Page 9 - Curriculum Visions Dynamic Book

Iron’s special oxide layers
The materials that most often show signs of corrosion are metals. The oxide coating that develops on the surface of some metals is so thin it is invisible.
Look at a clean iron or steel nail and the surface looks unaffected because the oxide layer is so thin. When the oxide coating is thicker, it may appear
as a discolouring, or tarnishing, of the surface.
However, unlike some other metals such as aluminium and copper, iron’s oxide coating is not able to keep water and oxygen out. On the contrary, it is a porous coating, which is why it rusts.
corrosion: the slow decay of a substance resulting from contact with gases and liquids in the environment. The term is often applied to metals. Rust is the corrosion of iron.
oxide: a compound that includes oxygen and one other element.
porous: a material containing
many small holes or cracks.
Quite often the pores are connected, and liquids, such as water or oil,
can move through them.
product: a substance produced by a chemical reaction.
 This rusty bolt shows many of the features of corrosion. Notice, for example, how the thread is much less clear. This is because when iron corrodes (rusts), oxygen and iron combine to make a bulkier substance than pure iron. So, as the sides of
the threads rust and swell, the gap between threads becomes smaller. This is one reason why a rusty bolt is so hard to undo: the oxide has jammed the threads.
 Old iron anchoring chain and a new galvanised chain.
 The anchor chain is not attacked evenly. The surface of the iron is pitted in places and shows rust scales in other places.
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