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Materials 121 > Chapter 9 - Corrosion > Flashcards

Chapter 9 - Corrosion Flashcards

1
Q

True/False: Corrosion is a purely chemical process.

A

False. Corrosion is an electrochemical process.

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2
Q

What is a positively charged ion callled?

A

A cation.

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3
Q

What is a negatively charged ion called?

A

An anion.

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4
Q

What is oxidation?

A

The reaction that forms cations that go into aqueous solution. Also called the Anodic Reaction.

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5
Q

What is reduction?

A

Reaction where a metal or non-metal cation accepts electrons. Also called the Cathodic Reaction.

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6
Q

Where does oxidation occur?

A

At the anode, that is why it is called the anodic reaction.

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7
Q

Where does reduction occur?

A

At the cathode, that is why it is called the cathodic reaction.

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8
Q

Oxidation is the ____ of electrons. Reduction is the ____ of electrons.

A

Oxidation is the loss of electrons. Reduction is the gain of electrons.

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9
Q

True/False: Oxidation and reduction must happen at the same time and at the same rate.

A

True. They are both halves of the same reaction.

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10
Q

What is the basic form of an acidic reaction?

A

2H + 2e => H2 (gas)

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11
Q

What is the basic form of an oxidation reaction?

A

O2 + 4H + 4e => 2H2O

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12
Q

What is the basic form of an alkaline or neutral reaction?

A

O2 + 2H2O + 4e => 4OH-

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13
Q

What kind of reaction is the most common for our purposes?

A

The neutral/alkaline reaction. As we assume that the liquids are neutral.

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14
Q

What kind of reaction is the rust reaction?

A

A neutral/alkaline reaction.

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15
Q

True/False: Different metals corrode at different rates.

A

True.

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16
Q

How do we determine how fast different metals will corrode?

A

One method for comparing the tendency for metals to corrode is to compare their half-cell oxidation or reduction potentials to that of a known reference.

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17
Q

How is the electrical potential of the cell calculated?

A

Potential difference between the reduction half-cell potential and the oxidation half-cell potential.

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18
Q

What metal is usually used as the reference metal for corrosion testing?

A

Platinum, as it is chemically inert and does not corrode.

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19
Q

What voltage is the hydrogen reference electrode assigned?

A

0V. It is taken as ground and the potential of the tested half cell is measured against it.

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20
Q

True/False: Metals that are more reactive than the hydrogen reference are assigned negative potentials.

A

True. Negative is more reactive than the hydrogen reference, positive is less reactive.

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21
Q

When two metals are used in a cell, which will oxidise and which will reduce?

A

The one with the more negative electrode potential will oxidise, and the one with the more positive electrode potential will be reduced.

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22
Q

Why is the electrode potential table not useful for real world situations?

A

The electrode potential table will only explain how elements will corrode under the exact conditions of the experiment.

  • 25*C
  • Pure Elements only
  • In a solution of its own ions at one mol
  • Does not consider metals that form passive films or oxide layers.
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23
Q

Since the electrode potential table is not useful for real world situations what is used instead?

A

A galvanic series. Which is simply a relative table.

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24
Q

What is a galvanic series?

A

A table of metals including common alloys, listed in order of least reactive to most reactive relative to each other. Metals that have the same reactivity are shown with brackets.

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25
Q

True/False: Alloys in brackets have the same tendency to corrode.

A

True. No compositional galvanic corrosion.

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26
Q

Why are some alloys in the galvanic series listed twice?

A

Because some alloys have both passive and active conditions.

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27
Q

What is a passive condition for an alloy?

A

It is a conditions where the alloy can form a stable protective oxide.

28
Q

What is an active condition for an alloy?

A

It is a condition where the alloy cannot form a stable protective oxide.

29
Q

What are the four components of a galvanic cell?

A
  1. Anode.
  2. Cathode.
  3. Physical Electrical connection.
  4. A liquid electrolyte.
30
Q

What function does the anode perform within a galvanic cell?

A

The anode gives up electrons to the circuit, this means it is where the anodic/oxidation reaction occurs.
This is what undergoes corrosion.

31
Q

What function does the cathode perform within a galvanic cell?

A

Receives electrons from the circuit, this means this is where the cathodic/reduction reaction occurs.
Ions that combine with the electrons produce a byproduct at the cathode.
The cathodic reaction depends on the environmental conditions.

32
Q

What function does the physical electrical connnection perform within a galvanic cell?

A

The anode and cathode must be electrically connected usually by a physical connection, to permit the electrons to flow from the anode to the cathode to continue the reaction.

33
Q

What function does the liquid electrolyte perform within a galvanic cell?

A

The liquid electrolyte must be in contact with both the anode and cathode in order to complete the circuit.

34
Q

What is a composition cell?

A

A composition cell is a galvanic cell formed with any two dissimilar metals.

35
Q

In a composition cell which metal will be the anode?

A

The metal that is lower on the galvanic series table will become the anode.

36
Q

In a composition cell what will determine the rate at which the metal will corrode?

A

The further the two metals are apart from each other on the galvanic series the faster the corrosion will occur.

37
Q

True/False: In a composition cell corrosion only occurs at the anode.

A

True. The cathode is protected from corrosion by the process.

38
Q

True/False: When two metals are coupled in a composition cell the corrosion acts at the same rate as it would if the metals were isolated.

A

False. The metal that forms the cathode is protected from the corrosion. Whereas the metal that forms the anode undergoes significantly accelerated corrosion. DON’T MIX METALS.

39
Q

Why would we use a material with less corrosion resistance as a coating on a material?

A

Because if the surface is not completely covered, the material with less corrosion resistance will form the anode and corrode more quickly. Whereas the material being coated will form the cathode and will not corrode.

40
Q

What other factor controls the effect of the galvanic corrosion?

A

The relative size of the anode and the cathode.

41
Q

What happens to the galvanic corrosion if the anode is smaller than the cathode?

A

The corrosion on the anode will be increased.

42
Q

What happens to the galvanic corrosion if the anode is larger than the cathode?

A

The corrosion on the anode will be reduced.

43
Q

What is the equation for the rate of metal loss to corrosion?

A 
m = (I x t x M) / (n x F)
m = Mass in grams of metal lost.
I = Corrosion current in amps.
t = Time in seconds.
M = Atomic mass of the metal (g/mol)
n = number of electrons exchanged in the anodic reaction.
F = Faraday's number (96,500 C/mol or amp x sec/mol)
44
Q

What is the current density?

A

The current density is the corrosion current divided by the area over which the corrosion occurs.

45
Q

What is a concentration cell?

A

A concentration cell is a corrosion reaction that develops due to differences in the concentration of the electrolyte.

46
Q

In a concentration cell which part of the metal will be the cathode?

A

The part of the metal in contact with the electrolyte with higher concentration. As it is the cathode it will be protected from corrosion.

47
Q

In a concentration cell which part of the metal will be the anode?

A

The part of the metal in contact with the electrolyte with lower concentration. As it is the anode it will undergo corrosion.

48
Q

What is the most common location to find concentration cells?

A

During crevice corrosion.

49
Q

What is crevice corrosion?

A

During crevice corrosion a crevice is filled with electrolyte. Deeper into the crevice this electrolyte becomes stagnant and depleted in oxygen, whereas at the surface it is rich in oxygen. This causes a concentration difference.

50
Q

What is pitting corrosion?

A

Pitting corrosion works the same as a crevice corrosion, except it occurs in a pit in a metal, rather than in a crevice between two pieces of metal.

51
Q

What is “Water Line” corrosion?

A

A type of concentration cell based corrosion that occurs at the meniscus of some water. The water just at the water line has a high concentration of O2, but just a bit deeper the concentration lowers enough to cause corrosion.

52
Q

How does corrosion occur in Buried Steel structures?

A

Damp soil forms an electrolyte, and the soil at the surface has a lot of O2, deeper in the concentration lowers.

53
Q

What is “droplet” corrosion?

A

The surface of the droplet has high levels of O2, but in the centre it has low levels of O2.

54
Q

How does corrosion occur in Buried Steel pipes?

A

One side of the soil is a low permeability clay, whereas another has high permeability sand, the O2 concentration is higher in the higher permeability areas.

55
Q

How do we protect against galvanic corrosion?

A

Remove one of the four components that must be present for a galvanic cell to form.

  1. Electrolyte.
  2. Anode.
  3. Cathode.
  4. Physical Electrical Connection.
56
Q

How do we remove the electrolyte from a galvanic cell?

A

Do not allow water to sit/stagnate. Avoid formation of crevices. Clean surface deposits, so that no deposit corrosion can occur.

57
Q

How do we remove the anode?

A

Through cathodic protection. This works by continuously supplying electrons to the metal to be protected, forcing it to be the cathode.
This can by done either with a sacrificial anode, or through an impressed voltage.

58
Q

How do we remove the cathode?

A

We can cover the cathode (or anode) with a protective coating to isolate it from the system.

59
Q

What kinds of protective coatings exist?

A
  1. Inert Coatings.
  2. Metal Coatings.
  3. Oxide Layers.
60
Q

How does an inert coating work?

A

An inert coating (e.g. paint) forms a physical barrier to the electrolyte. However if the material is scratched the protection fails quickly. It must be consistently maintained.

61
Q

How does a metal coating work?

A

Galvanising or metal spraying a metal covers the metal with a physical barrier but also provides cathodic protection if scratched by preferentially corroding.

62
Q

How does an oxide layer work?

A

This requires the formation of stable, adherent, non-porous, oxide layers, that exclude the electrolyte. This can reform if damaged.

63
Q

How do we remove the electrical connection between the anode and cathode?

A

We can include an insulating material between the two dissimilar metals, to stop them from forming a cell.

64
Q

What is a sacrificial anode?

A

A metal that is more reactive on the galvanic series is attached to the metal to be protected and acts as a sacrificial anode. As it corrodes it provides electrons to the metal to be protected. As it is consumed it must be replaced.

65
Q

What is an impressed voltage?

A

A power supply/battery is used to supply electrons to the part to be protected. Must be careful to ensure corrosion is not accidentally accelerated. Specialist work.

66
Q

What is passivation?

A

Passivation means treating a metal in such a way that it forms a protective oxide layer.

Materials 121 (8 decks)

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