- Email: [email protected]
Influence of substrate on the cathodic electrodeposition behavior of waterborne epoxy resins
Progress in Organic Coatings 54 (2005) 292–295
Influence of substrate on the cathodic electrodeposition behavior of waterborne epoxy resins Zahra Ranjbar a,∗ , Siamak Moradian b b
a Iran Color Research Center, Department of Surface Coatings and Corrosion, Tehran, Iran Amirkabir University of Technology, Faculty of Polymer & Color Engineering, P.O. Box 15875-4413, Tehran, Iran
Received 6 April 2005; accepted 27 June 2005
Abstract A modified epoxy–amine adduct was prepared and was then emulsified in water, and was subsequently deposited on a cathode substrate at constant voltage (200 V) or constant current (1 mA/cm2 ) by the aid of a DC power supply. The cathode was made of different substrates such as bare steel, phosphated steel and aluminum. The results show that the film conductance was greatest on the aluminum substrate which was followed by bare steel and phosphated steel. © 2005 Elsevier B.V. All rights reserved. Keywords: Substrate; Electrocoating; Film growth; Epoxy; Aluminum; Steel; Phosphated steel
1. Introduction Since the introduction of electrodeposition (ED) processes for the painting of metal objects in the 1960s, these processes have been adopted worldwide in the automotive, industrial and appliance areas to provide corrosion resistant coats to a variety of products [1]. Polymer coatings provide corrosion protection by acting as a barrier layer between the substrate material and the environment [2–4]. All polymer coatings are to some degree permeable to water, oxygen and various ions depending on the characteristics of the polymer coating, the type of substrate, the surface treatment and the metal/coating interface [5]. Electrocoatings offer superior performances by the aid of processes and materials that are environmentally friendly and economically efficient. Major advantages of the electrocoating processes include highly automated, total coverage of metal parts having complex shapes with unsurpassed film uniformity, material transfer efficiencies normally in the 95–99% range. Closed-loop systems with excellent productivity and low operating costs, fast line speeds, high part racking densi∗
Corresponding author. E-mail address: [email protected] (Z. Ranjbar).
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.06.016
ties, very low air and wastewater emissions that foster environmental compliance, and totally enclosed systems leading to cleaner and safer paint application methods [6]. Corrosion protection of metallic substrates by organic coatings is a complex process and depends on many different factors such as electrical, chemical and mechanical properties of the coating, adhesion of the coating to the substrate, sorption characteristics of the coating (water and oxygen uptake), ion penetration through the coating and surface characteristics of the metal substrate. It is well known that surface characteristics are an important factor determining the corrosion stability of the organic coatings [7,8]. This factor also affects the deposition behavior of the electrocoatings. The aim of this work was to investigate the deposition behavior of an epoxy coating electrodeposited on bare steel, phosphated steel and aluminum substrates.
2. Experimental 2.1. Materials The epoxy resin used in this study was Epon 1004 (EEW of 806-909 from Shell Chemical Co.), diethanolamine
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
(ANALR) and acetic acid (ANALR), ethylene glycol monon-butyl ether (Butyl CelloSolve-BCS from BASF), glycidyl ester of versatic 10 acid (Cardura E10 from Shell Chemical Co.) and hexamethoxymethyl melamine (HMMM, Mapranal MF900 from Hoechst Co.), aluminum (JIS# 5000). 2.2. Equipments and instruments 2.2.1. Reactor assembly A four-necked round bottom glass flask (1 l capacity) equipped with a mechanical stirrer, a thermometer, a reflux system and a dropping funnel, was used as the basic reactor unit for resin preparation. The heating was carried out in a 1 l heating mantle, up to reaction temperatures of 180 ◦ C. 2.2.2. Electrodeposition cell The electrodeposition cell consisted of a 500-ml glass container containing 350 ml of depositable solution. The anode was made of a rectangular stainless steel plate 8 cm × 3 cm, equal in size and shape to the cathode (i.e. a bare mild steel panel). The anode and the cathode were connected to the respective terminals of a DC power supply (0–450 V, 4 A from EmenTablo Co.). The distance between the anode and the cathode was about 8 cm. The solution was magnetically stirred for the complete duration of the electrodeposition process. 2.2.3. Practical work The cataphoretically applicable epoxy resins were prepared by reacting the stoichiometric amounts of Epon 1004 (at 90% solid content in butylcellosolve) with diethanolamine. The amine equivalent weight of this resin was about 1000, the actual value which is recommended in the literature for such purposes [9]. In order to improve the corrosion resistance of the resin, it was then reacted with Cardura E-10 [10]. Dispersions of the resin in deionized water were prepared using a homogenizer at 85% neutralization level of ionizable groups by adding 10% acetic acid. The cross-linking of the resin was made to occur by using 20% HMMM (based on the epoxy–amine adduct solid). The prepared dispersions were further diluted by distilled water (i.e. to about 8–10% solid content). Electrodeposition can be carried out at a constant current or at a constant voltage on bare steel, phosphated steel and aluminum. The current densities generally used in electrodeposition are in the order of a few mA/cm2 . When the current is applied, the following electrochemical reactions start. The main reactions are: at the cathode : at the anode :
293
where Cx = 0 is the concentration of hydroxyl ions at the interface, Cb the bulk concentration, j the current density, t the time, F the Faraday’s constant and D is the diffusion coefficient of hydroxyl ions. From the Sand’s equation, it is possible to calculate the interfacial concentration of hydroxyl ions and the pH at the electrode surface. It is only after this critical concentration has been reached that the film will start to form at the cathode. In contrast to galvanostatic experiments where a preliminary time of a few seconds is needed before the formation of the polymer film commences, in constant voltage experiments, the electrode is completely covered with a polymer film within fractions of a second. This is caused by the flows of large currents at the beginnings of the electrodeposition process at constant voltage [1,11,12]. Epoxy coatings were electrodeposited on bare steel, phosphated steel and aluminum substrates, the resin concentration in the electrodeposition bath was 8 wt.% solid in water at pH 5.68, the temperature was 28 ◦ C, applied voltage was 200 V (at constant voltage experiments) and current density was 1 mA/cm2 (at constant voltage experiments) after 3 min. The electrodeposition behavior of each resin was studied by measuring its attained dry film thickness, the voltage and current density variations with time and its corresponding throwing power.
3. Results A weighed substrate panel (the cathode) along with a stainless steel anode of the same size was immersed into the above composition in the electrodeposition cell. After connecting the electrodes, the required time and voltage were set. Freshly deposited films on the panels were rinsed with mildly flowing distilled water and were then cured for 20 min at 180 ◦ C. The voltage–time behavior of the films for different substrates is shown in Fig. 1. It can be seen that in the case of the
2H2 O + 2e− → 2OH− + H2 2H2 O → 4e− + O2 + 4H+
The concentration of these ions at the electrode/electrolyte interface is given by the Sand’s equation [1]: Cx=0 = Cb +
2j F (t/πD)0.5
Fig. 1. The effect of substrate on the variation of the voltage–time function at constant current density (T = 30 ◦ C, t = 180 s, D = 8 cm, j = 1 mA/cm2 and solid content = 8%).
294
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
Fig. 2. The variation of current density–time at constant voltage for the different substrates (T = 30 ◦ C, t = 180 s, D = 8 cm, V = 200 V and solid content = 8%).
phosphated steel, the voltage rises as soon as the circuit is connected. However, for bare steel and aluminum, the case is different and the voltage rises only slowly. The study of electrodeposition behavior was followed by observing the current flow as a function of time at constant voltage (Fig. 2). The current density for all curves decayed towards a residual current, which is the maximum for aluminum. The drop in current density with time was steepest for the phosphated steel. The decrease was more gradual for the aluminum substrate. At constant voltage, the hydroxide ions migrate to bulk solution through the gaps and pores in the film and the film grows at the film/bulk solution interface. The electrodeposited organic film consists of a conductive polymeric gel containing ionic sites. Evidently, hydroxyl ions jump from site to site in the gel. The sites are tertiary ammonium groups attached to the organic polymer chains [1]. The preset voltage is a main factor determining the deposited weight and the film thickness on the substrate. Increasing the voltage leads to increased deposited weight as well as increase in the film thickness. Fig. 3 shows the variation of the deposited weight, the film thickness and the throwing power for different substrates. Throwing power is the ability of a coating system to deposit films in recessed
Fig. 3. The effect of substrate on the throwing power, the deposited weight and the dry film thickness (T = 30 ◦ C, t = 180 s, D = 8 cm, V = 200 V and solid content = 8%).
areas. Many methods have been proposed to measure the throwing power. We measured the throwing power by the method discussed by Yang and Chen [13]. In the measurement procedure, the epoxy–amine adduct resin dispersion was poured into a stainless steel cylindrical vessel up to a height of 26 cm from the bottom, and the bath was maintained at 25 ◦ C by the aid of uniform magnetically induced stirring. The test strips (8 mm × 220 mm × 0.8 mm) and the hollow glass tube were immersed to a depth of 23 cm. The coating time was constant at 180 s and the voltage was set at a predetermined desired value (between 50 V and 250 V). The distance measured in centimeters from the bottom of the strip upwards is the throwing power of each epoxy–amine adducts dispersion. Film resistance is a main factor affecting the throwing power.
4. Discussion The results show that the slope of the voltage–time curve is the least steep for the aluminum substrate (see Fig. 1). The current cut-offs also occur at longer times (see Fig. 2). Furthermore, the throwing power was minimum and the deposited weight and dry film thickness were maximum for the aluminum substrate (see Fig. 3). When the electrodeposition of a coating starts initially, only the electrolysis of water occurs, which results in hydrogen gas evolution and hydroxyl ion production at the surface of the cathode increasing the local value of pH to 12–13. In the case of an aluminum substrate, cathodic corrosion takes place in addition to the chemical reaction of hydroxyl groups with the protecting oxide layer. The overall reaction being as follows: Al + 2H2 O + e− → AlO2 − + 2H2 Thus, as can be seen this process may also generate hydrogen gas evolutions, therefore the overall hydrogen gas evolution can increase to three-folds [14]. Excessive gassing at the cathode results in porous film formation. When a porous film is formed, the conductance of the resin is so much lower than the conductance of the electrolyte, so most of the conduction is through the pores in the film and the polymer conduction could be neglected [1]. As the electrodeposition continues, the fraction of cathode area covered by pores diminishes and the voltage starts to rise. Eventually, the electrode will seal itself off and voltage will rise rapidly. In the case of phosphated steel, highly porous and electrical resistant zinc and iron phosphate layers of a few microns thickness are formed onto the steel substrate prior to the electrodeposition process. Consequently, the current will flow only through the pores of the inorganic layer, a polymeric film will form on the substrate, sealing the pores and so the voltage will rise rapidly. The throwing power of the electrocoating is the highest for the phosphated steel; this is because the conductance of the polymeric film and underlying phosphated layer is the least. In the case of aluminum substrate, since the conductivity of
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
the deposited film is the highest, the throwing power is least and the dry film thickness is the highest. The behavior of the bare steel lies somewhere in between the behavior of aluminum and the phosphated steel substrates.
5. Conclusion It can be concluded that the underlying substrate affects the properties of the deposited film. On the aluminum substrate, because of the cathodic corrosion, films that are more porous and conductive were obtained. In the case of phosphated steel, a less porous film forms on the surface, so the throwing power is greater (current cut-offs at shorter times) and the voltage rise occurs more rapidly. The behavior of the bare steel lies between the aluminum and the phosphated steel.
295
References [1] P. Pierce, J. Coat. Technol. 53 (672) (1981) 53. [2] U. Rammelt, G. Reinhard, Prog. Org. Coat. 21 (1992) 205. [3] T. Monetta, F. Bellucci, L. Nicodemo, L. Nicolais, Prog. Org. Coat. 21 (1993) 353. [4] P.L. Bonora, F. Deflorian, L. Fedrizzi, Electrochim. Acta 41 (1996) 1073. [5] J.E.O. Mayne, Official Digest 24 (1952) 127–136. [6] J.J. Oravitz, Prod. Finish. 220 (1997). [7] E.M. Geenen, E.P. Mewsting, J.H. Wwit, Prog. Org. Coat. 19 (1990) 295. [8] F. Deflorian, L. Fedrizzi, P.L. Bonora, Corrosion 50 (1994) 113. [9] S.B.A. Qaderi, K.R. Carduner, D.R. Bauer, J. Coat. Technol. 59 (747) (1987) 37. [10] W. Raundendusch, Epoxy Resin Chemistry, ACS, 1979, p. 57. [11] F. Beck, Prog. Org. Coat. 4 (1976) 1. [12] G.E.F. Brewer, Electrodeposition of Coatings, ACS, 1973, p. 151. [13] C.-P. Yang, Y.-H. Chen, J. Appl. Polym. Sci. 42 (1991) 1097. [14] P. Hulser, U.A. Kruger, F. Beck, Corros. Sci. 48 (1) (1996) 47.
Influence of substrate on the cathodic electrodeposition behavior of waterborne epoxy resins Zahra Ranjbar a,∗ , Siamak Moradian b b
a Iran Color Research Center, Department of Surface Coatings and Corrosion, Tehran, Iran Amirkabir University of Technology, Faculty of Polymer & Color Engineering, P.O. Box 15875-4413, Tehran, Iran
Received 6 April 2005; accepted 27 June 2005
Abstract A modified epoxy–amine adduct was prepared and was then emulsified in water, and was subsequently deposited on a cathode substrate at constant voltage (200 V) or constant current (1 mA/cm2 ) by the aid of a DC power supply. The cathode was made of different substrates such as bare steel, phosphated steel and aluminum. The results show that the film conductance was greatest on the aluminum substrate which was followed by bare steel and phosphated steel. © 2005 Elsevier B.V. All rights reserved. Keywords: Substrate; Electrocoating; Film growth; Epoxy; Aluminum; Steel; Phosphated steel
1. Introduction Since the introduction of electrodeposition (ED) processes for the painting of metal objects in the 1960s, these processes have been adopted worldwide in the automotive, industrial and appliance areas to provide corrosion resistant coats to a variety of products [1]. Polymer coatings provide corrosion protection by acting as a barrier layer between the substrate material and the environment [2–4]. All polymer coatings are to some degree permeable to water, oxygen and various ions depending on the characteristics of the polymer coating, the type of substrate, the surface treatment and the metal/coating interface [5]. Electrocoatings offer superior performances by the aid of processes and materials that are environmentally friendly and economically efficient. Major advantages of the electrocoating processes include highly automated, total coverage of metal parts having complex shapes with unsurpassed film uniformity, material transfer efficiencies normally in the 95–99% range. Closed-loop systems with excellent productivity and low operating costs, fast line speeds, high part racking densi∗
Corresponding author. E-mail address: [email protected] (Z. Ranjbar).
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.06.016
ties, very low air and wastewater emissions that foster environmental compliance, and totally enclosed systems leading to cleaner and safer paint application methods [6]. Corrosion protection of metallic substrates by organic coatings is a complex process and depends on many different factors such as electrical, chemical and mechanical properties of the coating, adhesion of the coating to the substrate, sorption characteristics of the coating (water and oxygen uptake), ion penetration through the coating and surface characteristics of the metal substrate. It is well known that surface characteristics are an important factor determining the corrosion stability of the organic coatings [7,8]. This factor also affects the deposition behavior of the electrocoatings. The aim of this work was to investigate the deposition behavior of an epoxy coating electrodeposited on bare steel, phosphated steel and aluminum substrates.
2. Experimental 2.1. Materials The epoxy resin used in this study was Epon 1004 (EEW of 806-909 from Shell Chemical Co.), diethanolamine
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
(ANALR) and acetic acid (ANALR), ethylene glycol monon-butyl ether (Butyl CelloSolve-BCS from BASF), glycidyl ester of versatic 10 acid (Cardura E10 from Shell Chemical Co.) and hexamethoxymethyl melamine (HMMM, Mapranal MF900 from Hoechst Co.), aluminum (JIS# 5000). 2.2. Equipments and instruments 2.2.1. Reactor assembly A four-necked round bottom glass flask (1 l capacity) equipped with a mechanical stirrer, a thermometer, a reflux system and a dropping funnel, was used as the basic reactor unit for resin preparation. The heating was carried out in a 1 l heating mantle, up to reaction temperatures of 180 ◦ C. 2.2.2. Electrodeposition cell The electrodeposition cell consisted of a 500-ml glass container containing 350 ml of depositable solution. The anode was made of a rectangular stainless steel plate 8 cm × 3 cm, equal in size and shape to the cathode (i.e. a bare mild steel panel). The anode and the cathode were connected to the respective terminals of a DC power supply (0–450 V, 4 A from EmenTablo Co.). The distance between the anode and the cathode was about 8 cm. The solution was magnetically stirred for the complete duration of the electrodeposition process. 2.2.3. Practical work The cataphoretically applicable epoxy resins were prepared by reacting the stoichiometric amounts of Epon 1004 (at 90% solid content in butylcellosolve) with diethanolamine. The amine equivalent weight of this resin was about 1000, the actual value which is recommended in the literature for such purposes [9]. In order to improve the corrosion resistance of the resin, it was then reacted with Cardura E-10 [10]. Dispersions of the resin in deionized water were prepared using a homogenizer at 85% neutralization level of ionizable groups by adding 10% acetic acid. The cross-linking of the resin was made to occur by using 20% HMMM (based on the epoxy–amine adduct solid). The prepared dispersions were further diluted by distilled water (i.e. to about 8–10% solid content). Electrodeposition can be carried out at a constant current or at a constant voltage on bare steel, phosphated steel and aluminum. The current densities generally used in electrodeposition are in the order of a few mA/cm2 . When the current is applied, the following electrochemical reactions start. The main reactions are: at the cathode : at the anode :
293
where Cx = 0 is the concentration of hydroxyl ions at the interface, Cb the bulk concentration, j the current density, t the time, F the Faraday’s constant and D is the diffusion coefficient of hydroxyl ions. From the Sand’s equation, it is possible to calculate the interfacial concentration of hydroxyl ions and the pH at the electrode surface. It is only after this critical concentration has been reached that the film will start to form at the cathode. In contrast to galvanostatic experiments where a preliminary time of a few seconds is needed before the formation of the polymer film commences, in constant voltage experiments, the electrode is completely covered with a polymer film within fractions of a second. This is caused by the flows of large currents at the beginnings of the electrodeposition process at constant voltage [1,11,12]. Epoxy coatings were electrodeposited on bare steel, phosphated steel and aluminum substrates, the resin concentration in the electrodeposition bath was 8 wt.% solid in water at pH 5.68, the temperature was 28 ◦ C, applied voltage was 200 V (at constant voltage experiments) and current density was 1 mA/cm2 (at constant voltage experiments) after 3 min. The electrodeposition behavior of each resin was studied by measuring its attained dry film thickness, the voltage and current density variations with time and its corresponding throwing power.
3. Results A weighed substrate panel (the cathode) along with a stainless steel anode of the same size was immersed into the above composition in the electrodeposition cell. After connecting the electrodes, the required time and voltage were set. Freshly deposited films on the panels were rinsed with mildly flowing distilled water and were then cured for 20 min at 180 ◦ C. The voltage–time behavior of the films for different substrates is shown in Fig. 1. It can be seen that in the case of the
2H2 O + 2e− → 2OH− + H2 2H2 O → 4e− + O2 + 4H+
The concentration of these ions at the electrode/electrolyte interface is given by the Sand’s equation [1]: Cx=0 = Cb +
2j F (t/πD)0.5
Fig. 1. The effect of substrate on the variation of the voltage–time function at constant current density (T = 30 ◦ C, t = 180 s, D = 8 cm, j = 1 mA/cm2 and solid content = 8%).
294
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
Fig. 2. The variation of current density–time at constant voltage for the different substrates (T = 30 ◦ C, t = 180 s, D = 8 cm, V = 200 V and solid content = 8%).
phosphated steel, the voltage rises as soon as the circuit is connected. However, for bare steel and aluminum, the case is different and the voltage rises only slowly. The study of electrodeposition behavior was followed by observing the current flow as a function of time at constant voltage (Fig. 2). The current density for all curves decayed towards a residual current, which is the maximum for aluminum. The drop in current density with time was steepest for the phosphated steel. The decrease was more gradual for the aluminum substrate. At constant voltage, the hydroxide ions migrate to bulk solution through the gaps and pores in the film and the film grows at the film/bulk solution interface. The electrodeposited organic film consists of a conductive polymeric gel containing ionic sites. Evidently, hydroxyl ions jump from site to site in the gel. The sites are tertiary ammonium groups attached to the organic polymer chains [1]. The preset voltage is a main factor determining the deposited weight and the film thickness on the substrate. Increasing the voltage leads to increased deposited weight as well as increase in the film thickness. Fig. 3 shows the variation of the deposited weight, the film thickness and the throwing power for different substrates. Throwing power is the ability of a coating system to deposit films in recessed
Fig. 3. The effect of substrate on the throwing power, the deposited weight and the dry film thickness (T = 30 ◦ C, t = 180 s, D = 8 cm, V = 200 V and solid content = 8%).
areas. Many methods have been proposed to measure the throwing power. We measured the throwing power by the method discussed by Yang and Chen [13]. In the measurement procedure, the epoxy–amine adduct resin dispersion was poured into a stainless steel cylindrical vessel up to a height of 26 cm from the bottom, and the bath was maintained at 25 ◦ C by the aid of uniform magnetically induced stirring. The test strips (8 mm × 220 mm × 0.8 mm) and the hollow glass tube were immersed to a depth of 23 cm. The coating time was constant at 180 s and the voltage was set at a predetermined desired value (between 50 V and 250 V). The distance measured in centimeters from the bottom of the strip upwards is the throwing power of each epoxy–amine adducts dispersion. Film resistance is a main factor affecting the throwing power.
4. Discussion The results show that the slope of the voltage–time curve is the least steep for the aluminum substrate (see Fig. 1). The current cut-offs also occur at longer times (see Fig. 2). Furthermore, the throwing power was minimum and the deposited weight and dry film thickness were maximum for the aluminum substrate (see Fig. 3). When the electrodeposition of a coating starts initially, only the electrolysis of water occurs, which results in hydrogen gas evolution and hydroxyl ion production at the surface of the cathode increasing the local value of pH to 12–13. In the case of an aluminum substrate, cathodic corrosion takes place in addition to the chemical reaction of hydroxyl groups with the protecting oxide layer. The overall reaction being as follows: Al + 2H2 O + e− → AlO2 − + 2H2 Thus, as can be seen this process may also generate hydrogen gas evolutions, therefore the overall hydrogen gas evolution can increase to three-folds [14]. Excessive gassing at the cathode results in porous film formation. When a porous film is formed, the conductance of the resin is so much lower than the conductance of the electrolyte, so most of the conduction is through the pores in the film and the polymer conduction could be neglected [1]. As the electrodeposition continues, the fraction of cathode area covered by pores diminishes and the voltage starts to rise. Eventually, the electrode will seal itself off and voltage will rise rapidly. In the case of phosphated steel, highly porous and electrical resistant zinc and iron phosphate layers of a few microns thickness are formed onto the steel substrate prior to the electrodeposition process. Consequently, the current will flow only through the pores of the inorganic layer, a polymeric film will form on the substrate, sealing the pores and so the voltage will rise rapidly. The throwing power of the electrocoating is the highest for the phosphated steel; this is because the conductance of the polymeric film and underlying phosphated layer is the least. In the case of aluminum substrate, since the conductivity of
Z. Ranjbar, S. Moradian / Progress in Organic Coatings 54 (2005) 292–295
the deposited film is the highest, the throwing power is least and the dry film thickness is the highest. The behavior of the bare steel lies somewhere in between the behavior of aluminum and the phosphated steel substrates.
5. Conclusion It can be concluded that the underlying substrate affects the properties of the deposited film. On the aluminum substrate, because of the cathodic corrosion, films that are more porous and conductive were obtained. In the case of phosphated steel, a less porous film forms on the surface, so the throwing power is greater (current cut-offs at shorter times) and the voltage rise occurs more rapidly. The behavior of the bare steel lies between the aluminum and the phosphated steel.
295
References [1] P. Pierce, J. Coat. Technol. 53 (672) (1981) 53. [2] U. Rammelt, G. Reinhard, Prog. Org. Coat. 21 (1992) 205. [3] T. Monetta, F. Bellucci, L. Nicodemo, L. Nicolais, Prog. Org. Coat. 21 (1993) 353. [4] P.L. Bonora, F. Deflorian, L. Fedrizzi, Electrochim. Acta 41 (1996) 1073. [5] J.E.O. Mayne, Official Digest 24 (1952) 127–136. [6] J.J. Oravitz, Prod. Finish. 220 (1997). [7] E.M. Geenen, E.P. Mewsting, J.H. Wwit, Prog. Org. Coat. 19 (1990) 295. [8] F. Deflorian, L. Fedrizzi, P.L. Bonora, Corrosion 50 (1994) 113. [9] S.B.A. Qaderi, K.R. Carduner, D.R. Bauer, J. Coat. Technol. 59 (747) (1987) 37. [10] W. Raundendusch, Epoxy Resin Chemistry, ACS, 1979, p. 57. [11] F. Beck, Prog. Org. Coat. 4 (1976) 1. [12] G.E.F. Brewer, Electrodeposition of Coatings, ACS, 1973, p. 151. [13] C.-P. Yang, Y.-H. Chen, J. Appl. Polym. Sci. 42 (1991) 1097. [14] P. Hulser, U.A. Kruger, F. Beck, Corros. Sci. 48 (1) (1996) 47.