Rust is found everywhere and is a common occurrence.

Rust is a general term for various forms of iron oxide. 
They are formed by the chemical reaction between iron and oxygen in the presence of water. Oxygen is found in the air we breathe and water can be found in the humidity in the air or as liquids normally found around us.

Rust is actually Fe₂O₃, a reddish form of iron oxide. Iron has another oxide, Fe₃O₄, which is sometimes called black oxide or black rust. Red rust is found in oxygen rich environments and black rust is found in oxygen poor environments such as under water.  Black oxide is a good protection for steel. Like aluminum oxide, black oxide molecules are the same size as iron molecules, so black oxide does not grow or flake. Black oxide is true gun bluing and the oxide found on some drill bits. Black oxide is also seen on iron and steel that has been hot-worked.

Rust starts by appearing as a light dusting on a piece of unprotected steel or alloy of such.  As rusting progresses, the rust particle starts to grow and in the case of red rust, starts to flake off the base metal.  Rust actually removes the iron that the oxygen and water react with, leaving the base metal "pitted".  the pitting is not noticeable if the rust is light enough and can be objectionable if heavy or severe rusting occurs.

Rusting is the commonly known as corrosion of iron and its alloys, such as steel or cast irons. Other metals undergo similar corrosion, although the resulting oxides are not commonly called rust.

Given enough time, the oxygen, water, and any iron mass eventually converts entirely to rust and ultimately disintegrates. The corrosion of aluminum is extremely slow because the resulting aluminum oxide forms a semi protective barrier or coating, which protects the remaining aluminum.  The protective barrier is a process known as passivation.

Many materials react with oxygen to form a chemical compound that is a combination of that material and oxygen. When iron combines with oxygen, it forms iron oxide, or rust. Iron oxide is a larger molecule than iron, so if iron oxidizes, it often puffs up and may even flake. This is because the rust requires more physical space than the original iron.

The rust expanding or puffing causes cracks and voids, which expose more bare metal to the environment. So the rusting of iron can progress and is only limited by destruction of all solid iron. Other metals oxidize, but the oxides of some other metals are no larger than the metal themselves, so they don't puff up or flake. For example, aluminum doesn't puff up when it oxidizes. This helps make aluminum oxide a good protective coating, rather than the start of rapid degeneration.

Some things cause steel or iron to rust faster than others. Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals because of electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Salt water will cause rust faster than water because salt water is a better electrical conductor. Like most chemical reactions, heat also speeds rust.

Conditions Affecting RUST

It takes oxygen, water and iron to start and promote rusting. Therefore removing one of these items will cause the reaction to stop.  The following sources of water will increase this reaction and discussed below.

Humidity:  Is the amount of water found in the air we breathe.  The amount of humidity depends on temperature and a source of water like a large body of water. We can find the greatest amount of humidity on hot humid days. On these days, rusting can be greatly accelerated as compared to cold dry winter days. Dehumidification can greatly reduce the amount of water in hot humid air.

Water:  Water is all around us and can be found in air, rain, and other sources.  increasing the amount of water around iron parts will increase its rate of rusting.

Salt Water:  Salt water is especially corrosive to iron or steel.   Ships, marinas, coastlines and other similar areas are greatly bothered by rust corrosion. Winters in the north are culprits to rusting cars out prematurely due to "salting" the roads for traction.

How to Protect Metals from RUSTING

Painting:  Painting adequately protects metals from corrosion.  The key with painting is to paint over metal free of contamination.  The metal needs to be free of oils/greases and rust/carbons. even if you remove all the contaminants except for all of the rust, the rust will continue to rusting under the paint and prematurely peeling off the paint coating.

Rust Preventatives:  Preventatives do not offer the protection of paints.  This coating provides rust prevention if the coating is not washed off, left outside or similar.  Usually used in-between processes or in protected environments.

Rust Inhibitors:  Inhibitors are only used for short term to stop parts from rusting quickly and are easily removed.

VCI materials:  Vapor Corrosion Inhibitors some chemicals emit a vapor that resists moisture from the air from reaching the surface of the ferrous materials, thus inhibiting oxide formation.  Usually used in closed containers or wrapped objects. 

Cathodic Protection:  Used on automobiles to slow the rusting process.  Operates on cathode and anode principles.

HOW DO YOU REMOVE RUST?

Heat:  Heat up a part with high heat using a acetylene torch or similar.  This will remove most of the rust and may be aided  by hitting with a hammer.

Acids:  Acids have been typically used in the past.  They work well but are hazardous to the employees and your health.  Requires protective equipment and safety precautions.  Acids must be thoroughly rinsed with clean water several times to remove all the acids.  Disposal can be expensive and mandatory.

Electrolytic:  Uses electric current to set up an anode and cathode reaction with the iron oxide.  Can take up to a week to remove rust.  Also corners, sharp edges, etc. are not adequately cleaned of rust due to faraday effects.

Chelation:  New technology that uses non-hazardous chelant technology chemicals to remove the rust.  Chelant is pronounced "key-lant".

It always best to protect the metal in one way or another.