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Essential condition of rusting:
Prsence of Water and Air (Oxygen)
Without these two, rusting will not occur.
Factors that speed up rusting:
1. Presence of acidic solutions or soluble salts
Since these two contain mobile ions, it can speed up rusting as rusting is an electrochemical reaction (Fe will lose e- to air, forming Iron (III) ions and hydroxide ions). Presence of mobile ions can increase electroconductiuvity of water and hence speed up the rate of rusting.
2. High temperature
An increase in temperature always increases the rate of chemical rustings.
3. Connection of Iron and less reactive metals (e.g. Cu and Ag)
Take example as Cu. Compare the reactivity of Fe and Cu, Fe is more reactivity, more tendency to lose e-. When two metals connect together, Iron will lose e- to Cu, hence promotes the formation of Fe2+ (i.e. rusting).
4. Uneven or sharply pointed regions (Deform region)
The deform region will increase the contact surface area between Fe and air, water. As a result, it speeds up the rate of rusting.
Factors that slow down rusting:
1. Less contact surface between Iron and water, air
Apply a protective layer, coating are the examples to decrease contact surface between Fe and H2O,O2.It will slow down the rate of rusting.
2. Presence of more reactivity metal
Take Zn as examples. Galvanizing is the example to use Zn to slow down the rate of rusting. Zn will lose e- to Fe, to prevent the formation of Fe2+. It is so called Sacrificial protection.
3. Presence of electrons
Similar to Sacrificial protection, when an electrode is connected with Iron, the electrode will supply e- to Fe, to prevent Fe from losing e-.
