Page 290 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

268 CORROSION CONTROL AND PREVENTION

Phosphates and silicate corrosion inhibitors have been used with or without pH
control, to reduce the metal release and to prolong the service life of distribution sys-
tems or domestic installations. When the concentration is limited, the inhibitors may
not avert localized corrosion such as pitting or the corrosion of galvanized steel, steel,
cast iron, copper, or lead, sufficiently to extend the life of the system beyond 75–100
years. Corrosion inhibitors are useful when concerns about water quality deteriora-
tion have to be resolved. Unfortunately, there is no simple solution for balancing water
quality, health risks, system reliability, and environmental impact.

4.18.4 Corrosion Control in Water Storage Systems
After treatment of raw water in treatment facilities, clean water may be stored in
aboveground or underground tanks or underground clear wells. If left unattended,
both internal and external corrosion may pose a structural risk because of loss of wall
thickness. Hence the need for a periodical inspection of water tanks and towers. With
periodic maintenance, water tanks may have a life of over 100 years.

4.18.4.1 Internal Corrosion of Storage Towers and Tanks The predominant
forms of internal corrosion are general corrosion, galvanic corrosion and MIC in
standing water. The microbiological contaminants are regulated under the surface
water treatment rule (SWTR) and the total coliform rule (TCR). Corrosion control
for these types of corrosion is CP and lining or painting of the interior of the tanks.
CP is usually performed on a project basis, while painting generally is performed as
part of a long-term maintenance program.

4.18.4.2 External Corrosion Because of Weather Conditions External corrosion
originates from moisture, rain, and changes in weather. Generally, water tanks and
towers are designed with the so-called corrosion allowance, which is an allowable rate
of corrosion. The thickness of walls of the tanks is measured from time to time and the
corrosion rate determined. If the tanks are originally designed to withstand the loss in
thickness because of corrosion with time the tank is considered to be structurally fit
for service. The common corrosion control method consists of painting the tower or
storage tank. Deterioration of the appearance of water towers by external corrosion
is another consideration for painting the towers.
The costs of corrosion control for water storage tanks are determined by the type
of CP and the type of coatings utilized. Comparative case studies of the economics of
corrosion protections systems showed that it costs large sums of money to recoat and
repair interior coatings while CP would mitigate corrosion activity and prolong the
necessity of coating maintenance. The corrosion control method consists of painting
the towers to prevent its deterioration because of corrosion. Using economic models,
Robinson determined that long-term cost benefits can be realized by the application
of CP to water storage tanks (48).