Aluminium and stainless steel resist corrosion by the same trick. Neither metal is inert; both react instantly with oxygen to form a thin passive film that seals the surface, and both spend their service life depending on that film staying intact. Compare them on general corrosion and they look almost identical, which is why the comparison is usually made badly.
The difference that matters is in how the film fails, where it fails, and whether anyone notices. On that basis the two metals behave very differently, and the right choice depends on the environment and on who will be inspecting the structure.
What the passive film is doing
On stainless steel the protective layer is chromium oxide, which is why the chromium content rather than the iron defines the family. On aluminium it is aluminium oxide, which forms faster and thicker. Both reform automatically when scratched, provided oxygen is available at the damaged point, and that proviso is the origin of almost every failure mode described below.
Neither metal wastes away uniformly in the way unprotected carbon steel does. When either fails it does so at isolated points, which means a structure can look perfect and be losing section somewhere specific.
Where each one is vulnerable
Chlorides attack both. Salt water and de-icing salt break down the passive film locally and initiate pitting, and both metals suffer crevice corrosion wherever two surfaces sit close together with stagnant liquid between them and no oxygen to repair the film.
Beyond that the vulnerabilities diverge. Aluminium is sensitive to galvanic coupling, so contact with a more noble metal in the presence of an electrolyte drives attack quickly, and it is also attacked by strong alkalis, which is why contact with fresh concrete is a detailing issue. Stainless steel is far less bothered by galvanic contact, being usually the noble partner, but is susceptible to chloride stress corrosion cracking where tensile stress, chlorides and warmth combine, and to sensitisation in the heat-affected zone of a weld if the grade and procedure are wrong.
The failure you can see and the one you cannot
This is the practical difference for an asset owner. Aluminium corrosion announces itself: pitting is visible, and the corrosion product is a white powder that shows up on inspection long before section loss matters. The mechanism is slow and the warning is early.
Stainless steel failures are quieter. Crevice attack under a fitting, or a stress corrosion crack, develops out of sight and progresses without surface staining, and the first symptom can be a component parting. Structural failures of stainless fixings in swimming pool atmospheres are the classic documented case, and they are documented precisely because nobody saw them coming.
Neither metal is unsafe. The point is that an inspection regime that works for one is not automatically adequate for the other, and a specification that assumes stainless needs no attention is making an assumption the metal does not support.
Grade matters more than family
Comparing aluminium with stainless steel in the abstract is close to meaningless, because the spread within each family is wider than the gap between them. Among stainless grades, 304 is not suitable where 316 is required, and neither is a duplex grade. Among aluminium alloys, the magnesium-bearing 5xxx series is the marine family while the 6xxx series is chosen for extruded structural sections.
A specification that says stainless steel or aluminium and stops has specified almost nothing. Naming the grade, and for aluminium the temper alongside it, is what turns the requirement into something a fabricator can price and a client can check.
Weight, and what it buys
Density is where the two part company completely. Aluminium sits at roughly 2 700 kg per cubic metre against about 7 900 for stainless steel, so a comparable section weighs around a third as much. On a floating structure that difference converts directly into buoyancy that does not have to be bought, and on any structure it converts into smaller foundations and lighter lifting equipment.
Stiffness runs the other way. Aluminium’s modulus is about a third of steel’s, so sections are deeper for the same deflection, and on lightweight structures serviceability rather than strength often sets the size.
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Stainless steel earns its place in fasteners, fittings, handrail components, anything with small sections carrying concentrated load, and anywhere hygiene or a hard polished surface is the requirement. It is also the right answer where the component will be abraded, since a passive film on a hard substrate survives that better.
Aluminium earns its place in the structure itself: decks, frames, floats, walkways and spans, where weight is carried permanently and where sections are large enough that the stiffness difference is easily absorbed. That is the usual division on alumiiniset kelluvat rakenteet, and it creates the one thing both metals dislike, which is direct contact with each other in salt water.
The combination is the real subject
Most structures use both, and the interface is where the engineering happens. A stainless fastener through an aluminium plate is a small noble area against a large active one, which is the favourable direction and generally workable with isolation. Reverse the areas and the attack concentrates.
Getting that right is a detailing exercise rather than a material choice, and it is covered properly in the context of what actually ends an aluminium structure’s life in salt water, where contact with other metals sits near the top of the list.
Before the material is fixed
The useful question is not which metal resists corrosion better. It is which failure mode the structure can tolerate, which grades are being compared, and who is going to inspect what. Answer those and the material choice usually makes itself.
Our engineers will review a draft specification and say which components suit which family, and where an interface between them needs an isolation detail rather than a preference.