Cold-end corrosion will occur wherever the temperature of metal drops
below the sulfuric acid dew point of the flue gas. Most problems caused by
cold-end corrosion occur in relatively low temperature boiler components
such as the economizer, air preheater, induced-draft fan, flue-gas scrub-
bers, and stack.
In most combustion systems, flue-gas temperatures can range from 300O0F
(165O0C) in the flame to 25O0F (1210C) or less at the stack. This tempera-
ture change can cause numerous chemical and physical changes in the
components of the flue gas. Among the most troublesome changes is the
reaction between water vapor and sulfur trioxide to form sulfuric acid.
As flue gas cools, vapor-phase sulfuric acid forms. If the sulfuric acid
vapor contacts a relatively cool surface, it may condense as liquid sulfuric
acid. The temperature at which sulfuric acid first condenses (sulfuric acid
dew point) varies from 240 to 330 ° F (116 to 166 ° C) or higher, depending on
sulfur trioxide and water-vapor concentrations in the flue gas. The corro-
sion resulting from condensation of sulfuric acid on metal surfaces is
termed "cold-end corrosion" because it generally affects the cooler regions
of the combustion system.
In general, the problem is associated with the combustion of fuel that
contains sulfur or sulfur compounds. Sulfur in the fuel is oxidized to sulfur
trioxide in the following sequence:
S + O2 -»SO2
A small fraction (1% to 3%) of the sulfur dioxide produced is additionally
oxidized to sulfur trioxide by direct reaction with atomic oxygen in the
SO2 + O -» SO3
Catalytic oxidation to SO3 is also possible if ferric oxide, vanadium pen-
toxide, or nickel is present:
SO2 + %O2 + catalyst -> SO3
The quantity of sulfur trioxide and moisture in the flue gas affects the
temperature at which the dew point is reached. The graph in Fig. 12.1
illustrates the general relationship between sulfur trioxide concentration
and dew point.
Corrosion may occur