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Corrosion protection of carbon steel with thermoplastic coatings and alkyd resins containing polyaniline as conductive polymer
Progress in Organic Coatings 52 (2005) 151–160
Corrosion protection of carbon steel with thermoplastic coatings and alkyd resins containing polyaniline as conductive polymer Jos´e Ignacio Iribarren Lacoa,∗ , Francisco Cadena Villotab , Francisco Liesa Mestresc a c
Departament d’Enginyeria Qu´ımica E.T.S.E.I.B., Universitat Polit`ecnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain b Departamento de Materiales, Escuela Polit´ ecnica Nacional, Ladr´on de Guevara E 11-253, Quito, Ecuador Departament d’Enginyeria Mec`anica E.T.S.E.I.B., Universitat Polit`ecnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain Received 29 March 2004; received in revised form 20 September 2004; accepted 25 October 2004
Abstract Protection against corrosion was evaluated for specimens of carbon steel coated with conventional thermoplastic polymers as a blend of poly(methyl methacrylate) with poly(butylmethacrlylate), phenoxy resin and a poly(vinyl chloride-co-vinyl acetate) 90/10 copolymer and compared with an alkyd resin containing 0.2, 0.4 and 0.6% (w/w) of polyaniline, a conductive polymer extensively investigated for its ability to protect metals against aqueous corrosion. Previously, physicochemical and thermal characterization of the polymers was carried out and specimens of carbon steel were used in order to evaluate the protective power of these coatings. Field tests in urban and marine environments were compared with laboratory accelerated tests. Coatings degradation and corrosion products were related to the different mechanism of corrosion processes that take place in thermoplastic and alkyd resins. Equivalent time of laboratory tests with respect to field tests was also evaluated in order to validate the accelerated cyclic tests. In both cases, field and laboratory conditions, the presence of conductive polymer in alkyd resin improve the protection against corrosion of the metal and the degradation resistance of the coating, improving the overall performance of the coated steel. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion protection; Thermoplastic coatings; Alkyd resin; Conductive polymer
1. Introduction Protection of metals and alloys against corrosion by using organic coatings is an area of great interest. The development of new paint based coatings attempts to improve the corrosion resistance, increasing their performance. This performance is related to the rheological behavior [1], thermal properties [2], chemical constitution of the polymer systems, pigment and additive selection [3]. Thermoplastic polymers include several applications in coated systems, as powder coatings [4], anticorrosive paints in automotive industry and clean room applications [5]. Polyolefins (homopolymers and copolymers ethylene– propylene), polyamides (PA66), polyesters (PET, PBT) and
∗
Corresponding author. E-mail address: [email protected] (J.I.I. Laco).
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.10.005
poly(ether-ester)s as polycarbonate (PC) have been widely used for this purpose. In the field of alkyd resins, solvent-borne paints are the most used in the market [6]. Conductive polymers such as polyaniline have been used in order to improve the corrosion resistance of alkyd and epoxy resins. In all cases, passivation of the surface metal is the suggested mechanism for the protective activity of the polyaniline [7,8]. In a preceding paper [9], we have studied the corrosion of carbon steel in marine and urban environments by means of field and laboratory tests; a good correlation has been established by means of the existence of an equivalent time of laboratory accelerated tests with respect to field tests. In this work, three different thermoplastic systems (vinyl copolymer, a blend of two acrylic polymers, and the phenoxy resin) were selected in order to evaluate the corrosion resistance and compare the results with an alkyd resin modified with polyaniline as a conductive polymer.
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Scheme 1.
The vinyl copolymer is based in vinyl chloride and vinyl acetate, the blend of acrylic polymers is constituted by poly(methyl methacrylate) and poly(butyl methacrylate). Phenoxy resin is a commercial thermoplastic based in poly(hydroxy ether) of bisphenol-A, (repeat constitutional unit is represented in Scheme 1a). Polyaniline used as additive in alkyd resins is the variety named emeraldine, (repeat constitutional unit is represented in Scheme 1b). The corrosion study includes two different aspects of corrosion tests, the comparison of the field tests in urban and marine environments with the laboratory accelerated tests carried out in sodium hydrogen sulfite and sodium chloride solutions. Simultaneously, coating degradation and corrosion mechanism were suggested for the four coatings.
2. Experimental 2.1. Materials The elemental composition of the carbon steel used is: C < 0.08%, Mn = 0.25%, Si = 0.01%, P = 0,014% and Al = 0.014%. Other remarkable characteristics were evaluated in order to finish the material characteristics, such as density and rugosity. The results give a density of 7.86 × 103 kg m−3 , whereas the medium rugosity and the maximum rugosity are 1.9 m and 12.6 m, respectively. The metal specimens used in both field and laboratory tests are schematic shown in Fig. 1. These specimens were cleaned with trichloroethylene and stored in a dried atmosphere until the coating application, using calcium chloride as dryer in the recipient connected to the vacuum.
and phenoxy were supplied by Quimidroga, S.A. (Barcelona, Spain). Specimens used in field and laboratory tests were prepared by immersion of the steel plates into the paint (alkyd resin) or polymer solutions prepared in chloroform (phenoxy) or acetone (acrylic and vinyl polymers). 2.3. Instrumental techniques: corrosion evaluation Morphological and chemical aspects of the different rusts formed were studied by optical microscopy (Olympus BX-5 operating in the reflection mode with an Olympus C3030Z digital camera coupled) and scanning electron microscopy (JSM-6400 JEOL with energy-dispersive connected X-ray spectrometer SEM/EDS). The subsequent metallic surface state after the oxide cleaning was evaluated by measuring their rugosity with a Talysurf Clans 30 meter. Lost weights were registered in accordance with ISO 9226 and ASTM G-1 standards. The corrosion products were analyzed by X-ray powder diffraction (Siemens D-500, with Cu K␣ radiation of λ = 0.154 nm) and the contaminants were detected by X-ray scattering spectroscopy (EDS) connected to scanning microscopy. 2.4. Instrumental techniques: field and laboratory tests Two different stations were chosen at the metropolitan area of Barcelona (Spain) to carry out the field tests. The
2.2. Chemicals and sample preparation Chemical characteristics of coatings used in this work are as follows: (a) Commercial alkyd resin pure and modified with 2, 4 and 6 wt.% of polyaniline was supplied by Akzo-Nobel Coatings (The Netherlands) (b) Commercial thermoplastic polymers, as poly(methyl methacrylate) (PMMA), poly(butylmetacrylate) (PBMA), poly(vinyl chloride-co-vinyl acetate) copolymer
Fig. 1. Characteristics and dimensions of the specimens used in field and laboratory tests.
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Fig. 2. Running sequence of robotized device in laboratory tests: (a) ready for immersion; (b) drying.
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first one is located in the urban environment (Cornell`a de Llobregat, metropolitan area of Barcelona), far away from industrial pollution and the influence of the sea. The second one was selected near of the sea (breakwater of Barcelona), a typical marine environment. The equipment used in accelerated laboratory tests consists of a specimen support controlled by a programmable device, with a bath containing the corrosive solution (Fig. 2). Processes allowed are immersion, dropping, drying and cooling. Corrosive solution used in these tests contains 3% aqueous solution of sodium chloride (pH = 6.6) to emulate a marine environment. The urban environment is supplied by means of a sodium hydrogen sulfite solution acidulated with sulfuric acid to obtain pH = 3.2. The operating conditions of each test cycle were: (a) Immersion time: 15 min; (b) dropping time: 30 min; (c) drying time: 10 min; (d) cooling time: 5 min; (e) cycle overall time: 60 min. In the step (a) the specimens were totally immersed into the solution and the drying was forced by means of two lamps in order to obtain a temperature similar to the environment temperature in field tests. Furthermore, the coated specimens were painted on both sides, and the edges and the non-coated section were treated with a polyurethane resin in order to minimize the local attack avoiding the beginning of corrosion process. Other additional conditions, including the camera inside temperature (laboratory temperature) and the relative humidity were remained constant at 20 ◦ C and 50%, respectively. Selected solution temperature (immersion bath) was 30 ◦ C, whereas the heating temperature used to dry the samples with lamps was about 40 ◦ C. The resultant overall test time was 120 h. After the beginning of the test, the samples are removed at 30, 60, 90 and 120 h.
Fig. 3. TGA thermogram trace of alkyd resin (scan rate 10 ◦ C/min).
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2.5. Instrumental techniques: coatings characterization and degradation Physicochemical characterization and coating degradation studies were carried out by IR and NMR spectroscopy. IR spectra were recorded on a Perkin-Elmer 2000 FT-IR spectrometer for transparent specimens. For translucent and opaque specimens the same equipment was used with ATR technique. Alkyd resins were analyzed with an optical microscope IR-560 with a detector MST and an objective for ATR. NMR spectra were recorded on a Bruker AMX-300 spectrometer operating at 300.13 and 70.48 MHz for 1 H and 13 C, respectively. Size-exclusion chromatography (SEC) was performed on a Waters Associates instrument fitted with two columns with respective exclusions limits at 103 and 104 nm and a refractive
index detector. The (95:5) chloroform/o-chlorophenol mixture was used as the mobile phase, and molecular weights were estimated against polystyrene standards (Polysciences) using the Maxima 820 computer program. It should be stressed that molecular weights given in this work are only approximate, because no absolute measurements were made. Thermal analysis was carried out using a Perkin-Elmer Pyris 1 DSC and a Perkin-Elmer thermogravimetric analyzer TGA-6, both at 10 ◦ C/min under inert atmosphere. Corrosion degree evaluation was carried out using the ASTM D 610 “Standard Test Method for Evaluation Degree of Rusting on Painted Steel Surfaces”.
3. Results and discussion 3.1. Coatings characterization FT-IR spectrum of copolymer poly(vinylchloride-covinylacetate) shows the characteristic stretching bands at 3000 and 1730 cm−1 corresponding to CH and C O groups, respectively. Between 1420 and 1330 cm−1 (CH bending) is located the acetate CH band, at 1370 cm−1 , from its relative intensity the proportion of acetate groups can be evaluated. By integration of 1 H NMR signals about 5.0–5.5 ppm (CH of vinylacetate) and 4.0–4.5 ppm (CH of vinylchloride), overall composition of copolymer was established as 90% of vinyl chloride and 10% of vinyl acetate. Moreover, 1 H NMR reveals that the peak at 5.27 ppm corresponds to CH of the VCVAVC sequences (VC = vinylclhoride, VA = vinylacetate), the peak at 5.15 ppm is the CH of VAVAVC sequences and the peak at 5.0 ppm correspond to CH of VAVAVA sequences. Integration of these signals gives the composition 62% of VCVAVC, 23% of VAVAVC and 15% of VAVAVA sequences. The number average molecular weight estimated by SEC was 19,000 relative to polystyrene. Blends of polymethylmetachrylate (PMMA) and polybutylmetacrylate (PBMA) are an alternative to plasticizers of low molecular weight of PMMA. The addition of PBMA improves the flexibility of coatings and films, whereas the Table 1 Influence of addition of the conductive polymer in absorption intensity of FT-IR characteristics signals for alkyd resin
Fig. 4. Optical micrographs of specimens coated with alkyd resins, exposed during 12 months in field tests: (a) marine environment; (b) urban environment. Scale bar: 100 m.
Polyaniline (%)
t (months)a
OH
CH
C O
0
0 5 12
15.5 15.7 17.0
16.9 3.6 2.5
9.9 3.6 2.5
50.6 24.6 1.3
0.2
0 5 12
15.4 29.8 19.0
16.8 6.5 1.8
10.2 5.4 2.0
51.1 24.5 11.6
0.6
0 5 12
12.9 38.8 14.1
16.8 8.1 2.6
10.3 5.6 2.4
49.9 27.4 21.6
a
Time corresponding to field tests in marine environment.
CO3 2−
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Fig. 5. FT-IR spectrum of alkyd resin exposed in urban environment (A) and marine environment (B). (a) Initial state; (b) after 5 months; (c) after 12 months. Note the decreasing of carbonate signal (marine environment) as the result of resin degradation and the splitting in OH absorption (urban environment).
UV degradation resistance characteristic of PMMA does not change substantially. FT-IR of the blend exhibits characteristics signals between 2800–3000 cm−1 (CH stretching), 1730 cm−1 (C O stretching) and 1250 cm−1 (C O stretching). The additional signal at 1240 cm−1 (C O) is related with the syndiotactic configuration of the polymer chain. The absence of different signals to the pure homopolymers in 1 H and 13 C NMR may be interpreted as the existence of a true total miscibility, discarding the presence of copolymers in the blend. By integration of signals of OCH3 group in methyl ester and OCH2 group in butyl ester, a overall composition of 47% of PMMA and 53% of PBMA has been evaluated.
Number average molecular weight estimated by SEC results 138,000 with a polydispersity of 4.5. DSC trace at 10 ◦ C/min of the blend shows a glass transition temperature (Tg ) at 60 ◦ C, having an intermediate value between the two homopolymers (108 ◦ C for PMMA and 20 ◦ C for PBMA) [10]. The inflection point of the thermogram was selected as the reference temperature in order to evaluate the glass transition temperature Tg . Phenoxy resin is the current name of poly(hydroxyether) of bisphenol A, a thermoplastic polymer having a similar chemical constitution to epoxy resins, without the epoxy end groups and a greater molecular weight. In fact, this polymer is a thermoplastic with good barrier properties, adherence and flexibility and potential applications in surface coatings. This
Table 2 Relative quantities of iron oxides obtained in laboratory tests Corrosive medium
Polyaniline (%)
Lepidocrocite (%)
Goethite (%)
Lepidocrocite/goethite
NaHSO3 solution
0.0 0.2 0.4 0.6
100 100 100 99.9
47.6 74.1 80.0 100
2.10 1.35 1.25 0.99
NaCl solution
0.0 0.2 0.4 0.6
98.0 57.4 69.6 91.9
71.2 40.4 58.0 100
1.4 1.4 1.2 0.9
The indicated percentages of lepidocrocite and goethite are referred on the basis of X-ray intensities measured from the corresponding diffractogram, being 100 the observed maximum intensity.
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J.I.I. Laco et al. / Progress in Organic Coatings 52 (2005) 151–160 Table 3 Comparison of the corrosion degree between thermoplastic coatings and alkyd resins modified with polyaniline as conductive polymer t (h)
Vinyl copolymer
Acrylic blend
Phenoxy
Alkyd resin (%)a 0
0.2
0.4
0.6
100 200 300 400 500 600 700 800 900 1000
10 18 25 32 40 50 61 74 80 92
0.2 1 4 8 10 15 27 39 50 58
0.1 0.2 0.4 1 3 5 10 20 27 32
0.0 0.1 0.2 0.5 1.0 3.0 4.0 5.1 6.0 6.9
0.0 0.0 0.0 0.1 0.2 0.5 0.7 1.2 1.7 2.3
0.0 0.0 0.0 0.0 0.0 0.1 0.3 0.9 1.4 1.9
0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.4 0.6 0.9
Laboratory accelerated tests with NaHSO3 solution. a With different percentages of polyaniline as conductive polymer.
Fig. 6. Optical micrographs of specimens coated with alkyd resins exposed in laboratory accelerated tests (NaCl solution): (a) without conductive polymer; (b) with 0.6% of polyaniline. Scale bar: 100 m.
polymer has been widely characterized in precedent works [11], being their blends with another semi-crystalline and amorphous polymers of great interest [12]. FT-IR spectrum of this polymer shows a broad band in the region 3700–3100 cm−1 corresponding to OH stretching, the broadness of the signal being the result of a distribution of OH groups associated by hydrogen bonds. Other characteristic bands are presents at 1607, 1581 and 1510 cm−1 related with aromatic backbone. The symmetric bending of gem-dimethyl of bisphenol A appears at 1384 and 1360 cm−1 . Signals of 1 H NMR spectrum are located at 1.61 ppm (s), 2.55 ppm (s), 4.08 (m), 4.34 (m), 6.80 (d) and 7.11 (d). The characterization of alkyd resin was carried out by means of TGA, EDS, X-ray diffraction and FT-IR. Thermogravimetric analysis (Fig. 3) at 10 ◦ C/min is consistent with the presence of approximately 80% of inorganic pigments and extenders. Thermal transition between 750 and 900 ◦ C is related to the chemical reaction of calcium carbonate to generate calcium oxide and carbon dioxide. EDS analysis of the remain inorganic gives a elemental composition of 15.7% of sulfur, 30.1% of calcium, 9.0% of titanium and 44.4% of barium. Table 4 Comparison of the corrosion degree between thermoplastic coatings and alkyd resins modified with polyaniline as conductive polymer
Fig. 7. Influence of conductive polymer in alkyd resin degradation, through the intensity of carbonate signal in FT-IR spectrum. (a) Without polyaniline, (b) with 0.6% of polyaniline.
t (h)
Vinyl copolymer
Acrylic blend
Phenoxy
100 200 300 400 500 600 700 800 900 1000
15 30 42 54 71 80 95 100 100 100
8 20 31 40 52 60 72 83 100 100
4 6 8 10 20 35 48 60 75 91
Alkyd resin (%)a 0
0.2
0.4
0.6
0.0 0.2 0.6 1.0 2.0 8.0 13 19 23 29
0.0 0.0 0.1 0.2 0.4 1.2 2.2 3.1 5.5 7.1
0.0 0.0 0.0 0.1 0.2 0.6 1.0 1.5 2.2 3.4
0.0 0.0 0.0 0.0 0.1 0.2 0.4 0.7 1.0 1.2
Laboratory accelerated tests with NaCl solution. a With different percentages of polyaniline as conductive polymer.
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Fig. 8. Degradation mechanism in thermoplastic coatings. (a) Initial state of the surface; (b) pollutants access; (c) pollutants diffusion; (d) corrosion products obtained under the surface.
X-ray diffraction corroborates this result and the present inorganic pigments are barium sulfate, calcium sulfate and titanium dioxide. FT-IR spectrum contains bands at 3600–3000 cm−1 (OH stretching), 2900–2800 cm−1 (CH stretching), 1740 cm−1 (C O stretching), 1400 cm−1 (carbonate), 1200–950 cm−1 (sulfate). The signals of titanium dioxide are out of range of the apparatus scale. Emeraldine, one of structural forms of polyaniline was selected as conductive polymer additive. The proportions used in this work were 0.2%, 0.4% and 0.6% (w/w) of polyaniline. 3.2. Corrosion resistance and coating degradation In field tests, the general appearance of thermoplastic coatings does not undergo significant changes related with the color, surface wholeness and hardness, although the copoly-
mer of vinyl chloride and vinyl acetate shows surface crazing and the corrosion probably process begins quickly, that is, few months after the beginning (field tests) and for the second extraction in laboratory tests. In this case the corrosion rate is high and the thickness of the oxide film increases and the breaking of the polymer film is promoted in this process. The alkyd resin shows fissures, pulverization and discoloration, but these phenomena are clearly avoided in presence of conductive polymer. In all cases, the corrosive attack is greater for specimens exposed in marine environment (Fig. 4). Whereas the degradation process is not apparent in thermoplastic coatings containing additives, they improve the chemical and photochemical resistance, coating degradation is important particularly in alkyd resin. Changes in FT-IR
Fig. 9. Degradation mechanism in alkyd coating. (a) Initial state of the surface; (b) crazing of the coating surface; (c) pollutants access; (d) obtained corrosion products.
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(b) CH and C O stretching absorptions decrease with the experiment time. This behavior is practically the same in presence of conductive polymer. (c) In a similar form, the carbonate (CO3 2− ) signal decreases especially in marine environment and absence of conductive polymer (Fig. 5b)
Fig. 10. Photographic sequence of the mechanism explained in Fig. 9. (a) Crazing and formation of corrosion products; (b) progress in the coating breaking; (c) removing and surface total segregation of the coating. Scale bar: 20 m.
spectra have been evaluated by measuring the areas of signals, and can be summarized as follows: (a) The intensity of stretching band corresponding to OH (between 3000 and 3600 cm−1 ) increases with the test time and conductive polymer proportion. Twelve months after the beginning of the test, this signal shows a split (Fig. 5c).
In laboratory tests, the surface morphology and general appearance are very similar to obtained in field tests. Changes in FT-IR spectra are qualitatively the same in NaCl and NaHSO3 solutions, but quantitatively the numerical results are greater in laboratory accelerated tests (Table 1). Changes mentioned above in FT-IR can be explained in terms of photochemical influence of UV radiation as well as the influence of inorganic pigment migration in corrosive medium. Decreasing of intensity in CH absorption is related with chain excision reactions [13], whereas in the case of C O band the lower intensity is attributed to disappearance of ester groups by hydrolysis and oxidation reactions [14]. Changes produced in OH band are probably the result of two contributions, the binding medium degradation and pigment hydration. Moreover, the calcium carbonate signal decreasing is related to its aqueous solubility and its possibility to transform calcium sulfate [15]. In an urban environment the pigment migrates to the coating outside in alkyd resin [13]. The influence of additional of conductive polymer is conclusive to improve the resistance against corrosion. This fact is particularly apparent in marine environment (Fig. 6). The FT-IR spectra also are different comparing the carbonate (CO3 2− ) absorption intensity with 0% and 0.6% of polyaniline (Fig. 7). It is well known that the environmental resistance of polyaniline and the subsequent affinity with inorganic pigments [16]. These pigments avoid the photo-oxidative degradation in organic resin [17]. This fact contributes to improve the physicochemical protective mechanism, stopping the access of a great quantity of pollutants to metal surface. Moreover, the electrochemical mechanism is related to the migration of the electrons from the metal substrate to coating exterior, avoiding the generation of cathode zones on the coating–substrate interface [18,19]. Other additional effects are related with the formation of protective oxides on the metal surface. Corrosion products were analyzed by X-ray diffraction for alkyd resins only. The results show the presence of lepidocrocite (␥-FeOOH) and goethite (␣-FeOOH) in urban environment and hydrogen sulfite solution, whereas the marine environment and sodium chloride solution yield also akaganeite (-FeOOH). The results corresponding to 1000 h of laboratory tests are summarized in Table 2, showing the lower rate lepidocrocite/goethite as the percentage of conductive polymer is increased. This rate is important because the goethite is the most stable oxide of iron and therefore, the protective properties are better than the lepidocrocite in order to avoid a ulterior corrosive attack.
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Fig. 11. Degradation mechanism in alkyd resin exposed by means the cross test. (a) Initial state of the surface; (b) pollutants access; (c) pollutants diffusion; (d) obtained corrosion products.
The corrosion degree is also very sensitive to the presence of conductive polymer, decreasing to values of 0.5–1.0% with 0.6% of conductive polymer in alkyd resin. The results are very positive particularly in marine environment or sodium chloride solution. Tables 3 and 4 summarize the corrosion degrees, obtained from the percentage of attacked surface in the coating. Five measurements were taken in each point, being the error assumed about 0.1–0.5% in all cases. The corrosion resistance of alkyd resin is comparatively the best and only the phenoxy resin yields the same corrosion degree at 400 h after the beginning of the experiment in hydrogen sulfite solution. Moreover, the resistance of all thermoplastic coatings is very poor in sodium chloride solution. 3.3. Corrosion mechanism Thermoplastic coatings present a different mechanism than alkyd resin depending on the existence of surface crazing. Once the surface is broken, the metal substrate is exposed to corrosion process and oxide surface thickness increase with the time. The adherence of the coating seems to be a determinant factor on the corrosion rate control. The acrylic and phenoxy coatings do not present crazing, and the corrosion mechanism is represented in Fig. 8. The diffusion of pollutants across the coating surface is the beginning of corrosion process, because zones with a poor adherence are the suitable points to be attacked by electrochemical corrosion reactions. The alkyd coating undergoes surface crazing after the diffusion and adherence loss processes. After the beginning, water and oxygen attack directly to the metal substrate yielding iron complexes from OH groups and oxygen. These compounds precipitate allowing only the selective access of water. Local cathodes are placed on the oxide surface and anode domains remain inside the oxide film. The growth of resultant bubble leads to fracture and disappearance of coating. The corrosion mechanism is represented in Fig. 9 and a photographic sequence of this process can be visualized in Fig. 10.
Additionally, cross-cut test has been carried out in order to compare the operating mechanism in this particular case. Two cuttings similar to an X form are made on coating surface and rust under the paint has been located after the cyclic laboratory tests. This result corroborates the difference between the saline fog tests [20] (active and continuous conditions without growth of iron rust under the coating) and cyclic tests, where the alternating periods of wetting and drying lead to diffusion processes and subsequent precipitation of rust under the paint coating. The mechanism of overall process is schematized in Fig. 11. 3.4. Equivalent time of laboratory tests Results obtained from corrosion degree and calculated from percentage of attacked surface after 1 year of field tests can be used to obtain the necessary laboratory time to yield the same numeric value by means laboratory accelerated tests. This correspondence is calculated for each coating and exposition environment.
Fig. 12. Equivalent time between field and laboratory conditions. (a) Marine environment (NaCl solution); (b) urban environment (NaHSO3 solution).
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The result is represented in Fig. 12. Ideally, this graphic should be a horizontal line, but the different coatings do not have the same equivalent time. The higher test time was selected from the linear regression as the overall equivalent time. The correlation equations in marine an urban environment are: Marine environment (NaCl solution): t = 626 − 0.069 s Urban environment (NaHSO3 solution): t = 744 − 0.055 s where t is time in hours and s the percentage of attacked surface. These results allow us to conclude that 1-year of field tests is equivalent to 744 and 626 h, respectively of laboratory cyclic accelerated tests. Obviously, this conclusion is only a first approximation to reproduce the field conditions, because the conditions of tests are directly related to the involved phenomena complexity. In spite of these difficulties, the similarity between field and laboratory tests in attack mechanism, coating failure, and corrosion products leads us to conclude a comparable overall behavior for two classes of experiments.
4. Conclusions Corrosion resistance of carbon steel was evaluated using as protective coatings with three different thermoplastic polymers and an alkyd resin modified conveniently with low percentages of polyaniline, a conductive polymer widely used in relation with good anticorrosive properties. All coatings were physicochemically characterized by the usual techniques, as spectroscopy and thermal analysis. The more important conclusions derived from the obtained results in this work are: 1. Thermoplastic polymer coatings have a corrosion resistance very low in comparison with alkyd resin modified with conductive polymer. This resistance is particularly poor at short times of the field and laboratory tests. In spite of this behavior, the phenoxy resin shows a greater resistance that the acrylic blends and the vinyl copolymer. Although the characteristics of alkyd resin without conductive polymer are poor in relation to the thermoplastics polymers used in this work, the corrosion resistance is clearly improved by the presence of the conductive polymer. 2. Corrosion mechanism is very different in thermoplastic and alkyd resin, as a direct consequence of different degradation processes of the different coatings. The existence of crazing and failure is characteristic of alkyd resins based coatings. In these systems, the experimental results based
in spectroscopic information show the influence of the conductive polymer in the decreasing of the degradation, remaining the pigments in the coating and avoiding the access of pollutants to metal. Moreover, the corrosion mechanism based in the existence of cathode and anode zones is prevented by the electron movement of the electrons to metal surface. 3. Field tests in marine and urban environments may be properly reproduced by means accelerated laboratory tests, using sodium chloride and hydrogen sulfite solutions, respectively. In fact, all the results can be accurately compared qualitatively and quantitatively. Differences are related to the experimental procedure. The calculated equivalent for laboratory tests proves the validity of these cyclic accelerated tests.
Acknowledgements Financial support for this work was provided by MCYT of Spain, and FEDER with Grant No. MAT2003-00251.
References [1] M. Osterhold, Prog. Org. Coat. 40 (2000) 131. [2] C. Pagella, D.M. De Faveri, Prog. Org. Coat. 33 (1998) 211. [3] H.J. Jacobasch, K. Grundke, St. Scheneider, F. Simon, Prog. Org. Coat. 26 (1995) 131. [4] T.A. Misev, R. van der Linde, Prog. Org. Coat. 34 (1998) 160. [5] E. Lugscheider, S. B¨arwulf, C. Barimari, M. Riester, H. Hilgers, Prog. Org. Coat. 108/109 (1998) 398. [6] T. Schauer, A. Joos, L. Dulog, C.D. Eisenbach, Prog. Org. Coat. 33 (1998) 20. [7] D.W. De Berry, J. Electrochem. Soc. 132 (1985) 1022. [8] D.A. Wroblewski, B.C. Bencewicz, K.G. Thompson, C.J. Bryan, Polym. Prep. 35 (1) (1994) 265. [9] J.I. Iribarren, F. Liesa, F. Cadena, L. Bilurbina, Mater. Corr., in press. [10] J.M. Mark, Physical Properties of Polymers Handbook, AIP Press, Woodburg, New York, 1996. [11] J.I. Iribarren, M. Iriarte, C. Uriarte, J.J. Iruin, J. Appl. Polym. Sci. 37 (1989) 3459. [12] M. Iriarte, J.I. Iribarren, A. Etxeberr´ıa, J.J. Iruin, Polymer 30 (1989) 1160. [13] J. Hodson, J. Lander, Polymer 28 (1987) 251. [14] J. Lucki, J.F. Rabek, B. Ranby, C. Ekstr¨om, Eur. Polym. J. 17 (1981) 919. [15] G. Bierwagen, Prog. Org. Coat. 15 (1987) 179. [16] C. Gettinger, A.J. Heeger, D.J. Pine, Y. Cao, Synth. Met. 74 (1995) 81. [17] H. V¨olz, Photodegradation and photoestabilitation of coatings, ACS Symp. Ser. 136 (1981) 1511. [18] T. Schauer, A. Joos, L. Dulog, C. Eisenbach, Prog. Org. Coat. 33 (1998) 20. [19] Mc. Andrew, TRIP 5 (1997) 71. [20] J. Standish, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 357.
Corrosion protection of carbon steel with thermoplastic coatings and alkyd resins containing polyaniline as conductive polymer Jos´e Ignacio Iribarren Lacoa,∗ , Francisco Cadena Villotab , Francisco Liesa Mestresc a c
Departament d’Enginyeria Qu´ımica E.T.S.E.I.B., Universitat Polit`ecnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain b Departamento de Materiales, Escuela Polit´ ecnica Nacional, Ladr´on de Guevara E 11-253, Quito, Ecuador Departament d’Enginyeria Mec`anica E.T.S.E.I.B., Universitat Polit`ecnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain Received 29 March 2004; received in revised form 20 September 2004; accepted 25 October 2004
Abstract Protection against corrosion was evaluated for specimens of carbon steel coated with conventional thermoplastic polymers as a blend of poly(methyl methacrylate) with poly(butylmethacrlylate), phenoxy resin and a poly(vinyl chloride-co-vinyl acetate) 90/10 copolymer and compared with an alkyd resin containing 0.2, 0.4 and 0.6% (w/w) of polyaniline, a conductive polymer extensively investigated for its ability to protect metals against aqueous corrosion. Previously, physicochemical and thermal characterization of the polymers was carried out and specimens of carbon steel were used in order to evaluate the protective power of these coatings. Field tests in urban and marine environments were compared with laboratory accelerated tests. Coatings degradation and corrosion products were related to the different mechanism of corrosion processes that take place in thermoplastic and alkyd resins. Equivalent time of laboratory tests with respect to field tests was also evaluated in order to validate the accelerated cyclic tests. In both cases, field and laboratory conditions, the presence of conductive polymer in alkyd resin improve the protection against corrosion of the metal and the degradation resistance of the coating, improving the overall performance of the coated steel. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion protection; Thermoplastic coatings; Alkyd resin; Conductive polymer
1. Introduction Protection of metals and alloys against corrosion by using organic coatings is an area of great interest. The development of new paint based coatings attempts to improve the corrosion resistance, increasing their performance. This performance is related to the rheological behavior [1], thermal properties [2], chemical constitution of the polymer systems, pigment and additive selection [3]. Thermoplastic polymers include several applications in coated systems, as powder coatings [4], anticorrosive paints in automotive industry and clean room applications [5]. Polyolefins (homopolymers and copolymers ethylene– propylene), polyamides (PA66), polyesters (PET, PBT) and
∗
Corresponding author. E-mail address: [email protected] (J.I.I. Laco).
0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.10.005
poly(ether-ester)s as polycarbonate (PC) have been widely used for this purpose. In the field of alkyd resins, solvent-borne paints are the most used in the market [6]. Conductive polymers such as polyaniline have been used in order to improve the corrosion resistance of alkyd and epoxy resins. In all cases, passivation of the surface metal is the suggested mechanism for the protective activity of the polyaniline [7,8]. In a preceding paper [9], we have studied the corrosion of carbon steel in marine and urban environments by means of field and laboratory tests; a good correlation has been established by means of the existence of an equivalent time of laboratory accelerated tests with respect to field tests. In this work, three different thermoplastic systems (vinyl copolymer, a blend of two acrylic polymers, and the phenoxy resin) were selected in order to evaluate the corrosion resistance and compare the results with an alkyd resin modified with polyaniline as a conductive polymer.
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Scheme 1.
The vinyl copolymer is based in vinyl chloride and vinyl acetate, the blend of acrylic polymers is constituted by poly(methyl methacrylate) and poly(butyl methacrylate). Phenoxy resin is a commercial thermoplastic based in poly(hydroxy ether) of bisphenol-A, (repeat constitutional unit is represented in Scheme 1a). Polyaniline used as additive in alkyd resins is the variety named emeraldine, (repeat constitutional unit is represented in Scheme 1b). The corrosion study includes two different aspects of corrosion tests, the comparison of the field tests in urban and marine environments with the laboratory accelerated tests carried out in sodium hydrogen sulfite and sodium chloride solutions. Simultaneously, coating degradation and corrosion mechanism were suggested for the four coatings.
2. Experimental 2.1. Materials The elemental composition of the carbon steel used is: C < 0.08%, Mn = 0.25%, Si = 0.01%, P = 0,014% and Al = 0.014%. Other remarkable characteristics were evaluated in order to finish the material characteristics, such as density and rugosity. The results give a density of 7.86 × 103 kg m−3 , whereas the medium rugosity and the maximum rugosity are 1.9 m and 12.6 m, respectively. The metal specimens used in both field and laboratory tests are schematic shown in Fig. 1. These specimens were cleaned with trichloroethylene and stored in a dried atmosphere until the coating application, using calcium chloride as dryer in the recipient connected to the vacuum.
and phenoxy were supplied by Quimidroga, S.A. (Barcelona, Spain). Specimens used in field and laboratory tests were prepared by immersion of the steel plates into the paint (alkyd resin) or polymer solutions prepared in chloroform (phenoxy) or acetone (acrylic and vinyl polymers). 2.3. Instrumental techniques: corrosion evaluation Morphological and chemical aspects of the different rusts formed were studied by optical microscopy (Olympus BX-5 operating in the reflection mode with an Olympus C3030Z digital camera coupled) and scanning electron microscopy (JSM-6400 JEOL with energy-dispersive connected X-ray spectrometer SEM/EDS). The subsequent metallic surface state after the oxide cleaning was evaluated by measuring their rugosity with a Talysurf Clans 30 meter. Lost weights were registered in accordance with ISO 9226 and ASTM G-1 standards. The corrosion products were analyzed by X-ray powder diffraction (Siemens D-500, with Cu K␣ radiation of λ = 0.154 nm) and the contaminants were detected by X-ray scattering spectroscopy (EDS) connected to scanning microscopy. 2.4. Instrumental techniques: field and laboratory tests Two different stations were chosen at the metropolitan area of Barcelona (Spain) to carry out the field tests. The
2.2. Chemicals and sample preparation Chemical characteristics of coatings used in this work are as follows: (a) Commercial alkyd resin pure and modified with 2, 4 and 6 wt.% of polyaniline was supplied by Akzo-Nobel Coatings (The Netherlands) (b) Commercial thermoplastic polymers, as poly(methyl methacrylate) (PMMA), poly(butylmetacrylate) (PBMA), poly(vinyl chloride-co-vinyl acetate) copolymer
Fig. 1. Characteristics and dimensions of the specimens used in field and laboratory tests.
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Fig. 2. Running sequence of robotized device in laboratory tests: (a) ready for immersion; (b) drying.
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first one is located in the urban environment (Cornell`a de Llobregat, metropolitan area of Barcelona), far away from industrial pollution and the influence of the sea. The second one was selected near of the sea (breakwater of Barcelona), a typical marine environment. The equipment used in accelerated laboratory tests consists of a specimen support controlled by a programmable device, with a bath containing the corrosive solution (Fig. 2). Processes allowed are immersion, dropping, drying and cooling. Corrosive solution used in these tests contains 3% aqueous solution of sodium chloride (pH = 6.6) to emulate a marine environment. The urban environment is supplied by means of a sodium hydrogen sulfite solution acidulated with sulfuric acid to obtain pH = 3.2. The operating conditions of each test cycle were: (a) Immersion time: 15 min; (b) dropping time: 30 min; (c) drying time: 10 min; (d) cooling time: 5 min; (e) cycle overall time: 60 min. In the step (a) the specimens were totally immersed into the solution and the drying was forced by means of two lamps in order to obtain a temperature similar to the environment temperature in field tests. Furthermore, the coated specimens were painted on both sides, and the edges and the non-coated section were treated with a polyurethane resin in order to minimize the local attack avoiding the beginning of corrosion process. Other additional conditions, including the camera inside temperature (laboratory temperature) and the relative humidity were remained constant at 20 ◦ C and 50%, respectively. Selected solution temperature (immersion bath) was 30 ◦ C, whereas the heating temperature used to dry the samples with lamps was about 40 ◦ C. The resultant overall test time was 120 h. After the beginning of the test, the samples are removed at 30, 60, 90 and 120 h.
Fig. 3. TGA thermogram trace of alkyd resin (scan rate 10 ◦ C/min).
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2.5. Instrumental techniques: coatings characterization and degradation Physicochemical characterization and coating degradation studies were carried out by IR and NMR spectroscopy. IR spectra were recorded on a Perkin-Elmer 2000 FT-IR spectrometer for transparent specimens. For translucent and opaque specimens the same equipment was used with ATR technique. Alkyd resins were analyzed with an optical microscope IR-560 with a detector MST and an objective for ATR. NMR spectra were recorded on a Bruker AMX-300 spectrometer operating at 300.13 and 70.48 MHz for 1 H and 13 C, respectively. Size-exclusion chromatography (SEC) was performed on a Waters Associates instrument fitted with two columns with respective exclusions limits at 103 and 104 nm and a refractive
index detector. The (95:5) chloroform/o-chlorophenol mixture was used as the mobile phase, and molecular weights were estimated against polystyrene standards (Polysciences) using the Maxima 820 computer program. It should be stressed that molecular weights given in this work are only approximate, because no absolute measurements were made. Thermal analysis was carried out using a Perkin-Elmer Pyris 1 DSC and a Perkin-Elmer thermogravimetric analyzer TGA-6, both at 10 ◦ C/min under inert atmosphere. Corrosion degree evaluation was carried out using the ASTM D 610 “Standard Test Method for Evaluation Degree of Rusting on Painted Steel Surfaces”.
3. Results and discussion 3.1. Coatings characterization FT-IR spectrum of copolymer poly(vinylchloride-covinylacetate) shows the characteristic stretching bands at 3000 and 1730 cm−1 corresponding to CH and C O groups, respectively. Between 1420 and 1330 cm−1 (CH bending) is located the acetate CH band, at 1370 cm−1 , from its relative intensity the proportion of acetate groups can be evaluated. By integration of 1 H NMR signals about 5.0–5.5 ppm (CH of vinylacetate) and 4.0–4.5 ppm (CH of vinylchloride), overall composition of copolymer was established as 90% of vinyl chloride and 10% of vinyl acetate. Moreover, 1 H NMR reveals that the peak at 5.27 ppm corresponds to CH of the VCVAVC sequences (VC = vinylclhoride, VA = vinylacetate), the peak at 5.15 ppm is the CH of VAVAVC sequences and the peak at 5.0 ppm correspond to CH of VAVAVA sequences. Integration of these signals gives the composition 62% of VCVAVC, 23% of VAVAVC and 15% of VAVAVA sequences. The number average molecular weight estimated by SEC was 19,000 relative to polystyrene. Blends of polymethylmetachrylate (PMMA) and polybutylmetacrylate (PBMA) are an alternative to plasticizers of low molecular weight of PMMA. The addition of PBMA improves the flexibility of coatings and films, whereas the Table 1 Influence of addition of the conductive polymer in absorption intensity of FT-IR characteristics signals for alkyd resin
Fig. 4. Optical micrographs of specimens coated with alkyd resins, exposed during 12 months in field tests: (a) marine environment; (b) urban environment. Scale bar: 100 m.
Polyaniline (%)
t (months)a
OH
CH
C O
0
0 5 12
15.5 15.7 17.0
16.9 3.6 2.5
9.9 3.6 2.5
50.6 24.6 1.3
0.2
0 5 12
15.4 29.8 19.0
16.8 6.5 1.8
10.2 5.4 2.0
51.1 24.5 11.6
0.6
0 5 12
12.9 38.8 14.1
16.8 8.1 2.6
10.3 5.6 2.4
49.9 27.4 21.6
a
Time corresponding to field tests in marine environment.
CO3 2−
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Fig. 5. FT-IR spectrum of alkyd resin exposed in urban environment (A) and marine environment (B). (a) Initial state; (b) after 5 months; (c) after 12 months. Note the decreasing of carbonate signal (marine environment) as the result of resin degradation and the splitting in OH absorption (urban environment).
UV degradation resistance characteristic of PMMA does not change substantially. FT-IR of the blend exhibits characteristics signals between 2800–3000 cm−1 (CH stretching), 1730 cm−1 (C O stretching) and 1250 cm−1 (C O stretching). The additional signal at 1240 cm−1 (C O) is related with the syndiotactic configuration of the polymer chain. The absence of different signals to the pure homopolymers in 1 H and 13 C NMR may be interpreted as the existence of a true total miscibility, discarding the presence of copolymers in the blend. By integration of signals of OCH3 group in methyl ester and OCH2 group in butyl ester, a overall composition of 47% of PMMA and 53% of PBMA has been evaluated.
Number average molecular weight estimated by SEC results 138,000 with a polydispersity of 4.5. DSC trace at 10 ◦ C/min of the blend shows a glass transition temperature (Tg ) at 60 ◦ C, having an intermediate value between the two homopolymers (108 ◦ C for PMMA and 20 ◦ C for PBMA) [10]. The inflection point of the thermogram was selected as the reference temperature in order to evaluate the glass transition temperature Tg . Phenoxy resin is the current name of poly(hydroxyether) of bisphenol A, a thermoplastic polymer having a similar chemical constitution to epoxy resins, without the epoxy end groups and a greater molecular weight. In fact, this polymer is a thermoplastic with good barrier properties, adherence and flexibility and potential applications in surface coatings. This
Table 2 Relative quantities of iron oxides obtained in laboratory tests Corrosive medium
Polyaniline (%)
Lepidocrocite (%)
Goethite (%)
Lepidocrocite/goethite
NaHSO3 solution
0.0 0.2 0.4 0.6
100 100 100 99.9
47.6 74.1 80.0 100
2.10 1.35 1.25 0.99
NaCl solution
0.0 0.2 0.4 0.6
98.0 57.4 69.6 91.9
71.2 40.4 58.0 100
1.4 1.4 1.2 0.9
The indicated percentages of lepidocrocite and goethite are referred on the basis of X-ray intensities measured from the corresponding diffractogram, being 100 the observed maximum intensity.
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J.I.I. Laco et al. / Progress in Organic Coatings 52 (2005) 151–160 Table 3 Comparison of the corrosion degree between thermoplastic coatings and alkyd resins modified with polyaniline as conductive polymer t (h)
Vinyl copolymer
Acrylic blend
Phenoxy
Alkyd resin (%)a 0
0.2
0.4
0.6
100 200 300 400 500 600 700 800 900 1000
10 18 25 32 40 50 61 74 80 92
0.2 1 4 8 10 15 27 39 50 58
0.1 0.2 0.4 1 3 5 10 20 27 32
0.0 0.1 0.2 0.5 1.0 3.0 4.0 5.1 6.0 6.9
0.0 0.0 0.0 0.1 0.2 0.5 0.7 1.2 1.7 2.3
0.0 0.0 0.0 0.0 0.0 0.1 0.3 0.9 1.4 1.9
0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.4 0.6 0.9
Laboratory accelerated tests with NaHSO3 solution. a With different percentages of polyaniline as conductive polymer.
Fig. 6. Optical micrographs of specimens coated with alkyd resins exposed in laboratory accelerated tests (NaCl solution): (a) without conductive polymer; (b) with 0.6% of polyaniline. Scale bar: 100 m.
polymer has been widely characterized in precedent works [11], being their blends with another semi-crystalline and amorphous polymers of great interest [12]. FT-IR spectrum of this polymer shows a broad band in the region 3700–3100 cm−1 corresponding to OH stretching, the broadness of the signal being the result of a distribution of OH groups associated by hydrogen bonds. Other characteristic bands are presents at 1607, 1581 and 1510 cm−1 related with aromatic backbone. The symmetric bending of gem-dimethyl of bisphenol A appears at 1384 and 1360 cm−1 . Signals of 1 H NMR spectrum are located at 1.61 ppm (s), 2.55 ppm (s), 4.08 (m), 4.34 (m), 6.80 (d) and 7.11 (d). The characterization of alkyd resin was carried out by means of TGA, EDS, X-ray diffraction and FT-IR. Thermogravimetric analysis (Fig. 3) at 10 ◦ C/min is consistent with the presence of approximately 80% of inorganic pigments and extenders. Thermal transition between 750 and 900 ◦ C is related to the chemical reaction of calcium carbonate to generate calcium oxide and carbon dioxide. EDS analysis of the remain inorganic gives a elemental composition of 15.7% of sulfur, 30.1% of calcium, 9.0% of titanium and 44.4% of barium. Table 4 Comparison of the corrosion degree between thermoplastic coatings and alkyd resins modified with polyaniline as conductive polymer
Fig. 7. Influence of conductive polymer in alkyd resin degradation, through the intensity of carbonate signal in FT-IR spectrum. (a) Without polyaniline, (b) with 0.6% of polyaniline.
t (h)
Vinyl copolymer
Acrylic blend
Phenoxy
100 200 300 400 500 600 700 800 900 1000
15 30 42 54 71 80 95 100 100 100
8 20 31 40 52 60 72 83 100 100
4 6 8 10 20 35 48 60 75 91
Alkyd resin (%)a 0
0.2
0.4
0.6
0.0 0.2 0.6 1.0 2.0 8.0 13 19 23 29
0.0 0.0 0.1 0.2 0.4 1.2 2.2 3.1 5.5 7.1
0.0 0.0 0.0 0.1 0.2 0.6 1.0 1.5 2.2 3.4
0.0 0.0 0.0 0.0 0.1 0.2 0.4 0.7 1.0 1.2
Laboratory accelerated tests with NaCl solution. a With different percentages of polyaniline as conductive polymer.
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Fig. 8. Degradation mechanism in thermoplastic coatings. (a) Initial state of the surface; (b) pollutants access; (c) pollutants diffusion; (d) corrosion products obtained under the surface.
X-ray diffraction corroborates this result and the present inorganic pigments are barium sulfate, calcium sulfate and titanium dioxide. FT-IR spectrum contains bands at 3600–3000 cm−1 (OH stretching), 2900–2800 cm−1 (CH stretching), 1740 cm−1 (C O stretching), 1400 cm−1 (carbonate), 1200–950 cm−1 (sulfate). The signals of titanium dioxide are out of range of the apparatus scale. Emeraldine, one of structural forms of polyaniline was selected as conductive polymer additive. The proportions used in this work were 0.2%, 0.4% and 0.6% (w/w) of polyaniline. 3.2. Corrosion resistance and coating degradation In field tests, the general appearance of thermoplastic coatings does not undergo significant changes related with the color, surface wholeness and hardness, although the copoly-
mer of vinyl chloride and vinyl acetate shows surface crazing and the corrosion probably process begins quickly, that is, few months after the beginning (field tests) and for the second extraction in laboratory tests. In this case the corrosion rate is high and the thickness of the oxide film increases and the breaking of the polymer film is promoted in this process. The alkyd resin shows fissures, pulverization and discoloration, but these phenomena are clearly avoided in presence of conductive polymer. In all cases, the corrosive attack is greater for specimens exposed in marine environment (Fig. 4). Whereas the degradation process is not apparent in thermoplastic coatings containing additives, they improve the chemical and photochemical resistance, coating degradation is important particularly in alkyd resin. Changes in FT-IR
Fig. 9. Degradation mechanism in alkyd coating. (a) Initial state of the surface; (b) crazing of the coating surface; (c) pollutants access; (d) obtained corrosion products.
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(b) CH and C O stretching absorptions decrease with the experiment time. This behavior is practically the same in presence of conductive polymer. (c) In a similar form, the carbonate (CO3 2− ) signal decreases especially in marine environment and absence of conductive polymer (Fig. 5b)
Fig. 10. Photographic sequence of the mechanism explained in Fig. 9. (a) Crazing and formation of corrosion products; (b) progress in the coating breaking; (c) removing and surface total segregation of the coating. Scale bar: 20 m.
spectra have been evaluated by measuring the areas of signals, and can be summarized as follows: (a) The intensity of stretching band corresponding to OH (between 3000 and 3600 cm−1 ) increases with the test time and conductive polymer proportion. Twelve months after the beginning of the test, this signal shows a split (Fig. 5c).
In laboratory tests, the surface morphology and general appearance are very similar to obtained in field tests. Changes in FT-IR spectra are qualitatively the same in NaCl and NaHSO3 solutions, but quantitatively the numerical results are greater in laboratory accelerated tests (Table 1). Changes mentioned above in FT-IR can be explained in terms of photochemical influence of UV radiation as well as the influence of inorganic pigment migration in corrosive medium. Decreasing of intensity in CH absorption is related with chain excision reactions [13], whereas in the case of C O band the lower intensity is attributed to disappearance of ester groups by hydrolysis and oxidation reactions [14]. Changes produced in OH band are probably the result of two contributions, the binding medium degradation and pigment hydration. Moreover, the calcium carbonate signal decreasing is related to its aqueous solubility and its possibility to transform calcium sulfate [15]. In an urban environment the pigment migrates to the coating outside in alkyd resin [13]. The influence of additional of conductive polymer is conclusive to improve the resistance against corrosion. This fact is particularly apparent in marine environment (Fig. 6). The FT-IR spectra also are different comparing the carbonate (CO3 2− ) absorption intensity with 0% and 0.6% of polyaniline (Fig. 7). It is well known that the environmental resistance of polyaniline and the subsequent affinity with inorganic pigments [16]. These pigments avoid the photo-oxidative degradation in organic resin [17]. This fact contributes to improve the physicochemical protective mechanism, stopping the access of a great quantity of pollutants to metal surface. Moreover, the electrochemical mechanism is related to the migration of the electrons from the metal substrate to coating exterior, avoiding the generation of cathode zones on the coating–substrate interface [18,19]. Other additional effects are related with the formation of protective oxides on the metal surface. Corrosion products were analyzed by X-ray diffraction for alkyd resins only. The results show the presence of lepidocrocite (␥-FeOOH) and goethite (␣-FeOOH) in urban environment and hydrogen sulfite solution, whereas the marine environment and sodium chloride solution yield also akaganeite (-FeOOH). The results corresponding to 1000 h of laboratory tests are summarized in Table 2, showing the lower rate lepidocrocite/goethite as the percentage of conductive polymer is increased. This rate is important because the goethite is the most stable oxide of iron and therefore, the protective properties are better than the lepidocrocite in order to avoid a ulterior corrosive attack.
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Fig. 11. Degradation mechanism in alkyd resin exposed by means the cross test. (a) Initial state of the surface; (b) pollutants access; (c) pollutants diffusion; (d) obtained corrosion products.
The corrosion degree is also very sensitive to the presence of conductive polymer, decreasing to values of 0.5–1.0% with 0.6% of conductive polymer in alkyd resin. The results are very positive particularly in marine environment or sodium chloride solution. Tables 3 and 4 summarize the corrosion degrees, obtained from the percentage of attacked surface in the coating. Five measurements were taken in each point, being the error assumed about 0.1–0.5% in all cases. The corrosion resistance of alkyd resin is comparatively the best and only the phenoxy resin yields the same corrosion degree at 400 h after the beginning of the experiment in hydrogen sulfite solution. Moreover, the resistance of all thermoplastic coatings is very poor in sodium chloride solution. 3.3. Corrosion mechanism Thermoplastic coatings present a different mechanism than alkyd resin depending on the existence of surface crazing. Once the surface is broken, the metal substrate is exposed to corrosion process and oxide surface thickness increase with the time. The adherence of the coating seems to be a determinant factor on the corrosion rate control. The acrylic and phenoxy coatings do not present crazing, and the corrosion mechanism is represented in Fig. 8. The diffusion of pollutants across the coating surface is the beginning of corrosion process, because zones with a poor adherence are the suitable points to be attacked by electrochemical corrosion reactions. The alkyd coating undergoes surface crazing after the diffusion and adherence loss processes. After the beginning, water and oxygen attack directly to the metal substrate yielding iron complexes from OH groups and oxygen. These compounds precipitate allowing only the selective access of water. Local cathodes are placed on the oxide surface and anode domains remain inside the oxide film. The growth of resultant bubble leads to fracture and disappearance of coating. The corrosion mechanism is represented in Fig. 9 and a photographic sequence of this process can be visualized in Fig. 10.
Additionally, cross-cut test has been carried out in order to compare the operating mechanism in this particular case. Two cuttings similar to an X form are made on coating surface and rust under the paint has been located after the cyclic laboratory tests. This result corroborates the difference between the saline fog tests [20] (active and continuous conditions without growth of iron rust under the coating) and cyclic tests, where the alternating periods of wetting and drying lead to diffusion processes and subsequent precipitation of rust under the paint coating. The mechanism of overall process is schematized in Fig. 11. 3.4. Equivalent time of laboratory tests Results obtained from corrosion degree and calculated from percentage of attacked surface after 1 year of field tests can be used to obtain the necessary laboratory time to yield the same numeric value by means laboratory accelerated tests. This correspondence is calculated for each coating and exposition environment.
Fig. 12. Equivalent time between field and laboratory conditions. (a) Marine environment (NaCl solution); (b) urban environment (NaHSO3 solution).
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The result is represented in Fig. 12. Ideally, this graphic should be a horizontal line, but the different coatings do not have the same equivalent time. The higher test time was selected from the linear regression as the overall equivalent time. The correlation equations in marine an urban environment are: Marine environment (NaCl solution): t = 626 − 0.069 s Urban environment (NaHSO3 solution): t = 744 − 0.055 s where t is time in hours and s the percentage of attacked surface. These results allow us to conclude that 1-year of field tests is equivalent to 744 and 626 h, respectively of laboratory cyclic accelerated tests. Obviously, this conclusion is only a first approximation to reproduce the field conditions, because the conditions of tests are directly related to the involved phenomena complexity. In spite of these difficulties, the similarity between field and laboratory tests in attack mechanism, coating failure, and corrosion products leads us to conclude a comparable overall behavior for two classes of experiments.
4. Conclusions Corrosion resistance of carbon steel was evaluated using as protective coatings with three different thermoplastic polymers and an alkyd resin modified conveniently with low percentages of polyaniline, a conductive polymer widely used in relation with good anticorrosive properties. All coatings were physicochemically characterized by the usual techniques, as spectroscopy and thermal analysis. The more important conclusions derived from the obtained results in this work are: 1. Thermoplastic polymer coatings have a corrosion resistance very low in comparison with alkyd resin modified with conductive polymer. This resistance is particularly poor at short times of the field and laboratory tests. In spite of this behavior, the phenoxy resin shows a greater resistance that the acrylic blends and the vinyl copolymer. Although the characteristics of alkyd resin without conductive polymer are poor in relation to the thermoplastics polymers used in this work, the corrosion resistance is clearly improved by the presence of the conductive polymer. 2. Corrosion mechanism is very different in thermoplastic and alkyd resin, as a direct consequence of different degradation processes of the different coatings. The existence of crazing and failure is characteristic of alkyd resins based coatings. In these systems, the experimental results based
in spectroscopic information show the influence of the conductive polymer in the decreasing of the degradation, remaining the pigments in the coating and avoiding the access of pollutants to metal. Moreover, the corrosion mechanism based in the existence of cathode and anode zones is prevented by the electron movement of the electrons to metal surface. 3. Field tests in marine and urban environments may be properly reproduced by means accelerated laboratory tests, using sodium chloride and hydrogen sulfite solutions, respectively. In fact, all the results can be accurately compared qualitatively and quantitatively. Differences are related to the experimental procedure. The calculated equivalent for laboratory tests proves the validity of these cyclic accelerated tests.
Acknowledgements Financial support for this work was provided by MCYT of Spain, and FEDER with Grant No. MAT2003-00251.
References [1] M. Osterhold, Prog. Org. Coat. 40 (2000) 131. [2] C. Pagella, D.M. De Faveri, Prog. Org. Coat. 33 (1998) 211. [3] H.J. Jacobasch, K. Grundke, St. Scheneider, F. Simon, Prog. Org. Coat. 26 (1995) 131. [4] T.A. Misev, R. van der Linde, Prog. Org. Coat. 34 (1998) 160. [5] E. Lugscheider, S. B¨arwulf, C. Barimari, M. Riester, H. Hilgers, Prog. Org. Coat. 108/109 (1998) 398. [6] T. Schauer, A. Joos, L. Dulog, C.D. Eisenbach, Prog. Org. Coat. 33 (1998) 20. [7] D.W. De Berry, J. Electrochem. Soc. 132 (1985) 1022. [8] D.A. Wroblewski, B.C. Bencewicz, K.G. Thompson, C.J. Bryan, Polym. Prep. 35 (1) (1994) 265. [9] J.I. Iribarren, F. Liesa, F. Cadena, L. Bilurbina, Mater. Corr., in press. [10] J.M. Mark, Physical Properties of Polymers Handbook, AIP Press, Woodburg, New York, 1996. [11] J.I. Iribarren, M. Iriarte, C. Uriarte, J.J. Iruin, J. Appl. Polym. Sci. 37 (1989) 3459. [12] M. Iriarte, J.I. Iribarren, A. Etxeberr´ıa, J.J. Iruin, Polymer 30 (1989) 1160. [13] J. Hodson, J. Lander, Polymer 28 (1987) 251. [14] J. Lucki, J.F. Rabek, B. Ranby, C. Ekstr¨om, Eur. Polym. J. 17 (1981) 919. [15] G. Bierwagen, Prog. Org. Coat. 15 (1987) 179. [16] C. Gettinger, A.J. Heeger, D.J. Pine, Y. Cao, Synth. Met. 74 (1995) 81. [17] H. V¨olz, Photodegradation and photoestabilitation of coatings, ACS Symp. Ser. 136 (1981) 1511. [18] T. Schauer, A. Joos, L. Dulog, C. Eisenbach, Prog. Org. Coat. 33 (1998) 20. [19] Mc. Andrew, TRIP 5 (1997) 71. [20] J. Standish, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 357.