Dual corrosion protection system consisting of cold galvanizing and powder coating

As an alternative to the classic hot-dip galvanizing of steel substrates, a new type of dual corrosion protection system consisting of cold galvanizing and powder coating has been developed. It can be used as a more energy-saving variant in coating operations.

In addition to classic wet paint coatings, duplex systems (hot-dip galvanizing + organic coating, e.g. powder coating) have long been widely used for the corrosion protection of steel and have proven their worth. Recently, however, rising energy prices have driven up the costs of the very energy-intensive hot-dip galvanizing process. Experts are also repeatedly confronted with complaints or cases of damage and know that a good quality powder coating on galvanized steel components is not always easy to achieve.

In addition to the additional mechanical processing steps involved in surface preparation, such as fine fettling or sweep blasting, the steel grade, or more precisely certain silicon and phosphorus contents of the alloy, are also important for a flawless appearance without spots, craters and outgassing. However, the coater has little influence on this and can only try to minimize it using certain methods, such as tempering (pre-treatment of galvanized components in an oven at 180-200 °C before coating). Sometimes, however, this does not help either, so that for a supposedly simple and quick coating application, considerable additional work is required for sanding and overcoating.

With this in mind, Dr. Herrmann GmbH & Co. KG joined forces with Vestocor GmbH as a paint manufacturer for corrosion protection systems and Lackierzentrum Reichenbach GmbH (LZR) as a competent coater for wet and powder coatings to develop a new type of coating system consisting of a high-zinc corrosion protection primer based on wet paint and a resistant powder coating. The aim was to combine the best of both worlds and thus develop a high-quality corrosion protection alternative to hot-dip galvanizing that coating companies can use directly on their premises and thus expand their portfolio.

This research project was carried out between 2021 and 2023 and funded by the German Federation of Industrial Research Associations (AiF). First, however, the question had to be answered as to why powder coating primers containing zinc should not be used. In order to achieve a good corrosion protection effect, the zinc content must be very high (> 90 % in the dry film). Due to its chemical-physical properties, a powder coating cannot be filled to such a high level for this effect to take effect. In wet paint, on the other hand, a large amount of zinc can be introduced, which also achieves the desired corrosion protection effect.

The first step was to find a suitable binder system that could withstand the high temperatures of 180-200 °C during the powder coating curing process. The initial choice fell on high-zinc silicone resin and ethyl silicate systems. The compatibility between the primer and the subsequent polyester-based powder coating proved to be very good, but the results of the mechanical load tests and corrosion tests were not satisfactory. This was probably mainly due to the relatively brittle binder systems, which also required a very narrow coating thickness range to be maintained, which is often difficult to achieve in practice.

Subsequently, tests were carried out with an alternative epoxy-based binder, whereby the zinc was added in powder form on the one hand and in flake form on the other. After appropriate pre-treatment by tempering and powder coating with polyester topcoat, both systems showed very good flow, including edge coverage and very good mechanical properties as well as excellent corrosion protection in the neutral salt spray test.

The difficulty in making a direct comparison of the results with conventional systems, such as galvanizing + powder or steel + powder primer + powder topcoat, lies in the selection of the standard to be used for the evaluation. For wet paint systems, DIN EN ISO 12944 applies, which specifies a maximum corrosion of 1.5 mm at the scribe and does not provide any information on possible delamination. For powder coatings, DIN 55633 or 55634 applies, which specifies a maximum corrosion of 1.0 mm and a maximum delamination of 3.0 mm on steel and 8.0 mm on galvanized steel.

In the new system, the wet paint has direct contact with the steel substrate, which is why the limit value for corrosion (1.5 mm) in DIN EN ISO 12944 was applied. However, as the delamination also has a decisive influence on the appearance and further corrosion behavior, DIN 55633 and 55634 (3 and 8 mm respectively) were used as the limit values. The new dual system was able to achieve a high corrosion category of C4 and C5. Better results were achieved than various comparative variants on steel material, which were only coated with powder primer and top coat.

A direct comparison with hot-dip galvanized and powder-coated steel sheets also showed significantly less delamination. However, the new system did not quite match the outstanding corrosion protection properties of hot-dip galvanizing with cathodic remote effect. This is probably due to the fact that, despite the high zinc content, the individual particles are partially insulated by the binder matrix and therefore the electrical conductivity is limited.

Nevertheless, excellent results were achieved, making the system an interesting alternative for coating companies, which can expand their portfolio with a more energy-saving variant. (OM-7/24)