Metal Cladding Corrosion: Causes and Prevention

Metal cladding corrosion is a specification discipline, not a lab report. It belongs with the other design and specification guides on the metal wall coverings map. An engineer still owns unusual chemical or marine exposures.

How does each metal family corrode?

Coated steel wall cladding fails at scratches, cuts and wet laps. Aluminum wall panels fail at galvanic couples and some coastal pits. Stainless steel wall panels tea-stain when chlorides sit on a rough finish. Zinc wall cladding fails on wet reverse faces. Copper wall cladding and corten steel cladding stain what they drip on.

Three mechanisms explain most of that list.

Galvanic couples. Put two different metals in electrical contact, keep the joint wet, and the less noble metal corrodes to protect the more noble one. The classic facade couples: copper runoff onto zinc or aluminum, carbon-steel screws through aluminum trays, aluminum rivets in stainless flashings. Area ratio decides severity — a small anode feeding a large cathode fails fast, which is why the wrong fastener dies quickly and the wrong panel dies slowly. Dry contact is largely harmless; the electrolyte is the third partner, so the wet zone is where the rules apply.

Cut-edge corrosion. Coil-coated steel is protected by a metallic layer (galvanized or aluminum-zinc) under the paint. Every shear cut exposes bare steel at the edge; the zinc protects it sacrificially, but only across a narrow strip. Heavy gauges, coastal salt and standing water at bottom edges overwhelm that protection, and the familiar rust-fringe creeps up from laps and drips. Factory-cut, hemmed or treated edges resist; ragged field cuts left raw do not.

Filiform corrosion. Thread-like corrosion filaments creep under a coating from an edge, a scratch or a drilled hole, lifting the paint as they go. Aluminum and coated steel both show it, especially in humid coastal air. Pretreatment quality at the coil line and edge discipline on site are the controls; once filaments run, touch-up paint over the top does nothing.

On top of these, chlorides pit aluminum and stain the wrong stainless grade — the SSINA corrosion guidance is a good primer on pitting, crevice attack and grade selection — and runoff carries metal salts downslope: copper and weathering steel stain concrete, glass and lighter metals below them.

Which details cause the failures?

The wall’s flat field almost never starts the problem. The details do.

Trapped water. Sealant beads that dam a drainage path, horizontal girts that shelve water, unventilated cavities that keep the reverse face of zinc damp for months, laps that pump water in by capillarity and never dry. Metal rainscreen cladding cavities must drain and vent; insulated metal panels need treated openings so the foam joint never becomes a reservoir.

Dissimilar-metal fasteners. The cheapest screw is a corrosion specification. Exposed-fastener corrugated metal wall panels live or die on washer and edge quality, and a bright zinc-plated screw in an aluminum sheet at the coast is a countdown. Perforated metal panels multiply the cut-edge count by thousands, which is why their edge treatment and alloy choice matter more than their pattern.

Missing isolation. Aluminum on bare steel girts, stainless flashings lapped onto galvanized copings, treated lumber blocking against aluminum or zinc — all are couples waiting for the first wet week. Metal wall panel installation is where isolation tapes and washers are either installed or quietly skipped.

Interiors still corrode in pools, kitchens and cleaning closets, driven by chlorinated air and aggressive cleaners rather than rain. See interior metal wall panels and metal panel finishes.

What should the spec actually say?

Corrosion protection that lives only in the drawings gets value-engineered out. Put it in words:

• Exposure class. Name the environment — distance to salt water, de-icing spray, industrial fallout — so every downstream choice has a reason.
• Fastener metallurgy. List the allowed fastener and rivet metals per panel metal, and ban substitution without written approval. Stainless into aluminum, matched coated fasteners into coated steel, and never plain carbon steel in the wet zone.
• Isolation. Require butyl or EPDM isolation tape, nylon washers or coated sleeves wherever dissimilar metals meet and can stay wet — panel-to-girt, flashing laps, dissimilar copings.
• Edge treatment. Limit field cutting on coated steel, require cold-cut methods, and state whether exposed cut edges must be sealed, hemmed or capped. On perforated and heavy-gauge material, ask the manufacturer for their tested edge-protection detail.
• Runoff management. Draw drip edges that throw copper and weathering-steel runoff clear of glass, light masonry and lighter metals below.
• Coastal upgrade logic. State the trigger — typically proximity to breaking salt water — and what upgrades with it: coating class, aluminum alloy, stainless grade, or a different metal entirely. Exact grades and distances belong to the manufacturer’s tested data, not to a copied spec paragraph.

The International Zinc Association and the Whole Building Design Guide metal panel wall systems notes are context, not a fastener schedule — the binding numbers come from the system manufacturer.

How do coastal and de-icing chlorides change the shortlist?

Chlorides are the great accelerant. Near breaking surf, salt aerosol lands on everything; the surfaces rain rinses survive far better than the sheltered ones — soffits, screened balconies, the underside of copings — where salt accumulates and never washes off. That inversion surprises people: the protected-looking face is often the first to fail.

The upgrade logic runs in steps. First the coating class on coated steel moves up, or the substrate shifts from galvanized toward aluminum-zinc. Then aluminum with a marine-appropriate alloy and premium finish becomes the default — a common reason residential metal cladding near the shore lands on aluminum. Stainless remains viable if the grade and finish are chosen for chlorides and the surface stays smooth and washable. Weathering steel is the wrong answer near salt: the patina never stabilizes, and section loss continues. De-icing salt at grade creates a miniature coastal zone on the bottom few feet of any urban wall — treat panel bases, kick plates and parking-garage edges as chloride exposure regardless of geography. Every step up this ladder costs money; weigh it deliberately on metal cladding cost rather than discovering it in change orders.

Where should you look during inspections?

Corrosion inspection is boring and cheap compared to recladding. Look where it starts:

• Fastener lines — red staining, crusted washers, backed-out screws.
• Cut edges and laps — rust fringe at bottom edges, paint lifting in threads (filiform), white corrosion product on aluminum and zinc.
• Weeps and cavity openings — blocked weeps mean a wet cavity; check after every re-caulking campaign, because sealant crews block weeps routinely.
• Runoff paths — green or brown staining below copper and weathering steel, and whatever that runoff lands on.
• Sealant joints — failed joints feed water into laps that were never meant to stay wet.
• Sheltered faces — soffits and screened areas in coastal zones, which accumulate salt.

Annual walks are enough for benign exposures; coastal and de-icing zones justify twice yearly plus a fresh-water washdown of sheltered surfaces. Write the cadence into the O&M manual or it will not happen.

Is it a maintenance item or a replacement item?

Most of what owners call corrosion is staining, and staining is maintenance: wash it, fix the runoff detail that caused it, move on. Use this ladder.

Maintenance: surface staining, tea-stained stainless (clean and passivate), chalking coatings, isolated fastener rust (replace the fasteners with the correct metallurgy), small scratches touched up per the finish manufacturer’s rules.

Repair: localized cut-edge creep at a few laps (re-treat edges, replace individual sheets), failed sealant feeding a wet lap, one bad flashing couple (rebuild with isolation).

Replacement conversation: perforation anywhere, fasteners losing section, coating delaminating with corrosion running underneath it across whole elevations, or a zinc or coated-steel wall whose reverse face has been wet for years. At that point the question is no longer how to fix the metal but whether the detail set that failed it should be rebuilt — which sends you back to the materials hub with the exposure written down honestly. If the choice is only aluminum versus steel, use the comparison page.