When using galvanized sheet metal, always specify "mill phosphatized" (also called "bonderized") galvanized sheet metal. We is an issue with respect to membrane adhesion. Unless the contractor is particularly careful to use only bonderized product, what will be provided is probably passivated, also known as "chem-treated" and hexavalent chromium treated.
Passivated galvanized sheet metal is known to result in poor adhesion. Few people at the construction level are aware of requirements related to the removal of passivation treatments. Passivation is the use of hexavalent chromium on coil galvanized stock to prevent white rust stains during storage.
The supply chain for galvanized steel sheet used for architectural applications can have numerous links, including producers who hot dip or electrogalvanize coil stock, coating applicators that may bonderize coil stock, large scale distributors, regional or local warehousers, and finally, end users that include contractors and fabrication shops.
There is a remarkable lack of understanding among individuals representing various links in this supply chain about the end use of galvanized sheet metal for architectural applications and the issue of field painting.
The only reliable way for an architect to ensure membrane adhresion is to specify a bonderized finish, which is a phosphate pretreatment. Both zinc and iron phosphate are used. Iron phosphating is less durable but also less expensive.
Bondurized galvanized sheet metal is not readily available for fabricated products like fabricated structural roof/ceiling decking panels, so it is almost certain that these will be "passivated" or "chem-treated" with a chromate solution, which inhibits paint and coating adhesion. Chromium-based pretreatments may contain both trivalent and hexavalent chromium. Although there is a drive to phase these out for environmental reasons in favor of Zinc-phosphate pretreatments, and while hexavalent chromium is banned in Europe, the US has no such requirements. Hexavalent chromium leaching from PG&E cooling towers in Hinckly, CA, is what the movie "Erin Brockovitch" was about.
There are only three effective methods to prepare passivated galvanized steel for painting:
1. Weathering for 12-18 months to oxidize sufficiently.
2. Brush off blast cleaning
3. Chemical treatment with a product such as Henkel Galvaprep SG with a scotch-brite abrasive pad (Some consider this high risk)
None of these methods is easy for typical architectural sheet metal applications. Brush off blast cleaning is messy and expensive on a finished building, and few building owners are going to accept a 12-18 month wait for a building to be painted. Chemical methods are dependent on workmanship. But one of the three has to be done unless the fabricated components can be furnished non-passivated.
Bonderized finishes are not as widely available as passivated finishes, but they are readily available for sheet metal stock such as is used for fabricated flashings. There may be 10% to 15% cost premium for bonderized finishes, but the savings in paint-related costs is probably more than that. For more information:
• GalvInfoNote 2.10 Imparting Resistance to Storage Stain, GalvInfo Center email:
info@galvinfo.com 1-888-880-8802
• GalvInfoNote 2.11 Preparing Galvanize for Field painting
• GalvInfoNote 2.12 Pretreatments for Metallic Coated Sheet
