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.

Takeaway: 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?

Edges and wet joints are useful inspection starting points, but the exposed field and manufacturing or coating defects also need review.

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.

Takeaway: 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.

Takeaway: 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.

Takeaway: Every step up this ladder costs money; weigh it deliberately on metal cladding cost rather than discovering it in change orders.

How long will metal cladding last?

No single number describes all metal walls. Warranty term is the period and conditions of contractual coverage. Appearance life is the period before an owner’s agreed colour, gloss or staining threshold is exceeded. Functional service life concerns continued performance of the enclosure and its connections. A coating may need repair while the substrate and wall remain functional; a covered warranty period proves none of those outcomes by itself. Review condition, exposure and the manufacturer’s maintenance documents before estimating remaining life.

Five factors set the service life:

• Substrate metal. Steel needs its coatings to survive. Aluminum relies on its oxide film. Zinc and copper rely on their patinas, and corten on a patina that only stabilizes in wet-dry cycles. Each fails differently, as described above.
• Metallic coating (steel only). Galvanized or aluminum-zinc coating is what keeps coated steel from rusting. On the steel wall cladding guide, aluminum-zinc coatings back the longer perforation warranties, but they protect cut edges less.
• Finish or patina. Paint and powder coatings age by fading, chalking and losing film integrity. Living metals keep changing colour instead.
• Exposure. The American Galvanizers Association lists temperature, humidity, rainfall, sulfur dioxide pollution and air salinity as the drivers of zinc coating life in the atmosphere. It ranks industrial air as the most aggressive, tropical marine as almost as corrosive, and rural as the least aggressive. Its figures describe hot-dip galvanized steel, not coil-coated cladding sheet. Use the ranking of environments, not its years, for a panel.
• Details and maintenance. Cut edges, couples, wet cavities and blocked weeps shorten life whatever the metal. So does skipped washing in salt zones. Metal wall panel installation is where many of these are decided.

Coating warranty versus substrate life

A finish standard or warranty measures the finish, not the wall. AAMA 2605 is the FGIA performance specification for superior performing organic coatings on aluminum extrusions and panels. Coating maker PPG summarizes its weathering test as a 10-year South Florida exposure. To pass, the coating must show colour change of 5 ΔE or less, a chalk rating of 8 or better, and at least 50% gloss retention. That tells you how a qualified finish holds its appearance in a harsh sun test. It does not tell you when a panel perforates, when fasteners lose section, or when a lap starts to leak.

Read warranties the same way. Film integrity, chalk and fade, and substrate perforation are separate promises with separate terms. Each carries its own exclusions, typically exposure, distance to the coast and sheltered areas. Ask for each one in writing for the actual product and site. The finish tiers are covered on metal panel finishes.

Why “the most durable cladding” depends on the site

The same metal can be the best and the worst choice. Corten suits cyclic wet-dry inland air and is the wrong answer near salt. Copper does not depend on a paint film, but it stains what sits below it. Coated steel can serve well inland and fall into warranty exclusion zones at the coast. “Most durable” only means something once the exposure, details and maintenance are fixed.

To assess durability for a project, write down the exposure class and get the written warranty for every layer, with its exclusions. Then check the details that this page lists against the drawings. Last, agree the inspection and washing cadence the warranty assumes.

Takeaway: If two metals are still in play, compare them on the materials hub.

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.

Editorial planning suggestion: start discussion with an annual inspection, or more frequent checks in coastal and de-icing exposure. These are not proven service intervals.

Takeaway: Use the actual manufacturer’s O&M and warranty conditions to set inspection and approved fresh-water cleaning frequency; review that plan after findings or exposure changes.

Is it a maintenance item or a replacement item?

Use this copyable inspection record to support a maintenance or engineering decision. Surface appearance alone cannot establish remaining capacity.

Finding / location	Evidence to record	Next review
Staining or chalking	Panel ID, date, photos, extent, cleaner trial and coating condition	Approved cleaning procedure; recheck for underlying damage
Local edge or underfilm corrosion	Edge / lap ID, close-up and overview photos, spread since last inspection	Manufacturer repair advice; inspect substrate and water route
Perforation or suspected section loss	Part ID, accessible measurements by a qualified person, photos and limits of inspection	Design professional checks capacity and repair or replacement scope
Corroded or loose fixing	Fixing ID, attachment condition, photos and any measured section loss	Prompt professional connection review; do not infer capacity from staining

Record the reviewer, decision, repair method and follow-up date. Widespread damage may justify replacement, but coating delamination alone is not an automatic end-of-life test.

Takeaway: Compare any redesign on the materials hub or aluminum versus steel guide.