Steelwork Corrosion Protection: Paint vs Hot-Dip Galvanising

Steel that is exposed to moisture corrodes. The protection required depends on how much moisture the steel will encounter in service — and that varies significantly between a beam inside a heated room and a shelf plate buried in a cavity wall.

Internal Steelwork in Dry Conditions

For steelwork inside a heated building with a clean, dry atmosphere — the majority of domestic wall removal beams — primer only is sufficient. The steel is not exposed to moisture, the cavity is sealed, and a standard mill primer provides adequate protection throughout the design life of the building.

There is no need for galvanising internal beams in a dry domestic environment.

Steelwork Subject to Moisture

When steel is in a cavity wall, forms part of an external shelf plate, or is otherwise exposed to variable moisture conditions, primer alone is not adequate. The PorthouseDean specification requires:

Hot-dip galvanising to the relevant European Standards (typically 85 µm average coating thickness) — or a paint-applied system approved by Building Control.

Hot-Dip Galvanising: Pros and Cons

Advantages:
– Long design life (50+ years in a sheltered cavity environment)
– No re-coating or maintenance required
– The zinc coating bonds metallurgically to the steel, not just as a surface film

Disadvantages:
– Off-site process — steel must be transported to a galvanising plant and back
– Adds lead time to the fabrication programme
– More expensive than site-applied paint
– Complex assemblies may need ventilation holes to allow complete immersion

Paint-Applied Systems

A site-applied paint system can achieve acceptable corrosion protection for cavity and semi-exposed applications if the right products are specified. The system should be selected based on the NPL (National Physical Laboratory) recommendations for the specific exposure environment, and must be approved by Building Control.

Paint coatings require more maintenance over time than galvanising and are more susceptible to damage during installation.

The Bottom Line

For steelwork in a heated room: primer. For shelf plates, cavity lintels and anything exposed to moisture: galvanise or use an approved paint system. Check the specification on your PorthouseDean drawings and inform your fabricator accordingly before ordering.

Contact PorthouseDean if you are unsure what protection is required for specific elements on your project.