What started out in Europe and South America as an award-winning water based, zinc and aluminum-rich, chromium-free finish is now available to support North American automaker’s strategies to improve corrosion protection on their vehicles.
Metal Coatings International Inc. (Chardon, OH), a member the NOF Metal Coatings Group that originated zinc flake coatings in 1974, has released Geomet 321 as a high-performance finish that provides corrosion protection to ferrous metals, including powdered metal parts.
The new coating is used in a variety of markets including automotive, construction and wind energy, and was awarded the Pierre Potier 2009 Prize for ‘chemical innovation favoring sustainable development’. Potier, who died in 2006 71, was at the forefront of the French school of natural-products chemistry during the second half of the 20th century. His award is given to companies who develop innovative chemical products that respect the environment and for acting eco-responsibly in its anticorrosion activity.
Terry Dorsett, MCII’s Technical Director, says Geomet is a chromium-free, water-based coating, containing metallic zinc and aluminum flakes. The zinc and aluminum platelets align in multiple layers forming a metallic silver film. Applied as a liquid, the coating is completely inorganic after curing, Dorsett says.
“Geomet provides sacrificial protection as the zinc corrodes to protect the steel substrate,” Dorsett says. “The highly effective corrosion inhibitors, which are interspersed throughout the coating film, make the zinc less active and therefore prolong galvanic protection.”
Compared to the previous generation of Geomet, Dorsett says zinc passivation is improved, and the anti-corrosive coating provides extended corrosion protection, especially in cyclic or on-vehicle corrosion tests.
“We expect corrosion protection in excess of 1,000 hours salt spray per ASTM-B117,” he says.
Development of Geomet came about after the European ‘End-of-Life Scrap Vehicle Directive’ banned the use of hexavalent chromium in automotive applications after July 1, 2003. One year ahead of schedule, MCII completed a North American transition from their Dacromet finish, which contained a trace of hexavalent chromium, to their newly developed chromium-free finish Geomet by July 1 2002.
Subsequently, the European Union deadline to eliminate hexavalent chromium was moved back to 2007, and Dorsett says that NOF Metal Coatings Group members DACRAL in Europe and Metal Coatings Brazil in South America used the extra time to optimize Geomet and release an improved formula prior to the 2007 deadline. By 2006 Chrysler specified the first North American use of Geomet for one fastener trial application on a production vehicle.
“The North American transition to this new finish was lead by Chrysler,” Dorsett says. “When their validation testing began in 2006, their main goals were to increase on-vehicle corrosion performance without changing torque tension properties in order to maintain plant installation practices.”
For Chrysler finish approval, production quantities of bolts and nuts in all diameters ranging from M6-M16 were coated for a variety of tests. These parts were put through a rigorous testing program including Vehicle Proving Grounds, Torque Tension, SAE J2334 Cyclic Corrosion testing, Salt Spray, Heat & Salt Spray, Mechanical Damage Resistance, and Patch Compatibility.
As part of the validation process, Dorsett says an emphasis was placed on how well the new coating held up to mechanical damage. He says that, although a coated part may perform well in corrosion testing when tested immediately after coating application, this may not be the case when the part arrives at the point of assembly, and especially after assembled to a vehicle.
“Coated parts are typically subjected to a variety of secondary handling, such as inspection, sorting, patching or automated feed systems, which may cause damage to any finish,” he says. “The new USCAR 32 test provides a procedure to evaluate how a coating will perform after such handling.”
USCAR 32 outlines that one kilogram of coated parts are dropped through a one meter tube three times to simulate handling damage. The parts are subsequently tested in a cyclic corrosion or salt spray chamber. The specifications for making the tube are simple, and suppliers are now incorporating this test in their labs.
“A clear advantage of our chromium-free coating systems is their performance after mechanical damage,” says Dorsett. “By choosing high performance, chromium-free coatings now, suppliers avoid potential issues related to chromium enhanced products in the future.”
The Chrysler M6-M16 production coated parts were tested before and after USCAR 32 mechanical damage testing. The results showed that Geomet performed well, with corrosion results exceeding performance expectations, on parts both before and after mechanical damage. Additionally, Geomet far outperformed the previous generation of Geomet and a competitor’s product when corrosion testing was conducted following mechanical damage.
Dorsett says Chrysler’s full-scale transition to Geomet was completed in 2008 through the addition of Geomet to the PS-5873 material specification. Chrysler waived PPAP and other requirements in lieu of their validation testing, which made the transition easier for suppliers.
Ford released Geomet in 2008 by updating S438 (WSS-M21P39-A1), the standard finish designation for Geomet ML. General Motors approved Geomet ML to GMW 3359, which is used for the majority of GM’s high-performance fasteners. Geomet allows GM to upgrade fastener performance with a thin-film coating. The GMW 14 and GM 6173M specifications will also utilize this new basecoat. Available worldwide, global suppliers are using MCII’s latest offering to GM on new parts.
“In addition to satisfying automotive requirements, we’re satisfying the needs of the Tier One suppliers and have completed trials at all their licensed coating facilities,” Dorsett says. “The response from suppliers and coating applicators has been extremely positive in terms of an improvement noted in corrosion performance.”
For more information on Metal Coatings International Inc., please call 440-279-1483, or visit www.metal-coatings.com.
Zinc Alloy Plating
To read about how zinc alloys has increased and the production processes, visit http://www.pfonline.com/articles/pfd0313.html
The process of applying the Geomet coating requires the substrate to be clean bare steel. Sufficient cleanliness and coating weight is required for proper levels of adhesion and performance.
Alkaline Cleaning- In order to remove manufacturing oils or other contaminants, an alkaline wash by immersion or spray is used.
Mechanical Cleaning (Shot Blasting)- Mechanical cleaning uses steel grit, balls or glass beads to remove heat-treat scale and/or flash rust in addition to giving the part roughness to provide good adhesion.
Dip Spin: Small parts (fasteners, stampings, clips, springs, etc) are coated using the Dip Spin application method. Bulk parts are loaded into a basket, immersed into the Geomet coating, raised up and spun to remove the excess Geomet from the parts. Following coating, the parts are transferred onto a conveyor and proceed into a convection oven for curing. Parts coated using Dip Spin application method require two coats of the Geomet base coat.
Spray: Large parts (rotors, brackets, large fasteners, etc.) can be coated using the Spray application method. Parts are individually loaded onto a chain on edge or rack and continue past air or electrostatic spray nozzles that can be set to strict parameters to provide optimum coating weight on individual areas of the part. Following application of the Geomet coating, the parts proceed into a convection oven or induction-heating coil for curing. Parts coated using Spray application method require one coat of the Geomet base coat.
Dip-Drain Spin: Large parts (fuel filler tubes, stampings, large bolts, etc.) can be coated using the Dip-Drain Spin application method. Parts are individually loaded onto a rack, dipped into the Geomet coating, raised up and spun to remove the excess Geomet from the parts. Following coating, the parts proceed into a convection oven for curing. Parts coated using Dip-Drain Spin application method require one or two coats of the Geomet base coat.
Pre-cure: Parts are subjected to 150-250° F (66-121° C) for 10 minutes to set the coating. Pre-cure is an essential element in the curing process to achieve proper adhesion and appearance.
Main Cure: Parts are transferred from pre-cure into the main cure to provide the final cure for the Geomet coating. In order to properly cure Geomet , a peak metal temperature of 575°-600° F (300-315° C) must be maintained for 15 minutes.