Strategies to Inspect Thermal Spray Zinc Metalizing

In a technical paper presentation on Dec. 15, 2021 at Coatings+, Bernardo Duran of the International Zinc Association shared insight on “How to Inspect Thermal Spray Zinc Metalizing.” Duran, who is manager of technology and market development with the association, kept the presentation interactive and took questions from attendees.

According to Duran, thermal spray zinc metalizing and its alloys are used in many industries to provide corrosion protection. Some of these applications include bridges and other infrastructure, such as marine vessels, offshore wind towers and oil platforms, as well as application over concrete to protect the reinforcing bar. To ensure continued coating performance in these harsh environments, it is necessary to properly inspect the thermal spray metalizing.

The presentation at the Phoenix Convention Center began with a foundation examining how the surface preparation and thermal spray processes (arc spray and flame spray) are similar to and different from other corrosion protection systems, including video of the arc spray method.

ZTS_PVA from International Zinc Association on Vimeo.

Standards and specifications in the thermal spray zinc industry were discussed, including SSPC CS 23.00 (NACE No. 12) and SSPC-PA 18. The audience learned inspection criteria and became more familiar with inspection tools, techniques, and methods, including pre-job and daily operator qualifications. 

The presentation featured pictures and explanations of coating variations, and it also covered various safety requirements and personal protective equipment (PPE) needs.

Key takeaways from Duran’s presentation included:

Flame Spray and Arc Spray

• The flame spray method consists of a spray material being continuously melted in the center of an acetylene-propane-oxygen flame. Using compressed air or nitrogen, the spray particles in droplet form are accelerated onto the prepared surface.
• The arc spray method involves two electrically conductive spray material wires that are fed together at a constant speed. An arc is ignited between the two wires, which acts as an energy source for melting the spray material. With compressed air, the melt material is atomized, accelerated, and deposited on the steel.

• Application locations can be in the field or in a shop environment. Automation is increasingly becoming popular in shop applications due to its superior efficiency, especially on flat surfaces.

Health and Safety Considerations

• The arc spray method produces UV (ultraviolet) radiation and should be treated as a welding process. Metallizing requires eye, body, and ear protection, along with sunscreen. Other safety and PPE needs include a full respirator helmet with negative pressure.

• Surface preparation is critical to achieving good adhesion of the metalized coating to the substrate. The substrate must be free of oil and grease (SSPC-SP 1), as well as oxidation (abrasive blast). 
• To conduct work, the humidity should be less than 85%, while the steel surface should be at least 5 °F over the dew point.
• For immersion applications, SSPC-SP 5 (NACE No. 1) is suitable, while SSPC-SP 10 (NACE No. 2) is needed for atmospheric applications.
• A sharp, angular profile of 2.5 to 5.0 mils (65 to 125 microns) is necessary. This can be achieved using grit abrasive material.

Pre-Job Qualification, Equipment

• Equipment needs often include a compressor, surface preparation equipment, power supply, metalizing unit, and an application gun.

• Applicators should consider the appropriate distance from the surface; speed of movement and overall technique; and coating thickness specification requirements.

• Daily qualification steps include ensuring atmospheric variables are appropriate for coating, proper surface preparation, coating thickness checks, qualitative bend testing for ductility and adhesion; and quantitative adhesion testing.
• Test coupons and plates are used for daily qualification steps.

Coating Thicknesses

• Typical average coating thicknesses for zinc (Zn) or 85/15 zinc-aluminum (ZnAl) alloys are 3-6 mils (75-150 microns) for sealed and painted surfaces, and 8-16 mils (200-400 microns) for Zn or ZnAl metal-only (no topcoat).
• Each pass of the metallizing gun applies 2-6 mils (50-150 microns) of coating thickness.

After-Metalizing Inspection

• The after-metalizing inspection includes checks for coating thickness; appearance in the form of smoothness, uniformity, and any cracks, blisters, or bare areas; and adhesion. 
• Zinc should have a tensile adhesion of 500 psi minimum (3.45 MPa), while 85/15 Zn/Al should have a 700 psi minimum (4.83 MPa). Due to its nature of being a destructive test, adhesion is typically verified on the test plates used during daily qualification, but can be performed on the substrate per the specification requirements.

More information on the process is available at thermalsprayzinc.zinc.org, and Duran can be contacted by email at bduran@zinc.org.

Source: Coatings+ Show Daily, www.sspc.org/coatings-2021.