Hydrogen Embrittlement: Acid Baths and Patinas — Should We Be Concerned?!

[stoffi]

Hi,

I started talking about this topic in another thread of mine, but I felt it deserved its own.

I've recently learnt a bit about the concept of hydrogen embrittlement due to acid exposure (in my case a room temp 12h+ citric acid bath)
of high-HRC hardened steels (over 30-40 HRC), especially with low chromium content (non-stainless steels).

This seems to particularly concern us knife-nuts, since all of our blades fall into the high-hardened steel category, but especially if they are carbon steels.
We use apples, lemons, vinegar, citric acid baths and whatnot to achieve beautiful patinas or etchings or to simply remove rusted blades and so on.

After spending about six hours searching the net and reading countless report-pages last night, most of what I found on forums was repetitive hearsay and the reports I read didn't answer my questions; but they did worry me. So I've started to contact industry experts dealing with acid treatments and testing of hydrogen embrittlement and I'm waiting for their replies. There are several methods for testing hydrogen embrittlement, but they are often very expensive and sometimes they even need to destroy the item in order to test it. A no-go, it would seem.

The problem with hydrogen embrittlement when it does occur is that the hydrogen atom, which is released when the acid works its magic on the blade, may enter the steel structure and bond to the carbon and if not treated (baked at 200F for 2h) immediately after the exposure or within 4 hours, the hydrogen atom alloys with the carbon and thus weakens the steel. In practice, this can create microscopic fractures within the steel and can result in a catastrophic crack when forces are applied. What I don't know is how deep the hydrogen can penetrate the steel under certain conditions and how much of a problem it is when no significant heat (which speeds up ion activity) has been applied in the process.

So, I'm reaching out to all of you guys for help on this matter. What can you bring to the table?
Should we be concerned with hydrogen embrittlement when we acid etch, form patinas and remove rust in acid baths?
Is it stupid to leave a blade in a bath of acid over-night to remove rust and form a patina?

 

[BluntCut MetalWorks]

Blunt guesses from non-chemist/metallurgist - bring hands waving to the table It's well known that acid dull carbon blade. C taken by alkaloid, O combine with Fe=rust; catalystic loop until neutralize. People all over the world cut citrus fruits and not destroying(embrittle) their carbon knives. I think, reactions are on the blade few molecular surface layers.

It could be a telltale if you would bend/break the broken blade to test for brittleness. A strong citric acid (lot of O & H & OH) cascade react to eat away the steel and if the blade is poorly heat treated, the acid could fracture the blade along the large grain structure.

I force patina by rub a thin layer of acid or base agent on blade, wipe clean after 10 to 20 minutes <= limited amt & time. Soak knife overnight in a tub of acid (even plain water) is not a good idea.

 

[Obsessed with Edges]

Being that I'm 100% ignorant on this topic (I wanted to clarify this from the beginning ), I can only say this.

I'd think that if it were an issue serious and prevalent enough to cause frequent failures in knife blades, we would've seen a lot more well-documented accounts of it by now (aside from the 'hearsay' discussions, where it might seem convenient to blame a broken blade on some obscure failure mechanism, which is difficult for the layman to disprove otherwise). I could see how this would be a major concern in industry, especially in fields manufacturing and using steel parts subjected to extreme stresses. Something like the turbine blades in jet engines, and steel components used in the nuclear power industry, for example, which are designed to be used under extremes of mechanical stress and heat.

What tiny bit I've read of hydrogen embrittlement, it seems to suggest that electroplating of hardened steel parts seems to increase the likelihood of it. Apparently the electroplating process, utilizing acids high in hydrogen content, and also from immersion in water used in the process, provides a much more abundant source of the (very small) hydrogen molecules to migrate into the much larger and more porous steel lattice, and then be trapped there by the plating itself. Seems like hydrogen has to be in very abundant supply, and in constant contact with the steel over an extended period of time, to be a problem. I get the impression that typical, everyday exposure to atmospheric moisture and other small-scale contributors of hydrogen wouldn't be significant, at least under the typical conditions encountered by a knife blade. (Edit: The bolded text in the quote below suggests it basically requires conditions that actually force (inject) the hydrogen into the steel, so I'd think it shouldn't be much of an issue under the normal atmospheric conditions of daily use).

Conditions which cause severe corrosion of steel apparently also contribute to hydrogen embrittlement. This seems sort of redundant to me, in the sense that we already know the hazards of corrosion in steel.

I found this description on a web site dealing with corrosion (generally). Very interesting reading (bold emphasis added by me):
(from site: http://events.nace.org/library/corrosion/Forms/embrittlement.asp )

Hydrogen Embrittlement

This is a type of deterioration which can be linked to corrosion and corrosion-control processes. It involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal. Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions which inject hydrogen into the component. Presently this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys and aluminum alloys.

Sources of Hydrogen

Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below: (Defence Standard 03-30, October 2000)

a. by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed

b. as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction (Equation 2) acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed &#8220;sulphide stress cracking (SSC)&#8221;

c. the use of cathodic protection for corrosion protection if the process is not properly controlled.

Hydrogen Embrittlement of Stainless Steel

Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminum components.

If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is transgranular. At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.

Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the steel, so that ductility is restored.

To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ (in position) hydrogen introduced during the service life of a part.

Interesting topic, though I suspect it may not be too much of a worry for us. Curious to see where this discussion goes...

 

[ecos]

I'm not a metallurgist....and I'm most definitely not a chemist but heres my experience with acids on carbon steels:

I do a fair amount of hamon lined knives, mostly 1095, 1080, that spend about 10mins total in ferric chloride. I've noticed no apparent weakness in the several dozen pieces I've tested to destruction.

I used to do a fair amount of knives with acid etched patterns (I've since switched to electrochemical etching due to environmental/safety concerns) using mostly ferric chloride and soaking the knives for 2-3 hours. Steel used is mostly 1095, 1080, and 5160. Again in the half a dozen or so pieces I tested to destruction I noticed no apparent weakness.

I'm guesstimating there are a good 1,000+ pieces of these in customer hands. I've had no reports of abnormal breakage.

I'm not saying its not possible, this is just my experience.

Carbon steel is cheap. If someone is curious enough about the issue buy a thin batch of carbon steel, heat treat it together in the same batch, at the same temps....then soak some in acid for various lengths of time and test to destruction. If the acid ones seem to fail first repeat process to see if it was a fluke or if there is a definite trend there. If someone does this I'd be interested in seeing their results.

 

[JR42]

Fascinating thread! I recall reading years ago that hard-chromed steel can experience hydrogen embrittlement as a result of the plating process, which seems to be supported by ObsessedWithEdges' post above. It appears a properly-controlled plating process with a soak at low heat afterwards eliminates the risk. We certainly never experienced any failures on the various combine internals we had hard-chromed, though none of the parts were subject to much tensile stress.

The only knives I can think of with chrome-plated blades are the old M2 Gerbers, and Mad Dogs.

My entirely amateur guess would be that whatever embrittlement that occurs during patina formation is very shallow in high-quality clean steel, as already mentioned by other posters.

JR

 

[Obsessed with Edges]

(...)"The only knives I can think of with chrome-plated blades are the old M2 Gerbers, and Mad Dogs.(...)

There was a thread yesterday that reminded me of another. Some of Case's old carbon fixed blades (1970s and earlier, for the most part) were chrome-plated too. On the upside, I've never (ever) heard of one of those blades breaking.

I tend to believe also, that whatever patina or etching we may mess around with on our blades, it's probably too shallow to be significant, at least in the context of this discussion.

 

[Lagrangian]

I'm also no metallurgist, but I had one thought: Probably the key factor is the rate of diffusion.

How fast does the hydrogen diffuse into/through the metal, and how much hydrogen is available?

If the rate of diffusion is very slow, then at most, it would only affect a thin surface layer during etching. In most of the industrial applications that I've read about (like in Obsessed with Edges's post above), the metal was exposed for a very long time to a hydrogen source (such as piping in a coolant system). btw, hydrogen embrittlement is also a concern if we ever develop hydrogen powered cars; the fuel system would be continuously exposed to hydrogen. (Not to mention that hydrogen powered cars would have a zillion other problems.)

I think if this were really a problem with knives, people would see cracking on etched knives?

If any of you get a real answer (say from an engineer or a textbook), then I would be curious to hear about it.

Sincerely,
--Lagrangian

 

[KennyB]

Hmm... So just to interject here for a moment and think of a way one could possibly test this, and in a somewhat affordable way...

Perhaps buy a length of stock of some carbon steel. 1080 is generally pretty darn cheap... Then just cut them to length, I'd say about six inches or so, send them off to get heat treated. Now make three sample groups. One group of blades with no patina for control, another group with patina that have also been treated, and another which as suspect of "hydrogen embrittlement" and begin testing to see what kind of forces are required to snap each piece.

I would think this would be the only affordable way to get a sample size large enough (I'm thinking at least 10 blades for each sample group) and apply a test method extreme enough to really get answers for this. Personally, I think if one were to do this, they might find a difference in how much one can hold before snapping, but that it wouldn't be that significant. I say this because of all the people who have forced patinas on blades and then gone and had them perform admirably. The truth is that we can't know though if they really are more brittle, but still strong enough, or if the reports that have broken due to embrittlement were from other reasons.

That's just my take on it anyway. I think we could muck about with the logistical, theoretical side of it, but to get some hard empirical evidence I think this is the avenue that has to be explored.

 

[bruss]

i went to a seminar on hydrogen embrittlement..mainly on welding and the correct procedures on storage of rods in an oven when opened from a sealed container, the lecturer got his point across by welding a sample T joint in the morning with rods that had been out for a few days, he then put the sample in a jar with baby oil in it, you could actualy see bubbles slowly emerging from the weld being trapped in the oil, if some brave soul wants to test this with a blade..i dont see why the results would be any different..good/bad

 

[hardheart]

Used to use low hydrogen rods when welding, I doubt anything done at room temperature is having an effect.

 

[stoffi]

Interesting points everyone! Thanks for all the replies :thumb up:

I suppose a cheap and simple method would be to take a bunch of already heat-treated, mass-produced carbon- and stainless steel Mora knives and subject them to different acids, exposure-times and so on.
It'll be cheaper, but it's still a 50-100USD-project or more. For once those vidz from Cold Steel would come to some use, especially the segment were they bend the knife in a vise, with the support of a metal pipe.

 

[sandgrouper]

Hi Stoffi,

You really pricked my interest with this one. This paper was the one I found most interesting (if a bit worrying):

http://ftp.rta.nato.int/public/PubFullText/RTO/AG/RTO-AG-AVT-140///AG-AVT-140-20.pdf

I think I will be thinking about this a bit more from now on.

On a side note, one of the fastener manufacturers, who really worry about this stuff, noted that in a batch that had not been properly baked post acid treatment they would only expect to see an issue with 2 to 3% of the items. So there seems to be an fair amount of randomness associated, which means your kukri has a good chance of being ok. But you will be giving it a whacking anyway to find out, hey? It generally seems if you have the issue it isn't very subtle and you would find out pretty fast.

 

[stoffi]

Yeah, I'll find out soon enough if my Kukri has been damaged or not, but since I've never used this particular one yet (only sharpened it), I wanna learn as much as possible about this issue before putting it to the test.
Possibly, there could be something which would drive out hydrogen, but it's probably too late and it has most likely alloyed itself with the carbon grains -- if the hydrogen ever entered the blade.

 

[Bo T]

Most of the hydrogen embrittlement that I have read about delt with the use of existing natural gas pipelines to transport hydrogen. In this case the hydrogen exposure is much greater than anything we could subject our knives to as average users. Of greater concern is the chemical reaction that takes place in corroding the surface iron that is exposed to the acid. For us the much greater danger is having the steel eaten away until the knfe is no longer usable.

 

[teckdeck34]

I wish i found this article before i bathed my knife . i made a small knife out of a file, unknown carbon content, but pretty high. i had it hardened and tempered well, it wasn't the slightest bit brittle after tempering because had to aggressively hammer hammer it straight. I placed it into a bathe of vinegar, fully submerged for about 7 hours to take off what scale was left from heat treat. i do this with my bottle openers and it works great so i had no reason to suspect something bad. it started to build up what looked like scale on it so i took it out and polished it down and found a sizable crack in the blade. Heartbreak. so i googled it and found this article. im a beginner smith so i just took this as a lesson in what NOT to do. I would say yes, we should absoloutly be concerned with hydrogen embrittlement when you acid etch, patina, or remove scale and rust. i have a few pictures, even before and afters if i can figure out how to post them

 

[Bill DeShivs]

I would think the embrittlement would be more pronounced on the thinnest part of the bevel.
If you are concerned, why not just bake the steel?

 

[mete]

Here's an example we had . Made helicopter clutches which included some electroplating . Parts inspected and passed. shipped to military. Military opened the packages 1 month later found cracks !! That 1 month permitted the hydrogen to diffuse to the highest stressed area and the steel cracked ! I've known cases where equipment hung from the ceiling dropped causing fatal injury. Brackets were never baked !
So it takes steel at certain hardness , hydrogen introduced [electroplating a good way ! ] time for diffusion , stresses. In the helicopter clutch there was a disc with notches . The failure was at the root of the notches which created a stress concentration !

The clutches involved " delayed hydrogen embrittlement "
The automobile industry considers " hydrogen trapping " in alloy development . RA and dislocations tend to trap hydrogen.