The metalizing process always begins with proper surface preparation. Next, aluminum wire or zinc wire is continuously melted in an electric arc spray or gas flame spray gun. Clean, compressed air strips droplets of molten metal from the wire depositing these particles onto the steel forming the protective coating. This sprayed metal coating is both a barrier coating and a galvanic coating in one. A single metalized coating protects steel for 30 years or longer depending upon the application, coating thickness and sealing.
Compare the performance of metalizing and painting.
These are two 1988 photographs of a well-publicized railroad bridge over Ridge Avenue in Philadelphia. This is probably the best known of the U.S. bridges metalized more than 60 years ago.
The paint on the Interstate 71 overpass pictured below failed after only seven years.
I-71 overpass painted August 1992 with
zinc-paint-epoxy-urethane system.
Photographed in 1998.
If the endless cycle of re-blasting and re-painting every few years is to be controlled, the longest life coatings must be used. Owners should consider metalizing rather than painting, re-blasting and re-painting.
- The metalizing or thermal spray process consists of surface preparation by abrasive blasting, metal spraying and sealing or topcoating the sprayed metal.
- The three spray wires used for atmospheric or immersion service are pure aluminum, pure zinc or an alloy of these two metals 85/15, zinc/ aluminum. (The alloy is approximately 85% zinc and 15% aluminum by weight.) [See website section headed "The Coating" for typical applications for each material.]
- A metalized coating may be bare sprayed metal, sprayed-metal-plus-sealer or sprayed-metal-plus-sealer-plus-topcoat. The sprayed metal plus sealer is generally the most cost effective, lowest life cycle cost (LCC) option. [See website section headed "Sealer and Topcoats" for more information on sealing.]
- Coating thickness may vary according to application from .004" to thicker coats of zinc in the range of .012" - .014" for seawater splash zones.
Metallizing is considered a cold process in that the aluminum or zinc is deposited onto steel by spraying rather than by dipping the steel into a bath of molten zinc as with galvanizing. The steel remains relatively cool at about 250º-300ºF. There is virtually no risk of heat distortion or weld damage by metalizing.
There are no VOC's (volatile organic compounds) in the metalized coating. There is no cure time or temperature to limit metalizing, so metalizing may be applied throughout the year - in Houston during the summer or in Ohio in January and February.
Any steel structure that can be blast cleaned may be metalized. Applications include:
- Bridges
- Piers & shore facilities
- Lock & dam gates
- Pipelines
- Offshore oil platforms
- Communication or power transmission poles