Page 32 - Curriculum Visions Dynamic Book

Preventing corrosion
Corrosion is a major problem for anybody Corrosion-proof coating
using iron products. One of the simplest ways to protect iron is to seal it from damp air.
The most common sealants are paints. To be useful paints need to have “skins” that are gas and watertight.
Once iron has begun to corrode, covering
Steel-hulled ships and motor vehicles are very prone to corrosion because they are continuously exposed to wet or damp conditions. The traditional way to protect them was to apply coats of paint, but despite this it was common for rust to bubble up under the paint because the paint was not
it with paint will not stop the rusting. Anyone well bonded to the metal.
who has painted a rusty object will know that
the paint soon bubbles up and the rust shows
In recent years better coating materials have been applied to the steel used in vehicles to prevent
through. In fact the painting has increased oxygen reaching the steel. Up to six
the rate of rusting by making sure that the
rust (and the moisture it contains) remains
coatings are now applied, beginning with a coating of iron phosphate and including several layers of
in contact with the iron. The only way to cure plastic seal. These special coatings
rust is to treat it chemically, so that the rust is converted to an unreactive substance.
Rusting is actually a process of electrolysis, as shown on page 31. Rusting can also, therefore, be prevented by operating electrolysis in reverse, a process
are the reason many vehicles now have anti-rust warranties lasting for several years.
Another way to coat the steel is to use a coating of a metal that is not prone to corrosion because the oxide that forms on its surface is gastight. Chrome, tin and zinc are
called cathodic protection. commonly used for this purpose. Tin was traditionally used to protect
 Rust-inhibitors are compounds such as phosphoric acid. The reaction between the acid and iron oxide converts the iron oxide layer to iron phosphate, a glassy, insoluble material that is gastight when
it dries out.
EQUATION: Phosphoric acid rust-inhibitor
food cans as “tin-plate”, while zinc was mainly used for protecting steel exposed to the weather as “galvanised steel”.
Phosphoric acid + iron oxide ➪ iron phosphate + water 2H3PO4(aq) + Fe2O3(s) ➪ 2FePO4(s) + 3H2O(aq)
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