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Silver Tarnishing
Silver articles become black when exposed to air because they react with sulphur compounds in the atmosphere. This forms a coating of silver sulphide (Ag₂S) on the surface, giving it a black appearance.
2Ag + S → Ag₂S (black coating)
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Copper Corrosion
Copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface. It gains a green coating of basic copper carbonate [CuCO₃·Cu(OH)₂]. This is why old copper statues and vessels appear green.
The green patina actually protects the copper underneath from further corrosion!
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Iron Rusting
Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust. Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). Unlike other corrosion products, rust is porous and flaky, allowing further corrosion.
Both oxygen AND water are needed for iron to rust!
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Rusting Conditions
Activity 3.14 demonstrates that iron needs BOTH air and water to rust. Test tube A (air + water) shows rusting. Test tube B (only water, no air) shows no rust. Test tube C (dry air, no water) shows no rust. Both conditions must be present!
This is why iron doesn't rust in completely dry environments or underwater without dissolved oxygen!
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Prevention Methods
Corrosion can be prevented by: Painting (barrier coating), Oiling/Greasing (moisture barrier), Galvanizing (zinc coating), Chrome plating (chromium layer), Anodizing (oxide layer for aluminum), and Alloying (changing metal properties).
Galvanized iron is protected even if the zinc coating is broken because zinc corrodes preferentially!
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Alloys
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Pure iron is soft, but adding 0.05% carbon makes it hard and strong (steel). Adding nickel and chromium to iron creates stainless steel, which is hard and rust-resistant.
Alloying is an excellent way to improve metal properties and prevent corrosion!