Quick answer. Two doors can carry the same finish and still age completely differently on the coast. What decides it is what happens before the powder goes on (degrease → phosphate conversion → passivation), which powder chemistry is used (interior hybrid vs exterior TGIC/HAA) and the film build. Coastal environments are classified C5-M in ISO 12944 — the “very high” corrosivity class.
The standards behind a coastal specification
| What it covers | Standard | Why it matters on a quotation |
|---|---|---|
| Corrosivity of the environment | ISO 12944 | C5-M covers “coastal and offshore areas with high salinity” — the very high corrosivity class (the 2018 revision adds CX for extreme offshore conditions) |
| Salt spray testing | ISO 9227 / ASTM B117 | Neutral salt spray (NSS), 5% NaCl at pH 6.5–7.2; results are reported as hours to first corrosion product |
| Coating adhesion | ASTM D3359 | Tape test over a cut lattice, rated 0–5; Method A (X-cut) above 125 µm, Method B (cross-cut) below |
| Pre-treatment of the steel | ISO 9717 | Phosphate conversion coating — the layer that anchors the powder and slows under-film corrosion |
1. The two doors
Two black steel entrance doors are installed in the same month, on the same street, 400 metres from the Indian Ocean. Two years later, one has rust blooms along the bottom rail, around the fixing holes and on the hinge-side edge. The other still looks new.
The difference is almost never the colour, the brand, or even “paint versus powder”. It is what happened before the powder went on — and which powder was used.
2. How rust actually starts
Steel corrodes when moisture, oxygen and chloride ions reach bare metal. A coating system has two jobs: be a barrier, and stay stuck to the steel. Failure therefore starts at discontinuities — cut edges, weld seams, fixing holes and scratches — not in the middle of a flat panel.
Once moisture reaches the coating/steel interface, corrosion creeps sideways underneath the film and lifts it. That is why a small scratch can become a large blister. Coastal air, loaded with chloride, accelerates the whole process.
The industry does not leave “coastal” as a vague word. ISO 12944 classifies environments by corrosivity, and its C5-M category covers coastal and offshore areas with high salinity — the “very high” class (the 2018 revision adds CX for extreme offshore conditions). A door specified for Mombasa or Dar es Salaam is being asked to survive one of the harshest environments the standard describes.
3. Where the rules came from
Salt spray testing is one of the oldest ways the industry has quantified corrosion performance. Public references describe ASTM B117, first published in 1939, as the earliest internationally recognised salt spray standard; ISO 9227 is the corresponding international standard (alongside JIS Z 2371 and ASTM G85).
The test itself is deliberately boring: a 5% sodium chloride solution is atomised inside a closed chamber, held at a neutral pH of 6.5–7.2 (which is why it is called the Neutral Salt Spray, or NSS, test). Results are reported as hours in NSS before corrosion products appear — that is where a line like “720 hours” in a specification comes from.
Adhesion has its own standard: ASTM D3359, “Standard Test Methods for Rating Adhesion by Tape Test”. A lattice is cut through the film, tape is applied and pulled at close to 180°, and adhesion is rated on a 0–5 scale. Method A (X-cut) targets films thicker than 125 µm; Method B (cross-cut) targets thinner films.
The layer few buyers ask about is the one that does the quiet work: phosphate conversion coating. ISO 9717, “Phosphate conversion coating of metals”, specifies the process requirements for phosphate coatings on ferrous metals (and other substrates). Phosphate is what anchors the organic coating and slows under-film corrosion.
Then came the generations: liquid paint, then powder coating — applied electrostatically and cured at around 200 °C, with build-ups typically greater than 50 µm needed for an acceptably smooth film — then decorative wood-transfer (sublimation) films, and finally fluoropolymer (PVDF) topcoat systems for the harshest exterior exposure.
One trap deserves its own line: interior and exterior powders are not the same chemistry. Hybrid polyester/epoxy powders are formulated for indoor use; exterior exposure calls for TGIC-polyester or HAA systems. Putting an interior-grade powder on an exterior door is a specification error, not a question of wear.
AAMA 2605 is often quoted as the pinnacle of exterior coating performance — and it is, but note its scope: it is a voluntary specification for superior performing organic coatings on aluminium extrusions and panels. Treat it as the industry yardstick, not as a steel-door standard.
4. Five checks a dealer can run without a laboratory
1) Look at the discontinuities first — a cut edge, a weld seam, a fixing hole. Is there edge coverage, or bare grey steel?
2) Ask which pre-treatment steps are used, and whether they are documented per batch: degrease → phosphate → passivation.
3) Ask for the salt-spray report, and read three things: which standard (ISO 9227 or ASTM B117), how many hours, and on which colour/finish — matte black and dark metallics are harder to pass than white.
4) Ask whether the powder is an interior (hybrid) or exterior (TGIC/HAA) system, and what the cure temperature and time are.
5) Inspect the film where water actually sits and hands actually touch: corners, bottom rail, lock area. Look for orange peel, sags, thin patches; if you carry one, take a coating-thickness (DFT) gauge reading.
5. What we do, and what we will not claim
Our coastal-spec doorsets are produced on a phosphate pre-treatment line with exterior-grade powder systems, and the coating data sheet and process parameters travel with the order, so you can check them yourself.
We do not claim certifications we cannot document. If your project names a specific report or a local approval route, tell us and we will confirm what documentation we can provide for your specific assembly.
Next step: send us your door size, quantity and installation city (coastal or inland) and we will come back with a specification sheet and a quotation. → WhatsApp / enquiry form
6. Sources
ISO 12944 (corrosivity categories, incl. C5-M “coastal and offshore areas with high salinity”); ASTM B117 (1939, first internationally recognised salt spray standard) and ISO 9227 (NSS, 5% NaCl, pH 6.5–7.2, results as hours); ASTM D3359 (tape adhesion test, Method A >125 µm / Method B <125 µm, 0–5 rating); ISO 9717 (phosphate conversion coating of metals); powder coating cure ~200 °C and film build-up >50 µm (industry technical references); AAMA 2605 (voluntary specification for superior performing organic coatings on aluminium extrusions and panels — scope note applies).
Frequently asked questions
Why does a powder-coated steel door rust at the coast but not inland?
Coastal air carries chloride, which accelerates corrosion once moisture reaches the steel. The coating fails at discontinuities — cut edges, weld seams, fixing holes and scratches — where the film is thinnest. Pre-treatment quality and powder chemistry decide how fast that spreads.
Is powder coating better than liquid paint for exterior doors?
Generally yes for exterior steel doors, because a cured powder film is thicker and tougher than a thin solvent-based paint. But the powder family matters: hybrid polyester/epoxy powders are made for interior use, while exterior exposure calls for TGIC-polyester or HAA systems.
How many hours of salt spray testing should a coastal door have?
There is no single number that is valid for every project. Salt spray results are reported as hours in neutral salt spray (NSS) to a defined standard — ISO 9227 or ASTM B117 — and the figure depends on substrate, colour and film build. Ask which standard, how many hours, and on which finish; then match it to your own specification.
What should I ask a supplier before ordering coastal-spec doors?
Five things: which pre-treatment steps are used (and whether they are documented per batch); whether the powder is an exterior-grade system; what salt spray report exists and to which standard; the cure temperature and time; and the film thickness at the corners, bottom rail and lock area.
Why JinHong Doors
We have 39 years of door manufacturing experience in China and are now exporting to East and African markets — steel security doors, fire-rated doors and project doors. We quote with the specification sheet, packing plan and the documentation your project asks for.
Request a coastal specification sheet →
