Three products share the name
"Cold galvanizing" is a marketing term, not a process standard. Three unrelated things are sold under it, and the differences between them are larger than the differences between any of them and hot-dip.
| Product | How zinc is applied | Typical dry film | Zinc in dry film |
|---|---|---|---|
| Electroplated zinc | Electrodeposition from a zinc salt bath | 5–15 µm | ~99% |
| Zinc-rich paint | Zinc dust in an epoxy or silicate binder, brush or spray | 60–100 µm | 60–85% |
| Cold spray zinc | Zinc dust in a minimal binder, high pressure spray | 60–100 µm | 92–96% |
| Hot-dip galvanizing | Immersion in molten zinc at 445–465 °C, alloy layer forms | 45–85 µm | ~98% (with zinc-iron alloy) |
The articles that rank for this query mostly compare the first row against the last. That is a fair comparison — electroplating is thin, it is not designed for outdoor structural service, and nobody would specify it for a bridge. But it is not the comparison a fabricator is making when they ask whether a zinc-rich coating can replace a galvanizing bath on a 30-metre truss that will not fit in the tank.
If the article you are reading quotes 5–20 µm and a five-to-fifteen-year service life, it is describing electroplating. Put it down; the numbers do not apply to cold spray zinc.
Compare zinc mass, not film thickness
Film thickness is a convenient number because it is easy to measure on site. It is also the wrong basis for comparison, because the two coatings are not the same material all the way through.
Hot-dip galvanizing produces a metallurgical coating roughly 98% zinc by weight. Cold spray zinc is a zinc dust dispersed in a small amount of binder — at 96% zinc the binder is doing very little except holding particles against the steel and to each other. Zinc-rich epoxy is a different animal again: at 80% zinc, a fifth of the film is resin.
The useful figure is mass of zinc per square metre:
zinc mass (g/m²) = dry film thickness (µm) × dry film density (g/cm³) × zinc fraction × 10
| System | DFT | Density | Zinc | Zinc mass |
|---|---|---|---|---|
| Hot-dip, ISO 1461, steel ≥6 mm | 85 µm avg | 7.14 | ~98% | ~600 g/m² |
| Cold spray zinc, 96% grade | 80 µm | ~2.7 | 96% | ~200 g/m² |
| Zinc-rich epoxy primer, 80% grade | 75 µm | ~2.5 | 80% | ~150 g/m² |
| Electroplated zinc | 10 µm | 7.14 | ~99% | ~70 g/m² |
So the honest headline is not "equivalent to hot-dip". At typical structural thickness, cold spray zinc carries about a third of the zinc that hot-dip does. Any supplier claiming one-to-one equivalence on a bare coating is either quoting the alloy-inclusive density of molten zinc or hoping you will not do the arithmetic.
Thickness figures in ISO 1461 scale with steel thickness because a thicker section holds more heat and reacts with more zinc. A 6 mm plate averages 85 µm; a 1.5 mm section is allowed 55 µm. If you are comparing against a thin-wall section, do the calculation with that number, not with 85.
A third of the zinc — so what is it for?
A third of the zinc is still enough to do the job in a large part of the market, for a reason that has nothing to do with coating chemistry.
Hot-dip galvanizing is normally left bare. The zinc is exposed to the atmosphere, and its life is simply how long that mass takes to corrode away. Under ISO 9223, zinc loss in a C4 atmosphere runs about 2–4 µm per year and in C5 about 4–8 µm. At 85 µm that is decades of service with no maintenance — which is exactly what a transmission tower in a remote valley needs, because nobody is going back up there to repaint it.
Cold spray zinc is almost never left bare on a heavy-duty structure. It goes under a system: zinc-rich primer, then an intermediate coat, then a topcoat. In that arrangement the zinc is no longer the primary barrier — the intermediate and topcoat are. The zinc's job narrows to what only zinc can do: hold the steel at a protected potential at scratches, cut edges, weld seams and mechanical damage, where a barrier coating has failed by definition.
This is the same logic as a duplex system over hot-dip galvanizing, and it changes the arithmetic. You are not buying 25 years of zinc consumption; you are buying a coating system whose barrier layer carries the service life and whose zinc underlayer prevents the failure mode that ruins barrier coatings — rust creep from every defect.
Which is also why the "25 years" figure on a technical data sheet means less than it looks. It is a manufacturer figure for a stated system in a stated corrosivity class, normally C3–C4 with the full build-up. Taken as a bare coating in C5 immersion, no zinc-rich product at 80 µm reaches 25 years, and a supplier who says otherwise should be asked for the test report.
Why 96% and 80% are not "close enough"
Zinc-rich coatings protect steel cathodically only if the zinc particles form a continuous conductive path to the steel. That is a percolation problem, and it has a threshold: below a certain loading, the particles are isolated in a resin matrix and the film behaves as a barrier coating with some zinc filler in it.
SSPC Paint 20 sets the floor for a Type I zinc-rich primer at 85% zinc dust by weight in the dry film. In practice, coatings that reliably maintain cathodic protection at a scratch sit at 90% and above. This is why a 96% cold spray and an 80% zinc-rich epoxy are not substitutes for each other even though both are called zinc-rich:
| Property | 96% cold spray zinc | 80% zinc-rich epoxy |
|---|---|---|
| Primary mechanism | Cathodic, with barrier as secondary | Barrier, with cathodic as secondary |
| At a scratch | Zinc protects the exposed steel | Protection depends on loading and film build |
| Typical role | Stand-alone zinc layer, or primer under a system | Primer in a multi-coat system, always overcoated |
| Can be left exposed | Yes, in moderate corrosivity | Not recommended |
If a specification asks for "cold galvanizing" without a zinc content, it is not a specification. Two suppliers can quote the same thickness and deliver films with a 40% difference in zinc. Ask for the figure by ASTM D2371 or ISO 3549 and put the number in the document.
Where cold spray zinc genuinely replaces hot-dip
Four situations, all of them about geometry and logistics rather than chemistry.
1. The component will not fit in a bath
Galvanizing baths are finite. Long welded girders, oversized portal frames, and assemblies that are welded after fabrication all exceed what a plant can dip. The alternatives are redesigning the structure into shippable pieces, bolting it together on site with unprotected interfaces, or coating it where it stands. Only the third one protects the whole structure.
2. The structure is already erected
Retrofit, life extension and change of use all mean the steel is in place, often in service. Nothing is going back to a plant. A cold-applied zinc layer at ambient temperature is the only route to a metallic zinc surface without dismantling.
3. Repair of a galvanized surface
This is the application with a written standard behind it. ASTM A780 covers repair of damaged and uncoated areas in hot-dip galvanized coatings, and zinc-rich coatings are the recognised repair material. Welds, cut edges, handling damage and abrasion scars all fall here. Note the requirement that usually gets missed: the repair film has to match or exceed the surrounding galvanizing in zinc content and thickness, otherwise the repair becomes the first place the structure rusts.
4. Moderate corrosivity with a colour requirement
Hot-dip comes in one colour. Where the steel is visible — architectural steelwork, handrails, equipment frames in a plant the client walks through — a zinc layer that accepts a full topcoat in any RAL shade solves a problem the bath cannot. This is the duplex case, and it is the most common reason a galvanized structure gets painted anyway.
Where it does not
The same arithmetic that makes the substitution work in the cases above rules it out in these.
Buried or immersed steel
Soil and immersion are the most aggressive exposure a coating meets, and both remove the option of inspection and repair. In ISO 12944 terms this is the immersion category, and the zinc demand goes well beyond what 80 µm of a binder-based coating can supply. Hot-dip, or hot-dip plus a system, or cathodic protection — but not a cold-applied zinc layer on its own.
Chemical splash and strong electrolyte
Zinc is amphoteric: it dissolves in both acid and strong alkali. A coating whose protection depends on zinc being present is the wrong choice where the zinc will be consumed chemically rather than electrochemically. The binder also has to survive the chemistry, which is a separate question from zinc loading.
Abrasion-loaded surfaces
Ore chutes, hopper liners, aggregate handling, ice-loaded decks. A zinc dust film in a thin binder does not have the abrasion resistance of a zinc-iron alloy layer, and once the film is gone the protection is gone. Specify a lining or a suitable hot-dip thickness, not a paint.
Twenty-five years, no maintenance, no access
If nobody can return to the structure — remote towers, offshore above-water steel, confined spaces — then the coating has to survive on its own zinc for the whole design life. That is the case hot-dip was invented for, and no 200 g/m² coating wins it.
Cost: per square metre to equivalent protection
Comparing price per litre against price per kilogram of galvanizing is the most common error in coating selection, and it usually flatters the wrong option.
The comparison that matters is the total cost to put a specified protection on one square metre of finished, erected steel:
- Coating material at the specified dry film thickness, not at a nominal wet volume. Zinc-rich coatings lose volume on cure, and the consumption depends on surface profile — a Sa 2.5 blast with 60–80 µm profile takes noticeably more material than a smooth plate.
- Surface preparation to the same standard. Hot-dip does its own preparation in the pickling line; a cold-applied coating needs Sa 2.5 by blast, which is a real cost on site and often the largest line item.
- Transport both ways for hot-dip, plus the risk of coating damage in transit and the repair that follows.
- Dismantling, lifting and re-erection if the structure is already standing — frequently more than every other line combined.
- Production loss while the component is away.
- Repair of damage after erection. This cost exists in both routes and is routinely left out of the hot-dip quotation.
Run this properly and the pattern is consistent: for shop-fabricated components that fit in a bath and will never be repainted, hot-dip wins on cost per year of service. For large, erected or repaired structures, the transport and dismantling lines alone usually decide it before the coating price is even compared.
What to put in the specification
If the decision is cold spray zinc, six lines in the specification prevent most of the failures we see in service.
| Requirement | Write this |
|---|---|
| Zinc content | ≥96% zinc in the dry film by ASTM D2371 or ISO 3549. State the method — a percentage without a test method is not enforceable. |
| Surface preparation | Sa 2.5 to ISO 8501-1, profile 50–85 µm. St 3 only where blasting is physically impossible, and expect a shorter life if you accept it. |
| Dry film thickness | 80 µm nominal, with a stated minimum. Measure per SSPC-PA 2 on a defined grid; a single spot reading on a blasted surface proves nothing. |
| Adhesion | Pull-off per ASTM D4541 or cross-cut per ASTM D3359, at an agreed frequency. |
| Salt spray | Hours to a stated creep from the scribe, per ASTM B117 — not hours to first rust. The scribe result is the one that reflects cathodic protection. |
| Overcoating window | The interval within which the intermediate coat must go on. Zinc surfaces passivate in air; waiting too long costs adhesion, and the window is temperature-dependent, not a fixed number of days. |
Two more things worth writing down, because they get argued about on site. First, the dew point rule: steel temperature at least 3 °C above dew point, whatever the specification says about humidity. Second, whoever applies the coating should be the one who prepared the surface — a blast-and-coat handover between subcontractors is where most adhesion failures originate.
Questions that come up in every enquiry
Can cold spray zinc replace hot-dip galvanizing entirely?
No. It replaces hot-dip where the component cannot be dipped, is already erected, is being repaired, or sits in a moderate corrosivity class under a full coating system. It does not replace hot-dip for buried or immersed steel, abrasive service, or where the structure must go 25 years without access for maintenance. At 80 µm a cold spray carries about a third of the zinc mass of an 85 µm hot-dip coating, and that difference matters in exactly those cases.
How thick should cold spray zinc be?
80 µm dry film is the usual spec for structural steel, applied in two coats of 40 µm where the product allows it. Below 60 µm the zinc reserve is too small to be reliable in C4 and above, and the film becomes difficult to apply uniformly over a blasted profile. Above 120 µm you are mostly adding cost, with a higher risk of solvent retention and mud cracking.
Is a 96% zinc coating better than an 80% zinc-rich primer?
They do different jobs rather than one being better. Above roughly 90% zinc the particles form a continuous conductive network and the film protects cathodically at a scratch, which is what a stand-alone zinc layer needs. An 80% zinc-rich epoxy is a primer: it works as part of a system, relies mainly on barrier protection, and should always be overcoated. Specifying one where the other is needed is a common and expensive mistake.
Does cold spray zinc work over old galvanizing?
Yes, with preparation. The surface must be free of oil, salt, loose zinc and white rust; power-tool cleaning to St 2 or St 3 is normally enough, and abrasive blasting is not required over sound galvanizing. This is the basis of repair to ASTM A780. The failure mode to avoid is applying over a surface that is still chemically active or contaminated — the new zinc will not bond and will lift within a season.
What does the 25-year design life actually cover?
It is a manufacturer figure for a defined system in a defined corrosivity class, normally C3–C4 with a full build-up of primer, intermediate and topcoat. It is not a guarantee that an 80 µm bare zinc film survives 25 years in C5 marine exposure — the zinc consumption arithmetic does not support that. Ask for the exposure class and the system build behind any life figure you are quoted.
Send the structure, get a system
Tell us the component, the steel condition, the environment and the area. The reply is a coating system with film build and consumption — not a price list. If hot-dip is the right answer for your case, we will say so.