Coating
for Locks and Dam GatesExcerpt 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

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.