- There are some common methods used from preventing the rusting of iron which are described here below:
i) By surface coating
- Rusting of iron can be prevented by coating the iron surface with oil, grease, paint and varnish, etc.
Image source: Tampasteel
ii) By electroplating iron with corrosion-resistant metals
- Certain metals like nickel, chromium do not corrode. Nickel and chromium can be electroplated on iron.
- Nickel and chromium coatings shine and have very decorative finish.
- Most of the bathroom fittings e.g. taps, cycle parts, car bumper, door handles etc., are chrome plated.
Image source: slideplayer
iii) By alloying
- Iron can be prevented from corrosion by alloying iron with chromium and nickel.
- Stainless steel (iron 74% chromium 18% and nickel 8%) is corrosion resistant and used for making cutlery, knives, and domestic utensils, etc.
iv) By galvanization of iron
- Iron can be protected from corrosion by coating it with a thin layer of zinc.
- The process of depositing a thin layer of zinc on iron is called galvanization.
- The iron coated with a thin layer of zinc is called galvanized iron or simply G.I.
- Zinc is more electropositive than iron. So, as long as zinc is there on any part of the galvanized iron, iron does not corrode.
- The galvanized iron is used for making roof sheets, water tanks, buckets, etc.
Galvanization is done by either of the following methods.
- By spraying molten zinc on the iron surface.
- By dipping the iron sheet/object into molten zinc.
The protection of iron surface from rusting by galvanization can be explained as follows.
- The standard electrode potential s for Zn2+/Zn and Fe2+/Fe systems are,
EoZn2+/Zn = -0.76 V and EoFe2+/Fe = -0.44 V
- EoZn2+/Zn is lower (more negative) than EoFe2+/Fe. So, Zn has higher tendency for oxidation than Fe.
- Therefore, when iron and zinc are present together, zinc will show greater tendency for oxidation.
- So, as long as zinc is there on the surface of iron, iron will not get oxidized, hence will not rust.
- It is due to this reason that iron does not rust even when zinc coating gets broken.
- Zinc is more electropositive than So, zinc shows greater tendency for oxidation.
- If it is so, then zinc coating on iron should not stay for a long period. In fact, zinc coating on the galvanized iron stays for years together.
- This is because, zinc in the presence of moisture, air and carbon-dioxide forms an invisible, non-reactive layer of basic zinc carbonate (Zn(OH)2.ZnCO3).
- This layer of basic zinc carbonate protects zinc layer from further reaction, and therefore, as a result zinc coating on galvanized iron stays for a long time.
v) By tinning
- Tin (Sn) is not attacked by air and water. Tin is non-toxic. So, the containers used in the packaging of food items are coated with tin.
- Coating of any reactive metal with a thin layer of tin is called tinning.
- Iron sheet coated with tin is called tin plate. Tin plates are used for making containers for oils, ghee, milk powder, pickles and other food items.
- Tin is less electropositive than iron. So, if tin coating on iron gets scratched then, the exposed iron starts getting corroded at that place very rapidly.
- So, tin coating protects iron as long as the coating remains intact. This can be explained as follows:
EoFe2+/Fe = -0.44 V EoSn2+/Sn= -0.14 V
- This shows that iron has greater tendency towards oxidation. Tin protects iron because tin is not affected by air and moisture.
- If the tin coating gets scratched, then, the exposed iron undergoes corrosion.
- So, tin prevents the rusting of iron as long as the tin coating remains intact.
- However, tinning protects copper surface effectively from corrosion.
- This is because, tin is more electropositive than copper.
Eosn2+/sn = -0.14V Eocu2+/cu= 0.34 V
Image source: MDPI
vi) By connecting iron to a more electropositive metal: cathodic protection
- Corrosion of iron can be prevented by connecting iron to a highly electropositive metal such as magnesium.
EoFe2+/Fe = -0.44 V EoMg2+/Mg= -2.37 V
- Magnesium being more electropositive acts as anode, while iron acts as cathode.
- As a result, magnesium does not allow the oxidation of iron and protects it from rusting.
- This method, called cathodic protection is generally used for the protection of underground pipes and tanks from corrosion.
Prevention from Rusting