C6.1 part 3 :/ Flashcards
Corrosion
reaction of a metal w substances in its surroundings e.g. air, water
Silver corrosion
Doesn’t react easily w oxygen in air or water
BUT will corrode in the presence of HYDROGEN SULFIDE (o2 + water present)
- produces a thin layer of black silver sulfide (Ag2S)
= but you can clean
Iron/steel corrosion name
Rusting
Rusting
- REDOX reaction
iron + o2 = h20 –> hydrated iron(iii) oxide
Methods to reduce rusting
- painting
- coating w oil, grease or plastic
- galvanising = plating w zinc
- plating w tin
- sacrificial protection
paint/coating method
- if paint damaged then rusting will start on exposed metal
= will continue under paint, will eventually flake off
= not v reliable
Sacrificial protection method
Involves a metal that is MORE reactive than iron e..g MG/ZINC
- if iron is in contact w reactive metal the more reactive metal corrodes first! (sacrificing to protect iron/steel)
good on ships etc. (blocks on hulls below water)
*however do need to replaced as gradually corrode away
Sacrificial method half eq
Fe –> Fe3+ + 3e-
more reactive metal = easily loses electrons so more reactive metal is more EASILY oxidised than iron, more readily rusted
Metal plating method
Prevents air + water reaching iron/steel
Galvanising - coated in molten zinc which cools + solidifies
- stops air + water reaching
- if damaged, still acts as sacrificial metal = object protected
Tin - steel food cans etc. but NOT SACRIFICIAL METAL
:( so if chipped, rusting will happen faster than normal
Ceramics
hard, non metallic materials
e.g. Brick, china, porcelain, plastics
Ceramics bonding + properties
Metal + non metals combined to form GIANT IONIC LATTICES = - High m.p. - hard + stiff BUT brittle - Poor conductors (heat + elec) - mostly oxides = unreactive
glass properties
melting sand = cool + solidify
- Irregular giant structure W OUT CRYSTALS + transparent
Clay ceramics properties
e. g. China + porcelain
- clay heated to v high temp = tiny crystals form, joined together w glass
- usually coated in glaze + reheated = smooth, hard, waterproof surface
compressive strength
how easily it can be squashed
tensile strength
how easily it can be stretched
