Page 284 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

262 CORROSION CONTROL AND PREVENTION

A coating is selected on the basis of the location within the tank: bottom, water layer,
product exposed, vapor space and roof structure. In addition, coatings are sometimes
used to maintain product purity. Often the internal bottom surface must be able to
withstand the abrasive effects of slurry movement caused by internal flow patterns,
mixers, or inlet and outlet flows or by mechanical action, such as the movement of
roof drain hoses lying on the tank bottom. A benefit of a bottom liner is that it reduces
the cleaning effort when the tank is removed from service for repairs or for inspection.

4.15.2 Aboveground Storage Tanks – External Coatings

Painting the exposed external surfaces of an AST provides corrosion protection,
improved appearance, and reduced evaporation loss. Selection of coating type
depends on the tank operating temperature and the presence of insulation that
contains minerals and salts that may cause corrosion. External coatings must be
able to withstand the effects of weather, ultraviolet light, and industrial or marine
atmospheres.

4.15.3 Aboveground Storage Tanks – Cathodic Protection
AST forms have a network of CP rectifiers and anodes to protect the tank bottoms.
The design of CP for new or existing ASTs can be done according to the API Rec-
ommended Practice for CP of ASTs (41).
Design considerations include the proximity to other metallic structures and
existing CP systems, the type of grounding, the estimated remaining service life
of the tank, the type and temperature of the stored product, the amount of product
stored, the cycling rates, the method of tank bottom plate construction, the type of
tank foundation, the type of secondary containment, if any, and the backfill soil
characteristics.
There are two types of CP: (i) sacrificial anode CP, by zinc or magnesium ribbon or
ingot anodes; (ii) impressed-current CP, using perimeter, deep-buried, angle-drilled
anodes or vertical, loop or string undertank anodes. Depending on the above parame-
ters, the CP type, and the diameter of the tank, CP installation costs for an AST tank
bottom may range from $10,000 to $25,000 per tank and the average cost may be
$15,000.

4.15.4 Underground Storage Tanks – Corrosion Control
Corrosion control of the external surfaces of USTs (42) can be achieved with a combi-
nation of CP and dielectric coating. However, the external coating should be applied
when the tank is new. A buried tank cannot be retrofitted with an external coating
unless it is removed from the ground. Internal corrosion protection, when required
because of contamination or corrosive products, is commonly protected with an inter-
nal liner and at times in combination with galvanic CP.