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2nd Year Shit > Materials In Service > Flashcards

Materials In Service Flashcards

(92 cards)

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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
Ideal oxide during processing
1) thin external oxide layer 2) no internal oxidation 3) low adherence
26
Ideal oxide in service
1) slow-growing external oxide layer 2) no internal oxidation 3) low tendency to spall (flakes of material that are broken off)
27
Structure of carbon steel
Hematite - Fe2O3 Magnetite - Fe3O4 Wüstite - FeO Substrate (Fe alpha)
28
How does oxides grow?
Middle outwards
29
Factors affecting oxide morphology
1) surface finish 2) chemical composition 3) phases present 4) time 5) temp 6) atmosphere
30
Why does oxidation rate slow?
As the layer of oxide becomes thicker, longer for O2 to diffuse in and metal to diffuse out
31
Breakaway oxidation
= 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
32
LOI = limiting oxygen index
= 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
33
Mechanical Failure in polymers
1) fracture 2) creep 3) fatigue 4) impact 5) wear 6) yielding, crazing 7) distortion 8) environmental stress cracking 9) plasticiser bleeding 10) swelling
34
Thermal failed in polymers
1) degradation, depolymerisation 2) dimensional instability 3) shrinkage 4) fire, slow combustion 5) thermal fatigue
35
Chemical failure in polymers
1) oxidation, ozone attack 2) chlorinolysis 3) hydrolysis 4) stress corrosion cracking 5) other chemicals
36
Optical failure in polymers
1) UV light 2) Ionising radiation 3) photo-tendering (fading of colour)
37
Electrical failure in polymers
1) arcing | 2) electrostatic buildup (degrades)
38
Cyclic vs static loading
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
Factors affecting fatigue life
1) size - fatigue is controlled by the weakest link? 2) loading 3) surface finish - surface flaws 4) surface treatment 5) temperature 6) environment
40
How does surface treatment affect fatigue life
compressive residual stresses = beneficial to fatigue life , tensile opposite.
41
How does loading affect fatigue life
under rotating bending + axial loading, material volume subjecting high stress is different
42
How does temperature affect fatigue life
- high temp, endurance limit for steels disappears ∵ mobilising of dislocations - temp over half mp, creep becomes important - annealing -> remove beneficial residual compressive stresses
43
How does environment affect fatigue life
- corrosion - cyclic losing -> localised cracking of oxide layers
44
Are residual stresses permanent ?
No - high temp + overlord can cause stress relaxation
45
Examples of surface treatment
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
What is shot peening?
= 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
Prediction of Archard Model
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
Abrasive wear of metals
= wear due to hard particles contacting a surface
49
Differences between erosion and abrasive wear
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
Solid lubricants are useful for?
1) high temp 2) sealed units 3) vacuum
51
Are wear rates directly related to friction coeff?
No - see polymers
52
Transition between mild and severe wear for brass on satellite
1) wear debris for mild = fine oxide particles (0.01-1micronm) Large metallic particles for severe (20-200) 2) finish
53
Stages of fatigue fracture
1) initiation 2) fatigue crack propagation 3) catastrophic rupture
54
Endurance limit
- 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
How do we determine initial crack length of fatigue ?
- simple inspection - ultrasonic / xrays Crack- tolerant design = remaining life time assessed of crack is found
56
Magnetic particle inspection
= 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
How to make magnetic field in MPI
1) use permanent magnet for electro magnets 2) passing current through = circular magnetic field Or around component = magnetic field linear
58
How to detect defect in MPI/requirement
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
Different electrical current
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
Procedure for MPI
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
Advantages of MPI
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
Disadvantages of MPI
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
Eddy current (basic)
- 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
Problem with eddy current
Not shape/material independent ∴ understand relationship between shape and defect
65
How does eddy current work?
- 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
Types of NDT
1) dye penetrant 2) MPI 3) eddy current 4) ultrasonic testing 5) radiography
67
Other uses of eddy currrent
1) quality control 2) inspection 3) checking for corrosion 4) ID and sort materials 5) heat damage 6) measurement of coating thickness
68
Ultrasonic testing
- 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
Generation of ultrasound
- disc piezo electric material (transducer crystal) + AC Eg. Materials = quartz, barium titanate Disc into probe -> ultrasound Can act as transmitter and receiver
70
Dead zone
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
Near zone
Almost parallel sided beam = main region to detect defects Best sensitivity @ far end
72
Far zone
Beam spread ∴ detection sensitivity decreases w/ square of distance
73
Sound attenuation
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
Radiography
- 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
Concrete
Ceramic composite - strong in compression, weak in tension ∴ reinforce with steel (alkali environment = no corrosion)
76
Concrete corrosion
Overtime, obvs interact w/environment e.g. salts from atmosphere, acids from rain ``` Cl- SO4- HCO3- O2 H2O ```
77
Name layers ish of reinforced concrete | Rememebr pic
Carbonation zone Reinforced steel Aggregate Cement mix
78
What happens when the acid reaches steel in concrete
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
How to protect concrete
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
Creep
= Static mechanical stress + Elevated temp T>0.4Tm - Time dependent - Common in components subjected to constant Load/stress
81
Metal corrosions
1) Electrochemical wet corrosion | 2) high temp oxidation
82
Ceramic corrosion
Sea, water + acid attack
83
Polymer corrosion
1) oxidation of double bond links | 2) UV light catalysis of free radical cross linking
84
Endurance limit
Highest stress a material can withstand for an infinite no of cycles without breaking
85
What happens when polymers react with UV light ? | In terms of paint coating metal
Degrades -> brittles + cracks -> water ingress ∴ metal corrodes -> decrease bonding between metal and paint
86
Dye penetrant testing
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
Adv + disadvantage of Dye
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
Fatigue
Due to cyclic loading/stresses Key part = crack initiation
89
Fatigue life
No of cycles required for material to fail at given stress
90
Fatigue strength
Stress that produces failure in given no of cycles Usually 10^7
91
Well defined endurance limits (flatten out S-N curve)
Carbon steels + ductile cast irons If stress is below fatigue limit, component will not fail regardless of no of cycles
92
No obvious fatigue limit
Al, Ti, Cu Will fail at any stress and no of cycles

2nd Year Shit (5 decks)

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