What Causes Railings to Corrode, and How Do You Prevent It?

Aluminum railings do not rust. Rust is specifically iron oxide, and there is no iron in an aluminum extrusion. What aluminum does is corrode, through several distinct mechanisms that look similar from a distance and require completely different responses. The five that account for nearly every failure we see are chloride attack from salt, galvanic corrosion where dissimilar metals touch, crevice and poultice corrosion under trapped debris, chemical attack from concrete and cleaning products, and coating breakdown that exposes the metal in the first place. Steel and galvanized railings on older buildings genuinely do rust, and that adds a sixth failure mode with its own consequences. Diagnosing which one you have is the whole job, because cleaning fixes one of them and none of the others.

Chlorides: salt air and de-icing salt

Chloride ions break down the thin, self-healing oxide layer that protects aluminum, and they do it locally rather than uniformly. The result is pitting: small deep holes rather than general thinning. Coastal buildings get chloride from marine aerosol, which travels well inland on prevailing winds and settles on any surface that is not rinsed by rain. Inland buildings get it from de-icing salt, tracked onto balconies on boots, sprayed up from roads, and applied directly to parking structure decks all winter. Parkade guards frequently corrode faster than oceanfront guards for exactly this reason.

The tells are white powdery deposits, small blisters under the coating, and pinhole pitting concentrated on horizontal surfaces and at the base of posts where salt-laden water pools. The defence is a combination of alloy and finish selection, detailing that drains rather than collects, and a rinse schedule. On our Halifax work, including projects like SoQu and The Marlstone, marine exposure drives the specification from the beginning rather than being handled as an afterthought. Sheltered elevations that never see rain need the most attention, because rain is the free maintenance that keeps exposed faces clean.

Galvanic corrosion: dissimilar metals in contact

Put two different metals in electrical contact with an electrolyte between them, and the less noble one corrodes preferentially. Aluminum is less noble than stainless steel, carbon steel, copper and brass, which means aluminum is the sacrificial partner in almost every common pairing. The electrolyte is just water, and salt water is dramatically more effective at it. The classic failures are stainless fasteners bearing directly on an aluminum base plate, a steel shim left under a post, embedded steel plate in contact with an aluminum bracket, and copper flashing draining onto an aluminum rail. It concentrates at the contact point and eats the aluminum from the inside of the joint outward, which is why it is usually discovered as a loose post rather than a stain.

Prevention is entirely a detailing exercise: isolation washers and nylon bushings, non-conductive gaskets between plates, correct fastener selection, and never allowing a dissimilar metal upstream in the drainage path. It is cheap during fabrication and expensive to correct afterward. This mechanism deserves its own detailed treatment, but the short rule is simple: assume any aluminum touching any other metal outdoors needs a deliberate isolation detail.

Aluminum tolerates weather remarkably well. What it does not tolerate is being kept permanently wet, and almost every corrosion failure we investigate starts with trapped water.

Crevice and poultice corrosion: the balcony killer

This is the mechanism that ruins more condo guards than salt and galvanic action combined, and it is entirely preventable. In a tight gap where oxygen cannot circulate, or under a layer of trapped organic debris acting as a poultice, the local chemistry turns aggressive and the aluminum corrodes far faster than the surrounding exposed surface. On balconies the culprits are outdoor mats and interlocking deck tiles laid over the post base, planters sitting against a rail, leaves and dust packed into a base shoe, and any base plate installed flat on a surface that does not drain. Residents create these conditions after occupancy with the best intentions.

The fix is partly design and partly housekeeping. Design side: drainage at every base plate, weep paths that actually work, and avoiding details that create permanently damp gaps. Housekeeping side: lift the mats twice a year, clear the base shoes, and move the planters. A five-minute annual task at each balcony prevents a repair that costs hundreds of dollars per post.

Chemical attack from concrete, mortar and cleaners

Aluminum is amphoteric, meaning it is attacked by strong alkalis as well as strong acids. Fresh concrete, mortar, grout and slurry from slab-edge repairs are all strongly alkaline, and a splash left to dry on a railing will etch through the finish and pit the metal underneath. So will masonry cleaners, some acidic brick-wash products, and aggressive alkaline degreasers used by well-meaning maintenance staff. This one is almost always construction damage or maintenance damage rather than weathering, and it shows up as sharp-edged patches or drip runs rather than diffuse deterioration.

The prevention is procedural: protect installed guards during any concrete, masonry or restoration work, rinse any splash immediately with clean water, and restrict cleaning products to pH-neutral detergents. Never let a pressure washer with a chemical injector near a coated railing without checking what is in the tank.

Coating breakdown: the door that lets everything else in

A powder coat is a barrier. It protects the aluminum only as long as it is continuous. Once it is breached at a scratch, a drilled hole or a cut edge, moisture gets underneath and corrosion creeps outward from the breach in fine threads, which is why it is called filiform corrosion. UV exposure also degrades the coating itself over time, producing fading and chalking long before the metal is at risk. Anodizing behaves differently because it is a conversion of the surface rather than a layer on top of it, so it does not peel, but it can be worn through and it can be attacked by the same alkaline chemicals. Neither finish is maintenance-free; both last far longer with an annual rinse.

Specification is where this is won. Coastal and high-exposure projects justify a higher-performance coating class and, in some cases, a pretreatment upgrade before powder. Cut edges and field-drilled holes need touch-up, every time, without exception. And on any project where the guard is being installed before the building envelope is finished, protection during construction is not optional.

What about steel railings on older buildings?

Steel and galvanized guards genuinely rust, and on older buildings the consequence extends past the railing. Iron oxide occupies substantially more volume than the steel it came from, so a corroding embedded steel plate or a rusting post set into concrete expands and cracks the concrete around it. The visible symptom is a rust stain and a spalled slab nose; the actual problem is that the anchorage is losing its host. This is a common reason a guard replacement on a 1970s or 1980s building turns into a concrete repair project, and it is worth investigating before the budget is set rather than after the first post comes out.