Zinc Flake Organic Fastener Coatings
Zinc Flake Coating
Zinc Flake thread Coating is a generic term for the coatings marketed by different suppliers under their respective brand names. The coatings consist predominantly of Zinc Flakes usually with a small quantity of Aluminium. The flakes are about 1µm thick and are bonded together with inorganic or organic compounds.
Development of these coatings began around the 1970’s to give better corrosion resistance than Zinc Electroplated coatings. The coatings come under brand names such as Geomet® and Dacromet® to name two and are used extensively by the Automobile Industry to give salt spray resistances exceeding 600 hours with a thin coating, making them ideal for fasteners.
This coating does not induce Hydrogen Embrittlement, which means that high tensile fasteners can be safely coated.
Some of these coatings have integral lubricants to facilitate assembly of fasteners, or they can have lubricated topcoats.
All Zinc Flake materials are similar to paints, so they can be applied by dip coating or spraying.
Zinc Flake Dip spin coating
This process is ideal for small parts including small to medium size fasteners.
As with all coating processes the parts must first be cleaned to remove any oils, greases or other contaminants, usually by submersion in an alkaline cleaner. The scale or rust is then removed, either by submersion in an inhibited acid solution, but more likely to be blast cleaned in a belt type shot blaster, usually using a Wheelabrator type of shot accelerator with fine steel shot. (Some coatings are applied to a phosphate layer)
The parts are then dipped into the Zinc Flake mixture in a basket; the raised basket is then centrifuged to remove excess material. The parts are then dried and cured, (at temperatures between 200 and 300ºC).
When processed in bulk there will be touch marks, the process is repeated to ensure overall coverage. In some cases, further coats are applied to build up the thickness to improve corrosion resistance, or a topcoat used to alter lubricity or colour.
Zinc Flake Coating Large Fasteners
Fasteners that cannot be bulk coated due to damage problems; usually an issue with bolts above M16 and longer than 150mm. can be spray coated, or occasionally they are installed onto racks and dipped and spun similar to bulk processing.
Extra thickness can be applied to increase corrosion resistance, (up to 3000 hrs. salt spray) in some cases thread allowance may be required.
Specifications have been written by many OEM’s; the appropriate International specification is ISO 10683 –Fasteners: non-electrolytically applied zinc flake coatings.
ZINC FLAKE FASTENER COATINGS