Two Methods for Zinc Corrosion Protection
When it comes to protecting steel from corrosion using zinc, there are two primary methods: zinc arc wire spray (thermal spray zinc/metallising) and hot-dip galvanizing. Both provide effective protection but work in different ways and suit different applications.
Zinc is one of the most effective metals for protecting steel from corrosion, and the reason comes down to electrochemistry. Zinc acts as a sacrificial anode — it corrodes preferentially in place of the steel it is bonded to. Even if the coating is scratched or damaged, the surrounding zinc will continue to protect the exposed steel from rust. This principle underpins both zinc arc wire spray and hot-dip galvanizing.
At a high level, hot-dip galvanizing involves dipping steel into a bath of molten zinc, while zinc arc wire spray uses an electric arc gun to melt zinc wire and spray it onto a prepared surface. Both methods deposit a zinc layer that provides long-term corrosion protection, but the processes, the resulting coatings, and the situations they suit best are quite different.
The choice between the two methods matters most when you are specifying corrosion protection for structural steel, fabricated metalwork, or infrastructure components. Factors such as the size of the item, whether it can be transported to a galvanizing plant, whether a topcoat is required, and the environment it will be exposed to all influence the decision. In New Zealand, both methods are widely used across construction, infrastructure, and industrial applications.
The Hot-Dip Galvanizing Process
Hot-dip galvanizing involves immersing steel in a bath of molten zinc, creating a metallurgical bond between the zinc and the steel.
The galvanizing process follows a series of carefully controlled steps. First, the steel is cleaned — typically through a caustic degreasing bath followed by acid pickling to remove mill scale and rust. After cleaning, the steel is dipped in a flux solution that promotes the bond between zinc and steel. The item is then immersed in a bath of molten zinc, held at around 450 degrees Celsius. During immersion, the zinc reacts with the steel surface to form a series of zinc-iron intermetallic layers, topped with a layer of pure zinc. When the steel is withdrawn and cooled, a metallurgical bond has been established that is fundamentally different from a surface coating — the zinc is alloyed into the steel itself at the interface.
The distinctive crystalline pattern that often appears on freshly galvanized steel is known as "spangle" and is a natural result of the zinc solidification process. The appearance varies depending on the chemistry of the zinc bath and the cooling rate.
One of the practical limitations of hot-dip galvanizing is the size of the zinc bath. Items must physically fit inside the bath, which restricts the dimensions of what can be galvanized in a single dip. Larger fabrications may need to be double-dipped (immersed from each end), which can leave a visible overlap line. Extremely large structures may simply be too big to galvanize at all.
Galvanized steel can also develop white rust (technically called wet storage stain) if freshly galvanized items are stored in damp, poorly ventilated conditions before the zinc surface has had time to form its stable protective patina. Good storage practices — keeping items dry and allowing air circulation — help prevent this issue. In service, galvanized coatings perform well across a range of environments, with the rate of zinc consumption varying depending on whether the location is rural, urban, industrial, or coastal.
The Zinc Arc Wire Spray Process
Zinc arc wire spray involves melting zinc wire using an electric arc and propelling the molten particles onto the steel surface using compressed air.
In the zinc arc wire spray process, two zinc wires are fed into an arc spray gun where an electric arc forms between the wire tips, melting the zinc. A stream of compressed air then atomises the molten zinc into fine droplets and propels them at high velocity onto the steel surface. The droplets flatten on impact and build up into a dense, layered coating that mechanically bonds to the prepared substrate.
Surface preparation is critical for zinc arc wire spray. The steel must be abrasive blast cleaned to a minimum standard of Sa 2.5 (near-white metal) per ISO 8501, with an appropriate surface profile to provide a key for the molten zinc particles. Without thorough blast preparation, the coating will not achieve proper adhesion. This means that sandblasting is always the first step before zinc spray application.
One of the key advantages of zinc arc wire spray is that there is no size limitation. Unlike galvanizing, which requires the item to fit inside a bath, zinc spray can be applied to structures and assemblies of any size — they are simply coated in sections. This makes it the preferred method for large structural steelwork, bridge components, and fabrications that cannot practically be transported to a galvanizing plant.
The zinc coating produced by thermal spray is inherently more porous than a galvanized finish. For this reason, a sealer coat is typically applied over the zinc spray to close the surface porosity, improve the barrier properties of the system, and extend its service life. The zinc spray equipment is also portable, which means it can be used on-site for structures that are already erected or installed — a significant advantage for infrastructure and construction projects.
Key Differences Compared
Understanding the practical differences between these two methods helps you specify the right solution for your project.
The most fundamental difference is the application method itself. Galvanizing is an immersion process — the item is dipped in molten zinc and the coating forms through a chemical reaction with the steel surface. Zinc spray is a projection process — molten zinc particles are sprayed onto a prepared surface where they bond mechanically. This distinction drives most of the practical differences between the two methods.
Size is often the deciding factor. Galvanizing requires the item to fit inside a bath, placing a hard constraint on maximum dimensions. Zinc arc wire spray has no such limitation and can be applied to structures of any size, either in the workshop or on-site. Galvanizing requires transport to a galvanizing plant; zinc spray can be brought to the job.
When it comes to applying a topcoat such as powder coating or paint, zinc spray has a clear advantage. The slightly rough, porous surface of thermal-sprayed zinc provides an excellent key for subsequent coatings once sealed. Galvanized surfaces, by contrast, are smooth and chemically passive, which means they typically require additional surface preparation — such as sweep blasting or the application of an etch primer — before paint or powder coat will adhere reliably.
On complex shapes with internal angles, recesses, and hollow sections, galvanizing can achieve good coverage because the molten zinc flows into and around the item during immersion. Zinc spray, being a line-of-sight process, requires the operator to direct the spray gun into all areas that need coating, which demands skill and access on complex fabrications. For welded repairs and site modifications, zinc spray is more practical because it can be applied locally to repaired areas, whereas re-galvanizing would require the entire item to be re-immersed.
Cost Factors
The relative economics of each method depend on the project scale, location, and access requirements.
Galvanizing tends to be well suited to high-volume runs of standard-shaped fabrications that can be efficiently transported to a galvanizing facility. When you have a large batch of similar items that all fit in the bath, the process is straightforward and the per-item cost can be favourable.
Zinc arc wire spray is often the better fit for large or one-off structures, items that cannot be transported easily, and projects where the steel needs to be coated on-site. Transport logistics play a role — moving large fabrications to and from a galvanizing plant adds handling costs and risk of transit damage, while zinc spray can be applied at the workshop or mobilised to the project site.
Because every project is different in terms of size, complexity, access, and environment, the economics of each method should be assessed on a case-by-case basis. The best approach is to discuss your specific project requirements with a coating specialist and get a tailored quote. Contact us for a free quote and we can help you determine the most suitable and cost-effective approach for your job.
NZ Applications and Use Cases
In New Zealand, both methods see widespread use across infrastructure, commercial construction, and industrial applications.
Structural steel on bridges, pedestrian overpasses, and large-scale infrastructure projects is frequently specified with zinc arc wire spray, often in accordance with AS/NZS 2312 guidelines for the protection of structural steelwork. These projects typically involve large members that may not fit in a galvanizing bath, and the ability to apply zinc spray on-site or in a dedicated spray booth makes it the practical choice.
Fabricated steel gates, railings, bollards, and similar items are commonly hot-dip galvanized, especially when produced in batches. These are standard-sized items that transport easily and benefit from the all-over coverage that immersion galvanizing provides, including internal surfaces and hard-to-reach areas.
Coastal and marine structures in New Zealand face particularly aggressive corrosion conditions due to salt-laden air and high humidity. In these environments, the choice of zinc protection method and the specification of coating thickness are critical to achieving an adequate service life. Agricultural infrastructure — including structural steel in dairy sheds, fertiliser storage buildings, and livestock handling facilities — is also commonly protected with zinc-based systems due to the corrosive effects of moisture, ammonia, and chemical exposure.
Across all of these applications, the NZ Building Code and project specifications set out the corrosion protection requirements based on the exposure category of the location. Understanding which method and which coating thickness is appropriate for your project's environment is essential, and it is something we can advise on during the quoting process.
Which Method is Right for Your Project?
Choosing between zinc spray and galvanizing depends on your specific requirements.
Galvanizing is generally the better choice when you have standard-shaped items that fit inside a galvanizing bath, particularly for high-volume production runs of similar components. It is well suited to items that do not require a paint or powder coat topcoat, such as structural brackets, gate frames, and hardware fittings that will be left in their galvanized finish.
Zinc arc wire spray is the stronger option for large structures and assemblies that cannot be immersed in a bath, items that need to be coated on-site, and projects where a powder coat or paint topcoat is planned over the zinc layer. The combination of zinc spray plus a topcoat (known as a duplex system) provides both sacrificial cathodic protection from the zinc and barrier protection from the topcoat, resulting in a highly effective corrosion protection system.
Whatever the project, the best time to consult with a coating specialist is at the specification and design stage — before fabrication begins. Decisions made early in the process, such as designing drainage holes for galvanizing or specifying the right surface preparation for zinc spray, can significantly affect the quality and longevity of the finished coating.
If you are unsure which method is right for your project, get in touch with the Coating Lab team for advice. We can assess your requirements and recommend the most appropriate corrosion protection strategy for your specific application and environment.
Frequently Asked Questions
Hot-dip galvanizing involves immersing the entire steel item into a bath of molten zinc, creating a metallurgically bonded coating through a chemical reaction between the zinc and the steel. Zinc arc wire spray uses an electric arc to melt zinc wire, then compressed air propels the molten zinc particles onto a blast-cleaned steel surface where they form a mechanically bonded coating. The main differences are in how the zinc is applied, the type of bond formed, and the practical constraints of each method — particularly size limitations for galvanizing and surface preparation requirements for zinc spray. Contact us to discuss which method suits your specific project.
The longevity of both methods depends on several factors, including the thickness of the zinc coating, the corrosiveness of the environment (rural, urban, industrial, or coastal), and whether a sealer or topcoat has been applied over the zinc. Both methods provide long-term corrosion protection when properly specified for the exposure conditions. A duplex system — zinc spray sealed and overcoated with paint or powder coat — generally extends the service life beyond what either the zinc or the topcoat would achieve on its own. For guidance specific to your project and location, contact the Coating Lab team.
Yes, zinc arc wire spray equipment is portable and can be used on-site for structures, bridges, and large fabrications that cannot be transported to a workshop. The same surface preparation standards still apply — the steel must be abrasive blast cleaned to the required standard before zinc spray is applied, regardless of whether the work is done in the workshop or in the field. Contact the Coating Lab team to discuss your project requirements and the best approach for your situation.
Yes, powder coating can be applied over zinc arc wire spray, and this combination — known as a duplex coating system — is one of the most effective methods of corrosion protection available. The zinc spray is first sealed to close its surface porosity, then the powder coat is applied over the sealed surface. The result is a system that provides both sacrificial protection from the zinc and a hard, durable barrier from the powder coat. This approach is commonly used on structural steel, architectural metalwork, and fabricated items that need both corrosion resistance and a high-quality visual finish. Get in touch to discuss this option for your project.