Cold galvanizing, hot-dip galvanizing or metallizing: which to choose?

Hot-dip galvanizing suits new, removable parts that can be shipped and dipped in a zinc bath. Cold galvanizing, a zinc-rich coating applied by brush or spray, suits installed parts and touch-ups. Zinc metallizing, sprayed on site, gives a thick zinc layer without dismantling, at the cost of more demanding surface preparation.

All three protect steel the same way: zinc corrodes in its place, through galvanic action, as our galvanic series table shows. What sets them apart is where and how the zinc is deposited, the thickness achieved, the preparation required and whether a part can be treated without removing it. This comparison helps maintenance managers and project managers choose in Quebec and Ontario.

How does each process work?

All three processes deposit zinc on steel, but through very different mechanisms, which explain their strengths and limits.

• Hot-dip galvanizing (immersion). The part is chemically cleaned (degreasing, acid pickling, fluxing) and dipped in a bath of molten zinc at a galvanizing plant. The zinc reacts with the steel and forms zinc-iron alloy layers metallurgically bonded to it, topped with pure zinc. Requirements are set by ASTM A123 (fabricated products) and ISO 1461.
• Cold galvanizing. Despite the name, it is not galvanizing in the metallurgical sense: it is the application of a coating heavily loaded with zinc dust, bound by an organic or inorganic resin. As long as the zinc particles stay in electrical contact with each other and with the steel, the film provides galvanic protection. See our glossary entry on zinc-rich primer.
• Metallizing (thermal spray). Zinc or zinc-aluminum wire or powder is melted by an electric arc or a flame and sprayed onto blasted steel. The droplets lock mechanically into the profile. The process is governed by ISO 2063 and AWS C2.23; a sealer coat is often applied afterwards. The aluminum variant is described in the thermal spray aluminum entry.

Decision table: what to choose by part and by site

The first criterion is almost always the same: can the part be removed and shipped? The table below compares the three processes on the criteria that decide in practice. Cost terms are relative and qualitative; the real price depends on the job.

When is hot-dip galvanizing the best choice?

Hot-dip galvanizing is the best choice for new or removable parts that fit in the bath and can travel. It covers every face, including the inside of hollow sections when properly vented, edges and recesses, which no spray-applied process matches easily. The metallurgical bond stands up well to handling damage.

Its limits are just as clear:

• It cannot be done on site. An installed structure has to be dismantled, which is almost never realistic in an operating plant.
• It requires suitable design. Vent and drain holes for hollow sections (ASTM A385), care with thin or asymmetric parts that can distort, and with steels prone to embrittlement (ASTM A143).
• It depends on a third party. Lead time includes round-trip shipping and the galvanizing plant's schedule.

Areas damaged during erection or by welding are repaired per ASTM A780, which allows zinc-rich paint, thermal spray and zinc-based solders.

When is cold galvanizing the right answer?

Cold galvanizing is the right answer when the part is already installed, when the area to treat is limited or when touching up hot-dip galvanized steel. It is applied on site with simple equipment and can serve as a primer under an intermediate coat and a topcoat in a system designed to ISO 12944.

It needs two conditions to work. First, clean bare metal: a zinc-rich coating applied over rust or mill scale does not protect galvanically. Preparation follows the product data sheet and ASTM A780 for touch-ups, usually by blast cleaning or power tool cleaning to bare metal (SSPC-SP 11). Second, an adequate and verified dry film thickness, because that sets the amount of zinc available.

Its main limit is durability on its own in severe exposure: the film is thinner than hot-dip galvanizing or metallizing. In aggressive environments, it is topcoated with a full system. Our article on maintenance painting of steel structures shows how to fit it into a maintenance program.

When should you choose zinc metallizing?

Metallizing is the logical choice when you want a thick zinc layer, comparable to hot-dip galvanizing, on a structure that cannot be dismantled: bridge, conveyor structure, building frame, tank, pipeline. It has no size limit and does not heat the steel enough to distort it.

In return, it needs the most demanding preparation: abrasive blasting to a high grade (SSPC-SP 10 or SP 5 depending on the specification, Sa 2½ or Sa 3 under ISO 8501-1) with an angular profile, then prompt spraying before any contamination. AWS C2.23 describes the checks: thickness, pull-off adhesion and bend tests on coupons. A sealer closes the porosity, and a topcoat can be added for colour or a particular exposure. Because the work is done in place, the site conditions that govern blasting (humidity, dew point, access, containment) also govern metallizing, and they should be included in the planning from the start. Abrasive blasting is therefore an integral part of the job.

Can zinc and paint be combined?

Yes, and it is often the best solution in severe exposure. A duplex system (zinc by hot-dip galvanizing or metallizing, then paint) lasts longer than either layer alone, because the paint slows zinc consumption and the zinc stops rust from creeping under the paint at damaged spots.

Two precautions apply. The zinc surface has to be prepared to the paint system's requirements (cleaning, light abrasive sweep or a compatible tie coat), because the wrong paint on fresh zinc can peel. And metallizing must be sealed before topcoating. Selecting the full system falls under our protective coatings service. In immersed or buried environments, cathodic protection is sometimes better suited than applied zinc.

How to get a recommendation and a quote

To recommend the right process, we need to know whether the part is removable, its dimensions, its exposure, the surface condition and the work window. Send photos, dimensions and the service environment; we will indicate the process that fits and, for cold galvanizing or metallizing on site, a quote within 24 business hours. Pricing is on consultation.

Our crews and qualified subcontractors work from LaSalle, in Montreal, across Quebec and Ontario, including Montreal, the Côte-Nord, Abitibi-Témiscamingue and Ontario. The service is described on the cold galvanizing page. To send your documents, use the request a quote form.

Frequently asked questions

Is cold galvanizing as durable as hot-dip galvanizing?

Not on its own. The zinc film is thinner and bound by a resin rather than metallurgically. Topcoated with a full system, however, it provides solid protection on a part that cannot be removed.

Can an installed structure be hot-dip galvanized?

No. Hot-dip galvanizing requires immersion in a bath at the plant. On an installed structure, the options are cold galvanizing, zinc metallizing or a high-performance paint system.

How is damaged hot-dip galvanized steel repaired?

Per ASTM A780, with a zinc-rich coating, zinc thermal spray or a zinc-based solder, after preparing the area to bare metal. The repair thickness should be measured and recorded.

Does metallizing replace paint?

It can be used alone, with a sealer, in atmospheric exposure. In very aggressive environments or where colour matters, a topcoat is added to form a duplex system.