As it is well known, in order to adequately protect a steel item against corrosion, it is necessary to proceed both with a valid chemical conversion of the surface and with a suitable finishing. The figure here below schematically illustrates the relationship between most used steel pre-treatments and coating systems versus corrosion resistance.
It is clear, therefore, that a polyester coating cannot provide a high corrosion resistance if the used pretreatment is phospho-degreasing. On the other hand, most coating plants are not equipped with modular treatment systems, that allows to pass quickly from phospho-degreasing to iron or zinc-phosphating treatment. To solve this issue, ST Powder Coatings R&D department has developed a new polyester series, called Cor-Shield, with improved corrosion resistance, even in case of non-optimal pretreatments.
The following picture shows four steel panels without any pre-treatment, respectively coated with:
a) New Polyester System, Cor-Shield series (V1);
b) Typical Polyester System (V2);
c) Epoxy-polyester System (V3);
d) Epoxy System (V4).
After 210 hours of neutral salt spray test (ISO 9227), the film detachment of the new polyester system (first on the left) was very similar to the film detachment of the epoxy system (last on the right), and significantly lower than the detachment of the standard polyester system (second on the left).
In addition to its high corrosion resistance, the new polyester series still maintains a good resistance to weathering. The image below shows the trend in gloss retention after accelerated aging test (QUV-B).
The UV resistance features of the new Polyester Serie Cor-Shield are suitable for outdoor industrial applications.