Hot-Dip Galvanizing Explained: Process, Thickness and Life
How hot-dip galvanizing protects steel: process steps, ISO 1461 and ASTM A123 coating thickness, design for galvanizing, vent holes, life expectancy and duplex systems.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: Hot-dip galvanizing immerses cleaned, fabricated steel in molten zinc at about 450 °C, forming metallurgically bonded zinc-iron alloy layers and an outer zinc layer. It protects steel by barrier and sacrificial action. Standards such as ISO 1461 and ASTM A123 set minimum coating thickness, which increases with steel thickness.
Hot-dip galvanizing is the most common way to protect structural and fabricated steel outdoors. It's widely used across Saudi Arabia for structures, solar mounts, towers, handrails and grating.
The process
- Degreasing — removes oil and grease
- Pickling — acid removes rust and mill scale
- Fluxing — prepares the surface for zinc wetting
- Dipping — immersion in molten zinc (~450 °C)
- Cooling and inspection — thickness measured, defects repaired
How it protects
- Barrier: zinc seals the steel from the environment
- Sacrificial (cathodic): zinc corrodes preferentially, protecting small exposed areas
- Alloy layers: hard zinc-iron layers resist abrasion
Coating thickness (ISO 1461, minimum average)
| Steel thickness | Min. mean coating |
|---|---|
| > 6 mm | 85 µm |
| > 3 to ≤ 6 mm | 70 µm |
| ≥ 1.5 to ≤ 3 mm | 55 µm |
| < 1.5 mm | 45 µm |
ASTM A123 has comparable requirements by grade and thickness category.
Life expectancy
Life depends on coating thickness and environment. Zinc corrodes faster in coastal and industrial atmospheres than in dry inland air. ISO 9223/9224 corrosivity categories help estimate it — see corrosion protection in Saudi Arabia.
Designing for galvanizing
- Vent and drain holes in hollow sections and enclosed spaces — essential for safety and coating quality
- Avoid sealed overlapping surfaces
- Consider bath size limits for large members
- Use galvanizing-compatible steel chemistry (silicon and phosphorus affect coating thickness and appearance)
- Minimise distortion with symmetrical designs
Repair
Damaged areas and site welds are repaired with zinc-rich paint, zinc metal spraying or zinc solder, per the standard.
Duplex systems
Paint over galvanizing gives a combined life longer than either system alone — useful for coastal sites and colour requirements (paint systems for steel).
Sheet vs batch galvanizing
Continuous sheet galvanizing (e.g. Z275) applies much thinner coatings than batch hot-dip galvanizing of fabricated parts. See galvanized coatings explained.
Our fabrication services can include hot-dip galvanizing.