Galvanizing Structural Steel: When and Why

Hot-dip galvanizing extends steel life by decades in corrosive environments. For structural applications, the question is rarely "does it work" but "is the premium justified for this project."

What galvanizing does

Steel is dipped in molten zinc at ~450°C. The zinc bonds metallurgically with the steel and forms multiple alloy layers plus an outer pure zinc coating. The zinc corrodes preferentially (sacrificially) to protect the steel underneath.

Expected life

• Internal dry environments: 100+ years
• External urban/suburban: 60–80 years
• Coastal/industrial: 20–40 years

Cost premium

• Hot-dip galvanizing: R3,000–R5,600 per tonne of steel
• Adds roughly 10–18% to fabricated steel cost
• Compares to painted alternative: paint costs R800–R1,800 per m² over 20-year life cycle

When to specify

• External structural steelwork (canopies, balconies, walkways)
• Steel in damp environments (swimming pools, food processing)
• Coastal locations
• Where future maintenance access is difficult
• Roof trusses in unheated buildings

When it isn't worth it

• Internal heated buildings (paint or no coating sufficient)
• Steel that will be encased in concrete or behind cladding
• Steel that will be intumescent-painted (galvanizing complicates adhesion)

Design implications

• Vent and drain holes required for hollow sections (otherwise explosion risk during dipping)
• Tank size limits — largest South African tanks ~17m × 2m × 3m
• Steel grade and chemistry affect coating thickness
• Repair patches needed where galvanized surface is damaged on site

Alternatives

• Two-pack epoxy paint systems (zinc primer + epoxy + topcoat)
• Thermal sprayed zinc (for very large or repair work)
• Weathering steel (Cor-Ten (corrosion-resistant) steel) for specific architectural use
• Stainless steel for highest corrosion resistance

MCFAR specifies appropriate corrosion protection on all steel projects.

Frequently Asked Questions