C15 - Rusting
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corrosion
caused y chemical reactions between metal and substances in the environment - destruction of material - effects strength
rusting
corrosion of iron -becomes soft, crumbly, flakes off
oxide layer of aluminium
-protects it from further corrosion
what is needed for iron to rust
both air and water
rust chemical formula
hydrated iron oxide -water loosely bonded in structure -iron + water + o2
prevention of rust
coat with: - paint -oil/grease - plastic -less/more reactive metal
sacrificial protection or electroplating elements
-zinc, mg, al for iron
sacrificial protection or electroplating process
-iron is galvanised (protected) -reacts with water/o2 instead
why is zinc used in electroplating
zinc = more reactive and better reducing agent
electroplating used on
-equipment under sea - where coating may be scratched
why are alloys harder than pure metals
• regular layers in a pure metal are distorted by differently sized atoms in alloy
bronze
alloy of copper + tin -tough -resists corrosion -statues
brass
-alloy of copper + zinc - musical instruments
aluminium alloys
• low density • aircraft
why are pure metals alloyed
to make them harder - pure iron = too soft to be useful
gold in jewellery
• alloy of silver,copper, zinc
carat
proportion of gold in alloy is measured in this -24 = 100% - 18 = 75%
steel
allows of iron and carbon and other metals -added to change properties
carbon steels - high carbon properties
• strong but brittle
low carbon steel properties
softer easily shaped
stainless steels
• contain chromium and nickel • hard + resists corrosion
stainless steels eg
• cutlery -pipework
nickel steel alloy eg
bridges -bicycle chains
properties of polymer depends on
monomers affect this - changing reaction conditions