What's going on with my knife? Its R2 steel with stainless cladding but it looks like it reacted with something and developed a film.

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whywhywhy

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What is this on my knife? It's R2/SG2 steel. It's a Shibata knife. It appears to be concentrated mostly on the edge of the knife, not really the cladding. I bought it a while ago as a gift to myself to motivate me to cook more but my plans got put on hold and I've only used the knife a few times. I think I cut green onions, parsley, cilantro, celery, onion, maybe rhubarb, and some chicken. It seems pretty hard. Is the knife steel defective or something? How can I remove it? I've only washed it with a soft sponge with soap and warm water. Thank you very much!
 
whywhywhy said:
What is this on my knife? It's R2/SG2 steel. It's a Shibata knife. It appears to be concentrated mostly on the edge of the knife, not really the cladding. I bought it a while ago as a gift to myself to motivate me to cook more but my plans got put on hold and I've only used the knife a few times. I think I cut green onions, parsley, cilantro, celery, onion, maybe rhubarb, and some chicken. It seems pretty hard. Is the knife steel defective or something? How can I remove it? I've only washed it with a soft sponge with soap and warm water. Thank you very much!
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R2/SG2 has pretty high chromium content (14 - 16) but not as high as some stainless steels. For example, the 440 series is 16-18. SUS440B can be up to 19. Even something like CROMOVA 18 can be 17+. Although all of these would qualify as "stainless" steels (which require a minimum of about 11% chromium to be labeled as stainless), that does not mean truly stainless, as in completely oxidation/corrosion resistant. So I think what you're seeing there is just the core steel being a bit reactive and darkening some. It shouldn't mean that the knife is defective.

The other option, depending on your water quality, is that it could be some sort of mineral deposit or build up, but I've never actually seen that. So I'm just throwing out ideas. Mainly, I would primarily suspect that it's just the steel showing a bit of reactivity/patina. Nothing to worry about

[Edit]: If you want to remove it, you can try a non-abrasive polisher like simichrome or flitz. Should take it right off without leaving a scratch.
 
I would like to take it off if possible. In fact I'm thinking about eventually selling it since I dont really use it much! So I should use acetone? That won't ruin it?
 
whywhywhy said:
I would like to take it off if possible. In fact I'm thinking about eventually selling it since I dont really use it much! So I should use acetone? That won't ruin it?
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Acetone will just dissolve the coating if there is one. It's an organic solvent, not an acid. It's volatile so it'll just evaporate off.

Just don't breathe or drink too much of it and you and your knife will be a-ok.
 
Thank you very much everyone, I'll take your advice and report back when I get some time.
 
GeneH said:
Take note acetone will also remove any finish on your handle. It’s great stuff but do remember it’s a darn good solvent.
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And that is why I always ask before I do something I'm unsure of, haha. You can guess what I'd be writing soon if I hadn't. "I ruined the coating of my handle with acetone, is there anything I can do?" :)
 
I’ve had similar stuff happen when cutting lots of greens in particular. It almost seems like a mineral deposit which can be scraped off with a fingernail.
See gyuto below:
968380F8-B477-49AF-AE51-C17EAF9300C8.jpeg
 
whywhywhy said:
Hmm, I couldnt get it off with acetone. It does looks like hard water buildup or something similar to that. Any other ideas for getting rid of whatever it is? Here are a few more pics.
Click to expand...
While sg2/r2 is one of the best stainless on market , it’s corrosion resistance is not as good as Aebl or other lower carbon stainless steels . Most of the chromium gets tied up with carbon to get higher hrc .
You can use simichrome or flitz metal polish to get rid of stains , secure the knife , apply the metal polish with cotton pad or micro mesh , rub until it comes out , watch the edge not to cut yourself . If the stain is too deep you may have to use very fine sand paper 1500 grit and above
 
Thank you very much, I'll give it a try. Why would that happen so fast? I only used it a handful of times.
 
You mentioned rhubarb, which contains oxalic acid, which is also in bar keeper's friend. BKF created stubborn stains on my knives so the rhubarb could have done the same for you. A touch up with a fine stone will surely fix it.
 
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