# Passivation of Carbon Knives



## DrNaka (Sep 10, 2011)

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....


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## DrNaka (Sep 10, 2011)

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.


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## ecchef (Sep 10, 2011)

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.


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## ecchef (Sep 10, 2011)

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?


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## Lefty (Sep 10, 2011)

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.


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## phan1 (Sep 10, 2011)

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!


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## tk59 (Sep 10, 2011)

DrNaka said:


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


 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.


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## kalaeb (Sep 10, 2011)

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.


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## Benuser (Sep 10, 2011)

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.


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## Lefty (Sep 10, 2011)

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


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## ajhuff (Sep 10, 2011)

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.


 
No. Fe2O3 is what we commonly call rust.

-AJ


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## Benuser (Sep 10, 2011)

ajhuff said:


> No. Fe2O3 is what we commonly call rust.
> 
> -AJ


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


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## Larrin (Sep 10, 2011)

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


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## SpikeC (Sep 10, 2011)

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


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## Benuser (Sep 10, 2011)

SpikeC said:


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


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


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## SpikeC (Sep 10, 2011)

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


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## phan1 (Sep 10, 2011)

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.


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## Marko Tsourkan (Sep 10, 2011)

SpikeC said:


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


 
Haha. It can also turn black. 

M


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## wsfarrell (Sep 11, 2011)

phan1 said:


> I'm calling "fuzzy science" here.



+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


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## SpikeC (Sep 11, 2011)

Passivation occurs on all metals.


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## DrNaka (Sep 11, 2011)

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".


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## Eamon Burke (Sep 11, 2011)

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.


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## BraisedorStewed (Sep 12, 2011)

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.


 
+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.


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## James (Sep 12, 2011)

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


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## Larrin (Sep 12, 2011)

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.


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## Andrew H (Sep 12, 2011)

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


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


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## UglyJoe (Sep 12, 2011)

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".


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## tk59 (Sep 12, 2011)

+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.


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## UglyJoe (Sep 12, 2011)

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.


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## Eamon Burke (Sep 12, 2011)

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


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## Mike Davis (Sep 12, 2011)

I have a friend who works in an anodizing shop, they do passivation. As he explained to me when he did some SS for me, it is done in a controlled heated steam environment, and is used to remove impurities from the surface of the steel. I had considered having him do some blades and guards for me just to see what it would do. He said it definitely makes the metal easier to bring to a high luster polish, and it is visible when it is finished.


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## ajhuff (Sep 12, 2011)

UglyJoe said:


> 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 think it's simply an observation of changes in surface tension.

-AJ


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## DrNaka (Sep 13, 2011)

I think too that pure metal and metal oxides are hydrophobic.

But a not good maintained knife will have metal oxide+water like in form of FeOOH etc.
These will have affinity to water and the blade will be hydrophilic.

It is the same for glass.

SiO2 itself will be hydrophobic but the surface of glass has -SiOH and a well cleaned wine glass is hydrophilic.


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## memorael (Sep 13, 2011)

I guess this calls for an experiment, theres no point in arguing or debating since all anyone has to do is drop some water on top of a knife see if it spreads and then pour boiling water on it let it dry, pour water and see if it beads. If this does work and I have a feeling it does, the whole shigefusa ultra reactive thing is going to be a thing of the past. I will report tomorrow after polishing my yanagi and pouring hot water over it.


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## wsfarrell (Sep 13, 2011)

Once again, carbon knives do not passivate. Here's another reference, from wikipedia:

Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as passivation (rust, for example, usually grows to be much thicker, and so is not considered passivation, because this mixed oxidized layer is not protective). While this effect is in some sense a property of the material, it serves as an indirect kinetic barrier: the reaction is often quite rapid unless and until an impermeable layer forms. Passivation in air and water at moderate pH is seen in such materials as aluminium, stainless steel, titanium, and silicon.

A patina, whether forced with chemicals like baking soda or natural from cutting food, can help prevent further corrosion. It is not passivation.


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## Vertigo (Sep 13, 2011)

Dude I don't get what you just said.

You quote in the wiki:



> Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation.



This is patina, right?

The wiki then goes on to say that:



> When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon *is known as passivation*



This is what happens to carbon knives with an excellent patina built up, right? I mean, I suppose I could leave my main carbon knife in a bath of lime juice and get it to bunk up, but it is, for all intents and purposes (the conditions it will be used it), no longer reactive.

Going on to say then that:



> A patina, whether forced with chemicals like baking soda or natural from cutting food, can help prevent further corrosion. *It is not passivation.*


Directly contradicts the bit quoted from Wiki.


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## memorael (Sep 13, 2011)

wsfarrell said:


> Once again, carbon knives do not passivate. Here's another reference, from wikipedia:
> 
> Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as passivation (rust, for example, usually grows to be much thicker, and so is not considered passivation, because this mixed oxidized layer is not protective). While this effect is in some sense a property of the material, it serves as an indirect kinetic barrier: the reaction is often quite rapid unless and until an impermeable layer forms. Passivation in air and water at moderate pH is seen in such materials as aluminium, stainless steel, titanium, and silicon.
> 
> A patina, whether forced with chemicals like baking soda or natural from cutting food, can help prevent further corrosion. It is not passivation.


 
No where does this quote claim that carbon steel does not passivise. Just because it doesn't make direct mention of it doesn't mean it doesn't passivise.


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## ajhuff (Sep 13, 2011)

As Larrin stated above, patina is Fe3O4. Rust is Fe2O3. See the difference. Exposing steel to nitric acid is essentially the same as etching with Nital which is why I think this is all just a surface tension observation. Though I always used Nital for pearlite not martensite.

-AJ


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## Eamon Burke (Sep 13, 2011)

memorael said:


> I guess this calls for an experiment, theres no point in arguing or debating since all anyone has to do is drop some water on top of a knife see if it spreads and then pour boiling water on it let it dry, pour water and see if it beads. If this does work and I have a feeling it does, the whole shigefusa ultra reactive thing is going to be a thing of the past. I will report tomorrow after polishing my yanagi and pouring hot water over it.




Hey guess what? I have a Fowler blade in 52100 clad with Wrought Iron at my disposal, reactive foods in large amounts, and huge pots of boiling water at work.

I did this today, and will do it again in 2 days. It didn't seem to do a lot today, but I'll keep checking.


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## James (Sep 25, 2011)

Just wanted to bring this thread back up to see what kind of results you got John. Just to put it out there, I think it would be possible that what DrNaka is experiencing is a reaction between minerals/impurities in the water with the steel, but I don't have enough experience of knowledge to comment further on the topic.


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## tk59 (Sep 25, 2011)

I fooled around with my Shigefusa (kitaeji) for a bit. I have not noticed significant improvement. I still see rust forming in the time it takes to sharpen and similar reactivity to onions and acids, in general. I have by no means done any exhaustive testing, at this point. I've been using another iron clad knife the last couple of days and it's doing great though even without hot water treatment, lol.


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## The Edge (Sep 25, 2011)

Iron does passivate, though I'm not quite sure about the hydrophobic properties of the protective oxide coating. The tendency of a metal to form a protective oxide coating is indicated by an especially simple parameter known as the "Pilling-Bedworth ratio". If the number lies between 1 and 2, that material tends to form a protective oxide, while nonprotective oxides fall outside of the range. There are exceptions, such as silver and cadmium, where in addition to the ratio, a number of factors must be favorable to produce a protective coating. Here is a wiki link: http://en.wikipedia.org/wiki/Pilling-Bedworth_ratio 

Now on to the experiments, which should be fun!!


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## The Edge (Sep 25, 2011)

I guess I should have also mentioned that the ratio for Iron is 1.77


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## tk59 (Sep 25, 2011)

The Edge said:


> I guess I should have also mentioned that the ratio for Iron is 1.77


According to your article, that's only if it somehow sticks around as Iron II.


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## The Edge (Sep 25, 2011)

Iron II is the resulting oxide formed from the interaction of Iron to pure water or dry oxygen. Converting this layer to rust is usually a combination of water and oxygen that forms hydroxides.


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## DrNaka (Sep 25, 2011)

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD055740&Location=U2&doc=GetTRDoc.pdf
This is a nice read.


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## DrNaka (Sep 25, 2011)

http://www.misumi-techcentral.com/tt/en/surface/2010/08/052-passivation-of-metals.html



> In highly oxidizing acids such as concentrated nitric acid and concentrated sulfuric acid, iron will easily become passivated.



Who is saying that iron cannot be passivated?


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## DrNaka (Sep 25, 2011)

tk59 said:


> I fooled around with my Shigefusa (kitaeji) for a bit. I have not noticed significant improvement. I still see rust forming in the time it takes to sharpen and similar reactivity to onions and acids, in general. I have by no means done any exhaustive testing, at this point. I've been using another iron clad knife the last couple of days and it's doing great though even without hot water treatment, lol.


 
Please be sure that you have sharpened and cleaned from any residual rust.
And after sharpening you must apply hot water as soon as possible.

If it is your first time do not use the knife directly. let it stay in dry air for 1 day, apply hot water again and let it stay for a day again.
Do it till it get hydrophorbic and repells water.


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## tk59 (Sep 25, 2011)

DrNaka said:


> Please be sure that you have sharpened and cleaned from any residual rust.
> And after sharpening you must apply hot water as soon as possible.
> 
> If it is your first time do not use the knife directly. let it stay in dry air for 1 day, apply hot water again and let it stay for a day again.
> Do it till it get hydrophorbic and repells water.


I'll try that. I guess I can also try distilled or boiled water to remove impurties and/or oxygen.


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## Eamon Burke (Sep 25, 2011)

I would like to say that the wrought iron on the knife I was using was a bit less reactive when cutting onions and whatnot after 2 days of cleaning in boiling water. The 52100 core built up a little patina, so I can't really say how that does, but it did have a minor effect on the wrought iron cladding.


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## tk59 (Sep 25, 2011)

Are you saying that it is less reactive than it would have been if you were using room temp water?


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## Eamon Burke (Sep 25, 2011)

I'm saying it was less reactive. I used it for a few days, then started dunking it in the pots of boiling water that always sit on the stove, right after final wiping and before putting it down for a few hours. After 2 days, the iron didn't get the orangey-rust, it looked more yellow and didn't develop as quickly. It wasn't night and day, but it was a difference.


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