Chemical Engineering June 2015 - 38

FIGURE 4. The honeycomb structure
observed on the surfaces
inside the cave confirms MIC. This
is a scanning electron microscopic
view at 1,800x magnification
at a velocity below 3 ft/s, the acid can be
safely handled using carbon-steel pipe.
at higher velocities, the protective ironsulfate
layer is stripped away from the
pipe wall and corrosive failure occurs in
a short time.
one notable exception to the above
is microbiologically influenced corrosion.
Bacteria like stagnant water. flowing
water inhibits mic.
Temperature
the basic effect of temperature on the
corrosiveness of an environment is well
known by most process and project engineers.
as a rough estimate, the corrosion
rate doubles for every 10°c increase
in temperature. this is essentially true up
to the boiling point. if the operation is at
a temperature above the dew point, in a
gaseous state, the corrosion rate is essentially
nil.
the above statements explain why
condensing a corrosive gas results in corrosive
attack that would not occur if the
operation is kept above the dew-point
temperature.
the effect of temperature must be
carefully assessed for each environment.
many material failures occur because the
temperature evaluated was the mass
temperature of the corrosive material in a
vessel, not the higher inside-surface temperature
of a steam-jacketed vessel.
Cleaning/sanitizing operations
consideration should be given to the
procedure and frequency with which
the process equipment will be cleaned
and sanitized. it is not unusual for the
cleaning operation to corrode processing
equipment. periodic cleaning with
38
steam and cleaning agents can corrode
processing equipment even though the
basic production environment is noncorrosive.
when designing for a specific
operating environment, don't neglect the
cleaning operation.
Surface finish/passivation
surface finish is an important consideration
particularly when dealing with stainless
steels used in the pharmaceutical
and food-processing industries. the surface
finish must comply with u.s. food
and drug administration (fda; washington,
d.c.; www.fda.gov) requirements to
minimize the possibility of any bacterial
growth on surfaces.
another important finish consideration
for stainless steel is the issue of surface
passivation. process engineers are familiar
with the passivation of stainless steel
with an oxidizing acid, such as nitric or
citric acid. the passivation treatment produces
a passive protection chrome-oxide
film on stainless steels. this chromeoxide
film is responsible for the overall
corrosion resistance of the commonly
used austenitic stainless steels, such as
304 and 316. there is some controversy
over the necessity for using a passivation
treatment, either initially or at intervals
while the equipment is operating. it is this
author's opinion that the main reason for
passivation is not necessarily to form the
protective chrome-oxide film but rather to
thoroughly clean the surface of any deposits
on the surface that could initiate
pitting corrosion of the stainless steel.
normally, stainless steel will self-passivate
with intermittent exposure to the oxygen
in the atmosphere. if there is no intermittent
exposure of the stainless-steel surChemiCal
engineering www.Chemengonline.Com june 2015
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Chemical Engineering June 2015

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