Iron rusts in the presence of water and air (oxygen). The first step of rusting involves losing electrons from iron (oxidation of iron). The half-equation is shown below:
Fe(s) → Fe2+(aq) + 2e–
In light of that, there are 3 main approaches to prevent iron from rusting:
- Coat a layer on top of the iron surface to prevent the iron from contacting with air and water.
- Prevent iron from losing electrons.
- Alloy iron with other corrosion resistant metal.
The below will show the details of those approaches in action.
Coating
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- cheap
Disadvantage:
- can be scatched off easily
Examples:
- bridges
- ships
- fences
- car bodies
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- long-lasting
- good appearance
Disadvantage:
- more expensive than painting
Examples:
- coat hangers
- fences
- paper clips
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- does not fall off like paint or plastic
- has lubricating effect
Disadvantage:
- need to oil the object once in a while
Examples:
- moving parts of machines
- woodworking tools
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- in case the zinc coating is damaged, the iron is still protected by sacrificial protection of zinc
Disadvantage:
- zinc ions are poisonous
Examples:
- galvanized iron plate used in construction, buckets
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- tin is corrosion resistant
- tin and tin ions are not poisonous
Disadvantage:
- when the tin coating is damaged, rusting will occur more rapidly than iron alone
Examples:
- “tin cans” for storing food
Principle:
- the coated layer prevents iron from contacting with air and water
Advantage:
- beautiful appearance
Disadvantage:
- quite expensive
Examples:
- water taps
- car bumpers
Preventing iron from losing electrons
Principle:
- the negative terminal of an electric source is connected to the iron object, supplying electrons to prevent it from rusting (prevent the first step to occur)
Advantage:
- convenient
Disadvantage:
- not applicable to many objects
Examples:
- car bodies
- undergound water pipilines
- storage tanks
- steel
- pier legs
Principle:
- a more reactive metal in contact with the iron object loses electrons more readily and is “sacrificed” to form ions, preventing iron from losing electrons to form iron(II) ions
Advantage:
- effective
Disadvantage:
- the “sacrificed” metal needs to be replaced from time to time
Examples:
- galvanized iron
- zinc blocks attahced to the ship hull
- magnesium blocks attached to the underground pipelines
Alloying
Principle:
- Alloying with carbon, chromium, nickel and manganese, which are corrosion resistant
Advantage:
- beautiful
- effective
Disadvantage:
- most expensive
Examples:
- cookware
- cutlery
Try to find iron objects in the daily life and observe what method is employed to protect it from rusting!