Overview
Zinc is a moderately reactive, bluish-white metal most widely used as a protective coating for steel (galvanizing), in die-cast alloys (notably Zamak), as an alloying element in brass, and in batteries. Its defining engineering property is sacrificial cathodic protection of steel — zinc corrodes preferentially, protecting exposed steel even at coating defects. Zinc is also an essential trace element in biology.
Mechanical Properties (pure zinc, wrought, room temperature)
| Property | Value |
|---|---|
| Density | 7,140 kg/m³ (0.258 lb/in³) |
| Young's Modulus | 95 GPa (about half steel) |
| Yield Strength | 20 MPa (pure, cast) |
| Ultimate Tensile Strength | 110 MPa (pure, wrought); up to 280 MPa (high-strength alloys) |
| Elongation at Break | 10-40% (alloy dependent) |
| Hardness (Brinell) | 30-50 BHN |
| Melting Point | 419°C (low — easy to cast) |
| Boiling Point | 907°C |
| Crystal Structure | HCP (hexagonal close-packed) |
Physical Properties
| Property | Value |
|---|---|
| Thermal Conductivity (20°C) | 116 W/m·K (good conductor) |
| Specific Heat | 388 J/kg·K |
| Coefficient of Thermal Expansion | 30 × 10⁻⁶ /°C |
| Electrical Resistivity | 5.9 × 10⁻⁸ Ω·m |
| Electrochemical Potential | -0.76 V (very anodic — active sacrificial protection) |
Key Properties
- Corrosion protection: Zinc forms a protective patina (ZnCO₃, Zn(OH)₂) in atmospheric exposure; corrosion rate ~1 µm/year in rural air, 3-6 µm/year in industrial/marine
- Sacrificial anode: With electrode potential -0.76 V, zinc protects steel (which is -0.44 V) by acting as the anode in galvanic cells
- Castability: Low melting point (419°C), excellent fluidity, high dimensional accuracy and surface finish in high-pressure die casting
- Dimensional stability: Zamak alloys hold tight tolerances in thin sections
- Ductility: Reasonable ductility at room temperature; can be rolled into sheet, extruded, drawn
- Alloying: Zinc-aluminum (ZA) alloys offer elevated strength and bearing properties
Major Applications
Galvanizing (~50% of zinc use)
- Hot-dip galvanizing: Steel parts dipped in molten zinc (~450°C), forming metallurgically bonded Zn-Fe layers
- Typical coating thickness: 45-85 µm (G90/G60 sheets; heavier for structural steel)
- Service life: 50-100 years in rural/atmospheric environments; 20-50 years marine/industrial
- Continuous galvanizing: sheet steel for roofing, siding, automotive body panels, appliances
- Zinc protects steel two ways: barrier coating AND sacrificial (cathodic) protection at scratches/edges
Die Castings (~20% of zinc use)
- Zamak family (Zn + 4% Al + Mg/Cu): Zamak 3 (most common), Zamak 5 (higher strength)
- Automotive: carburetor bodies, door handles, trim, mirror housings (replaced by polymers in many cases)
- Hardware: locks, hinges, gears, bathroom fixtures, toys (model cars)
- Electronics: connector housings, RF shields
- High-pressure die casting produces complex shapes with excellent surface finish ready for chrome plating
Brass & Bronze Alloying (~15%)
- Zinc is the primary alloying element in brass (copper-zinc) — see [[brass|Brass]]
- Typical 70/30 and 60/40 (cartridge brass, Muntz metal)
Zinc Alloys
| Alloy | Composition | Use |
|---|---|---|
| Zamak 3 | Zn-4Al-0.04Mg | General purpose die casting |
| Zamak 5 | Zn-4Al-1Cu-0.05Mg | Higher strength/creep resistance |
| ZA-8 | Zn-8Al-1Cu | Higher strength; gravity casting |
| ZA-12 | Zn-11Al-1Cu-0.02Mg | Bearings, high-load parts |
| Zn-Al eutectoid | 78Zn-22Al | Superplastic forming alloy |
Other Applications
- Zinc anodes: Sacrificial anodes on ships, pipelines, storage tanks (cathodic protection)
- Batteries: Zinc-carbon (dry cell), alkaline (Zn-MnO₂), zinc-air (hearing aids), zinc-bromine flow (grid storage)
- Roofing/cladding: Rolled zinc sheet for standing-seam roofs, gutters, flashing (centuries-long service life in Europe)
- Zinc oxide: white pigment in paint/rubber; sunscreen; vulcanization of rubber; semiconductor
- Zinc die castings for costume jewelry, buckles, buttons, zippers
- Chemical compounds: zinc sulfate (fertilizer), zinc chloride (batteries, soldering flux), zinc sulfide (phosphors)
Production & Recycling
- Primary production from zinc sulfide ore (sphalerite), roasted then electrowon or smelted
- ~90% of zinc in scrap is recycled (galvanized steel, brass, die castings)
- Zinc is a relatively low-energy metal to recycle
Summary
Zinc (density 7140 kg/m³) is the fourth most common metal in use after iron, aluminum, and copper. Its primary engineering roles are galvanic corrosion protection of steel (50% of use, 50-100 year service life), high-pressure die casting (Zamak alloys for complex hardware and automotive parts), and alloying with copper to make brass. Low melting point (419°C), excellent castability, sacrificial electrochemistry, and natural patina formation make zinc irreplaceable for corrosion protection, while its low cost and good dimensional stability support the largest die-casting industry.