DM.5 Flashcards
Rusting and methods of protection against rusting
What kind of process is rusting?
An electrochemical process.
What is rusting?
Corrosion of iron.
What are the two reversible half equations involved in rusting?
Fe2+(aq) + 2e- —> Fe(s)
0.5O2(g) + H2O(l) + 2e- —> 2OH-(aq)
Which half equation has a higher tendency for reduction?
The oxygen and water half equation will go in the reduction direction as it has a more positive electrode potential compared to the iron half equation.
What happens to iron when the reduction of oxygen happens?
Fe is oxidised to Fe2+ ions. Where electrons produced flow to where they can be used to supply the reduction of oxygen.
What two conditions are important for rusting to occur?
Water and oxygen must both be present.
When a droplet of water is left in contact with the iron/steel surface, where does the reduction of oxygen take place?
Around the edges of the droplet where the concentration of dissolve oxygen is high, oxygen is reduced to hydroxide ions.
When a droplet of water is left in contact with the iron/steel surface, where does the oxidation of iron take place?
At the centre of the droplet where the concentration of dissolved oxygen is low. Fe2+ ions pass into the solution here.
How do you describe the movement of electron when rusting occurs when a droplet of water is in contact with the iron surface?
The electrons released from the oxidation of iron flow outwards towards the edge of the droplet where the reduction of oxygen happens.
Take the example of the water droplet, where does most corrosion happen on the surface of the iron?
Corrosion is always greatest at the centre, of the water droplet. Overtime you will observe a pit in the surface where Fe ions dissolved.
How can rust be formed from Fe2+ ions in the solution?
It is formed through a series of secondary processes in the solution as Fe2+ ions and hydroxide ions diffuse away from the metal surface.
What are the equations of the secondary processes that form rust from Fe2+ ions?
Fe2+(aq) + 2OH-(aq) –> Fe(OH)2(s), green precipitate
Fe(OH)2(s) –> Fe2O3.xH2O, oxidation of Fe2+ to Fe3+
Fe2O3.xH2O, iron(III) oxide is rust, because of the water trapped inside, it is irregular and crumbly, and flakes off to expose more Fe.
What kind of metal can be used as a sacrificial metal to prevent the rusting of iron?
Metals that have a more negative electrode potential value than iron.
What are three examples of sacrificial metal?
Zn, Al, Cr, Mg
What are three ways of protecting steel against rusting?
- Using a barrier.
- Using a sacrificial protection layer, such as galvanising using Zn.
- Impressed current to make the metal a cathode site.
