Performance of finish coated galvanized steel sheets for automotive bodies

Progress in Organic Coatings 62 (2008) 265–273

Performance of finish coated galvanized steel sheets for automotive bodies D. Santos a,∗ , H. Raminhos a , M.R. Costa a , T. Diamantino a , F. Goodwin b a

Instituto Nacional de Engenharia, Tecnologia e Inova¸ca˜ o, I.P. (INETI), Estrada do Pa¸co do Lumiar, 1649-038 Lisboa, Portugal b International Lead Zinc Research Organization Inc. (ILZRO), 2525 Meridian Parkway, Suite 100, Durham, NC 27713, USA Received 26 July 2007; accepted 7 January 2008

Abstract High performance and cost reduction are principal concerns for vehicle manufacturers. In addition to shape, the exterior finish is the first thing a consumer sees when looking at a vehicle. The quality and durability of the finish is a reflection of the quality and durability of the vehicle itself. In this work, the behavior of two finish coat systems was studied. Finish coat system 1 included a powder coat, a waterborne base coat and a clear coat, and finish coat system 2 included the same powder coat, a solvent-based base coat and a clear coat. These finish coats were applied on unprimed and pre-primed electrogalvanized, hot dip galvanized and galvannealed steel. Before application, for each finish coat two activation treatments were considered: a conventional zinc phosphate for automotive industry and an experimental pretreatment. Three primers were considered. Primer 1 is an organic zinc rich silicate with a low thickness (near 4 ␮m), primer 2 is a very thin (near 2 ␮m) water-based primer filled with graphite and primer 3 is a conventional electrophoretic applied primer system. To evaluate the performance of finish coats, the samples were submitted to a cyclic corrosion test, and the paint coating adhesion was evaluated, before and after exposure, by a cross-cut test method. Better adhesion was verified on the finish coat with a waterborne base coat, a system environmentally more convenient. Both finish coats presented good anticorrosive behavior, without significant defects on undamaged areas. Regarding the scribe area, the performance seems sometimes to be better with the application of the conventional zinc phosphate pretreatment compared to the experimental pretreatment. © 2008 Elsevier B.V. All rights reserved. Keywords: Finish coat; Waterborne; Metallic coatings; Automotive; Pretreatment

1. Introduction Lower costs, reduced air pollution, and better performance continue to be key drivers in automotive coatings development. Corrosion resistance, forming, welding and adhesive bonding properties are important performance factors. Automobile finishing requires critical surface appearance and corrosive protective properties. Automotive finishing technology encompasses the highest level of surface treatment technology and auto coatings have the best quality in the coating industry [1]. The traditional finishing process includes solvent-based surface primer + base coat + clear coat, which is featured by its high quality, easy application, etc. but with large VOCs (volatile organic compound) emission. Increasingly strict regulations

∗

Corresponding author. Tel.: +351 21 0924782; fax: +351 21 7166569. E-mail address: [email protected] (D. Santos).

0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.01.003

for environmental protection will cause the traditional solventbased coating systems to be replaced. Waterborne systems will reduce the VOCs, powder coating will eliminate the use of solvents and can be recycled. Also, high solid coatings can utilize the existing production lines for application and reduce the emission of VOCs. Therefore, waterborne, powder and high solid coatings are the leading alternative technology to replace the solvent-based intercoat or topcoat and become the dominant of environment compliant auto coatings. Already, waterborne and powder coatings are widely used in Europe and North America [1]. Experiments with paint using waterborne systems began between the years 1975 and 1985. The late 1980s and early 1990s brought rapid, extreme changes in the industry. The amounts of VOCs were lowered. Urethane and polyurethane blends, along with custom hybrids were dominant [2]. Initially, these new paint systems began flaking away and were damaged by ordinary waxes and polishes. Today’s, paint systems, mostly base coat/clear coats and base coat/tint coats, provide optimum

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Table 1 Mean coating thickness of finish coated samples Substrate or substrate + primer

Mean coating thickness (␮m) With finish coat 1

E H G E + primer 1 H + primer 1 G + primer 1 E + primer 2 H + primer 2 G + primer 2 E + ELPO H + ELPO G + ELPO

With PT1

With PT2

With PT1

With PT2

163 158 153 165 163 153 163 150 154 189 177 169

162 155 157 167 162 155 162 149 152 199 179 184

138 136 132 146 137 135 128 161 127 161 155 157

142 134 133 141 132 134 133 169 129 169 154 156

performance. Newly built finishing lines in Europe are using waterborne surface primer and base coat, and in North America, some of the new lines are using waterborne intercoat and some powder coatings [1]. The base coat is the coating layer that provides color and aesthetic effects. Waterborne base coat is based on acrylic and polyurethane resin systems [1]. According to Rink and Mayer [3] the high technological demands of a waterborne base coat for automotive refinishing could be achieved by the use of tailor-made polyurethane resins connected with an intelligent three-component concept. Waterborne base coats are now used together with a solvent based clear coat and powder clear coat and its properties are comparable with those of solvent-based base coat [1]. The auto manufacturers use clear coats for a number of reasons, such as: appearance, durability, pollution reduction, reparability, and cost of application. Waterborne clear coating is mainly composed of water-soluble melamine formaldehyde and water borne acrylic or alkyd resin. As an environment compliant alternative with high price, the waterborne is still to be improved in its property and has not been widely used yet [1]. By the end of 2000, 65% of production lines were using waterborne base coats, but the top clear coats are still solvent-based systems, with only few of them trying to use waterborne or powder clear coats [1]. Waterborne coatings in the automotive industry are primarily used for electrocoat, base coat and primer applications [4]. The developing in waterborne paint is aimed to obtain an alternative, having properties, price and Table 2 Mean coating thickness of primer 1 samples Sample type

E + primer 1 H + primer 1 G + primer 1 a

With finish coat 2

Mean coating thickness (␮m)a Metallic coating

Primer 1

8 11 10

4 4 2

From pickling and measurements according to ISO 2808, Method 7C.

applicability equivalent to its solvent counterpart and lower VOC. For this work, the performance of two finish coats, with waterborne base coat and with solvent-based base coat, was studied. The same powder intercoat was included in the two finish coat systems. 2. Experimental 2.1. Materials 2.1.1. Substrates Three metallic zinc coatings applied on steel, were used as substrates: Zn electrogalvanized, hot-dip galvanized and galvanneal, codified by E, H and G, respectively. 2.1.2. Primers This study included: two different conductive primers (with different conductive fillers), codified by 1 and 2, and a conventional electrophoretically applied primer system used as baseline, codified by ELPO. Primer 1 was Grancoat ZE, a zincbased conductive primer, applied in the USA. Primer 2 was Granocoat X, a very thin conductive experimental primer containing graphite, applied in Germany. Before the application of a primer, all the panels were passivated with a new environment-friendly (non-chrome) treatment, codified by Bonderite 1456X. 2.1.3. Finish coats On pre-primed samples, two finish coat systems were applied. For each finish coat, two pretreatments were considered. Pretreatment 1 (PT1) was Bonderite 958—a conventional zinc phosphate for automotive industry. Pretreatment 2 (PT2) was an experimental pretreatment under development, codified by Bonderite X. Finish coat 1 (FC1) was a system used at the Windsor, Canada assembly plant of Daimler Chrysler, a PPG system. It included waterborne base and clear system, being composed by PCV70118 Powder, stone white basecoat HWBS83542 and

D. Santos et al. / Progress in Organic Coatings 62 (2008) 265–273 Table 3 Mean coating thickness of primer 2 samples Sample type

E + primer 2 H + primer 2 G + primer 2 a

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Table 4 Mean coating thickness of electrophoretic primer system samples

Mean coating thickness (␮m)a

Sample type

Mean coating thickness (␮m)

Metallic coating

Primer 2

ISO 2808, Method 7C

SEM micrograph

6 10 9

2 2 2

Total

Metallic coating

Electrophoretic primer

33 35 35

6 – –

26 – –

From SEM micrographs of samples obtained by cross section.

clearcoat DCT 555. Finish coat 2 (FC2) was a system used at the Warren, Michigan truck plant of Daimler Chrysler, a PPG system, composed by PVC 70118 Powder, white basecoat NHU90394E and clearcoat CSRC 8002E. Panels with two shapes were considered: flat panels and “pie pan” deformed panels. Deformation was made after the application of primers 1 and 2, and in the case of conventional ELPO before application of any surface treatment, and before applying the finish coat, just like at the current automotive plant. 2.2. Studies 2.2.1. Thickness Total coating thickness of finish coated samples was measured using an Elcometer 300 SP instrument, following the method 7C included in the ISO 2808 standard [5]. Before that, the thickness of pre-primed samples was evaluated by two different methods: following the method 7C included in the ISO 2808 standard [5] and by scanning electron microscopy (SEM).

E + ELPO H + ELPO G + ELPO

Total thickness of primers plus corresponding metallic coating substrate was also measured using the Elcometer 300 SP instrument [5]. After that, the primer was removed by pickling and the thickness of the metallic coatings was measured. Samples obtained by cross-section of pre-primed flat panels, were observed by scanning electron microscopy (SEM) and the thickness of substrates and primers was evaluated. A high resolution Philips XL 30 FEG/EDAX NX scanning electron microscope was used. Samples obtained by cross-section of finish coated panels were also observed by SEM and the three coats included in both finish coat system were analyzed by energy dispersive spectrometry (EDS). 2.2.2. Adhesion Coating adhesion was evaluated, before cyclic corrosion test and after 10 cycles, by “cross-cut test”, according to EN ISO 2409 [6]. In this test method a right-angle lattice pattern is cut

Fig. 1. Aspects observed by SEM and corresponding EDS spectra of sample with finish coat 1.

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Fig. 2. Aspects observed by SEM and corresponding EDS spectra of sample with finish coat 2.

into the coating, penetrating through to the substrate. This test was performed 3 times in each sort of pie pan panels, with a maximum pressure of 6 bar with 3 mm of spacing of cuts taking into account the coating thickness [6]. The method was carried out as a six-step classification test. A multifunction scratching Braive Instruments 1535 M001 instrument was used. 2.2.3. Corrosion mechanisms 2.2.3.1. Cyclic corrosion test. The finish coated samples were submitted to a corrosion cycle test according to the VDAprocedure 621-415 [7] contained in the SEP 1160 [8]. The cycle consists of: • 1 day salt spray test according to ASTM B117 [9];

• 4 days cyclic humidity, being each daily cycle composed by 3 h at 40 ◦ C and 98% relative humidity and 16 h at 23 ◦ C and 50% relative humidity; • 2 days ambient climate according to DIN 50014 [10]. Before testing, a low amount (0.8 g m−2 ) of lubricant Fuchs Anticorit RP 4107 S was applied on the surface of the samples. Studies previously performed for the uncoated substrates and pre-primed substrates showed that lubricant had a small effect [11]. For a better evaluation of finish coated samples performance, before exposure, a controlled accident (a scribe) was made on some panels. So, two of each kind of three “pie pan” panels and one of each kind of two flat panels under test, were submitted

Table 5 Adhesion results obtained for pie pan deformed samples with finish coat 1 Substrate or Substrate + primer

Finish Coat 1 With PT1

E H G E + primer1 H + primer 1 G + primer 1 E + primer 2 H + primer 2 G + primer 2 E + ELPO H + ELPO G + ELPO

With PT2

Before corrosion test

After corrosion test

Before corrosion test

After corrosion test

0 0 0 0 0 0 1 0 0 0 0 0

0 0 0 0 0 0 1 1 1 0 0 0

2 2 0 0 0 0 1 0 0 0 0 0

0 0 0 0 0 0 1 0 1 0 0 0

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Table 6 Adhesion results obtained for pie pan deformed samples with finish coat 2 Substrate or Substrate + primer

Finish Coat 2 With PT1

E H G E + primer1 H + primer 1 G + primer 1 E + primer 2 H + primer 2 G + primer 2 E + ELPO H + ELPO G + ELPO

With PT2

Before corrosion test

After corrosion test

Before corrosion test

After corrosion test

2 2 2 2 2 2 2 2 2 0 0 0

5 5 3 2 2 2 2 2 3 0 0 0

2 2 1 2 1 1 2 2 2 1 0 0

4 5 1 1 2 3 1 1 2 0 1 0

Table 7 Results obtained after exposure to 10 corrosion cycles [7] on the scribe of samples with finish coats applied on not primed substrates

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Table 8 Results obtained after exposure to 10 corrosion cycles [7] on the scribe of samples with finish coats applied on substrates with primer 1

to a single scribe into the coating before exposure. This scribe was 1 mm wide and 50 mm long, penetrating through the substrate with an increasing pressure from 0 to a maximum of 4 bar. The scribe was performed in accordance with the international standard EN ISO 12944-6, Annex A [12]. 2.2.3.2. Evaluation of corrosion severity. During and after cyclic corrosion tests, visual inspections were performed. Blisters found around the scribe were classified in accordance with EN ISO 4628 Standard part 2 [13]. 3. Results and discussion Table 1 contains the mean thickness values of finish coated samples. Results of mean thickness measurements of metal-

lic coatings and primers, previously performed, are presented in Tables 2–4. For primer 1, the evaluation of primer thickness from SEM micrographs was not feasible, due to difficult observation of the interface between the primer and the resin used for the preparation of cross section samples. For the electrophoretic primer, due to its difficult removal by pickling, the individual thickness was measured only on SEM micrographs. Total coating thicknesses of the two schemes for finish coating are very similar (Table 1), nevertheless, with values somewhat higher for Scheme 1 than for Scheme 2. This fact is also shown in Figs. 1 and 2, which present the aspects, observed by SEM, of the three coats of systems 1 and 2, respectively. Considering all the samples (Table 1), thicknesses of finish coat 1 range from 151 to

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Table 9 Results obtained after exposure to 10 corrosion cycles [7] on the scribe of samples with finish coats applied on substrates with primer 2

199 ␮m and thicknesses of finish coat 2 range from 127 to 169 ␮m. Tables 5 and 6 present the results of adhesion evaluated according to EN ISO 2409 [6], on finished coated samples before and after 10 corrosion cycles exposure. Finish coat 1 (waterborne base and clear system) applied on uncoated substrates and on pre-primed substrates, generally, presented better adhesion than finish coat 2 (solvent borne base and clear system). After 10 exposure cycles, the adhesion of finish coat 1 did not change significantly. However, the adhesion of finish coat 2 decreased, more or less, depending on the substrate it was applied on. When primers were applied, the adhesion of finish coat 2 did not decrease or decreased less than that verified for uncoated substrates, after 10 exposure cycles.

Primers increase the adhesion of the painting scheme to the substrate. Finish coat 1 presented good adhesion either on the uncoated substrates, either on the pre-primed substrates. Regarding finish coat 2, better adhesion was verified when electrophoretic primer system was applied than when primers 1 and 2 were applied. Regarding activation treatments, the exchange of pretreatment 1 by experimental pretreatment 2 under development, seems not to influence the adhesion of the paint scheme. Results of visual inspections performed on the scribes, during exposure and after 10 cycles according to VDA procedure [7], are presented for the several combinations of substrates, primers, pretreatments and finish coats in Tables 7–10.

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Table 10 Results obtained after exposure to 10 corrosion cycles [7] on the scribe of samples with finish coats applied on substrates with electrophoretic primer system

No significant changes were observed, on undamaged areas for all the samples. The performance of a paint system is the result of a complex interdependency among the substrate, pretreatment, adhesion primer and finish coat. The pigment component in any formulation can either enhance or degrade the overall performance of the protective coating. According to results of EDS studies, the two base coats, both with white color, seem to be pigmented with titanium dioxide (Figs. 1 and 2, spectra B). After 10 exposure cycles, white and red corrosion, and severe blistering or delamination were observed on the scribe and from the scribe, respectively, on all samples (Tables 7–10). For finish coats applied on not primed substrates, significant differences were not verified (Table 7). However, for finish coats applied on

pre-primed samples, in many cases the maximum width of the defect (blisters or delamination) across the scribe was higher for electrogalvanized steel substrate than for hot dip galvanized steel or galvannealed substrates. This in agreement with the results of corrosion resistance previously obtained for uncoated and pre-primed substrates [11,14]. For finish coats applied on pre-primed electrogalvanized steel, the degradation on the scribe seems sometimes to be lower when the pretreatment 1, a conventional zinc phosphate, was applied than when the experimental pretreatment 2 was used. This fact was verified for electrogalvanized steel with primer 2 and finish coat 2 (Table 9) and for electrogalvanized steel with electrophoretic primer, either with finish coat 1 or finish coat 2 (Table 10).

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4. Conclusions The results of adhesion tests performed on finish coated samples, including not pre-primed and pre-primed substrates, generally showed better adhesion of finish coat 1 (FC1) including waterborne base coat, than of finish coat 2 (FC2) with solvent borne base coat. The use of primers increased the adhesion of the painting scheme to the substrate. For finish coat 2 (applied on not primed substrates), better adhesion was observed for samples with galvannealed substrate. Concerning the finish coat 2 applied on pre-primed substrates, better adhesion was seen when electrophoretic primer system was used, and no differences were noted between the three substrates. Finish coat 1 presented good adhesion on the three substrates, without and with the three primers. The use of two different activation treatments seems not to affect all the adhesion results. After 10 exposure cycles (10 weeks), all the finish coats samples without damage, presented good performance without significant defects. Regarding the behavior observed on the scribe area, it was verified in many cases worst behavior of finish coated samples with electrogalvanized steel substrate than of finish coated samples with the other two substrates. For finish coats applied on pre-primed electrogalvanized, the performance seems sometimes to be higher with the application of the conventional zinc phosphate pretreatment comparatively to the experimental pre-treatment. Similar corrosion resistance was verified for finish coated samples including the three primers. Summarizing, the two finish coats presented good performance, with better adhesion of finish coat system including a waterborne base coat, an environmentally more convenient. Acknowledgements This work was sponsored by the Galvanized Autobody Partnership program of International Lead Zinc Research Orga-

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nization, Inc. The authors acknowledge with thanks the staff of Henkel Surface Technologies, Madison Heights, Michigan USA who applied the pretreatments and Primers 1 and 2 to the test panels. References [1] Chen Muzu, Present, Status and Future. Development of Automotive Finishing, Materials, Coatings and Finishing Consultation, Copyright (C) 2001 ASIACOAT.COM, http://www.asiacoat.com/eng /emarkets/emarkets3.htm. [2] A brief History and Trends in Vehicle Paints, 1999–2006 Protect All, Inc., Anaheim, CA, USA, last revised July 24, 2006. http://www.protectall. com/artpaints.htm. [3] H.-P. Rink, B. Mayer, Prog. Org. Coat. 34 (1998) 175. [4] L. Prendi, P. Henshaw, Edwin K.L. Tam, Int. J. Environ. Stud. 63 (4) (2006) 463. [5] ISO 2808, Paints and varnishes—Determination of Film Thickness, ISO, Geneva, 2007. [6] EN ISO 2409, Paints and varnishes—Cross-cut test, CEN, Brussels, 2007. [7] VDA 621-415, Testing of Corrosion Protection of Vehicle Paint by Alternating Cycles Test, VDA, Frankfurt, Germany, 1982. [8] SEP 1160, Evaluation of Weldable Corrosion Protection Primers for the Automotive Industry. Part 1: Corrosion Performance, Stahlinstitut VDEh, D¨usseldorf, Germany, 2004. [9] ASTM B-117, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM, PA, USA, 2007. [10] DIN 50014, Climates and their technical application; standard atmospheres, DIN e. V., Germany, 1985. [11] F.E. Goodwin, D. Santos, Effect of lubrication and comparative formability performance of pre-primed coated automotive sheet steels, Galvanized Steel Sheet Forum, D¨usseldorf, Germany, May, 2006, ILZRO. [12] EN ISO 12944, Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 6: Laboratory performance test methods, CEN, Brussels, 1998. [13] EN ISO 4628-2, Paints and varnishes. Evaluation of degradation of coatings, Designation of quantity and size of defects, and of intensity of uniform changes in appearance. Part 2: Assessment of degree of blistering, CEN, Brussels, 2003. [14] D. Santos, H. Raminhos, M. Costa, T. Diamantino, F. Goodwin, Performance of conductive pre-primers applied on galvanized steel sheets for automotive bodies, in: SAE World Congress & Exhibition, Detroit, MI, USA, April, 2007.