Rust takes a rest

Feb. 5, 2004
New corrosion-control coatings are simple to apply and better withstand harsh environments than zinc phosphating.
Lord Corp.'s two-part MetalJacket coating passes the rigorous ASTM B117 1,500-hr salt-spray test for both scribed and unscribed samples. Creep from scribed lines is less than 1 mm. Unscribed samples show no coating degradation. In contrast, the same parts treated by zinc-phosphating alone last about 96 hr. To go longer, they require post painting with an epoxy enamel. The MetalJacket coating meets GM, Ford, DaimlerChrysler, Freightliner, and Nissan corrosion specs. It also meets automotive substance-use restrictions, and the European end-of-life recyclability directive.
Coating deposition thickness is primarily influenced by immersion time, and to a lesser degree, by thickness of the underlying metal treatment. Total deposition thickness ranges from 18 to about 41 µm.

Senior Editor

Improved protection against rust and corrosion is one reason automakers have been able to offer longer vehicle warranties. These efforts go beyond cosmetic body panels to include hidden components such as metal-elastomer isolation and motor mounts, strut mounts, and brackets. Protecting the latter parts from environmental assault and preparing their metal surfaces to be bonded used to be complicated. Preparation entailed special, multistep (and environmentally unfriendly) processes of chemical rinses and surface treatments.

But the Lord Corp. MetalJacket coating system does away with all that. It instead "autodeposits" barrier coatings without high voltage, rinses, or the heavy metals of the ubiquitous zinc-phosphate plating process. Lord says MetalJacket aqueous coatings contain no hazardous, volatile organics and generate no hazardous waste or sludge. MetalJacket coatings work on clean ferrous steels, zinc-phosphated steels, or cast iron. Parts to be coated are first induction heated to remove surface contaminants. This eliminates grit blasting and the acid-pickle and alkaline-rinse steps of conventional methods.

The MetalJacket process is either one or two-part, depending on the application. The first part, an autodepositing metal treatment called MetalJacket 1100, replaces the zinc-phosphating step and prepares mild steel for part two, the coating. MetalJacket 2100 coating is a low-viscosity, aqueous solution that reacts with steel surfaces to form a soft gel when parts are dipped into it. Gel deposition is driven by diffusion of iron ions at the part surface. Diffusion slows as a film builds which helps control thickness. The self-limiting process applies uniform films tens of microns thick, even on complex surfaces. Parts coming out of the coating bath are induction heated (to a lower temperature than used for cleaning) to drive off water. The coating serves two purposes: it acts as a primer for rubber-to-metal bonding and it protects exposed areas of bonded and nonbonded parts.

For some rubber-to-metal applications, a one-part metal treatment called MetalJacket 1200 can be applied to clean, unphosphatized steel. The approach replaces zinc phosphating and rubber-to-metal primers. The 1200 coating provides both a surface with which rubber adhesives can bond as well as moderate corrosion protection for exposed surfaces. And it's highly flexible to resist cracking during aggressive swaging operations common in the manufacture of certain isolation mounts. Zinc-phosphate coatings, in contrast, tend to be brittle and can tolerate only about 5% reductions without fracturing.

Besides meeting stringent environmental regulations and cutting cycle times, the technology is well suited for automation. "Robotic, cellular manufacturing spaces no larger than a conference table can rapidly apply corrosion-control coatings to single or multiple parts," says Lord's Mark Weih. Coated parts can pass to subsequent manufacturing stages without breaking a process flow.

Along those lines, Lord and factory-automation firm Hahn Automation have developed a turnkey corrosion-control system for lean manufacturers. These systems integrate with existing production operations, so it may be possible to bring raw materials in one end of a plant and send finished product out the other, "lights-out and 24/7," says Weih. Additional savings come from reduced movement of parts within a plant, smaller inventories, and the elimination of transportation costs to third-party coaters.

Make contact

Lord Corp., Cary, N.C., (919) 469-2500, www.metaljacket.comHahn Automation, Erlanger, Ky., (859) 283-1820,

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