The electroplated galvanized steels have the pure zinc outer layer but lack the intermediate alloy layer that mixes the steel and zinc.
Galvanized surfaces will lose the spangle and form a gray zinc oxide film with a slight tooth or fuzziness to them. Sometimes, streaks of red rust will develop on galvanized steel surfaces where the steel is no longer getting the benefit of the galvanic protection.
Spangle Galvanized Steel
One of the visual effects resulting from galvanization of steels is an appearance of crystalline spangling across the surface. The spangle is a unique and interesting formation on the galvanized surface. It is created by use of trace metals in the molten zinc, and by controlling the cooling process of the hot-dip zinc coating on the steel.
The process involves running a sheet, plate, structural shape, or constructed shape into a molten bath of high-purity zinc. Additional trace metals are put into the molten zinc to assist in the formation of the zinc crystal on the surface. As the hot, coated steel exits the bath of zinc, it is blasted with air to remove excess zinc from the surface. The zinc surface cools with the steel base material. As the surface cools, crystals of zinc develop across the surface.
Spangled Galvanized Steel: Environmental Concerns
Large spangles can be achieved on galvanized sheet, but it comes at a cost: these materials often contain lead content. There are other methods to create spangle on galvanized materials, including the use of Antimony, a lustrous grey metalloid for alloying metals. However, Antimony-alloyed galvanized steels are not typically of the same size as the lead-bearing galvanized steels.
Most manufacturers of galvanized sheet have ceased to produce galvanized steel with the high lead content required for a spangled appearance.