I think I accidentally did this to a bottle filler once by leaving it sitting in Star San (phosphoric acid solution) for too long.
As a knifemaker, I feel the need to add: you should also neutralize the acid anywhere it isn’t easily rinsed off – just dissolve a bit of baking soda in water and soak like you did the knife. Lightly coat the blade with a good food-safe oil or wax to increase it’s rust resistance. Never machine-wash your blade, hand wash with mild detergent, then re-oil the blade. Hone the blade whenever it stops cutting easily. These basics will keep your knife usable for decades.
Same thing happened to me, bottle of star san leaked into my equipment bin and corroded all kinds of stuff.
My blades get rusty fast enough without encouragement, so I’ll skip this. I don’t do pre-stressed or pre-worn clothing, either 
But anyway, I didn’t get a chance to add my endorsement to your previous post before it closed, so I’ll do that now! Opinels are good pocketknives for the price, I’ve had a couple and liked them.
I think you’re missing the point completely, Medievalist. This process actually protects the blade; it’s not just for appearance, unlike “prestressed” clothing. It’s more akin to prestressed steel or other structural materials, which actually strengthens the final structure.
Thank you. You answered my question which was, “Why?”.
Nobody reads the article anymore! “A big complaint about carbon steel is that it rusts quickly, while stainless steel does not. […] Adding a patina, or acquiring one via use, on carbon steel will slow bad rust.”
I feel sorry for that lemon, it never had a chance to be squeezed over a piece of fish. I found that white vinegar will do it in less than an hour. Simply put a knife in a narrow glass, blade pointing down, pour vinegar up to where the handle starts, let it sit. Within minutes it will start to change color.
Also, before the jump: “Examining one of my carbon steel blades I was disappointed to find it had not yet formed a protective patina.”
I still have the opinel knife I got in the boy scouts over 20 years ago. And it still gets regular use, though these days it’s mostly opening parcels and the like.
I recently bought the identical blade but in stainless steel (for a buck or two more).
For the relatively low price of the blade, I can be honest with myself that I’m unlikely to put much care or maintenance into it. And as I live in a wet environment, I may as well go with built in red-rust protection…
Alternate title: “How to keep unruly citrus in line.”
Naw, I understood, just making a wisecrack. Poorly, though, I guess! Sorry about that. Acid patinas are corrosion, or rust, as Jason pointed out himself (he also points out that a few of the iron oxides are mildly passivating, which is particularly important to cooks).
Black or grey rust will hold some oil and can keep your steel from developing red rust when exposed to moisture. But it doesn’t strengthen the blade in any meaningful way, and isn’t a substitute for keeping the blade clean between uses. It mostly just makes it prettier, bringing out any grain in the metal, and prevents mildly acidic foods from reacting with the raw iron and generating “off” tastes.
My blades and other metal accoutrements get patinated with use pretty quickly; for one thing, my sweat seems to be more corrosive than that of other people, for another, blood does the job up a treat (I’m a meat-eater as well as a tool user and occasional smith). Most of my armor has a heavy brown patina, most of my blades have a dark grey to black patina, with mirror edges.
This isn’t a “protective patina,” it’s just corrosion. You’re just abusing the blade for no result other than you like the purty color that results.
Look, you can passivate stainless steel with citric acid (the stuff that’s actually used for that is rather more concentrated than what you’ll find in a lemon, though), because the “stainless” part of the steel is the chromium. The chromium atoms at the surface react slowly with atmospheric oxygen to produce chromium oxide in a passive surface layer which resists further corrosion. Other crud on the surface, like the iron that’s in the steel, interferes with that, so stainless steels can be passivated with acid treatments that remove iron from the surface and allow the chromium to react with a subsequent treatment with an oxidizing agent.
But that’s not what you’re doing. You have carbon steel, not stainless, and all you’re doing is eating some of it away with a dilute acid and forming some thin layer of iron (II) or iron (III) citrate which doesn’t do a darned thing to protect the blade. You literally might as well leave the thing out in the rain and pretend that the red rust coating that develops is a “protective patina.” It isn’t, it’s just rust.
If you want to blue the thing and convert the surface to magnetite, there are a number of treatments that will result in an actually protective surface. Just knifing a lemon isn’t one of them.
have you read/heard of Alexander Weygers “Modern Blacksmith”?
I feel, as a fellow motorcyclist and tool junkie, you might enjoy it very much.
Ordered. Thank you. Will report back, I am sure.
As a habit, I cut my apples with my knife when I eat them. After 2-3 you’ll end up with a patina that doesn’t involve soaking your cutlery and leaves a much more even finish. Don’t wipe it right away, and it’ll happen faster. BTW #8 opinel…best low cost folder ever made.
Best stuff I’ve used for this purpose is Birchwood Casey’s Presto Black. Goes on very evenly, and helps to hold on to whatever sort of oil you want to use.
"Never machine-wash your blade, "
blackanvil, I machine washed a knife once and it shredded my clothing.
