Generally, in-service acid corrosion is confined to water-cooled tubes in
regions of high heat flux; slanted or horizontal tubes; locations beneath
heavy deposits; and heat-transfer regions at or adjacent to backing rings at
welds, or to other devices that disrupt flow.
Although relatively rare, a general depression of bulk-water pH may occur
if certain contaminants gain access to the boiler. Boilers using water of low
buffering capacity can realize a bulk pH drop to less than 5 if contaminated
with seawater, hydrochloric acid, or sulfuric acid.
The concern of this chapter, however, is with the more common creation
of localized pH conditions. Two circumstances must exist simultaneously
to produce this condition. First, the boiler must be operated outside of
normal, recommended water-chemistry parameters. This may happen if
condenser in-leakage occurs when seawater, or water from a recirculating
water system using cooling towers, is used. Another source of contamina-
tion is the inadvertent release of acidic regeneration chemicals from a
makeup-water demineralizer into the feedwater system.
The second condition that must exist to produce low-pH conditions is a
mechanism for concentrating acid-producing salts. This condition exists
where boiling occurs and adequate mixing is hindered by the presence of
porous deposits or crevices. Where deposits or crevices are present, a
concentration of acid-producing salts may induce hydrolysis to produce
localized low-pH conditions, while the bulk water remains alkaline.
M+Cl- + H2O -> MOHi + H
Wherever low-pH conditions exist, the thin film of magnetic iron oxide
is dissolved and the metal is attacked. The result is gross metal loss. This
loss may have smooth, rolling contours similar in appearance to caustic
gouging. The gouged area will frequently be covered with nonprotective
Two critical factors contribute to low pH. The first is the availability of
free acid or acid-p