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ENG10002 > Week 11: Corrosion > Flashcards

Week 11: Corrosion Flashcards

1
Q

What is corrosion?

A

Destruction of a metal or alloy because of CHEMICAL or ELECTROCHEMICAL REACTIONS with SURROUNDING ENVIRONMENT or MEDIUM

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2
Q

Why does corrosion matter?

A

It degrades a material and can change its properties leading to failure

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3
Q

What is a requirement for corrosion to occur?

A

Ions that combine with the electron have to be electrically connected

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4
Q

Why does corrosion occur?

A

Because the electrolyte is in contact with the anode and cathode

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5
Q

Anode

A

Gives up electrons to the circuit and corrodes (oxidation)

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6
Q

Cathode

A

Receives electrons from the circuit (reduction)

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7
Q

Path

A

Anode and cathode need to be electrically connected

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8
Q

Electrolyte

A

The medium where metallic ions leave anode and move to cathode to accept electrons

Must be in contact with anode and cathode

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9
Q

What are the 2 types of iron oxides that rust is composed of?

A

Ferrous and ferric oxide

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10
Q

Reduction

OIL RIG

A

Occurs at the cathode

(Reduction is gain)

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11
Q

Oxidation

OIL RIG

A

Occurs at the anode

(Oxidation is loss)

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12
Q

Galvanic series

A

Ranks reactivity of metals/alloys in seawater

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13
Q

How does corrosion occur?

A

An electrochemical process where the electrons from one chemical species transfer to another

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14
Q

What are the forms of corrosion?

UA - UA
G - gives
C - courteous
P - people
I - in
SL - SL
E - extra
S - space
HE - here

A
  • Uniform attack
  • Galvanic corrosion
  • Crevice corrosion
  • Pitting corrosion
  • Intergranular corrosion
  • Selective leaching
  • Erosion-corrosion
  • Stress corrosion
  • Hydrogen embrittlement
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15
Q

Uniform Attack

A

Oxidation & reduction reactions occur uniformly over surfaces.

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16
Q

Galvanic Corrosion

A

Occurs when two metals or alloys having different compositions (inert or active) are in electrical contact while exposed to an electrolyte.

This leads the anode to give electrons out to the cathode and corrodes the anodic side where the cathodic remains unharmed

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17
Q

Localised Corrosion

A

Crevice or pitting corrosion

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18
Q

Crevice Corrosion

A

Narrow and confined spaces causing high in ion concentrations which accelerate oxidation

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19
Q

Intergranular Corrosion

A

Occurs in specific alloys along grain boundaries in particular environments

Precipitates eat away at the chromium and make the material less resistant to corrosion

20
Q

Pitting Corrosion

A

Downward propagation of small pits and holes, microns in size

21
Q

Selective Leaching

A

Preferred corrosion of one element of an alloy.

This occurs in brass where zinc is corroded away to leave only the copper, impairing the properties of the alloy

22
Q

Erosion Corrosion

A

Combined chemical attack and mechanical wear

23
Q

Stress Corrosion

A

Corrosion at crack tips when a tensile stress is present

24
Q

Hydrogen Embrittlement

A

Loss of strength due and metal becoming brittle due to Hydrogen absorbed through a metal surface

25
Q

How can you prevent/protect against corrosion?

MS - might sell
C - candy
I - in
T - there

A
  • Material selection
  • Coating
  • Inhibitor
  • Thermodynamics method
26
Q

Passivate

A

Metals that form thin oxide layer that slows corrosion

27
Q

Coating

A

Paints, plastics/rubbers/glass/ceramics, metals

28
Q

Inhibitor

A

Substances which decrease corrosiveness when added in low concentrations to the environment

29
Q

What does an inhibitor depend upon?

A

Alloy and corrosive environment

30
Q

Anode Inhibitor

A

Slows down the anodic half-reaction of corrosion by the metal surface become passive

31
Q

What is a material called if it’s easy to give out electrons?

A

Active

32
Q

Cathode Inhibitor

A

Slows down the rate of reduction reaction at a cathode

33
Q

Cathodic Protection

A

By using a sacrificial anode or external voltage

34
Q

Swelling

A

Liquid or solute diffuses into and is absorbed within the polymer

35
Q

Dissolution

A

A continuation of swelling when a polymer is completely soluble

36
Q

Scission

A

Rupture of molecular chain bonds in polymers

37
Q

Why is rupture in polymers bad?

A

Reduces the molecular weight and lowers mechanical strength and corrosion resistance

38
Q

How can bond rupture occur?

A

Radiation, heat or chemical reactions

39
Q

What are examples of weathering?

A
  • UV rays starts oxidation
  • Water can reduce hardness and stiffness if absorbed into polymers
40
Q

What is a material called if it’s difficult to give out electrons?

A

Inert

41
Q

What do swelling and dissolution both lead to?

A

A polymer with a lower Tg and E, but with more viscous behaviour

42
Q

How to prevent swelling and dissolution?

A
  • Increase molecular weight
  • Increase degree of crystallinity
  • Decrease temperature
43
Q

Why does chromium in stainless steels make them corrosion resistant as compared to carbon steel?

A

The chromium forms an oxide layer to prevent corrosion.

In carbon steels, rust forms instead of a protective coating,

44
Q

Briefly explain why cold-worked metals are more susceptible to corrosion than noncold-worked
metals.

A

CW is more susceptible to corrosion because of increase in dislocation density.

More dislocation density means there’ll be a high energy state there so more corrosion can occur

45
Q

What possible mechanism accounts for an inhibitor’s effectiveness?

A

The formation of a very thin and protective coating on the corroding surface.

46
Q

Which metal is typically used in the sacrificial cathodic protection of steel?

A

Zinc

ENG10002 (17 decks)

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