Passivation of Carbon Knives

[Mike Davis]

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.

 

[ajhuff]

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

 

[DrNaka]

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.

 

[memorael]

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.

 

[wsfarrell]

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.

 

[Vertigo]

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.

 

[memorael]

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.

 

[ajhuff]

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

 

[Eamon Burke]

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.

 

[James]

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.

 

[tk59]

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.

 

[The Edge]

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!!

 

[The Edge]

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

 

[tk59]

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.

 

[The Edge]

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.

 

[DrNaka]

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

 

[DrNaka]

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?

 

[DrNaka]

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.

 

[tk59]

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.

 

[Eamon Burke]

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.

 

[tk59]

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

 

[Eamon Burke]

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.