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2nd Year Shit > Materials In Service > Flashcards

Materials In Service Flashcards

1
Q

What is galvanised steel?

A

Steel dipped in molten zinc (oxidises in place of steel)

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2
Q

Corrosion

A

Interaction of a materials with its environment in an engineering context leading to degradation and ultimately failure of structure/assembly

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3
Q

Give examples of why a material might fail?

A

Material issues, manufacturing, design, installation, in service conditions

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4
Q

Wet corrosion

A

Water takes an active role

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5
Q

High temperature corrosion

A

Oxidation w/atmospheric O2

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6
Q

Black rust

A

Fe3O4
Magnetite
Limited O2

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7
Q

Red rust

A

Fe2O3
Haematite
Water? (Salt)

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8
Q

What’s needed for electrochemical reaction?

A

1) anode + cathode
2) electrolyte
3) electronic conductor = connector between anode and cathode

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9
Q

Electrode potential

A

= when a perfect ideal metal is placed in an electrolyte, an EP is developed = measure of tendency a metal has to give up e BUT
driving F for oxidisation is offset by an equal and opposite F for reduction reaction ∴ no net overall reaction occurs

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10
Q

Half cell

A

Overall no reaction is happening

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11
Q

No corrosion

A

∵ noble metal, protective layer, electrolyte could be deionised water

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12
Q

General attack

A

Corrosion tends to be localised

I.e. not homogenous

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13
Q

Inter-granular corrosion

A

Starts @ surface —> GB ∵ GB = anodes, grain centre = cathode

GB = anodes ∵ : 1) pure metals - due to impurities 2) alloys - due to particles + precipitates

∴ different composition

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14
Q

Exfoliation/layer corrosion

A

= ends of heavily deformed materials

E.g.

1) heavy rolling —> grains elongated
2) corrosion products = ↑ V than metal -> pushes metal apart

Similar to Inter granular but difference is:

Metals starts to delaminates/open up

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15
Q

Selective corrosion

A

Eg.

1) brass - dezincification (dissolving of Zn in H2O leaving porous Cu.
∴ add arsenic + lead to stabilise material (resistant alloy)

2) cast iron - graphite flakes (cathode) ∴ metal corrodes leaving graphite flakes

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16
Q

Stress corrosion cracking

A

= Inter-granular/transgranular cracking of a metal by combined action of a static tensile stress + specific environment

= delayed failure.

= stress (linear) speeds up corrosion

Stress raisors -> small cracks, precipitate @ surface, notch?

Corrosion -> crack grow faster

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17
Q

Corrosion fatigue

A

= cycle stress

Corrosion -> prevent crack healing
Fatigue crack -> corrode faster

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18
Q

Crevice corrosion

A

= occurs ∵ part of metal surface is in a shielded/restricted environment as opposed to the rest of the metal which is exposed to a larger V or electrolyte

= water trapped between 2 components / slots

Water stagnates in crevice + forms corrosive cell

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19
Q

Deposit corrosion

A

= creation of a crevice due to something on the component

E.g. mud of car

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20
Q

Pitting corrosion

A

= localised corrosion

Pit = anode, surrounding metal = cathode

How it start e.g. : Break in protective film, stress raisers/ emerging dislocation caused by residual stresses, compositional differences (inclusion, segregation/precipitates)

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21
Q

Bitmetallic corrosion/ galvanic corrosion

A

More than 1 metal

Large diff of PD between 2 metals -> more corrosion

Only corrode if diff > 0.3V

Large anode + small cathode = slower corrosion ∵ insufficient area of cathode to sustain corrosion cell.

Other way round = much faster

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22
Q

How to prevent bimetallic corrosion

A

1) select metals close to each other in electrochemical series
2) avoid small anode to large cathode
3) insulate dissimilar metals
4) apply coatings w/ caution
5) add corrosion inhibitors
6) design longer service life?
7) install 3rd metal = anodic both metals of interest (sacrificial anode)

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23
Q

Hydrology

A

Autocataltioc reaction?

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24
Q

High temp oxidation

A

≠ corrosion

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25
Q

Ideal oxide during processing

A

1) thin external oxide layer
2) no internal oxidation
3) low adherence

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26
Q

Ideal oxide in service

A

1) slow-growing external oxide layer
2) no internal oxidation
3) low tendency to spall (flakes of material that are broken off)

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27
Q

Structure of carbon steel

A

Hematite - Fe2O3
Magnetite - Fe3O4
Wüstite - FeO
Substrate (Fe alpha)

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28
Q

How does oxides grow?

A

Middle outwards

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29
Q

Factors affecting oxide morphology

A

1) surface finish
2) chemical composition
3) phases present
4) time
5) temp
6) atmosphere

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30
Q

Why does oxidation rate slow?

A

As the layer of oxide becomes thicker, longer for O2 to diffuse in and metal to diffuse out

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31
Q

Breakaway oxidation

A

= Sudden increase in oxidation rate

Dunno acc reasons but:

1) breaks in oxide
2) increase in oxygen diffusion (I.e. increase T, porosity, GB)
3) eg of stainless steel - depletion of protective elements I.e. Cr

AVOID changes in T

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32
Q

LOI = limiting oxygen index

A

= measures the min conc. of O2 (%) that will support combustion (polymer)

Test @ room + elevated T to see how material changes

> 26% air = self extinguishing polymer
really high values = fire retardant

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33
Q

Mechanical Failure in polymers

A

1) fracture
2) creep
3) fatigue
4) impact
5) wear
6) yielding, crazing
7) distortion
8) environmental stress cracking
9) plasticiser bleeding
10) swelling

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34
Q

Thermal failed in polymers

A

1) degradation, depolymerisation
2) dimensional instability
3) shrinkage
4) fire, slow combustion
5) thermal fatigue

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35
Q

Chemical failure in polymers

A

1) oxidation, ozone attack
2) chlorinolysis
3) hydrolysis
4) stress corrosion cracking
5) other chemicals

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36
Q

Optical failure in polymers

A

1) UV light
2) Ionising radiation
3) photo-tendering (fading of colour)

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37
Q

Electrical failure in polymers

A

1) arcing

2) electrostatic buildup (degrades)

38
Q

Cyclic vs static loading

A

Static: until applied K reaches Kc crack will not grow

Cyclic: K applied can be well below Kc and crack still has potential to grow over time

Kc = 30MPa.m^(1/2)

39
Q

Factors affecting fatigue life

A

1) size - fatigue is controlled by the weakest link?
2) loading
3) surface finish - surface flaws
4) surface treatment
5) temperature
6) environment

40
Q

How does surface treatment affect fatigue life

A

compressive residual stresses = beneficial to fatigue life , tensile opposite.

41
Q

How does loading affect fatigue life

A

under rotating bending + axial loading, material volume subjecting high stress is different

42
Q

How does temperature affect fatigue life

A
  • high temp, endurance limit for steels disappears ∵ mobilising of dislocations
  • temp over half mp, creep becomes important
  • annealing -> remove beneficial residual compressive stresses
43
Q

How does environment affect fatigue life

A
  • corrosion - cyclic losing -> localised cracking of oxide layers
44
Q

Are residual stresses permanent ?

A

No - high temp + overlord can cause stress relaxation

45
Q

Examples of surface treatment

A

1) chrome + nickel plating -> 60% reduction in endurance limits
2) carburising, nitriding etc. Improve fatigue strength
3) hot rolling + forging -> surface decarburisation-> lower strength + residual tensile stresses
4) cold rolling + shot peening - produces compressive stresses -> Improve fatigue life time

46
Q

What is shot peening?

A

= cold working process. Surface bombarded w/ spherical media called shot -> indentation -> net result = state of residual compression

Crack ≠ initiate/propagate in a compressively stressed zone

47
Q

Prediction of Archard Model

A

True:

1) Wear rate usually prop to load
2) “ “ = independent of contact area for given load, or for given contact stress increases linearly w/apparent contact area
3) “ “ independent of sliding speed

Untrue:
1) loss of material through wear = prop to sliding length/time

48
Q

Abrasive wear of metals

A

= wear due to hard particles contacting a surface

49
Q

Differences between erosion and abrasive wear

A

1) strain rates much high (liquid drops = Severe erosion, espec on brittle materials)
2) fluid dynamics of gas/liquids flow may be important
3) angle of incidence = important variable

50
Q

Solid lubricants are useful for?

A

1) high temp
2) sealed units
3) vacuum

51
Q

Are wear rates directly related to friction coeff?

A

No - see polymers

52
Q

Transition between mild and severe wear for brass on satellite

A

1) wear debris for mild = fine oxide particles (0.01-1micronm)

Large metallic particles for severe (20-200)

2) finish

53
Q

Stages of fatigue fracture

A

1) initiation
2) fatigue crack propagation
3) catastrophic rupture

54
Q

Endurance limit

A
  • if stress below -> component = infinite life

Steels + copper alloys limit = 0.35 - 0.5ts

Nf = 10^7 - used well endurance limit isn’t well defined

55
Q

How do we determine initial crack length of fatigue ?

A
  • simple inspection
  • ultrasonic / xrays

Crack- tolerant design = remaining life time assessed of crack is found

56
Q

Magnetic particle inspection

A

= detects surface defects/shallow subsurface in Ferro-magnetic components

Detection of field using fine magnetic particles = dry (magnetite Fe3O4)
Or wet (petroleum based carrier)

Collect @ point of discontinuity/leakage forming magnetic bridge = shows location, size and shape of defect

57
Q

How to make magnetic field in MPI

A

1) use permanent magnet for electro magnets
2) passing current through = circular magnetic field

Or

around component = magnetic field linear

58
Q

How to detect defect in MPI/requirement

A

1) If magnetic field in same direction/parallel to defect = can’t detect (not visible)

Weak or strong detection depending

2) needs to be near enough to surface to disrupt magnetic

59
Q

Different electrical current

A

1) AC - strong magnetic field on surface

2) Pulsed direct current (HVDC) - rectified single phase AC
Magnetised in depth

3) DC - large components/defects

60
Q

Procedure for MPI

A

1) clean surface (corrosive products magnetic) - wire brush, sand blast
2) demagnetise sample
3) apply paint or uv active die in particulate solution = contrast with magnetic particles
4) magnetise
5) apply magnetic particle solution

61
Q

Advantages of MPI

A

1) dependable/sensitive (not as die)
2) simple/rapid
3) inexpensive
4) sub-surface flaws can be detected (die cant)
5) flaws directly visible
6) unaffected by contaminated flaws (unlike die)
7) no special surface prep

62
Q

Disadvantages of MPI

A

1) ferromagnetic materials only
2) deep defects not always highlighted
3) burn scars
4) direction of magnetic fields need to be considered
5) can be difficult to interpret results

63
Q

Eddy current (basic)

A
  • don’t have to be magnetic material, but conductive
  • induced within material

Detects: surface + subsurface defects

thickness of coatings, structural features (grain size, heat treatment conditions
Physical properties (electrical conductivity, hardness, magnetic permeability)
64
Q

Problem with eddy current

A

Not shape/material independent ∴ understand relationship between shape and defect

65
Q

How does eddy current work?

A
  • coil carrying AC place in proximity of sample -> eddy current induced
  • no defects = continuous flow
  • defect = deviation of eddy current

Changes indicated via metre, chart record or display screen

66
Q

Types of NDT

A

1) dye penetrant
2) MPI
3) eddy current
4) ultrasonic testing
5) radiography

67
Q

Other uses of eddy currrent

A

1) quality control
2) inspection
3) checking for corrosion
4) ID and sort materials
5) heat damage
6) measurement of coating thickness

68
Q

Ultrasonic testing

A
  • looks @ Internal defects + small surface cracks

Between - 0.5-20MHz

Sound waves only reflected if object = or > than wavelength

Difficult for surface defects due to ringing

69
Q

Generation of ultrasound

A
  • disc piezo electric material (transducer crystal) + AC

Eg. Materials = quartz, barium titanate

Disc into probe -> ultrasound

Can act as transmitter and receiver

70
Q

Dead zone

A

Oscillating region of material = can’t detect defects

Can get rid of it by putting a block between probe and material so block = dead zone

This is called dampening block

71
Q

Near zone

A

Almost parallel sided beam = main region to detect defects

Best sensitivity @ far end

72
Q

Far zone

A

Beam spread ∴ detection sensitivity decreases w/ square of distance

73
Q

Sound attenuation

A

Loss of energy in sound wave as it travels through material

Scattering @ GB, precipitates, inclusions

Internal friction effects

  • greater attenuation @ High hz but reducing hz = reduce penetration depth ∴ need to balance
  • attenuation depends on material
74
Q

Radiography

A
  • use of xrays + gamma -> penetrate media
  • detection via film/plate/ screen

Type of defects: porosity, voids, inclusions, Internal flaws

Difficult to detect planar defects like cracks

Welds + casting

Most materials except high or low density (polymer)

75
Q

Concrete

A

Ceramic composite - strong in compression, weak in tension ∴ reinforce with steel (alkali environment = no corrosion)

76
Q

Concrete corrosion

A

Overtime, obvs interact w/environment e.g. salts from atmosphere, acids from rain

Cl- 
SO4-
HCO3-
O2
H2O
77
Q

Name layers ish of reinforced concrete

Rememebr pic

A

Carbonation zone
Reinforced steel
Aggregate
Cement mix

78
Q

What happens when the acid reaches steel in concrete

A

Electrochemical cell is set up -> localised anode and cathode sites are set up

-OH produced @ cathode + starts to increase (similar to crevice corrosion)

Rust = bigger volume than steel -> cracks + decrease distance to reach steel + penetrate deeper

79
Q

How to protect concrete

A

1) sacrificial electrode (e.g. zinc) - will need to be replaced
2) replacing carbon steel w/ stainless steel but increases cost (price of Ni?)
3) replace w/ glass fibre but not good in alkaline condition (beginning stage)

80
Q

Creep

A

= Static mechanical stress
+ Elevated temp T>0.4Tm

  • Time dependent
  • Common in components subjected to constant Load/stress
81
Q

Metal corrosions

A

1) Electrochemical wet corrosion

2) high temp oxidation

82
Q

Ceramic corrosion

A

Sea, water + acid attack

83
Q

Polymer corrosion

A

1) oxidation of double bond links

2) UV light catalysis of free radical cross linking

84
Q

Endurance limit

A

Highest stress a material can withstand for an infinite no of cycles without breaking

85
Q

What happens when polymers react with UV light ?

In terms of paint coating metal

A

Degrades -> brittles + cracks -> water ingress ∴ metal corrodes -> decrease bonding between metal and paint

86
Q

Dye penetrant testing

A

1) clean surface + dry
2) spray dye + leave 10mins
3) wash excess + dry
4) spray developer = bleeds dye back out
5) use uv light to observe

87
Q

Adv + disadvantage of Dye

A

Adv:

1) easy to use
2) more sensitive than MPI
3) no shape problems
4) in situ
5) cheap

Dis:

1) only surface
2) no pourous materials/rough surfaces

88
Q

Fatigue

A

Due to cyclic loading/stresses

Key part = crack initiation

89
Q

Fatigue life

A

No of cycles required for material to fail at given stress

90
Q

Fatigue strength

A

Stress that produces failure in given no of cycles

Usually 10^7

91
Q

Well defined endurance limits (flatten out S-N curve)

A

Carbon steels + ductile cast irons

If stress is below fatigue limit, component will not fail regardless of no of cycles

92
Q

No obvious fatigue limit

A

Al, Ti, Cu

Will fail at any stress and no of cycles

2nd Year Shit (5 decks)

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