Lecture 3

Question 1

Dissimilar metal corrosion

Answer 1

The more active metal will dissolve at a faster rate when coupled to the other metal compared to if the metal was just corroding on its own, uncoupled.

Question 2

Damages of Cl- ions

Answer 2

• Chloride ions increase the conductivity of water electrolytes, encourage ionic conduction through the solution, accelerate corrosion.
• Chloride ions breakdown oxide layers present on metal surfaces that protect corrosion
• Chloride ions can form complexes with metallic ions that can generate low pH (acidic) conditions that will worsen corrosion.

Question 3

Case study - Ships with copper sheets and iron nails (nails were corroding)

Answer 3

Use wax paper to cover the sheets and break the electrical connection between the copper and iron

Question 4

Case study - Iron aerial next to chimney on boat, soot covered the aerial and iron corroded rapidly

Answer 4

• Carbon in soot good conductor of electricity and acts as cathode for oxygen reduction
• Need to relocate the aerial

Question 5

Case study - Car manufacture uses brass screws to fasten steel sheets, steel rapidly corrodes

Answer 5

• Brass (copper) in contact with steel and promotes the iron corrosion.
• Use plastic inserts to break the connection between steel and brass

Question 6

Designing against dissimilar metal corrosion

Answer 6

• Eliminate one of the 4 components
• Make cathode small in comparison to anode
• Keep anode upstream
• Use of sacrificial anodes/supply of electrons