R-ATCHE-2

Graph

Metals can react with $O_{2}$

e.g. $4Al + 3O_{2} \to 2Al_{2}O_{3}$

The formation of $Al_{2}O_{3}$ means that pure $Al$ metal is sealed off from the oxygen and prevents further oxidation.

It forms an oxide layer, which protects the metal from further oxidation.

$Fe$ with $O_{2}$ and $H_{2}O$

$Fe \to Fe^{2+} + 2\overline{e}$ ox
$O_{2} + 2H_{2}O + 4\overline{e} \to 4OH^{-}$ red
- Using some sort of basic indicator (phenolphthalein) can be used to identify corrosion, as it produces hydroxide.

Then:

$Fe^{2+}+ 2OH^{-} \to Fe(OH)_{2(s)}$
- Notice $Fe^{2+}$ is green.
$Fe(OH){2} + OH^{-} \to Fe(OH){3} + \overline{e}$
$Fe(OH){3} \to Fe{2}O_{3} \ x \ H_{2}O$
- This is a red, flaky rust
  - Flaky is important: the strength of the metal has severely degraded. If the metal was used within structures, the structural integrity decreases significantly.
  - This is a problem, as iron is used everywhere

Seal - Paint, grease, etc

Not that great. Any crack or hole in the seal will allow rusting to occur. Alloy - e.g. Stainless steel
Better oxidisers within the metal structure, hence rusting does not occur.
Expensive, hence not a good long term situation. Coat with $Zn$ - Galvanisation
This is good as you both seal the substance, and even if it's scratched, the $Zn$ oxidises preferentially.
i.e. Seals and offers a sacrificial anode. Coat with $Sn$
Tin good cause it doesn't react with your food.
Good seal, but not good as something to oxidise preferentially. If the coating is breached, the iron oxidises, furthermore the $Sn$ extends the electrical conductivity of the metal, accelerating the oxidation of iron. Sacrificial Anode: $Mg$ or $Zn$
Often found in boats/hot water system. Cathodic Protection