FAILURE ANALYSIS OF A LEADED BRASS OXYGEN BOTTLE VALVE
IVO BLAČIĆ
Military Technical Institute, Belgrade, Serbia
LJUBICA RADOVIĆ
Military Technical Institute, Belgrade, Serbia, [email protected]
DUŠAN VRAČARIĆ
Military Technical Institute, Belgrade, Serbia
Abstract: An oxygen bottle valve of an aircraft failed during bottle filling was received with the request to perform a
failure investigation. The valve was made of an extra-high-leaded brass (62Cu-35.5Zn -2.5Pb) in the extra hard
conditions (150 HV5), coated with nickel. The valve was about 28 years in service. The fractographic and
metallographic analyses were performed by means of stereo and scanning electron microscopy (SEM/EDS), as well as
hardness measurement. Nickel coating had unequal thickness (3-7 μm) and their fracture and spalling in some region
was observed. The microstructure of the high leaded brass consists of a mixture of alpha and beta phases and lead
particles. The fractographic examination revealed that the crack was originated on the grain boundary. The crack
propagation exhibited both intergranular and transgranular mechanism. Results suggest that the stress corrosion
cracking of the leaded brass was the main fracture mechanism.
Key words: oxygen bottle valve, extra-high-leaded brass, stress corrosion.
thickness of protective film, concentration of corrosion
products, and stress concentration. Grain boundary
segregation or precipitation can cause the passive layer on
the surface of the metal to be locally thinned, allowing
pitting or grain boundary corrosion to start. This corrosion
pit or trench, produced by chemical attack on the metal
surface, may act as a stress raiser and thus serve as a site
for initiation of SCC. A preexisting mechanical crack,
surface defect, fabrication flaw, or other surface
discontinuity may also trigger SCC [4].
1. INTRODUCTION
Brass is a metal alloy of copper and zinc, with copper
content ranging from 58% to 95%. Zinc is the major
alloying element, but the other alloying elements are
added (less than 5%) to modify the properties. Brasses
containing a minimum of 63% copper are termed alpha
(α) brasses or cold working brasses. Brasses containing
35%-45% zinc are known as alpha-beta (α+β) or duplex
brasses because they contain a mixture of the original
solid solution (α-phase) and a new solid solution of
higher zinc content (β-phase) [1].
The fracture surfaces usually contain easily identifiable
regions ordinarily observable of crack initiation, slow,
subcritical crack propagation, and final rupture. Final
rupture usually occurs by tensile overload. Thus, the
region of final fracture often shows some evidence of
ductility, such as a shear lips or a herringbone patterns
emanating from the zone of slow cracking. The region of
slow crack propagation often contains corrosion products
or is stained or otherwise discolored with respect to the
region of final fast fracture. [4]
Brasses are susceptible to stress corrosion cracking.
Alloying additions have influence on this property. The
corrosion resistance of the brasses is reduced when there
is a mixture of α+β phase [2].
Stress-corrosion cracking (SCC) is a failure process that
occurs because of the simultaneous presence of tensile
stress, an environment and a susceptible material. Stresscorrosion cracking is a subcritical crack propagation
phenomenon involving crack initiation at selected sites,
crack propagation, and overload final fracture of the
remaining section. Stress-corrosion cracking is a dangerous
and severe degradation mechanism, but with proper
understanding and care, failures can be avoided [3].
In some metals cracks propagate intergranularly and in
others, transgranularly. In certain metals, such as highnickel alloys, iron-chromium alloys, and brasses, both
types of cracking can occur, depending on the metalenvironment combination. The path of SCC in some
metals is not governed completely by composition and
structure of the metal but also is influenced by the
The site of initiation of SCC may be submicroscopic and
determined by local differences in metal composition,
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are given in Table 2. Results show that the structure consists
of two phases with different Zn content: α-phase (weaker
etched) and β-phase (strong etched), i.e. duplex (α+β) and
dispersed lead (Pb) particles (white particles) [2, 5].
environment. For example, the path of cracking in copperzinc alloys can be made transgranular or intergranular by
adjusting the pH of aqueous solutions in which these
alloys are immersed [4].
Careful correlation of microscopic fracture surface
topography with macroscopic fracture surface features is
essential. The crack initiation region and the directions of
crack propagation must be identified accurately so that
information concerning the sequence of events and the
micromechanisms of fracture, as observed with electron
microscopy, can be correlated with the circumstances of
crack initiation and the mechanism of crack propagation.
Frequently several different fracture micromechanisms
are observed on a single fracture surface. Accordingly,
correct identification of the initiating fracture mechanism
and of any changes in micromechanism during fracture
propagation is of vital importance in arriving at a correct
understanding of the failure. [4].
The aim of this work was to perform a failure
investigation of an oxygen bottle valve of an aircraft
failed during bottle filling.
a)
2. EXPERIMENTAL
The chemical composition was analyzed by x-ray
spectrometer Philips PW 1404.
Macroscopic observations were carried out using a
stereomicroscope. Fractographic examination was
performed by scanning electron microscope SEM JEOL
JSM-6610LV equipped with EDS analyzer.
The microstructures were characterized by a Leitz optical
microscope and SEM-JEOL JSM-6610LV.
Metallographic samples were prepared using traditional
grinding and polishing techniques up to 5 μm diamond
paste .To reveal the grain structure, the specimen were
electrolytic polished and etched in phosphorous acid. The
Vickers hardness tester was used for hardness
measurement.
b)
3. RESULTS
3.1. Chemical composition, hardness and
metallography
Chemical composition of the oxigen bottle valve obtained
by x-ray fluorescence spectroscopy is given in the Table 1.
c)
Table 1. Chemical ciomposition of the oxigen bottle
valve, (mass %).
Zn
Pb
Fe
Ni
Sn
Cu
36.9
2.5
0.08
0.024
0.11
Ball.
Avarage hardness of the the oxigen bottle valve was 150
HV 5. The chemical composition and hardness of the
oxigen bottle valve are very closed to leaded brass
C35600 according to ASTM B 121M-01.
Microstructure of the analyzed material is shown in the
Figure 1a. EDS spectra of the strong etched phase, weak
etched phase, white particles and whole area are shown in
Figures 1 b-d. Corresponding content of alloying elements
d)
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(B and C): the lighter (B), 1 mm in length, without shear
lips and, darker with shear lips (C). The region B
corresponds to slow crack propagation and C to final
fracture. In the area B, at higher magnification, the third
area 150 μm in length was observed (A). It is shown in
the Figure 3b. This surface is perpendicular to the valve
surface. The crack is originated in this region. The
corrosion products are detected on the fracture surface
and they are caused by oxygen flow during fracture.
e)
Figure 1. Microstructure of the material and
corresponding EDS spectra: a) microstructure of α+β
brass; b) EDS spectra of strong etched phase; c) EDS
spectra of weak etched phase; d) EDS spectra of white
particles; e) EDS spectra of the whole region.
Table 2. Chemical composition of the phases (EDS),
(mass %).
Strong etched phase
Weak etched phase
White particles*
Whole region
Cu
Zn
55.13
64.02
5.10
58.18
44.87
35.98
38.89
Pb
a)
91.13
2.93
* 3.77 mass % P
The surface protected with Ni coating is shown in Figure
2. Coating had thickness between 3 μm to 7 μm. In the
region of the crack origin the thickness was 3 μm.
b)
Figure 3. SEM fractography of the fracture surface: a) slow
crack propagation region (B) and final fracture region (C)
with crack initiation point, b) crack initiation zone.
The results indicate that fracture and spalling of the Ni
coating had occurred before the crack initiation (Figure 4).
Figure 2. Nickel coating on the valve surface.
3.2. Fractography
Representative fracture regions were photographed in the
SEM at various magnifications. Fracture surface of the
one valve part which has not been placed in the casing
was shown in the Figure 3.
Fracture surface shows region of macroscopic brittle
crack propagation, indicated by radial ridges, that are
oriented to the crack origin (O), and two regions of crack
propagation (B and C). Two regions of the crack
propagation were observed by macroscopic observation
Figure 4. Fracture of the Ni coating.
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The region of the crack initiation is shown in the Figure 5.
The results shown that the fracture of the oxygen bottle
valve happened after protected Ni coating fractured.
Coating fracture allowed chemical attack (aggressive
products and Cl− ions) on the brass.
The crack initiation region (A in Figure 3b), slow crack
propagation region (B in Figure 3a), and final fracture
region (C in Figure 3a) are shown in the Figure 6 a-c.
a)
a)
b)
b)
c)
c)
Figure 6. Micromechanisms of fracture processes in
region of: a) crack initiation; b) slow crack propagation;
c) final fracture.
Figure 6a shows a region of intergranular fracture.
Intergranular fracture also dominates in Figure 6b, but
small amount of ductile dimples is observed. In the region
of finale fracture (Figure 6c) dimple fracture dominates,
while intergranular fracture is less pronounced.
d)
Figure 5. Crack initiation region with corresponding EDS
spectra: a) appearance of fracture surface; b) EDS
spectrum of the corrosion products; c) EDS spectrum of
the brass; d) EDS spectrum of the Ni coating.
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4. DISCUSSION
5. CONCLUSION
The brittle fracture of the oxygen bottle valve occurred
after fracture of corrosion-preventive Ni coating.
Therefore, the material of the valve, (α+β) leaded brass
was attacked by corrosive environment with Cl− ions. The
presence of three macroscopic regions, initiation and slow
propagation of the crack, as well as finale fracture region,
indicate that the stress corrosion cracking is the main
failure mechanism. Intergranular fracture in the crack
initiating region and dominating intergranular brittle
fracture in slow crack propagation region are
characteristics of the stress corrosion in the brass.
Fracture of the leaded brass oxygen bottle valve was
governed stress corrosion cracking mechanism.
The fracture or spalling of the Ni coating should be
criteria for acceptance or rejection the oxygen bottle valve
during quality control.
References
[1] Shah C.G.: Failure Analysis of Brass Components,
Microsc. Microanal. 17 (Suppl 2), Microscopy
Society of America, 2011.
[2] Borggren U., Selleby M.: A Thermodynamic
Database for Special Brass, Journal of Phase
Equilibria, Vol. 24, No. 2, 2003.
The crack was initiated under rubber seal (band), so the
nondestructive inspection, i.e. visual inspection is not
possible. The crack length in the slow propagation region
(Figure 3a) is approximately ~ 1 mm, that is very close to
minimal detectable crack value by Eddy current method
(~ 0.6 mm) [4]. Therefore, Eddy current method is not
enough reliable for cracking detection in (α+β) brass,
which is very susceptible to stress corrosion cracking.
These are the reasons that the criteria for acceptance (or
rejection) of the oxygen bottle is established fracture and
spalling of Ni coating.
[3] ASM Metals Handbook, Corrosion, Vol. 13, Ohio:
ASM Metals Park, 1987.
[4] ASM Metals Handbook, Failure Analysis and
Prevention, Vol. 11, Ohio: ASM Metals Park, 2002.
[5] ASM Metals Handbook, Metallography and
Microstructure, Vol. 9, Ohio: ASM Metals Park,
2004.
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