Why Do Knives Rust? [And 5 Ways To Deal With It]



What is Rust?

Last year, I left my brand new Condor knife outside in the rain for a week while I was on holiday. And dare I say that I regretted it.

When I came back, the blade was covered with orange/reddish stains. And so I learnt about rust, which is not he best part about owning a knife, but which is still an important aspect of it that one must deal with.

What is rust? Well, rust is primarily Iron(III) Oxide, which has the chemical formula Fe2O3. It is formed when the Iron from your steel blade combines with the Oxygen from the atmosphere.

Here is a short post from NCH Europe which gives a good description of what rust actually is:

WHAT EXACTLY IS RUST?

Most of us know rust as a reddish-brown flaky coat on metal and think nothing more of it, however rust is the term commonly used for the corrosion and oxidation of iron and its alloys, such as steel. Technically rust is Hydrated Iron (III) Oxide, also known as iron oxide (Fe2O3), as it is caused when iron reacts with oxygen and water – this reaction is known as oxidizing. If a piece of iron is left long enough, with exposure to water and oxygen, its rusting is inevitable – it could take days, weeks, months, or even years depending of the intensity of its exposure, however it will rust if it is not protected in any way. Rust is very common, as iron reacts easily with oxygen.

NCH Europe

 


What Causes Rust?

Okay, so now I know that rust is actually Iron Oxide. But what are the conditions that cause a knife’s blade to rust?

For a knife to rust, three things are required: oxygen, iron and water.

Oxygen comes from the atmosphere and is present in the air around us. Iron is the main material of steel blades. 

Moisture (water) favors the formation of rust. It does not have to be water as a liquid. A humid air will suffice to cause a knife to rust.

This can give us an insight in how to present a knife from rusting. If we can prevent these three ingredients from coming into contact, your knife will not rust!


Do some steels rust more easily than others?

The big divide in types of blade steel is that between Carbon steel and Stainless steel. Most types of blade steels can be classified as being either Stainless or Carbon steel.

As the name suggests, Carbon Steel is steel with additional carbon. Many steels can contain different metal alloys. However, it is the percentage of carbon making up the blade that plays the main role here.

Usually, Carbon Steel blades contain between 0.5% and 1.5% carbon. This makes the blade very strong and durable. Carbon Steel is thus a favourite for experts as it makes very good survival knives.

However Carbon steel blades will be prone to corrosion if you don’t take good care of it! They will rust easily in humid and acidic environments.

Stainless Steel blade contain Chromium, which usually makes up between 12.5% and 13.5% of the blade. This creates a strong oxide layer on the surface of the blade, which prevents the steel from further oxidation. As a result, the corrosion of the blade is slowed down, which is why the steel is called” Stainless”.

Hence, Stainless Steel knives are a good choice for corrosive environments, such as the sea.

Just remember that stainLESS does not mean stainPROOF. Stainless steel blades will also rust if you don’t maintain them properly, even if they take longer than Carbon steel ones.

There are other blade materials out there such as Titanium or Ceramic. Because these don’t contain any iron, they will not rust, and your knife will be completely corrosion-proof.

Another factor to consider is the coating of the blade. Some blades have specific coatings that make them more durable and corrosion-resistant.

For example, some knives come with Titanium or Aluminium Oxide coatings, which prevent the inner steel blade from rusting.


Why is Rust Bad?

Some knife experts say that rust is a sign that the knife is good quality. I’m not an expert here, but to me a rusty knife is quite ugly.

Rust in itself is not dangerous, and you won’t die if you eat something cut with a rusty knife.

However, if often reduces the quality of the blade, and can make it less versatile in the field. 

In any case, I really don’t want any rust stains on my knife, and I always aim to keep it “stainless”.


So How do I Remove Rust From my Knife?

Fortunately, there are many ways that you can remove rust from your old knife.

These methods vary quite a lot. While some are more traditional, some of the natural methods to remove rust will surprise you!

METHOD 1: CHEMICAL SOLVENTS

There are many types of chemicals out there that can be used to remove rust from your knife. A popular one is WD-40 (however, this one is toxic, so make sure that you don’t intend to prepare food with that knife).

Step 1: clean the knife with oil to remove any other stains or impurities.

Step 2: spray the blade with the solvent.

Step 3: use thin sandpaper to scrub the rust stains of the blade.

Step 4: repeat if needed, then wipe, rinse and dry the knife,

METHOD 2: BAKING SODA

Step 1: wet the rusted area with water, and sprinkle baking soda on it. The baking soda should stick to the wet area. Gently tap the knife to make the excess fall off.

Step 2: use a wet scrubbing pad to scrub the baking soda-covered area. Make sure that the scrubbing pad remains wet, so that you don’t damage the blade. 

Step 3: after scrubbing for a few minutes, the rust should have come off. Repeat if required, then rinse the knife with water and dry it with a cloth.

METHOD 3: VINEGAR

Step 1: Pour white vinegar into a wide container. I typically use a pan for this as it is wide enough for my knife.

Step 2: Soak the knife (or just the blade) in the vinegar. Do this for about 5 minutes. Don’t try to leave it too long, as it might just stain or damage your blade.

Step 3: Wipe down the blade with a cloth. Then rinse and dry the knife.

METHOD 4: SANDPAPER

Sandpaper usually works out well with small rust stains. Just take a small piece of fine sandpaper and rub the stain gently with it until is disappears. 

You can also use sandpaper after having gone through other methods, if there is still some rust left on the blade.

However, make sure that you are using very fine sandpaper. Otherwise, you might really damage your blade, remove the finish, scratch it or dull it.

I would typically use a 3000 grit product to scrub off the rust stains.

METHOD 5: NATURAL PRODUCTS

We’ve already covered many ways to remove rust stains from your knife. 

But there are actually tons of methods that don’t require any special equipment. 

Here are 4 methods that might help to remove rust from your knife:

#1 – THE POTATO METHOD

Potatoes contain oxalic acid, which can dissolve rust. Just stick your knife into a potato for a few hours. After that, you should be able to wipe the rust off with a cloth!

I tried this on a pocket knife which had small rust stains, and to my surprise, it actually did work! I left it overnight, and the rust was gone the next day.

#2 – THE DIRT METHOD

This one is also quite cool. It involves repeatedly plunging the rust-covered knife into rich soil before wiping it clean and rinsing it.

I haven’t actually tried this one out myself. If you have and would like to share your thoughts on it, please leave a comment at the end!

Just make sure that you don’t use this method to much, especially in rough soil, as this might make you knife dull or even scratch the blade.

#3 – THE LEMON JUICE METHOD

The citric acid in lemon juice can be used to dissolve the rust on your blade. 

Simply cut a lemon is half and rub it across the rust stain. If the stain is small, you should be able to remove is with a scrubbing pad.

Otherwise, let the half-lemon sit on the stain for about half an hour before attempting to scrub it off.

#4 – THE ONION METHOD

Onions contain sulfenic acids, which can help dissolve iron oxide (the main compound of rust).

You can either stab an onion with your knife and let it sit for a while or saw back and forth into the onion.

After some time, you will see that the rust starts coming off by itself!


How do I Prevent my Knife from Rusting?

We’ve talked a lot about what to do to remove rust from your knife. However, remember that prevention is better than cure.

You’ll always want to keep your knife rust-free. So what should you to to ensure that?

TIP #1 : KEEP YOUR KNIFE CLEAN AND STORE IT PROPERLY

This one is the most important thing in this entire post. If you care for your knife properly, it won’t rust!

So how do you care for a knife?

Firstly, wash it after use. Whether you’ve been cutting wood or gutting fish, you will always have some degree of dirtiness on your blade.

Clean the residue off with water and the soft side of a sponge, before dry the knife thoroughly with a cloth. Don’t skip that step, otherwise your knife will definitely rust!

Secondly, store it appropriately. If you are indoors, store is out of its sheath, in some sort of dry drawer or cupboard. If you are outdoors, keep it in its sheath. 

You can also use knife oil to prevent contact between the blade and the atmosphere (hence preventing rusting). The traditional oil used for storing knives is tsubaki oil.

TIP #2 : CATCH RUST EARLY AND GET RID OF IT

Another great way to prevent significant rust to accumulate on your blade is to check it often.

As soon as you see any orange stain appear on your knife, try to give is a rub with very fine (3000 grit) sandpaper to remove it.

TIP #3 : BUILD A PATINA

What is a patina?

Simply put, a patina is a thin protective layer that forms on knife blades over time. A patina is unreactive, and will prevent your blade from rusting.

A patina is actually also Iron Oxide, although it is a different, more stable form, with the chemical formula Fe3O4.

Patinas can form naturally with age and use, or can be forced onto a blade. You can force a patina onto a blade with vinegar, potatoes or even mustard.

A naturally-formed patina on an Opinel knife

Conlusion

I hope this helped you to remove rust from your knife. Remember that the best way to keep a knife rust-free is to prevent rusting in the first place.

Make sure that you always keep your knife in peak condition, so that it does it job the best it can.

Now that your knife is clean, it’s time to sharpen it. I have compiled for you a 5-step guide to knife sharpening.

Any comments, questions, or contributions you would like to offer? Please share in the section below. Looking forward to hearing from you!

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