Prevention from Rusting

• 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.

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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.

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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.

1. By spraying molten zinc on the iron surface.
2. 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,

         E^o_Zn²⁺_/Zn = -0.76 V   and E^o_Fe²⁺_/Fe = -0.44 V

• E^o_Zn²⁺_/Zn is lower (more negative) than E^o_Fe²⁺_/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:

          E^o_Fe2+/Fe = -0.44 V   E_oSn2+/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.

       E^o_sn2+/sn = -0.14V   E_ocu2+/cu= 0.34 V

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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.

          E^o_Fe2+/Fe = -0.44 V   E_oMg2+/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.

References:

i) https://www.industrialmetalsupply.com/blog/6-tips-for-preventing-rust

ii) https://www.wikihow.com/Prevent-Rusting-of-Iron

iii) https://www.metalsupermarkets.com/how-to-prevent-rust/

Prevention from Rusting