DM - Rusting Flashcards
What is rusting all down to?
Electrochemical processes
What happens if iron is exposed to oxygen and water?
It will turn into crumbly, flaky stuff called rust.
Explain how iron is turned into rust
The Fe^2+ (aq) and OH- (aq) ions produced in the overall equation combine to form a precipitate or iron(II) hydroxide, Fe(OH)2.
The Fe(OH)2 is further oxidised to Fe(OH)3 by oxygen.
Iron(III) hydroxide gradually turns into hydrated iron(III) oxide, Fe2O3.xH2O - this is rust.
Formula for rust
Fe2O3.xH2O
When is rust less likely to form?
In alkaline conditions.
Why is rust less likely to form in alkaline conditions?
Because of the way the two-half equations (in the process of rusting) shift when you add OH- ions.
The equilibrium of the half-reaction between water and oxygen moves left to get rid of the extra OH-.
The equilibrium of the iron half-equation is pushed right, to get rid of the extra electrons this produces.
So there’s less Fe^2+ around to take part in the reaction to produce Fe(OH)2, which ultimately means less rust.
How many main ways are there to prevent rusting?
2
What are the 2 main ways of preventing rusting?
- Coat the iron with a barrier to keep out either oxygen, water or both, e.g. by painting/coating with a polymer or by oiling/greasing.
- Sacrificial method. Placing a more reactive metal with the iron so that the water and oxygen then react with this sacrificial metal instead of with the iron.
When would you use painting/coating with a polymer to prevent rusting?
Ideal for big and small structures alike. It can be decorative too.
When would you use oiling/greasing to prevent rusting?
This has to be used when moving parts are involved, like on bike chains.
What is often used as a sacrificial metal?
Zinc
How can a sacrificial method be used to prevent rusting?
Sacrificial method. Placing a more reactive metal with the iron so that the water and oxygen then react with this sacrificial metal instead of with the iron.
Zinc is often used as a sacrificial metal:
- The Zn/Zn^2+ system has a more negative electrode potential value than the Fe/Fe^2+ system.
- This means zinc will be oxidised to Zn^2+ ions in preference to the iron.
- A coating of zinc can be sprayed onto the object (this is known as galavising).
- Alternatively, blocks of zinc can be bolted to the iron. This is used on the hulls of ships, or on underground iron pipes.
What is galvanising?
A coating of zinc can be sprayed onto the object (this is known as galavising).
