How Fast do Japanese Carbon Steel Knives Rust?

A frequently asked question in Knifewear stores is, “How quickly do these knives rust?”. If you’re new to Japanese knives, some are made with high-carbon steel, which means they can rust. If you’d like to learn more about steel, check out this article from Mike. If you want to dig right in, stick around! I’ll start with a summary, then read as far as you dare:

The carbon knives we carry start to visibly rust when exposed to moisture after a rough minimum of 6 minutes. That sais, oxidation time can vary wildly; leaving your blade covered in onion juice can rust it in a matter of seconds, whereas  drippings from a roast will affect the steel far less. Using your carbon steel knife will build a patina, which makes it rust much, much slower- think 45 minutes to a couple of hours. Stainless steel will only rust under some super-specific conditions, but you’re more or less okay leaving it to air dry. Go stainless if you don’t want to think about it; go carbon if you’re a romantic and a maintenance nerd. That’s it! You can read on for extra nerdy stuff or duck out at this point. I won’t be offended.

Greetings, nerds. 

Let’s start with a definition of what rust is. According to trusty ol’ Wikipedia, “Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture.” Or, in more plain language, it’s a substance that can corrode your carbon steel knife if allowed to hang out on a blade for a super long time. If you'd like to learn how to care for carbon steel, check out this article by Adam.

With that in mind, whenever I’m not supposed to do the thing, my lizard brain wants to do the thing. Every week, I tell dozens of folks not to put their knives in the dishwasher, not to leave them wet for too long if they’re made with carbon steel, not to go through super hard foods/bones/ice, so now I’m on a mission to do all of those things, one by one. So I got enough volunteers and shop demo knives and a whole bunch of lemon juice and got to work doing exactly what you’re not supposed to do with some very fancy knives. I shot a bunch of timelapses that condense a pretty boring 45 minutes into 38 seconds.

Hindsight 20/20, I went into this experiment expecting dramatic blooming rust, but what I got most of the time was ultimately pretty tame! I started with comparing a full carbon blade against a stainless one, coating each with water, saltwater, and lemon juice. Over a period of 45 minutes, the most I got from either was some darkening around the saltwater and just a bit of staining. 

The biggest culprit of rust is also the most common: water. Maybe you washed your knife and let it air dry, maybe you cut up something juicy and didn’t wipe off your knife, or maybe there was some spray from the sink that ended up on your blade. In any of these situations, the rusting will be really, really mild, most likely a spot here or there. Lemon juice, tomato juice, and other acidic things will be a similar story, but just a bit quicker. Saltwater is also extremely corrosive on carbon steel so that you might have issues in coastal areas or if you’re storing your knives right above where you cook your pasta. 

Next, I tried some super blue carbon with a full patina for another 45 minutes with almost no change. I can’t overstate how much a good patina will make your life easier in the long run- the more you use your knife, the less likely it is to rust as it builds that coating! If you want to force your own patina, we have a separate blog on that.

At this point, I was itching for some dramatic rusting action, so I picked two of the knives I see most often coming back with rust on them: The Fujiwara Maboroshi, for a very pure carbon core, and the Moritaka Ishime, fully clad in yellow carbon and chock-full of rust-friendly divots in the finish. 

A Fujiwara Maboroshi that came in post-dishwasher :(

Whereas the Maboroshi got some light patina going on, I finally got the drama I wanted from the Moritaka.

Because rust loves the combination of moisture and oxygen, you’ll usually see it appearing at the edges of a droplet where those two meet. On a smooth, polished knife, the droplets can be exactly just that- little circles here and there that shrink as they dry, which can cause some rust. Then, there are knives with tsuchime (hammered), or ishime (rock face) finishes with a million little peaks and valleys for moisture to settle into, and it dramatically increases the amount of area where liquid meets air. It’s also easier to clean a smooth surface, just like how it’s easier to clean off a plate than a cheese grater! So, combining soft yellow carbon cladding, a textured surface, and plenty of moisture, you’re going to end up with a pretty high-maintenance knife. 

Compare that to a smoother fully carbon knife, and you won’t see nearly as much action:

To the best of my knowledge, stainless steel works by adding enough chromium (at least 10.5%) to steel, which forms an initial layer of thin/stable oxidation that is so small our eyes can’t see it, kind of like an invisible patina. Literally, this is corrosion on an atomic level, which is honestly kinda bad. This layer is what prevents rust from creeping in further on stainless steel knives. This layer forms extremely quickly, which is why stainless knives don’t become rusty after we sharpen them. 

The only time you’ll see this kind of steel rust is when damage occurs to the blade in the presence of the regular rust culprits (oxygen and moisture). This damage can be physical, when a knife gets banged around in a drawer or dishwasher which usually looks like a tiny dark spot surrounded by a ring of rust. The damage can also be chemical, wherein that protective layer gets continuously corroded by cleaning with bleach or other substances. Chemical corrosion to a stainless steel knife will make a very spotty/pock-marked rusting effect, but even then, it requires tons of time to be left out and often continuously damaged. 

NOW THAT I’VE RUSTED EVERYTHING…

So! Now that everything is gross and rusty, how do I fix it? As it turns out, it's pretty simple. Your first defence against rust is the rough side of a kitchen sponge. That’s the kind of rust where you go “oops” after leaving it wet for a little longer than it should be. If you’ve got a little more than that, maybe “I put this away wet in a box and forgot about it for a week,” I like to use a paste made of Bar Keepers Friend and water and rub that in with a wine cork or sponge, using gentle pressure.

If you’ve got a metric sh*t ton of rust, like, “I brought my carbon steel santoku to the Caribbean and frolicked on the beach with it and then left it to salty waves and lemon wedge garnishes for six months,” maybe it’s time to step up your rust removal game. A rust eraser is the tool for the job. If there’s deep pitting in the blade, that generally only gets fixed by grinding away the corroded steel, and that means it’s probably time to bring it to the professionals, although with a coarse enough set of stones, you could do it yourself too. 

So, to recap on rust:

• Carbon steel is way more likely to rust if it has a bumpy surface, is brand new (e.g. no patina), or is continually mistreated.
• The quicker you wipe off your knife, regardless of steel, the better off it’ll be in the long run!
• Never EVER put your knife in the dishwasher.

I hope that helped you to understand rust better! If you’d like to learn more about Japanese knives, visit one of our shops, shoot us a message online, or click the link below!

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