Zinc flake systems provide long-lasting and reliable protection for offshore structures

Corrosion protection for external conditions

2018 saw the updating and supplementation of DIN EN ISO 12944 for steel structures. In part 9 of the standard the corrosivity category “CX” was introduced for coatings in offshore use. Current laboratory investigations at Dörken illustrate that zinc flake systems are also an advantageous alternative to the commonly-used wet coatings named in the standard for these extreme areas.

What is covered by DIN EN ISO 12944?

DIN EN ISO 12944 “Corrosion protection of steel structures by protective paint systems” standardises corrosion protection on load-bearing steel structures with a material thickness of at least 3 mm of unalloyed or low-alloyed steel via organic wet coating systems – i.e. coatings that dry or cure under atmospheric ambient conditions. The standard forms the basis for the planning and execution of coating work on steel structures such as bridges, electricity pylons or wind energy or offshore units.

What is new in the updated standard?

The majority of DIN EN ISO 12944, which dates from 1998, has been completely updated and supplemented. This saw the former DIN ISO 20340 integrated into part 9 of DIN EN ISO 12944. Part 9 describes the precise implementation of cyclical corrosion protection testing, respectively performance testing procedure in the laboratory for structures in offshore use. In the current version a new category “CX”/ “CX Extreme” has been introduced for these structures situated on the open sea – previously covered by the corrosivity category “C5-M”. In addition, the previous categories “C5 M” and “C5 I” were combined and now refer exclusively to structures on land.

What changes have been made to corrosion protection testing?

An altered laboratory test to determine the performance of corrosion protection has been introduced for all offshore structures of the new corrosivity category “CX” (corrosion stress: extreme). This means: all systems employed in these extreme areas are required to undergo considerably more demanding cyclic aging testing according to DIN EN ISO 12944-9 in the laboratory. In this, over a period of 4,200 hours the samples are subjected to a combination of salt spray test, UV test and freezing at -20°C. The test takes a total of 7 days, which is designed to simulate 25 weeks of external weathering under real-life conditions. This simulation – as also noted in the standard – should be viewed critically, however, as cyclic aging does not necessarily have the same effect as natural aging.

One of the most important changes compared to the “old” DIN ISO 20340 also concerns the performance criteria for the disbonding of the coating. The standard now specifies that the subsurface corrosion of areas subjected to heavy stress such as tidal areas, splash zones, helicopter decks/emergency escapes and floor coatings may be a maximum of 8.0 mm and a maximum of 3.00 mm for all other category “CX” applications.

Do zinc flake coatings satisfy the requirements of “CX”?

Against this background, Dörken subjected the company’s zinc flake system in various base coat /top coat combinations to cyclic aging testing in accordance with DIN EN ISO 12944-9 to investigate its suitability for the requirements of the new “CX” corrosivity category. This involved the preparation of 13 coated steel sheets, which were subjected to a horizontal score with a length of 50 mm and thickness of at least 2 mm prior to undergoing the test cycle. The result: with all systems, red rust could be determined on the scoring after 4,200 hours. 7 of the 13 zinc flake systems tested could fulfil the requirements in full and correspond to corrosivity category “CX” according to DIN EN ISO 12944-9. The combination of the Delta-Protekt KL 120 base coat with the Delta-Seal GZ top coat proved to be the solution with the least debonding.

What other advantages does zinc flake technology have?

In addition to the very high protective performance against red rust in combination with very low coating thickness, zinc flake systems offer further advantages. The precise tailoring of the top coat to the zinc flake base coat enables additional multifunctional characteristics such as chemical resistance to cleaning media to be achieved.

This can be achieved by “classic” zinc flake products that develop their full functionality in an annealing process or through innovative room temperature curing zinc flake coatings that cure in ambient air. In both of these application forms no hydrogen is offered and the risk of application-related, hydrogen-induced stress corrosion cracking is not present. For this reason, zinc flake coatings are especially suited for high-tensile steel (> 1000 MPa). The wafer thin protective film of just 8–20 μm means that the surface coating also saves resources and is economical.

Conclusion

In the scope of extensive aging testing in accordance with DIN EN ISO 12944-9 it proved possible to demonstrate the performance, respectively the suitability of zinc flake systems individually tailored to requirements for extreme use in offshore areas (corrosivity category “CX”). This means they are a suitably economical and resource-saving alternative to wet coatings that are commonly used or named in the standard.

By Emre Kocak (Product Engineer Dörken Coatings)