Powder Coating, Hot-Dip Galvanising or Duplex? Specifying Steel for Outdoor Play Equipment

Powder coating is an excellent finish. It is hard, it takes any colour, and it has essentially no solvent emissions. What it is not is a corrosion protection system on its own. It works as a barrier: while the film is continuous and intact, the steel underneath is dry and nothing happens. The moment the film is breached — a scratch from a strimmer, a chip from a stone, the thin coverage that never forms properly on a sheared edge — water reaches bare steel and rust begins. Because rust occupies several times the volume of the steel it consumes, it lifts the surrounding coating and creeps underneath it. A powder-coated structure that has one scratch in year two has a visible rust trail along the whole panel by year five.

This is why a quotation that says "powder coated steel" and nothing else is almost always cheaper, and why it is not comparable with one that says "hot-dip galvanised and powder coated". The two read very similarly on a specification sheet. They are not the same product, and the difference only becomes visible several years after the invoice is paid.

The one exception worth knowing: powder over a properly prepared, blasted surface with a good pretreatment can perform well in sheltered and indoor conditions. It is an acceptable finish for indoor play equipment with no weathering exposure, provided the pretreatment is real — degreasing and a conversion coating, not a wipe-down.

Galvanising works in two ways at once. The zinc layer is a physical barrier, and it is also sacrificial: where the coating is damaged and steel is exposed, the surrounding zinc corrodes in preference to the steel, and the corrosion product tends to seal the damaged area rather than spread under it. That is the property powder coating does not have, and it is why galvanised steel tolerates the small damage that outdoor equipment inevitably receives.

The sequence of fabrication is what makes or breaks it. Everything — cutting, drilling, punching, welding — must be completed before the item goes into the zinc bath, because galvanising after fabrication coats the edges and the insides of the holes. When holes are drilled afterwards, the raw steel inside the hole is unprotected, and that is precisely where water collects and where corrosion starts. Hollow sections need vent and drain holes so that air can escape during dipping and water cannot sit inside afterwards. If a supplier cannot describe the vent and drain provision, the internal surfaces are probably unprotected.

EN ISO 1461 sets minimum coating thicknesses that scale with steel thickness, from around 45 µm on thin sections to around 85 µm on steel 6 mm and thicker, with minimum local values slightly below the mean figures. It also permits the repair of small damaged areas with zinc-rich paint. Ask for the coating thickness readings and for batch traceability, and check that the repair paint used is zinc-rich rather than an ordinary topcoat.

In a duplex system the galvanising does the corrosion protection and the powder coat does the appearance. The two together last longer than either alone — the powder reduces the rate at which the zinc is consumed, and the zinc protects the steel wherever the powder is damaged. Published guidance on duplex systems commonly puts the combined life at around 1.5 to 2.5 times the sum of the two individual lives, with the largest benefit in aggressive environments; the exact figure reflects local conditions and should not be treated as a guarantee.

The practical advantage is maintenance. The layer that degrades first in sunlight is the powder, which chalks and loses gloss — particularly in dark colours, and particularly with UV. When that happens, the structure is still protected by zinc underneath, and it can be recoated without stripping to bare steel. Compare that with a powder-only structure, where chalking and a breach at the same time means the steel is already corroding.

For a coastal site, duplex over galvanising is the normal specification, with stainless fixings and brackets. It is worth being explicit in the specification that the powder coat is applied *over* the galvanising rather than instead of it, because those two options are frequently both written as "powder coated steel" on a quotation.

Galvanised steel that has been stored or shipped wet develops white rust — a bulky white or grey zinc oxide and hydroxide deposit, also called wet storage stain. It is a surface condition caused by moisture with restricted air circulation, which is exactly what a sea container in a humid season provides. It is usually cosmetic: it does not mean the coating has failed, and it normally weathers away once the equipment is out in the open and drying cycles resume.

It nonetheless causes disputes, because it arrives looking like a defect. The controls are ordinary shipping practice: keep the steel dry before loading, avoid loading during rain, use dunnage so the load is not sitting in condensation, ventilate the container where possible, and use container desiccants on long humid routes. A supplier who has shipped galvanised equipment before will have a routine for this. It is a fair question to ask, and the answer is informative.

Both finishes are measurable, so neither has to be taken on trust. A magnetic coating thickness gauge gives a direct dry film thickness reading on galvanising, on powder, or on both layers if you measure before and after the topcoat. Salt spray testing to ISO 9227 is commonly quoted, but treat the hours as an indicator rather than a prediction: correlation between salt spray hours and real service life is weak, and it is easy to quote hours from a flat test panel that does not represent a formed, welded, edge-exposed production item.

The most useful things to put in a purchase specification are therefore concrete and checkable: the galvanising standard and the coating thickness class, whether powder is applied over galvanising, the treatment of edges and holes made after dipping, the powder type and the dry film thickness range, and the touch-up procedure and paint for site repairs. Add a pre-shipment inspection clause covering thickness readings and a visual check for damage. None of this is unusual, and a supplier who routinely produces to specification will already have the records.