Zinc is the fourth most-used metal globally after iron, aluminium, and copper, with annual production around 13 million tonnes. Roughly half of this output goes into galvanizing, the process of coating steel with zinc to prevent rust. The remaining half splits among die-casting alloys, brass production, rolled zinc sheet for building, and zinc oxide for rubber and paint.
Galvanizing Processes
Hot-dip galvanizing remains the dominant industrial method. Cleaned steel components are immersed in a bath of molten zinc at approximately 450 degrees Celsius. A series of iron-zinc intermetallic layers forms at the interface, topped by a pure zinc outer layer. The total coating thickness typically ranges from 50 to 150 microns depending on steel section and immersion time. These layers are metallurgically bonded to the steel, meaning they resist mechanical damage better than paint alone. Hot-dip galvanized structural steel in a moderate inland environment can achieve a service life of 50 to 70 years without maintenance.
Electrogalvanizing, also called zinc electroplating, deposits a thinner, more uniform zinc layer using an electric current in an aqueous zinc sulphate or zinc chloride bath. Typical coating thicknesses range from 5 to 25 microns. The process suits sheet steel for automotive body panels and appliance cabinets where a smooth, paint-ready surface matters more than maximum corrosion protection. The absence of intermetallic layers makes electrogalvanized coatings less abrasion-resistant than hot-dip equivalents.
Continuous galvanizing feeds steel strip through a zinc bath at high speed, producing the familiar spangle-finished sheet used for roofing, ducting, and agricultural buildings. Production lines at steel mills including ArcelorMittal and POSCO can coat strip over 2 metres wide at speeds exceeding 100 metres per minute.
Sacrificial Protection
Zinc protects steel galvanically because it sits lower on the galvanic series, with a standard electrode potential of negative 0.76 volts versus the standard hydrogen electrode. When the coating is scratched, exposing bare steel, the surrounding zinc corrodes preferentially while the steel remains cathodically protected. This sacrificial mechanism distinguishes zinc coatings from barrier coatings like paint, which fail locally at any breach. The area of steel that can be protected depends on the conductivity of the electrolyte: in seawater the protection distance may reach several millimetres, while in freshwater the effect is more localised.
Architectural Zinc Roofing
Rolled zinc sheet for roofing and cladding typically uses an alloy of 99.995% pure zinc with small additions of titanium and copper to improve creep resistance and reduce thermal expansion. The two dominant brands, VM Zinc and Rheinzink, both originated in the Rhineland region of Germany, where zinc roofing tradition dates to the early 19th century. Zinc roofs develop a protective patina of zinc carbonate and zinc hydroxide that evolves from bright metallic to matte blue-grey over three to five years. This self-healing patina, combined with the material's malleability allowing complex folded-seam detailing, has made zinc the roofing material of choice for architects from Daniel Libeskind to Herzog and de Meuron. A properly installed standing-seam zinc roof can last 80 to 100 years with zero coating maintenance.