Page 32 - Curriculum Visions Dynamic Book

(Left) Anodized duralumin alloy dog collar – used almost entirely for its decorative effect and light weight.
Galvanic protection is also used to guard the hulls of ships, which
are constantly bathed in seawater. To protect a hull, large blocks of
magnesium are attached to the hull. The blocks, the seawater, and the hull
act as a “battery,” protecting the steel hull by sacrificing the magnesium blocks
rather than the hull. From time to time the blocks have to be replaced because they
quickly become corroded.
Anodizing
Some metals protect themselves by naturally forming a thin oxide coat when exposed to air. Oxides do not react with water, and so they form a natural barrier
to more corrosion. The layer of oxide is often so thin it is invisible. Stainless steel and aluminium protect
themselves with oxide films of this kind. Aluminium is also often given added protection (and sometimes
colour) by anodizing.
During anodizing the object to be treated
is dipped in a bath of electrically conducting liquid. It is usually dilute sulphuric acid. The object is connected to an electricity
supply in such a way as to make it the positive connection, or anode, in a circuit. As
the electricity flows, it causes a layer of oxide to be deposited, or plated, on the object being
anodized. By putting a dye in the acid, the plating can be coloured and the objects made more decorative.
((Left))Anodized duralumin alloy climbing clips (carabiners).
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