By Linc York and Kendall Smith, Induron Sales Reps
The previous blog post, The 9 Lives of Galvanized Steel, Part I, describes the stages or conditions of galvanized metal as it ages or weathers. The pure zinc layer on the surface first reacts with oxygen to form zinc oxide, then that oxide layer reacts with moisture in the environment to form zinc hydroxide. Finally, zinc hydroxide reacts with carbon dioxide in the atmosphere to yield a thin, quite insoluble dull grey layer of zinc carbonate.
In typical environmental exposures, this process can take one or two years. Many environments can alter this normal progression. For example, in the presence of alkaline (high pH) or acidic (low pH) environments this reaction is accelerated, hence the reason to protect the zinc with top coats suitable for the environment. Simply stated, when galvanized metal is not top coated in corrosive environments your investment is quickly corroded away.
The most critical step in protecting galvanized metal in corrosive environments is to understand what exactly you are painting (pure zinc, zinc oxide, white rust, zinc-iron alloy, etc.) and properly treat and/or prepare it to receive a suitable protective coating. Following are several methods specific to the condition of the galvanizing.
NEW GALVANIZED METAL
When possible, tell your galvanizer that you intend to apply a protective coating. They should not apply a chromate coating or any protective material over the galvanizing. It is advantageous to have the galvanizer employ a phosphating treatment to passivate the metal and create a profile.
- Brush Blasting. In many situations, the galvanized metal (especially for large areas) is prepared by brush blasting to provide a profile. This can be effective, but caution is required to not destroy or remove too much of the layer of galvanizing. When brush blasting with abrasive media, be sure to use low pressures, approximately 40 PSI. If 4 mils of new galvanizing are specified, and 3 mils get blasted off, the coating system you specify will have to do most of the work protecting the steel underneath.
- Vinyl Butyral Wash Primers. If chromate quenching or water quenching are specified, proper surface preparation is even more important. In these cases, brush blasting or proprietary power-tool cleaning is often used. In many cases vinyl butyral wash primers are still used with good results, but are only as effective as the cleanliness of the galvanized surface. Post galvanizing phosphate treatments are also used effectively to stabilize and profile new galvanized metal; but may reduce the cathodic protection of the zinc layer.
- Acid Etching. This is commonly used to provide a profile for coatings adhesion over galvanized metal. Application of a weak phosphoric acid, or similar low pH solution, will suffice. Needless to say, all precaution and safety measures are in order when applying an acid. It is recommended to apply the acid to a small area first as a test. The acid should not bead up. If it does, there is a contaminant or preservative that must be removed to allow the acid to work. After the acid has been in contact for a few minutes it should be rinsed with potable water and the surface of the galvanized metal should appear dull gray. When this appearance is achieved the protective coating should be applied within 2-4 hours.
- Power Tool or Hand Sanding. These are excellent means of preparation for small areas and will provide a surface profile suitable for the adhesion of the protective coating.
PARTIALLY WEATHERED GALVANIZED METAL
The appearance of this condition will be mostly dull and gray/white in color, which indicates the presence of zinc oxide/hydroxide/carbonate. There should be very few shiny silver colored spots in evidence. Proper surface preparation must begin by pressure washing with potable water to remove the water soluble salts, zinc oxide and zinc hydroxide. When that is completed, employ abrasive blasting, acid etching and hand/power tool cleaning as mentioned above.
FULLY WEATHERED GALVANIZED METAL
This condition is visually evident with a dull gray to dark gray, salty appearance to brownish, orange and yellow colors from the several different layers of zinc-iron alloy being exposed. Each “color” has a fairly predictable percentage of zinc/iron.
The salty appearance is indicative of zinc carbonate salts. These salts are generally seen as beneficial since they are tightly adherent, water insoluble and actually form a profile that is beneficial for coatings adhesion. Surface preparation methods should employ pressure washing with potable water to remove any residual water soluble salts. Allow to dry and apply the recommended protective coating(s).
Generally speaking, any coloration other than dull, dark gray indicates the presence of iron oxides and suggests that the protective layer of galvanized has practically reached the end of its protective capabilities. This means it’s best to paint before all of the zinc-iron alloy layers are gone, because now you are painting rusted carbon steel. At this point proper surface preparation consists of abrasive blast cleaning, possibly in combination with a salt neutralization solution, although in some cases a 100% solids, penetrating epoxy (Induron E-Bond 100) can be applied in lieu of abrasive blast cleaning.
Further information for surface preparation of galvanized metal can be found in ASTM D6386 but are summarized here.
As always, please consult your Induron Coatings representative for further information and assistance with surface preparation and protective coatings recommendations. Stay tuned to this blog for “Protective Coatings Systems for Application Over Galvanized Metal”.