Corrosion

Question 1

What is corrosion?

Answer 1

The natural phenomenon whereby a metal converts to oxides, hydroxides and sulphates

Question 2

What two actions cause corrosion?

Answer 2

Chemical

Electrochemical

Question 3

How does corrosion occur?

Answer 3

The tendency for metals to return to their natural state

Question 4

What sort of metals do not corrode and why?

Answer 4

Noble metals as they are already in their stable state

Question 5

What are the two chemical changes that happen during corrosion?

Answer 5

The metal that is attacked/oxidised undergoes an anodic change, whilst the corrosive agent is being reduced and undergoes a cathodic change

Question 6

What is electrochemical action?

Answer 6

Otherwise known as galvanic action

When two dissimilar metals are in the presence of an electrolyte, they corrode

Question 7

What is microbiological corrosion?

Answer 7

Corrosion due to bacterial/ fungus/mould attacks

Question 8

What is steel corrosion?

Answer 8

An intergranular cracking of the metal which is caused by a combination of stress and corrosion

Question 9

What is filiform corrosion?

Answer 9

Special form of oxygen crevice corrosion which occurs on metal surfaces having an organic coating system

Question 10

What happens if filiform is not treated asap?

Answer 10

Can cause intergranular corrosion

Question 11

How can filiform corrosion be removed?

Answer 11

Using glass bead-blasting material with portable abrasive equipment and/or sanding

Question 12

How can ALclad be susceptible to corrosion?

Answer 12

If the surface is scratched , the underlying material is attacked by corrosion
Too many solution heat treatments can affect the thickness of protection

Question 13

How can passive film be made to be resistant to corrosion?

Answer 13

By adding a tight non-porous layer of oxide to the surface

Question 14

How can corrosion take place in alloy grain structures?

Answer 14

The boundaries may be prone to corrosion which can lead to pitting and intergranular corrosion

Question 15

What are the effects of corrosion on steel?

Answer 15

Red rust will form on its surface

Question 16

What are the effects of corrosion on aluminium?

Answer 16

Experience crevice, stress and fretting corrosion. It appears mainly as a white/grey powder

Question 17

What rapidly corrodes with aluminium?

Answer 17

Mercury

Question 18

What effects does corrosion have on magnesium ?

Answer 18

Magnesium is already very easy to corrode. It creates grey powders when it is corroded and severe pitting is seen on the surface

Question 19

What effect does corrosion have on copper?

Answer 19

Green powder is seen

Question 20

What are the influencing factors that make a material more susceptible to corrosion?

Answer 20

Metal type
Dissimilar metals
Exposed surface areas
Temperatures
Electrolytes are present
Oxygen and organisms are present
Mechanical stresses
Exposure time

Question 21

What is filiform usually found on?

Answer 21

acidic surfaces

Question 22

How do you identify stress corrosion?

Answer 22

Dark rings around the rivet heads or fasteners
Wearing away of parts/components
Smoking rivets - black round the rivet and give off black burnt looking streaks
Intergranular cracking caused by corrosion
Failure beyond yield point

Question 23

What does cold expansion and shot peening do?

Answer 23

Increase cracking

Question 24

What is the list of called to show how much certain metals will react and corrode?

Answer 24

Galvanic scale

Question 25

What does the rate of galvanic corrosion depend on?

Answer 25

The size of the surface of metals

Question 26

What is pitting corrosion most common in?

Answer 26

Magnesium and aluminium alloys

Question 27

How is pitting corrosion identified?

Answer 27

White powder

Question 28

How do you describe pitting corrosion?

Answer 28

It starts on the surface and shows up as a white powdery substance and it extends down vertically

Question 29

What can pitting corrosion do to the strength of the structure?

Answer 29

Decreases its strength

Question 30

What is intergranular corrosion?

Answer 30

An attack along the grain boundaries of the metal with little or no indication on the surface

Question 31

What is exfoliation corrosion?

Answer 31

An advanced form of intergranular corrosion where surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries below the surface

Question 32

What is fretting corrosion?

Answer 32

When vibration causes the surface to rub together resulting in abrasive wear.
It can cause severe pitting

Question 33

Where does fretting corrosion form?

Answer 33

Forms between 2 highly loaded surfaces which are not designed to move against each other

Question 34

How can you reduce fretting corrosion?

Answer 34

Use of lubrication or dampening methods

Question 35

What is microbial corrosion?

Answer 35

The attack of a surface sure to bacteria, fungi or mould

They produce corrosive chemicals such as hydrogen, sulphide, ammonia or inorganic acids

Question 36

What can bacteria be?

Answer 36

Aerobic or anerobic

Question 37

What do aerobic bacteria require to live and how do they accelerate corrosion?

Answer 37

They need oxygen to live

Accelerate corrosion by oxidising sulphur to produce sulphuric acid

Question 38

How do anaerobic bacteria create corrosion?

Answer 38

They oxidise inorganic compounds which result into chemical reactions that cause corrosion

Question 39

How can you treat microbial corrosion?

Answer 39

Use of biocide

Question 40

What is crevice corrosion?

Answer 40

A form of concentration cell corrosion

Corrosion of metal in a metal to metal joint and corrosion at the edge of a joint

Question 41

How can crevice corrosion be reduced?

Answer 41

Sealing of joints and use of protective coatings in the fraying surface areas