Iron and steel corrode when exposed to oxygen in the air, forming rust. Rust occurs through a chemical reaction where iron reacts with oxygen to form iron oxide. The iron oxide molecules are larger than the original iron atoms, causing rust to flake off and weaken the metal over time. Some elements added to steel, such as copper, can slow the rusting process by forming protective oxide layers. Stainless steel is an alloy that is highly resistant to rusting due to the formation of a thin, stable chromium oxide film that protects the underlying metal.

1. Rust in Iron and Steel
Iron and steel, the most commonly used metals, corrode in many media
including most outdoor atmospheres. Usually they are selected not for their
corrosion resistance but for such properties as strength, ease of fabrication, and
cost. These differences show up in the rate of metal lost due to rusting. All steels
and low-alloy steels rust in moist atmospheres. In some circumstances, the
addition of 0.3% copper to carbon steel can reduce the rate of rusting by one
quarter or even by one half.
Iron is a pure element. Every atom in a chunk of iron is an iron atom. Iron is nice,
but it’s not very strong. Steel is even better than iron. To make steel, they melt
down iron and mix in small amounts of other elements. There are different
types of steel based on what they mix in and how much. Each type of steel has
different advantages, such as stength, durability, chemical reactivity, and many
other qualities. Typical elements that are added to iron to make steel include
Carbon, Manganese, Silicon, Chromium, and Molybdenum. But most of the
atoms of steel re still iron, because they only usually add in a few percent of
other elements
(Rust in screw)

2. Many elements like to go through chemical reactions with other elements.
These are 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. We can’t keep oxygen away from the iron
in our cars since much of the atmosphere is oxygen. Rust weakens the iron and
can make parts break. Steel has the same problem, but some of those elements
that are added during the steel production can help prevent rust or at least slow
it down. For example, "stainless steel" is designed so that it will not rust, but it is
more expensive that other types of steel.
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.
If steel starts to rust, it will puff up because iron oxide is a larger molecule than
iron. The 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.
Rust is really Fe2O3, a reddish form of iron oxide. Iron has another oxide, Fe3O4,
which is sometimes called black oxide, black rust, or hammerscale. 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.

3. Steel can be coated with black oxide by a careful regimen of rusting the right
amount and boiling the rusted metal in water to convert it. This is how non-
caustic gun bluing is done, and although it is tedious, it produces very attractive
and durable results after several treatments.
Rapid rust of steel needs water, low pH (preferably with HCl (available at
hardware stores as Muriatic acid [caution: very corrosive liquid and vapor), and
oxygen. Moderate heat (about 60-80 C) also helps.s
Once rust is formed, there is mobile oxygen in the metal, and the oxygen can
move deeper into the metal causing further rust. Techniques to kill rust are
described below. All rely on dissolving rust or converting rust into black oxide
and then coating the metal with something that inhibits the formation of more
Stainless steel does not rust:
Stainless steel remains stainless, or does not rust, because of the interaction
between its alloying elements and the environment. Stainless steel contains
iron, chromium, manganese, silicon, carbon and, in many cases, significant
amounts of nickel and molybdenum. These elements react with oxygen from
water and air to form a very thin, stable film that consists of such corrosion
products as metal oxides and hydroxides. Chromium plays a dominant role in
reacting with oxygen to form this corrosion product film. In fact, all stainless
steels by definition contain at least 10 percent chromium.
The presence of the stable film prevents additional corrosion by acting as a
barrier that limits oxygen and water access to the underlying metal surface.
Because the film forms so readily and tightly, even only a few atomic layers
reduce the rate of corrosion to very low levels. The fact that the film is much
thinner than the wavelength of light makes it difficult to see without the aid of
modern instruments. Thus, although the steel is corroded on the atomic level, it
appears stainless. Common inexpensive steel, in contrast, reacts with oxygen
from water to form a relatively unstable iron oxide/hydroxide film that
continues to grow with time and exposure to water and air. As such, this film,

4. otherwise known as rust, achieves sufficient thickness to make it easily
observable soon after exposure to water and air.
So stainless steel does not rust, because it is sufficiently reactive to protect itself
from further attack by forming a passive corrosion product layer. (Other
important metals such as titanium and aluminum also rely on passive film
formation for their corrosion resistance.) Because of its durability and aesthetic
appeal, stainless steel is used in a wide variety of products, ranging from eating
utensils to bank vaults to kitchen sinks.
Reference:
http://www.clihouston.com/knowledge-base/iron-and-
steel-rust.html