Powder Coating vs Hot-Dip Galvanising
Galvanising and powder coating protect steel by opposite mechanisms. Galvanising is sacrificial, the zinc corrodes preferentially and protects exposed steel even at a scratch. Powder coating is a barrier: it keeps moisture away from the steel, but offers no protection where the film is breached.
That difference is why the two are so often combined rather than chosen between.
How each protects
Hot-dip galvanising immerses cleaned steel in molten zinc at around 450 °C, forming metallurgically bonded zinc-iron alloy layers 45–85 µm thick. Zinc is less noble than iron, so in the presence of an electrolyte it corrodes instead of the steel. Scratch through to bare metal and the surrounding zinc still protects the exposed steel. This is cathodic protection, and no organic coating can do it.
Powder coating forms a continuous polymer barrier 60–120 µm thick. Intact, it is an excellent moisture and chemical barrier. Breach it and corrosion begins at the breach and creeps sideways under the film, which is exactly what a salt spray test measures.
Side by side
| Hot-dip galvanising | Powder coating | |
|---|---|---|
| Mechanism | Sacrificial / cathodic | Barrier |
| Protects at a scratch? | Yes | No |
| Thickness | 45–85 µm zinc | 60–120 µm polymer |
| Bond | Metallurgical | Adhesive, via conversion coating |
| Colour | Grey only; weathers dull | Any colour; full RAL range |
| Appearance | Spangle, runs, variable | Uniform, controlled gloss |
| UV | Unaffected | Needs a UV-stable chemistry |
| Chemical resistance | Poor in acids and alkalis | Good; excellent with pure epoxy |
| Abrasion | Hard, but zinc is soft | Good; H–2H |
| Process temperature | ~450 °C, can distort thin sections | 140–200 °C |
| Dimensional change | Adds thickness; threads need allowance | Minimal |
| Life, inland | Decades | Depends on film integrity |
| Life, coastal or industrial | Reduced, zinc consumed faster | Depends on pre-treatment |
| Repair | Zinc-rich paint | Strip and recoat |
Choose galvanising when
- Appearance is irrelevant, structural steel, framing, buried or concealed work
- The surface will be scratched or impacted in service
- Very long maintenance-free life is the priority
- The part is hard to inspect or re-coat once installed
- Transmission towers, guardrails, structural framing, agricultural structures
Choose powder coating when
- Colour and appearance matter
- The environment is chemically aggressive, zinc dissolves in acids and alkalis
- The part is thin, precise or would distort at 450 °C
- Threads, bearings or close tolerances must be preserved
- Indoor or moderate exposure where cathodic protection is not needed
Duplex systems: usually the right answer for demanding exterior steel
Galvanise first, then powder coat over it. The two protect synergistically, and a duplex system typically outlasts the simple sum of its parts. The coating shields the zinc from the atmosphere so it is consumed far more slowly, while the zinc provides cathodic protection anywhere the coating is breached.
You also get colour, which galvanising alone cannot give.
Getting duplex right
The failure mode is well known and entirely avoidable:
- Outgassing. Zinc layers release gas at cure temperature, producing pinholes and blisters. Preheat above cure temperature before coating. This is the step that gets skipped.
- Sweep blast to remove zinc oxide and white rust and provide a mechanical key.
- Use a conversion coating suited to zinc: not a plain steel phosphate process.
- Coat promptly. Fresh zinc oxidises quickly.
- Watch runs and spangle from galvanising, they telegraph through the coating.
Where each fails
Galvanising fails when the zinc is consumed: faster in coastal, industrial or acidic conditions. It also fails where acids or alkalis attack zinc directly, and it will not hold a colour.
Powder coating fails at breaches: a chip, a sharp edge with thin film, or a failure of the conversion coating. Corrosion starts there and creeps sideways under the film. Which is why pre-treatment and edge preparation matter more than the powder. Breaking sharp edges to a radius does more for edge corrosion than any coating upgrade.
Frequently asked questions
Which lasts longer?
Galvanising, in unpainted structural exposure, decades of maintenance-free life inland. Powder coating alone is more variable because it depends on film integrity and pre-treatment. Duplex outlasts either.
Can galvanised steel be powder coated?
Yes, and it is standard practice: provided the part is sweep blasted, correctly pre-treated, and preheated to degas before coating. Skipping the preheat is the commonest cause of pinholing on galvanised work.
Is powder coating over galvanising worth the cost?
For exterior steel with a long expected life, generally yes. You get cathodic protection, a colour, and a slower rate of zinc consumption.
What about zinc-rich primers under powder?
Zinc-rich powder primers exist and give partial cathodic protection, though less than hot-dip galvanising. A reasonable middle option where full galvanising is impractical.
Which for coastal India?
Duplex, if budget allows. Chloride consumes zinc quickly and attacks breached coatings quickly, so combining a sacrificial layer with a barrier is the sound approach. Specify the corrosivity category using ISO 12944 rather than describing the site in words.
Specifying a system for exterior steel? Tell us the environment and the design life you need and we will advise on the coating. Including where galvanising alone is the better economic answer.