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Degradation mechanism of an acrylic water-based paint applied to steels
Progress in Organic Coatings 47 (2003) 164–168
Degradation mechanism of an acrylic water-based paint applied to steels M. Bethencourt a , F.J. Botana a,∗ , M.J. Cano a , R.M. Osuna a , M. Marcos b a
Departamento de Ciencia de los Materiales e Ingenier´ıa Metalúrgica y Qu´ımica Inorgánica, Facultad de Ciencias del Mar, Universidad de Cádiz, Avda. República Saharaui s/n, Apdo. 40, Puerto Real, 11510 Cádiz, Spain b Departamento de Ingenier´ıa Mecánica y Diseño Industrial, Universidad de Cádiz, Escuela Superior de Ingenier´ıa, C/Chile s/n, 11003 Cádiz, Spain Received 12 December 2002; accepted 13 May 2003
Abstract The behaviour, in conditions of total immersion, of an acrylic water-based paint applied to a St 35.8 steel, has been studied using electrochemical techniques. The data obtained have enabled a three-step mechanism to be proposed. In the first step, there is an increase in the activity of the system, as a consequence of the entry of water. In the second step, once a critical level in the water content has been passed, the inhibitor pigments are activated, and this leads to a decrease in the electrochemical activity. Finally, once that the pigments have been exhausted, there is a loss of the protective properties of the paint, with the result that the activity detected is similar to that of the bare metal. © 2003 Published by Elsevier B.V. Keywords: Water-based paint; Steel; Acrylic resin; Linear polarisation; EIS
1. Introduction Organic coatings, particularly paints, constitute one of the most widely employed methods of protection to prevent the corrosion of steel. Among the reasons accounting for their widespread use are their low cost, the ease of application and their aesthetic functionality. As a result, it is estimated that approximately 90% of all metallic surfaces are protected with paints [1]. Traditionally, the most frequently used paints are those that utilise volatile organic compounds (VOCs) as solvents. However, the employment of this type of substance presents serious disadvantages in respect of both environmental and occupational health considerations. In recent years, this has given rise to intensive research activity directed towards the development of paints in which the VOC content is reduced or even eliminated. Among the various alternatives developed are the paints that utilise water as solvent. In parallel to the studies on the substitution of the type of solvent, there are studies in which attempts have been made to eliminate from the formulation of the paints those pigments that incorporate heavy metals, such as chrome and lead, and to replace them with others that are neither toxic nor harmful to the natural environment [2].
∗ Corresponding author. Tel./fax: +34-956-016-154. E-mail address: [email protected] (F.J. Botana).
0300-9440/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0300-9440(03)00124-3
The new generation of paints that employ non-contaminating inhibitors and water as solvent usually suffer from the problem that they do not achieve as good an anticorrosive performance as the contaminating paints that they are intended to replace. This problem becomes especially critical in sectors like the shipbuilding industry where high specification products must be used. This study reports the behaviour of a commercial acrylic water-based paint applied to a pressure-blasted naval steel; a variety of electrochemical techniques are used for this. This study is part of a research project aimed at evaluating the anticorrosive behaviour of different ecological paints employed in the shipbuilding industry.
2. Experimental The behaviour of “Intercryl”, a commercial acrylic paint produced by International Paints Ltd., has been studied. Table 1 gives the more important technical characteristics of this paint. A naval steel, St 35.8, has been employed as the metallic substratum. The samples studied were rectangular test pieces of 100 mm × 150 mm × 2 mm, dry pressure blasted with copper shot until a finish of type Sa 3 was obtained. Prior to being painted, the samples were degreased with acetone. The paint was then applied using a short-pile roller up to a thickness of 75 m, the optimum thickness recommended in the technical data sheet of the paint.
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Table 1 Technical sheet of Intercryl 506 paint from International Marine Coatings Product description
Water-based acrylic primer
Intended uses
For use at new building and maintenance and repair behind linings, on internal accommodation and store rooms
Product information
Colour: WPA300-Gray Volume solids: 46 ± 2% (ISO 3233:1998) Typical film thickness: 75 m dry (163 m wet) Theoretical coverage: 6.13 (m2 /l) Method of application: airless spray, brush, roller
Regulatory data VOC
51 g/l as supplied (EPA Meted 24)
The protective capacity of the coating was evaluated by monitoring the behaviour of the samples in immersion tests, using electrochemical techniques. For this, electrochemical cells were constructed in which an area of 13 cm2 of one of the painted surfaces of the test pieces was exposed. The test piece thus served as the working electrode. A solution of 3.5% NaCl was utilised as the aggressive medium. A saturated calomel electrode was used as the reference electrode, and a graphite bar as the counter-electrode. A 1287 potentiostat coupled to a 1255 frequency response analyser (FRA), both from Solartron, was used to make the electrochemical measurements. The scanning velocity for recording the polarisation curves was 0.1667 mV/s. The range of frequency measurement employed for recording the impedance spectra was 106 to 10−3 Hz and the amplitude of the signal in respect of the corrosion potential was 10 mV.
3. Results and discussion 3.1. Evolution of the corrosion potential Fig. 1 shows the evolution of the corrosion potential, in function of the time of exposure, for a painted sample, immersed in NaCl for 90 days. In accordance with [3], the evolution of this parameter can, together with other methods, provide information on the mechanism of degradation of the paint during the period of exposure. In this figure, it can be observed how, during the first 3 days of exposure, the corrosion potential undergoes a displacement towards more active potentials in comparison with the initial potential. This type of displacement can be interpreted as the consequence of a progressive increase of the fraction of the area that presents an anodic behaviour. As the time of exposure is increased, a displacement of the potential towards more noble values is observed, and this must be interpreted as an increase in the fraction of the surface area that presents a cathodic behaviour. This inversion in the activity can be interpreted, in accordance with [3], as a consequence of the water and oxygen having reached the metal–paint interface, after
Fig. 1. Time evolution of the corrosion potential in a painted sampled immersed in a 3.5% NaCl solution.
this period of time. The process of reduction of the oxygen generates alkaline conditions that limit the oxidation of the metal beneath the coating. This situation is maintained for the full 19 days of exposure, and over this period a very sharp decrease of the corrosion potential occurs. This is when the paint loses its corrosion-resistance properties, and is completely degraded such that the corrosion potential tends towards values typical of bare steel in 3.5% sodium chloride. 3.2. Linear polarisation In Fig. 2, the anodic polarisation curves corresponding to samples subjected to tests for increasing periods of exposure are represented. In the same figure, the curve corresponding to the bare metallic substratum in NaCl at 3.5% is included as reference. As can be observed, all the curves are characterised by presenting a limiting value of anodic current density, im . In this way, the level of protection provided by the paint may be evaluated using this parameter. It can be
Fig. 2. Anodic polarisation curves corresponding to the indicated samples immersed in a 3.5% NaCl solution.
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Fig. 3. (a) Nyquist diagrams and (b) Bode diagrams corresponding to the indicated samples immersed in a 3.5% NaCl solution.
effectively confirmed how, for the recently immersed sample, I value has various orders of magnitude less than that for the bare metal, which gives an idea of its level of protection. As the time of exposure is increased, it is observed that there is a progressive increase in the values of I, hence for very prolonged times of exposure, values of I close to those corresponding to the bare metal are reached. This last finding demonstrates that the paint has lost its protective properties. 3.3. Electrochemical impedance spectroscopy Fig. 3 shows the EIS diagrams corresponding to tests made on painted samples after being subjected to tests of different periods of duration. The data obtained demonstrate that, in the first 12 h of immersion, there is a considerable decrease of the arch that appears in the Nyquist diagrams, Fig. 3a. In parallel, a decrease is observed in the modulus of the impedance in the Bode diagrams, Fig. 3b. This decrease in the impedance suggests that, during the first hours of immersion, there is an increase in the activity taking place in the system in this period, a result that is also evident from the diagrams of the evolution of the corrosion potential against time, Fig. 1.
The Nyquist diagrams of the system studied are characterised by presenting a single capacitive arch, Fig. 3. For this reason, the equivalent circuit of Fig. 4 has been selected to simulate the response of the system [4]. In this circuit, Re represents the resistance between the working electrode and the reference electrode, generally associated with the ohmic resistance of the electrolyte. Cp is related to the capacity of the paint, Rpo is the resistance of the pores and is a measurement of the porosity that arises as a consequence of the degradation of the paint. Finally, Zif represents the impedance of the electrochemical processes that take place at the metal–paint interface. Study of the evolution of the diagrams of EIS with time of immersion enables an analysis to be made about the
Fig. 4. Equivalent circuit representing the coating system.
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Fig. 5. Time evolution of Cp and Rpo values calculated from EIS spectra. Fig. 6. Time evolution of water uptake calculated from Brashner and Kinsbury equation.
variation of the protective capacity of a specific organic coating applied onto a metallic substratum [4]. In our case, from the fit of the experimental diagrams to the equivalent circuit proposed, the values of the capacity, Cp , and of the resistance, Rpo , associated with the layer of paint, have been calculated. Fig. 5 presents the evolution of these parameters during the first 12 h of exposure. In this figure, it can be observed how, as the time of exposure is increased, there is an increase in the capacity of the paint, and a decrease of the associated resistance. In accordance with [5,6], this type of behaviour is due to an increase of the conductivity of the polymeric film, as a consequence of the increase in the number of defects in the paint, because of these defects, the water reaches the substratum more rapidly. Similarly, in [7,8] the decrease of Rpo , during the first hours of exposure, is associated with the entry of water in the pores of the film, which provides the electrolyte necessary to initiate the processes of corrosion. The end result of all these effects is that the paint loses part of its protective properties by barrier effect, during the first hours of immersion. In the case of the paint studied here, this rapid decrease in the protection by barrier effect may be explained by taking into account that this is a water-based acrylic resin, with linear polymers, that therefore presents a high permeability to the electrolyte. The loss of the protection by barrier effect that a paint experiences can be evaluated by measurement of the volume of water that enters, and the rate at which this water penetrates the system [9]. The entry of water not only affects the value of the resistance but also modifies the dielectric properties of the organic coating, causing an increase in the value of its capacity. Thus, the time for the coating to be saturated by the intake of water coincides with that necessary for the value of the capacity of the coating to stabilise. In the case of the paint studied, the time to saturation is about 12 h, Fig. 5. Further, assuming that there is a uniform distribution of water in the polymeric matrix, the percentage of water entering (φ) at each moment can be calculated making use of the Brashner and Kinsbury ratio [10], by means
of the following formula φ (%) =
log(Ct /C0 ) × 100 log 80
where φ is the percentage by volume of water that enters into the film. Ct and Cor are the values of the capacity of the paint at t seconds and for t = 0, respectively, and 80 is the relative permittivity of the water at 20 ◦ C. Fig. 6 shows the values of φ obtained in function of exposure time. The results indicate √ that, in the system studied, φ does not vary linearly with t, from which it must be concluded that the process of diffusion of water in the paint is not ideally Fickian. This type of behaviour is customary in paints, since paint is a heterogeneous mixture in which each of its components interacts in a different way with the entry of water [11]. In such cases, in [12,13] it is proposed that the denominated apparent coefficient of diffusion (Dapp ) should be calculated using the following expression Dapp =
(2d)2 τ
with d being the thickness of the paint (cm) and τ being the time constant that corresponds to the process of saturation of the film (s) obtained from the data included in Fig. 5. The value of Dapp obtained is 5.21 × 10−9 cm2 s−1 , which is slightly higher than the values proposed in [14,15] for solvent-based paints. The increase in the value of Dapp can be explained by the fact that water is utilised as the solvent. Further, in accordance with the calculated values of Dapp , in this type of paint the entry of water could be facilitated, improving the functioning of the anticorrosive pigments, that requires the presence of the electrolyte in order to inhibit the process of corrosion of the metal [16]. For prolonged periods of exposure, it can be confirmed from Fig. 3 how, after the first 4 days of immersion, there is an improvement in the protective properties of the paint. This improvement can be evaluated through the increase detected in the impedance of the system, by which the value
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of Rpo increases up to 107 cm2 at 16 days of testing. This behaviour can be associated with the effect of the inhibitor pigments present in the paint. The entry of water that takes place in the first hours of immersion facilitates the solution of the pigments. Once dissolved, the pigments may act either by promoting the formation of surface films of oxides on the metal, or else by forming insoluble compounds as a result of their reaction with corrosion products or with by-products of the agglutinating ingredient [17]. One of the specific pigments utilised in the formulation of the paint studied is zinc phosphate. The mechanism of actuation of this compound has not been clearly established. However, several authors [18,19] propose that, when the electrolyte penetrates the paint, the solution of the pigment takes place. The solution formed would cause the phosphatation of the metallic substratum. In this way, a film would be formed that would passivate the metallic surface. The passivation of the metallic surface would explain the increase observed in the values of impedance. On the other hand, other authors [20,21] propose that the inhibitory action of the zinc phosphate is based on the precipitation of a film of very stable ferric phosphate on the metallic surface. This film partially adheres to the steel and is capable of blocking the pores, by means of the diffusion of the ferric phosphate that is deposited, thus diminishing the amount of water and oxygen that reaches the metal–paint interface. The final degradation of the paint is produced at 92 days of exposure and can be detected through the sudden decrease of the diameter of the capacitive arch in the Nyquist diagrams, Fig. 3. This degradation can be explained by the disappearance of the inhibitory effect of the anticorrosive pigment. As the time of exposure is increased, conduits for the electrolyte through the layer of paint are being continually formed. The protective effect is maintained all the while that the pigment is able to act. When the pigment is exhausted, and there is an increase in the number of channels by which the electrolyte can penetrate the coating, the pores cannot be blocked nor the surface kept passivated, and the paint finishes by losing its protective capacity. This conclusion is demonstrated by the sudden decrease in the values of Rpo , which drop to below 103 cm2 .
4. Conclusions The behaviour, in conditions of total immersion, of an acrylic water-based paint applied to a St 35.8 steel, has been studied using electrochemical techniques. The set of data obtained has enabled a mechanism for the degradation of the paint in three steps to be proposed. In accordance with the discussion presented, the first step corresponds to the process of entry of water into the system. The analysis of the data
of electrochemical impedance spectroscopy has enabled the non-Fickian characteristics of this process, together with the time required for saturation, to be established. This step is characterised by an increase in the electrochemical activity of the system that takes place at the same time. The second step of the mechanism proposed coincides with the period during which a decrease in the activity is observed. This decrease has been associated with the solution of the inhibitor pigments present in the paint. Finally, the third step is characterised by the sudden increase produced in the activity of the system, which has been associated with the exhaustion of the inhibitor pigments and the continuous increase in the porosity of the layer of paint.
Acknowledgements This work has been financed by the Interministerial Commission for Science and Technology, project MAT2001-3477 and by the Junta de Andaluc´ıa. References [1] K. Barton, Protection Against Atmospheric Corrosion, Wiley, New York, 1976. [2] M.C. Perez Perez, Tesis Doctoral, Universidad de Vigo, 1998, pp. 20–23. [3] O. Ferraz, E. Cavalcanti, A.R. di Sarli, Corros. Sci. 37 (1995) 1267– 1280. [4] P.L. Bonora, F. Deflorian, L. Fedrizzi, Electrochem. Acta 41 (7–8) (1996) 1073. [5] G.W. Walter, Corros. Sci. 32 (10) (1991) 1041. [6] E. Potwin, L. Brossard, G. Larochelle, Prog. Org. Coat. 31 (1997) 363–373. [7] F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Mater. Sci. Forum (1998) 289–292, 337–346. [8] J.A. González, E. Otero, A. Bautista, E. Almeida, M. Morcillo, Prog. Org. Coat. 33 (1998) 61–67. [9] S. Feliu, J.C. Galván, M. Morcillo, Prog. Org. Coat. 17 (1989) 143– 153. [10] D.M. Brashner, A.H. Kinsbury, J. Appl. Chem. 4 (1954) 62. [11] M.C. Perez Perez, Tesis Doctoral, Universidad de Vigo, 1998, p. 156, 203. [12] L. Diguet, Tesis Doctoral, Univ. Paris VI, 1996. [13] S.J. Downey, O.F. Devereux, Corrosion 45 (8) (1989) 675. [14] V.B. Miskovic-StanKovic, D.M. Drazic, M.J. Teodorovic, Corros. Sci. 37 (2) (1995) 241–252. [15] V.B. Miskovic-StanKovic, D.M. Drazic, Z. Kacarevic-Popovic, Corros. Sci. 38 (9) (1996) 1513–1523. [16] W. Funke, Prog. Org. Coat. 31 (1997) 5–9. [17] S. González, R.M. Souto, Materiales y Procesos Electródicos 1 (2002) 83–130. [18] G. Adrian, A. Bittner, J. Coat. Technol. 58 (740) (1986) 59. [19] R. Romagnoli, V.F. Vetere, Corrosion 51 (2) (1995) 116. [20] J. Caprari, A. Di Sarli, B. del Amo, Pigment Resin Technol. 29 (1) (2000) 16–22. [21] A. Guenbour, A. Benbachir, A. Kacemi, Surf. Coat. Technol. 133 (1999) 36–43.
Degradation mechanism of an acrylic water-based paint applied to steels M. Bethencourt a , F.J. Botana a,∗ , M.J. Cano a , R.M. Osuna a , M. Marcos b a
Departamento de Ciencia de los Materiales e Ingenier´ıa Metalúrgica y Qu´ımica Inorgánica, Facultad de Ciencias del Mar, Universidad de Cádiz, Avda. República Saharaui s/n, Apdo. 40, Puerto Real, 11510 Cádiz, Spain b Departamento de Ingenier´ıa Mecánica y Diseño Industrial, Universidad de Cádiz, Escuela Superior de Ingenier´ıa, C/Chile s/n, 11003 Cádiz, Spain Received 12 December 2002; accepted 13 May 2003
Abstract The behaviour, in conditions of total immersion, of an acrylic water-based paint applied to a St 35.8 steel, has been studied using electrochemical techniques. The data obtained have enabled a three-step mechanism to be proposed. In the first step, there is an increase in the activity of the system, as a consequence of the entry of water. In the second step, once a critical level in the water content has been passed, the inhibitor pigments are activated, and this leads to a decrease in the electrochemical activity. Finally, once that the pigments have been exhausted, there is a loss of the protective properties of the paint, with the result that the activity detected is similar to that of the bare metal. © 2003 Published by Elsevier B.V. Keywords: Water-based paint; Steel; Acrylic resin; Linear polarisation; EIS
1. Introduction Organic coatings, particularly paints, constitute one of the most widely employed methods of protection to prevent the corrosion of steel. Among the reasons accounting for their widespread use are their low cost, the ease of application and their aesthetic functionality. As a result, it is estimated that approximately 90% of all metallic surfaces are protected with paints [1]. Traditionally, the most frequently used paints are those that utilise volatile organic compounds (VOCs) as solvents. However, the employment of this type of substance presents serious disadvantages in respect of both environmental and occupational health considerations. In recent years, this has given rise to intensive research activity directed towards the development of paints in which the VOC content is reduced or even eliminated. Among the various alternatives developed are the paints that utilise water as solvent. In parallel to the studies on the substitution of the type of solvent, there are studies in which attempts have been made to eliminate from the formulation of the paints those pigments that incorporate heavy metals, such as chrome and lead, and to replace them with others that are neither toxic nor harmful to the natural environment [2].
∗ Corresponding author. Tel./fax: +34-956-016-154. E-mail address: [email protected] (F.J. Botana).
0300-9440/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0300-9440(03)00124-3
The new generation of paints that employ non-contaminating inhibitors and water as solvent usually suffer from the problem that they do not achieve as good an anticorrosive performance as the contaminating paints that they are intended to replace. This problem becomes especially critical in sectors like the shipbuilding industry where high specification products must be used. This study reports the behaviour of a commercial acrylic water-based paint applied to a pressure-blasted naval steel; a variety of electrochemical techniques are used for this. This study is part of a research project aimed at evaluating the anticorrosive behaviour of different ecological paints employed in the shipbuilding industry.
2. Experimental The behaviour of “Intercryl”, a commercial acrylic paint produced by International Paints Ltd., has been studied. Table 1 gives the more important technical characteristics of this paint. A naval steel, St 35.8, has been employed as the metallic substratum. The samples studied were rectangular test pieces of 100 mm × 150 mm × 2 mm, dry pressure blasted with copper shot until a finish of type Sa 3 was obtained. Prior to being painted, the samples were degreased with acetone. The paint was then applied using a short-pile roller up to a thickness of 75 m, the optimum thickness recommended in the technical data sheet of the paint.
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165
Table 1 Technical sheet of Intercryl 506 paint from International Marine Coatings Product description
Water-based acrylic primer
Intended uses
For use at new building and maintenance and repair behind linings, on internal accommodation and store rooms
Product information
Colour: WPA300-Gray Volume solids: 46 ± 2% (ISO 3233:1998) Typical film thickness: 75 m dry (163 m wet) Theoretical coverage: 6.13 (m2 /l) Method of application: airless spray, brush, roller
Regulatory data VOC
51 g/l as supplied (EPA Meted 24)
The protective capacity of the coating was evaluated by monitoring the behaviour of the samples in immersion tests, using electrochemical techniques. For this, electrochemical cells were constructed in which an area of 13 cm2 of one of the painted surfaces of the test pieces was exposed. The test piece thus served as the working electrode. A solution of 3.5% NaCl was utilised as the aggressive medium. A saturated calomel electrode was used as the reference electrode, and a graphite bar as the counter-electrode. A 1287 potentiostat coupled to a 1255 frequency response analyser (FRA), both from Solartron, was used to make the electrochemical measurements. The scanning velocity for recording the polarisation curves was 0.1667 mV/s. The range of frequency measurement employed for recording the impedance spectra was 106 to 10−3 Hz and the amplitude of the signal in respect of the corrosion potential was 10 mV.
3. Results and discussion 3.1. Evolution of the corrosion potential Fig. 1 shows the evolution of the corrosion potential, in function of the time of exposure, for a painted sample, immersed in NaCl for 90 days. In accordance with [3], the evolution of this parameter can, together with other methods, provide information on the mechanism of degradation of the paint during the period of exposure. In this figure, it can be observed how, during the first 3 days of exposure, the corrosion potential undergoes a displacement towards more active potentials in comparison with the initial potential. This type of displacement can be interpreted as the consequence of a progressive increase of the fraction of the area that presents an anodic behaviour. As the time of exposure is increased, a displacement of the potential towards more noble values is observed, and this must be interpreted as an increase in the fraction of the surface area that presents a cathodic behaviour. This inversion in the activity can be interpreted, in accordance with [3], as a consequence of the water and oxygen having reached the metal–paint interface, after
Fig. 1. Time evolution of the corrosion potential in a painted sampled immersed in a 3.5% NaCl solution.
this period of time. The process of reduction of the oxygen generates alkaline conditions that limit the oxidation of the metal beneath the coating. This situation is maintained for the full 19 days of exposure, and over this period a very sharp decrease of the corrosion potential occurs. This is when the paint loses its corrosion-resistance properties, and is completely degraded such that the corrosion potential tends towards values typical of bare steel in 3.5% sodium chloride. 3.2. Linear polarisation In Fig. 2, the anodic polarisation curves corresponding to samples subjected to tests for increasing periods of exposure are represented. In the same figure, the curve corresponding to the bare metallic substratum in NaCl at 3.5% is included as reference. As can be observed, all the curves are characterised by presenting a limiting value of anodic current density, im . In this way, the level of protection provided by the paint may be evaluated using this parameter. It can be
Fig. 2. Anodic polarisation curves corresponding to the indicated samples immersed in a 3.5% NaCl solution.
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Fig. 3. (a) Nyquist diagrams and (b) Bode diagrams corresponding to the indicated samples immersed in a 3.5% NaCl solution.
effectively confirmed how, for the recently immersed sample, I value has various orders of magnitude less than that for the bare metal, which gives an idea of its level of protection. As the time of exposure is increased, it is observed that there is a progressive increase in the values of I, hence for very prolonged times of exposure, values of I close to those corresponding to the bare metal are reached. This last finding demonstrates that the paint has lost its protective properties. 3.3. Electrochemical impedance spectroscopy Fig. 3 shows the EIS diagrams corresponding to tests made on painted samples after being subjected to tests of different periods of duration. The data obtained demonstrate that, in the first 12 h of immersion, there is a considerable decrease of the arch that appears in the Nyquist diagrams, Fig. 3a. In parallel, a decrease is observed in the modulus of the impedance in the Bode diagrams, Fig. 3b. This decrease in the impedance suggests that, during the first hours of immersion, there is an increase in the activity taking place in the system in this period, a result that is also evident from the diagrams of the evolution of the corrosion potential against time, Fig. 1.
The Nyquist diagrams of the system studied are characterised by presenting a single capacitive arch, Fig. 3. For this reason, the equivalent circuit of Fig. 4 has been selected to simulate the response of the system [4]. In this circuit, Re represents the resistance between the working electrode and the reference electrode, generally associated with the ohmic resistance of the electrolyte. Cp is related to the capacity of the paint, Rpo is the resistance of the pores and is a measurement of the porosity that arises as a consequence of the degradation of the paint. Finally, Zif represents the impedance of the electrochemical processes that take place at the metal–paint interface. Study of the evolution of the diagrams of EIS with time of immersion enables an analysis to be made about the
Fig. 4. Equivalent circuit representing the coating system.
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Fig. 5. Time evolution of Cp and Rpo values calculated from EIS spectra. Fig. 6. Time evolution of water uptake calculated from Brashner and Kinsbury equation.
variation of the protective capacity of a specific organic coating applied onto a metallic substratum [4]. In our case, from the fit of the experimental diagrams to the equivalent circuit proposed, the values of the capacity, Cp , and of the resistance, Rpo , associated with the layer of paint, have been calculated. Fig. 5 presents the evolution of these parameters during the first 12 h of exposure. In this figure, it can be observed how, as the time of exposure is increased, there is an increase in the capacity of the paint, and a decrease of the associated resistance. In accordance with [5,6], this type of behaviour is due to an increase of the conductivity of the polymeric film, as a consequence of the increase in the number of defects in the paint, because of these defects, the water reaches the substratum more rapidly. Similarly, in [7,8] the decrease of Rpo , during the first hours of exposure, is associated with the entry of water in the pores of the film, which provides the electrolyte necessary to initiate the processes of corrosion. The end result of all these effects is that the paint loses part of its protective properties by barrier effect, during the first hours of immersion. In the case of the paint studied here, this rapid decrease in the protection by barrier effect may be explained by taking into account that this is a water-based acrylic resin, with linear polymers, that therefore presents a high permeability to the electrolyte. The loss of the protection by barrier effect that a paint experiences can be evaluated by measurement of the volume of water that enters, and the rate at which this water penetrates the system [9]. The entry of water not only affects the value of the resistance but also modifies the dielectric properties of the organic coating, causing an increase in the value of its capacity. Thus, the time for the coating to be saturated by the intake of water coincides with that necessary for the value of the capacity of the coating to stabilise. In the case of the paint studied, the time to saturation is about 12 h, Fig. 5. Further, assuming that there is a uniform distribution of water in the polymeric matrix, the percentage of water entering (φ) at each moment can be calculated making use of the Brashner and Kinsbury ratio [10], by means
of the following formula φ (%) =
log(Ct /C0 ) × 100 log 80
where φ is the percentage by volume of water that enters into the film. Ct and Cor are the values of the capacity of the paint at t seconds and for t = 0, respectively, and 80 is the relative permittivity of the water at 20 ◦ C. Fig. 6 shows the values of φ obtained in function of exposure time. The results indicate √ that, in the system studied, φ does not vary linearly with t, from which it must be concluded that the process of diffusion of water in the paint is not ideally Fickian. This type of behaviour is customary in paints, since paint is a heterogeneous mixture in which each of its components interacts in a different way with the entry of water [11]. In such cases, in [12,13] it is proposed that the denominated apparent coefficient of diffusion (Dapp ) should be calculated using the following expression Dapp =
(2d)2 τ
with d being the thickness of the paint (cm) and τ being the time constant that corresponds to the process of saturation of the film (s) obtained from the data included in Fig. 5. The value of Dapp obtained is 5.21 × 10−9 cm2 s−1 , which is slightly higher than the values proposed in [14,15] for solvent-based paints. The increase in the value of Dapp can be explained by the fact that water is utilised as the solvent. Further, in accordance with the calculated values of Dapp , in this type of paint the entry of water could be facilitated, improving the functioning of the anticorrosive pigments, that requires the presence of the electrolyte in order to inhibit the process of corrosion of the metal [16]. For prolonged periods of exposure, it can be confirmed from Fig. 3 how, after the first 4 days of immersion, there is an improvement in the protective properties of the paint. This improvement can be evaluated through the increase detected in the impedance of the system, by which the value
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of Rpo increases up to 107 cm2 at 16 days of testing. This behaviour can be associated with the effect of the inhibitor pigments present in the paint. The entry of water that takes place in the first hours of immersion facilitates the solution of the pigments. Once dissolved, the pigments may act either by promoting the formation of surface films of oxides on the metal, or else by forming insoluble compounds as a result of their reaction with corrosion products or with by-products of the agglutinating ingredient [17]. One of the specific pigments utilised in the formulation of the paint studied is zinc phosphate. The mechanism of actuation of this compound has not been clearly established. However, several authors [18,19] propose that, when the electrolyte penetrates the paint, the solution of the pigment takes place. The solution formed would cause the phosphatation of the metallic substratum. In this way, a film would be formed that would passivate the metallic surface. The passivation of the metallic surface would explain the increase observed in the values of impedance. On the other hand, other authors [20,21] propose that the inhibitory action of the zinc phosphate is based on the precipitation of a film of very stable ferric phosphate on the metallic surface. This film partially adheres to the steel and is capable of blocking the pores, by means of the diffusion of the ferric phosphate that is deposited, thus diminishing the amount of water and oxygen that reaches the metal–paint interface. The final degradation of the paint is produced at 92 days of exposure and can be detected through the sudden decrease of the diameter of the capacitive arch in the Nyquist diagrams, Fig. 3. This degradation can be explained by the disappearance of the inhibitory effect of the anticorrosive pigment. As the time of exposure is increased, conduits for the electrolyte through the layer of paint are being continually formed. The protective effect is maintained all the while that the pigment is able to act. When the pigment is exhausted, and there is an increase in the number of channels by which the electrolyte can penetrate the coating, the pores cannot be blocked nor the surface kept passivated, and the paint finishes by losing its protective capacity. This conclusion is demonstrated by the sudden decrease in the values of Rpo , which drop to below 103 cm2 .
4. Conclusions The behaviour, in conditions of total immersion, of an acrylic water-based paint applied to a St 35.8 steel, has been studied using electrochemical techniques. The set of data obtained has enabled a mechanism for the degradation of the paint in three steps to be proposed. In accordance with the discussion presented, the first step corresponds to the process of entry of water into the system. The analysis of the data
of electrochemical impedance spectroscopy has enabled the non-Fickian characteristics of this process, together with the time required for saturation, to be established. This step is characterised by an increase in the electrochemical activity of the system that takes place at the same time. The second step of the mechanism proposed coincides with the period during which a decrease in the activity is observed. This decrease has been associated with the solution of the inhibitor pigments present in the paint. Finally, the third step is characterised by the sudden increase produced in the activity of the system, which has been associated with the exhaustion of the inhibitor pigments and the continuous increase in the porosity of the layer of paint.
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