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Metalizing for Corrosion Control
Introduction to Metalizing
85-15 metallized tainter gate at Racine Locks and Dam, Ohio River
85-15 metalized tainter gate at Racine Locks
and Dam, Ohio River
Coating for Locks and Dam Gates
Excerpt from Metalizing for Corrosion Control contributed by Tim Race Vice President Corrosion Control Consultants and Labs

Water or hydraulic structures are demanding facilities to maintain. Coatings must perform reliably for long periods of time. Inspection and maintenance are infrequent because dewatering is usually required. Taking critical components off-line for inspection or recoating can be expensive. Temporary outages during maintenance and repair of navigation structures such as locks and dams can result in costly delays for shipping companies. Disruptions at hydropower facilities can lead to lost revenues. Other important types of water use structures include flood control pumps and gates, water storage tanks, water treatment plants, and irrigation projects.

Water structures are typically protected using conventional paint coatings such as vinyls and epoxies. Vinyl coatings such as SSPC Paint No. 9, Bureau of Reclamation VR-6, and Corps of Engineers VZ-108 and V-766 provide excellent long term protection in fresh water immersion. Unfortunately, traditional high performance vinyl resin coatings are becoming an orphan technology, that is to say they are disappearing from the market. This is because vinyl coatings formulated for immersion contain large amounts of strong solvents and as such they don't meet the requirements of tightening VOC regulations. Epoxy coatings, on the other hand, can be formulated to meet VOC requirements. However, epoxy coatings are much more brittle than are vinyl coatings, and are subject to mechanical damage in immersion caused by floating ice and debris. Impact and abrasion damage to coatings is a common problem for many owners of water structures. Water structures are often only partially immersed or are immersed only during certain seasons. Epoxy coatings exposed to the sunlight on these structures may not perform adequately because of their generally poor resistance to ultraviolet radiation. The Corps of Engineers and other water users have struggled in recent years to find adequate replacements for traditional paint coatings for immersion applications. Users have experimented with a number of technologies including thermal spray coatings.

Innovative project engineers and researchers in the Corps of Engineers have experimented with a wide variety of VOC-compliant technologies in recent years, including high and 100-percent solids epoxies, moisture cure, plural-spray, and multi-component polyurethanes, high-build polyureas, and thermal spray coatings. The most promising technology to emerge from these trials has been thermal spray or metalizing.

Ironically the Corps first experimented with wire flame-sprayed aluminum coatings in the 1930s. The record is sketchy, but available documents indicate that the coating outperformed all other technologies being evaluated at the time. It's unclear why the technology was not adopted at that time. However, the most likely reason was the high cost of thermal spray coating application at that time. The Corps revisited thermal spray in the 1980s, performing field trials and then publishing the organizations first guide specification for metalizing in 1992. Since that time a number of structures have been protected using flame-sprayed 85-15 zinc-aluminum alloy coating. An early controversy in the Corps guidance was the restriction on the use of arc spray for applying 85-15. Based on subsequent evaluations and improvements in commercially available arc spray equipment, Corps districts are now allowing the use of arc spray for applying 85-15 on a case-by-case basis. Arc spray has always been approved for applying aluminum and zinc metalizing.

The Corps has continued to experiment with metalized coatings and in general they have concluded that 85-15 is the preferred VOC-compliant alternative to vinyl coatings for fresh water immersion applications. For extreme exposures involving severe impact and abrasion, 85-15 is now preferred over vinyl paints. Zinc and 85-15 metalized coatings have been identified as the most practical coatings for preventing the attachment of zebra mussels on immersed fresh water structures. Aluminum thermal spray is now regarded as a superior coating for high temperature steel where traditionally high-VOC aluminum pigmented silicone paints were used.

Other water structure owners have experimented with or adopted thermal spray coatings as well. Navigation gates for the Panama Canal were recently metalized with aluminum. The Salt River Project in Arizona has been a long time user of zinc metalizing on their structures and has many valuable lessons learned to convey. The Bureau of Reclamation has used aluminum thermal spray to protect gates at major power dams in the western states. The Tennessee Valley Authority is experimenting with the use of 85-15 on trash racks.

locks and dam gates
locks and dam gates

The use of thermal spray coatings on water projects is likely to grow as owners learn to focus more on long-term performance and the life cycle cost advantages of thermal spray. A recent Federal Highway Agency report concluded that for bridge structures in severe exposure environments including marine atmospheric and de-icing salt exposures, thermal spray coatings are more cost effective than other types of protective coatings. A similar life cycle cost analysis performed by the Corps for immersed structures found that for severe exposures thermal spray coatings were more cost effective than vinyl coatings.

The water user market for thermal spray coatings promises to grow. Improved application equipment, higher awareness of the technology by owners, appropriate emphasis on life cycle cost, and better industry standards should all contribute to this growth.



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