20'1 Corrosion prevention

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• 20.1 Corrosion prevention
• Provide drainage

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• Avoid stresses

ASM Handbook vol 20.
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Corrosion prevention Summary

• Avoid bimetal corrosion.
• Sacrificial protection.
• Coating.
• Bias voltage.
• Design for drainage.
• Design to avoid crevices.
• Avoid stress concentration.
• Welding practice.
• Prompt treatment.

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20.2 Stress assisted corrosion cracking

• Stress causes local corrosion.
• Loss of material intensifies stress
  concentration.
• Cracking forms crevice, corrosion intensifies
• Accelerated corrosion failure

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20.3 An example of corrosion limited application
the exhaust system of cars
Selection and Use of Engineering Materials, J.A.
Charles and F.A.A. Crane, Butterworth-Heinemann,
1989
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Muffler/silencer
Selection and Use of Engineering Materials, J.A.
Charles and F.A.A. Crane, Butterworth-Heinemann,
1989
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Operating condition of the exhaust

• Corrosive environment inside and outside
  electrolyte
• Consisting of multiple components welded
  together bimetal process and thermal stress at
  weldments.
• High temperature and oxidation mediated wet
  corrosion gaseous reduction process.
• Vibratory stresses stress assisted corrosion
  cracking

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Corrosion at the weldment

• Thermal stress due to welding stress relief
  treatment
• Chemical inhomogeneity due to heating alloy
  selection
• Local Cr depletion in case of SS.
• Changing load carrying cross-section stress
  concentration

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20.4 Fretting corrosion

• Two metal surfaces rubbing against each other.
• Abrasion of protective surface oxide.
• Oxide debris acting as abrasive powder.
• A form of dry corrosion.

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20.5 Chemical attack of polymers
Nature of chemical attacks

• Breakage of covalent bonds, leading to
  disintegration of the polymer chains.
• Acid attack
• Alkali attack
• Dissolution in solvents
• Radiation damage

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Common acid resistant plastics

• Resistant to all acids
• PTFE
• Resistant to weak acids
• PE
• PP
• PVC
• ABS
• PS
• Epoxy

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Common alkali resistant plastics

• Resistant to all alkalies
• ABS
• PTFE
• PE
• PP
• PVC

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Common solvent resistant plastics
Strong solvents methylene chloride, acetone

• Highly resistant
• PTFE, Melamine
• Generally resistant
• Epoxy, PE, PP

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20.6 Radiation degradation

• Particularly serious in transparent plastics
• Forms of radiation damage
• Bond breakage by UV (photo-oxidation).
• Defect production.
• Void formation.
• Results of radiation damage
• Embrittlement.
• Cracking.
• Discoloring