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MILLENNIUM STEEL 2008 231 Corrosion protection mechanism of chromate-free coil coating systems Chromate-free primers for strip coating are now preferred to chromate-containing primers for environmental reasons. Accelerated corrosion tests on commercial galvanised coated strip produced with both primers have demonstrated equivalent performance. T he colour of facades, roofs, construction equipment and domestic appliances is, in many cases, applied to flat steel or aluminium in the coil coating process. “Finish first – fabricate later” is the slogan, which describes this process where galvanised steel strip is coated (painted) and re-coiled prior to transport to the fabricators for processing including cutting, forming and assembling. Figure 1 shows a schematic of the paint layers from the coil coating process. Where high levels of humidity and climatic stresses prevail, hot-dipped galvanised steel is used. Prior to painting, the substrate first has to be prepared for painting by adequate cleaning and pre- treatment. The first coat to be applied on the topside is a 5-8µm thick primer then, following the first drying cycle, the topcoat, approximately 20µm thick, is applied. This gives the material its desired colour. At the same time the underside is also painted. Here, too, a primer is frequently applied, with a backing coat as a second layer. In some cases a functional single layer backing coat is sufficient. The primer ensures good adhesion to the substrate and it is also the adhesion promoter for the topcoat. A balanced combination of these adhesion properties ultimately controls the performance and corrosion protection of the coated metal. Today, modern coil coating lines use chromate-free universal primers which are suitable for virtually all applications. All outdoor and indoor building applications and the materials for domestic appliances, are coated with just one primer, which is based on high molecular, saturated polyester resins combined with active non- chromate anti-corrosive pigments. This formulation is in compliance with tight environmental legislation like RoHS (Restriction of the use of certain Hazardous Substances) in electrical and electronic equipment) and provides a sustainable product for the forthcoming REACH (Registration, Evaluation and Authorisation of Chemicals) process. In the course of development, and with the use of the chromate-free universal primer on coil coating lines[1] Authors: Michael Dornbusch, Markus Hickl, Kristof Wapner and Lothar Jandel BASF Coatings AG r Fig 1 Standard coil coating system on galvanised steel FINISHING PROCESSES on four continents for more than 15 years, countless investigations have been carried out and proved its long- term corrosion protection suitability. The substitution of corrosion-protective hexavalent chromium pigments in old primer formulations by less hazardous chemicals is a necessity for the sustainable future production of pre-coated metal, but the novel third generation of chromate-free universal primer technology is gaining further market share. The systematic use of the latest micro-analytical methods has helped to find new chemical compositions which give this primer its outstanding corrosion protective properties. In this paper a comparison between the corrosion protection provided by old-fashioned chromate-containing primer versus the third generation of chromate-free universal primer – today’s standard – will be reported. MATEriALS And METhodS The experimental part of the investigation was set a
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Corrosion protection mechanism of chromate-free coil coating systemsChromate-free primers for strip coating are now preferred to chromate-containing primers for environmental reasons. Accelerated corrosion tests on commercial galvanised coated strip produced with both primers have demonstrated equivalent performance.

The colour of facades, roofs, construction equipment and domestic appliances is, in many cases, applied to

flat steel or aluminium in the coil coating process. “Finish first – fabricate later” is the slogan, which describes this process where galvanised steel strip is coated (painted) and re-coiled prior to transport to the fabricators for processing including cutting, forming and assembling.

Figure 1 shows a schematic of the paint layers from the coil coating process. Where high levels of humidity and climatic stresses prevail, hot-dipped galvanised steel is used. Prior to painting, the substrate first has to be prepared for painting by adequate cleaning and pre-treatment. The first coat to be applied on the topside is a 5-8µm thick primer then, following the first drying cycle, the topcoat, approximately 20µm thick, is applied. This gives the material its desired colour. At the same time the underside is also painted. Here, too, a primer is frequently applied, with a backing coat as a second layer. In some cases a functional single layer backing coat is sufficient. The primer ensures good adhesion to the substrate and it is also the adhesion promoter for the topcoat. A balanced combination of these adhesion properties ultimately controls the performance and corrosion protection of the coated metal.

Today, modern coil coating lines use chromate-free universal primers which are suitable for virtually all applications. All outdoor and indoor building applications and the materials for domestic appliances, are coated with just one primer, which is based on high molecular, saturated polyester resins combined with active non-chromate anti-corrosive pigments. This formulation is in compliance with tight environmental legislation like RoHS (Restriction of the use of certain Hazardous Substances) in electrical and electronic equipment) and provides a sustainable product for the forthcoming REACH (Registration, Evaluation and Authorisation of Chemicals) process.

In the course of development, and with the use of the chromate-free universal primer on coil coating lines[1]

Authors: Michael Dornbusch, Markus Hickl, Kristof Wapner and Lothar Jandel BASF Coatings AG

r Fig 1 Standard coil coating system on galvanised steel

Finishing Processes

on four continents for more than 15 years, countless investigations have been carried out and proved its long-term corrosion protection suitability.

The substitution of corrosion-protective hexavalent chromium pigments in old primer formulations by less hazardous chemicals is a necessity for the sustainable future production of pre-coated metal, but the novel third generation of chromate-free universal primer technology is gaining further market share. The systematic use of the latest micro-analytical methods has helped to find new chemical compositions which give this primer its outstanding corrosion protective properties.

In this paper a comparison between the corrosion protection provided by old-fashioned chromate-containing primer versus the third generation of chromate-free universal primer – today’s standard – will be reported.

MATEriALS And METhodSThe experimental part of the investigation was set a

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up with the materials shown in Table 1. Before the coated panels were treated in the various corrosion tests the quality of the laboratory coating of panels was checked through the standard quality assurance procedure involving mechanical tests like bending and adhesion tests. The curing stage of the coatings was indicated by a solvent resistance test using methyl ethyl ketone. All panels passed the specification. The following tests were selected for the evaluation of the corrosion protection performance:` Salt spray fog test (DIN EN 13523-8): 500 hours and

1,000 hours` Climate change test (VDA 621-415) 5 cycles and

10 cycles ` Scanning Kelvin probe (manufacturer: K&M Soft

Control, Germany)` Impedance spectroscopy (IM6, manufacturer: Zahner-

Messsysteme, Germany)

To analyse some of the panels after the corrosion test the following methods were applied:` Focused ion beam cut (FIB)` Scanning electron microscope (SEM)` Transmission electron microscope (TEM), energy

dispersive X-ray analysis (EDX)

rESuLTS And diScuSSionThe comparison of the chromate-free universal primer with the chromate containing primer is the main focus of this paper. The experimental investigations are still ongoing and additional results with the experimental products will be published in due course.

Salt spray fog test The salt spray test is the standard

Substrate hot dipped galvanised steel Z275, 0.60mm thick(1)

Pre-treatment Chromate-free(2) Chromate containing(2)

Primer Chromate-free Chromate containing Chromate-free Chromate containing universal primer, universal primer, universal primer, universal primer, COILTEC®Top (commercial) COILTEC®Top (commercial) (commercial) (commercial)Dry film 5 5 thickness, µmDrying schedule 33 sec, 38 sec, PMT 224˚C PMT 224˚C Topcoat POLYCERAM®, Polyester topcoat for building applications, whiteDry film 20 20 thickness, µmDrying schedule 38 sec, 38 sec, PMT 235˚C PMT 235˚C

(1) Voestalpine Stahl GmbH, Linz, Austria; (2) Henkel Surface Technologies, Düsseldorf, Germany

r Table 1 Materials for evaluation

r Fig 2 Cathodic de-lamination mechanisms with a defect area (left side) as local anode and a delaminated area (middle) as local cathode

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method within the industry to evaluate the corrosion protection properties of a coating system. It was designed to accelerate corrosion processes under wet conditions and the influence of electrolytes. It is an empirical method but is widely accepted due to its relatively short testing time and the simplicity of the experiment. The coated panels for this test were prepared with a scratch in the middle and freshly cut edges. The scratch was applied carefully to avoid any injury of the zinc layer due to the following consideration:

In principle, on galvanised steel substrates both anodic and cathodic de-lamination mechanisms can be observed. If a scratch exposes the steel surface to the electrolyte, a galvanic cell between iron and zinc is established. The zinc then acts as the anode, providing a sacrificial function and cathodically protecting the steel substrate. The cathodic part of the reaction, oxygen reduction, is then located on the steel surface as well as on the corroding zinc surface. Therefore, two mechanisms of de-lamination have to be considered on such samples, the anodic dissolution of the zinc layer and a destruction of the polymer/zinc interface due to the cathodic oxygen reduction.

In the tests described here the scratches in the middle of the test plate remained fully zinc coated, so no galvanic cell between zinc and iron could be formed. Typically, only the cathodic reaction of the oxygen reduction then takes place underneath the polymer coating in the form of a local cathode, whereas the anodic zinc dissolution takes place as a local anode in the defect (see Figure 2).

In this de-lamination cell, a thin electrolyte layer, which ingresses beneath the de-laminating coating, couples the

r Fig 3 Panels after 500hr salt spray fog test (left part of each panel: primer + topcoat; right part of each panel: primer only) (a) Chromate containing; (b) Chromate-free

r Fig 4 EIS measurements of the water uptake during immersion in Harrison electrolyte

Finishing Processes

a

(a)

(b)

(a)

(b)

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where Ct is the Cc at time (t), C0 is the initial capacitance and C∞ capacitance at saturation, D is the effective diffusion coefficient and SC, the absorption coefficient.

The first part of the equation was introduced to describe a Fickian-like starting period where initially water diffuses into interstitials/pores without interaction with polarisable groups. An estimate of the water content in the coating after this initial phase φ (t) was calculated from the fitted results using the Brasher-Kingsbury (BK) relation. The case 2 absorption process, describing the swelling of the polymer on longer exposure times due to interaction with the penetrant, was considered by the incorporation of a linear absorption term SC x t with the absorption coefficient SC.

For both primer systems shown in Figure 4a and 4b, the water uptake was largely independent of pre-treatment. Remarkably, both analysed systems differ strongly in the capacitance evolution during water uptake. The chromate-free universal primer initially shows a Fickian-like absorption behaviour with a quite slow diffusion coefficient of approximately 2.9 x 10-10cm2s-1 and a water content of 2.9% (BK). There then follows a significant further swelling process at about 0.07min-1 (SC).

Surprisingly, measurements of the chromate-containing primer showed no, or only extremely short, periods according to a Fickian absorption process (see the small graph in the right-hand figure). Thus, no reliable calculations of diffusion coefficients during this short period were possible. Instead, an approximately linear increase in the Cc was obtained during the whole duration of the measurement, with absorption coefficients SC similar or slightly lower than the chromate-free version. The reproduction of the measurement at a higher frequency finally gave the chance to see the expected Fickian-like absorption behaviour in the very initial phase of the test. The diffusion coefficient could be calculated at 6.3 x 10-10cm2s-1 and is approximately twice that of the chromate-free system.

Measurement of corrosion rates by means of the Scanning Kelvin Probe (SKP) experiments were performed in-situ for the measurement of corrosion rates and corrosion mechanisms on actively corroding samples. With the SKP method it is possible to detect corrosion beneath a coating film without destroying it. The principle of the SKP method[5,6,7] as well as the principle of the height control[8] are described in literature. A coated panel is prepared as shown in Figure 5.

In the process the coating on galvanised steel is being thoroughly removed, but the zinc surface needs to remain fully intact. The gap is then filled with a defined electrolyte solution. With the Kelvin probe, which is a very precisely manufactured flattened needle (see Figure 6), a defined

anodic metal dissolution in the defect to the cathodic oxygen reduction occurring near the de-lamination front. An alkaline environment is formed at the cathode (OH– as reaction product) and suggested mechanisms of coating de-bonding include the dissolution of an amphoteric metal oxide film, a base-catalysed or radical-catalysed polymer degradation and a base-catalysed hydrolysis of interfacial bonds[2]. The driving force for the development of cathodic de-lamination is the potential difference between the intact polymer/metal interface and the active corroding area.

The results of 500 hours of salt spray test are shown in Figure 3. It can be seen clearly that the chromate-free universal primer (b) shows a slightly better corrosion performance than the chromate containing one (a).Electrochemical impedance spectroscopy (EiS) Measurements of water uptake into, and swelling of, the coil coating films were performed by means of electrochemical impedance measurements of changes in the coating capacitance (Cc) during exposure in an electrolyte solution. Ccs were calculated from impedance spectra using a simple equivalent circuit for an intact coating without apparent degradation, consisting of the Cc in parallel with the ohmic resistance of the coating (Rc), both in series with the resistance in the electrolyte (Rel).

For all measured coil coated samples, after the initial increase in the Cc during immersion, no saturation plateau in the capacitance evolution was achieved, as would be expected for a pure Fickian behaviour (see Figure 4). Instead, a close to linear further increase of the Cc was obtained. Therefore, a model combining an ideal Fickian adsorption process with a non-Fickian case 2 absorption, as already introduced by van Westing[3] and Nguyen[4], was used for fitting the water uptake, see equation below:

r Fig 5 Sample preparation for SKP measurement

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area next to the gap is being scanned in order to calculate an electrochemical potential from voltage potential differences. The probe is specially equipped with a height control which ensures a constant distance between the probe and the coating during scanning.

The reading of the potential is dependent on the adhesion of the coating to the galvanised steel. An intact coating shows a certain level of electrochemical potential, but once there is some undercreepage of the coating due to the start of corrosion, the level of the potential will drop. Figure 7 shows for a model system the electrochemical potential at the beginning of the test (a), -200mV. After 20 hours a clear creepage of 2mm from the electrolyte filled gap has started (b), and the potential decreased to approximately -650 mV. Further creepage can be seen after 40 hours (c) and 60 hours (d) with the front progressing to 4mm and 7mm, respectively. The corrosion mechanism follows a cathodic de-lamination process.

The SKP measurement of the chromate-containing and the chromate-free systems did not follow the fast changes of the model system. Both reacted similarly but did not show any effect after 6 weeks of testing – the potential remained at the initial level and there was no creepage. After 8 weeks some slight creepage was detected, but even after more than 12 weeks the creepage was only a little more

r Fig 6 The SKP apparatus

r Fig 7 Results of SKP measurement from a model system

Finishing Processes

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Finishing Processes

developed and did not cover the full front. Figure 8 shows the results for the system of the chromate-free primer on the chromate-free pre-treatment.

This result is exceptionally good. It shows the very high degree of corrosion resistance of both chromate-free and chromate-containing coil coating systems. These investigations confirm the long-term experiences of the market, that even under harsh conditions a very reliable corrosion resistance is obtained on galvanised steel facades and roofs, which are pre-coated with the chromate-free universal primer and the appropriate topcoats.

concLuSionS And ouTLookUse of chromate-containing primers is of concern due to the high toxicity of the chromate pigments. The durability of chromate-free universal primer, as determined by empirical test methods like the salt spray fog test and cyclic climate change tests, is well known. Comparable results between chromate-free and chromate-containing primers can now also be shown by micro-analytical methods like the SKP, impedance spectroscopy, and others.

Chromate-free universal primer is the sustainable product for the future and represents best practice in the coil coating industry. Its properties are the guideline for future developments, which will lead to products with fewer volatile organic compounds (VOCs) and which require less energy consumption during the coating

r Fig 8 Results of the SKP measurement from the system with chromate-free primer on chromate-free pre-treatment

process. New water-based primers and curable primers are currently being investigated and optimised to supplement the primer portfolio for the coil coating industry. MS

Michael Dornbusch, Markus Hickl, Kristof Wapner and Lothar Jandel are with BASF Coatings AG, Muenster, Germany.

conTAcT: [email protected]

rEfErEncES [1]FM Androsch et al, ECCA 33rd General Meeting, ECCA Conference transcript, Monte Carlo (June 1999).[2]W Fürbeth, M Stratmann, Corrosion Science 43 p207 (2001).[3]EPM van Westing and JHW deWit, Corrosion Science 36(6) p957(1994).[4]VN Nguyen and FX Perrin, Corrosion Science 478 pp397-412 (2005).[5]W Fürbeth, M Stratmann and J Fresenius, J. Anal. Chem. 353 p337 (1995). [6]M Stratmann, R Feser and A Leng, Electrochim Acta, 39 p1207 (1994).[7]M Stratmann, R Feser and A Leng, Farbe+Lack 100 p93 (1994).[8]K Wapner, B Schoenberger, M Stratmann and G Grundmeier J. Electrochem. Soc. 52 E114 (2005).


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