Why security door frames corrode near the sea, and what to do

The short answer: most frame corrosion starts where the coating is broken

On a powder coated aluminium frame, corrosion usually begins at a breach in the finish, and how far it has spread decides whether it's a wash, a repair or a new door. The Australian Aluminium Council says a thin layer of oxide forms on aluminium in air and protects the metal beneath. So bare aluminium isn't helpless. The trouble is that a coating has edges, and salt and water collect there.

BRANZ, New Zealand's building research body, says scratches or gouges in a coating expose the base metal. ScreenGuard, which makes the SCREENGUARD system, says abrasive pads scratch through the coating and corrosion starts at the scratch. Dulux Powder Coatings adds that a powder coat that isn't cared for can show scratches and film integrity loss.

A light chalky film is usually the coating ageing. Bubbles, thread-like lines or a crusty build-up at a chip or a screw point to corrosion of the metal underneath, and those need more than a bucket.

What frame corrosion looks like, sign by sign

The look of the mark tells you most of what's happening, so check where it sits before you reach for a cleaner. A film spread evenly across a whole face of the frame is a different problem from a spot that grows out of one chip or one screw.

Chalking first. BRANZ says powder coating loses gloss over its first few years because a very thin chalk film develops, more noticeably on darker colours and most quickly on surfaces in direct sun. That's the coating weathering, not the frame failing.

Bubbles and wandering lines under the coat are different. BRANZ describes filiform corrosion, also called underfilm or wormtrack corrosion, and says wet painted aluminium may suffer it, particularly at coating defects. It says salt particles on the metal and humidity both raise the risk. BRANZ calls it a cosmetic form of corrosion that doesn't significantly weaken metal components, though it spoils the look and won't wash off.

The table below matches each sign to its likely cause and a sensible first step.

Why coastal frames corrode: salt that stays wet and never gets rinsed

Salt does the damage on a coastal frame because it holds moisture against the metal. The Australian Stainless Steel Development Association (ASSDA) says sea salt deposits start to dissolve once relative humidity passes 34 percent. Above that, a dusting of salt on a frame becomes a salty film.

Where the frame sits matters as much as how close it is to the beach. BRANZ lists areas not washed by rain, such as under eaves, among the things that affect powder coated aluminium, along with crevices where dirt and salt build up and aren't easily washed away. ScreenGuard makes the same point about doors: one under a deep verandah never gets rinsed by rain, so deposits just accumulate.

That's why the sheltered door on the back patio can look worse than the one facing the weather. It's also why ScreenGuard says to wash after a storm has driven sea spray inland, whenever you last cleaned.

Screws and mixed metals: where corrosion gathers

Corrosion ringing a screw or a rivet is often galvanic corrosion, which happens where two different metals touch in wet conditions. The Australian Aluminium Council describes it as corrosion that takes place when different metals are coupled together in the presence of an electrolyte. ASSDA says salt makes water more conductive, so galvanic effects are normally more severe near the coast.

Which two metals shouldn't be used together? Two pairings the industry bodies warn about are these. The Australian Aluminium Council says aluminium and copper are a long way apart in the galvanic series, and using them together in a marine environment isn't good practice. ASSDA says aluminium fasteners in stainless steel aren't acceptable, because the aluminium corrodes quickly.

The reverse is common. ASSDA notes a stainless screw in aluminium is frequently used, although corrosion of the aluminium right around the screw is quite possible. Plastic strips, washers or sleeves that keep the metals apart stop galvanic corrosion, ASSDA says. Paint isn't a reliable insulator, especially under screw heads.

Clean, repair or replace: what you can do and where to stop

A wash is always the first step, and for chalking or fresh salt it may be the only one. Dulux Powder Coatings recommends removing loose deposits with a wet sponge, then a soft non-abrasive brush or cloth and a mild household detergent, and a rinse with clean water. BRANZ says coated aluminium should never be cleaned with an abrasive or cutting compound or a solvent-based cleaner.

Once the metal itself is corroding, BRANZ is frank that remedial treatment is difficult. Its method for removing bulky corrosion uses wetted stainless steel wool or, as a last resort, a chemical etch cleaner, and it warns both further damage the finish. That's refinishing work, so on a screen still under warranty, ScreenGuard says to check with the licensed supplier who installed it before reaching for anything harsher.

A full recoat isn't a home job either. Dulux says powder coating is a factory applied finish and can't be applied on site, and that coating over corroding metal only masks it.

1. Rinse the frame with fresh water to lift loose salt and grit.
2. Wash with a soft brush or cloth and a mild detergent, then rinse with clean water.
3. Dry it and look again: note whether marks sit across a whole face or grow from a chip, cut end or screw.
4. Photograph anything under the coat or around fixings, with the date, before you touch it further.
5. If the screen is under warranty, call the installer with the photos before using abrasives, polishes or touch-up paint.
6. If the coat is lifting widely or the metal is crumbling at corners, book an inspection to decide between repair and replacement.

Warranty care: on SCREENGUARD, the wash schedule is a condition

For SCREENGUARD doors and screens, washing on schedule is part of keeping the warranty, so frame corrosion and warranty questions are linked. ScreenGuard says its warranty runs for 10 years from installation to the original purchaser, provided the care and maintenance recommendations have been complied with. Regular maintenance with ScreenGuard PowaWash is required to comply.

ScreenGuard sets the interval by environment. Within 15 km of the coast in rural or suburban areas it's every six months, within 10 km it's every three months, and within 5 km of the coast, marine or heavy industrial areas it's every two weeks. ScreenGuard also says the warranty doesn't apply to damage caused by abuse, accident or alteration to the product, and that nothing in it limits your rights under the Australian Consumer Law.

If you think a frame has a defect, ScreenGuard says to start with the licensed supplier who measured and installed it, because they hold the record of the job. Keep a note of when you wash, too. Dulux likewise ties regular removal of residues to fulfilling powder coat warranty requirements.

Preventing it: a checklist for frames near the sea

Most frame corrosion is preventable with regular washing, a careful eye on the coating and the right fixings at install time. Dulux says powder coated surfaces should generally be cleaned every twelve months, and more often in coastal areas. On a SCREENGUARD screen, follow the PowaWash schedule above, which is tighter near the water.

At the fabrication end, Dulux says any section cut to expose raw metal must be sealed to protect it, and BRANZ says cut ends of extrusions rely only on their natural oxide film, so they need protecting too. Those are questions for the maker, not jobs for the homeowner. Our cleaning guide covers the full washing routine for mesh and frames.

• Wash sheltered doors as often as exposed ones, and often more, because rain never rinses them.
• Keep the bottom track and drainage slots clear so water doesn't sit against the frame.
• Use soft brushes and cloths only: no scourers, steel wool, solvents or harsh degreasers on powder coat.
• Check corners, joints and screw heads when you wash, and photograph anything new.
• Ask your installer what the fixings are made of and what keeps different metals apart.
• Don't patch chips with touch-up paint before asking the coating supplier: BRANZ says touch-ups usually differ from the original coating and fade differently.

Next step: settle the mesh grade, then book an inspection

If the frame is sound and you're weighing a replacement screen for a coastal house, the mesh grade is the next decision, and our 316 vs 304 stainless steel mesh guide covers it. If you're choosing a frame metal from scratch, see steel vs aluminium security doors, and for how long screens tend to last, how long security screens last.

When a frame has gone past a wash, request a quote or see our onsite repairs. We repair and replace security doors across Cairns and the areas we serve, and every new door is made to measure and manufactured in Cairns. Our SCREENGUARD frames are T6 extruded aluminium, manufactured and finished in Queensland.

Fabricating and installing security screens and doors falls within the scope of QBCC's glass, glazing and aluminium licence class. We hold QBCC licence 1199847.