Three ways to keep steel from rusting, and they are not alternatives to each other. The difference is thickness, bond and what happens at a cut edge.
These three are not competing options for the same job. Hot dip galvanizing puts 75–85 microns of metallurgically bonded zinc on the steel. Electroplated zinc puts under 25 microns, often under 10, deposited electrically. Paint puts an organic film on top that adheres but does not bond.
The practical consequence: electroplated zinc is not an outdoor coating. If a quotation for outdoor structural steel offers “zinc plated” at a lower price, it is not a cheaper version of galvanizing — it is a different product for a different application, and it will not last outside.
| Hot dip galvanizing | Electroplated zinc | Paint | |
|---|---|---|---|
| Typical thickness | 75–85 µm | under 25 µm, often under 10 | Depends on system; 100–300 µm for a full outdoor system |
| How it attaches | Metallurgical bond — zinc-iron alloy layers grown into the steel | Electro-deposited layer | Adhesion to a prepared surface |
| Cut edges and drilled holes | Zinc protects sacrificially across small gaps | Sacrificial but very little zinc to work with | No protection — a cut edge is bare steel |
| Inside hollow sections | Coated, because the section is dipped | Generally not | Generally not |
| Outdoor structural service | Yes — the standard choice | No | Yes, with the right system and maintenance |
| Maintenance | None for decades in normal atmospheres | Not applicable outdoors | Periodic recoating |
| Handling damage | Hard alloy layers resist site knocks | Thin and easily worn | Chips and scratches at every handling point |
| Typical use | Towers, gantries, OHE masts, solar structures, line hardware | Indoor fasteners, appliance and automotive components | Architectural finish, colour, or over galvanizing as a duplex system |
Thickness figures for galvanizing are our own published range; the electroplating figure is the general industry range for zinc plating on fasteners and small parts. Paint thickness varies entirely with the specified system.
Steel does not usually start rusting in the middle of a plate. It starts at a cut edge, a drilled hole, a weld toe or a scratch — the places where a coating is thinnest or absent.
Zinc is sacrificial. Where the steel is exposed at a small scratch or a cut, the surrounding zinc corrodes preferentially and protects the bare steel across the gap. Paint cannot do this: a scratch through a paint film is a site for rust to start and then creep underneath. Electroplated zinc is sacrificial too, but there is so little of it that it is consumed quickly.
Hot dip galvanizing also draws the coating around edges and into holes rather than bridging them, because the article is immersed rather than sprayed. That is why the corners of a galvanized member are the last place it fails and the corners of a painted one are the first.
The two are not mutually exclusive. Painting over hot dip galvanizing is a recognised approach where colour, visual identity or an especially aggressive environment is involved, and the combination lasts longer than either coating alone — the paint shields the zinc and the zinc protects the steel wherever the paint is breached.
It needs the right surface preparation on the galvanizing, so tell us at enquiry if a piece is going to be painted afterwards. It changes how we finish it.
“Zinc coating” is an ambiguous phrase. Typed into a search engine it very often returns electroplating shops, because that industry uses the term more than galvanizers do. If you are looking for structural coating, search for hot dip galvanizing — and if a supplier is quoting “zinc coating” for outdoor steel, ask which of the two they actually mean and what thickness they will certify.
Answered from our own line, not from the standard's cover page.
No. Both put zinc on steel, but hot dip galvanizing immerses the article in molten zinc and forms a metallurgical bond at 75–85 microns, while electroplating deposits zinc electrically at under 25 microns and often under 10. Zinc plating is an indoor coating for fasteners and small parts.
Galvanizing, in almost every structural case, and it needs no maintenance. Paint protects a surface but not a cut edge or a scratch, and it has to be recoated. Where colour or an especially aggressive environment is involved, paint over galvanizing outlasts either on its own.
Yes — it is called a duplex system and it lasts longer than either coating alone. It needs the right surface preparation on the galvanizing, so tell us at enquiry if the piece will be painted.
Because zinc is sacrificial. At a small scratch or cut edge the surrounding zinc corrodes in preference to the exposed steel and protects it across the gap. Paint has no such mechanism, which is why rust creeps under a damaged paint film.
No. We hot dip galvanize, on our own 7-metre kettle at Howrah, at 450–460 °C. For outdoor structural steel that is the appropriate coating; electroplating is a different process for different applications.
Tell us the standard, the coating requirement and the component, and we will confirm in writing what we can hold and how we will certify it. Galvanizing is on our own floor, so the answer comes from us and not from a sub-contractor.
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