Mechanical

• 1.Erosion Corrosion(cavitation)

  influenced by high relative movement between corrosive fluid + metal

  Apperance- grooves,waves on surface with direction pattern

  Occurs most metals when speed of light=high

  metals depend on passive film- force can damage passive film=corrosion

  direction of fluid --> towards tip- opposite of tip pointing

  
  

  How patterns:

  -bubble burst form force

  -film destroyed

  -Film reformed- not flat

  -Bubble again
• 1.Erosion Corrosion

  • combination of chemical and mechanical attack
  • failure in short time + unexpected in static conditions
  • solid particle in fluid=severe damage

  Special type: Fretting-contact with vibration + slip

  
  

  Prevention:

  • Material selection
  • Design - shape, increase diameter to decrease speed
  • Decrease velocity
  • Inhibitors
  • Coatings/surface hardening
  • Cathodic Protection
• 2. Stress Corrosion cracking

  • Dangerous form of failure caused by simultaneous tensile stress and specific corrosion medium
  • Almost no rust while crack progresses therefore brittle (sudden)

  Important Factors:

  1. special combination of materials and corrosive environment- not necessarily high corrosivity
  2. Tensile stress- compressive does not cause SCC
  3. Above certain temperatures

  Environments/stress sensitive cracking:

  • Fittings for high pressure air bottle in slightly corrosive environent=cracked
  • stress relieving heat treatment - uncracked=no more failure
• Classic examples of stress corrosion cracking

  • Brass in amonia containing environment
  • C steel in OH- solution
  • Austenite SS in cl- containing H2O- like seawater
• Stress corrosion cracking

  • Brittle fractures can be intergranular or transgranular or both
  • Cracks vary with degree of branching
  • Not well understood - hard to predict
• Stress corrosion cracking

  1. Pit on surface = high stress concentration at notch tip (small radius)

  2. Stress concentration= crack mechanically "jump" on tip

  3. Stress + corrosion = crack propagation

  4. Cathodic protection stops corrosion- can stop crack moving

  5. During crack propagation- tensile stress= breaks passive film

  6. Does not reheal due to corrosion crack growing
• Protection for SCC:

  • avoid sensitive combinations of material and environment
  • Annealing = stress decreases
  • Lower load/thickening section
  • Cathodic protection
  • Inhibitor= risk of pitting decreases, risk of SCC decreases
  • surface coating
  • Introduce compressive stress - eg shot peening
• 3. Corrosion Fatigue

  decreased fatigue strength due to corrosive medium

  Fatigue: tendancy of metal to fail under repeated cyclic stress

  failure below yield point

  Like SCC, depends on metal + environment

  environment

  • large smooth area covered with rust and small rough "crystal" aea
  • affected by test frequency

  Protection:

  • stress decreases by annealing
  • shot-peening (comp stress)
  • coating
  • inhibitor
• 4. Hydrogen Damage

  Mechanical failure caused by presence of H2/interaction

  Forms of Damage:

  1. Hydrogen blistering

  Product of cathodic reaction diffuse into metal rather than release H2

  pressure increase=rupture metal

  Protection:

  • Clean steels (less defects)- H blister resistance
  • coatings
  • inhibitor
  • material selection
  • low temp annealing

  2.Hydrogen embrittlement

  3.Decarburisation

  4. Hydrogen attack
• Corrosion failure analysis:

  investigation to determine cause of failure with the aim of fixing failure & avoiding it happening again
• Corrosion failure Analysis steps:

  1. Get original sample before and after failure
  2. Obtain info: materials
  3. Photos of damaged parts and surrounding
  4. Optical/low magnification mirographs of top/surfaces to see the morphology of rust/failure
  5. high magnification microstructure & composition analysis
  6. Work out corrosion form/course/mechanism
  7. Suggest methods of prevention/control