Lecture 9

Question 1

Sacrificial Anodes

Answer 1

Make our structure the site of the cathodic reaction by introducing a sacrificial material which is more reactive (e.g. aluminium is more reactive than steel)

Question 2

Impressed current corrosion protection (ICCP)

Answer 2

• A three electrode cell
• Anode on the outside of vessel and connected to the circuit through a discrete connection
• rest of anode insulated from the vessel wall
• Vessel wall acts as the cathodic site
• potential of anode controlled with respect to the cathode by means of a third reference electrode
• Power supply forced between anode and cathode
• electrons are forced in to the bulk metal which becomes negatively charged (cathodically protected) with respect to the anode which becomes positively charged

Question 3

What potential to use for ICCP

Answer 3

• Protection potential typically -850mV vs SSC
• important not to go to low potential as it is possible to evolve hydrogen on protected surface which could damage surface

Question 4

Principles of Impressed current protection

Answer 4

1)Insulation anode for protection
2)Non consumable elecltrodes (anodes designed for life as anode reaction is the oxidation of water at the surface of the anode, 2H2O = O2 + 4H+ + 4e)
In sea water oxidation of chloride to chlorine is observed
3)Control electronics (need potentiostats and computer controls, can be costly)

Question 5

Anodic protection

Answer 5

• Reverse of cathodic impressed current
• If potential of metal is increased a passive oxide layer will form
• potentiostat used to maintain potential in passive region
• If electrical supply fails very aggressive corrosion will start due to increase in corrosion potential