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Citric Acid to blacken SS knife?
- Thread starter KenHash
- Start date
It could work, but I am not sure if you don't have to pre-treat the blade somehow. Citric acid is mild oxidizer and is used in treating SS steel even in industry (as an alternative for less environmentally friendly and way more toxic solutions). The treatment is called passivation and it's essentially what happens on surface of SS steel in normal conditions, but way more quicker and tailored to suit the industry standard when it comes to boilers, pipes and so on. It's generally done on SS steel. It would be generally good to see some patents and some literature regarding those treatments and try them on some cheap SS steel after the theoretical research.
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Isn't that done to get rid of stuff like steel dust left over from machining so the powder doesn't start rusting the stainless part? Using tooling on carbon steel then a stainless part can cause rust on even very stainless steel types as well as titanium which normally won't.
That's what is called pickling and that is often done with hydrochloric or phosphoric acid. These acids however are either non oxidizing or they just don't oxidize the steel how you want it to be oxidized.
I don't know if that's the desired blackening. It doesn't have oxidizing properties. It would form chlorides from oxides already on the steel.
I would strongly suggest phosphating (special solutions of salts of phosphoric acid are used for this) after the steel has been treated with some hydroxide pre-treatment (only to get rid of oils and so on on the steel). It can't damage it as some oxidizing acids and I bet that those solutions even come with a manual.
Industry standard for SS steel is nitric acid, but I wanted the author to search for himself. It's very dangerous to use it when you are unskilled with it and you don't have enough ventilation. The gases emerging from the reacting nitric acid are no joke and the nitric acid is also no joke. You can even blow yourself up if you mix it with substances that you shouldn't mix it with. It's even used in rocket fuel in it's purest form. I would really be concerned with losing the blade as last thing with it.
Bill DeShivs
Knifemaker / Craftsman / Service Provider
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Why, oh why does everyone want to blacken steel?
SOME of the methods offer corrosion protection, but anything you do will wear off with use.
SOME of the methods offer corrosion protection, but anything you do will wear off with use.
I guess that it has got something to do with how tacticool the knives look afterwards.
Citric acid treatment is called passivation.
The reason that stainless does not rust is that it immediately forms a hard oxide coating on exposure to air which looks just like the non-oxidized stainless and protects it from further oxidation. When stainless is machined, the coating immediately forms on newly exposed surfaces.
However, metal dust from tooling can collect, microscopically and otherwise, and stop the oxygen from getting to all of the stainless. The result can be pitting, microscopic and otherwise. The citric acid is to oxidize the non-stainless dust so that is can be easily washed away.
It's often done to parts used in food handling/packing which have to have a perfect finish with no pits.
I did not say that citric acid doesn't cause passivation. Basically any acid can be used for the pickling (cleaning the dust and surface), but not every acid will cause passivation. The passivation can be caused by oxygen in the air or any other oxidizing agent. The only difference would be the structure. It's good to look into ACS handbook for those kind of stuff because it's really sort of bible for steel manufacturing.
The passivation can also be caused by basic solutions of hydroxides or salts. The only disadvantage is that it won't cause the blackening of the steel and can in fact result in corrosion.
It has worked just fine on plenty of my Damascus blades that lost their blackening near the bevel due to scratching and wear. I redid a pile of them years ago, they still look great.
Am I saying anywhere that the citric acid wouldn't work? Regarding the hydrochloric acid. I know that it can stain stainless steel, but the desired passivation mechanism just isn't there. Chlorides are generally very soluble in water and the stains are just spots where corrosion took place more intensively. Chlorides are also relatively more susceptible to oxidation than oxides so it wouldn't really work for long time if they wouldn't be soluble in the first place. You need to form something that is very resistant to oxidizing agents on the surface like phosphates, oxides or maybe some high-tech resin. The blade would just continue to corrode otherwise.
To take a page from your book: Am I saying anywhere anything about citric acid?
Again: hydrochloric works for many stainless steels. Ferric chloride works for many carbon steels. I have done it many times. It works, and works well, whether your mind would like to believe it possible or not. Here are a couple that got dipped several years ago. The Benchmade was ground to bare steel halfway up the blade to thin it a bit BTE then blackened again. It has a Hakkapela Damasteel blade, which is a blend of two stainless steels. I used plain hydrochloric acid available at most hardware stores. The etching matched the rest of the original factory etch perfectly.
The other is Case damascus, from Devin Thomas I believe. That one was etched with ferric chloride all the way down to the edge, then microbevelled.
Both ended up great and have held up for many years of use and pocket riding.
Also, after posting this I took a quick second to search google and found this etching guide for Damasteel. Their recommendation is 95% hydrochloric acid.
This really goes nowhere. I've been intensively studying and directly surface treating steel for a year or so now, so I know what I am talking about.To take a page from your book: Am I saying anywhere anything about citric acid?
Again: hydrochloric works for many stainless steels. Ferric chloride works for many carbon steels. I have done it many times. It works, and works well, whether your mind would like to believe it possible or not. Here are a couple that got dipped several years ago. The Benchmade was ground to bare steel halfway up the blade to thin it a bit BTE then blackened again. It has a Hakkapela Damasteel blade, which is a blend of two stainless steels. I used plain hydrochloric acid available at most hardware stores. The etching matched the rest of the original factory etch perfectly.
The other is Case damascus, from Devin Thomas I believe. That one was etched with ferric chloride all the way down to the edge, then microbevelled.
Both ended up great and have held up for many years of use and pocket riding.
Also, after posting this I took a quick second to search google and found this etching guide for Damasteel. Their recommendation is 95% hydrochloric acid.
The ferric chloride will work because it is mild oxidizing agent. The citric acid is also mild oxidizing agent. They cover the steel with little bit of oxides that can't be oxidized further. So it's basically like waging war with rust that is caused by oxidation (Fe2O3 and it's hydration products) with other oxidation (Fe3O4, Cr2O3...) products. Hydrochloric acid on the other way is not oxidizing agent. At least not in the way that you want it to be.
You see, there are a few types of surface treating of steel and I will get what happens with the HCl and explain the etching guide also.
Passivation
Process of oxidation by oxidizing agents that form oxides or hydroxides with a metal. That oxide or hydroxide also has to be insoluble in water, can't be easily oxidized further and can't be easily scraped from the surface (that however is relatively speaking) or degrade by reaction with water. Oxidizing acids, oxidizing agents and hydroxides can be used to do this. Or some relatively special mixtures of chemicals (that however still contain the oxidation agent). This is also what is called "blackening" as the Fe3O4 is black.
Pickling
Process that is used to basically "clean" the steel of dust particles and imperfections after machining and working with the steel. This also dissolves any passivation or rust layer so it can also be used as pretreatment to dissolve those oxides and basically any other chemical on the surface. Hydrochloric acid, sulfuric acid or phosphoric acid are the ones that are used the most.
Etching
This is process that utilizes some acid with oxidizing agent to dissolve the surface of a metal to uncover its microstructure. It can also be used to change the appearance of metal as you pointed out.
Conversion and other types of coatings
Conversion coatings are basically coating of salts on a surface of steel that can't be oxidized and are not soluble in water. Here are mainly used phosphates. There was some use of chromates also but they are really carcinogenic and they are not safe to work with at all (the solutions are just a chore to work with and it changes colour of everything it touches). These coatings are done mainly by specially designed solutions of their salts. There are also other coatings of steel like epoxy coating, silicating (pre treatment for some reason coatings or some special steels) and so on.
Why is steel more resistant to corrosion after treatment with hydrochloric acid then?
It's because you are essentially pickling the steel so that the hydrochloric acid "eats away" all imperfections and makes all of the rust soluble. It also eats away a little bit of the surface steel that has been damaged by a corrosion. Now you basically have something brand new surface in pristine condition. Maybe better from factory. What is recommended after this is also dipping the steel in NaOH solution with pH slightly above 10 to passivate the surface. You are right. I've seen that this works, but it won't make the steel rustproof.
What about the etching?
Firstly, there is the thing that there is no 95 % hydrochloric acid. The conc. Hydrochloric acid is somewhere between 36-37%. There are some sources that will say it's either that or that. It doesn't matter though. And what they say is ferrochloric acid is basically addition of ferric chloride that is oxidizing agent. You see, the hydrochloric acid eats away the steel and ferric chloride oxidize it. Both of those things do the etching and uncover the surface layer of steel to see the microstructure. But I think that this etching bath really isn't meant to do that. It's rather for the looks of the etched steel, but I am not an expert in Field of steel etching.
Edit: The ACS handbook is very handy for this type of stuff, but there isn't sadly explained the chemistry because it's what I would call a "industry handbook".
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(sigh)
A quick scan of the search will show site after site stating...
"Passivation of stainless steel is a process that removes free iron from the surface of a stainless component and at the same time promotes the formation of a thin, dense oxide protective barrier."
A quick scan of the search will show site after site stating...
"Passivation of stainless steel is a process that removes free iron from the surface of a stainless component and at the same time promotes the formation of a thin, dense oxide protective barrier."
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HydroChloric acid is a reducing acid. Good for removing rust (an Oxide). Not good for stainless steel because it goes after the Chrome which lessens the stainlessness. HCl is used to measure the thickness of Chromium plate by timing the dissolution of the Chrome layer (about 1 sec/1 millionth of an inch at room temp). Most exterior Chrome plating calls for 10 millionths of an inch deposit thickness.
Birchwood Casey may have a cold blue or black treatment for stainless steel.
Birchwood Casey may have a cold blue or black treatment for stainless steel.
That's not how the percentages are used like when speaking of chemicals.