CORROSION
SCIENCE
135
corrosion
studies,
the
chemical state information
from
XPS,
and the
compositional change within
the
very
thin passive
film
obtained
by-
sputter
depth profiling.
The
versatility
of a
scanning Auger microscope
fitted
with
an
energy
dispersive
X-ray detector
has
been illustrated
by
studies
of the
initiation
of
pitting
corrosion
in a
stainless steel
at
oxide
and
manganese sulphide
inclusions.
A
system
of
this type
is
illustrated
in the
next
Chapter,
(Figure
6.3,
p.
169).
In
order
to
follow
the
corrosion process
as a
function
of
time
it
is
necessary
to
re-establish
the
specimen repeatedly
in the
Auger system
for
analysis, following exposure
to the
corrosive environment. This
is
best
achieved
by the
judicious
use of
microhardness indents around
the
inclusion
group
of
interest. Although inclusions
in
steels will have good
contrast
in
optical
microscopy this
is not the
case with
electron
micro-
scopy and, until
corrosion
features
are
evident
in the
secondary electron
image,
relocation
of the
specimen
is a
very uncertain process. Hence
the
need
for
physical markings such
as the
microhardness indents.
A
typical oxide inclusion
is
shown
in
Figure 5.21
(a)
following
1 day
of
exposure
to
acidified sodium chloride solution, whilst Figure 5.2l(b)
is
the
same inclusion
after
63
days exposure.
A
complementary
set of
SAM
and EDX
images, taken
after
30
days exposure
is
presented
in
Figure
5.22.
The
micrographs
of
Figure 5.21 indicate that pitting
has
initiated,
as
expected,
at the
oxide inclusion/metal boundary,
and the
X-ray
images
of
Figure 5.22 show that
the
inclusion
is a
mixture
of
Mn/Ti/Al
oxides.
The SEM
images show
the
presence
of
corrosion
deposits adjacent
to the
inclusion,
the
surfaces
of
which
are
enriched
in
oxygen,
chlorine,
silicon,
titanium,
and
managanese,
as
indicated
by
the
Auger images. Clearly there
has
been
a
reduction
of pH
within
the
crevice
which
has
developed
at the
oxide/metal interface which
has
lead
to the
partial dissolution
of the
inclusion
and the
deposition
of
titanium
and
manganese ions
on the
adjacent regions. These zones
have
also been decorated
by
silicon which
is
thought
to be the
result
of
the
dissolution
of a
soluble silicate which
is a
minor component
of
the
oxide inclusion.
A
similar approach
has
been used
to
study
the
very early
(after
10s
exposure
to
saline solution) dissolution
of
manganese sulphide
inclu-
sions
in
steel.
The
model
of
Figure 5.23, indicates
the
reactions
and
transport processes
at, and
around,
a
pit,
as a
function
of
time, asso-
ciated
with such inclusions
and was
deduced
from
Auger analyses.