Zinc molybdenum phosphate belongs to the so called second generation phosphate pigments and is claimed to have equal or greater anticorrosive properties than chromates and better than zinc phosphate alone. Little Information is available in the literature about its anticorrosive performance.
The aim of this research was to stucfy the anticorrosive performance of zinc molybdenum phosphate in solvent borne epoxy paints employing two anticorrosive pigment loadings. The effect of incorporating zinc oxide as complemetary pigment was also studied SAE 1010 Steel ponéis were primed and coated with three different paint systems containing the anticorrosive paint and this paint plus a sealer cmd/or a topcoat. The anticorrosive efftciency of the different paint systems was assessed by accelerated tests (salí spray, humidity and accelerated weathering). Electrochemical measurements were done employing the anticorrosive paints alone.
Results showed that the highest anticorrosive effect was obtained employing 30% of zinc molybdenum phosphate. Polarization measurements showed that the anoche film formed on Steel blocked the active sites for oxygen reduction. The incorporation of zinc oxide to pigment formula was detrimental due to its high water absorption and to the fact that it reduced zinc molybdenum phosphate solubility by the common ion effect. Polarization curves of pigments mixtures could be used as a guideline to precüct the anticorrosive coating performance in accelerated and electrochemical tests. However, the final decisión on pigment selection musí be taken on the basis of accelerated triáis

del Amo, Beatriz

Romagnoli, Roberto

Vetere, Vicente

Véleva, L.

Undetermined

Centro de Servicios en Gestión de Información

HIGH PERFO RM ANCE ANTICO RRO SIVE EPO X Y  PA IN TS PIG M EN TED  W ITH ZINC  M O L Y B D E N U M  PHOSPHATEPINTURAS EPOXIDICAS ANTICORROSIVAS DE ALTA EFICIENCIA PIGMENTADAS CON FOSFATO DE CINC MODIFICADO CON MOUBDATO DE CINCR. Rom agnoli1, B. del Amo1 2 , V. F. Vetere3, L. Véleva4SUMMARYZinc molybdenum phosphate belongs to the so called second generation phosphate pigments and is claimed to have equal or greater anticorrosive properties than chromates and better than zinc phosphate alone. Little Information is available in the literature about its anticorrosive performance.The aim o f this research was to stucfy the anticorrosive performance o f zinc molybdenum phosphate in solvent borne epoxy paints employing two anticorrosive pigment loadings. The effect o f incorporating zinc oxide as complemetary pigm ent was also studiedSAE 1010 Steel ponéis were prim ed and coated with three different paint systems containing the anticorrosive pa in t and this paint plus a sealer cmd/or a topcoat. The anticorrosive efftciency o f the different paint systems was assessed by accelerated tests (salí spray, humidity and accelerated weathering). Electrochemical measurements were done employing the anticorrosive paints alone.Results showed that the highest anticorrosive effect was obtained employing 30% o f zinc molybdenum phosphate. Polarization measurements showed that the anoche film  form ed on Steel blocked the active sites fo r  oxygen reduction. The incorporation o f zinc oxide to pigment form ula was detrim ental due to its high water absorption and to the fa c t that it reduced zinc molybdenum phosphate solubility by the common ion effect. Polarization curves o f pigm ents mixtures could be used as a guideline to precüct the anticorrosive coating perform ance in accelerated and electrochemical tests. However, the fin a l decisión on pigm ent selection musí be taken on the basis o f accelerated triáis.Keywords: zinc molybdenum phosphate, epoxy anticorrosive paints, zinc oxide, accelerated tests, electrochemical tests.1 Miembro de la Carrera del Investigador del CONICET; Profesor Adjunto, UNLP2 Miembro de la Carrera del Investigador del CONICET3 Profesor Titular, UNLP; Jefe Area Estudios Electroquímicos Aplicados a Problemas de Corrosión y Antioorrosión4 Investigador del Centro de Investigación y Estudios Avanzados (CINVESTAV), México215INTRODUCTTONZinc molybdenum phosphate belongs to the so called second generation phosphate pigments. It is basically composed by zinc phosphate added with zinc molybdate up to 1 % (expressed as M0 O3) and is claimed to have equal or greater anticorrosive behaviour than chromates and undoubtely better than zinc phosphate alone [1-6]. The active inhibitive species in this pigment is molybdate anión which is thought to repassivate corrosión pits in Steel [7].Little information is available in the literature about zinc molybdenum phosphate anticorrosive performance. Adrián and Bittner [3, 5, 6 ] reported the behaviour of zinc molybdenum phosphate in alkyd paints in comparison with zinc phosphate and zinc chromate. The performance of zinc molybdenum phospahte and other pigments belonging to the second generation phosphate pigments series in compliant primers was also tested [6 ].The purpose of the present research was to study the anticorrosive properties of zinc molybdenum phosphate in solvent borne epoxy paints employing two anticorrosive pigment loadings. The effect of incorporating zinc oxide was also studied. Zinc oxide was selected because of its ability to polarize cathodic areas by precipitating sparingly soluble salts [8]. Painted panels with either the anticorrosive paint alone or with a complete paint system were subjected to the salt spray, humidity chamber and accelerated ageing tests. Electrochemical tests were also performed on Steel panels coated only with the anticorrosive paint to avoid higher barrier effects due to the complete paint system.EXPERIMENTALBinder. The film forming material was an epoxy bisphenol polyamide resin.Pigment. Micronized zinc molybdenum phosphate (average particle diameter 1 pm) was employed as anticorrosive pigment with two different loadings, 10 and 30 % by volume with respect to the total pigment contení. The complementary pigments were ferric oxide and barium sulfate and the mixture resulting on partially replacing these pigments (2 0 % by volume) by zinc oxide. Ferric oxide was also introduced in the pigment formula because its colloidal particles are said to internet with metallic substrates increasing its coverage [9-11]. Solids contained in the tested anticorrosive paints is shown in Table I.Table ISolids in anticorrosive paints compositions (%  by volume)Paint 1 2 3 4Zinc molybdenum phosphate 12.1 4.1 12.1 4.1Ferric oxide 14.1 18.1 11.3 14.5Barium sulfate 14.1 18.1 11.3 14.5Zinc oxide — 5.6 7.2Epoxy resin 59.7 59.7 59.7 59.7Note: The solvent mixture employed for epoxy paints was toluene/ methyl isobutyl ketone/ butyl alcohol (36/52/12, by weight)216The inhibitive properties o f pigments mixtures were evaluated by means o f anodic and cathodic polarization curves. In each case the swept began in the vicinity o f corrosión potential at a sweep rate of 3 mV.s"1. The electrolytic cell was composed by an iron electrode (working electrode), a saturated calomel electrode (SCE) as reference and a platinum counterelectrode. The electrolyte was 0.5 M sodium perchlorate solution. Cathodic curves were obtained immediately after the anodic swept.Paints m anufacture and application. It was carried out employing a ball mili with a 3.3 liters jar. The resin solution was added firstly while the pigments were incorporated later; the system was dispersed for 24 hours to achieve an acceptable dispersión degree, grade 4 in Hegman’s scale [12].A sealer and a topcoat, whose composition may be seen in Table EL, were prepared with a similar method. In the case o f the sealer, the non leafing aluminium was incorporated after pigment dispersión to avoid reaction with chlorinated rubber resin.Table nSealer and topcoat compositions expressed as % by volumeComponents PercentajeSealer TopcoatFerric oxide 2.2 —Barium sulfate 2.2 —Non leafing aluminium 4.7 —Zinc oxide 0.8 —Titanium dioxide — 7.3Alkyd resin (solids) 5.6 20.8Chlorinated rubber (RIO) 16.8 4.8Chlorinated paraffin (42%) 7.2 2.1Pine oil 0.5 0.5Xilene 40.0 43.0Aromasol H 20 21.5Test panels were previously sandblasted to Sa 2 1/2 (SIS 05 59 00-67) attaining 20 ± 4 jim máximum roughness, degreased with toluene and coated with a wash primer (SSPC-PT 3- 64). Paints applied by means o f a spray gun on SAE 1010 Steel panels (15.0 x 7.5 x 0.2 cm) and three different paint Systems were studied (Table n i) to assess their anticorrosive performance.Salí spray test (ASTM B 117). After 1500 hours exposure panels were evaluated to establish the oxidation degree according to ASTM D 610. In all cases experiences were camed out in triplicate, detennining the mean valué o f the results obtained in the test. Accelerated test were done on panels coated with the anticorrosive paint alone and with different paint systems (Table IH).217T ab le  mTested paint systemsIdentificationPaint systems (thickness, between brackets)1 Anticorrosive paint alone (75 ± 5 jim )2Anticorrosive paint (40 ± 5 jim)+Sealer (30 ± 5 jim)3Anticorrosive paint (40 ± 5 jim)+Sealer ( 30 ± 5jim)+Topcoat (40 ± 5 jim)Hum idity cabinet test (ASTM D 2247). Panels were placed in the humidity chamber at 38 ± Io C for 2650 hours. The blistering degree was established according to the ASTM D 714 standard specification.Accelerated ageing test (ASTM G 26). The accelerated degradation of painted samples was canied out in a Atlas Weather Ometer (Xenón are type). The test program consisted of a 102 minutes üght eyele followed by 18 minutes light water spray eyele. The overall time of each eyele was 2 hours and that of the complete test 1100 hours. The degree of rusting was evaluated according to the above mentioned standard specification.Corrosión potential measurements. The cells to perform these measurements were constructed by delimiting 3 cm2 circular zones on the painted surface. A cylindrical open acrylic tube, with one fíat end, 7 cm long, was then placed on the specimen and the electrolyte (0.5 M sodium perchlorate solution) placed in the tube. The measurements of the corrosión potential of the painted Steel substrate with respect to the SCE, were made employing a high impedance voltmeter. Electrochemical tests were carried out on Steel panels covered only with anticorrosive paints.Ionic resistance measurements. The resistance between the Steel substrate and a platinum electrode was al so measured employing the cells described previously and an ATI Orion, model 170, conductivity meter which operates at a 1000 Hz frequeney.Polarization resistance measurements. The polarization resistance of painted specimens was determined as a fimetion of immersion time employing an electrochemical cell with three electrodes. The reference electrode was the SCE and the counterelectrode a platinum grid. The voltage swept was + 10 mV, starting from the corrosión potential. Measurements were carried out employing an EG&G PAR Potentiostat/Galvanostat, Model 273A and the software SOFTCORR 352. Polarization resistance of uncoated Steel was also monitored as a function of the immersion time.218RESULTS AND DISCUSSIONPigments m ixtures. Anodic polarization curves (Fig. 1) reveal that pigments mixtures 1 and 3 (with the highest zinc molybdenum phosphate contení) have the best inhibitive properties because they have the lowest passivation current for the second peak. In this sense, the presence o f zinc oxide was detrimental because it made difficult to achieve an effective Steel passivation (pigment mixture 2). However, zinc oxide restrained the effect of lowering the zinc molybdenum phosphate content (pigment mixture 4). It is thought that the passive film formed by corrosión is similar to that formed at potentials previous to the second peak (which corresponds to the oxidation of ferrous species to ferric ones); in the case of mixtures 1 and 3 a more protective film seemed to be formed because, as it was said before, the current corresponding to the second peak decreased notably.F ig.l.- Anodic polarization corves o f the iron electrode in the difíerent pigment mixtures tested. Swept rate 3 m V^1.From the cathodic polarization curves (Fig. 2), it may be seen that the higher the zinc molybdenum phosphate content in the pigment mixture the lower the oxygen diffiision current is. The presence o f zinc oxide in mixture 3 resulted detrimental because it increased the oxygen difiiision current. It must be remembered that the highest the oxygen diffiision current the higher the corrosión rate would result.As the cathodic curve was obtained after performing the anodic one, it was concluded that the film formed on the electrode during the anodic sean blocked the actives sites for oxygen reduction.According to the foregoing discussion, it may be expected that paints formulated with the pigment mixture 1 must show the best anticorrosive performance and paints incorporating mixtures 2 and 4 the worst one.219Fig.2.- Cxthodic polarization curves of tbe iron dectrode ín the different pígmcnt mixtiLres tested in this research. Swept rale 3 m V j1.Salí spray test (ASTM B 117). Results obtained in the salt spray cabinet after 1500 hours of testing are shown in Table IV. The anticorrosive protection was more efficient for paints with the highest zinc molybdenum phosphate contenís (paint 1). The incorporation of zinc oxide did not improve coating performance as it could be expected from data in the literature [12] and previous results obtained with alkyd paints [13]. This is due to the fact that zinc oxide has a relatively high water absorption [14] and that it reduces zinc molybdenum phosphate solubility by the common ion effect. Evidently, these characteristics prevailed over the catodic protection provided by this pigment.Paint system 2 (anticorrosive paint+sealer) proved to be as efficient as system 1 (two coats of the anticorrosive paint); this may be attributed to initial high barrier effect developed for anticorrosive paints as it will be said later. The presence of a topcoat (Paint system 3) enhanced the barrier effect and all systems showed high effciency including those containing zinc oxide. Results were confirmed after paint system removal.Hum idity cabinet test (ASTM D 2247). Paints blistering results are presented in Table V. Paint with the higher anticorrosive pigment content ( paint 1, Table I) did not present blistering after 2640 hours exposition . Blistering increased as the zinc oxide content did. It was not observed signs of corrosión under the blisters except for paint 4.3 after 1500 hours (Table IV and V). In all cases the blistering was higher in system 3 but blisters originated between at the anticorrosive sealer paint interfase.220T a b le lVRusting degree (ASTM D 610) after exposare in the salt spray testPaintSystems Time (hours)360 720 960 1300 15001.1 10 10 10 9 81.2 10 10 10 10 91.3 10 10 10 10 102.1 10 10 10 8 72.2 10 10 9 8 62.3 10 10 10 10 103.1 10 10 10 8 63.2 10 10 10 9 73.3 10 10 10 10 104.1 10 10 10 7 64.2 10 10 10 7 64.3 10 10 10 10 8Note. In the paint system oolumn, the first number corresponds to  thetested antioorrosive paint (Table 1)and the seoond one idenlifies the paint system.Table VBlistering degree (ASTM D 714) after exposure in humidity testPaintSystems Time (hours)360 720 960 1500 1872 26401.1 10 10 10 10 10 101.2 10 10 10 8F 8F 8F1.3 10F 8F 8F 8F 8F 8F2.1 8F 8M 8M 8M 8M 8M2.2 6F 6F 6M 6M 4MD 4MD2.3 4F 4MD 4D 4D 4D 4D3.1 8F 8F 8F 8F 8F 8F3.2 8F 8MD 8MD 8D 6F 6F3.3 6F 6F 6F 6F 6F 6F4.1 8F 8MD 8MD 8MD 8MD 8MD4.2 8F 8D 8D 6F 6F 6F4.3 6F 6MD 6MD 6MD 6MD 6MDNote: The numeral represents blister size: 10 no blistering, 8 the smallest size blister, etc. Letters indícate the frequency:D, dense; MD, médium dense; M, médium; Fiew .Accelerated ageing test (ASTM G 26). All paints exhibited a good behaviour after 1100 hours exposure. No signs of corrosión were observed neither on the painted surface ñor221on the Steel substrate. Blistering was, as a general rule, low and increased slightly for paints containing zinc oxide. It may be expected that these paint systems would perform acceptably in outdoor exposure for at least three years without showing signs of corrosión because, as it was stated in the literature, because 1100 hours of accelerated wheathering may be considered, as an average, equivalent to 3 years outdoor exposure [15].Corrosión potential measurements.The best anticorrosive protection was afforded by paints 1 and 3 and the high barrier effect showed by these paints made it impossible to measure accurately the painted panel potential. As it was predicted by polarization curves obtained with the iron electrode dipped into the different pigment suspensions, the diminishing o f zinc molybdenum phosphate contení impaired the paint performance (paint 2) as well as the incorporation of zinc oxide. In both cases corrosión potential of painted panels derived towards negative valúes during the test period (Fig. 3).Fig. 3.- Corrosión potential of Steel panels as a fnnction of the exposure time,in 0.5 M sodinm perchlorate solation.Ionic resistance measurements The measured resistance is composed by two contributions: the solution resistance and the paint film resistance. As the solution resistance is low (84 Q) the paint film resistance is responsible o f the measured valúes. Polarization effects may be neglected at the measuring frequency employed in this test.The ionic resistance of paints 1 and 3 was higher than 108 and 107 Q.cm'2 respectively, giving fifil protection to the Steel substrate by barrier effect. Paints 2 and 4 showed high ionic resistance at the beginning of the test but this barrier effect was lost as time elapsed (Fig.4). The more impervious films corresponded to the highest anticorrosive pigment contení; this led to think that pigment-binder interaction is responsible for higher barrier effect of paints 1 and3. In the case of paint 3 this was slightly impaired by the presence of zinc oxide in the film, tuming it more susceptible to electrolyte penetration.222Fig. 4.- Resistance o f painted Steel as a  fdnction of tfae exposmre time in 0.5 M  sodimn perclorate sohitíon.Polarízation resistance measurements. The high bairier effect showed by paints 1 and 3 controlled paint behaviour. In these cases a linear response is to be obtained and it makes no sense to measure the variation o f polarization resistance as a function of time. However, once paints 2 and 4 lost part o f their barrier properties polarization resistance was measured and it was found that it exceeded the ionic resistance indicating that zinc molybdenum phosphate inhibited Steel corrosión [16].CONCLUSIONS1. The obtained results showed that the highest antácorrosive effect is obtained employing 30 % by volume o f zinc molybdenum phosphate in the pigment composition. Polarization measurements show that the anodic film formed on Steel blocked the active sites for oxygen reduction.2. The ferric oxide and barium sulfate employed as extenders gave an additional barrier effect.3. Zinc oxide is not recommended in epoxy paints pigmented with zinc molybdenum phosphate. This may be due to its high water absorption and to the fací that it reduces zinc molybdenum phosphate solubility by the common ion effect.4. Polarization curves of pigments mixtures may be used as a guideline to predict the probable anticorrosive coating performance in accelerated and electrochemical tests. However, the final decisión on pigment selection must be taken on the basis o f accelerated triáis.223AGKN OWLEDGEMENTSThe authors are gratefiil to  CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas ), CIC (Comisión de Investigaciones Científicas de la Provincia de Buenos Aires) and UNLP (Universidad Nacional de La Plata) for their sponsorship to do this research. The authors also want to thank Colores Hispania for providing the anticorrosive pigment.REFERENCES[1] Meyer, G.- Farbe+Lack, 69 (7), 528 (1963).[2] Meyer, G.- Farbe+Lack, 71 (2), 113 (1965).[3] Adrián, G.; Gerhard, A.; Bittner, A ; Gawol, M - European Supplement to PolymerPaint Colour Journal, 62 (1981).[4] Leidheiser (Jr.), H.- J. Coat. Tech., 53 (678), 29 (1981).[5] Gerhard, A.; Bittner, A - J. Coat. Tech., 58 (740), 59 (1986).[6] Bittner, A - J. CoatTech., 61 (777), 111 (1989).[7] Ambrose, J.R.- Corrosión (NACE), 34 (1), 27 (1978).[8] Szklarska-Smialowska, Z.; Mankowsky, J.- Br. Corros. J., 4 (9), 271 (1969).[9] Andrade, E.M.; Molina, F.V.; Posadas, D.- X Colloid and Interface Science, 165,450 (1994)[10] Andrade, E.M.; Gordillo, G.J.; Molina, F.V.; Posadas, D.- X Colloid and Interface Science, 173,231 (1995)[11] Andrade, E.M.; Molina, F.V.; Gordillo, G.J.; Posadas, D.- X Colloid and Interface Science, 165,459 (1994)[12] Giúdice, C.; Benítez, J.C.; Rascio, V.- J.Oil Col. Chem Assoc., 62 (3), 151 (1980).[13] del Amo, B.; Romagnoli, R.; Vetere, V.F.- Corrosión Reviews, 14 (1-2), 121-133(1996).[14] Payne, H.F.- Organic Coatings Technology, vol II: Pigments and Pigmented Coatings, p.1095, N.Y., Wiley, 1961.[15] Rascio, V.; Caprari, J.J.; del Amo, B.; Ingeniero, R - JOCCA, 62 (12), 475(1979).[16] Szauer, T - Prog. Org. Coatings, 10, 157 (1982).224

High performance anticorrosive epoxy paints pigmented with zinc molybdenum phosphate

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High performance anticorrosive epoxy paints pigmented with zinc molybdenum phosphate

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