material & processes for ndt part 5
TRANSCRIPT
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7/25/2019 Material & Processes for NDT part 5
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7/25/2019 Material & Processes for NDT part 5
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32 Materials
and Processes
for
NDT
Technology
and not
tied
closely
together.
In
this
liquid
state, the
materials
have
fixed
volume
but
assume
the
shape
of
the container
in
which they
are
placed.
As
the energy level
is further
decreased, the mobili-
ty
of
the
atoms
decreases.
There
are
at least
four
different
mechanisms
by
which the atoms can
assume
positions
well fixed
enough
that
for
practical
pur-
poses
the
material
could
be called
so/id.
Of the
materials
of
interest
to
manufacturing,
all the
metals
occur as crystalline
solids.
METALI,IC
STRUCTURE
Definition
of a
Metal.
Metals are usually defined
as materials
having
some
degree
of
plasticity.
relatively
high hardness
and
strength,
good
electric and
thermal
conductivity,
crystallinity
when solids,
and opacity.
A
definition
based
on atomic
structure is
more
pre-
cise. A
metallic
solid
is
one
that
has free
electrons
available
in
the
structure
to
carry a current
and that
has
a
negative
coefficieni
of conductivity
with
in-
creasing
temperature.
States
of Matter.
Figure
4-1
shows
the
relation-
ship
that
exists among
the
three
states of
matter
for
a
crystalline
material.
At
the
intersection
of tempera-
ture
?t
and
pressure
P1
on
the curve,
notice
that
an
increase
of temperature
of a
material
for
which
this
curve
is valid
would cause
the
material
to
change
directly
from
a
solid to
a
gas.
Similarly,
a reduction
of
pressure
(a
shift toward
the left)
would
also cause
the
same
change.
Such
a
change
of
state
from solid
directly
to
gas
is
known as
sublimation. Arsenic is
the
only
metallic
material
that
sublimates
at atmospheric
pressure.
When
the
temperature
is
raised
to
T2 at
pressure
P2,
the
atoms
of the
material
will
become
sufficiently
active
that
a change
is
made from a solid
to
a liquid.
A further
increase
in
temperature
at this
same pressure
to
point
73
will
cause a second
change
Pl
Px
PRESSURE
------f
Figure
4-1
States
of
matter
from a liquid
to a
gas.
The
intersecting
point
of the
curves
at
the
temperature
Ty
and.
pressure
Ps
is
known
as the
triple
point
and occurs
at
the
tempera-
ture
and
pressure
conditions
under which a material
may
exist as a solid, a liquid, a
gas,
or
partially
all
three
at the same time. For
most
metals, this
point
occurs
below normal
temperatures
and
well
below
atmospheric
pressure;
consequently,
most
metals
upon
being
heated go through the
changes
from
solid
to liquid
to
gas
as
the temperature increases.
Space
Lattices.
As the energy
of
a liquid
metal is
reduced
by
taking
away heat,
the
attraction
between
atoms
increases
until
they
arrange themselves
in
definite
three-dimensional
geometric
patterns
that
are
characteristic
of
the metal.
These
structures
are
called
space
lattices
and
consist
of nctwork
groupings
of
identical
ttnit
cells
that
are aligned
in
parallel planes.
There
ere fourteen
types of
crystal latl,ices,
but
rnost of
the common
and
commercially
important
metals
exist,
in
the
solid
state,
in
one
of
three
struc-
tures.
These
are,
as
shown
in
Figure
4-2,
body-cen-
tered
cubic, face-centered
cubic, and hexagonal
closed-
packed.
In the illustrations
ofunit
cells,
the
dots repre-
senting
atoms
should be
considered
as
centers
of
activ.
ity for
the
atoms
and
not
as
graphic
illsutrations
of the
atoms
themselves.
I
/l / \
---l-r---'a
/\
\
EOOY-CENTERED CUBIC
LATTICE
FACE
-
CENTERED
OUBIC
LATTICE
\r-A
iJ,t..,'i
i,r,l l,
,i
,,
\t-;.;--)..',
T
I
I
lrj
E
:f
k
(E
lrj
o-
=
d
F
HEXAGONAL
CLOSE.PACKED LATTICE
Figure
4-2
Common
metallic
space lattice
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7/25/2019 Material & Processes for NDT part 5
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7/25/2019 Material & Processes for NDT part 5
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34
Materials
and Processes
for
NDT
Technology
costs
will
be added.
The methods.
other than
solidifi-
cation,
that
can
be
used
for
grain-size
control
involve
solid-state
changes.
As has already
been
indicated, coarse
grains
in
the
harder materials
have
lower
strength
than
fine
grains.
Coarse-grained
materials
machine
more
easily,
requir-
ing
Iess
power,
although
the
quality
of
surface
pro-
duced will
not be
as
good
as
with
a
finer-grained
material.
Coarse-grained
ferrous material
is
easier
to
harden
by
heat treatment
than
fine-grained
material
of
the
same
composition
but
has
increased
suscepti-
bility
to
cracking
under
the thermal
loads.
Coarse-
grained
material
will
caseharden
on
the surface
more
readily
than
fine-grained.
It
is
evident,
then,
that
coarse
grains
may
sometimes
be
desirable
during
processing,
but
fine
grains
are usually
necessary
in the
final
product
to
provide
the
best
mechanical
proper-
ties.
Some deformation
processes
of
shaping tnaterials
can be used
so as
to
cause
grain-size
reduction
auto-
matically durir,g
the
shaping
process with
little or
no
additional
cost involved.
SOLID STATE CHANGES
IN
METALS
In
the
previous
section
the
process
of
metal
solidifi-
cation was briefly
described.
The
properties
of a
mate-
rial
are
derived
from the crystalline
structure,
includ-
ing the atomic
arrangement and
the
crystal
sizes,
and
are
affected by
the
boundary
layers that
join
the
grains
together. The atomic
arrangement
is
primarily
a
func-
tion
of
the
material
composition, which may consist
of
a
single
material or
a
combination
of
materials
that
are
completely
soluble,
partially
soluble,
or
totally
insolu-
ble
in each other
in the solid
state.
The
structure
and
grain
size also
may be
influenced by
the operating
tem-
perature
changes
and by
mechanical
loads
that
stress
the
material sufficiently
to
cause
plastic flow
in
com-
bination
with
time and
heat effects.
Some materials,
particularly
those
that
are
cast
to
shape,
may be used with
the
structure
in
which
they
solidify,
but
some
of
the
cast materials
and
nearly
all
metals
processed
by other
methods
are
treated
in
some
way
in
the
solid
state
to
obtain
improved
mechanical
properties.
These treatments
include
work
hardening,
recry-
stalization,
age
hardening,
and
heat treating
of
allo-
tropic
materials
to
cause crystal
transformations.
In
many
cases,
treatment
may
be
inherent
in
the
process.
This may be beneficial,
as
in many
cases
of
deformation shaping
with
associated
work
hardening,
or may be detrimental,
as
in other
cases
in which
cold
working
develops directional
properties
in
a material
to
make some
kinds
of
further
cold work
difficult
or
impossible.
WORK
HARDENING
Effects
of
Deformation.
The
application
of
loads
to a
solid
material
in
processing
or
in
service
can
cause
two
kinds
of
deformation.
If the
load
does
not
stress
the
material
past
its
elastic
limit,
the
deforma-
tion
is
"elastic,"
and
the
material
returns
to
its
original
position
upon
removal
of
the
load' If,
how-
ever.
the
elastic
limit is exceeded,
the
material
does
not
return
completely
to
its original
position
when
the
load is removed
and
is
permanently
deformed
by
plastic
flow within
its
crystalline
structure.
When
the
elastic
limit
is
passed,
elastic
properbies
are
not
lost,
but
instead
are
enhanced,
providing the deformation
is
produced
by
cold
work.
The
strength
of
metal
is
increased
by
plastic
flow
and
the
elastic
Iimit
is
raised.
Some
of
the
deformation
processes
produce
improved
properties at
the same
time
the
shaping
is
being
performed.
PLASTIC
DEFORMATION
Permanent
deformation
of
metallic
crystals
occurs
in
three ways:
slip,
twinning,
and
rotational
deforma-
tion.
The degtee
of
each is dependent largely
on
the
characteristics
of the
particular
metal.
UNSTRAINEO
GRAIII
SLIP DEFORIIED
GRAIN
Figure 4-3
Slip
Slip
Deforrnation.
Slip deformation
is
illustrated
in
Figure 4-3 and
occurs
by
translation or sliding
be-
tween the
atomic
platres
within
a
grain.
If
the defor-
mation causes
more
than
a
very minor
shift,
a
large
number
of
atomic
planes
in
each
grain
will
slide
over
adjacent
planes
to
occupy
new
locations
with
new
neighbors.
The
planes
through
the
crystal
that
are
usually
most
subject
to slip
are those
of
the
greatest
atomic
population
and
gteatest
distance
between
planes.
The
orientation
of the
planes
along which
slip
takes
place
most
easily will,
of
course, be
different
for
different
types
of
crystal
lattices.
Because
of
the
usual random orientation
of the
crystals,
the
slip
planes
of
many
will
not
be
in
line
with
the direction
of
loading.
When
the
best
slip
planes
are
completely
-
7/25/2019 Material & Processes for NDT part 5
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7/25/2019 Material & Processes for NDT part 5
6/14
36
Materials and
Processes
for
NDT
Technology
by treatment
for
recovery,
most
of the
distorted
crystalline
lattice
remains
as
it
was
produced
by
cold
work.
The
elastic
limit
for
the
material
has
been
raised close
io
the ultimate
strength,
and
further
deformation
will cause
fracture
failure. Recovery
of
ductility
to
permit
further
change
of shape
by defor-
mation
can
be obtained
only
by
elimination
of
the
deformed
grains,
and
this can
be
accomplished
by
recrystallization.
By this
heat-treating
process'
new'
smaller,
unstrained
glains
with
fully
recovered
capac-
ity
for
plastic
flow
can be
formed
by
solid-state
change
in
the metal.
It
is
important
to
note
that
in
the
absence of
allotropic
changes,
which
will
be dis-
cussed
later,
no
grain-size
changes
by heating
metal
to
any
temperature
below
the
melting
point
can
be
accomplished
unless
the
strained
condition
of
cold-
worked
metal
is
present.
Recrystallization
is the
nucleation
and
growth
of
new,
strain-free
crystals
from the
strained
crystals
of
a
cold-worked
material.
Recrystallization
Temperatures.
The
phenomenon
occurs
over a wide temperature
range
with
the
length
of
time
required
for complete
recrystallization
in'
versely
related
to
the
temperature
and
to
the
degtee
of
strain
present.
For
practical
purposes,
recrystalli-
zation
temperatures,
such as
shown
in
Table
4-I,
ate
temperatures
which
will
permit complete
recry-
stallization
in
a
time
period
of
approximately
t
hour
for
metals
that
have
been
fully
hardened
by
previous
cold work.
TABLE
4-1
Recrystallization
Temperatures
for
Some
Common
M"t"lt
""d
All"ys
Material
Theory
of Recrystallization.
It is
belierud
recrystallization
takes
place
by
the nucleation d
grains
mainly
about
the
high
energy
points
of
cation
in
a
work-hardened
grain.
They then
grow
until
they
fill the
old
grain
space and
elimuqH
the
existing
strain
by
realignment
of
the
atoms
into
e
new
crystal
lattice. Recrystallization
can
thus
be
a
grain-refining
process
as
well
as
a
method
for
recovery
of ductility,
if it
is discontinued
as
soon
as
complete
recrystallization
has taken
place.
The new
gSains
formed during
recrystallization
are
likely to
take
positions
with
preferred orientations'
Directional
properties
caused
by
preferred
orientation
are
objectionable
for most
manufacturing
operations.
This
tendency
can
be reduced
and
more
random
orientation
obtained
by
the
addition
of
small
amounts
of
an
alloying
element
or
by
recrystallizing
before
maximum
work
hardening
has
been
per-
formed.
Recrystallization
Seldom
Terminal.
In a
few
cases,
recrystallization
may be
used
as
an end
process
to
leave a
product
in its
most
ductile condition
or
with
its
best
electrical
and
chemical
properbies,
but
more
often
it is an
in-process
treatment
for ductility
improvement
or
for
grain
refinement.
In
many
cold
deformation
processes,
such as deep
drawing,
the
ductility
of the
material
may be
reduced
by
cold
working
to
the
point
where
fracture
failure is
immi-
nent.
Ductility
may
be
retumed
to
the
material
any
number of
times
by repeated
recrystallization
be-
tween
steps
of
the
forming
operation.
In
most cases
tire
last
forming
operation
will
not
be
followed
by
recrystallization,
in order
that
the
higher
hardness
and
strength
of
the
cold-worked material may
be
re-
tained
in the
product.
Although
heating
for
recovery
is
a
stress-relieving
process,
recrystallization
at
a
higher
temperature
is
sometimes
also
called
stress
relieving.
The
same
proc-
ess
may
be
referred
to as
process
annealing,
particular-
ly when
performed
in conjunction
with
deformation
processes.
GRAIN
GROWTH
If
a
metal
is
kept
heated
at
or
above
its
recry-
stallization
temperature
after the
new,
unstrained
grcins
have
formed,
the
tendency
is for
some
of
the
new
grains
to
absorb others and grow
to
larger
size.
Large
grains
are more
stable
than
small
grains
because
of
the
higher
grain-to-boundary-area
ratio,
which
is a
lower
energy
state.
If
fine
grain
structure
is desired
after
the
recrystallization
process,
it
is
necessary
to
reduce
the
temperature
quickly
to
prevent
sub-
sequent
grain
growth.
This
is usually
performed by
some
kind
of
quench.
Grain-Size Control
During
processing, small
grain
size is
not always
wanted
because
large
grains
usually
exhibit
gteater
ductility,
better
machinability,
and
require
less
pressure
to
be
deformed.
The
final
oF
"c
Aluminum
(pure)
.
Aluminum
alloys
.
Copper
(pure)
.
Copper
alloys
lron
(pure)
Lowcarbonsteel,.....
Magnesium
(purel
......
Magnesium
alloys ......
Zinc
Tin
80
316
120
316
400
540
65
232
10
-4
-4
750
1000
150
175
600
250
600
450
50
25
25
ead .
The
table
shows
that
zinc,
tin, and
lead
re-
crystallize
at
temperatures below
room
temperature.
This
means
that
these
metals
in the
pure
state
cannot,
at ordinary
temperatures,
maintain
a work-hardened
condition.
The
normal
use
of
deformation
processes
on
these
materials would
be hot
working
rather
than
cold working
since
it would
be
performed
above
their
recrystallization
temperatures.
Examination
of
the
table
also
reveals that
contamination
of
a
pure
metal
with
other
elements
makes
it
more
difficult
for
re-
crystallization
to occur,
and the
temperatures
must
be
increased
for
completion
to occur
in a
reasonable
length of
time.
-
7/25/2019 Material & Processes for NDT part 5
7/14
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-
7/25/2019 Material & Processes for NDT part 5
8/14
38 Materials and
Processes
for NDT
Technology
from
a
body-centered
cubic
(BCC)
lattice
to
a
face-
centered
cubic
(FCC)
lattice. A
second
phase
change
occurs
with
further
heating
to
1,394o
C
(2541'
F),
where
the
lattice
structure
retums to
the
body-
centered
cubic
form.
The
reverse
transformation
occurs
on
cooling through
the
same
temperatures.
Iron
in
the temperature
range
up
to
912"
C
is called
alpha
iron;
from 912'
C
to
1,394"
C,
gamma
iron;
and
above
1,394' C
to
1,538o
c
(2,800'
F), the melt-
ing
point,
delta
iron. Little
attention
is
given
to delta
iron
because
the changes
that
occur
in this
range
have
little
or no
effect
in commercial
practice
of
treatment
for
properties.
The
changes that
take
place
between
alpha
and
gamma
iron
at
912"
C,
however,
are
ex-
tremely
important.
The
most effective
change
is
the
difference
of
carbon
solubility
in
the
two
phases,
which
serves
as the
basis
for
all
heat-treat
hardening
and most
grain-size
control
for steel.
HEAT TREATMENT
OF STEEL
Steel
has been
treated
by heating
and
cooling
methods
to vary
its
properties
ever
since
its discovery,
but
even
today
the
exact mechanism
by
which
these
variations
take
place
cannot
be completely
explained
by
fully accepted
theories.
Most
of
the treatments
have
been
developed
empirically.
Various
theoretical
explanations
have been
used to
describe
the
mechan-
ism, but
it
has been
only
in
recent
years
that
the
theory
has advanced
to
the
point that
it is a
prime
source of
new development
of
commercial
heat-treat-
ing methods.
NDT
and Other
Control
Methods. Change
of
pro-
perties
of
steel
can be accomplished
by
cold working,
by
precipitation hardening, and
by allotropic
changes.
Cold working
changes
are
important
in most
of
the
cold
deformation
processes
and,
in
some
cases,
may
be the
only
treatment
received
by
the
metal.
Precipi-
tation
hardening
is seldom
used
intentionally,
except
for stainless
steels,
although
it may
be
an accidental
occutrence with
some of
the
processing treatments.
Causing
allotropic
changes
by
heat treating
proce-
dures
is the
most effective
and
most
easily
accom-
plished
method of
varying
mechanical
properties
of
steel and therefore
is the most
frequently
used way
of
obtaining the
desired properties.
Heat treating
is often
defined
as
intentional
heating
and cooling
for
control of
properties.
Such
a defini-
tion
is
perfectly
good,
but
it must
be remembered
that the
effects
of temperature
changes
are no
less
important
when they
are
caused
by
unintentional
heat transfer
during
a
process
such
as
fusion
welding
or
during
a
service
use
in high environmental
tempera-
tures
such
as
in a fumace or
gas
turbine.
Assessment
of thermal
treatment,
whether
inten'
tional
or
not,
is
often amenable
to
nondestructive
test'
ing techniques
that
are
capable
of
measuring
subtle
changes
in electrical
conductivity.
The
heat treatment
processes
described
in
this
chapter
produce
various
physical
property
changes
including
electrical
con-
ductivity.
Both eddy current
and
thermo'electric
meth-
ods are capable
of
indicating
changes
in
electrical
con-
ductivity
and
to some
extent can
provide
absolute
measures of electrical
conductivity.
However,
both
methods
only
probe
relatively
small
volumes
of
the
test material
essentially
at
an exposed
surface.
During
heat
treatment,
exposed
surfaces
tend to
heat and
cool
at
a
different
rate
from the
interior.
Thus,
measure-
ments
of
surface
characteristics
do
not
necessarily
characterize
the condition
of
the interior,
but
in many
practical
cases
can
provide
adequate
information
for
process
control
purposes.
APPROXIMATE
EQUILIBRIUM
HEAT
-
TREATMENT
PROCESSES
Several
heat-treating
processes place
the material
in
either
a
complete
or
an
approximate equilibrium
energy condition. These
processes
include
austenitiz-
ing, annealing, normalizing, and spheroidizing. Except
for
the
first,
all
are
finalized
at
room
temperature
but since
austenitizing
consists
of diffusion
of
carbon
into face-centered
cubic
iron
that
exists
at
a
mini-
mum
temperature
of
727" C
(eutectoid
composition
only,
all
others
higher),
stability,
or
equilibrium,
in
this state
can be
maintained
only
at the
higher
temp-
eratures.
Austenitization
is
therefore
not
a fina
process
but only
a
step
in one of several
heat-treating
procedures. For
these
approximate
equilibrium
processes,
it
is
possible
to
predict
the
material
be
havior
from
the
equilibrium
phase diagrams.
AUSTENITIZATION
When steel
is
heated to
or
above its
critical temp-
erature
(transformation
temperature range),
the
value
of
which
is dependent
upon
the alloy
percentages,
and held
at temperature
for some
period
of time
carbon
unites
in
solid
solution
with
iron
in
the
gamma
or face-centered
cubic lattice form. In
this
phase,
as much
as
27" carbon can dissolve
at the
eutectic
temperature
of 1,1-48"
C
at which
the
widest
range
of
gamma
composition
exists.
Grain-Size
Control.
It
is
important
that
the
austenitization temperatures
not be exceeded
more
than necessary
to accomplish
the
work
in a reason
able
length of time
because
grain
growth
can occu
rapidly
as the
temperature
is
increased. One
of
the
important
features
of austenitization
is
grain
refine
ment that occurs
with
the
formation of
the
new
face
centered
cubic
lattice. These
new
small
grains
are
nucleated with
the raising
of the
metal
temperature
through
the austenite
range and will
remain
small
i
the
temperature
is
not
raised
too high
or
maintained
too long.
With
lowering temperature
and decompo
-
7/25/2019 Material & Processes for NDT part 5
9/14
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7/25/2019 Material & Processes for NDT part 5
10/14
40 Materials and
Processes
for
NDT
Technology
Distortion
and Cracking
Minimized
by
Martemper-
ing.
Several
special
types
of
quench are
conducted
to
minimize
quenching
stresses and
decrease
the
ten-
dency
for
distortion
and
cracking.
One of
these,
is
called
martempering
and consists
of
quenching
an
austenitized
steel
in
a
salt bath
at
a
temperature
about
that
needed
for the start of
martensite
formation.
The
steel being
quenched
is
held
in this
bath
until
it is of
uniform
temperature
but is
removed before there
is
time for the
formation of bainite
to
start. Completion
of the cooling
in air
then causes
the same
hard
marten-
site
that would
have formed with
quenching
from
the
high temperature,
but
the
high thermal
or
"quench"
stresses
that are
the
primary
source
of
cracks
and
warping
will
have
been
eliminated.
Austempering
-
a Terminal
Step.
A
similar
proc-
ess
performed
at a
slightly higher
temperature
is
called
austempering,
ln
this
case
the steel
is
held at
the
bath
temperature
for a longer
period,
and
the
result of the
isothermal treatment
is the
formation
of
bainite. The bainite
structure
is
not
as
hard
as the
martensite
that
could
be formed
from
the
same
com-
position,
but
in
addition
to reducing
the
thermal
shock to which
the
steel
would
be
subjected
under
normal
hardening
procedures,
it
is
unnecessary
to
perform
any
further
treatment
to
develop
good
impact
resistance
in the high
hardness
range.
TEMPERING
A
third
step
usually
required
to
condition
a hard-
ened steel
for service
is
tempering,
or
as
it is
some-
times
referred
to, drawing.
With
the
exception
of
austempered
steel,
which
is
frequently
used
in
the
as-hardened
condition.
most
steels are
not
serviceable
"as
quenched".
The drastic
cooling
to
produce
mar-
tensite causes the
steel to
be
very
hard and
to
contain
both macroscopic
and
microscopic
intemal
stresses
with
the result that the
material
has
little
ductility
and extreme
brittleness. Reduction
of
these
faults
is
accomplished
by reheating
the
steel to
some
point
below
the
lower transformation
temperature.
The
structural
changes caused
by
tempering
of
hardened
steel are
functions
of both time
and
temperature,
with
temperature
being
the most
important.
It should
be
emphasized
that tempering
is not
a
hardening
process'
but
is,
instead,
the
reverse. A tempered
steel
is
one
that
has
been hardened
by
heat
treatment
and
then
stress
relieved,
softened, and
provided with
increased
ductility
by
reheating
in the tempering
or drawing
pro-
cedure.
CORROSION
Corrosion
Definition. In
general,
conosion
is the
deterioration
of
metals
by the chemical
action
of
some
surrounding
or contracting
medium which
may
be
li-
quid, gas,
or some combination
of
the
two.
To
some
de-
gree,
corrosion
can
influence
all
metals,
but
its effect
varies widely
depending
upon
the combination
of
metal
and
corrosive
agent.
The term
"corrosion" is
used
to describe
action
that
is normally considered
to be
detrimental,
but
the
prin-
ciple
is
actually
used
for benefit
in some
cases.
For
ex-
ample, acids
and alkalies
are
used
to corrode
metal
away
in the
manufacturing
process
of chemical
milling.
Also, aluminum alloys
are
frequently
anodized
to
pro-
duce an
oxide coating
that
resists
further
oxidation
and,
in addition,
may
serve
as an
improved
surface
for
paint
adhesion.
Corrosion
attacks
metals
by direct
chemical action,
by
electrolysis
(electrochemical
action),
or
commonly
by
a combination
of
the
two. The subject
is
complex
and
many
persons have
devoted
their
lives
to its study.
This
discussion
will summarize
some
of
the
known
facts
concerning
the subject
in
order
to develop
some
understanding
of corrosion,
its
detection,
and
preven-
tion.
DIRECT
CHEMICAL
ACTION
Theoretically,
all
corrosion
phenomena
are
electro-
mechanical
because
a
transfer
of electrons
takes
place
but the term
direct chemical
action
is used
to
describe
those reactions where
coupled
anodes
and cathodes
ex-
isting
in an electrolyte
are
not identifiable.
The chemi-
cal
milling mentioned above
is
direct
chemical
action.
Another
example
is
pickling of
steel,
a
process
in
which
heated
dilute
sulpheric acid
baths
are
used
to
dissolve surface
scale
withut
leaving
a
residue
and
pro-
ducing only
minor chemical
attack
on
the
steel
proper.
Figure 4-5
illustrates
another
example
of direct chemi-
cal action.
Figure
4-5
Direct chemical
attack
of
nitric acid
in
which
a
magnesium
alloy
product
is immersed.
Direct
chemical
reaction
is usually
evident
from
bubbles
formed bY
gas
evolution'
Galvanic
Series.
Table
4-3 shows
a
list of
metals
ar-
ranged in order
of their
decreasing
chemical
activity
in
sea water.
This
is a special
arrangement
of
the electro-
mechanical
and
the
electromotive
force
series.
It
-
7/25/2019 Material & Processes for NDT part 5
11/14
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