Salt-air corrosion protection

The mechanism, because it explains every decision

Concrete protects steel chemically: the alkaline environment forms a passive layer on the bar. Chloride from salt-laden air penetrates through the concrete over time and destroys that passivity. Once the steel starts corroding, the products of corrosion occupy substantially more volume than the original steel, and that expansion cracks and then detaches the cover from the inside.

Two things determine how long that takes: how far the chloride has to travel (cover depth) and how easily it travels (concrete permeability). Both are specification decisions made before the pour, and neither can be corrected afterwards.

That is why the answer to "how do we stop this" is almost never a coating applied to a finished building. It is cover and mix design, decided at the start.

Specification on a new building

• Cover appropriate to a marine exposure class, not a mainland default — and checked and recorded before every pour, because cover is only real if the spacers are right and nobody has stood on the reinforcement.
• A low-permeability mix. Water-cement ratio and cement type matter more than headline strength. A high-strength permeable concrete is worse in this environment than a moderate-strength dense one.
• Proper curing. A poorly cured surface is a permeable surface exactly where the chloride arrives first. In this climate that means protection from sun and wind, which is a site discipline issue rather than a specification one.
• A316 stainless at every fixing within reach of spray — balustrade bases, brackets, anchors, handrail fixings, hidden connections. A304 pits in marine exposure.
• Detailing that sheds water. Drips, throats and falls that keep water moving off surfaces rather than sitting on them or running down the face.
• Protective coatings to the most exposed elevations, as an additional measure rather than a substitute for the four above.

Repairing what has already gone

The wrong repair is extremely common: chip off the loose material, fill with mortar, paint over. Within a year or two the patch cracks and the corrosion continues behind it, frequently worse than before, because a patch can set up a difference in the concrete that accelerates corrosion at its edges.

1. Survey the extent. Sounding, and where warranted testing for chloride penetration and cover depth. Repairing only the visibly spalled areas usually misses material that is already contaminated.
2. Break out well beyond the visible damage, and fully behind the bar — not just to its face. Corrosion continues on the hidden side otherwise.
3. Clean the reinforcement back to bright steel and treat it. Replace or supplement bars where section loss is significant, which is an engineer's call.
4. Reinstate with a repair mortar compatible with the parent concrete in strength and thermal behaviour. A much stronger patch is not a better patch.
5. Reinstate cover to what it should have been, which sometimes means building the profile out.
6. Apply a protective treatment to the wider elevation, because the areas that have not spalled yet are the same age and the same specification.

Everything else salt attacks

Concrete gets the attention; the failures owners actually notice are usually smaller.

• Window and door hardware — hinges, rollers, locks — seizing. See windows and doors.
• Balustrade base fixings, which are a safety item on a caldera terrace. See glass balustrades.
• Air conditioning condenser coils losing efficiency and then failing in August. See air conditioning.
• Brackets for services — solar, satellite, lighting, cameras — which are usually the cheapest metal on the building and mounted where nobody inspects them.
• Rust staining down whitewashed walls, which is almost always a fixing rather than the component it holds. See metalwork.