Steel Corrosion and protection

Question 1

Corrosion is

Answer 1

degradation in properties and appearance

Question 2

The corrosion of steel is simply iron in steel returning to

Answer 2

a hydrated iron hydroxide

Question 3

where does corrosion occur?

Answer 3

surface of the metal

Question 4

Direct losses due to corrosion are

Answer 4

component replacement
money spent on expensive corrosion resistant materials

Question 5

indirect losses due to corrosion are

Answer 5

plant shutdown
loss of product in pipes
loss of efficiency
contamination of production

Question 6

Corrosion is an [] effect

Answer 6

electrochemical (electrons are transferred/ REDOX reaction)

Question 7

metals get oxidised at the []

Answer 7

anode

Question 8

outline the steps of how corrosion forms rust

Answer 8

when wet, it is energetically favourable for iron to rust:

iron oxide
V
blast furnace redox reaction
V
Iron
V
reformation of oxide in electrochemical process
V
back to iron oxide again

Question 9

What does the galvanic series describe?

Answer 9

relative reactivity of common materials in seawater

Question 10

What is the order of the galvanic series from inert to active?

Answer 10

platinum
gold
graphite
titanium
silver
stainless steel
nickel
copper
tin
lead
iron
aluminium
cadmium
zinc
magnesium

Question 11

inert materials are

Answer 11

cathodic

Question 12

active materials are

Answer 12

anodic

Question 13

steel pipes can be coated in a polymer called []. Why is this good?

Answer 13

epoxy

excellent adhesion to steel and high chemical resistance

Question 14

What are the main methods of avoiding corrosion?

Answer 14

physical barrier (paint covering)
galvanic protection
cathodic protection (circuit)

Question 15

what issues might arise with using a physical barrier for avoiding corrosion?

Answer 15

coating itself may not last

Question 16

How does galvanic protection work?

Answer 16

nearby a steel pipeline a rod of magnesium is installed

the magnesium is the sacrificial anode which loses electrons and will get consumed in the process

these electrons travel using the moisture in the soil as an electrolyte

the steel pipe, being a cathode will be protected by the reduction of oxygen and moisture in the soil.

(instead of corrosion of steel, we sacrifice an anode instead)

Question 17

How does Cathodic protection work?

Answer 17

e- are supplied externally via power supply

-ve end of power supply attached to metal we are protecting (cathode)

+ve end attached to inert anode

power supply flows e- from inert anode to cathode, completing the circuit.

anode corrodes in place of steel

Question 18

What is hot dip galvanic coating?

Answer 18

steel dipped into Zinc produces many protective intermetallic layers by reacting with steel itself

Question 19

What happens if galvanised coating gets damaged?

Answer 19

galvanised coating continues to protect steel by providing a barrier and sacrificial protection

Question 20

What are the advantages of hot dip galvanic coating?

Answer 20

zinc weathers slowly

coating corrodes preferentially to provide superficial protection to exposed steel

if damage is large, sacrificial protection prevents sideways creep as to not undermine coating

Question 21

Explain the chemistry of rust formation

Answer 21

inside a water droplet on the surface of iron, where O2 from the atmosphere is also entering the droplet:

anodic site of iron: Fe > Fe2+ 2e-

electrons travel from anode to cathode

cathodic site of iron: O2 + 2H20 +4e- > 4OH-

precipitation caused by free iron and hydroxide ions: Fe2+ _2OH- > Fe(OH)2

rust formation caused by oxidation of iron hydroxide:
4Fe(OH)2 + O2 > 2FeO3H2O + H2O