Emerald Group Publishing Limited
Copyright © 2011, Emerald Group Publishing Limited
New high-performance alternative to zinc plating
Article Type: Methods From: Anti-Corrosion Methods and Materials, Volume 58, Issue 6
NOF Metal Coatings North America Inc. and Curtis Metal Finishing Company have launched ZnCoat, a revolutionary new finish that offers effective solutions for the problems that have been encountered by users of zinc plating for fasteners and small parts. ZnCoat, a zinc-rich coating for ferrous metals, offers several major advantages for high-strength fasteners when compared to electroplated zinc. The finish is entirely chromium-free (no hexavalent or trivalent chromium), and provides superior corrosion resistance with a thin film, without risk of hydrogen embrittlement. Most importantly, these features and more are available at no extra cost when compared to zinc plating, baked for hydrogen embrittlement relief. ZnCoat is applicable for industries such as agriculture, construction, appliance, recreation, marine, etc.
The zinc in the ZnCoat is passivated without the use of toxic materials. ZnCoat is free of chromium, nickel, cadmium, lead, barium and mercury, and meets RoHS, REACH and WEEE environmental regulations. By comparison, conventional zinc plating is passivated by the use of hexavalent or trivalent chromates. Chromates provide a thin-film passivation layer, but only on the surface of the zinc plating. Subsequently, during installation or in-use, this fragile film may be damaged and lead to premature zinc oxidation. ZnCoat offers a solution to this problem: environmentally friendly passivation agents and corrosion inhibitors are found throughout the film, and so the coating is far less susceptible to early failure. Furthermore, the unique corrosion inhibitors found in ZnCoat allow for continuous release throughout the life of the coating.
ZnCoat provides sacrificial protection as the zinc corrodes to protect the steel substrate. The coating is further enhanced by a self-repairing mechanism. If a coated fastener is abraded during installation, zinc oxides and carbonates form at a controlled rate at the damaged area, sealing the breach in the coating, and thus restoring barrier protection. Compared to conventional zinc plating, ZnCoat consistently achieves over 240 hours of ASTM-B117 salt spray corrosion protection, as well as higher cyclical corrosion performance and real-world outdoor weathering resistance.
ZnCoat is applied to steel parts through a non-electrolytic dip spin coating process. In contrast, the process of zinc electroplating produces hydrogen as a byproduct. Some of this hydrogen can be trapped beneath the plated layer and cannot escape. For hardened parts, this can cause hydrogen embrittlement of the steel substrate, leading to delayed fracture of the part after installation. For this reason, high-hardness parts must be baked to relieve the entrapped hydrogen, but this step adds cost, and does not guarantee the part is free from embrittlement. The ZnCoat process is intrinsically free from the risk of hydrogen embrittlement, thus reducing a processing step for high-hardness fasteners.
For threaded fastener applications, ZnCoat has a tight, consistent friction coefficient (CoF). Lower CoF can be achieved with conventional wax-type topcoats, as is common with electroplated fasteners. ZnCoat is compatible with microencapsulated, anaerobic and nylon type thread adhesive and sealant patches. Because the dry film is thin (8-10 microns/0.3-0.4 mils), no adjustments to thread tolerances are necessary.
ZnCoat provides excellent UV protection for outdoor applications. The coating performs to greater than 2,000 hours with no change in appearance in Q-Panel ultraviolet (QUV) testing (UVB at 158°F/70°C). Fluorescent UV lamps simulate the damaging effects of sunlight. In a few days or weeks, the QUV test can produce damage that would occur over months or years of outdoor exposure. ZnCoat resists fading, chalking, cracking, crazing, hazing, blistering, gloss loss, strength loss and embrittlement.
ZnCoat is applied under license by Curtis Metal Finishing, which took the lead in bringing ZnCoat to market. “ZnCoat has a number of performance advantages for high-strength fasteners in comparison to zinc trivalent plating, at no additional cost. The superior rheology of this coating allows for consistent, uniform application – a definite must for thin films,” says Brian Lowry, Vice President, Technical Services at Curtis Metal Finishing. “The finished product has an aesthetically pleasing, metallic silver appearance and it retains that appearance much longer than electroplated zinc.” Curtis provides the coating at two locations, Sterling Heights, Michigan, and Machesney Park, Illinois.
For more information, go to www.zncoat.com