Hex-chrome-free process turns the screws on corrosion

Dec. 12, 2002
As manufacturers try to balance market demands for increased performance, recent environmental mandates such as ELV (End-of-Life Vehicle Directive), have forced companies to reexamine how they use corrosion-resistant coatings.
Depending on the appropriate surface treatment, passivate (clear/silver), and top coat, fasteners have sustained more than 120 hr to "white corrosion" compared to only 20 hr for their Cr(VI) coated counterparts.

This examination has trickled down to fasteners in applications including bolted or riveted joints.

With new ELV standards, the auto industry must look at lead, cadmium, and hexavalent-chromium replacements to reduce and/or eliminate the use of these materials in coating processes. The ELV has several objectives for primary manufacturing and secondary handling of new and old vehicles. First, ELV wants to put fewer chemicals in the waste stream. Second, it aims to eliminate restricted and hazardous materials to ease recycling of vehicle components.

In Europe, with the exception of certain components such as lead in batteries and steel wheel rims, vehicles sold after July 1, 2007 will no longer use hexavalent chromium. The European standard's goal will be a totally hexavalent-chromium-free vehicle.

The North American automotive industry is also implementing new standards to better meet global specifications. Several major automotive manufacturers and their suppliers have adopted either the European standard or created even more stringent guidelines to be phased in over time.

Hex-chrome-free coated parts
Hexavalent-chromium-free processes let component finishers deliver parts with the right color, corrosion resistance, and proper torque tension while meeting environmental directives and recyclability standards.

DIN 946 results for Enseal hexavalent-chromium-free system topcoats versus alternative products for torque tension control.

The first step in the process is a metallic undercoat such as Enthobrite zinc or Zincrolyte zinc alloy from Enthone Inc., New Haven, Conn. (www.enthone.com). Undercoat thickness ranges from 8 to 15 mm and is followed by a conversion coating or passivate to replace the former hexavalent conversion coating. Passivates are applied by solution immersion, lasting 30 to 60 sec. Hexavalent-chromium-free passivate processes such as Perma Pass are available in black, iridescent yellow, or clear/silver.

The final step in a hexavalent-chromium-free system is the application of an Enseal topcoat or seal to complete the finish. This is also an immersion process lasting up to a minute. Fasteners then dry either in a centrifuge spin dryer or on a conveyor belt dryer. Topcoats and seals also lower friction levels letting threaded fasteners meet tighter torque tension specification.

Corrosion specs
The newer hexavalent-chromium-free system finishes can meet and often exceed required neutral salt-spray performance standards and accelerated corrosion tests. Depending on the appropriate surface treatment, passivate, and topcoat, tests on parts have sustained more than 120 hr to "white" corrosion and exceeded 400 hr to "red." There is reportedly also no loss of performance after 24 hr at temperatures ranging from 120 to 150°C.

Prevention of contact corrosion between dissimilar metals and disassembly properties are additional benefits. Hexavalent-chromium-free finishes also don't fill fastener threads or recesses. And the process can be configured with a controlled torque tension range for the proper handling and setting of threaded fasteners.

Proper application of hexavalent-chromium-free finishes to fasteners can also control torque tension levels. Newly developed topcoat chemistries meet torque tension targets and better define the maximum and minimum ranges in tighter and more acceptable measurements critical to assembly and manufacturing operations versus alternative coatings.

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