1 March 1997
7. CORROSION CONTROL.
a. Corrosion process. Corrosion is the natural deterioration and loss of material due to a chemical or
electrochemical reaction with its environment. Electrons in metals are loosely bound to their atoms and
easily removed in the presence of an electrolyte such as water. The basic elements of corrosion are an
anode (the corroding metal), a cathode (a noncorroding metal), an electrolyte (a solution such as water
which conducts electricity), and a closure path for the current connecting the anode and cathode. In
corrosion, galvanic cells are formed in which certain areas become anodes and others cathodes. Electric
current flows through the electrolyte and metal as the anode is corroded. Corrosion does not occur if any
of these elements are missing. To prevent corrosion, all steel water structures shall have a protective
coating system, which prevents the current from flowing between the metal and electrolyte, and an
impressed-current cathodic protection system.
b. Cathodic protection. Coating systems can not be applied completely without flaws. Cathodic
protection systems arrest corrosion at such flaws by reversing the electric current and turns the metal to be
protected into the cathode. In an impressed-current system, an electrical current, from an outside source,
flows into anodes located inside the tank, through the water, and into the structure. The impressed current
system shall be designed in accordance with AWWA D104 and TM 5-811-7.
8. COATING SYSTEMS.
a. General. The paint and coating systems industry changes rapidly with new products regularly
becoming available and the use of many products being restricted or eliminated changing EPA, OSHA, or
health laws and regulations. OSHA has set highly restrictive limits on the use of products containing lead,
chromate, or mercury. When specifying a system, the number and thickness of each coat, primer, surface
preparation method, and curing conditions (temperature, time, etc.) must be given.
b. Interior. Coatings used on interior surfaces of a tank, including that of risers and all other surfaces in
contact with the water, must not add taste, odor, toxicity, or impurities to the water; must readily adhere to
the tank's surface while continuously submerged; must have a low rate of permeability; and must meet the
minimum requirements of NSFStandard 61. For potable water tanks, the systems must also be acceptable
to the appropriate regulatory agencies. Several standards are available for use as references - two of the
more popular standards used when painting steel tanks are "Coatings For Potable Water Tank Interiors" by
the Steel Structures Painting Council and AWWA D102. Common interior systems include epoxies, vinyl,
and coal-tar systems.
c. Exterior. While the exterior of concrete tanks are generally painted only for aesthetics, steel tanks
must be painted or coated to protect the tank. Exterior coating systems must be selected based on local
climatic conditions. Alkyd enamel systems are good for rural areas, mild to fairly heavy industrial
atmospheres and mild atmospheres. Vinyl, epoxies, urethane, and polyurethane systems are better for
moderately severe to the most severe atmospheres, such as seacoasts and corrosive industrial areas.
Vinyl paints have the advantage of being very fast drying and should be used in congested areas where
overspray could damage property (vinyl paint overspray from elevated tanks will dry before reaching the
ground) but fade easily and should be specified only in white or light gray colors. Polyurethanes have
widespread application and retain their color and gloss well, are highly abrasion resistant, and easy to clean
of graffiti. Silicone alkyd systems are also weather resistant, high gloss coatings with good color and gloss
retention and are resistant to many chemicals. Alkyd and silicone alkyd systems are generally not
recommended in locations subject to excessive condensation. Applicable standards include AWWA D102,
AWWA D103, and the Steel Structures Painting Council.
9. BID DOCUMENTS. Design of large tanks, standpipes, and reservoirs, especially prestressed concrete
structures and elevated tanks, is complex, demanding a wide range of special and unique knowledge and
experience. Subsequently the majority of the Government's water storage structures will be bid using
performance specifications requiring the bidding contractor to prepare detailed designs and specifications.