Lecture 2

Question 1

6 types of corrosion

Answer 1

1) corrosion of a simple metal surface
2) dissimilar metal corrosion
3) selective attack
4) crevices and pitting corrosion
5) flow induced corrosion
6) stress induced corrosion

Question 2

What are the cathodic and anodic reactions?

Answer 2

Cathodic is the reduction reaction (re-plating)

Anodic is the oxidation reaction (dissolution)

Question 3

Corrosion in terms of electrode potential

Answer 3

The more negative electrode potential the more likely it is to corrode, doesn’t tell us the rate.

Question 4

General Anodic reaction

Answer 4

M = Mn+ + ne

Question 5

General Cathodic reaction (normal pH)

Answer 5

2H2O + O2 + 4e = 4OH-

Question 6

General Cathodic reaction (low pH)

Answer 6

2H+ + 2e = H2

Question 7

General steps of corrosion

Answer 7

• Metal exposed to atmosphere containing water
• Anode set up, metal dissolves into solution leaving excess electrons (oxidation)
• Electrons flow from anode to another site on metal surface o set up cathodic site
• At cathode, electrons combine with water to reduce oxygen at the cathode creating hydroxide ions.
• Metal ions and hydroxide ions created then flow through the electrolyte, combine and chemically react to form the corrosion product

Question 8

Four components of wet corrosion

Answer 8

1. Anode - Metal oxidation
2. Cathode - reduction reactions
3. Electron flow from anode to cathode
4. An electrolyte or aqueous medium (water salt etc.)