Brass Cracking
Transcript of Brass Cracking
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FAILURE(Environmental Fracture)
James Marrow
Manchester
Materials Science
Centre
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Environment Assisted Cracking 2
Environmental Assisted Cracking
Example of EAC
Hydrogen assisted
fatigue of a
compressed hydrogenstorage tank.
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Environment Assisted Cracking 3
Stress Corrosion Cracking
Example:
Failure of austenitic
stainless steel roof
supports.
Cause?
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Stress Corrosion Cracking
Chloride SCC of
Austenitic Stainless
Steels
Requires:
Stress
Corrosive EnvironmentSusceptible Material
Note the typical branching crack character of SCC
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Stress Corrosion Cracking
Anodic Mechanism
Cracking encouraged
when chemical
conditions are on theborderline between
activity and passivity.
Accelerated anodicdissolution before
crack tip repassivates
Requires plastic strain to disrupt passive film
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Environment Assisted Cracking 6
Hydrogen Embrittlement
Example:
Blistering and cracking
of electroplated steel.
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Hydrogen Embrittlement
Hydrogen assisted
cleavage of steels
Hydrogen reduces
critical tensile stress forcleavage.
This shifts the BDTT to
higher temperature.
Hydrogen also embrittles other materials, including magnesium alloys,
zirconium alloys, intermetallics and ceramics.
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Mechanisms of EAC
Cracking of high
strength steel
Hydrogen crackinitiated at a corrosion
pit
To reduce the risk of hydrogen embrittlement.
Avoid: Hydrogen
Susceptible Materials
Stress Concentrators and local plastic deformation
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Mechanisms of EAC
Cracking of high
strength steel
Hydrogen diffuses to
the hydrostatic tensilestress maximum ahead
of crack tip.
Cracking rate depends
on transport of atomichydrogen
Cracking rate influenced by chemical kinetics and stress concentration
Cracking threshold depends on stress and hydrogen concentration
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Hydrogen Assisted Cracking
H (atomic hydrogen)
produced by corrosion
e.g.
Or
Then
Examples:
Oxygen-free neutral
water (Cooling water
systems)Hydrolysis of corrosion
products (pits and
crevices)
2H2OH2eO2H -
2
2e8HOFeO4H3Fe 4322
ad2H2e2H
Cracking tends to be
branched and can be
confused with anodic SCC
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Environment Assisted Cracking
Static Fatigue
Stage 1:
da/dt=AKn
Stage 2:Reaction rate limit.
(affected by crack size,
shape and environment
Stage 3:K1Capproached.
K
da/dt
Stage 1
Stage 2
Stage 3
Log axes
Stress corrosion cracking
K1SCC
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Environment Assisted Cracking
Corrosion Fatigue
Acceleration of
cracking and reductionof threshold.
Decreasing strain rate
increases
environmental effect.
K
da/dN
Air
Log axes
Environment
True corrosion fatigue
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Environment Assisted Cracking
Corrosion Fatigue
Acceleration of
cracking above K1SCC.Decreasing strain rate
increases
environmental effect.
DK
da/dN
Air
Log axes
Stress corrosion fatigue
Environment
Kmax>K1SCC
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Case Study: EAC
Cracking of Brass
(Season Cracking)
Cracking caused by
ammoniac environment
and residual stress
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Case Study: EAC
Cracking of Brass
Cracking is intergranular
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Case Study: EAC
Cracking of Brass
4OHNHZnNHCu
O2HO8NHZnCu2
43
2
43
223
Anodic dissolution of metal
4OH4eO2HO22
Cathodic reaction
Soluble reaction product, and concentration of Cu and Zn ions does not rise
So the reaction does is not stifled by polarisation
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Case Study: EAC
Cracking of Brass
(cont.)
Grain boundaries areanodic to grains due to
segregation of Zn
Large Cathode and
small Anode
Dissolution at
grain boundaries
leads to stress
concentration and
stress corrosion
cracking
Crack growth rate depends on the
current density of the anodic
reaction at the crack tip
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Tests for EAC
Static Tests
e.g. bolt loaded
Dynamic Tests
e.g. slow strain rate
Tests material for serviceenvironment, or using severe
environment can rank materials for
resistance
Very severe
test for ranking
materials
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Tests for EAC
Static Tests Dynamic Tests
Time
Stress Stress
Strain
Tensile Data (air)
Slow strain rate test
in environment
sscc
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Summary.
Environment can enable failure.
Failure can require both crack initiation
and crack propagation. These may be
influenced differently by environment.
Many service failures are caused byresidual stress.