TM 5-815-1/AFR 19-6
severe pitting in stainless steels. This condition is fre-
resistant to oxidizing acid environments, but are
quently encountered in an incinerator which burns
attached by acids under reducing conditions. The
large quantities of disposable polyvinyl chloride (PVC)
equipment designer should select materials based on
materials.
individual case conditions including temperature, abra-
b. Temperature. Corrosion rates generally increase
sion, pH, etc.
with increases in exhaust temperatures. This is due to
7-6.
Auxiliary equipment
the increased mobility of ions and increased reaction
a. Gas transport.
rates. However, in cases where the corrosion process
(1) Ducts and stacks. Large boiler plant stacks
is accelerated by the presence of oxygen, increasing the
have a wind shield of reinforced concrete or
acid temperature eventually boils out dissolved oxygen,
of steel, with a separate inner flue or
rapidly diminishing corrosion rate. This is the case with
numerous flues of steel, acid-resistant brick,
Monel, a nickel-copper alloy.
and occasionally, stainless steel. The space
between the inner flue and the outer wind
depends on the existence of an adhering oxide layer on
shield may be insulated with a mineral wool
its surface. A high exhaust gas velocity can remove or
wrapping. This is to prevent the condensa-
erode the surface layer. Once removed, this layer can-
tion of acid dew on the inside of the metal
not be renewed because the oxide film is washed away
chimney, which occurs below dew point
as it forms.
temperature, and also to prevent acid "smut"
d. State of oxidation. Under reducing condition,
from being blown out of the chimney. Acid
Monel is very resistant to moderate sulfuric-acid con-
smut is a term for ash particles contaminated
centrations. Under oxidizing conditions, or in the pres-
with acid. It is heavy and tends to fall out of
ence of oxidizing ions, however, very rapid corrosion
the gas plume soon after exiting from the
occurs. The reverse is true of stainless steels which are
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