This article originally appeared in the issue of “Der Maschinenschaden”
for December 1965, published by the Allianz Versicherungs-Aktiengesellschaft,
Munich, and is reproduced by kind permission of that Company.
Embrittlement due to hydrogen is known in Germany as “Wasserstoffkrankheit,”
the literal translation of which is “hydrogen disease.” This term
is not used in this country, the phenomenon being referred to as “hydrogen
embrittlement” or “gassing.”
Much research has been carried out on the hydrogen embrittlement of
copper and copper alloys. The onset of the trouble cannot be recognised macroscopically
and evidence regarding the cause of failure cannot be judged from the appearance
of components which have failed. It is for this reason that hydrogen embrittlement
can be such a source of danger.
The term “hydrogen embrittlement” describes the brittleness
affecting copper and copper alloys containing oxygen which develops during
heat treatment at temperatures of about 400°C. and above in an atmosphere
containing hydrogen. It arises from the hydrogen which diffuses through the
metal to combine with either dissolved oxygen, or oxygen present at the grain
boundaries as cuprous oxide to form water vapour. The latter cannot diffuse
through the metal and therefore high vapour pressures are generated at the
sites where the reaction takes place (such pressures may attain a value of
5120 atmospheres = 75,000 psi).
Intercrystalline cracks appear subsequently and their development is influenced
by such factors as the temperature and duration of the heat treatment, the
hydrogen content of the atmosphere and the oxygen content and constitution
of the copper or alloy.
specimens of copper strip which were bent after being heated in a stream of
hydrogen for a half, two, and six hours respectively to result in varying
degrees of hydrogen embrittlement.
It can be seen that a relatively short reaction time results in only a slight
reduction in the elongation and that a higher hydrogen content in the copper
is needed in order to cause serious deterioration of the properties of the
of electrolytic copper showing pronounced deposits of cuprous oxide (Cu2
O) at the grain boundaries is depicted. A similar sample, after annealing
for five hours in hydrogen at a temperature of 850°C., is shown in
. It will be apparent that
the treatment has reduced the amount of cuprous oxide at the grain boundaries
and it has been replaced by voids and cracks.
Fig. 1 Test pieces bent after heating for
, 2 and 6 hours in hydrogen.
Fig. 2 Copper showing cuprous oxide at grain boundaries. × 320.
Fig. 3 As
figure 2. after
annealing in hydrogen. × 130.
In order to provide some further clarification of the problems of hydrogen
embrittlement, the influence of the annealing temperature has been investigated
on plates and rods made of oxygen-containing copper.
Annealing for one hour in an hydrogen stream at 500°C. did not produce
any cracks in the material but heat treatment for the same time at 800°C.
caused severe damage. A network of cracks developed at the outer surface of
the copper plate as illustrated inFigure 4
. One of the copper rods was severely attacked, separation of
the individual grains taking place as is shown in
Fig. 4 Surface attack of copper plate after heating for 1 hour at 800°C
in hydrogen. × 65.
Fig. 5 Surface attack of copper rod after heating for 1 hour at 800°C
in hydrogen. × 65.
Hydrogen embrittlement only develops after a certain period of time,
the so-called “incubation period.” It is characterised by surface
deoxidation of the material which, according to Ransley,
results from the diffusion of oxygen to the metal surface and its subsequent
reaction with hydrogen and this provides an explanation of the pronounced
which is characteristic
of hydrogen embrittled components.
This effect is depicted in
. If the depth of the deoxidised zone exceeds a critical value, the
rate at which the oxygen reaches the surface will be less than that of the
hydrogen adsorption. Hydrogen will penetrate into the metal and the reaction
between it and cuprous oxide will commence.
Fig. 6 Intercrystalline cracking and deoxidised rim. × 125.
The influence of the hydrogen content of the atmosphere and the annealing
temperature upon the incubation for an annealing time of
hours is shown in
The upper curve
corresponds to the values determined experimentally but curve AA is recommended
for practical purposes since the onset of embrittlement is also related to
the structure of the material. The lower the annealing temperature and the
content of hydrogen in the atmosphere, the longer will be the incubation time
and the slower the formation of cracks. An increased oxygen content in the
copper will have the same delaying effect, but once penetration of hydrogen
has commenced the amount of water vapour formed and the extent of the cracking
will be greater the higher the oxygen content of the metal.
The embrittlement which occurs when copper
is annealed in an atmosphere containing carbon monoxide was, for a long time,
thought to result from diffusion of this gas into the metal,
but today it is fairly evident that the embrittlement is, in all cases, due
to the diffusion of hydrogen present as an impurity in the carbon monoxide.
The diffusion rate of hydrogen is more than
105 times greater than that of carbon monoxide at the same temperatures.
Fig. 7 Influence of hydrogen and temperature on incubation for heating periods
In some instances, the hydrogen driven off from a residue of drawing
or lubricating grease on a metal surface by heating subsequently is sufficient
to induce hydrogen embrittlement.
Insulation breakdown in the stator windings of a turbo-generator was found,
on dismantling, to be associated with damage to the copper conductors, partial
fracture having taken place.
damaged parts were bent and showed gaping brittle cracks as seen at “R”
Fig. 8 Brittle cracks in copper conductors.
Temper colours and traces of burning on the surfaces indicated that
excessive heating of the conductor had taken place. A photo-micrograph of
a section through the fractured conductor indicated the presence of intergranular
cracks as depicted in
where A, represents the surface of the fracture, B, fine intergranular cracks,
and C, some of the larger inclusions that were present. Examination at higher
magnification showed the presence of a system of interconnected cracks and
oxides mostly adjacent to the surface as is evident in
. The cuprous oxide, one of the prerequisites
for hydrogen embrittlement, was present in the material and had been reduced
by hydrogen. The temper colours and the traces of burning on the surface of
the conductor led to the conclusion that failure of the insulation had occurred.
The material used for the insulation, as a result of the temperature attained,
gave off hydro-carbons, hydrogen and tar substances to result in hydrogen
embrittlement of the bars.
Fig. 9 Section through one of the fractured bars shown in
figure 8. (× 35).
Fig. 10 Intergranular cracks and oxides adjacent to surface. (× 400).
The possibility that hydrogen embrittlement may occur requires to be
considered during welding operations. The presence of the usual de-oxidation
additives to copper reduces the electrical conductivity and a small amount
of oxygen is unavoidable in materials for conductors in electrical engineering.
In gas welding even a neutral oxygen-acetylene flame will cause “gassing”
as seen in
the use of a shielding gas will serve to prevent this to a large extent, a
phenomenon similar to hydrogen embrittlement may be observed in oxygen-bearing
copper (more than 0.2% O2
). The cuprous oxide which is present
at the grain boundaries originates from the melting process and is soluble
in liquid copper to form, with copper, a strongly embrittling eutectic at
O (approximately 0.4% O2
Fig. 11 Effect of welding oxygen-containing copper in neutral oxy-acetylene
flame. (× 160).
The two test pieces shown in
were welded under a protective gas, (a
an oxygen-containing copper and (b
) an oxygen-free copper.
While test specimen (b
) could be bent through 180°, specimen
) fractured after slight deformation, failure occurring
in the transition zone between the weld and the parent material. The respective
structures are illustrated in
, that from specimen
) showing the presence of the Cu-Cu2
at the grain boundaries, this not being present in specimen (b
Fig. 12 Effect of welding under protective atmosphere. (a) oxygen-containing
copper. (b) oxygen-free copper.
Fig. 13 Photomicrograph of specimen (a) in
figure 12. (× 80).
Fig. 14 Photomicrograph of specimen (b) in
figure 12. (× 160).
A further example is provided by a drainage cock made of (α + β)
brass which tore at the threads to result in considerable damage. Embrittlement
of the material was due to the presence of cuprous oxide the crack running
along a chain of Cu2
O particles. The critical position in which
the eutectic formed is shown diagramatically in
. The lower photo-micrograph shows that Cu2
present in the material and the formation of the resulting grain boundary
eutectic is apparent from the upper illustration.
For welding by shielded-arc processes, oxygen-free materials are therefore
to be recommended.
Fig. 15 Showing influence of welding on formation of eutectic.
Embrittlement can develop in copper or copper alloys containing oxygen
if they are brazed in a reducing flame containing an excess of hydrogen. The
bars of an 80 kW squirrel cage rotor broke off in serve at the junction with
the end rings, as seen in
The oxidised fracture surfaces
marked A, had a hammered appearance and the exposed crack surfaces B were
oxidised. The uncracked bar marked C was also broken off. Other bars in the
same position showed incipient cracks and microscopical examination of a specimen
taken tangentially from an end ring in the region of a cracked bar showed
that the cracks originated at the end of the brazed area and developed in
an arc-like manner through the ring in the manner depicted in
. The material of the end ring showed the presence
of gross oxide impurities and, in the brazed area, numerous fine cracks were
present as seen in
, the former being unetched
and the latter etched. The type and location of these cracks were related
to the numerous inclusions of cuprous oxide and indicated that failure resulted
from hydrogen embrittlement. Brazing of the rings, which contained cuprous
oxide, in a reducing flame containing an excess of hydrogen caused intercrystalline
cracks to develop which subsequently extended in service leading to fracture
at the junction with the rotor bars.
Fig. 16 Failure of bars at junction to end ring.
Fig. 17 Showing crack origin associated with brazing metal.
Fig. 18 Unetched. Intercrystalline cracking adjacent to brazing (×
Fig. 19 Intercrystalline cracking adjacent to brazing. (× 200)
Failure of Copper Parts due to causes which resemble Hydrogen Embrittlement
At elevated temperatures, certain temperature ranges and deformation
rates exist where the ductility of copper and its alloys is diminished and
cracks and fractures readily occur at slight degrees of deformation. At a
superficial glance, these may resemble those resulting from hydrogen embrittlement.
The curve showing the elongation of electrolytic copper passes through a minimum
in the temperature range 550–600°C., as shown in
Microscopical examination of specimens heated in this range showed the presence
of numerous pores and separation had taken place at those grain boundaries
which lay perpendicular to the direction of the stresses as indicated in
specimen A was broken at 400°C, B at 600°C and C at 800°C.
The pores that form initially extend and unite as the result of deformation
to result in cracks. The degree of deformation necessary for this process
to occur is lower the lower the deformation rates. The ductility increases
again at higher temperatures as soon as re-crystallisation prevents the formation
of pores and cracks and transfers separation to within the grains.
Fig. 20 Showing minimum elongation of copper in region of 550°–600°C.
Fig. 21 Intergranular cracks in specimen heated to 550° 600°C. ×
Fig. 22 Microstructure of × 200 specimens broken at a. 400°C, b.
600°C and c. 800°C.
Several theories have been proposed regarding the causes of the nucleation
One such theory assumes inclusions to be responsible.
Another concludes that grain boundary slip is the cause. Precipitations, especially
those at the grain boundary, can often be invoked to explain the development
of cracks. Bismuth for instance even in the smallest amounts (less than 0.003%)
occupies the grain boundaries and results in considerable embrittlement of
The effect of antimony is
controversial at the moment. According to American investigations, specimens
of tough-pitch copper containing 0.02% arsenic were not subject to hot cracking,
but at a content of 0.06% small cracks appeared.
Tellurium has been found to favour hot cracking. Other failures which in a
macroscopical manner resembles hydrogen embrittlement, may be due to brittle
fracture. Under the microscope, however, it is sometimes possible in such
cases to observe definite deformation of the structure as seen in
The former illustrates an unetched microsection, whereas the latter is of
a further portion of the same specimen in the etched condition.
Fig. 23 Section through fracture-unetched. (× 40).
Fig. 24 Section through fracture showing deformation near crack-etched. (×
In one example, the bars of an AC motor fractured where they where brazed
to the short circuit ring as shown in
The fracture surface
B was grainy and banded on those regions which had not been damaged. Microscopical
examination showed that incipient cracks were present in some bars, these
being situated in the same region as other bars in which fracture had occurred,
as depicted in
higher magnification this cracking was shown to be of an inter-crystalline
nature as shown in
No defects resulting from oxides were present and the appearance and course
of the cracks suggested that they most probably originated as a result of
the simultaneous effect of mechanical stress and high temperature possibly
associated with overloading of the cage.
Fig. 25 Failure of bars of a.c. Motor at brazed ends.
Fig. 26 Section through incipient cracks in unbroken bars. (× 3.5).
Fig. 27 Intergranular nature of cracking. (× 80).
, the cracking
on the bore of a copper tube resulting from a combination of vibration and
corrosion is shown.
specimen showed the cracks to be of the form illustrated in
. Such fine, parallel cracks usually result
from alternating stresses, possibly caused by pressure fluctuations or the
effects of water hammer, as well as simple vibration. Repeated expansion and
contraction as a result of considerable temperature fluctuations may show
similar effects. Corrosion fatigue cracks, which result from this cause, are
often initiated at small corrosion spots and develop in a transcrystalline
manner in contrast to cracks resulting from stress corrosion which are mainly
of the intercrystalline type.
Fig. 28 Corrosion fatigue cracking on bore of copper tube. (× 7).
Fig. 29 Section through cracking shown in
figure 28. (× 50).
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