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Rust prevention
Rust prevention
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, copper, titanium, zinc, aluminium.
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.
Several other methods are available to control corrosion and prevent the formation of rust, colloquially termed rustproofing: .
Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by supplying an electrical charge that suppresses the electrochemical reaction.
If correctly applied, corrosion can be stopped completely.
In its simplest form it is achieved by attaching a sacrificial anode thus making the iron or steel the cathode in the cell formed.
The sacrificial anode must be made made from something with a more negative electrode potential than the iron or steel, commonly zinc, aluminium or magnesium.
Bluing is a technique that can provide limited resistance to rusting for small steel items, such as firearms; for it to be successful, water-displacing oil is rubbed onto the blued steel.
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.
Such treatments also contain rust inhibitors.
Covering steel with concrete provides protection to steel by the high pH environment at the steel-concrete interface.
Another method to avoid rust is to control the environment.
Controlling the humidity, if possible, below a certain thereshold can reduce or stop the corrosion process.
Rusting can be controlled also by proper design, avoiding for example areas of stagnant water, galvanic coupling with more noble materials...
Corrosion inhibitors, like gas phase or volatile inhibitors can be used to prevent corrosion in closed systems.
A simple and inexpensive way to remove rust from steel surfaces by hand is to rub the steel with aluminium foil dipped in water.
Aluminium has a higher reduction potential than the iron in steel, which may help transfer oxygen atoms from the iron to the aluminium.
The aluminium foil is softer than steel and will not scratch it, as steel wool will, but as the aluminium oxidizes, the aluminium oxide produced becomes a fine metal polishing compound. .
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Source: CIA Factbook, Wikipedia
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