Zinc arc spray is widely used in industries where long-term corrosion protection is needed, especially for large or complex steel structures that are difficult to galvanize. It is frequently used in:
Bridges and marine structures, where on-site application is essential.
Oil and gas facilities, for coating pipelines and offshore structures.
Utility and transmission towers, which require protection in harsh environments.
Steel structures in industrial plants, where the process can be used as part of a duplex system, with additional coatings for enhanced durability
Twin Wire Arc Spray Process
In the twin wire arc spray process, two zinc wires are fed into a spray gun where an electric arc melts the zinc. Compressed air is then used to atomize and project the molten zinc particles onto the prepared surface of the steel. This process builds up a lamellar (layered) coating on the steel, which provides effective corrosion protection.
Surface Preparation
Proper surface preparation is critical to the effectiveness of the zinc coating. The steel must be cleaned to a near-white metal finish, typically achieved through abrasive blasting (NACE No. 2 or SSPC-SP 10). Any contaminants, such as rust, scale, or old coatings, must be completely removed to ensure good adhesion of the zinc. The surface should be sprayed within a few hours of cleaning to prevent re-oxidation, which could reduce the bond strength of the coating.
Tube Mill Applications
Arc spray is widely used in tube mills to apply protective zinc coatings for corrosion resistance, especially in harsh environments. The twin wire arc spray method is efficient for high-speed, large-scale production, offering uniform coatings over extensive surface areas. This process is ideal for automated systems, making it cost-effective for long-term corrosion protection.
Watch the video for an explanation of tube mills.
Key Benefits for Tube Mill Applications:
High-speed coating application: Twin wire arc spray can keep up with the demands of a tube mill’s high production rates.
Automation compatibility: The process can be automated, reducing labor costs and increasing consistency.
Uniform coatings: It provides a consistent, even coating over the entire length of the tube, crucial for corrosion protection.
On-site flexibility: Arc spray systems can be set up in the production line, reducing downtime and transportation costs.
When preparing steel tubes for the twin wire arc spray process, proper surface preparation and managing the temperature of the tubes are critical factors to ensure optimal coating adhesion and longevity.
Tube Surface Preparation
In the tube mill process, a scarfing process is used to prepare the seams of welded tubes before further processing, such as applying coatings like arc spray. Tube seams, especially on longitudinally welded pipes, often have excess weld material or flash on both the internal and external surfaces of the tube. The scarfing process removes this excess material to create a smooth, uniform surface along the seam. If the seam is left rough or uneven, the zinc coating may not adhere consistently, leading to potential corrosion issues in those areas.
How Scarfing Works:
External Seam Scarfing: After the tube is welded, a scarfing tool is used to remove the excess weld bead from the outside of the tube. This ensures a smooth, continuous surface, allowing coatings like zinc or aluminum to adhere properly without creating weak spots.
Internal Seam Scarfing: Some tube mills also use internal scarfing, particularly for tubes requiring strict internal dimensional tolerances or that need to withstand internal pressure. The internal weld bead is removed to create a smooth inner surface, preventing disruptions to fluid flow or stress concentrations.
Scarfing is also useful for automated systems in tube mills, as it ensures that subsequent manufacturing steps, such as bending or coating, can proceed without disruption due to inconsistencies in the tube's surface quality.
The surface preparation must be precise, as surface oxidation can quickly form on exposed steel if not coated within a short window after cleaning (usually within 4 hours). Ensuring the tube surface is dry and free of any moisture is equally important to prevent coating defects
Temperature of the Tube During Coating
Temperature is crucial for zinc adhesion in the arc spray process. Although arc spray is a "cold" process compared to hot-dip galvanizing, surface temperatures above 200°F can improve bonding strength. Preheating the steel to 200-400°F (or increasing line speed to maintain temperature after the weld process) enhances zinc adhesion by ensuring a dry surface and promoting better cohesion. The heat helps molten zinc particles bond more effectively, especially on rough, blasted surfaces, leading to a stronger, more uniform coating.
Why Higher Temperatures Help Zinc Adhesion:
Improved Bonding Strength: Higher temperatures promote better bonding of molten zinc particles, which enhances the coating’s adhesion.
Prevents Condensation: Preheating above 200°F ensures there’s no moisture or condensation that could interfere with the zinc adhesion.
More Consistent Coating: It allows for more uniform deposition of zinc, reducing the chances of defects such as peeling or blistering.
Have a new line installation or aging process in need of upgrading? We’ve worked with multiple tube mills to spec out and deliver turn-key arc spray systems, including enclosures and dust collection. Contact us to see how we can help: sales@thermach.com, www.thermach.com.
Remember, Thermach is not your competitor; your other options are. We don’t coat; we only supply thermal spray equipment and parts. You can proceed with confidence, knowing that our team is dedicated to creating a custom system that meets your exact specifications.
Manufacturer of Equipment & Parts for the Thermal Spray Industry. Backed by Reliable, Timely Service & Support.
Thermach manufactures thermal spray coating systems for the coating industry. We don't coat. We provide the equipment and parts necessary for you to run a successful coating operation. Our mission is to ensure your coating process is running smoothly with Thermach equipment.
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