Most iron-containing (ferrous) metals can be hot-dip galvanized. The chemical composition of the material influences the galvanized coating’s characteristics. During galvanizing, the molten zinc reacts with iron in the material being galvanized to form a series of highly abrasion-resistant, corrosion-inhibiting zinc-iron alloy layers, which are normally covered by a layer of almost pure zinc. Cold-rolled, hot-rolled, cast-iron, weathering, and even 300 series stainless steels can be and are galvanized.
HDG Fasteners
Fasteners and small parts are also commonly galvanized for corrosion protection, and hot-dip galvanized fasteners are recommended for joining galvanized structural assemblies. Using galvanized fasteners for assembling hot-dip galvanized steel structures eliminates complications related to contacting dissimilar metals.
Best practice suggests bolted assemblies be sent to the galvanizer unassembled. And there are guidelines to follow for undersizing male threads and overtapping nuts to allow for the dimensional increase in the threads from the zinc coating, which can be found on the AGA website.
Steel Chemistry
Knowing the steel chemistry of the material you plan to galvanize is also important. For information on recommended steel composition, refer to ASTM A 385 Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip). Silicon especially can have a profound effect on the growth of galvanized coatings. The Sandelin Curve (below) shows the recommended levels of silicon to produce typical hot-dip galvanized coatings. For highest-quality galvanized coatings, silicon levels should be less than 0.04% OR between 0.15% and 0.23%. Steels outside these ranges, considered reactive steels, can be galvanized, and typically produce an acceptable coating; however, these steels often form a thicker coating, thus a darker coating should be expected.
