Passivation of Carbon Knives

I do not know how the education in other countries are but in Japan we have chemistry lecture in middle or high school (I forgot which because it was about half century ago) about reaction of metal with acid.

A piece of iron or aluminum is reacted with concentrated or diluted hydrochloric, nitric and sulfuric acid.
At middle or high school level the schoolchild will find out that the piece of iron or aluminum does not react with concentrated nitric acid (68%) and the teacher will explain that it is passivation. The concentrated nitric acid works here as oxidant and makes a thin layer of insoluble metal oxide at the surface. So the underneath metal cannot react with the acid anymore.

If you go to University and learn a bit about corrosion you will have a lecture about Pourbaix diagram.
Here is the Wiki about it:
http://en.wikipedia.org/wiki/Pourbaix_diagram



The region of Fe2O3 nH2O is the region where iron does get passivated.

This is about corrosion in water at room temperature.

But how about SS?

Chrome and nickel in the alloy help to make the Fe2O3 nH2O region get bigger. Also it helps the alloy to get a oxidated surface in the air.

These passivation are not visible to the human eye.
A SS knife will have similar metallic shine as a carbon knife though the SS has a oxidation layer.

Continued to next part....

As I explained passivation is at surface only. It is not visible to eye.

The question here is how can you make it visible.

I think I found a easy method. Just use water and see if the surface is hydrophilic or hydrophobic.

After sharpening all my knives surface are hydrophilic that is the water will spread on the knife surface.
If you just wipe the blade and let it stay dry for some hours the surface of the SS knife get hydrophobic that is it will repell water.
The carbon knife will be still hydrophilic.

Now if you poor hot water on the carbon knife and let it dry it becomes hydrophobic.

All my carbon knives surface are hydrophobic.

I would like to know how the surface of the carbon knives are of the members here.

Thank you Dr. Naka. I think I understand this somewhat.

Unfortunately, most of my high school years were spent experimenting with nitrous oxide, so my brain is a little passivated these days.

About this hot water thing, Dr.N...
How hot should the water be? I'm assuming that the threshold between oxydation and passivation is thermally dependent.
Should the water be allowed to dry by evaporation or should it be wiped off immediately?
Will other substances achieve the same result?

As an experiment, I cord wrapped a carbon steel kinfe & soaked the handle in boiling water for about 30 seconds to tighten the wrap & let it air dry. I fully expected to see rust, but nothing happened. Is this an example of what you're refering to?

Hmmm. I got a great tip from P Tiger a while back about rinsing my carbon knives in the hottest water
I could handle, and then to dry it off as usual. Since I started doing this, I haven't had a sunless speck of rust pm my blades, even in our crazy humid summers in between the great lakes.
I (we) figured it was to expedite evaporation, but I'm thinking we (or was it I?) missed the mark on that one.

I've never heard of this "hot water making carbon hydrophobic" thing. Honestly, it sounds a bit far fetched, but it doesn't mean it's no worth trying!

DrNaka said:

...I would like to know how the surface of the carbon knives are of the members here.

Click to expand...

Guys, I think Dr. Naka is asking whether your knives are hydrophobic or hydrophilic and whether they are reactive or not. I, for one, am curious enough to take my Shige to the lab next week to test all of this stuff.

My well used Fuji fh is hydrophobic and is no longer reactive, but the patina is well set. So it would make sense that is it a hydrophobe.

Fe2O3 is what we know as patina I guess. When forcing a patina with musterd and vinegar I noticed an almost immediate reaction during the cleaning afterwards with, indeed, very hot water. The steel turns grey.

I grew up with a pool, so my knives are naturally going to be fond of the water.

Benuser said:

Fe2O3 is what we know as patina I guess. When forcing a patina with musterd and vinegar I noticed an almost immediate reaction during the cleaning afterwards with, indeed, very hot water. The steel turns grey.

Click to expand...

No. Fe2O3 is what we commonly call rust.

-AJ

ajhuff said:

No. Fe2O3 is what we commonly call rust.

-AJ

Click to expand...

Thank you, ajhuff! And does the further oxidation create patina?

The patina is fe3o4. Which is also what forms when you "blue" steel, which is a type of passivation.

I "blued" some steel while grinding a while ago! Now I know it was just "patina"!

SpikeC said:

I "blued" some steel while grinding a while ago! Now I know it was just "patina"!

Click to expand...

Please excuse my ignorance, SpikeC, but what do you mean by blueing steel? Do you mean getting a blue shine with red meat?

When grinding a piece of hardened steel if you over heat it it turns blue, destroying the temper. It was a joke.

I'm calling "fuzzy science" here. How the heck can knives that are made 90% of the same stuff be either hydrophobic or hydrophobic? I find the finish that I leave on knives to be a more telling factor. If it's shiny and polished, water beads up on it and rolls off. If it's more textured or hazy, water will want to spread out a cling to the knife. But it has nothing to do with whether or not a knife is hydrophobic or not.

SpikeC said:

When grinding a piece of hardened steel if you over heat it it turns blue, destroying the temper. It was a joke.

Click to expand...

Haha. It can also turn black.

M

phan1 said:

I'm calling "fuzzy science" here.

Click to expand...

+1, fuzzy science indeed. Passivation only applies to stainless steel, not carbon steel. You can do things to carbon steel to make it more rust-resistant (coat it, blue it, paint it), but passivation isn't one of them.

Here's a reference: http://www.finishing.com/118/22.shtml

Passivation occurs on all metals.

Reading the responses I realize that in some countries there is no lecture in school about iron in concentrated nitric acid.

BTW most reaction speeds including the oxidation of iron surface by air will increase by 2 to 3 times per 10K (or 10C).
So the reaction speed is about 1000 times at 100C than at 20C.
If you heat further you can see the "blue".

Indeed there is no such lecture in American schools. Also, American schools don't bother trying to get kids to retain anything into adulthood. Chemistry class is basically memorization of some basic, disconnected topics, and I've yet to meet any adults who have studied it no further and retained any of it into adulthood.

I'd trust Larrin on this topic, I wish he'd put his 2 cents in, after all, he's in fancy-pants steel-school.

johndoughy said:

Indeed there is no such lecture in American schools. Also, American schools don't bother trying to get kids to retain anything into adulthood. Chemistry class is basically memorization of some basic, disconnected topics, and I've yet to meet any adults who have studied it no further and retained any of it into adulthood.

I'd trust Larrin on this topic, I wish he'd put his 2 cents in, after all, he's in fancy-pants steel-school.

Click to expand...

+1. I even did a year of college chem and barely remember a damn thing, probably more than most but its really quite pitiful. The teaching is towards the standardized tests in the low level classes and geared to the AP test in higher classes, if they can get all the info needed into your head long enough to pass the test most teachers feel their job is done.

Pourbaix diagrams aren't even covered in general chemistry, even in expensive, highfalutin schools like mine (think NYC, $$$ and Obama went here). Luckily, I had to spend hours pouring over one for a high school research project. I'm not quite sure about the change in hydrophobicity though.

From what I'm understanding right now, the boiling/hot water poured over the carbon steel heats the blade and vastly increases the kinetics of the iron to iron (II, III) oxide. The change in hydrophobicity seems to be due to the formation of the new surface oxide, and perhaps it is more hydrophobic than the raw iron particles...I need Larrin to chime in here haha

I would comment but I've never heard of such a thing. As James said the difference would have to be between iron and iron oxide, but I don't know how/where to find out whether one or the other is hydrophobic. Also, as phan1 said surface roughness is a factor. I don't know if it's all that useful of a test, really.

James said:

Pourbaix diagrams aren't even covered in general chemistry, even in expensive, highfalutin schools like mine (think NYC, $$$ and Obama went here). Luckily, I had to spend hours pouring over one for a high school research project. I'm not quite sure about the change in hydrophobicity though.

From what I'm understanding right now, the boiling/hot water poured over the carbon steel heats the blade and vastly increases the kinetics of the iron to iron (II, III) oxide. The change in hydrophobicity seems to be due to the formation of the new surface oxide, and perhaps it is more hydrophobic than the raw iron particles...I need Larrin to chime in here haha

Click to expand...

King's College, these new schools have such low standards. (William and Mary, 1693)

From an intermolecular forces standpoint, iron metal is going to be hydrophobic. There simply are very little intermolecular forces between iron and water (ion-induced dipole interactions and london forces only). Depending on how the iron oxide is structured, there should be more intermolecular forces between the iron oxide and water, and in fact there could be hydrogen bonding between the oxygen of the iron oxide and water. My guess is an iron oxide layer on the metal would be much, much more hydrophilic than pure iron metal... but it is also much less reactive. As for the idea that the kinetics of a reaction increase as much as 1000x for a 100 K increase in temperature, this is simply almost assuredly not the case. The natural log of k is inversely related to temperature in Kelvin. A 100 K increase in temperature is not all that much. You are looking more at like a 10x increase in the rate constant for a reaction over a decade change in temperature with "normal" kinetics. I'm not saying it's impossible, but very unlikely, and certainly not the "norm".

+1. I will add a couple of things though: in Dr. Naka's defense, doubling the rate of for every 10 K is a decent rough estimate (and a commonly used reference) for processes that occur near room temperature. What makes me skeptical is the fact that many metal oxides are at least somewhat soluble in acid.

I just did some quick calculations (and really I should have been able to do this off the top of my head - getting rusty with the math), and having a reaction increase in rate 1000x over the temperature range of 0 C - 100 C would take an activation energy of about 15 kcal/mol and a pre-exponential factor of about 3E15. So not likely indeed.

Back on topic, if passivation occurs at the area described in your diagram, then you definitely aren't getting passivation with concentrated nitric acid because its pH is way below zero, which puts it well out of the Fe2O3/H2O portion of your diagram.

Also, the explanation of hydrophobic vs. hydrophilic doesn't really jive with what makes sense to me. You are stating that after sharpening the surface is hydrophilic. After sharpening you should have removed the oxides at the surface, making it more hydrophobic. As the oxides build up the surface should become more hydrophilic. What you are observing empirically seems to be the other way around, which doesn't make any sense to me.

I wonder how this relates to the forge scale left on some Kuro-uchi knives. Does that provide enough energy?