Corrosion electrochemical behavior of epoxy anticorrosive paints based on zinc molybdenum phosphate and zinc oxide

Progress in Organic Coatings 36 (1999) 211±216

Corrosion electrochemical behavior of epoxy anticorrosive paints based on zinc molybdenum phosphate and zinc oxide a

L. Velevaa,*, J. China, B. del Amob

Appl. Physics Dept., Centro de InvestigacioÂn y de Estudios Avanzados del IPN (CINVESTAV-IPN), Unidad MeÂrida, Km. 6, Ant. Carr. a Progreso, C.P. 97310, Merida, Yuc., Mexico b Centro de InvestigacioÂn y Desarrollo en TecnologõÂa de Pinturas (CIDEPINT), CIC-CONICE, Calle 52 e/121 y 122. (1900) La Plata, Argentina Received 24 September 1998; accepted 21 May 1999

Abstract Electrochemical impedance spectroscopy (EIS) was applied as a principal tool to describe the ef®ciency of anticorrosive epoxy paints (primers) based on zinc molybdenum phosphate (ZMP) pigment. Steel-coated samples were exposed to a 0.5 M NaCl solution. During the study the corrosion potential (Eoc) and Rp values also were monitored every 24 h. It is discussed the incorporation of micronized ZnO (1 mm) pigment to the base mixture and its positive, reinforcement effect on the protective properties of ZMP primer. The explanation is related to the izoelectric point (IEP) of ZnO particles (pH < 9), which determines their positive surface charge and electrostatic attraction with the molybdate anion. In this case the charge of the formed double layer capacitor is very high. Moreover, the mentioned attraction inhibits and saves ZnO particles from their rapid dissolution to hydroxide. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Zinc molybdenum phosphate; Epoxy anticorrosive paints; Zinc oxide effect; Corrosion mechanism; Electrochemical impedance spectroscopy; Corrosion potential; Resistance polarization

The problems of environmental protection set strict limits for the use of toxic pigments (lead and other heavy metals), historically introduced as corrosion inhibitors in anticorrosive paints. At present a new generation of pigments, called ``environmental friendly'', is looked for elimination of these heavy metals. Zinc molybdenum phosphate seems to be one good possibility and is claimed to have equal or superior anticorrosive behavior than chromate and zinc phosphate alone [1±6]. It is basically composed by zinc phosphate added with zinc molybdate up to 1% (expressed as MoO3). According to the literature, it is claimed to have anticorrosive properties when employed in anticorrosive paints applied on metals. The active inhibitive specie in this pigment is the molybdate anion, which is thought to repassivate the corrosion pits in steel [7]. Little information is available in the literature about zinc molybdenum phosphate (ZMP) introduced in anticorrosive paints. Bittner and coworkers [3,5,6] reported the behavior of ZMP in alkyd paints, compared with zinc phosphate and zinc chromate.

The aim of this work is to study the electrochemical behavior and corrosion protection mechanism of four epoxy anticorrosive paint systems (primers) applied on steel, using electrochemical impedance spectroscopy (EIS) as a principal electrochemical technique. The primers were ®lled with ZMP pigment (employing two different loads) and additionally zinc oxide particles were incorporated. Zinc oxide was selected because of its ability to polarize cathodic areas by precipitating sparingly soluble salts [8]. These paint systems (primers and topcoats) were designed by CIDEPINT (Argentina) and steel-coated samples were exposed also to natural tests in a humid tropical climate of Southeastern Mexico (Peninsula of Yucatan), to obtain more complete information about their anticorrosive performance in this environment. The principal environment of this region is marine coastal, which is very aggressive, due to the combination of high annual Time of Wetness1 and air temperature. Additionally, the presence of chloride ions accelerates the metal corrosion process. Electrochemical impedance spectroscopy (EIS) has already been in use for the evaluation of the performance

*Corresponding author. Fax: +52-5599-812917 E-mail address: [email protected] (L. Veleva)

1 Time of Wetness (TOW) is the time during which the surface is covered with an electrolytic layer; this is the real time when the corrosion occurs.

1. Introduction

0300-9440/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 9 9 ) 0 0 0 4 7 - 8

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of organic coatings and has proven to be a powerful tool in obtaining system speci®c parameters of organic coatings, providing very useful kinetic and mechanism information [9±20]. One speci®c EIS area of investigation is the ef®ciency and mechanism of anticorrosion pigments in coatings [11±13]. 2. Experimental 2.1. Paint (primer) composition The ®lm forming material (binder) was a solvent-borne epoxy-bisphenol polyamide resin and several pigments were used. The principal one, micronized ZMP (average particle diameter 1 mm) was employed as an anticorrosive pigment, with two different loads, 10% and 30% by volume with respect to the total pigment content. The complementary pigments were ferric oxide and barium sulfate and 20% by volume of the resulting mixture were partially replaced by zinc oxide, in the case of two primer systems. The ferric oxide was introduced in the total pigment formula, because its colloidal particles are said to interact with metallic substrates increasing its coverage [21±23]. The solid composition of the four tested anticorrosive paints is shown in Table 1. The low carbon steel (SAE 1010) panels (100  150  2 mm) were previously sandblasted to Sa 2 1/2 (SIS 05 59 00-67), attaining 20  4 mm maximum roughness, degreased with toluene and coated with a wash primer (SSPC-PT 3-64). The paints were applied by means of a spray gun. The total primer thickness for each system was 70 mm. Four samples of each type of primers were evaluated simultaneously. 2.2. Electrochemical measurements and test solution

namic anodic and cathodic curves were done for corrosion state characterization of the metal-coated surface (passive or active state). The measuring cell consisted in a working electrode (coated metal), a Pt counter-electrode and a saturated calomel reference electrode. The area of the painted metal surface was limited by cylindrical open acrylic tube (delimiting 15.2 cm2 circular area) which contained the electrolyte (the mentioned above solution). (All reported potential values in this study are vs. SCE). For these electrochemical measurements potentiostat PC
3 and software CMS 100 GAMRY instruments were used. The frequency EIS range was between 0.01 Hz to 5 kHz and the impedance spectra were obtained with a perturbation voltage in the range of 10± 100 mV. Electrochemical potential (Eoc), Rp values and impedance spectra (EIS) were collected every 24 h, for four coated samples of the same primer type, until the appearance of the steel corrosion potential. 3. Results and discussion It can be seen from Table 1 that the studied systems are dif®cult to discuss, due to the presence of several kinds of pigments particles, distributed in the ®lm of poorly conducting polymer. The existence of voids between these particles leads the paint to behave as a porous electrode, to which a transmission line model is applicable [24,25]. In this mixture of pigments, as an anticorrosive one is reported in the literature the ZMP (its molybdate anion). In two primers (nos. 3 and 4) besides ZMP, Zn was introduced, but as ZnO and therefore cannot provide cathodic protection effect. That observation that the measured potential value (Eoc) of all system samples is positive and similar in the initial state (between 20 and 40 mV vs. SCE; see Fig. 1), suggests an anodic passive effect due to the presence of

For each of these four systems (Table 1), EIS was carried out in order to assess the corrosion phenomena and mechanism of the coating anticorrosion pigments. Additional information was collected, measuring the change of the corrosion potential in an open circuit (Eoc), the polarization resistance (Rp in a 20 mV range) during the exposure time of the coated samples in a highly aggressive 0.5 M NaCl solution (at 22±238C). Previously, in the initial period, potentiodyTable 1 Solids (% by volume) in the anticorrosive paints compositions Primers

1

2

3

4

ZMP Fe2O3 BaSO4 ZnO Epoxy resin

12.1 14.1 14.1 ± 59.7

4.1 18.1 18.1 ± 59.7

12.1 11.3 11.3 5.6 59.7

4.1 14.5 14.5 7.2 59.7

Note: The solvent mixture employed for epoxy paints was toluene/ methyl isobutyl ketone/butyl alcohol (36/52/12, by weight).

Fig. 1. Time dependence of open circuit potential (Eoc) for ZMP primers applied on steel, immersed in a 0.5 NaCl solution.

L. Veleva et al. / Progress in Organic Coatings 36 (1999) 211±216

ZMP. The anodic polarization curves con®rmed this fact, showing a passive state for the metal. In the same time, the cathodic reduction of oxygen was highly controlled by its diffusion through the pores of the polymer matrix. Initially all systems presented a very high Rp in order of G . As can be seen from Fig. 1, during the ®rst 24 h the Eoc of the system no. 2 reached the steel corrosion potential, while those of system nos. 1 and 4 decreased monotonically (after 190 and 140 h, respectively) until 500 h, when the steel corrosion potential also was detected. Only the primer no. 3 was keeping its Eoc value at ÿ190 mV (vs. SCE) during approximately 1460 (61 days) and after that showed the corrosion potential value. The primer no. 3, according to Table 1, is characterized with a high ZMP concentration (similar to no. 1 system) and incorporated ZnO particles. Its good anticorrosion behavior means that the presence of micronized ZnO reinforces the ZMP epoxy system. However, probably there is a necessary ZMP concentration when this positive effect appears, since primer no. 4 (with less that half the ZMP than no. 3) does not present a good property, even in the presence of high ZnO content. The poor corrosion behavior of system no. 2 shows that the ZMP concentration (with less that half that of system no. 1) is not suf®cient to insure the metal passivity effect. One possible explanation of the excellent resistance of the system no. 3 could be explained with reference to the surface electric charge of the ZnO particles, which is positive or negative, depending of the isoelectric point (IEP) of the ZnO particles. It is reported in the literature [20] that the IEP is at pH9: below this pH the ZnO surface has a positive charge which leads to an electrostatic attraction with the negatively charged molybdate anion and, thus, the formed barrier on the steel surface is reinforced. Moreover, due to this electrostatic attraction, the ZnO surface particles are inhibited from corrosion formation of Zn(OH)2 and evolution of H2 during this reaction, which may lead to a dangerous pressure buildup in the polymer ®lm. With exposure time, when the pH value in the pores reaches > 9 (IEP) (due to the cathodic reduction of O2 e.g., formation of OHÿ), the surface of the ZnO particles will be negatively charged and the reinforcement of the passive layer will not occur (the molybdate anion will be rejected now). This conjecture will accelerate the corrosion process of the ZnO and the formation of Zn(OH)2, which is water soluble. An additional information about the anticorrosion ef®ciency of the studied four epoxy systems could be obtained by analyzing the EIS spectra. Fig. 2 shows EIS diagrams (Nyquist plots) of steel samples coated with primers nos. 1, 3 and 4, after 48 h of exposure in a 0.5 M NaCl solution. The no. 2 plot is not presented here, because in the ®rst 24 h this primer reaches the steel corrosion potential value. As can be seen initially, when the corrosion process is not yet detectable, the systems have similar behavior and their maximum impedance values are between 10 and 20 M . The EIS Bode-magnitude diagrams (Fig. 3) demonstrate that primer systems nos. 1, 3 and 4 give straight lines, like a typical

213

Fig. 2. Nyquist diagrams after 48 h of exposure in a 0.5 NaCl solution.

Fig. 3. Bode-magnitude plots obtained after 48 h of exposure in a 0.5 NaCl solution.

capacitive response, with phase angle ÿ908 (Fig. 4), that could be attributed to the passive metal surface. For comparison, here are presented the Bode diagrams of system no. 2 (poor in MZP and without ZnO content), which due to its corrosion state shows very low and constant impedance value in all frequency range (Fig. 3), with phase angle 08 (Fig. 4), that con®rms its pure resistive behavior. After 200 h of exposure in a chloride solution, the EIS spectra (Nyquist forms) of the studied paints had more

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L. Veleva et al. / Progress in Organic Coatings 36 (1999) 211±216

Fig. 4. Bode-phase plots obtained after 48 h of exposure in a 0.5 NaCl solution.

Fig. 5. Nyquist diagram for primer no. 1 (high ZMP content), no. 3 (high ZMP content ‡ ZnO) and no. 4 (low ZMP content ‡ ZnO) after 200 h of exposure in a 0.5 NaCl solution.

de®ned semicircles, indicating a change from pure capacitive behavior to charge transfer controlled corrosion process and partial diffusion control (deformed semicircle). A difference was observed between EIS spectra of primer nos. 1 and 3 (with ZnO and the same ZMP content) presented in the Fig. 5. At this stage the corrosion potential (Eoc) of system no. 1 was reaching the steel corrosion potential and that of primer no. 3 was still more positive (Fig. 1). It can be seen that the introduction of ZnO in the pigment mixture aids the increasing of the real impedance of ZMP, which reinforces

Fig. 6. Nyquist diagrams for primer no. 3 (high ZMP content ‡ ZnO) at different exposure times in a 0.5 NaCl solution.

the total coating resistance of the formed barrier on the metal surface. At the same time of exposure the EIS diagram spectra for system no. 4 (with a low ZMP content, but in the presence of ZnO) shows a well formed loop at high frequencies, corresponding to the charge transfer process control and the second part (at low frequencies) due to the oxygen diffusion control (Fig. 5). As was mentioned above, after 500 h (Fig. 1) only the system no. 3 (with high ZMP content and ZnO particles) kept a good resistance. The changes of its complex plane plots (Nyquist diagrams) during the rest of exposure, until the appearance of the corrosion steel potential, are presented in Fig. 6. It can be seen that the real impedance values do not change so much; however, when the corrosion potential is reached, the impedance falls abruptly and a high frequency loop appears with an oxygen diffusion tail, due to the formed corrosion products. From the corresponding impedance spectra in Bode diagrams (Fig. 7) can be seen that the approaching of the corrosion state is followed by increasing of the horizontal part at low frequencies (phase angle 08) and shorter straight line (capacitive behavior) with ÿ908 phase angle. These changes indicate the primer deterioration and ®nal loss of anticorrosion properties of system no. 3. Probably, as was mentioned above, the corrosion will occur when the local pH reaches value>9 (IEP) and ZnO particles are not protected by the electrostatic attraction of molybdate anions. The dissolution of the ZnO will start and their product Zn(OH)2 is soluble in water. Due to this fact, the pores will be more clean and the acceleration of the corrosion process will be done. Our interest was to interpret the EIS data spectra of the best primer no. 3 and the in¯uence of ZnO addition, using an electrical equivalent circuit and its comparison with the

L. Veleva et al. / Progress in Organic Coatings 36 (1999) 211±216

Fig. 7. Bode-magnitude and Bode-phase plots obtained for primer no. 3 (high ZMP content ‡ ZnO) at different exposure times in a 0.5 NaCl solution. (The thick lines show the fit).

215

Table 2 are presented the values of the parameters included in the equivalent circuit (Fig. 8) after 200 h and the ®nal parameters for the system no. 3 (after 1460 h). As can be seen, due to the additional introduction of ZnO particles in the mixture pigments, after 200 h of exposure the system no. 3 has three orders bigger values of capacitance of the double layer (Cdl) and one order more of pore resistance (Rpo), than those of the system no. 1. These fact aids principally to the increasing of the anticorrosion ef®ciency of system no. 3. Some possible explanation was done above. When the system ZMP‡ZnO (no. 3) reached the corrosion potential, the parameter that changed was the Cdl (two orders more) and some increase was detected in the Rct value. However, also there is a little decreasing of Rpo, due to the cleaning of the pores, as a consequence of Zn(OH)2 dissolution, as was mentioned above. In an attempt to learn more about the corrosion behavior of epoxy anticorrosive paints based on ZMP and ZnO, we are continuing our investigation studying these systems in solutions with a presence of sulfate ions or in acid solutions. 4. Conclusions

Fig. 8. Equivalent electrical circuit for the primer systems no. 1 and no. 3. (Rs ± solution resistance; Cc ± capacitance of the intact coating; Rpo ± pore resistance; Cdl ± capacitance of the double layer; Rct ± charge transfer resistance of the corrosion process at the metal/coating interface).

system no. 1 (in an absence of ZnO). A good ®t was obtained (between experimental and theoretical model data) using the electrical equivalent circuit (Fig. 8) for both primers. In

Using electrochemical techniques, such as potential change at open circuit (Eoc) and EIS, is interpreted the anticorrosive mechanism protection of the zinc molybdenum phosphate (ZMP) pigment introduced in an epoxy primer. In the presence of ZnO the anticorrosion effect of the ZMP system is reinforced, possibly due to the electrostatic attraction between the molybdate anion and the positively charged surface of the ZnO particles (at pH < 9). In such way, the dissolution of ZnO particles to Zn(OH)2 could be inhibited and at the same time the ZMP protected from dissolution. Thus, the formed complex barrier on the metal surface, by molybdate anion and ZnO charged particles, presents increased capacitance (Cdl) of the double layer. However, there is an optimum (necessary) concentration

Table 2 Values of the parameters included in the equivalent circuits Fig. 8 for systems no. 1 and no. 3 after exposure in a 0.5 M NaCl solution Primers

No. 1 (200 h)

No. 3 (200 h)

No. 3 (1460 h)

Rs ( ) Cc (F) Rpo (M ) Cdl (F) Rct (M )

394.5 2.510ÿ92.810ÿ11 1.6 4.710ÿ120.5 0.70.2

394.5 2.010ÿ92.6310ÿ11 16 5.710ÿ91.3 0.80.2

394.5 2.910ÿ93.3610ÿ11 1.2 3.510ÿ70.3 1.80.03

Rs ( ) Cc (F) Rpo (M ) Cdl (F) Rct (M )

394.5 210ÿ9 1.6 4.6910ÿ12 0.7

394.5 210ÿ9 16 5.6910ÿ9 0.8

394.5 210ÿ9 1.2 3.510ÿ7 1.8

The approximate value of Cc could also be calculated using Cc ˆ 0A/d, where  is the relative dielectric constant (about 3.0 for dried epoxy resin), 0 the absolute dielectric constant of vacuum (8.8510ÿ12 F mÿ1), A the electrode surface area and d the thickness of the dielectric layer of the coating.

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ratio ZMP/ZnO that guarantees the good anticorrosion property of the studied paints.

[9] [10] [11] [12]

Acknowledgements

[13]

The authors are grateful to Mexican CONACYT (Project 2667-PA), CINVESTAV-IPN (MeÂrida) and Argentinean CIC for their sponsorship to do this research and to Colores Hispania for providing the anticorrosive pigments to CIDEPINT.

[14]

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