Galvanised, zinc plated, powder coated or anodised — what is the difference?
Five ways of stopping steel and aluminium corroding, what each one actually is, how long each survives outdoors, and what happens the moment you cut or drill through one.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
- Safety
- caution
Short answer
Hot-dip galvanising dips steel in molten zinc and gives a thick, sacrificial, long-lived coating. Zinc plating is a much thinner electroplated version for indoor use. Sherardising is a diffused zinc coating for small components. Powder coating is a baked-on polymer film that is decorative and protective but not sacrificial. Anodising thickens aluminium's own oxide layer. Only the zinc coatings protect steel that has been exposed at a cut.
Two screws can look identical in the tin and last two years or twenty outdoors. The difference is the coating, and the labels on the boxes are not always as informative as they look — the word galvanised covers processes that differ by an order of magnitude in life.
Safety
Step by step
- Understand what sacrificial means.Zinc is less noble than steel, so where a zinc coating is scratched or drilled through, the zinc around the damage corrodes preferentially and protects the exposed steel. That is why a small scratch in galvanising does not start a spreading rust patch the way a scratch in paint does.
- Recognise hot-dip galvanising.Steel dipped in molten zinc. The coating is thick, often visibly crystalline or spangled, slightly rough, and it builds up at edges and threads. This is the coating for outdoor structural steel, gate hardware, joist hangers and anything in treated timber.
- Recognise zinc plating.An electroplated coating, thin, smooth and often bright or slightly yellow-tinted. It is what most bright screws from a general hardware tin are. It is an indoor coating and it will rust outdoors within a season or two.
- Recognise sherardising.A dull grey diffused zinc coating applied by heating components in zinc dust. It coats threads and complex shapes evenly without the build-up hot dipping causes, so it is used on nails, fixings and small ironmongery meant to last outside.
- Recognise powder coating.A dry polymer powder applied electrostatically and baked into a hard film. Tough, available in any colour, and used for railings, garden furniture, aluminium windows and appliance panels. It is a barrier, not a sacrificial coating, so damage exposes bare metal.
- Recognise anodising.An electrochemical process that thickens the natural oxide film on aluminium. The coating is part of the metal rather than applied on top, so it does not chip or peel, and it can be dyed. It does not apply to steel at all.
- Think about what a cut edge does.Cutting a galvanised post or drilling a galvanised bracket exposes steel. The surrounding zinc will protect a small exposure, but a large one needs a zinc-rich cold galvanising paint. Cutting a powder-coated or painted item exposes bare metal that will rust from that line outwards.
- Match the coating to the exposure, not to the price.Indoors and dry: plating is fine. Outdoors inland: hot dip, sherardised or A2 stainless. Coastal, chlorinated, or in treated timber: hot dip or A4 stainless. Marine or continuously wet: stainless of the right grade.
Tips
- Galvanised surfaces need a specific primer before painting. Ordinary primers do not key to zinc, which is why paint on a galvanised gate so often comes off in sheets.
- Anodised aluminium cannot be repaired locally. A scratch through the anodising stays visible, which is a reason to think about handling before installation rather than afterwards.
- Zinc-rich cold galvanising paint in an aerosol is worth keeping for touching in cut ends of galvanised posts and rails. It is not as good as the original coating, and it is far better than nothing.
Common mistakes
- Using bright zinc-plated screws outdoors — The coating is a few microns thick and made for dry conditions. It rusts within a season or two, and the rust stains whatever it is fixed to.
- Painting galvanised metal with ordinary primer — The zinc surface is passive and smooth and nothing keys to it. Use a mordant solution or a primer specifically for galvanised surfaces.
- Grinding or welding galvanised steel without extraction — Zinc fumes cause metal fume fever, which is genuinely unpleasant and entirely avoidable. Remove the coating locally first or work with proper extraction.
If it doesn't work
Rust running from the cut end of a galvanised post or rail
Cause: Steel exposed at the cut with too large an area for the surrounding zinc to protect — Fix: Wire brush back to bright metal and apply a zinc-rich cold galvanising paint. Do it when the cut is made rather than when the rust appears.
Powder coating lifting and blistering along an edge
Cause: Corrosion under the film, starting from a damaged edge or a poorly prepared one — Fix: It will keep spreading under the sound coating. Strip back to sound material and bare metal, treat the rust, and repaint — patching over a blister traps the corrosion.
Anodised window frames looking patchy and chalky
Cause: Weathering and surface deposits rather than coating failure — Fix: Clean with a non-abrasive cleaner made for anodised finishes. Abrasives and strong alkalis attack the anodised layer and make it permanently worse.
Questions people ask
How long does hot-dip galvanising last outdoors?
It depends entirely on the coating thickness and the atmosphere — rural, urban, industrial or coastal all consume zinc at different rates, and there are published corrosion rates for each. Decades is normal in a rural or suburban setting; far less at the coast or in a heavily polluted one.
Is stainless better than galvanised?
It is different rather than simply better. Stainless resists corrosion by forming its own passive film, but it is not sacrificial, so it does nothing for the steel it touches — and it can make galvanised or aluminium parts around it corrode faster.
Can something be galvanised after it is made?
Yes, and that is the normal way for fabricated steelwork — welded up first, then dipped, so every surface including the inside of hollow sections is coated. Cutting or drilling afterwards is what breaks that.
What is Dacromet or a geomet coating?
Proprietary zinc-flake coatings applied and cured on fixings, giving good corrosion resistance in a thin film without hydrogen embrittlement risk. They are common on high-strength construction screws and are specified by name on the packaging.