Coastal Corrosion Risk for Aluminum Window and Door Systems
Coastal rainfall and airborne dust carry a high chloride and alkali load, and some sites still use sea sand that has only been desalinated rather than fully washed. Aluminum is an active metal whose corrosion resistance depends on the compact oxide film on its surface; when that film or the applied protective coating is damaged, pitting corrosion starts quickly in a chloride-rich atmosphere. A coastal facade should be specified as an ISO 9223 corrosivity category C4 or C5 environment instead of an ordinary inland installation.
Alloys, Tempers and Standards for Coastal Profiles
Architectural extrusions are supplied under GB/T 5237 or ASTM B221, with 6063 in the T5 or T6 temper as the standard choice for frames and sashes and 6061-T6 used for structural mullions and load-bearing members. Table 1 lists the minimum mechanical values we guarantee on the mill certificate.
| Alloy and temper | Wall thickness | Rp0.2, MPa | Rm, MPa | Elongation, % |
|---|---|---|---|---|
| 6063-T5 | 1.4 to 2.0 mm | 110 min | 160 min | 8 min |
| 6063-T6 | 1.4 to 2.5 mm | 170 min | 205 min | 8 min |
| 6061-T6 | 2.0 to 3.0 mm | 240 min | 290 min | 8 min |
Finishes that control chloride attack
Anodic oxidation to ISO 7599 or GB/T 8013, class AA15 with 15 microns average and 12 microns minimum, or AA20 for facades directly facing salt spray.
Organic coatings applied to AAMA 2605 or EN 12206, with dry film thickness of 40 to 80 microns for PVDF systems and 60 to 100 microns for polyester powder.
Mill finish is only acceptable for interior members that never see rain or condensation.
Typical Applications
Sliding doors, balcony railings and curtain wall frames within one kilometre of the shoreline.
Hotel and resort fenestration in continuous salt fog and high humidity.
Port control rooms, marine terminal offices and seaside residential towers.
Coastal industrial buildings where the frames are washed regularly by wind-driven rain.
Installation, Handling and Supply Requirements
Field corrosion is normally caused by water trapped inside unprotected cavities rather than by the alloy itself. The following rules are written into our supply documents and should also govern site work:
Store profiles in a ventilated, dry and clean warehouse; the top face of the sleepers must be at least 100 mm above the ground.
Each stack must not exceed 15 molds or fans, with a soft pad between every two molds so the protective film is not crushed.
Every opening must have a working drip line that is water tested before the frames are fixed, because rain running down sea-sand render is strongly corrosive.
Use card-type fixing clips instead of drilled steel plates so the protective layer is disturbed as little as possible.
Seal joints between profiles with neutral sealant to stop rain entering cavities that have no protective layer; acidic sealant attacks the anodic film.
Remove the protective tape once installation is complete and never leave it exposed to strong sunlight for long periods.
We supply 6 m standard lengths, cut-to-size blanks and fully machined frames, with alloy, temper, wall thickness and coating class agreed on the drawing before extrusion.
Quality Control and Common Misconceptions
Every delivery is accompanied by a chemical composition report, a tensile test report and a coating thickness record. Salt spray testing to ISO 9227 or ASTM B117 is normally run for 1000 hours on anodized and coated samples, and coating adhesion is verified by cross-cut or impact test. The most common misconception is that anodized aluminum cannot corrode: the oxide layer is only microns thick and is easily bridged by galvanic contact with steel or copper, so isolation gaskets and stainless steel fasteners must be used at every dissimilar-metal interface.
Frequently Asked Questions
Q: Which alloy is best for doors and windows within 500 m of the sea?
A: 6063-T5 or T6 with an AA15 anodic film is the most economical choice, while 6061-T6 is preferred for structural mullions and AA20 is recommended where the facade faces the prevailing wind.
Q: How thick should the anodized layer be for coastal use?
A: ISO 7599 class AA15 requires 15 microns average and 12 microns local minimum. For direct sea front exposure we recommend AA20, which raises the average to 20 microns.
Q: Does the drip line need a water test before installation?
A: Yes. Test every opening before fixing the frames, because rain running over sea sand render is strongly alkaline and will attack any cavity that drains poorly.
Q: Can aluminum frames be fixed directly to steel brackets?
A: Direct contact is not recommended in a chloride atmosphere. Use isolation pads and stainless steel fasteners to avoid galvanic corrosion between the two metals.
Q: Which sealant should be used at profile joints?
A: A neutral curing silicone or MS polymer sealant. Acid curing products release acetic acid that attacks both the anodic film and the organic coating.
Q: What is the maximum stack height for delivered profiles?
A: No more than 15 molds per stack, with soft pads between molds and the lowest profile at least 100 mm above the floor.
