19.4.1 Corrosion and the Prevention of Corrosion

Question 1

Corrosion and the Prevention of Corrosion

Answer 1

• Corrosion is the spontaneous environmental oxidation of a metal.
• Salt and acid can speed up corrosion.
• Corrosion can be prevented by passivation and by connection the metal to be protected to a sacrificial anode.

Question 2

note

Answer 2

• Corrosion is the spontaneous environmental oxidation of a metal.
• Rusting is the corrosion of iron. The final product of the environmental oxidation of iron is rust, Fe 2 O 3 ·H 2 O.
• When iron rusts, the iron acts as both the anode and the wire of a galvanic cell. Since electrons are free to flow through the iron, the oxidation of iron can take place anywhere in the piece of metal. This results in pitting, which lowers the structural integrity of the piece of iron.
• Salts such as NaCl speed up corrosion by acting as an electrolyte and facilitating the transfer of charge through water to the metal.
• Acid speeds up the corrosion of iron. In the presence of H + , the reduction reaction O 2 (g) + 4H+(aq) + 4e –
  2H 2 O(l) can occur. This half-reaction has a larger standard reduction potential than the reaction in the absence of H + , and therefore acts as a better cathode.
• Passivation is one method of preventing the corrosion of metals. Passivation involves coating the metal to be protected with a thin metal oxide layer, thus blocking the metal from further oxidation. Galvanized nails are coated with zinc, which forms a thin zinc oxide coating to protect the nail from corrosion.
• Another method of preventing corrosion is the use of
  sacrificial anodes. Sacrificial anodes are pieces of strongly reducing metals (such as magnesium) in electrical contact with the metal to be protected (such as iron). When the iron is oxidized, electrons are transferred from the magnesium to the iron, preventing damage. Sacrificial anodes are used in cases where it would be difficult to replace the iron, such as the hulls of ships and underground pipes.

Question 3

How is a galvanized nail protected from corrosion?

Answer 3

• Passivation
• Zinc acts as a sacrificial anode.
• The iron is coated to prevent exposure to air.

Question 4

What is the role of the bulk iron metal in the corrosion process?

Answer 4

Conductor

Question 5

Under anaerobic conditions, which of the following can oxidize iron?

Answer 5

H2O

Question 6

In a corroding nail, what is used to reduce dissolved oxygen?

Answer 6

Electrons given up in the oxidation half-reaction

Question 7

Bars made of which of the following can be bolted to a ship hull to prevent corrosion?

Answer 7

Magnesium

Question 8

Which of the following is not a stronger reducing agent than iron?

Answer 8

Oxygen

Question 9

In which of the following could corrosion occur?

Answer 9

Pitted metal surface

Question 10

Which reaction best represents the formation of rust?

Answer 10

2Fe(OH)3(s) –> Fe2O3 * H2O(s) + 2H2O(l)

Question 11

All of the following are examples of electrochemical reactions. Which is an oxidation reaction?

Answer 11

2Fe –> 2Fe2+ + 4e-

Question 12

Which of the following could not be used as a sacrificial anode for an iron pipe?

Answer 12

Hydrogen