus, TMDD promotes electrode wetting and probably favours TiO2 incorporation by superficial tension diminution at the electrode j solution interface. 3.1.4. TiO2 electrolyte concentration The experiments were carried out in a range of 200 mg l 1 to 100 g l1 of TiO2. The results are shown in Fig. 3.
Fig. 2. Chronopotentiometric curve of composite film synthesis in 0.1 M KNO3 medium using 0.1 M pyrrole and 2.3/10 3 M TMDD, j/5 mA cm 2.
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Fig. 3. Relationship between TiO2 concentration in PPy/TiO2 composite film (cc) and its concentration in the electrolyte (ce). Electrolyte: 0.1 M KNO3, 0.1 M pyrrole and 2.3/10 3 M TMDD, j/ 5 mA cm 2 and t /20 min.
The maximum TiO2 concentration in the composite film from the KNO3 medium containing 100 g l 1 of TiO2 in the electrolyte was around 7%. However, it is also possible to obtain a similar composition using 10 g l 1 of TiO2 in the electrolyte because of its saturation within the PPy matrix, as shown by Beck and Dahlhaus [26]. 3.2. H2C2O4 medium (pH 2) In this medium a polymerization time of 30 min was used with a current density of 5 mA cm2 and a monomer concentration of 0.1 M pyrrole. This was
related to previous work that used magnetic stirring [20]. Using different current densities than 5 mA cm 2 in H2C2O4 medium, the TiO2 concentration in the composite film and the current efficiency will be lower. However, even for 5 mA cm 2, the current efficiency in this medium was lower than in KNO3 (around 66%) due to the presence of an induction time during which only iron corrosion and an iron oxalate layer formation occurred [10]. The induction time is illustrated in Fig. 4. After the first 2 min the potential stabilizes at /0.25 V and iron corrosion takes place. After this period pyrrole polymerization starts and the potential stabilizes at around
Fig. 4. Chronopotentiometric curve of composite film synthesis in 0.1 M oxalic acid medium using 0.1 M pyrrole, j/5 mA cm 2.
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1.0 V. The thickness of the PPy/TiO2 composite film synthesized in H2C2O4 medium under these conditions was around 25 mm. The influence of polymerization parameters on TiO2 incorporation was also investigated. 3.2.1. TiO2 electrolyte concentration Experiments with 0.1 M oxalic acid were carried out in electrolytes that contained from 200 mg l 1 to 50 g l 1 of TiO2 (Fig. 5). It is possible to incorporate around 14 wt.% of TiO2 in a composite with a concentration slightly greater than 10 g l1 of pigment in the electrolyte. Above this concentration TiO2 incorporation no longer increases. The same was observed in the KNO3 medium. 3.2.2. Electrolyte pH When the pH value of the oxalic acid medium was increased to 4 the TiO2 incorporation was found to be similar to that at pH 2 (Fig. 4). There was no marked difference in the current efficiency when the pH was changed. Experiments carried out with 0.1 M sodium oxalate solution (pH around 6) gave a similar percentage of TiO2 incorporation into the PPy matrix as for oxalic acid under the same experimental conditions. However, the current efficiency decreased and as a consequence the films had a lower thickness. The results showed that the use of oxalic acid as an electrolytic medium promotes the formation of a PPy/ TiO2 composite film that is visibly smoother and
brighter. The film also has a greater TiO2 incorporation and a stronger adhesion (Gr0 Grade, as DIN 53151) than PPy/TiO2 composite films synthesized in a KNO3 medium. The low roughness and high adhesion of the composite films obtained in this medium, as compared to those obtained in a KNO3 medium, could be due to the formation of a thin layer of iron oxalate during the induction time. Iron oxalate may provide favourable nucleation sites for electrocrystallization of PPy. PPy grows as microhemispheres and TiO2 deposits in the valleys or summits leading to smooth films [10]. Thus, the organized growth of PPy in a H2C2O4 medium is more significant than polymeric roughness found in PPy synthesized in a KNO3 medium for TiO2 incorporation.
3.3. Surface analysis by XPS and SEM XPS and SEM analysis were carried out on the PPy/ TiO2 composite films synthesized in the oxalic acid medium which contained 14% by weight of TiO2 particles. Samples were obtained by cutting the electrode into three (2 /2 cm) parts. The signal for titanium was observed at the surface of the PPy/TiO2 composite film (459 eV for Ti2p3/2 and 465 eV for Ti2p1/2) and each the three parts displayed a constant Ti/C ratio. Because the TiO2 is uniformly distributed throughout the sample, any fraction of the sample will be representative of the sample as a whole.
Fig. 5. Relationship between TiO2 concentration in PPy/TiO2 composite film (cc) and its concentration in the electrolyte (ce) Electrolyte: 0.1 M H2C2O4 pH 2.0 and 0.1 M pyrrole, j/5 mA cm2 and t /30 min.
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Fig. 6. SEM image of the PPy/TiO2 composite film.
After approximately 2 h of Ar ion bombardment, corresponding to 5 mm of composite film removal, the amount of TiO2 was found to be slightly higher than it was at the surface of the composite. However, the TiO2 concentration was observed to decrease further below the surface of the film, as was observed after further exposure to Ar ion bombardment [27]. SEM micrographs confirmed the homogeneous distribution of TiO2 at the surface of the PPy/TiO2 composite film (see Fig. 6, which shows the pigment particles as white dots). 3.4. Corrosion tests All samples used for these tests were synthesized in 0.1 M oxalic acid (pH 2.0) and contain 14% by weight of
titanium dioxide in the PPy/TiO2 composite film. The dedoping process */polymer transition from oxidized to reduced state [5] */was performed by sample immersion after polymerization in 0.1 M NaOH applying /0.7 V (SCE) for 15 min. 3.4.1. Weight loss tests The weight loss test determined the quantity of iron released from mild steel during immersion in a 3.5% NaCl solution. Fig. 7 shows the behaviour of the steel after three different treatments: (1) as degreased, (2) coated by a dedoped PPy film and (3) coated by a dedoped PPy/TiO2 composite film. With degreased steel iron oxidation takes place quickly when in contact with NaCl solution. This results in the formation of a thin layer of iron oxides and
Fig. 7. Iron concentration (mg Fe l1) as a function of immersion time in 3.5% NaCl solution. j degreased 1010 steel; m PPy without TiO2; and ' PPy with TiO2.
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hydroxides. But this protection is limited because the thin layer is easily damaged and the sample then undergoes a severe corrosion process. For the steel coated by dedoped PPy/TiO2 composite films there is at first an acceleration of the oxidation process, probably due to initial iron oxidation through the pores and/or defects of the dedoped polymer film. In this case, the dedoped process did not yield a homogeneous passive layer throughout the sample. After approximately 96 h no further iron is released from the steel and the corrosion process stabilizes at 12.5 mg of iron per litre of NaCl solution. For the steel coated with dedoped PPy film, the initial oxidation rate, attributed to iron oxidation, is slightly higher than for steel coated with PPy/TiO2 composite film. Thereafter the corrosion rate is slowed down but no stabilization is observed. After 7 days the samples released 25 mg of iron per litre of NaCl solution. This behaviour can be explained by the low porosity of PPy/TiO2 composite films due to the filling of polymer pores by TiO2 particles as was observed by SEM. This may contribute to the better performance of PPy/TiO2 composite films in these tests. Some experiments were performed with doped PPy and PPy/TiO2 composite films. The weight losses were slightly greater than those for the dedoped samples; however as before the presence of TiO2 in the polymer film contributed to a decrease in the amount of the corrosion. There are several explanations for the better performance of the dedoped ECP films. Ahmad and MacDiarmid [4] have concluded that during the reduction of a chemically synthesized PANI, iron corrosion inhibition occurs by stable oxide layer formation at the metal j PANI interface. Schauer et al. [9] have reported that the polyaniline transition from the conductor to the insulator form (dedoping process) also improves the barrier properties of the conductive film. This transition also enhances the pore resistance of the coating due to the high resistance and sealing effect of the dedoped polymer.
3.4.2. Salt spray tests (ASTM B117) In spite of giving only qualitative information about the behaviour of the metal/coating system when exposed to an aggressive medium (5% NaCl at 35 8C), the salt spray test is widely used for comparing the performance of coatings. Samples of doped films were submitted to 5 and 24 h of exposure to salt spray. Some of the samples received an X-cut in order to evaluate the underlayer corrosion. After 5 h of exposure to salt spray (Fig. 8a and b), the samples showed evidence of corrosion and the attack was more severe in the samples which had the X-cut. The corroded area with an intense red colour was always
larger for the samples that did not contain TiO2 particles. For films without an X-cut, the red corrosion within the total exposed area was about 10% in electrodes coated by the PPy films (Fig. 8a) and only 0.5% for those coated by the PPy/TiO2 composite films (Fig. 8b). Fig. 8c and d show the results obtained for samples exposed for 24 h. The same trends as for 5 h were observed. As expected the PPy and PPy/TiO2 samples with the X-cut showed intense red corrosion. Samples without the X-cut showed a corroded area that was approximately 70% of the total exposed area for the PPy/TiO2 samples and 100% for the PPy samples. No blistering was found on the coating in the zone outside the region of corrosion in all samples tested. This indicates an excellent adhesion of both the PPy and PPy/TiO2 films, to the substrate.
4. Conclusion In this work we showed that it is possible to synthesize PPy/TiO2 composite films that show good characteristics for the corrosion protection of mild steel. Some parameters greatly influence the synthesis and favour pigment incorporation e.g. looping stirring (there is no significant incorporation due to electrophoresis phenomena), pigment concentration in electrolyte, electrolyte nature (oxalic acid is better than potassium nitrate medium) and the applied current density (around 5 mA cm 2). XPS experiments have shown that the pigment is located throughout the sample thickness but mainly at the superior half part of the composite film. SEM images confirm that the TiO2 particles are homogeneously distributed at the composite film surface with a probable decrease in the number of polymer pores as compared to PPy films alone. Corrosion tests such as weight loss and salt spray tests show that TiO2 significantly improves the anticorrosion properties of PPy. The quantity of iron released from the electrodes coated by PPy/TiO2 composite film showed a stabilization of the corrosion process after 96 h of immersion in 3.5% NaCl solution. The electrodes coated by PPy films showed a continuous increase in the quantity of iron released. The dedoped PPy/TiO2 samples showed a 50% reduction of the iron quantity released as compared to the dedoped PPy samples after 7 days of immersion in 3.5% NaCl solution. Salt spray tests show that the PPy/TiO2 samples without the X-cut displayed a reduction of about 95 and 30% in the corroded area as compared to the PPy samples exposed for 5 and 24 h, respectively. PPy/TiO2 composite film coatings are a feasible method for corrosion protection and can be applied as a primary coating that can replace the conventional
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Fig. 8. Salt spray corrosion test: (A) after 5 h of exposure for PPy films; (B) after 5 h of exposure for PPy/TiO2 composite film. Left, sample after exposition; Right, X-cut sample after exposition. (C) After 24 h of exposure for PPy films; and (D) after 24 h of exposure of PPy/TiO2 composite film. Left, sample before exposition; Centre, sample after exposition; Right, X-cut sample after exposition.
system of phosphatized layers on mild steel. These layers satisfy the demand for an environmentally acceptable solution.
Acknowledgements The authors are grateful to CNPq and CAPESCOFECUB (project 184/96) for their financial support.
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