6.4 Flashcards

Does not include corrosion susceptibility

1
Q

what is corrosion

A

a natural phenomenon where chemical or electro chemical reactions cause metal to oxidise

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2
Q

what needs to be present for corrosion to happen

A

difference in electrode potentials and an electrolyte

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3
Q

what metals are less likely to corrode

A

noble metals such as gold and platinum

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4
Q

what metal can corrode easily when in contact with concrete covered in water and why

A

aluminium because the water washes lime out of the concrete and lime is alkali which corrodes the aluminium

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5
Q

what are the 2 chemical changes in corrosion

A

anodic and cathodic

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6
Q

out of anodic and cathodic what loses material in corrosion

A

the anodic metal loses the material

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7
Q

what metals have a passive film to prevent corrosion and what is it

A

titanium and stainless steel because they have a thin oxide layer

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8
Q

what metals have a loose and porous film which can be easily penetrated by electrolyte

A

magnesium and aluminium

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9
Q

what is the best way to control corrosion

A

to have good cleanliness

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10
Q

what is galvanic corrosion

A

2 dissimilar metals make electrical contact in the presence of an electrolyte

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11
Q

what kind of metals corrode easier

A

less noble

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12
Q

what is the difference between galvanic and electrolytic corrosions

A

electrolytic requires an electrical current to be passed through both metals for any corrosion to take place

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13
Q

where is microbiological corrosion usually found

A

in fuel tanks

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14
Q

what does the micro organisms do to causes corrosion

A

produce corrosive chemicals

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15
Q

how can microbiological corrosion be minimised

A

maintaining a maintenance programme which includes water checks looking for a milky appearance

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16
Q

what is stress corrosion

A

intergranular cracking caused by the combination of stress and corrosion

17
Q

how can stress corrosion be revealed

A

through ndt methods other than visual inspections

18
Q

what does BVID mean

A

barely visible impact damage

19
Q

what is filiform corrosion

A

worm like corrosion under paint surface which leads to intergranular

20
Q

how can filiform corrosion be removed

A

bead blasting or sanding at the early stages

21
Q

what is pitting corrosion and what materials does it occur in

A

white powdery deposit that extends vertically happens in magnesium and aluminium

22
Q

what is intergranular corrosion

A

corrosion that spreads along the metal grain boundaries and seriously affects structural integrity

23
Q

what is exfoliation corrosion

A

advanced form of intergranular the surface grains gets lifted by the expanding corrosion products

24
Q

what is fretting corrosion

A

where 2 surfaces rub the protecting oxide layer off each other allowing fresh active metal on the surface

25
how is fretting corrosion identified
a black or grey deposit
26
what is crevice corrosion
where stagnant solutions get in the crevices causing corrosion
27
how is crevice corrosion reduced
by sealing joint
28
what is mercury corrosion
mercury is very reactive on aluminium and eats through it the process is amalgamation
29
what affects corrosion
what different metals are in contact, environmental and operational conditions, size of material, operational stress, composition of metal alloys
30
what is a sign of corrosion in copper and copper alloys
green to blue copper salts
31
what is a direct chemical attack
when alkaline or acidic liquid contacts metal
32
what affects the rate of corrosion for a direct chemical attack
the formation of the protective film, if the film has been mechanically removed, flexing of the metallic surface and the changes in temperature
33
what are some key areas prone to corrosion
exhaust, battery compartment, toilet and galley areas, bilge areas
34
why does corrosion happen in dry corrosion
there is direct chemical attack in the absence of moisture
35
If the corrosion takes place due to electrochemical attack in the presence of moisture or conducting medium what type of corrosion is this
wet corrosion