6.4 Corrosion

Question 1

What is corrosion?

Answer 1

A natural phenomenon whereby chemical or electrochemical reactions cause metals to be oxidised, forming salts and oxides of the metal.

Question 2

If corrosion occurs at boundaries between regions what is the result?

Answer 2

Pits and intergranular corrosion.

Question 3

For corrosion to form what three condition must be met?

Answer 3

An electrical potential difference within the metal.
A conductive path between the two areas of potential difference.
Some form of electrolyte or fluid covering the two areas.

Question 4

What is electrolytic corrosion due to?

Answer 4

Due to an electrolyte (for example water) and the flow of an electrical current between the two metals

Question 5

What two categories of bacteria are there?

Answer 5

Aerobic
Anaerobic

Question 6

How do aerobic bacteria accelerate corrosion?

Answer 6

Oxidising sulphur to produce sulphuric acid.

Question 7

What are the ideal growth conditions for micro-organisms?

Answer 7

Temperatures of 20 °C - 40 °C and relative humidity of 85% - 100%.

Question 8

What is stress corrosion?

Answer 8

Growth of crack formation in a corrosive environment.

Question 9

What is filiform corrosion?

Answer 9

Worm like trace, oxygen concentration cell corrosion.

Question 10

Where does filiform corrosion occur?

Answer 10

Metal surfaces with an organic coating

Question 11

What conditions are needed for filiform?

Answer 11

78-90% humidity and slightly acidic surface

Question 12

What can filiform lead too?

Answer 12

Intergranular

Question 13

How is filiform removed?

Answer 13

Glass beading

Question 14

Is there any indication of pitting on the surface?

Answer 14

No

Question 15

What is exfoliation corrosion?

Answer 15

An advanced form of intergranular.

Question 16

Where is exfoliation more common?

Answer 16

Wrought products

Question 17

How does fretting corrosion occur?

Answer 17

Due to vibration and slipping

Question 18

What can help prevent fretting corrosion?

Answer 18

Lubricant or sealant

Question 19

How can crevice corrosion be prevented?

Answer 19

Shims or sealants

Question 20

`What is the chemical attack of mercury called on aluminium?

Answer 20

Amalgamation

Question 21

Why are thicker structural parts more prone to corrosive attacks?

Answer 21

Variations in their composition.

Question 22

What can occur when metals are not cleaned?

Answer 22

Black sulphide tarnish

Question 23

What kind of metal surface does wet corrosion occur on?

Answer 23

Heterogeneous

Question 24

On wet corrosion where does the corrosion take place and accumulate (anode/cathode)?

Answer 24

Occurs at the anode but accumulates at the cathode.

Question 25

What is a direct chemical attack?

Answer 25

Alkaline or acidic contacts metal.

Question 26

What can affect the rate of chemical attack?

Answer 26

Protective film
Flexing of the surface
Changes in temp

Question 27

What can skydrol do to titanium?

Answer 27

Cause severe corrosion and embrittlement of titanium alloys at temperatures above 120 °C.

Question 28

What are some key areas to check when servicing the aircraft?

Answer 28

Exhaust
Battery compartment
Lavatories and galleys
Bilge areas
Wheel well

Question 29

What three types of corrosion on aluminium are the worst?

Answer 29

Pit
Stress
Intergranular

Question 30

At what temp do coloured surface oxides develop on titanium?

Answer 30

370 degrees c

Question 31

What is the easiest metal corrosion to detect?

Answer 31

Magnesium

Question 32

Where are nickel alloys normally used?

Answer 32

Challenging environments that demand good strength resistance against high and low temps.

Question 33

How is attack on cadmium evidenced?

Answer 33

White to brown mottling of the surface