Coastal Salt Corrosion

Coastal salt corrosion is the faster deterioration of metal roof components near the ocean, where wind carries salt spray onshore. Salt deposits attract moisture and create a conductive film on metal surfaces, which speeds electrochemical corrosion. Corrosion is usually most severe close to the shoreline and on surfaces that are not rinsed by rain, such as the undersides of eaves and lapped joints.

Why salt matters

Chloride ions from sea salt can break down the thin protective (passive) oxide films that normally protect aluminium and stainless steel, leading to pitting and crevice corrosion in confined spaces such as under washers, laps and sealants. On galvanized or metallic-coated steel, the zinc or zinc-aluminium coating corrodes sacrificially to protect the steel, but the coating is consumed faster in marine air.

Galvanic corrosion

When two different metals touch in the presence of an electrolyte such as salt water, the less noble metal corrodes faster. Corrosion guidance suggests that in harsh outdoor or salt-exposed conditions the difference in anodic index between metals in contact should be kept to about 0.15 V or less. Roofing examples include:

• copper flashings or runoff onto galvanized steel or aluminium;
• carbon-steel fasteners in aluminium or stainless sheet;
• aluminium in contact with treated lumber or wet concrete.

The Statue of Liberty is a well-known example: its copper skin corroded the iron support frame after the insulation between them failed.

Material choices near the coast

• Aluminium forms a self-healing oxide layer and is widely used in coastal roofing, with compatible fasteners.
• Copper develops a stable patina but will corrode less noble metals it drains onto.
• Stainless steel fasteners resist general corrosion, though chlorides can still cause pitting in some grades.
• Metallic-coated steel (galvanized or zinc-aluminium) loses its sacrificial coating faster in marine air, so coating weight and paint systems matter; product warranties may restrict coastal use.
• Clay, concrete tile and slate are not affected by salt corrosion, though their fasteners and flashings are.

Maintenance

Rinsing sheltered surfaces with fresh water, inspecting fasteners and cut edges, touching up damaged coatings, and avoiding dissimilar-metal contact all extend service life in coastal environments. Hurricane connectors and other structural steel near the coast also benefit from heavier galvanizing or stainless steel.