Acid corrosion removal

Gofor

Mark
Corporate Member
A couple of recent threads on using citric acid or vinegar as a rust remover has brought to my mind a question concerning hydrogen embrittlement. While I was working in the aircraft maintenance field, one thing we were taught was to never use acid on high strength steel parts due to the possibility of causing stress corrosion cracking from hydrogen embrittlement. I was wondering if this would also apply to the steel in modern plane blades or good quality chisels. Has anyone noticed more chipping or cracking in the tool edges after using the acid corrosion removers? Not being a chemist nor metallurgist by trade, I don't know if this could be a factor in the low-strength solutions most are using, but I would caution against soaking Grade 8 bolts, etc.

One safety tip is to "Always Add the Acid" when mixing a solution. In short, pour the acid into the water, not vice versa. Pouring water into a concentrated acid can cause an explosive reaction, splashing it into your eyes, on clothes, face, etc.
 
Last edited:

marinosr

Richard
Senior User
This is an interesting question, following this. Certainly you are introducing hydrogen to the surface of the steel. I think that hydrogen embrittlement causes susceptibility to failure in tension, though. Plane blades, chisels, etc are never in tension during use.

Edit: maybe plane blades have a degree of indirect tension
 

Graywolf

Board of Directors, President
Richard
Staff member
Corporate Member
Mark, I used an acid on a saw plate once to clean it. It changed the surface of the steel to the point I can not raise a polished surface. At least without removing more steel than I would like. Now the saw is still in use and has held up well, but she ain’t pretty. I haven’t tested how brittle the steel is, but I’m certain that it’s not the same. I for one will not use an acid product to clean a tool again after that.
 

Graywolf

Board of Directors, President
Richard
Staff member
Corporate Member
Chris on that saw I used Evapo rust. The exposer was short, just a few minutes but it was enough to change the saw. After that I switched back to Simple Green and sandpaper.
 

Oka

Board of Directors, Vice President
Casey
Staff member
Corporate Member
The problem with Muriatic, Hydrochloric or Sulfuric acid is there is a fine line between what cleans and what etches.
 

Hmerkle

Hank
Corporate Member
Mark,
Short answer: No - acid will not induce HE (Hydrogen Embrittlement) in a a plane blade or other other (unformed) high-strength steel when it is subjected to an acid bath.

Hydrogen is a naturally occurring product in everything.
It becomes concentrated when a high strength (read: hardened RC 32 or greater for fastener products) product is subjected to an acid bath.
An acid bath is used to etch scale and byproducts of the heat treatment process from the hardened part.
The answer is a baking process prior to plating to relieve or release some of the trapped hydrogen. (you cant get rid of all of it)
The greatest concern is when that high-strength, acid dipped part, is then coated or plated as it is typically done in an electrolytic bath. (Electricity used to deposit a coating or plating onto a product.)
This operation traps the hydrogen in the metal. In the case of a high strength bolt for example, the hydrogen is trapped an migrates to the highest point of stress in the bolt (under the head and into the neck of the threads) When subjected to a tensile pull - i.e. when it is torqued and the bolt's head is put under strain, you can have a HE Hydrogen Embrittlement failure. (The head pops off in a dramatic pop!)

While there are stress points in a plane blade and in a casting, neither is coated (effectively trapping the hydrogen) and there are numerous paths for the hydrogen to escape the metal, so there is no concentration of the hydrogen in the metal to create a stress fracture.
 

Hmerkle

Hank
Corporate Member
Chris on that saw I used Evapo rust. The exposer was short, just a few minutes but it was enough to change the saw. After that I switched back to Simple Green and sandpaper.
Yes, Evaporust is a chemical process called Chelation and does create a Grey or black surface after the process is complete.
I have not been able to find an explaination of what that "product" or surface is - but you can mechanically remove some of it (scotchbrite pad, steel wool etc.), but as Richard points out you will not get back to a shiny surface...

here is a link to further information and a power point presentation at the bottom of the page:
 

Gofor

Mark
Corporate Member
Thank You, Hank, for the informed reply.

I retrospect, after grinding and honing after rust removal, most of the embrittled metal would most likely be removed anyway.
 

Hmerkle

Hank
Corporate Member
Thank You, Hank, for the informed reply.

I retrospect, after grinding and honing after rust removal, most of the embrittled metal would most likely be removed anyway.
nope! unfortunately embrittlement is throughout the metal, not just on the surface, and it is believed that in a flat material like a sheet (or blade in this case) where the hydrogen can escape it is of no issue, especially because it is not coated or plated "trapping" the hydrogen in the metal...
 

Chris C

Chris
Senior User
I think I'll throw a saw plate in the citric acid and see what happens. I've got an old one laying around here somewhere...
 

Oka

Board of Directors, Vice President
Casey
Staff member
Corporate Member
Hank, I believe this and Naval Jelly use phosphoric acid as its main ingredient.

Yes, Evaporust is a chemical process called Chelation and does create a Grey or black surface after the process is complete.
I have not been able to find an explaination of what that "product" or surface is - but you can mechanically remove some of it (scotchbrite pad, steel wool etc.), but as Richard points out you will not get back to a shiny surface...

here is a link to further information and a power point presentation at the bottom of the page:
 

Hmerkle

Hank
Corporate Member
Hank, I believe this and Naval Jelly use phosphoric acid as its main ingredient.
It is funny, I only did a cursory search, but some of the stuff I stumbled on were comments on what the "Active" ingredients of Evaporust were and how the MSDS sheets don't outline it...
I am not sure if that has changed since there are more rules about disclosure, but I for one am a believer in "if you have a secret sauce, and you developed it, you shouldn't be compelled to disclose it"
(unless there are (proven) harmful consequences)
 

mpeele

michael
User
Yes, Evaporust is a chemical process called Chelation and does create a Grey or black surface after the process is complete.
I have not been able to find an explaination of what that "product" or surface is - but you can mechanically remove some of it (scotchbrite pad, steel wool etc.), but as Richard points out you will not get back to a shiny surface...

I use Caswell cold black oxide product after de-rusting with Evapo Rust which they recommended. Product support guy there said that Evapo Rust leaves carbon and a bit of iron on the surface after chelation. He also said the leaving Evapo Rust on after cleaning will prevent re-rusting unlike other rust removal processes.
 

Oka

Board of Directors, Vice President
Casey
Staff member
Corporate Member
I did a quick look, Naval jelly is phosphoric acid based, but Evapo-Rust appears not. In further reading, it looks like Evapo-Rust uses a chelation method to remove rust that is sulfur based.

In general, based on some further reading, all products like Evapo-Rust work by removing Iron (Fe II) from Iron Oxide and binding it in a ligand (an ion or molecule attached to a metal atom by coordinate bonding) complex. The Selective-Chelator is not strong enough to remove Iron-to-Iron bonds from un-corroded steel and does not harm the underlying metal. The Chelator is too expensive to use large quantities in the finished product. An organic chemical that easily loses sulfur to form ferric sulfate was added to remove iron from the Iron-Chelator complex. This allows the Chelator to remove more Iron from Iron Oxide. The sulfur-bearing compound is much less expensive than the Chelator and makes Evapo-Rust economical to use. Evapo-Rust has a chemical carrying capacity of 18.75 oz of pure, dry rust per 5.2 qts of liquid.


This is from a materiel reference book I have:
The chelation reaction corresponds to an equilibrium (or equilibria). If the chelating agent has acidic H:
FeO(s) + n HL (chelating comp) <===> [FeLn] + H2O + (n-2) H+
Therefore, these equilibria depend on the pH. If there are more than one stable species Fe-L, all of them could be present in solution, that is, [FeL], [FeL2], and so on. Usually, it is used an excess of the chelating agent to assure the formation of [FeLn].
Iron hydroxides are very insoluble, especially iron(III), with pKs ~10-38 for [Fe(OH)3]. The chelate species can maintain iron ions soluble, even at higher pH, depending on its formation constants. Ex.:
Fe3+ + edta4- ⇄ [Fe(edta)]- Kf = 1.3 x 1025
Fe3+ + 6 CN- ⇄ [Fe(CN)6]3- Kf = 1.0 x 1042



It is funny, I only did a cursory search, but some of the stuff I stumbled on were comments on what the "Active" ingredients of Evaporust were and how the MSDS sheets don't outline it...
I am not sure if that has changed since there are more rules about disclosure, but I for one am a believer in "if you have a secret sauce, and you developed it, you shouldn't be compelled to disclose it"
(unless there are (proven) harmful consequences)
 

Hmerkle

Hank
Corporate Member
I did a quick look, Naval jelly is phosphoric acid based, but Evapo-Rust appears not. In further reading, it looks like Evapo-Rust uses a chelation method to remove rust that is sulfur based.

In general, based on some further reading, all products like Evapo-Rust work by removing Iron (Fe II) from Iron Oxide and binding it in a ligand (an ion or molecule attached to a metal atom by coordinate bonding) complex. The Selective-Chelator is not strong enough to remove Iron-to-Iron bonds from un-corroded steel and does not harm the underlying metal. The Chelator is too expensive to use large quantities in the finished product. An organic chemical that easily loses sulfur to form ferric sulfate was added to remove iron from the Iron-Chelator complex. This allows the Chelator to remove more Iron from Iron Oxide. The sulfur-bearing compound is much less expensive than the Chelator and makes Evapo-Rust economical to use. Evapo-Rust has a chemical carrying capacity of 18.75 oz of pure, dry rust per 5.2 qts of liquid.


This is from a materiel reference book I have:
The chelation reaction corresponds to an equilibrium (or equilibria). If the chelating agent has acidic H:
FeO(s) + n HL (chelating comp) <===> [FeLn] + H2O + (n-2) H+
Therefore, these equilibria depend on the pH. If there are more than one stable species Fe-L, all of them could be present in solution, that is, [FeL], [FeL2], and so on. Usually, it is used an excess of the chelating agent to assure the formation of [FeLn].
Iron hydroxides are very insoluble, especially iron(III), with pKs ~10-38 for [Fe(OH)3]. The chelate species can maintain iron ions soluble, even at higher pH, depending on its formation constants. Ex.:
Fe3+ + edta4- ⇄ [Fe(edta)]- Kf = 1.3 x 1025
Fe3+ + 6 CN- ⇄ [Fe(CN)6]3- Kf = 1.0 x 1042
I thought I hated Chemistry when I was compelled to take it - you reminded me that isn't true - I am a equal opportunity Chemistry hater!
but thank you for the deep dive into chelation... (I do like that word...)
 

amuller

New User
Alan
I did a quick look, Naval jelly is phosphoric acid based, but Evapo-Rust appears not. In further reading, it looks like Evapo-Rust uses a chelation method to remove rust that is sulfur based.

In general, based on some further reading, all products like Evapo-Rust work by removing Iron (Fe II) from Iron Oxide and binding it in a ligand (an ion or molecule attached to a metal atom by coordinate bonding) complex. The Selective-Chelator is not strong enough to remove Iron-to-Iron bonds from un-corroded steel and does not harm the underlying metal. The Chelator is too expensive to use large quantities in the finished product. An organic chemical that easily loses sulfur to form ferric sulfate was added to remove iron from the Iron-Chelator complex. This allows the Chelator to remove more Iron from Iron Oxide. The sulfur-bearing compound is much less expensive than the Chelator and makes Evapo-Rust economical to use. Evapo-Rust has a chemical carrying capacity of 18.75 oz of pure, dry rust per 5.2 qts of liquid.


This is from a materiel reference book I have:
The chelation reaction corresponds to an equilibrium (or equilibria). If the chelating agent has acidic H:
FeO(s) + n HL (chelating comp) <===> [FeLn] + H2O + (n-2) H+
Therefore, these equilibria depend on the pH. If there are more than one stable species Fe-L, all of them could be present in solution, that is, [FeL], [FeL2], and so on. Usually, it is used an excess of the chelating agent to assure the formation of [FeLn].
Iron hydroxides are very insoluble, especially iron(III), with pKs ~10-38 for [Fe(OH)3]. The chelate species can maintain iron ions soluble, even at higher pH, depending on its formation constants. Ex.:
Fe3+ + edta4- ⇄ [Fe(edta)]- Kf = 1.3 x 1025
Fe3+ + 6 CN- ⇄ [Fe(CN)6]3- Kf = 1.0 x 1042
Last night I was looking at a rusty 12" saw plate and dado set, and wondered about the safety of using a phosphoric acid product and the potential to induce cracking. This thread is very much on point but after working through it I still don't know the answer to my concern. Appearance is not so important but avoiding flying bits is a high priority. What do people think? Thanks for any guidance!
 

Mike Davis

Mike
Corporate Member
Cooking Vinegar is 5% and the citric acid some use is about the same?
Are these concentrations even in the realm of the other acids you mention?
 

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