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Thread: Passivation of Carbon Knives

  1. #1

    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....

  2. #2
    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.

  3. #3

    ecchef's Avatar
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    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.
    Though I could not caution all I still might warn a few; Don't raise your hand to raise no flag atop no ship of fools. - Robert Hunter

  4. #4

    ecchef's Avatar
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    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?
    Though I could not caution all I still might warn a few; Don't raise your hand to raise no flag atop no ship of fools. - Robert Hunter

  5. #5
    Canada's Sharpest Lefty Lefty's Avatar
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    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.
    09/06

    Take a look around at: www.sharpandshinyshop.com

    Email me at: tmclean@sharpandshinyshop.com

  6. #6
    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!

  7. #7
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    Quote Originally Posted by DrNaka View Post
    ...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.

  8. #8

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    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.

  9. #9
    Senior Member Benuser's Avatar
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    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.

  10. #10
    Canada's Sharpest Lefty Lefty's Avatar
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    I grew up with a pool, so my knives are naturally going to be fond of the water.
    09/06

    Take a look around at: www.sharpandshinyshop.com

    Email me at: tmclean@sharpandshinyshop.com

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