Hot-Dip Galvanizing for Steel Structures

Technical introduction

Hot-dip galvanizing protects steel by immersing the fabricated member in molten zinc. The zinc reacts with the steel to form a series of zinc-iron alloy layers topped with pure zinc, bonded metallurgically to the steel.

How it protects

• Barrier protection: the zinc coating separates steel from the environment.
• Sacrificial (cathodic) protection: zinc corrodes preferentially, protecting small areas of exposed steel at scratches or cut edges.
• Complete coverage: immersion coats inside hollow sections, edges and corners that are hard to paint.

Design for galvanizing

• Hollow sections need vent and drain holes so molten zinc and gases can flow.
• Member size is limited by the galvanizing bath; long members may need splices.
• Avoid overlapping surfaces that trap pickling solutions, or seal-weld them.
• Thin and thick parts in one assembly can distort; detail accordingly.
• Bolts and nuts should be galvanized to a matching standard.

Coating thickness

Minimum coating thickness depends on the steel thickness. For example, ISO 1461 specifies a minimum mean coating thickness of 85 µm for steel thicker than 6 mm. Actual thickness depends on steel chemistry and thickness and is measured after galvanizing.

Duplex systems

Painting over galvanizing (a duplex system) is used for aggressive exposures, where colour is required, or to extend life. Surface preparation of the galvanized surface is critical for paint adhesion.