As we watch the dawn of the U.S. offshore wind industry, one thing is certain: Corrosion protection that worked in Texas will not work in the Atlantic Ocean. Almost in answer to the challenge, a recent coating from France-based NOF Metal Coatings (www.nofmetalcoatings.com), Geomet 321, has passed corrosion tests by Germanischer Lloyd (GL) and ISO-20340. The coating has also found favor with Siemens Wind Power A/S which has approved it use on fasteners for its wind turbine blades. For this application, Siemens requires the coating survive salt spray for 1,400 hours (C5-H in ISO-12944) without corrosion of the substrate material. For the application, Geomet 321 is applied to a 15 micron thickness.
Geomet 321 is a non-electrolytically applied thin film coating for steel, cast iron, and other iron metals. It is a chrome and heavy-metal-free alternative to other coatings. Its corrosion resistance depends on the thickness and is due to mechanisms such as:
- A barrier effect due to the tile-like structure of the film
- Controlled sacrificial protection of the zinc in relation to the metal substrate
- Integral film passivation which reduces the speed at which the zinc and aluminum are consumed.
In addition, GE Power & Water, the division responsible for its wind turbine business, has approved the zinc-flake systems Geomet and Geoblack in its latest standard P14A-AL-0218. It calls for meeting 1,000 hours, salt-spray resistance, also described in ASTM-B117 or ISO-9227. Furthermore, the coating passes mechanical damage tests described by D24 1312 and USCAR 32.
NOF says it invented zinc flake corrosion protection, and that the technology is the only one in the world with a water-based chemical compound. The company adds that the coating can be combined with other topcoats for a range of friction coefficients, values that can be adjusted from 0.06 to 0.20. A few other coating plusses include it being a non-electrolytic thin dry film and patented chemistry that passivates with zinc and aluminum flakes in a binder.
Just as important, the coating is self-repairing, so if damaged, zinc oxides and carbonates migrate to the damaged area of the coating to self-repair and restore the barrier protection. Ductility of the coating is sufficient for elastic deformation of devices such as spring and clips.
The company adds that the coating can be used with or without a topcoat and produces no hydrogen embrittlement. A lubricated topcoat is said to provide an excellent assembly and multi-tightening behavior.
The company also claims the coating works well at temperatures up to 300°C, and it’s paintable. It’s electrically conductive for most application processes and is bimetallic compatibility with aluminum
Two other coating systems from the company – Geomet 321 with Plus VLh, and Geomet 500 with Plus XL – have also been certified by GL regarding the corrosion protection of fasteners in oceanic and offshore atmospheres. This means a resistance to salt spray from 1,440 to 2,016 hrs without the appearance of red rust. GL certification tests take a full week: three days of UV exposure with condensation, three days of salt spray tests, and one day of low-temperature exposure (-20°C).
The French Corrosion Institute has also certified the Geomet coating in 2010. Its ability to pass ISO 20340 (performance requirements regarding coating systems for the protection of offshore and associated facilities), which corresponds to the level C5-M. NOF Metal Coatings says it is the first company to have a coatings certified by two different institutes. Geomet 321 can be applied by dip-spin, spray, dip-drain-spin using bulk or rack. WPE
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