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mod 6 > 6.4 > Flashcards

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
Q

how is fretting corrosion identified

A

a black or grey deposit

26
Q

what is crevice corrosion

A

where stagnant solutions get in the crevices causing corrosion

27
Q

how is crevice corrosion reduced

A

by sealing joint

28
Q

what is mercury corrosion

A

mercury is very reactive on aluminium and eats through it the process is amalgamation

29
Q

what affects corrosion

A

what different metals are in contact, environmental and operational conditions, size of material, operational stress, composition of metal alloys

30
Q

what is a sign of corrosion in copper and copper alloys

A

green to blue copper salts

31
Q

what is a direct chemical attack

A

when alkaline or acidic liquid contacts metal

32
Q

what affects the rate of corrosion for a direct chemical attack

A

the formation of the protective film, if the film has been mechanically removed, flexing of the metallic surface and the changes in temperature

33
Q

what are some key areas prone to corrosion

A

exhaust, battery compartment, toilet and galley areas, bilge areas

34
Q

why does corrosion happen in dry corrosion

A

there is direct chemical attack in the absence of moisture

35
Q

If the corrosion takes place due to electrochemical attack in the presence of moisture or conducting medium what type of corrosion is this

A

wet corrosion

mod 6 (10 decks)

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