pf_812_anwendung-Fallbeispiele-CoilCoatings-BASF



 

Grundlagen Technik im Aufbau
Phänomen Farbe

Anwendung - Fallbeispiele - Coil Coatings -BASF

 

Chromate-free Coil Coating
and
One Year of Production Experience



F. M. Androsch, K.-H. Stellnberger, M. Wolpers (VOEST-ALPINE STAHL LINZ GmbH)
Jandel, D. Drescher (BASF Coatings AG)
J. Sander, R. Seidel (Henkel Surface Technologies)


1. Introduction

In 1990, VOEST-ALPINE STAHL LINZ GmbH was using state of the art high quality technology for coil coated products.


Figure 1: The coil coating line at VOEST-ALPINE STAHL LINZ GmbH

As base material for typical coil coated applications, cold-rolled steel, hot-dip galvanized zinc, Galfan and electrogalvanized zinc combined with polyester, polyurethane or PVDF coatings - depending on the customer requirements - are typically used. A very conventional pretreatment concept was in use at the beginning of the project. The zinc coated substrates were degreased in one step without brushing, chromated (spray) and chromate rinsed (spray). The cold rolled steel was degreased in three steps including brushing and for the pretreatment, iron phosphating (dip) with a chromate rinse (spray) was applied. Different chromate containing primers from each top coat paint supplier were also in use. A representative qualitative description of relevant product properties at this stage is

shown in fig. 2. We were never really satisfied with the wet adhesion results (blistering value m1-2g1 according to ISO 4628/2 and adhesion Gt1B according to ISO 2409 after 500h wet adhesion test according to ECCA T9 or ISO 2812), which we find typical for chromate containing primers.
Figure 2: Relevant properties for chromate pretreatment and chromate containing primer

As a company strategy, VOEST decided to make a change to a chromate-free coil coated production due to some of the customers signalizing their preferences for a chromate-free, less ecologically harmful product, due to the health hazard for line personnel in contact with the chemicals and chromate containing paints and due to imminent governmental restrictions regarding Cr(VI). This alone was not enough to drive a longer development project. It was just as important if not a main target to simultaneously streamline the process and therefore lower production costs. This was to be accomplished by introducing a universal primer for all top coats and by reducing the complexity and consumption of pretreatment chemicals. A chromium-free pretreatment would also eliminate the necessity for chromate reduction in our process waste water.

A first focus was put on the change to a universal primer beginning in 1992. To accomplish this, all relevant paint suppliers were invited to submit universal primers - chromate or non chromate, which to some extent were still development products at that time. Laboratory and line samples were then prepared (hot dip galvanized Z 275 with chromate pretreatment) and all suppliers and also ourselves tested the material in a round robin test according to standardized tests we found relevant for deciding which primer suited our needs best.

From the two chromate containing - and the two chromate-free systems tested, a chromate-free primer from BASF was chosen as best suited according to our requirements. Fig. 3

again gives a qualitative picture of the product properties from the best primer, statistically well substantiated by the round robin test results. We saw an only very slightly higher corrosion attack in the salt spray test - still according to requirements (from 0-1 to 1-2 mm after 504 h according to ISO 9227 - NSS with a scribe to the zinc surface) - but a significant improvement in wet adhesion. As mentioned earlier, we explained this by the possibility, that the chromate containing pigments are to some extent water soluble, whereas the
pigmentation in the chromate-free primers is not - or at least to a much lesser degree. Since we never saw any correlation between the salt spray test (severe and critical) and outdoor exposure (not so severe and in comparison uncritical), we decided to not overrate this slightly higher result in corrosion from the salt spray test. We rated the improvement in wet adhesion as more important.


Figure 3: Relevant properties for chromate pretreatment and chromate-free universal primer

The change to the chromate-free universal primer in production was completed in 1994. In the following, the efforts of BASF are described.

2. The chromate-free universal primer

For BASF, the decision to develop a chromate-free universal primer to substitute the previous standard products, which contained toxic chromate pigments, goes back to 1990. In compliance with BASF's product guide-lines, the new product not only had to meet the current market expectations, but should also find long-term acceptance among coil coaters and end users. Only a wide variety of applications can offer the greatest benefits and the high efficiency to all companies involved into the processing chain.

The arguments for not using chromate any more are very simple: the elimination of the carcinogenic potential ensures easier handling and greater environmental compatibility. From the legal aspect this provides the foundations for the acceptance of the products in future (fig. 4).


Figure 4: Arguments for chromate-free primers


Figure 5: Arguments for universal primer
technology

The reasons for developing a primer for universal use are very simple as well (fig. 5): the universality of the primer makes the logistics easier, because just one primer is used instead of a various number of different primers for different purposes. Less changes on the primer coater save manpower and a faster colour change gains production time and therefore gives higher efficiency. Handling only one primer continuously gives the chance to optimize the production process and leads to a fast growing knowledge on the learning curve. This results in higher coating line speeds on average. All these aspects save cost and improve the coil coaters profitability.

The first chromate-free universal primer was introduced to the coil coating market in 1991. Development stages and comparable results in handling and corrosion resistance were reported during the ECCA meeting 1994 ^[1]. VOEST was amongst the first users of the new primer - in the beginning only with topcoats from BASF. From experiences in an intensive development program the chromate-free universal primer showed its global concept for coating different substrates and the compatibility to nearly all kind of topcoat technologies as shown in fig. 6.

Figure 6: Compatibility of the chromate-free universal primer

Cold-rolled steel, electrogalvanized steel, hot-dip galvanized steel, Galfan and aluminium show the suitability of the chromate-free universal primer for adhesion and anti-corrosive protection parameters. All the different topcoat technologies on the market (polyester, high durable polymer, polyurethane, PVF2, siliconized polyester, pvc-plastisole, acrylates) provided by BASF or other paint suppliers for the coil coating market can be coated on top of the primer. In 1993 these results finally led to the decision at VOEST to include this chromate-free universal primer fully into their coil coating operations.

Not available:
Figure 7: Coil coating weathering station at BASF Coatings in Münster
Figure 8: after more than 5 years: Panels with chromate-free universal primer

While the project was running, the properties of the line coated materials with the chromate-free primer were tested and assessed over the full time. The salt spray test results after 360 hours show that the materials were always in compliance with the specifications for the building industry like DIN 55928, part 8. Production samples from 1994 to 1998 show for the creepage from the scribe a median value of 0.69 mm and a standard deviation of 0.64 mm. Additionally to the most critical and worst case test, the salt spray test, a large number of line-coated panels were exposed in weathering stations like the one at BASF Coatings in Münster shown in fig. 7and 8. A close look to some of those panels, which are in the weathering station for more than 5 years, show no creepage from the cut edges and no creepage from the scribe.


The new requirement in the common project between Voest, Henkel, BASF and the University of Erlangen was the inclusion of a chromium-free pretreatment as the next consequent step. The new target for the advanced primer development was the adjustment to chromium-free pretreatment by adapting the control parameters, especially for dry and wet adhesion. Of course the universal properties had to stay integrated into the new product. As the result of laboratory work, a totally new formulation was developed. The universal primer CP 21-09X7 is based on polyester in combination with silica pigments.

By comparing the properties of this new chromate-free universal primer with the prior product according to BASF evaluation, the big step in the compatibility to the chromium-free pretreatment becomes obvious, quantitatively described in fig. 9. Other parameters like flexibility, dry and wet adhesion could be optimized. Therefore the performance parameters of different corrosion tests show statistically better results. The increase in coverage ensures an excellent relation between high technical performance and coating costs.

Figure 9: Comparison of initial and new chromate-free universal primer

After laboratory development, the next step was a scale-up to the production state. This includes not only the paint manufacturing but also the experiences on the coil coating line. From this transfer into technical scale, minor adjustments for high speed coating and stability performance finalized the development work.

Since more than 6 years BASF has been supplying a chromate-free universal primer to the coil coating line in Linz. It is used universally for claddings, roofs, frames, light fixtures, appliances, etc. Today we look back to 3 years of experience with the new class of chromate-free universal primer. Since one year, this technology is working in the total chromate-free coil coating system on VOEST’s coil coating line.

3. The chromium-free pretreatment

The process of coil coating requires a clean and well-prepared surface of the metal strip, if one is to generate a long-lasting product with an aesthetic appeal and high durability in all environmental conditions.

Cleaning and pretreatment of the strip is done in sections of the coil coating line immediately preceding the painting operation. Once a clean, wettable surface has been obtained by the cleaning process, the highly reactive metallic surface must be covered by a corrosion-protective layer that also provides adhesion to the subsequent paint coat. This is achieved by chemical processes for conversion treatment of the metal surfaces.

Historically, the unique properties of chromium compounds have been the key factor of both pretreatments and paints. Chromium, in its oxidation states + VI and + III, serves as an electrochemical couple that can inhibit most corrosive reactions on a metal surface. Therefore, most pretreatments of metal strip in coil coating lines, operated in dip or spray applications, comprise at least one processing step with chromate containing chemicals to obtain the necessary corrosion resistance of the final product.

Tab. 1: Conventional Coil Coating Pretreatments for Architecture, White Goods etc.

Table 1 summarizes this prior art pretreatment technology. In particular, hot-dip galvanized (HDG) strip is commercially pretreated either with alkaline passivation or acidic, chromate passivation processes, while cold-rolled steel (CRS) is usually treated with iron phosphating solutions; all of these pretreatments, however, are completed by chromate final rinses.

A turn for the better, the so-called no rinse technology allows most effective utilization of the chemical which is preferably applied by roll-coating and spares the environment large amounts of effluents, since, as the term indicates, no subsequent rinsing step is involved. Yet, existing no rinse technology also involves chromate-based solutions for pretreatment.

The ultimate goal of the process conversion that was undertaken by the three partner companies Voest, BASF and Henkel, was therefore to invent a chromium-free pretreatment for coil coating of steel and galvanized steel. This pretreatment would have to match the state-of-the-art quality obtained by chromate-containing operations.

It was found that a chromium-free pretreatment chemical could be formulated that provides sufficient corrosion protection. The chemical is an aqueous solution of complexes of a mixture of non-toxic transition metals and additionally contains a special polymer.

It can be used for the pretreatment of a variety of substrates, such as aluminium, zinc, zinc alloys and cold-rolled steel. Preferably used in a roll-coat operation, the chemical solution is applied at an appropriate dilution onto the clean, dry strip surface and immediately reacts with the metal to form transparent, uniform layers upon drying off at temperatures above 50°C. There are no excess chemicals nor products of side reactions on the surface, hence no rinsing is required after pretreatment.

The reactions of the cleaning and pretreatment processes were studied in co-operation with Prof. Stratmann of the Institute of Corrosion and Surface Technology, University of Erlangen-Nürnberg, Germany, as part of a joint industrial research project of the three partner companies.

In one series of experiments, the surface reactions were monitored by means of the electrochemical quartz crystal microbalance (EQCM). The working principle of


Figure 10: The experimental set-up of the Electrochemical Quartz Crystal Microbalance
(EQCM) ^[3-4]

this analytical device is the fact that the frequency of the transverse oscillation of a plane-parallel piezo-electrical quartz crystal is a function of the crystal thickness or its mass.

This was first described by the Sauerbrey equation ^[2]

D f = -2f_o².(r _q.u _q)^-1.D m = - C_f.j

where f₀ is the resonance frequency of the crystal in the fundamental shear mode and r _q, u _q are the area density and the shear wave velocity of the quartz respectively. The frequency shift D f is equal to negative mass sensitivity C_f times surface changes of mass j .
A mass growth is indicated as a decrease of the oscillation frequency and vice versa. The sensitivity of the method allows the monitoring of thickness changes on the atomic scale, so that molecular layers can be detected while forming.

The unique feature of this research tool to electrochemists is its capability of recording mass and charge simultaneously and in situ. The schematic presentation of the experimental set-up of the EQCM for registration is given in fig. 10 ^[3-4]. For frequency counting a Racal Dana 1991 Frequency counter was used. With a special oscillator circuit, designed from Krug and Fili (University of Erlangen) the mass changes in solutions, e.g. alkaline cleaning and chromium-free pretreatment, were measured with high accuracy. The registration of potential and frequency transient diagrams was achieved with a computer system. The electrochemical cell is shown in fig. 11.


Figure 11: Electrochemical flow cell, including the quartz crystal microbalance

The EQCM detector is a special quartz crystal (AT cut) with a resonance frequency of 10 MHz, which is covered with a set of thin galvanic layers. The first layer is formed by chromium to provide adhesion to the next layer, which is made of gold. This metallic layer serves as electrical conductor for the oscillation driver circuit. For the studies of this paper, the upper surface was then covered with a thin copper interface, on top of which a galvanic zinc layer was deposited. The crystal was mounted on the bottom of a flow cell and was then subjected to solutions of the cleaning and pretreatment chemicals which were pumped through with a special flow system ^[5]. The frequency changes that occurred in the crystal´s oscillation were monitored versus the reaction time. Reaction parameters like temperature, concentration, chemical composition or flow rate were varied ^[5].

Figure 12: Frequency-time transient for Zn-coated 10 MHz quartzes in industrial alkaline
cleaner with increasing age: ( ^______ ) new, ( - - - - ) used.

In fig. 12, only the frequency transient during alkaline degreasing is illustrated. The different mass curves correspond to a Henkel industrial cleaner with increasing age. Two reactions in the course of chemical cleaning of zinc surfaces in alkaline solutions could be clearly distinguished (see fig. 12):


Stage A: Dissolution of the natural oxide / carbonate / hydrate layer (chemical reaction)
Stage B: Dissolution of metallic zinc and formation of a defined zinc oxide
or zinc hydroxide layer (electrochemical reaction)

The natural oxidized surface layer will be dissolved according to the following reactions:

ZnO + 2OH^- + H₂O ® Zn[(OH)₄]^2- (1)
Zn(OH)₂ + 2OH^- ® Zn[(OH)₄]^2- (2)
Zn₅(OH)₆(CO₃)₂ + 14 OH^- ® Zn[(OH)₄]^2- (3)

In addition, the bare zinc will electrochemically dissolve according to the following reactions:

anodic reaction:
Zn + 4OH^- ® Zn[(OH)₄]^2- + 2e^- (4)
cathodic reaction:
O₂ + 2H₂O + 4e^- ® 4(OH)^- (5)

While the first reaction is largely unaffected by the process conditions, the latter can be profoundly influenced by aeration, alkalinity, "age" of the cleaning solution and kinetic effects.

To differentiate between the chemical dissolution of the oxide layer and the anodic dissolution of bare zinc, frequency transients with different atmospheres were detected with the EQCM (see fig. 13). In a nitrogen-purged cleaner solution, there is only chemical dissolution, whereas in an oxygenated solution, electrochemical reactions are also possible. In the alkaline cleaning of a coil coating line, the pH value and the flow rate are kept constant, the zinc, aluminum and iron concentration increase continuously. In this work, the kinetic effect of different cations (Zn, Al, Fe) was also checked with the EQCM.

The results of the laboratory experiments show, that only the zinc concentration of the cleaning bath has an influence on alkaline zinc dissolution. Fig. 14 shows typical frequency versus time curves of zinc coated 10 MHz quartzes with different zinc amounts. Because - during the anodic dissolution of zinc - the solution will be locally saturated and zinc hydroxide for „fresh" or zinc oxide for „used" solutions are precipitated again, increasing the zinc concentration therefore only influences the electrochemical stage (B) of the alkaline cleaning.

Fig. 15 shows the Zn(2p_3/2) X-ray photoelectron spectra (XPS) of zinc coated quartzes after different immersion times in an „aged" alkaline cleaning solution. The zinc binding energy of the untreated sample is typical for alkaline zinc carbonates and zinc hydroxides. After only 3 seconds treatment, the zinc surface was covered with zinc oxide. In a fresh solution only zinc hydroxide was precipitated on the zinc coated quartz and also on technical samples.

Bath ageing can be interpreted as the build-up of dissolved zinc ions and carbon dioxide absorption from the air.



Figure 13: Behaviour of zinc-coated 10 MHz quartzes in alkaline cleaner under different atmospheres:
O₂ ( - - - - -); air (^________ ); N₂ ( .... .)



Figure 14: Behaviour of zinc-coated 10 MHz quartzes in alkaline cleaner with different
zinc concentrations:
without zinc ( ^______ ); 20 mg.l^-1 Zn( ......); 140 mg.l^-1 Zn ( - - - - );




Figure 15: Zn(2p_3/2) XPS spectra of electrogalvanized zinc treated with „contaminated"
alkaline cleaning solutions (1 g.l^-1 CO₂; 50 mg.l^-1 Zn):
untreated ( ^____ ); 3 sec. ( ^{___ _} ); 10 sec. ( - - - - .); 20 sec. ( - - ^__ )

Figure 16: Paint adhesion distribution of SP-coated coils with different cleaning bath age.

In turn, these reactions influence the quality of the final precoated product. Performance criteria, such as paint adhesion, can be correlated with the conditions of the cleaning reaction. Fig. 16 shows the trend of adhesion results, as studied in a statistical evaluation on standard chromate pretreatment: A fresh cleaning bath gives slightly better T-bend ratings on average.



Figure 17: Frequency transient for Zn-coated 10 MHz quartzes during the sequence:
rinsing (water), chromium-free pretreatment and rinsing (water).

Unlike a conventional chromating reaction, the chromium-free pretreatment results in a net mass decrease, as was also shown in microbalance experiments (fig. 17). This result must be attributed to the fast etching reaction in the acidic environment.

Nonetheless, a distinctive layer is formed that can be analytically characterized. The surface of pretreated HDG panels was studied by Auger and photo electron spectroscopy. The depth profile in fig. 18 shows the distribution of the elements present in the layer. Zinc, originating from the substrate, is found in all parts of the layer, albeit in higher concentration close to the interface between metal and pretreatment. This also applies to alloying elements of the galvanizing layer, like aluminium. Phosphorus, the transition metals and fluorine that are ingredients of the pretreatment bath, are generally distributed throughout the layer. Carbon, which indicates the presence of the organic polymer, is also found all over the layer, however its concentration increases sharply close to the outer surface of the layer.

These findings are consistent with a reaction model that can be described as follows:
The acidic chemical attacks the substrate surface, thereby generating, by interfacial precipitation, an oxide layer in which crystal lattice cations are incorporated, such as metal ions present in the substrate and in the chemical, and phosphorus. The polymer provides several functions, like fixing inorganic parts of the layer by complexation, and forming an inhibition layer.


Figure 18: Depth profile of chromium free pretreatment

The new chromium-free pretreatment was thoroughly tested in the laboratory in combination with the chromate-free primer and polyester topcoat that had been in place earlier. After fine-tuning, the complete system yielded adhesion (ECCA T-Bend test) and corrosion test results (500 hours of neutral salt spray test) on hot-dip galvanized, Galfan, cold-rolled steel and electrogalvanized panels that were similar to chromate no rinse and standard pretreatments. Fig. 19 displays the results, showing the maximum creepage values from the scribe mark that were observed.


Figure 19: Laboratory testing on corrosion resistance


4. The pretreatment process

The preferred application method for no rinse technology, as mentioned above, is the application of pretreatment chemicals to the steel or zinc-coated steel strip surface by a roll-coater. The acidic pretreatment chemicals react with the metallic surface and a following film formation takes place by drying the strip immediately (dry-in-place-principle). No reaction side-products can therefore change the process chemistry and a following water rinse is not necessary. Consequently, the main advantages are found in the efficiency of chemical usage and in substantial waste water savings.

The development of the new, chromium-free pretreatment was aimed at the roll-coater application. Therefore it was obvious that adjustments would be necessary, because the infrastructure of the coil coating line of VOEST does not offer this technique. To nevertheless utilize the potential of the chromium-free pretreatment, the established techniques of dipping or spraying were investigated. Laboratory experiments and line trials led to the insight that by using the dipping or spraying technique, a significantly reduced concentration - as compared to the roll-coater process- must be used. By this way of operating the product quality is not compromised at all.

The surface reaction of zinc-coated steel is characterized by acidic attack of zinc and subsequent film formation. Owing to these reactions, the acid is consumed and zinc dissolves. By using a dip or spray technology and removing excess solution from the strip surface with exit squeeze rollers, the composition of the bath would thus slowly change with operating time. In practice, it is therefore necessary to correct the pH and the ingredients of the bath by a replenisher. The correct bath composition is continuously maintained by feeding the correct amount of replenisher product.



Figure 20: Relevant parameters and influence on product properties

It was found that zinc ions do not build up to a critical concentration, hence properties like corrosion resistance and paint adhesion are not affected. However, the investigations have shown that the reaction time, pH value and iron in the bath do have a direct, adverse impact on performance. This can be seen in fig. 20. When producing CRS strip, the spraying technique is used because iron disturbs the process and deteriorates the product properties.


5. Conclusions

The development steps for VOEST up to today’s chromate-free product are summarized in fig. 21.

Figure 21: Development steps to chromate-free coil coating

Thus the chromate-free universal primer has already been supplied by VOEST for many years. We have had positive reactions from customers and no negative results from the field. A number of samples were randomly taken from production coils and were tested in different types of outdoor exposure and cyclic corrosion tests including also the salt spray test. Fig. 22 and 23 show exemplary results. Hoeck van Holland as a very severe outdoor exposure site shows almost no corrosion after the three years the samples have been exposed and also in the cyclic corrosion tests performed, there is no relevant difference between chromate-free and chromate containing (VDA-test and it’s relationship to the saltspray test, see [6], prohesion, see [7], SFW is a cyclic humidity test according to DIN 50 017). One can furthermore see that there is no correlation between the salt spray test and outdoor exposure - even on the sea coast with industrial climate. The salt spray test also does not fit to the other cyclic laboratory tests performed. But because it is so severe and easy to perform, most customer requirements still include it.

Figure 22: Creepage values from outdoor exposure (Hoek van Holland) versus
saltspray test for 13 coil coated samples (chromate - and chromate-free primers with a scribe to the zinc substrate).

Figure 23: Correlation of creepage values for chromate to chromate-free coated samples.

For the complete chromate-free coil coated system primer and pretreatment fig. 24 shows the situation for line samples tested in the salt spray test in comparison to samples with chromate pretreatment for randomly selected line samples.


Figure 24: Creepage distribution of SP-coated coils with different pretreatments
(chromate passivation, chromium-free pretreatment) after 360 h saltspray
test for samples with a scribe to zinc substrate.

Dry adhesion is another important product property directly of interest to every customer. The comparison chromate-free to chromate containing is shown in fig. 25.


Figure 25: Paint adhesion distribution of SP-coated coils with different pretreatments
(chromate passivation, chromium-free pretreatment).

In introducing the chromate-free universal primer and the chromium-free pretreatment into the production process at VOEST, a high degree of partnership, trust and flexibility was necessary from all sides. The drive to continuously adapt the technology to requirements from the customers, to jointly solve problems that cropped up in line trials and to further optimize the products in the early production stages were another factor for success. We are well satisfied with the product property - and quality situation - after one year of production experience with a totally chromate-free system for coil coating steel and so are our customers.

Acknowledgement: The authors especially wish to thank Prof. Dr. M. Stratmann of the Institute of Corrosion and Surface Technology, University of Erlangen-Nürnberg, Germany, for his assistance on the scientific part of this work.

References

1] Jandel, Chromate-free Primers, New Standards and Advantages für Coil Coating, ECCA-conference, Nov 21st - 22nd, 1994
[2] G. Sauerbrey, Z. Phys., 1955, 155, 206
[3] R. Schumacher, Angew. Chem. 1990, 102, 347
[4] W. Stöckel and R. Schumacher, Ber. Bunsenges. Phys. Chem., 1987, 91, 345
[5] K.-H. Stellnberger, M. Wolpers, T. Fili, C. Reinartz, T. Paul and M. Stratmann,
Faraday Discuss,. 1997, 107, 307
[6] F. Androsch, K. Kösters, W. Schiefermüller, IBEC Conference Detroit USA,
21 (1996), p. 65
[7] B. Skerry, C. Simpson, NACE 1991, 412

Quelle:
BASF Coatings AG
Coil Coatings
Glasuritstraße 1
48165 Münster
Germany
Telephone +49 (0) 2501/14-0
Telefax +49 (0) 2501/14-3373
E-mail: coil.coating@coatings.basf.org

Or contact directly
Market Segment Manager
Uwe Pelchen
Telephone +49 (0) 2501-14 3844
Telefax +49 (0) 2501-14 3401
E-mail: uwe.pelchen@coatings.basf.org

Technical Manager
Dr. Lothar Jandel
Telephone +49 (0) 2501-14 2078
Telefax +49 (0) 2501-14 3882
E-mail: lothar.jandel@coatings.basf.org

Marketing
Martin Henke
Telephone +49 (0) 2501-14 3414
Telefax +49 (0) 2501-14 3401
E-mail: martin.henke@coatings.basf.org