Corrosion

Question 1

What is corrosion?

Answer 1

Degradation of a metal within its environment

Question 2

Which part of the electrochemical cell gains electrons in wet corrosion?

Answer 2

Cathode

Question 3

what is the galvanic series?

Answer 3

Ranking of the reactivity of metals/alloys in seawater

Question 4

What are some corrosive environments?

Answer 4

Atmosphere:
- Moist air & oxygen
- Sodium Chloride
- Sulphur compounds
Aqueous Solutions:
- Dissolved oxygen in seawater
- Seawater (3.5% salt)
- For iron/steel corrosion rate is tripled in seawater compared to freshwater
Soil:
- Variable include: type, moisture, oxygen, salt content etc
- Concrete
- Acids
- Bases and Alkalis
- Solvents
- Liquid Metals

Question 5

What does wet corrosion require?

Answer 5

• Anode
• Cathode
• Metal connection
• Electrolyte

Question 6

What is Galvanic Corrosion

Answer 6

• Results from 2 different metals being in contact in the presence of an electrolyte
• More reactive metal (anode) will corrode
• More inert metal (cathode) will be protected

Question 7

What is Uniform corrosion?

Answer 7

• Attacks metal uniformly over entire surface
• Commonly referred to as rust on steel surfaces
• Can cause strength and weight loss

Question 8

What is pitting corrosion?

Answer 8

• Most common corrosion on Al and Mg alloys
• Appears first as white/grey dots then as white/grey powder.

Question 9

What is crevice corrosion?

Answer 9

Occurs in areas where 2 metals are joined and moisture works way into crevice

Question 10

What is Intergranular corrosion?

Answer 10

• Very serious
• Starts at exposed grain boundary then eats out everything in its way along grain boundary
• Occurs below surface, so can go unnoticed.

Question 11

What is Exfoliation corrosion?

Answer 11

• Advanced form of intergranular corrosion
• Surface grains of a metal are lifted up by the force of expanding corrosion occurring at grain boundary just below surface.

Question 12

What is Filiform corrosion?

Answer 12

• Begins at breaks in paint
• Recognised by worm like traces beneath grain surface which can lead to intergranular corrosion

Question 13

What is stress corrosion cracking?

Answer 13

• Caused by co,binational of stress and corrosion
• Intergranular cracking of metal in specific environment

Question 14

What is erosion corrosion?

Answer 14

Combined action of mechanical wear amid chemical reaction due to motion of a fluid

Question 15

What is concrete cancer?

Answer 15

concrete deteriorates due to corrosion of steel reinforcement

Question 16

Types of corrosion detection?

Answer 16

• Visual/Physical Inspection
• Thermography: infrared imaging
• Ultrasonic scanning
• Pulsed eddy currents
• Corrosion Potential Surveying

Question 17

What is ultrasonic scanning?

Answer 17

• Form of corrosion detection
• High frequency sound waves
• Thickness measurements identify corrosion
• Use3d to inspect pipework, pressure vessels, storage tanks and reactors.
• Direct contact needed

Question 18

What are pulsed eddy currents?

Answer 18

• Form of corrosion detection
• Probe positioned near surface
• Coil generates magnetic field
• Thickness interpreted to identify corrosion
• Doesn’t have to be in contact

Question 19

What is corrosion potential surveying

Answer 19

• Form of corrosion detection
• Used to detect corrosion in reinforced concrete
• Potential difference between steel and surface locations

Question 20

What are some ways of preventing corrosion?

Answer 20

• Use an alternative material
• Physical barriers (Paint, Polymer films, galvanising, electroplating, anodising)
• Change the nature of the environment ( lower temp.)
• Inhibitors
• Design (avoid trapping of water, avoid contact of dissimilar metals)
• Cathode protection ( galvanic - sacrificial anode)(Impressed Current Catholic Protection (ICCP))

Question 21

What is Galvanic cathodic protection?

Answer 21

• Sacrificial anode metal is more electrochemically reactive than the metal to be protected
• The anode metal is consumed whereas the cathode metal is largely protected
• Anode must be regularly inspected and replaced before it is fully consumed.
• Typically used where the cathode is largely inaccessible (underground tank) or Large and in a very corrosive environment (ship in sea)
• Zinc and magnesium are commonly used anodic materials

Question 22

What is ICCP cathodic protection?

Answer 22

• A small direct current I supplied by an external power source.
• This current is the source of the electrons to the system and prevent the anode from oxidising
• As such, the anode corrodes extremely slowly and lasts longer
• It is not necessary for the anode material to be more reactive than cathode
• Useful for protecting larger structures where the driving current from a galvanic protection would be insufficient.
• Graphite often used
• This system provides more control and improved monitoring