Mechanical Aspects of Corrosion

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Mechanical Aspects of Corrosion

• Bob Cottis
• Corrosion and Protection Centre, UMIST

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Mechanical Aspects of Corrosion

• Static stress
• stress-corrosion cracking
• hydrogen embrittlement
• liquid metal embrittlement
• Dynamic stress
• corrosion fatigue
• fretting corrosion

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The Effect of Stress and Strain on Corrosion

• Stress per se does not affect corrosion processes
  much
• Plastic strain can have a large effect
• increased dislocation density
• rupture of passive films

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Stress-Corrosion Cracking

• Cracking of a metal under the combined effects of
  a static stress and a specific chemical
  environment
• Several possible mechanisms, still not fully
  understood
• Cause of major industrial costs and safety hazards

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Stress-Corrosion Cracking

• Mechanisms
• Anodic dissolution
• Hydrogen embrittlement
• Film-induced cleavage

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Anodic Dissolution
1 The walls and tip of the crack are passive
2 The passive film at the crack tip is ruptured
by the plastic strain, and active corrosion
occurs
3 The crack tip repassivates
4 Go back to 1
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Film-Induced Cleavage
1 The walls and tip of the crack are covered by a
brittle film (either an oxide film or a
de-alloyed layer)
2 The film at the crack tip is ruptured by the
plastic strain
3 The brittle crack continues into the metal
4 The crack is blunted by plastic strain
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Hydrogen Embrittlement
1 Hydrogen produced by the cathodic reaction
2 Hydrogen diffuses to region of tri-axial
tensile stress ahead of the crack
3 Hydrogen causes brittle fracture
4 Crack blunts by plastic deformation as it runs
out of hydrogen
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Hydrogen Embrittlement

• sources of hydrogen
• welding
• electroplating
• contact with gaseous hydrogen
• corrosion, especially in the presence of
  sulphides
• higher strength materials are more susceptible to
  hydrogen embrittlement

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Hydrogen Induced Cracking

• Internal cracking of lower strength steels (e.g.
  pipeline steels) due to high pressure hydrogen
  collecting at inclusions.

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Hydrogen Cracking

• Internal cracking of steels at higher
  temperatures due to reaction of dissolved
  hydrogen with carbon to form methane

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Stress Corrosion Cracking Systems

• Brass and ammonia (often in local atmospheres).
• Austenitic stainless steels and chloride
  solutions (70oC).
• Carbon steels in caustic, carbonate/ bicarbonate,
  nitrate and phosphate.
• High strength aluminium alloys in water or water
  vapour.

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Stress-Corrosion Testing
Constant Load Testing
Stress
Threshold stress
log(Time to Failure)
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Stress-Corrosion Testing

• Slow strain-rate testing (or constant extension
  rate testing)
• Extend a plain or pre-cracked specimen at a slow
  constant rate
• Then assess by
• fracture surface
• change in elongation or reduction in area
• time to failure

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Stress-Corrosion Testing

• Fracture mechanics testing
• apply a constant load to a pre-cracked specimen
• measure crack growth rate as a function of stress
  intensity factor (K)

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Fracture Mechanics Testing
Fast fracture starting to occur as K approaches
KIc
Plateau crack velocity typical range of values
1011 to 103 m/s
log (crack growth rate)
Threshold stress intensity factor, KIscc
Stress Intensity Factor
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Control of Stress-Corrosion

• Remove stress (often difficult, especially for
  residual stresses)
• Avoid the necessary environment
• Apply electrochemical protection where possible
• Use a different material
• Live with it

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Liquid Metal Embrittlement

• Liquid metals can permeate down grain boundaries
  and cause intergranular cracking
• mercury on brass and aluminium alloys
• liquid zinc on stainless steel

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Corrosion Fatigue

• Metal fatigue results in crack propagation due to
  a cyclic stress
• Corrosion makes both crack initiation and
  propagation easier

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Corrosion Fatigue - S-N Curve
Stress Amplitude
log (cycles to failure, Nf)
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Corrosion Fatigue - Crack Growth
log (Crack Growth Rate, da/dN)
Log (Stress Intensity Factor Range, ?K
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Fretting Corrosion

• Rubbing of two metals removes oxide film and
  allows oxidation
• The oxide may also act as an abrasive
• Prevention of relative motion and allowing larger
  relative movement may prevent the problem

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