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Why Doesn’t Stainless Steel Rust?

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Steel has been an important metal in the human industry for thousands of years.  In fact, the earliest known production of steel dates back to an archaeological site in Anatolia nearly 4,000 years ago. The Roman military was known to use steel, and early civilizations used it in weapons, tools, and construction.  As prized as steel was by our ancestors, both for its strength and relatively lightweight, it possessed one key flaw that was a tremendous hindrance: it was incredibly prone to rust.

Thankfully, stainless steel was developed more than a century ago; due to this metal’s amazing corrosion resistance, it has ushered in a new era of industry and design.  Builders and manufacturers can now create all kinds of products and structures and no longer worry that the metal will succumb to rust when exposed to moisture.  Certain stainless steel alloys are even suitable for marine and other extreme environments.

So, what is it about stainless steel that protects it from rust?

Iron is naturally susceptible to corrosion

People will often associate any form of corrosion with rust, but the truth is that actual rust only occurs with iron and iron bearing materials.  This is because rust is by definition iron oxide.  This (normally) red oxide is the result of the reaction taking place between iron, oxygen, and water.  Even moisture in the air can be enough to corrode iron, as it is naturally susceptible to this reaction.

Over time, iron will eventually be converted entirely into rust in the presence of oxygen and water until it completely disintegrates.  This is due to the fact that the iron oxide surface is extremely flaky and offers no protection to the underlying iron. While pure iron will be the quickest to corrode, most metals that contain iron will eventually succumb to rust; this includes carbon steel, which is an alloy consisting mainly of carbon and iron.

The chemical process of rusting starts with the penetration of the iron’s surface by water molecules.  It may seem like the metal is solid, but its microscopic cracks leave it exposed. This allows the hydrogen atoms in the water to combine with molecules in the metal.  As the process proceeds, more and more of the metal becomes exposed.  If chloride is present in the water, as with salt water, the corrosion occurs much more quickly.

As this is happening, the oxygen atoms bind with the iron atoms to form the destructive iron oxide compound.  This causes the metal to weaken and become flaky and brittle.  Its typical red color has become known as rust.

Stainless steel was developed as a solution to rust

Stainless steel is characterized by the presence of chromium as an alloying agent.  Typical stainless steel will contain at least 10.5 percent chromium versus only 1.5 percent carbon. The presence of the chromium and other alloying agents effectively prevents the rusting that normally occurs with carbon steel.

Scientists first became aware of the corrosion resistant properties of iron-chromium alloys as far back as the 1820’s.  For many years metallurgists of the time were not able to find the right proportion of carbon and chromium, and the early alloys were far too brittle to be sold on the market.  It wasn’t until the 1870’s that John T. Woods and John Clark discovered the first modern example of stainless steel, which was patented in England.

Over the next several decades researchers continued to experiment, developing different versions of early stainless steel; in 1912, Harry Brearley developed the first martensitic stainless steel alloy that was commercially viable.  His company, Firth Vickers, sold the metal under the Staybrite brand in England and it was used for the new entrance canopy of the Savoy Hotel in London.  The American patent was granted in 1915, and the era of stainless steel had begun.

How does stainless steel prevent corrosion?

It is usually assumed that stainless steel is impervious to its environment in a way that normal steel is not, but the reality is that alloying agents are actively interacting with the environment around the metal.  It is this reaction that actually protects the metal from rusting.  The alloying agents in the stainless steel, such as chromium and molybdenum, mix with the oxygen in the water or air to form a thin but stable film of metal oxide.

This film forms a protective layer on the surface of the metal and protects it from more visible and damaging corrosion such as rust.  It does so by creating a barrier that prevents the oxygen and water from interacting with the iron content of the metal. The film is so thin that the wavelengths of light can’t even interact with it, and it is typically only visible with the aid of modern instruments.  This means that the metal retains its aesthetically pleasing appearance and gives it the illusion of being “stainless.”

When the surface is scratched or damaged in some way, the exposed inner layer will quickly react with the air or water to form a new protective layer; this is because the protective layer is not something that has been added but is actually a property of the metal itself.  In this way stainless steel is self-repairing.

A Trusted Technical Partner

While every kind of stainless steel offers protection against rust, each alloy has its own set of unique properties.  Knowing which alloy is right for your application is essential.  Thanks to the increasing diversity in the many stainless steels on the market, it is becoming harder and harder to keep track of them all.

Clinton Aluminum does more than just supply raw materials.  Our goal is to be a true partner to our clients, working with them during every step of their procurement process. As the Midwest’s leading supplier of aluminum and stainless steel products, our success is a direct result of our commitment to always doing the best we can for our customers. Please contact a member of our knowledgeable and friendly sales team today to learn more about how Clinton Aluminum can help you.

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