What Causes Rust? Mechanism of what causes Rust

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What Causes Rust?

Rust is the name of iron oxides of different colors, usually red or black, that form when iron or steel corrodes.

What causes rust is the reaction of iron with oxygen dissolved in an electrolyte, usually it's rainwater, condensing water from the atmosphere or freshwater.

The corrosion of iron and steel is an electrochemical reaction, it means that there is an exchange of electrons between the metal that is oxidized and corrodes and oxygen that is reduced.

There are always two different reactions in a corrosion process:

1) oxidation of Iron

2Fe ---> 2Fe²⁺ + 4e^-

2) reduction of Oxygen

O2 + 4 e^- + H2O ---> 4OH^-

The overall reaction is:

2Fe²⁺ + 4OH^- ----> 2Fe(OH)2

Fe(OH)2 reacts further with oxygen to form Iron oxides: Fe2O3 (hematite, red rust), Fe3O4 (magnetite, black)

iron corrosion, mechanism of iron corrosion

Is possible to stop rusting of iron or steel by removing what causes rust, i.e. water. A metal placed in a dry environment corrodes less than a metal placed in a wet environment, like a marine atmosphere.

One of the most used method to stop rust, is to apply a coating that isolate the metal from the environment and prevent contact with water.

To summarize:

What is rust:

- iron oxides of different colors

What causes rust:

-oxygen dissolved in water reacts with iron

What is needed to rust iron:

-water must be in contact with metal surface

Want to know more on Corrosion & Rust?

see Corrosion Fundamental

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