Why Do Knives Rust?


Whether you cook everyday, or just as a hobby, we believe you should always take care of your tools properly no matter their initial cost.

One of the more obvious signs that a knife needs more tending to, is rust. Even though perhaps you wash them everyday, you might spot some rust or through incorrect storage you may have seen a knife turn completely black!

You're not alone, though. Many people have trouble taking care of and managing rust. As a result, there's many rusty knives around people are hesitant to use around or on food.

Why do knives rust though? This article will explain that - as well as everything else you need to know about rust on kitchen knives.


Why do knives rust?

There are two main culprits that are the cause of knife rust - water and oxygen.

That said, these two must work in tandem for that to happen. If you only have one of these, a knife will almost never rust. When the two mix together though, it creates a chemical reaction as they oxidize on the surface of particular metals - that chemical reaction is rust. In this case, iron is a processed metal and rust is that iron returning to its natural state as it combines with the elements needed to do this.

As the diagram below shows, as the O2 molecules hit water which is in contact with Iron, it forms as Rust or Iron Oxide (Fe2O3.)

We can't easily just fix this scenario, as we can't take oxygen away. So as a result we need to remove water, hence the importance behind keeping knives, especially carbon steel ones dry or drying them as fast as possible after use. Cleaning your knives is of course important, but drying them is even more so. Ultimately, even though this is a natural phenomenom - it's very troublesome and annoying for knife owners.

Once a knife becomes rusty, the blade may dull or even chip, plus the rust on the knife may transfer to the food you cut with it or even your end dishes. So it pays to be particular with your maintenance.


Types of Rust

"Rust" itself is a broad term and there is many different ways of rusting that can occur on metal products, dependant on the type of metal and the situation. Additionally, multiple kinds of rust can occur at the same time.

In the case of kitchen knives, there's two main kinds of rust to be aware of (but others can occur too.)

Red Rust

When people first think of rust, this style of rust is probably what comes to mind.

The rust is a reddish-brown in colour, and the surface of it crumbles when it's touched.

This is absolutely a rust we want to avoid, as it corrodes the iron itself and destroys knives. If left unattended to, the knife's lifespan and durability will be vastly affected for the worse.

This is a good reminder that the most important thing about knife maintenance is keeping them from getting rust.

Once a knife gets rusty, the damage cannot be fully reversed. Never let your knife get rusty.

Black Rust

Black rust is another chemical reaction we call rust, but unlike red rust this is a very stable style of rust, and significantly less damaging. Some even refer to it as a benign rust.

Black rust creates an oxide film on the surface of the iron and coats it, so it doesn't spread or fall apart and crumble like red rust does.

For this reason, black rust rarely converts into red rust. Some people even say that in order to handle a carbon steel knife properly, you should maintain them so that only black rust forms.

The difficulty behind this is black rust rarely ever occurs naturally, and almost always needs to be processed or caused artificially.


Does stainless steel rust?

Many people think stainless steel doesn't rust - and who can blame them? The name somewhat implies it!

However, while stainless steel is resistant to rust, if not properly maintained or worse even neglected rust can still form.

Why does stainless steel resist rust?

Stainless steel is an alloy mainly comprised of iron, which also contains more than 10.5% chromium, and is characterised as an incredibly corrosion resistant metal.

This chromium forms a very thin film around knives, which combines with oxygen before the iron does, creating oxidised film instead and preventing rust from happening. This can also be called a "passive film" thanks to the occurance of passivation.

While this sounds like the perfect solution, this film is incredibly thin - less than ¹⁄₁₀₀ of 1 micrometer (μ), which in itself is one millionth of a meter. Continuous abrasion or wear from external forces can cause this film to be destroyed and rust to form as a result. Of course though, there are ways to prevent this film from being damaged such as good maintenance, and not scratching or dropping the knife.

What causes stainless steel to rust?

Three main factors can contribute to stainless steel rusting, so be wary of the following possibilities:

Water and dirt are always on the knife

If moisture or other dirty materials are only on a stainless steel knife for a little while, there shouldn't be any issues. However, leaving these materials on the blade without wiping them off for some time may cause the protective film to break, beginning the rusting reaction.

Exposure to salt

Red rust often has a degree of involvement with salt. Around coastal areas you'll often see more rusted products. This is because the salt in sea water makes it easier for products that come in contact with it to absorb water - a key component of rust.

Stainless steel has a significantly stronger resistant to salt than other steels, but if the exposure is continous and not cleaned off rust can still occur.

Contact with other rust

Rust, especially red rust can transfer from one rusted product to an unrusted one and turn it rusty very quickly.

If a stainless steel knife comes into contact with rust, the protective film is easily destroyed and the rusting process will begin.

Rust on stainless steel is troublesome to take care of, and you must make absolutely sure to keep other rusted products safely away from stainless steel products.

On stainless steel, the most common style of rusting is referred to as pitting corrosion.

Where pitting corrosion differs to other rusts is that the rust fully penetrates into the metal. This causes extreme damage to the knife.

This rust style can't be saved, as it effectively eats into the metal - so even after removal and sharpening, the rust pits remain.

Effectively, even though stainless steel knives are rust-resistant when they do start rusting the knife deteriorates significantly more quickly and in a way that can't be recovered - so take care of them!

Don't let your stainless steel knives rust! Keep them clean, dry and salt-free!


Isolating rusted knives

Rust is something that you unfortunately at some stage will likely encounter. In a sense, due to the nature of where kitchen knives are used (in the kitchen where there's water sources and other plumbing) and what they can be cutting (acidic or salty products,) rust can almost feel inevitable. It's very difficult to keep knives from rusting unless you perform maintenance on them like cleaning and properly drying them, as well as storing them in dry environments that aren't near salt.

Once rust starts however, it cannot be stopped. Even if you clean it all off it will eventually return. Make sure if a knife starts rusting to isolate it and never use it near any unrusted metal products as rust will spread across to your other knives.

If you do have a knife that has rusted, we can help remove that rust. We also have an entire article dedicated to rust removal, so be sure to read that. Or contact us!