1
Q
when does iron and mild steel corrode
A
- they corrode readily in aerated water plainly visible in the form of rust
2
Q
what are the components of an electrochemical corrosion cell
A
- the anode - part of the metal thats corroding
- the cathode - metal surface provides site for metal to react
- the electrical connection between the anode and cathode allowing the flow of electrons
- the electrolyte which must be in contact with both the anode and cathode to provide a route for ionic conduction
3
Q
what are the 4 methods to prepare the surface of steel
A
- manual preparation - cheap but effective (30%)
- mechanical preparation - quicker than manual but only marginally more effective (35%)
- acid pickling - immersing the steel in a bath of acid can be 100% effective
- blasting - high speed projections of either spherical (shot) or angular (grit) particals
4
Q
when does corrosion not occur
A
- corrosion does not occur in the absence of water, oxygen and if the electrochemical reactions are supressed
5
Q
what happens when a piece of iron/steel is exposed to aerated water
A
anode reaction (oxidation)
- Fe → Fe²⁺ + 2e-
cathode reaction (reduction)
- O₂ + 2H₂O + 4e- → 4OH-
overall equation
- 2Fe + O₂ + 2H₂O → 2FeO.H₂O
6
Q
why is wet corrosion rapid
A
- hydrated iron oxide is deposited loosely or away from the corroding iron thus giving little or no protection
- Fe²⁺ and OH- diffuse in the liquid state very rapidly
- iron conducts electrons very easily
7
Q
A
