{"id":2248,"date":"2026-10-07T11:44:42","date_gmt":"2026-10-07T09:44:42","guid":{"rendered":"https:\/\/unitearquitectura.com\/proyectos-unite\/oxido-junto-al-mar-corrosion-pilares-visita-tecnica\/"},"modified":"2026-10-07T11:44:44","modified_gmt":"2026-10-07T09:44:44","slug":"oxido-junto-al-mar-corrosion-pilares-visita-tecnica","status":"publish","type":"unite_proyecto","link":"https:\/\/unitearquitectura.com\/en\/proyectos-unite\/oxido-junto-al-mar-corrosion-pilares-visita-tecnica\/","title":{"rendered":"Rust by the sea: what we found beneath a seafront building"},"content":{"rendered":"

On the coast, rust makes no noise. It works slowly, in the dark and almost always where nobody is looking: in a basement, in a ventilated crawl space, behind the cladding of a column or under the edge of a terrace. By the time you can finally see it from outside, it has usually been advancing for years.<\/p>\n\n\n\n

In this article we describe a real case we know first-hand: a residential building from the late seventies, a few metres from the sea on the Alicante coast, with more than a hundred steel columns resting on base plates inside a ventilated crawl space. Out of respect for the owners' association we won't name it. What we will tell you is what was found, why the previous repairs didn't last and how to carry out a technical inspection<\/strong> that actually helps you make a decision.<\/p>\n\n\n\n

Why the sea eats steel<\/h2>\n\n\n\n

Steel rusts with water and oxygen. By the sea there is a third ingredient that speeds everything up: the chlorides<\/strong> in salt. The wind carries them inland as an aerosol and they settle on every surface. There they attract moisture, break down the layers that protect the metal and make corrosion advance through pitting, in depth, rather than more or less evenly.<\/p>\n\n\n\n

The standard that classifies how aggressive an environment is (UNE-EN ISO 9223) puts it in numbers. Bare steel in a C4 coastal environment loses 50 to 80 microns of thickness in the first year. In C5, 80 to 200. In a dry inland town, just a few.<\/p>\n\n\n\n

C2 \u00b7 low (inland, dry countryside)<\/text>1.3\u201325<\/text>C3 \u00b7 medium (city, mild coast)<\/text>25\u201350<\/text>C4 \u00b7 high (coast, industry)<\/text>50\u201380<\/text>C5 \u00b7 very high (seafront)<\/text>80\u2013200<\/text>0<\/text>50<\/text>100<\/text>150<\/text>200<\/text>microns of steel lost in the first year (\u00b5m)<\/text><\/svg>
How much thickness unprotected carbon steel loses in its first year, depending on the environment. Source: UNE-EN ISO 9223:2012 (atmospheric corrosivity categories). A seafront building may be in C4 or C5; inside a damp, poorly ventilated crawl space, local conditions can be even worse.<\/figcaption><\/figure>\n\n\n\n

Something similar happens in concrete structures, though more hidden. Concrete protects the reinforcement as long as it stays alkaline and no chlorides reach it. That is why the C\u00f3digo Estructural (Spanish structural code) treats above-ground structures generally located less than 5 km from the coast as a marine environment (class XS1<\/strong>) and requires more cover and denser concrete. A building from the seventies was not designed to those requirements.<\/p>\n\n\n\n

And one detail explains why rust ends up breaking things: it takes up much more space than the steel it comes from.<\/p>\n\n\n\n

Steel<\/text>1<\/text><\/marker><\/defs>water + oxygen<\/text>+ chlorides<\/text>\u00d7 2<\/text>\u00d7 6<\/text>Rust: between 2 and 6 times the volume<\/text>pushes,<\/text>cracks<\/text>and splits<\/text>into layers<\/text><\/svg>
Why rust breaks whatever surrounds it: corrosion products take up considerably more volume than the steel they come from. Inside concrete, that pressure cracks and spalls the cover; in a steel plate, it splits it into layers.<\/figcaption><\/figure>\n\n\n\n

The case: a crawl space right by the beach<\/h2>\n\n\n\n

The building is U-shaped, with three entrances and between twelve and sixteen storeys. Below the ground floor there is a suspended ground floor (forjado sanitario)<\/strong>: a void about 80 cm high (65 in places) that separates the building from the ground. Down there are the base plates of the steel columns, the beams and joists, and also the building's downpipes and drainage pipes.<\/p>\n\n\n\n

It is a perfect place for corrosion: salty air, little ventilation, condensation, total darkness and, as was later confirmed, water from leaks dripping for years. It is also a place nobody goes into except to work, so the deterioration advanced unseen.<\/p>\n\n\n\n

2016<\/text>First defects report on the suspended ground floor<\/text>2017<\/text>First repairs to plates and joists<\/text>2018<\/text>At the review, rust appears again<\/text>2019<\/text>Trials of different systems on specific plates<\/text>2024<\/text>Plate-by-plate inspection: more corrosion points than in 2019<\/text>2024<\/text>Diagnosis: leaking downpipes and condensation in the crawl space<\/text>Afterwards<\/text>Works in the right order: water first, then steel<\/text><\/svg>
Eight years of repairs on the same building. As long as water kept getting in, each new coat of paint bought a few years and little more.<\/figcaption><\/figure>\n\n\n\n

It's a familiar story: you repair what you can see, you paint it, after a while the rust comes back, you try another product\u2026 and so on for years. In the 2024 inspection the same plates were compared with the 2019 photos, and in 2024 there were clearly more corrosion points<\/strong>. The repairs weren't badly done; the problem was that nobody had cut off the cause.<\/p>\n\n\n\n

What a useful technical inspection looks like<\/h2>\n\n\n\n

A technical inspection isn't walking around and saying \u201cthis looks bad\u201d. To be able to decide, and to compare in five years' time, you need to leave a record. In this case there were three visits, one per area, and in each one:<\/p>\n\n\n\n

    \n
  1. The crawl space was lit<\/strong>. With a torch you can see one plate; to work you need proper light. For the later works, temporary lighting of at least 300 lux was specified throughout the crawl space.<\/li>\n\n
  2. Every column was numbered<\/strong> and each plate was photographed separately, with the number in view. That way every photo can be located on the plan and compared with the next inspection.<\/li>\n\n
  3. The dust was cleaned<\/strong> off the plates before assessing them. A layer of dirt hides a pit perfectly.<\/li>\n\n
  4. Each plate was graded<\/strong>, from 1 to 4, using written criteria (we'll look at them shortly).<\/li>\n\n
  5. Everything was transferred to a plan<\/strong> in colours: green, yellow, orange and red. On the same plan, the damp areas and the leaking downpipes were marked.<\/li>\n<\/ol>\n\n\n\n
    \"Pilar
    Every column, numbered and photographed. It seems a minor detail, but it is what allows the condition of each plate to be compared year after year. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n

    The result was a plan that showed, at a glance, that the worst-preserved plates were not randomly spread: they matched the damp areas<\/strong> caused by plumbing and drainage leaks, the areas most exposed to the marine environment, and the poorly ventilated corners with condensation.<\/p>\n\n\n\n

    Reading rust: four grades<\/h2>\n\n\n\n

    Not all rust is equally serious, and not all of it needs the same repair. The scale used is simple and works for almost any metal element:<\/p>\n\n\n\n

    Grade<\/th>What you see<\/th>What to do<\/th><\/tr><\/thead>
    1 \u00b7 Very minor<\/td>Some edges with slight corrosion, very isolated pitting. The plate is fine.<\/td>Monitoring. No short-term action needed.<\/td><\/tr>
    2 \u00b7 Minor<\/td>Isolated pitting, rust on edges, nuts or bolts, not widespread. Paint peeling in places.<\/td>Repair the protection before it spreads. If left, it moves to grade 3.<\/td><\/tr>
    3 \u00b7 Moderate or severe<\/td>Widespread pitting, corrosion at edges and anchors, blistered paint with rust underneath in several areas.<\/td>Thorough cleaning back to bare metal and a full protection system.<\/td><\/tr>
    4 \u00b7 Very serious<\/td>Loss of section in the plate, stiffeners, bolts or sections. Loss of structural function.<\/td>Urgent: prop, replace the lost material and protect.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    \"Placa
    Grade 1: plate repaired and in good condition. This is the benchmark for how everything should look. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n
    \"Placa
    Grades 2 and 3: rust coming out at the bolts and edges, exactly where the paint is thinnest. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n
    \"Base
    Grade 4: the plate has split into layers. The steel has turned into sheets of rust, and what is left no longer has its original thickness. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n
    \"Perno
    Grade 4: an anchor bolt almost completely eaten away. This is what ties the column to the foundation. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n

    Grades 1 to 3 are a maintenance problem. Grade 4 is already a structural problem: there were beams whose section was flaking away in layers, and a column support where the sections that spread the load had lost material.<\/p>\n\n\n\n

    \"Ala
    Layered rust on a beam flange, trapped under the brickwork. Where steel is in contact with damp material and can't be painted, corrosion carries on by itself. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n

    The lesson: if you don't stop the water, you'll be repainting every few years<\/h2>\n\n\n\n

    This is the most important conclusion from the case, and it applies to almost any building: paint protects against the environment, but not against water dripping onto it<\/strong>. A downpipe leaking onto a plate for years will corrode it, however good the product.<\/p>\n\n\n\n

    \"Mach\u00f3n
    Salts and damp at the base of a pier, in one of the areas affected by leaks. Water carries salts and leaves them behind as it evaporates. Photo: Unit\u00e9 Arquitectura.<\/figcaption><\/figure>\n\n\n\n

    Two ways of preparing the steel in the 2019 repairs were also compared, with the same paint system: mechanical sanding and sandblasting. Both showed rust again, but the sandblasted plates held up better. It makes sense: paint grips clean, roughened steel, not the rust left in the pores.<\/p>\n\n\n\n

    \n

    Repairing without a diagnosis means paying twice. In this building, the right order was water first and steel second, and for years it was done the other way round.<\/p>\n<\/blockquote>\n\n\n\n

    How to repair it properly<\/h2>\n\n\n\n

    Once the diagnosis was done, the repair project was fairly clear. In short:<\/p>\n\n\n\n

    First, stop the water<\/h3>\n\n\n\n

    All the downpipe and pipe leaks marked on the plan were located and repaired, replacing the damaged lengths. Without this, everything else is temporary.<\/p>\n\n\n\n

    Then, measure<\/h3>\n\n\n\n

    Before deciding anything, measure the remaining thickness of each element: plate, stiffeners, bolts, nuts and sections. The criterion was simple: if more than 20% of the section has been lost<\/strong>, the element is replaced; if not, it is cleaned and protected.<\/p>\n\n\n\n

    Grades 1 to 3: clean back to bare metal and protect<\/h3>\n\n\n\n