brass cracking

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  • 8/12/2019 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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    Environment Assisted Cracking 4

    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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    Environment Assisted Cracking 5

    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

  • 8/12/2019 Brass Cracking

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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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    Environment Assisted Cracking 15

    Case Study: EAC

    Cracking of Brass

    Cracking is intergranular

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    Environment Assisted Cracking 16

    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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    Environment Assisted Cracking 17

    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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    Environment Assisted Cracking 18

    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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    Environment Assisted Cracking 19

    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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    Environment Assisted Cracking 20

    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.