Lecture 5 Flashcards
1
Q
Concentration cell
A
Composition of the electrolyte (O2 levels) is not uniform and determines the location of the anode and cathode
2
Q
Differential aeration corrosion
A
Concentration cell where oxygen levels are not uniform
3
Q
Crevice Corrosion
A
Defined as attack that occurs because part of a metal surface is in a shielded or restricted environment compared to the rest of the metal that is exposed to a large volume of electrolyte.
4
Q
The Fontana Greene Mechanism
A
- Initially anodic and cathodic occur at equal rates all over surface
- As processes occur, electrolyte becomes depleted in oxygen close to surface
- Oxygen diffuses from surrounding electrolyte to replenish loss (much easier outside of crevice)
- Now there are areas of differential aeration
- Interior of crevice becomes site of anodic activity (rapidly fills with positive metal ions)
- Negative chloride ions flow into crevice to balance charge
- Chloride ion are encourage corrosion (3 reasons)
- Metal chloride complexes form hydrogen to produce very low pH
- Autocatalytic process which is self sustaining and becomes more and more aggressive
5
Q
Pitting corrosion
A
- Starts on a metal surface where there is a defect
- Initiated by metallurgical processes instead of differential concentrations
- Breach of oxide layer and corrosion will initiate at that point and small cavity will form
- Cavity essentially then follows the FG mechanism
- Localised pit chemistry maintained by the pit cap (formation of corrosion products)
- This restricts oxygen diffusion but allows Cl- diffusion
- If the pit cover ruptures the pit will die
6
Q
Raindrop model
A
- Anodic and cathodic activity occurs all over metal
- Cathodic activity consumes oxygen
- Diffusion of oxygen to surface easiest at edge of droplet
- Centre of droplet becomes focal anode and outer edge cathode
- metal ions diffuse out and hydroxide ions diffuse inwards where they meet to form a ring of corrosion product
