rusting*

a)
Steel is an alloy containing more than 99% of Fe (iron). When steel is kept in contact with Mg (magnesium), since Mg is more reactive than Fe (Mg is at a higher position in ECS), Mg is corroded readily before the rusting of Fe.

----

b)
Both the car and the bin are made of sheet of steel. The steel of the car is coated with Sn (tin), while that of the bin is coated with Zn (zinc).

For the bin, Zn is more reactive than Fe (Zn is at a higher position in ECS), Zn is corroded readily before the rusting of Fe.

For the car, Sn is less reactive than Fe (Sn is at a lower position in ECS), Fe undergoes rusting readily before the corrosion of Sn, and thus the rate of rusting is faster.

2006-12-10 15:23:10 補充：
To prevent iron/steel from rusting by keeping it contact with a metal more reactive than iron, the technique is known as 'sacrificial protection'.