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Alexander Hood 28/03/12
Rust is permeable to air and water, therefore the interior iron continues to corrode. Rust prevention
thus requires coatings that preclude rust formation. Stainless steel forms a passivation layer of
chromium(III) oxide. Similar passivation behavior occurs with magnesium, titanium, zinc, zinc oxides,
aluminium, polyaniline, and other electroactive conductive polymers.
An important approach to rust prevention entails galvanization, which typically consists of an
application, on the object to be protected, of a layer of zinc by either hot-dip galvanizing or
electroplating. Zinc is traditionally used because it is cheap, adheres well to steel, and provides a
cathodic protection to the steel surface in case of damage of the Zinc layer. In more corrosive
environments (such as salt water), cadmium is preferred. Galvanization often fails at seams, holes,
and joints where the coating is pierced. In these cases, the coating provides cathodic protection to
metal, where it acts as a galvanic anode rusting in preference. More modern coatings add aluminium
to the coating as zinc-alume; aluminium will migrate to cover scratches and thus provide protection
for longer. These approaches rely on the aluminium and zinc oxides protecting the once-scratched
surface rather than oxidizing as a sacrificial anode. In some cases, very aggressive environments or
long design life, both zinc and a coating are applied to provide corrosion protection.
Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by
supplying an electrical charge that suppresses the electro-chemical reaction. If correctly applied,
corrosion can be stopped completely. In its simplest form, it is achieved by attaching a sacrificial
anode, thereby making the iron or steel the cathode in the cell formed. The sacrificial anode must be
made from something with a more negative electrode potential than the iron or steel, commonly
zinc, aluminium, or magnesium.
Coatings and painting
Flaking paint, exposing a patch of surface rust on sheet-metal
Rust formation can be controlled with coatings, such as paint, that isolate the iron from the
environment. Large structures with enclosed box sections, such as ships and modern automobiles,
often have a wax-based product (technically a "slushing oil") injected into these sections (see
Rustproofing). Such treatments also contain rust inhibitors. Covering steel with concrete can provide
some protection to steel by the high pH environment at the steel-concrete interface. However
rusting of steel in concrete can still be a problem.
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Alexander Hood 28/03/12
Rusting can completely degrade iron. Note the galvanization on the unrusted portions.
Zinc plating (galvanized iron/steel): iron or steel coated with zinc metal layer. Hot-dipped method or
zinc-blasting method may be used.
Tin plating: mild steel sheet coated with a tin layer.
Chrome plating: wherein a thin layer of chromium is electrolytically applied to the steel, providing
both rust protection and a highly polished, bright appearance. Often used on the silver-coloured
components of bicycles, motorbikes, and automobiles.…read more