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NOF METAL COATINGS GROUP has developed thin layer anti-corrosion coatings that provide worldwide wind power companies with an alternative for protecting fasteners.

The zinc flake anticorrosion technology that the group has been developing for over 30 years is used on a global scale. But why is the wind power sector interested in this technology developed by the NOF METAL COATINGS GROUP? Michel Almosnino, Chemical Engineer in charge of the wind power market at NOF METAL COATINGS EUROPE in France explains: "The wind power market is one of the most  dynamic industries for which our water-borne  zinc flake anticorrosion technology is recognised as efficient by the European and international professionals of the sector. It is a fast-growing market with an installed capacity increasing from 25 to 30% yearly worldwide." The anticorrosion performance of coatings are indeed essential as in big mechanical structures such as wind turbines, the fastening of the different elements together provides the installation with safety and integrity. Michel Almosnino wishes to add: "on a 1 megawatt wind turbine, there is about 5 tons of bolts.”

What does this innovation consist of?

The structural element of a wind turbine, the nacelle mounted on top of the mast, connected to the rotor by the hub, is home to mechanical and pneumatic components as well as some electrical and electronic components necessary for its running. The corresponding fastenings, whose diameter is between 24 and 64 mm and which are subject to mechanical stress, physico-chemical and climatic constraints have to be protected. Thanks to its anticorrosion protection principles (barrier effect, sacrificial protection and passivation) already demonstrated in the fields of automotive, trucks, railway and construction equipments, the GEOMET® zinc flake coatings meet these requirements.

Regarding the control of coefficients of friction, the NOF METAL COATINGS GROUP has developed with several wind turbine manufacturers an alternative system for lubricating agents used generally when tightening and untightening bolts. It consists in a PLUS® XL lubricated topcoat ensuring the tribological functions needed for the screw-bolt assembly, which when associated with GEOMET® 500, provides an anticorrosion protection of at least 1000 hours when salt spray tested according to ISO 9227 standard. Two laboratories, one French, the other Spanish have tested and validated the anticorrosion and tribological properties. 5 assemblies/disassemblies are simulated without altering the properties. The result is a reduction of the assembly costs. Michel Almosnino provides details: "Owing to this system, the coefficient of friction is very low, ranging between 0.06 and 0.09. The tightening torque is therefore lower, thus making the assembly and disassembly easier, particularly during maintenance operations. The coefficient of friction is an essential parameter to control the integrity of the bolted joints."

The geographical situation of wind farms is part of the factors that have to be taken into account. Around the great American lakes for instance, where winter temperatures are very low, the damage incurred by the parts is not the same as that for the offshore wind turbines.Some specific tests have to be implemented. In France, the Institute of Corrosion in Brest, French subsidiary of the Sweden Institute of Corrosion has certified the NOF METAL COATINGS GROUP coatings.

Application processes require expertise

Fasteners for wind turbines are generally large-sized, can weigh up to ten kilograms and have often complex geometries. It is important to have appropriate industrial technology for application. There are several application processes among which the spraying of parts on racks which is the most used owing to its easy implementation on big-sized screws. Another common technology is the dip-spin process on racks. It enables covering hollow spaces and thus avoiding the “Faraday cage" effect linked to the electrostatic spraying application.

It is useful to be reminded that prior to being coated, the parts are degreased and shotblasted (and not pickled in order to avoid hydrogen embrittlement).

The GEOMET® 500 + PLUS® XL anticorrosion system is a coating formed from aqueous solutions. It is composed of zinc and aluminium flakes in an inorganic matrix, without chromium.
The application of PLUS® XL topcoat after the already self-lubricated GEOMET® 500 coat reduces substantially the coefficient of friction to values that are appropriate for the wind sector.

The PLUS® XL topcoat enables, on the one hand to ensure the required tribologic functions for the screw/bolt assembly and on the other hand to avoid the additional stage of further lubrication which is used frequently and costly. According to specifications, parts are coated with a very thin film, whose thickness is comprised between 8 and 10 µm. The last application process step consists of a drying and curing of the coating.

The product (chemical compounds) / application processes association is indissociable. NOF METAL COATINGS devotes substantial resources to develop new application possibilities. The licensed applicators provide their experience and knowhow regarding industrial application processes.

Land or sea-based wind farms are booming, particularly in the United states, China, India and in Europe, especially in Spain, Germany and United Kingdom with the offshore wind power. Thanks to its global organisation, NOF METAL COATINGS GROUP is able to accompany these changes by bringing its expertise in the thin layer anticorrosion field.

For more information and any technical enquiry, please contact Michel Almosnino (in charge of non-automotive markets) by email at: or by phone at: +33 (0)3 44 64 63 62

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