Home >> Science & Nature >> Rocks & Minerals

What Type of Metal Is Galvanized Steel?

Named for inventor Luigi Galvani, galvanization refers to a process whereby a thin layer of zinc metal (or aluminum-zinc alloy) is applied to the entire surface of a metal substrate. However, while the interior of galvanized steel is pure steel and the outermost layer is pure zinc, there is a thin dividing layer that actually consists of zinc-steel alloy. To prevent erosion and promote durability, I-beams, fasteners, corrugated sheets and other steel construction materials undergo either traditional "hot-dip galvanizing" or the newer "thermal diffusion galvanizing."

Types of Steel Substrate
- While galvanization improves resistance to chemical corrosion, the piece's steel interior determines its mechanical properties. Therefore, manufacturers group steel substrates into several different strength categories, including commercial steel (CS), forming steel (FS), structural steel (SS), deep drawing steel (DDS), extra deep drawing steel (EDDS), solution hardened steel (SHS), bake hardenable steel (BHS), high-strength low-alloy steel (HSLAS), advanced high-strength low-alloy steel (AHSS) and full hard steel (FHS).
Hot-Dip Galvanizing
- The traditional method for ferrous galvanization, hot-dip galvanizing consists of submerging a steel substrate into a molten bath of zinc and lead, heated to over 460°C. While the lead doesn't bond with the steel, it helps keep the liquid zinc fluid, thus preventing excess surface accumulation. The piece is then removed and cooled.
Thermal Diffusion Galvanizing
- Producing thinner layers than hot-dip techniques, thermal diffusion galvanizing consists of rotating the piece inside a 400°C oven filled with a special zinc powder for four hours. The resulting zinc layer has strong adhesion with the steel interior and is easier to paint than hot-dip galvanized steel pieces.
Weight Designations
- The zinc layer in galvanized steel can come in a wide range of thicknesses. Known as "grade," this thickness is measured as the amount of zinc coating per unit of area. For example, in the Imperial/American measurement system, common galvanization grades include G20, G30, G40, G60, G90, G115, G165 and G210, which stand for hundredths of an ounce per square foot, i.e. G60 = 0.60 oz/ft^2. For the SI/metric system, common grades include Z90, Z120, Z180, Z275 and Z350, which stand for grams per square meter, i.e. Z180 = 180 g/m^2.
Mechanism
- When exposed to oxygen in the atmosphere, the zinc crystals on the surface of galvanized steel form a uniform layer of zinc oxide (ZnO). If the piece is continually exposed to water, the ZnO can become zinc hydroxide (Zn(OH)2). Over time, the carbon dioxide in the air reacts with the zinc hydroxide to form zinc carbonate (ZnCO3), a durable, water-insoluble compound. Unlike iron, which forms patches of corrosion ("spalls") that expose deeper parts of the metal to the elements, zinc corrodes in a continuous layer that effectively seals off the entire interior of the piece.

Rocks & Minerals