corrosion protection and prevention
Rusting
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 Iron Plate)
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.
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
No Rust Occurs In Stainless steel:
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, 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
