Steel Corrosion and protection Flashcards
Corrosion is
degradation in properties and appearance
The corrosion of steel is simply iron in steel returning to
a hydrated iron hydroxide
where does corrosion occur?
surface of the metal
Direct losses due to corrosion are
component replacement
money spent on expensive corrosion resistant materials
indirect losses due to corrosion are
plant shutdown
loss of product in pipes
loss of efficiency
contamination of production
Corrosion is an [] effect
electrochemical (electrons are transferred/ REDOX reaction)
metals get oxidised at the []
anode
outline the steps of how corrosion forms rust
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
What does the galvanic series describe?
relative reactivity of common materials in seawater
What is the order of the galvanic series from inert to active?
platinum
gold
graphite
titanium
silver
stainless steel
nickel
copper
tin
lead
iron
aluminium
cadmium
zinc
magnesium
inert materials are
cathodic
active materials are
anodic
steel pipes can be coated in a polymer called []. Why is this good?
epoxy
excellent adhesion to steel and high chemical resistance
What are the main methods of avoiding corrosion?
physical barrier (paint covering)
galvanic protection
cathodic protection (circuit)
what issues might arise with using a physical barrier for avoiding corrosion?
coating itself may not last
How does galvanic protection work?
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)
How does Cathodic protection work?
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
What is hot dip galvanic coating?
steel dipped into Zinc produces many protective intermetallic layers by reacting with steel itself
What happens if galvanised coating gets damaged?
galvanised coating continues to protect steel by providing a barrier and sacrificial protection
What are the advantages of hot dip galvanic coating?
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
Explain the chemistry of rust formation
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
