Lecture 6

Question 1

Flow of electrolytes can accelerate degradation by…

Answer 1

• physical abrasion
• the removal of protective oxides layers
• enhanced transport of corrosion enhancing chemicals (e.g. chloride) or oxygen to the surface

Question 2

4 flow mechanism for erosion corrosion

Answer 2

laminar flow
turbulent flow
impingement
cavitation

Question 3

Design against erosion corrosion

Answer 3

• Avoid turbulent flow conditions
• Ensure smooth fluid paths
• Minimise obstructions and eliminate restrictions
• Avoid very rapid transport of abrasive slurries
• Minimise potential cavitation by careful design and selection of a suitable material (stainless steels are resistant to cavitation damage due to their hard surface

Question 4

Stress Corrosion Cracking

Answer 4

Cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses.

Question 5

How Does Stress Corrosion Cracking Occur?

Answer 5

• Initiation (highly localised anodic attack resulting in pit formation)
• Propagation (three proposed routes)
  a) pre existing active paths
  b) Strain generated active path mechanisms
  c) Adsorption related mechanisms

Question 6

pre existing active paths

Answer 6

crystal boundaries act as anodic sites. Corrosion combines with residual stress can result in cracks growing along boundaries

Question 7

Strain generated active path mechanisms

Answer 7

In absence of stress metal does not corrode due to growth of passive film. In these cases if crack propagation starts, anodic activity is focused at the crack tip and therefore rapid damaging penetration

Question 8

Adsorption related mechanisms

Answer 8

1) Aggressive electrolyte ions

2) Hydrogen embittlment

Question 9

Aggressive electrolyte ions

Answer 9

• At tip of crack, aggressive ions form metal complexes by adsorption
• Formation of metal ion bonds weakens the metal structure and results in rupture of metal-metal bonds
• successive adsorption removes metal atoms from surface

Question 10

Hydrogen embrittlement

Answer 10

• Intense pit condition result in pH falling to 1-2
• In these region cathodic activity becomes the reduction of hydrogen ions to hydrogen gas
• Two step reaction
• Initially hydrogen ions are reduced to hydrogen atoms that adsorb onto surface. Some H atoms diffuse into the metal.
• This can weaken the metal-metal bond
• can form metal hydrides causing embrittlement
• can combine to form H2 molecules which are unable to diffuse out of a cavity and increase stress in material

Question 11

Designing Against Stress Corrosion Cracking

Answer 11

• Prevent residual stress in materials by careful annealing
• Take care when welding not to introduce stresses and to anneal to relieve potential stress
• Eliminate possible sources of aggressive ions