Zinc coatings provide cathodic (sacrificial) protection to steel — the zinc corrodes preferentially, protecting the steel even at cut edges and scratches. Several distinct processes exist, each suited to different component types and performance requirements.
Hot-Dip Galvanizing (Batch)
The most robust zinc coating process, producing metallurgically bonded zinc-iron alloy layers. Components are immersed in molten zinc at 445–465°C per EN ISO 1461:
- **Typical thickness**: 45–200 µm depending on steel thickness and silicon content\n- **Adhesion**: Metallurgical bond — the strongest of all zinc coating methods\n- **Limitations**: Component must fit in the galvanizing bath (maximum typically 12–16 m length), must tolerate 450°C without distortion, requires vent and drain holes for hollow sections\n- **Service life**: 25–70+ years in C3 environment (EN ISO 9224)
Continuous Hot-Dip Galvanizing (Coil)
Sheet steel is passed continuously through a molten zinc bath at high speed. The coating thickness is controlled by air knives:
- **Typical thickness**: 7–42 µm per side (Z100–Z600 designation per EN 10346, where number = total coating mass in g/m²)\n- **Applications**: Automotive body panels, building cladding, white goods, HVAC ductwork\n- **Coating types**: Pure zinc (Z), zinc-aluminum (ZA — Galfan), zinc-aluminum-magnesium (ZM — MagiZinc) with significantly improved cut-edge corrosion resistance
Electrogalvanizing
Zinc is electrodeposited from an acidic or alkaline bath at room temperature. Provides precise thickness control and excellent surface finish:
- **Typical thickness**: 5–25 µm (thinner than hot-dip)\n- **Advantages**: No heat distortion, excellent dimensional control, smooth bright finish suitable for painting, uniform thickness on complex shapes\n- **Limitations**: Thinner coatings mean shorter bare corrosion protection. Primarily for indoor or mildly corrosive environments unless used as paint primer\n- **Applications**: Automotive fasteners, electronic enclosures, precision components
Sherardizing (Diffusion Coating)
Components are tumbled in zinc dust at 380–400°C (below zinc's melting point of 419°C). Zinc diffuses into the steel surface forming zinc-iron intermetallic layers:
- **Typical thickness**: 15–75 µm of uniform zinc-iron alloy\n- **Key advantage**: Extremely uniform coating on threads, internal surfaces, and complex geometries — no build-up or bridging\n- **Applications**: Fasteners (especially where thread tolerance is critical), small castings, chain links, springs\n- **Standard**: EN ISO 17668 (Sherardizing of iron and steel articles)
Mechanical Plating
Zinc powder is mechanically bonded to components by tumbling with glass beads in a chemical solution at room temperature:
- **Typical thickness**: 5–75 µm\n- **Advantage**: No hydrogen embrittlement risk — essential for high-strength fasteners (Grade 10.9 and above, hardness > 320 HV) where electroplating and hot-dip galvanizing may cause hydrogen-induced cracking\n- **Standard**: ASTM B695
Selection Guide
**Heavy structural steel**: Batch hot-dip galvanizing (EN ISO 1461). **Sheet steel products**: Continuous hot-dip (EN 10346). **Precision components, painting base**: Electrogalvanizing. **Threaded fasteners**: Sherardizing or mechanical plating. **High-strength fasteners (≥10.9)**: Mechanical plating or sherardizing to avoid hydrogen embrittlement.