July 6, 2009

Painting Galvanized Steel

by Bruce Snyder

Hot Dip Galvanizing is the process of “dipping” a steel structure in a molten vat of zinc.¬† This zinc will then metallurgically bond to the steel, protecting the steel substrate from corroding.¬†¬† The process consists of dipping the steel in a caustic bath to remove the organic contamination, followed by dipping the steel in an acid bath to remove the inorganic contamination.¬† After the steel has been prepared, it is dipped in flux to help promote adhesion and remove any oxides, followed by dipping the steel into the molten zinc.¬† The last phase of the galvanizing process can include a post treatment.¬†¬† These surface treatments include corn oil, palm oil, waxes, and chromic acid, just to name a few.¬† These treatments are typically a source of coating delamination and the contract specification should dictate this not be completed if the galvanized steel is to receive a coating system.

In order to determine if any of these post treatments are present and what type of surface preparation is needed to remove them, follow the test method below.

Make about a 10% solution of Copper Sulfate (normally available at a good drugstore) in water.¬† “Root Kill”, available at a hardware store, or “Pond Blue” available at local Farm Bureaus, are also copper sulfates.¬† Be aware some of these chemicals are copper sulfate free.¬† If these are used, you will get false readings with the test.

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To prepare the areas to be sampled, mark out a big enough area in order to test several methods of surface preparation.¬† In one area, take a rag with vinegar, and wash it.¬† In other areas, use acetone, MEK, or lacquer thinner (do not use an oily solvent such as VMP Naphtha or Mineral Spirits as they will leave an oily residue behind), a third area soap (like “Simple Green”), and in a fourth area, sand with sandpaper.¬† This will give you five test areas (the last being unprepared galvanized) with different preparation methods (acid washing, solvent washed, soap washed, abrasion and new galvanized).¬† Take the Copper Sulfate solution and apply it to all five test areas.¬† You will see a color change from blue to brown.¬† The color changes indicate that there is no surface treatment present and that the galvanized is paintable with the appropriate surface preparation.

The industry standard for the surface preparation of galvanized steel used to be to allow the structure to weather for 6 months.  The theory behind this was that after 6 months the zinc in the galvanizing has begun to degrade, producing a profile for the coating system to adhere to.  Since most of the time this scenario is not practical, mild acidic cleaners have been developed to not only clean the steel, but also to provide a profile for the coating to adhere to.

Never paint galvanized with an alkyd coating.  The alkyd coating will react with the high pH of the galvanized and will saponify.  Acrylic and epoxy coatings are better choices when you are looking to paint galvanized steel.

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