Although the painted surface was scratched down to the steel, exposing the steel surface, the steel was able to withstand the corrosion. Also, the paint held on strongly to the steel, slowing down any further corrosion. "A better corrosion resistance reduces the need of frequent repainting and maintenance and provides a more efficient usage of resources", said Robert F. Wesdijk at SSAB.
In 2012, SSAB began testing painted samples of SSAB Weathering Steel in a harsh outdoor environment, in order to evaluate how the steel resisted corrosion when compared to conventional painted steel. The results, which were collected in 2018, exceeded expectations. The SSAB Weathering Steel samples remained almost unchanged.
“We are very happy to see results that will offer great advantages to our customers. A better corrosion resistance reduces the need of frequent repainting and maintenance and provides a more efficient usage of resources”, commented Robert F. Wesdijk at SSAB.
The tests revealed that the Weathering Steels’ protective patina also forms on the steel surface when there is a scratch through the paint layer. This protective layer slows down the penetration of corrosion under the paint and improves corrosion resistance, which dramatically increases intervals between maintenance and repainting.
”To conduct the test, we painted 100 x 150 mm test pieces of five different weathering steels, as well as a reference steel without weathering steel alloying, with an epoxy-polyurethane paint system,” explained Esa Virolainen, Senior Specialist at SSAB. “We then made a scratch on each sample to expose the steel to corrosion and left them at an outdoor testing facility for over six years.”
Further analysis showed serious corrosion and loose paint layers around the scribe on basic carbon steel samples. Only minor corrosion could be seen in a few of the SSAB Weathering Steel samples. The samples were then evaluated according to standard ISO 4628 after visual inspection.
”You could see from the paint layer that there was visible delamination on the conventional steel, but the SSAB Weathering Steel looked more or less the same with an unbroken paint layer”, said Virolainen. “ A small amount of rust was found under the paint of the weathering steel samples, but the paint maintained good adhesion to the rust. In fact, force had to be applied in order to remove the paint from these corroded areas.”
Testing was carried out on Bohus Malmön island, which is located around 100 km north of Gothenburg, which lies on the western coast of Sweden. The test site is administered by RISE Kimab (Research Institutes of Sweden), and is continually exposed to extremely fluctuating weather conditions, as well as highly corrosive saltwater sprays. For nearly the entire duration of the test, the site upheld a corrosivity classification of C5, the highest on the internationally recognized scale.
The steels tested included SSAB Weathering 355, 550, 700 and 960. The steel grade of S420 was chosen to represent conventional carbon steels without weathering steel alloying as a reference material.
SSAB Weathering Steel is a high strength, low alloyed carbon steel that offers natural corrosion resistance, thanks to a protective patina that forms on the surface of the steel. It is often used in structures, bridges and the railway industry, as well as in poles for electrical grids.