danger of hydrogen embrittlement if removing rust with phosphoric acid (naval jelly)

Uncle Bill · Oct 11, 1999

Does anyone have experience or knowledge regarding the use of naval jelly to remove blade rust? Particularly, how easily it causes hydrogen embrittlement and to what degree this injures the knife?

I soaked an old carbon blade in naval jelly for about five minutes and the rust is all gone. Now there's a grey coating that easily wipes off leaving a nice patina.

But have I now ruined the blade? Will it snap the next time I use it? Will it hold an edge now? Chip out?

------------------
Bill

"They'll say I was a musician, won't they?"
--Beethoven on his deathbed (and you thought nobody but you was insecure . . . )

Walter · Oct 11, 1999

As far as I know the only time that hydrogen can adversly effect steel is when hydrogen is under pressure in say a steel containor,after a period of time hydrogen will migrate through the steel weakening it, I will look into the matter more as I'm not familiar with navel jelly.
Walter

klut · Oct 11, 1999

With most stainless and high carbon steel alloys, hydrogen embrittlement is only a problem when in a container at high pressure/high temperature conditions. Not sure how phosphoric acid would affect steel however. Will research at work tomorrow.

Sal Glesser · Oct 12, 1999

Hydrogen embrittlement usually does its dirty work during heat treat. The samples that I have seen are obvious when a problem exists. Usually a crack, but not for some time (months) after the actual hydrogen atom migrates in the steel.

sal

Uncle Bill · Oct 12, 1999

Sal:

Thanks for your reply. So are you saying that cleaning rust spots with naval jelly shouldn't be a problem, shouldn't lead toward hydrogen embrittlement?

Of course I'm not talking about soaking a blade all night. Still, my concern is about what might be happening at the molecular level along the edge.

Any thoughts from anyone with experience are appreciated.

------------------
Bill

"They'll say I was a musician, won't they?"
--Beethoven on his deathbed (and you thought nobody but you was insecure . . . )

Ed Severson · Oct 12, 1999

The naval jelly simply acted as an acid. By applying it you simply ate away the rust and slightly etched the blade. To get hydrogen embrittlement you need energy (heat) to get the atoms to migrate into the steel. At room temperature an acid is simply corrosive.

Uncle Bill · Oct 12, 1999

Here's where I first heard of the connection between naval jelly and hydrogen embrittlement:
http://www.loganact.com/mwn/howto/rust/rust.html

I'll put some text in another post.

Uncle Bill · Oct 12, 1999

From the site linked above:

"Another technique for removing rust is etching with Phosphoric Acid. Phosphoric Acid has a unique property of dissolving iron oxide quickly while etching iron very slowly. This means that you can leave metal in Phosphoric Acid for much longer than necessary with very little damage. The acid will attack bare metal slowly and will start the process of hydrogen embrittlement, so use the minimum etch time that removes all rust.

Another unique advantage of Phosphoric Acid is that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents rust. However, the iron phosphate coating is not very thick and not durable. Some additional protection is still required.

Phosphoric Acid etch will leave a hard, bright metal finish. This is because it will etch the surface slightly, exposing new, bare metal. Often this is desirable. It leaves an attractive surface and a surface ready to paint. A common product which contains Phosphoric Acid is Naval Jelly. The soft drink Coca-Cola contains Phosphoric Acid, so Coke will etch rust. But Coke also contains carbonic acid and other nasty things. You're going to drink that stuff?

Auto body shops treat metal with acid metal wash, a solution of Phosphoric Acid and alcohol before painting. This removes waxes and oils, removes slight amounts of rust that form between sand blasting and painting, and leaves a thin protective coat of iron phosphate. One commercial solution for this is DuPont Quick-Prep. Sherwin Williams has a similar product called Metal Prep.

Other acids will etch rust, but not as selectively. Hydrochloric acid can etch rust very quickly. Oxalic acid has also been used. However, if you leave metal in these acids a bit too long, you will lose a significant amount of metal. Also, neither acid leaves a protective film behind.

I read of using a solution of 1 part black molasses and 9 parts water to remove rust. Perhaps this works because the molasses is slightly acidic. The procedure is very slow. After treatment, metal starts rusting quickly, so this is probably not a phosphoric acid treatment.

All acids contribute some hydrogen to the metal structure. This weakens steel by a process called hydrogen embrittlement. If the metal is a cutting tool or gun barrel, for example, this weakening can be dangerous. One person claimed that if you bake the metal at 400F for an hour after acid cleaning, then you can drive out the hydrogen and prevent this embrittlement. With all acids, use rubber gloves and splash goggles, no matter how weak the solution. When thinning acid, add acid to water. Never add water to acid."

Comments please.