Why Does Stainless Steel Rust & Corrode?

I can’t tell you how many times I’ve heard asked, “If its ‘stainless’ why does it rust?”  While it is true that under many natural conditions stainless steel’s natural resistance protects it from rust some factors can still corrode and corrupt the material.

It is impossible – yes, impossible – to keep menacing little particles of iron out of stainless steel during its processing.  Those tiny iron deposits act like a magnet for rust and corrosion, especially under extreme (and not so extreme) environmental conditions.

Rust is a natural chemical process in which an anode (such as a metal surface) comes into contact with an electrolyte (a liquid, such as water) and a cathode (such as another metal surface).  The iron particles within the steel and the steel itself rapidly trade electrons when water is introduced.

With so many electrons in motion, some of the metal surface is lost.  Rust is effectively the disintegration of metallic objects.  The tell-tale reddish-brown color is what the chemical process leaves behind.

Some conditions that will cause stainless steel to rust include:

• Oxygen – Oxygen is, of course, a natural oxidizer – a key element in the rusting process.  Iron naturally combines with oxygen, causing the rusting process.  Applications that require a high pressure oxygen atmosphere can expect to replace stainless steel parts regularly.

• Water – Any stainless steel used in water is particularly susceptible to rust and corrosion. This is especially true in the ocean as salt acts as a natural chemical booster to the process of corrosion.

• Humidity – Moisture in the air, i.e. humidity, acts on the iron in stainless steel in the exact same way submerging it in water does.  The unfortunate truth of humidity, however, is that it is much more difficult to notice than a large body of water, and is therefore harder to protect stainless steel against it.

• Ocean Breeze – Humidity in the ocean air carries particles of salt along with it, and accelerates rusts and corrosion in stainless steel.

• Rain – If rain is anything, its water.  But rain also carries an extra surprise – pollution.  As rain falls, it carries air pollution down with it.  The make-up of a single raindrop over an industrial area is a soup of corrosion-accelerating chemicals.

• Electricity – Remember the chemical process of exchanging electrons through water?  That is technically an electrochemical process, and adding a few extra volts of electrical current to the metal only makes it that much easier for those pesky electrons to escape, carrying metal material with it.

Extreme amounts of any of the above substances will cause stainless steel to rust and corrode quickly, but over time even small amounts of water, electricity, and even the air we breathe will lead to rusting.

The Benefit Of Stainless Steel Passivation

Passivation is another chemical process that creates an oxide layer to protect stainless steel surfaces against rust and corrosion.  Stainless steel’s most vulnerable points of corrosion lie in its grain boundaries, where iron deposits tend to settle during the manufacturing process. Passivation in an acidic compound helps to smooth out those boundaries, releasing some of the iron that can later cause rust.

But, the true benefit of passivation comes from the forming of an inert surface layer, a kind of natural film known as an oxide layer.  Many metals, including silver and stainless steel, have a thin oxide layer already, making it naturally resistant to corrosion and rust.  Passivation thickens that naturally occurring oxide layer, making the chemical process of rust and corrosion much more difficult.

Passivation gets its name from the fact that it makes metal surfaces “passive” – in other words, less likely to suffer from natural chemical processes that eat away at metal.

Learn More About Passivation From The Experts

If you’d like to know more about passivation, or are looking for a way to protect your own metal surfaces or parts, contact the expert team of passivation technicians at CELCO today.  For us, it’s not just passivation, it’s a passion.