14
ii.
Calculating
Corrosion
Rate
As
explained
previously,
ER
probes
and
instruments
report
Metal
Loss
.
However,
the
value
that
is
of
ultimate
interest
is
Corrosion
Rate.
The
Corrosion
Rate
is
essentially
Metal
Loss
over
time,
so
the
Corrosion
Rate
can
be
calculated
using
the
following
formula:
Corrosion Rate mpy
∆MetalLoss mils
∆Time days
365
where:
∆
MetalLoss(mils)
is
the
difference
between
two
Metal
Loss
readings
Time(days)
is
the
time
difference
between
those
two
readings
It
is
a
common
practice
to
program
this
formula
into
the
control
(DCS/PLC)
system
and
have
it
calculate
Corrosion
Rate
on
a
continual
basis
from
the
Metal
Loss
data.
The
challenge
in
doing
this
is
selecting
an
appropriate
time
interval.
Using
an
interval
that
is
too
short
may
give
erratic
results,
while
selecting
an
interval
that
is
too
long
may
give
results
that
are
insensitive
to
system
upsets.
The
ideal
time
period
depends
on
many
factors,
and
will
vary
from
system
to
system.
It
may
take
some
trial
and
error
to
settle
on
the
best
time
period
for
your
installation.
In
some
cases
it
may
be
necessary
to
review
the
raw
Metal
Loss
data
and
manually
apply
the
Corrosion
Rate
formula
to
periods
of
interest.
For
example,
look
at
the
graph
below
and
see
how
much
the
calculated
Corrosion
Rate
can
vary
depending
on
the
time
period
used.
Figure
8.
Corrosion
Rates
calculated
from
Metal
Loss
data
While
each
of
the
results
is
valid
for
the
selected
time
period,
the
one
of
most
interest
is
the
value
of
11.81
mpy
which
represents
some
type
of
system
upset.
When
the
Corrosion
Rate
is
calculated
automatically
on
a
pre
‐
selected
time
period,
there
is
no
guarantee
that
the
selected
time
period
will
always
coincide
with
system
upsets
such
as
this.
That
is
why
manual
review
and
interpretation
of
Metal
Loss
data
is
also
helpful.
