Zinc, zinc-aluminum or aluminum metallizing: which to choose?

Zinc suits atmospheric exposure and fresh water, zinc-aluminum alloy suits severe marine and industrial atmospheres, and aluminum suits seawater immersion and high temperatures. In every case, metallizing requires high-grade abrasive blasting (SSPC-SP 5 or SP 10 depending on the standard and the service), controlled thickness and, in most cases, a sealer.

Metallizing, or thermal spray, melts a metal wire with an electric arc or a flame and sprays it onto blast-cleaned steel. It gives a thick metallic layer on an installed structure, with no dismantling and no size limit. The choice of metal, however, decides how long the protection lasts. This guide helps maintenance managers and project managers choose in Quebec and Ontario, alongside our comparison of cold galvanizing, hot-dip galvanizing and metallizing.

How does metallizing protect steel?

Metallizing protects steel in two ways: as a barrier, because the metal layer isolates the steel from the environment, and galvanically, because zinc and, in some environments, aluminum corrode in place of the steel at damaged spots. The share of each effect depends on the metal and the environment.

• Zinc. Strongly anodic to steel, as our galvanic series table shows. It protects scratches and holidays, but is consumed faster in acidic or highly alkaline environments.
• Zinc-aluminum alloy. Combines the galvanic protection of zinc with the stability of aluminum's corrosion products, which slow consumption of the layer.
• Aluminum. Forms a stable oxide layer that mainly acts as a barrier. In seawater, it keeps a useful galvanic action. See the thermal spray aluminum entry.

The sprayed layer is porous. That is why a sealer is almost always applied afterward, and sometimes a topcoat.

Which metal for which environment?

The service environment is the deciding criterion. The table below summarizes generally recognized trends; the project specification, ISO 2063 and SSPC-CS 23.00 / AWS C2.23 set the metal, thickness and sealer for each service.

Thermal spray aluminum is often chosen for insulated steel and hot equipment, as explained in our article on corrosion under insulation. For an atmospheric structure in a plant, zinc or zinc-aluminum are more common. In permanent immersion, metallizing can also be combined with cathodic protection.

What surface preparation does metallizing require?

Metallizing requires high-grade abrasive blasting, with an angular profile deep enough for the droplets to anchor mechanically. The required grade (white metal SSPC-SP 5 or near-white SSPC-SP 10) depends on the service and is set by SSPC-CS 23.00 / AWS C2.23 and the project specification, which decides.

Three points set metallizing preparation apart from preparation for paint:

• The abrasive. It has to be angular and clean to create the required surface profile; a rounded abrasive is not suitable. Profile is measured (for example per ASTM D4417) and recorded.
• Timing. Spraying has to follow blasting before the surface is recontaminated or oxidizes. If flash rust appears, the surface is blasted again.
• Ambient conditions. Surface temperature has to stay above dew point by the margin the standard sets.

Cleanliness grades are detailed in our NACE 2 / SSPC-SP 10 surface preparation guide. Abrasive blasting is therefore a major step of the job, planned together with containment and waste handling.

Is a sealer and a topcoat needed?

A sealer is almost always recommended, and a topcoat is added depending on the environment or the desired appearance. The sealer penetrates the porosity of the sprayed layer, slows moisture ingress and extends the life of the metallizing; the topcoat forms a duplex system.

• Sealer. A low-viscosity product applied as a thin coat shortly after spraying, to penetrate the pores rather than build a thick film. It has to be compatible with the metal and the service temperature: a heat-resistant sealer is needed on hot equipment.
• Topcoat. A compatible paint system applied over the sealer, for colour, UV resistance or a particular chemical exposure. It is part of a complete protective coatings system, often defined per ISO 12944.

One watch point: a thick topcoat applied directly over unsealed metallizing can trap air and blister. Sequence and thicknesses follow the specification.

Which standards and checks apply?

Two references govern most projects: ISO 2063, which defines thermally sprayed zinc, aluminum and alloy coatings, and SSPC-CS 23.00 / AWS C2.23, the joint North American standard covering preparation, application, sealing and inspection. The specification states which one applies.

Usual checks include:

• Before spraying. Surface cleanliness and profile, ambient conditions, abrasive condition, time since blasting.
• During and after spraying. Layer thickness by non-destructive measurement, following the standard's sampling plan; recorded in a dry film thickness log.
• Adhesion. Pull-off tensile testing using the specified method (for example ASTM D4541), on the structure or on witness panels.
• Bend test. On coupons prepared and sprayed under the same conditions, to check layer quality before or during the job.
• Sealer and topcoat. Thickness, appearance and recoat times per the data sheets.

These checks form a file that stays with the structure. It supports future inspection and maintenance planning.

Which mistakes cause metallizing to fail?

Most metallizing failures come from preparation and sequencing, not from the metal chosen. The sprayed layer only performs if it is anchored to a clean, angular profile and sealed in time.

• Insufficient profile or a worn abrasive. The layer bonds poorly and can lift in sheets, sometimes long after the job.
• Too long between blasting and spraying. Moisture, dust or flash rust settles on the steel and weakens adhesion.
• Spraying below dew point margin. Condensation, even invisible, compromises the bond of the first passes.
• Uneven thickness. Thin areas are consumed first; overly thick areas can crack or lose adhesion. The thickness range comes from the specification.
• No sealer, or the wrong one. The porous layer lets moisture in faster, and an unsuitable sealer can fail at service temperature.
• Edges, welds and connections neglected. Sharp edges and weld spatter are dressed before blasting, otherwise the layer is thin and fragile there.

Our article on coating failure modes shows how to diagnose a coating that is lifting or blistering.

How to choose and get a quote

You choose by starting from the service environment, then checking temperature, insulation, whether the required blast grade is achievable and the work window. If high-grade blasting is not possible, metallizing is not the right answer: a zinc-rich paint system suited to the achievable preparation should be considered instead.

Our cold galvanizing page presents the zinc protection options, from zinc-rich paint to metallizing. Our crews and qualified subcontractors work from LaSalle, including Montreal, the Côte-Nord and Ontario. Send photos, dimensions, the service environment and operating temperature through the request a quote form: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Is zinc metallizing the same as hot-dip galvanizing?

No. Both deposit zinc, but hot-dip galvanizing is done by immersion at the plant and forms zinc-iron alloys, while metallizing sprays zinc on site and anchors mechanically to the surface profile.

Why choose aluminum over zinc?

Because in seawater, in the splash zone and at high temperature, aluminum is consumed more slowly and holds up better. Zinc remains more common for atmospheric structures.

Can metallizing be done without abrasive blasting?

No. The sprayed layer anchors mechanically to the angular profile created by blasting. Without that profile and high-grade cleanliness, adhesion is not assured.

How is damaged metallizing repaired?

The area is prepared to the required grade, then the same metal is sprayed again, or a zinc-rich coating is applied if the specification allows, before sealing. The repaired thickness is measured and recorded.