Why stainless steel can rust

You would think that stainless steel does not rust. But this is not true. Almost all stainless steel can rust under certain conditions. This sounds contradictory and is often considered a defect or material flaw, but this is very rarely the case. Therefore, we would like to explain how and why stainless steel can rust and how you can avoid and clean possible rust.

There are two ways to explain this phenomenon: the long and the short one. Let’s start with the short version. If this is interesting enough, you can then read the long version.

In short:

Stainless steel is an alloy of various elements that can make and keep a knife sharp. These elements ensure that the steel is strong, but not quite stainless. The better the knife stays sharp, the more sensitive it becomes to rust. You can consider different aspects, but you always have to make concessions. In general, stainless steel will remain rust-free with careful maintenance, but sea air, salt water, and acids always have a major impact. This is true with any type of steel, regardless of the price of the knife. The most important thing is to always keep the blade clean and dry. You can easily grind out any rust spots with your preferred grinding method.

The detailed Explanation:

That was the brief explanation of a complicated matter, but there are many more aspects to this story. We have tips on how to prevent rust. There are types of steel that are completely rust-free and still remain sharp. Are you ready for a detailed explanation? Then you’re good to go!

How is a Knife Steel grade constructed?

To understand why different types of steel have different properties, it is essential to know how a type of steel is constructed. Steel is not only made of iron. Most of it is, of course, but iron alone is not enough for a knife. Therefore, depending on the type of steel, different components are added; carbon, chromium, nickel, molybdenum, silicon, tungsten, cobalt, manganese, or one of the many other options. By varying the percentages of all these ingredients, steelmakers develop different alloys. Then one alloy is tough, another is very hard; one alloy rusts just by looking at it the wrong way, and another can be left on the ocean floor without anything happening.

It’s all about Carbon

Knife steel contains carbon. This is the component that ensures that the knife can be sharpened and partly ensures that the knife stays sharp. That’s why knife brands have been using terms like “high-carbon steel” for decades. For many, this is a mark of quality, and in some ways it is. Many not-so-high-quality alloys have lower carbon content, which means the steel grade is less likely to stay sharp.

A lot of carbon in a steel grade is therefore usually a good sign. Unfortunately, carbon has another property: it makes steel rust quickly. Therefore, a high carbon content also provides a higher risk of rust and discoloration.

Knife steels often have a carbon content of 0.5% to 1.5%. There are also very high values of 3% or very low values of 0.15%. However, these are then rather unusual steel grades.

Addition of Alloys and the importance of Chromium

This is precisely why steel manufacturers vary the ratios between the alloying elements. Chromium in particular plays a major role here. By adding a large amount of carbon as well as a high dose of chromium, the steel becomes stainless. In practice, the limit is set at a chromium content of 13%. More than 13% means stainless, below it is carbon steel. There is another category: tool steel. Since the chromium content of this type of steel is about 13%, it is difficult to assign it to a category.

So you could say: add as much chromium as possible, and you get the perfect steel! In practice, however, it looks more difficult. In fact, a high chromium content reduces the quality, at least in terms of sharpness resistance and hardness. To prevent this, many other alloying elements are added to the steel, for example, vanadium.

Böhler M390 is a steel grade that is pushed to the limit. Because of its sharpness and corrosion resistance, this steel is very popular. With 1.9% carbon, this is very high-carbon steel; however, it also contains 20% chromium. This is an extremely high value. You would think that the knife would not stay sharp for long then. To counteract this, the steel also contains 1.0% molybdenum, 0.6% tungsten, and 4.0% vanadium. These are the elements that ensure that the cutting edge is extremely hard.

So does the M390 steel not have any drawbacks? There is something worth mentioning, but not really a disadvantage. For example, M390 is too hard to be break-resistant; so not very good for large knives. Also, M390 is sometimes difficult to grind without good grinding products. The alloys added to the steel only make the job more difficult.

Why Stainless Steel can rust after all

There are a few external factors that cause steel to rust: Moisture, acid, and salt. These are exacerbated by heat. “But I don’t store my knife in a salt shaker, so it can’t rust, right?” – unfortunately, it’s not that simple. Salt is found in many more places than you might think. Sweat on the hand, certain foods, and sea air are good examples.

Sea air is the most common cause of rust in stainless steel. This air is a mixture of salt and moisture: a disastrous combination for knife steel. We often read on knife forums that collectors who live by the sea complain of rust stains, no matter how careful they are.

Fruit acids are another catalyst. Peeled an apple and forgot to wipe your knife down afterward? That may go well a few times, but eventually, rusty spots will appear. VG10 steel is stainless and is used for pocket knives and kitchen knives. This steel is very resistant to corrosion as long as you take a little care of it. For example, you should not put such a knife in the dishwasher. The combination of temperature, salt from the dishwasher tab, water (of course), and aluminum in the dishwasher is disastrous for this type of steel. If there is also a steak knife with aluminum rivets in the dishwasher, it is the perfect environment for rust to develop. It has nothing to do with the quality or heat treatment of the steel. It can happen, is part of it, and can be easily avoided or remedied.

How to prevent Rust on Stainless Steel?

All you have to do is clean and use oil. Why make it more complicated? If possible, you should clean and dry your knife as soon as possible. An occasional drop of oil won’t hurt either. You can use a maintenance oil like Ballistol, but also sunflower oil or similar. This way, you don’t give rust a chance.

How to remove Rust?

There are several ways to remove light rust. Depending on the type of rust and how deep it is in the steel, you can first rinse the knife with hot water. If necessary, you can also use a sponge. Be careful not to scratch the blade.

If that is not enough, you can try a tube of Flitz. This is a polishing compound. If you apply it with a cloth, it can often remove rust spots. Make sure you move the cloth in the right direction. If there are lines, scratches, or other scuffs on the blade, you should not move the cloth against these marks.

You should also know that the use of such products can cause a reaction on the surface of the blade. For example, a satin finish blade can become dull. And everyone knows: better safe than sorry.

Conclusion

There are many factors that have an impact on steel rusting; from salty sea air to salt in food. It will rust faster in one situation than another. This means you need to take good care of your knives, so rust doesn’t happen in the first place. Is it already too late? Don’t panic, just clean or sharpen your knife.