Thread: Hardening Steel with Acid?

So I've been struggling with a Japanese blog post for a bit, but the gist of it seems to be this:

This guy took an old kitchen knife, dunked it in a mild (pH 5) acid solution and pumped oxygen and electricity through it for an hour and a half. It resulted in lots of black pitting, like we call devil's spit. Interestingly, the acid solution was made from natural hone slurry...one with a high level of sulfur

Then, he took the other end of the knife, put it in a stronger acid solution (pH 3.5--made by adding vinegar to the previous solution) and did the same.

This time, it resulted in cloudy kasumi effect, which under magnification showed tons of tiny little specks of that black rust. IN fact, it looked identical to the finish from a high end natural stone.

Now, one of the more "legendary" effects of a Japanese finisher is that it actually hardens the edge; someone actually tested an edge and said that it bumped up the rockwell by 2-3 points. I myself have always been skeptical. However, this guy speculates that perhaps, as I have seen myself, some stones are chemically prone to promote oxidation, and since Iron Oxide is harder than the Iron element of a blade, it can lead to hardening as it hones.

I'm not at all sure of the science, but it seems an interesting idea.

For those who can read Japanese, or like to look at pretty pictures of steel, here's a link to the blog: Å·Á³ÅÖÀФβâÌÏÍ͡ʺ½ÌÏÍ͡ˤòºÆ¸¡¾Ú - ÍïÇÀ²È¤Î¥Ö¥í¥° - Yahoo!¥Ö¥í¥°

So...what do you think? Anything meaningful or interesting here? I must admit, the idea is intriguing. If there is any reality to the "natural stones hardening steel" thing, it would be pretty cool.

This is a new item to me. I'll ask my friend to take a look since he has the metallurgical Japanese phrases in his head.

What I do know: The knife is a typical sushi/kitchen knife. High carbon core with stainless steel slabs on the outside. An acid treatment of that blade would passivate the stainless steel. That would make the SS more resistant to oxidation. I don't know without a text translation if that is what is meant.

Clearly the acid has darkend the carbon steel, as would happen over a few weeks of using it in the kitchen. I'm not aware of any method using electricity and chemicals to change the crystalline structure of steel of any kind in a way to produce martensite. That has to occur deep in the steel and the acid would not penetrate.

I could bring up hydrogen embrittlement. It's not a real strong possibility, but would affect the material. The hydrogen could slip between the steel molecules.

Good question. Like mental beef jerky...

Here's what Chris responded

The guy has a few theories as to what causes the white misting on the blade from polishing with stones. He has three theories and sets out to test them.

He compares the corrosion that results from the slurry from polishing stones. He compares this to the corrosion from a vinegar solution. He notes that the stone particle slurry produces a greenish colored rust only along the edge. He pumps up the acidity with a vinegar solution and gets a nice haze over the entire blade.

He makes comments at the end that the iron oxide is harder than the steel. Would seem a weak case hardening like effect....

He is not using electricity, just an air pump to speed the reaction....

There isn't enough information as to how he is testing hardness or judging that. I don't know if he means resistance to polishing/abrasion or cutting action.

The white misting could be something as simple as incidence of refraction of light reflected from the finer scratches from polishing. An interesting puzzle without the other questions answered.

Thanks Mike. I guess I misunderstood the energy part, but I got the rest, I guess.

He is only referencing the Mohs hardness of Iron Oxide vs. Iron. His conclusion is "It can be thought that the changing of Iron to Iron Oxide could lead to an increase in the hardness of the steel." Mohs is based on abrasion resistance, right?

That would be my understanding of the Mohs scale. But it's really more for evaluating minerals than metals. Iron oxide does not harden the same way or to the same degree as iron carbide (steel). Maybe a slight misunderstanding on his part there.

I was thinking Nitrogen embrittlement. What is the acid? What effect does the Sulpher have? I forget and am to lazy to go look it up. I suspect the electricle charge combined with the chemicals could create a chemicaly altered "crust" which could be harder and more brittle.

Just a thought
Jeff

Jeff, you've brought up one of the best reasons for avoiding the use of Liquid Nitrogen on steel.

The acid used by this experimenter was simply vinegar. Not nitric. If there was such an oxide crust, it would be molecules thick and probably wiped away by the next honing. Sometimes the iron oxide scale I get on billets can be pretty hard stuff, but it's formed at very high temperature. Not something seen in this kitchen safe trial.

Yeah but I was thinking that it could be related to the eletrification of the metal. I know all we are really be talking about is passivation and electrolsys. However, if there is a change to the blade then??

Another posibility is the electron flow may leave the blade along the edge and cause a refinement of the crystaline structure. Similar to drawing the steel? Allthough that should make it less hard. This would be interesting and quite usable on a large scale.

In the case of Nitrogen enbritlement what I ment to say was Nitrogen enrichment. I am not sure of the process but it is what they do to Nitronic 50 and 60 to make such a nasty stainless. Its not super hard but it is very wear resistant and REAL crappy to work with, Some of our guys say it is worse than the Inconels.

Jeff

Chris' translation said he was not using electricity but only bubbling air through the mix to agitate things.

But, Nitriding would really help. I have a good friend who has been doing some nitrogen enrichment experiments during the heat treatment phase of making blades. It's a complicated setup and requires delicate thermal control (which all equals expensive). I do not know that he routinely does this on his knives because of that. This type of treatment, does the whole blade all the way through.

More commonly is the plasma deposition type which gives all the pretty colors and a very curable corrosion resistant surface. The one exception is at the edge, because as soon as the blade is honed, that nice coating is abraded away.