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Engineering Materials > Corrosion and Degradation > Flashcards

Corrosion and Degradation Flashcards

1
Q

Define corrosion.

A

The loss of volume from a material due to a chemical reaction with its environment.

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2
Q

Define polymer degradation.

A

The structure and property change of polymers through interactions with
their environment.

Physical and/or chemical changes with or without loss of volume.

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3
Q

Name four types of polymer degradation.

A

Swelling
Dissolution
Bond rupture
Weathering

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4
Q

Describe swelling.

A

Partial dissolution of polymer in the solvent.

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5
Q

Describe bond rupture.

A

Polymer exposed to radiation.

Breaks down bonds and rearranges atoms.

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6
Q

Describe weathering.

A

Aging of polymer due to the natural phenomenon like: daylight, temperature, moisture and oxygen.

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7
Q

What is stress corrosion cracking (SCC)?

A

Accelerated corrosion localized at cracks in
loaded components.

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8
Q

What is corrosion fatigue?

A

The mechanical degradation of a material under corrosion and cyclic loading.

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9
Q

What is erosion corrosion?

A

Degradation of material surface due to mechanical action.

e.g. Water or wind removes the loose corrosion particles from the surface, exposing layers of material.

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10
Q

Define durability.

A

The material’s ability to function in its environment for a period of time.

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11
Q

Name four types of different chemical environments.

A

Type of fluid
pH
Temperature
Static or flowing

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12
Q

Name the conditions required for corrosion (REDOX) to occur.

A

Anode

Cathode

Electrical connection between anode and cathode.

Electrolyte with oxidising species.

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13
Q

What metals are most reactive and what is their reduction potential?

A

Li, Na, Mg

-3.05V

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14
Q

What metals are least reactive?

A

Au, Pt, Ag

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15
Q

What metal acts as the anode?

A

Metal with the lower potential (more negative).

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16
Q

How does corrosion take place with one metal present?

A

Oxygen concentration gradient exists
between the metal at the surface of
the water and metal in the water.

More oxygen at the top of the metal.

Electrons flow to the top.

Ions go to solution and corrosion takes place in the metal.

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17
Q

Name five types of selective corrosion.

A

Intergranular (IGC)

Pitting

Galvanic attack

Stress corrosion cracking

Corrosion fatigue

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18
Q

What is intergranular corrosion (IGC)?

A

When grain
boundaries have different chemical
properties to the grain.

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19
Q

What is pitting corrosion?

A

Preferential
attack at breaks in
the natural oxide on metals or at
precipitated alloys in certain
compounds.

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20
Q

What is galvanic attack?

A

When two or more different metals are brought into electrical contact under water and REDOX occurs.

21
Q

What is passivation?

A

Thin, nonporous, self healing film on the surface of the metal used to separate metal from fluid electrolyte, thus, preventing corrosion.

22
Q

What is oxidation?

A

Dry corrosion due to metals reacting to oxygen, forming surface oxide film.

23
Q

Name two metals good for passivation.

A

Aluminium
Stainless steel

24
Q

Pilling-Bedworth ratio?

A

V oxide / n V metal

25
Q

What does P-B ratio indicate?

A

If an oxide film will remain protective.

26
Q

What does it mean if P-B ratio < 1?

A

Oxide coating is too thin or porous so will give no protection.

e.g. Mg

27
Q

What does it mean if P-B ratio > 2?

A

Oxide coating chips off and provides no
protection.

e.g. Iron

28
Q

What does it mean if P-B ratio is between 1 and 2?

A

Oxide coating provides protection against
further surface oxidation, thus is passivating

e.g. aluminium, titanium, chromium-containing steels.

29
Q

Name three types of environmental degradation.

A

Photo induced
Chemical
Thermal

30
Q

What is photo induced degradation?

A

Polymer cracks or
disintegrates when exposed to
sunlight.

e.g. polypropylene, LDPE

31
Q

What is chemical degradation?

A

Change of properties in polymer due to chemical reactions with the environment.

32
Q

What is thermal degradation?

A

Molecular
deterioration due to overheating.

33
Q

Name five factors to consider when picking a material and how it will corrode.

A

Standard reduction potential

Composition

Temp

Flow

Plant location (near a coast?)

34
Q

Equation for corrosion penetration rate (CPR)?

A

CPR = K W / ρ A t

W = weight loss
t = time

35
Q

Units of CPR

A

mL / year

mm / year

36
Q

Faraday’s equation?

A

W = I t M / n F

W = weight gained / lost
I = current
t = time
M = atomic mass
n = charge
F = Faraday’s constant (96,500 C/mol)

37
Q

Name three corrosion issues in the oil and gas industry.

A

Sulfidation corrosion

Wet H2S cracking

High temperature hydrogen attack

38
Q

What is sulfidation corrosion?

A

Naturally occuring sulfur in crude oil corrodes metals at high temperatures.

39
Q

What is wet H2S cracking?

A

Corrodes carbon steel equipment.

40
Q

What is high temperature hydrogen attack?

A

Hydrogen at high temperature dissolves into steel then reacts with carbon to form methane.

Causes fissures and cracking.

41
Q

What are the four main design considerations for corrosion control?

A

Design

Coatings

Inhibitors

Monitoring

42
Q

What to avoid when designing to prevent corrosion?

A

Fluid trapping

Galvanic attack

Crevices

Stress corrosion and fatigue

43
Q

What to consider when designing to prevent corrosion?

A

Uniform attack

Cathodic protection

Inspection and maintenance

44
Q

What is cathodic protection?

A

Preventing a metal from corroding by making it an electrical cathode and using a sacrificial metal as an anode.

45
Q

What is a lining?

A

Different material like PTFE or glass that acts as a physical barrier to separate corrosive environment and piping.

46
Q

What are corrosion inhibitors?

A

Reduces the
rate of attack when dissolved in a corrosive medium.

46
Q

What is a passive coating?

A

Separates material from corrosive
environment.

Very corrosion
resistant.

47
Q

What is an active coating?

A

Sacrificially corrodes to protect the desired metal from
corrosion.

48
Q

What are self-generated coatings?

A

Rely on alloying in sufficient concentrations so
that a protective film forms spontaneously.

Engineering Materials (8 decks)

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