stainless steel tubing, ladders, safety rails nozzles,
noncompatible weld materials, and the like.
Te exterior surfaces of the tank shell and roof
exposed to the atmosphere are best protected from
corrosion by the use of protective coatings. Many
coating systems that have been successfully used
include epoxies, polyurethanes, and alkyds, some
of which incorporate zinc-rich primers. Section
4.3 of AWWA D102-14 “Coating Steel Water-
Storage Tanks” describes seven outside coating
systems. For the external surfaces of a f at bottom
tank, consideration should be given to the appli-
cation of cathodic protection to prevent corrosion
of the soil side. Accelerated corrosion can occur
because of corrosive soils or tank pad material,
high moisture content, or connection to copper
grounding. Applying a protective coating alone to
the soil side of the bottom plates is not an e fec-
tive alternative because the coating will be dam-
aged during placement and welding. Information
on tank bottom cathodic protection can be found
in NACE International’s RP 0193-2001, “Exter-
nal Cathodic protection of On-Grade Carbon
Steel Storage Tank Bottoms.”
Inside the tank, corrosion is even more challeng-
ing. Connection (metal-to-metal contact) to cop-
per, brass, and stainless steel appurtenances must
be avoided. Protective coatings and cathodic pro-
tection should both be used. Coating the inside
of a tank is not an easy process. Proper surface
2
preparation is essential. Humidity and temperature
for proper cure must be closely monitored, recog-
nizing that the side of the tank exposed to the sun
can be quite a bit hotter from radiant heating. Sur-
face preparation and quality application of coating
the roof, support columns, and purlins is di f-
cult because of impediments to inspection, includ-
ing access, crevices, and sharp edges. Section 4.4
of AWWA D102-14 “Coating Steel Water-Storage
Tanks” describes fve inside coating systems con-
sisting of epoxies, polyurethanes, or polyurea. Te
discussion that follows addresses corrosion and
cathodic protection of the internal wetted steel sur-
faces of carbon steel water storage tanks.
CORROSION OF INTERNAL
WETTED SURFACES
In fresh water tanks, corrosion activity on
internal wetted surfaces usually results in concen-
trated pitting attack, which leads to quicker wall
penetration than if the corrosion was more uni-
formly distributed on the metal surface. Tis is
particularly true on tank interiors that are coated,
where the corrosion attack is accelerated at holi-
days or voids in the coating. Te attack is initiated
by the development of anodic and cathodic areas
on the submerged metal surfaces. Te anodic
areas (e.g., location of coating holiday) will suf-
fer accelerated corrosion (metal loss), whereas the
cathodic areas will not corrode (see Figure 1-1).
3
FIGURE 1-1 Anodic and Cathodic Areas on Tank Wall
Te corrosion is often made even worse by the
small anode-large cathode area e fect (see Coating
Pinhole Corrosion).
Tere are a number of mechanisms that can
initiate and sustain corrosion of the submerged
steel in water tanks.
Uniform Corrosion
Although steel visually appears to be homoge-
nous, close inspection reveals that it is quite irregu-
lar, consisting of numerous grains of metal that are
electrically di ferent from each other. Tus, some
will be anodes, whereas others will be cathodes.
