Here is what goes on.... I bend a thin wall brass tube to shape then I have to melt the bending medium out of the pipe with a torch. This causes a black scaling to show up on the outside of the pipe. To remove this I dip the pipe in a 50/50 mix of muratic acid and water. This in turn plates the pipe with very thin copper that I have to buff off.
I know the acid is leaching copper and plating later pipes but what is the science behind this? Why does it plate?
LLB in Laredo
It isn't plating. It's de-zincifying the brass. Muriatic eats zinc fast and leaves a porous copper mess behind, which is very weak. When you buffed it, you polished right through the porous copper again and got down to the parent metal.
-- Ed Huntress
Ed, Thanks for your answer, I would be 100% convinced that your answer is correct except this doesn't happen when the muratic acid is a new clean batch. Why would that be?
LLB
Ah, well, now you've got me. It may be that there are copper ions in the acid, if you've used it for that purpose before, and that they'll plate out like electroless nickel. Now you'll need a chemist, which I definitely am not.
I have had a lot of experience with dezincified brass, however, and what you're describing sounds like it to me.
-- Ed Huntress
My first guess would be a lack of observational rigor. Coupled with slight differences in the situations, e.g., length of exposure.
The idea that "... acid is leaching copper and plating later pipes ..." doesn't make sense. Why would it leach one, but not the other & why would it plate one, but not the other?
"de-zincifying" is what I have always heard is happening. In support of this is the fizzing that takes place. This is the hydrogen being released from the muriatic acid (HCl) as zinc chloride is being formed.
Also not a chemist, Bob
My first guess would be a lack of observational rigor. Coupled with slight differences in the situations, e.g., length of exposure.
The idea that "... acid is leaching copper and plating later pipes ..." doesn't make sense. Why would it leach one, but not the other & why would it plate one, but not the other?
"de-zincifying" is what I have always heard is happening. In support of this is the fizzing that takes place. This is the hydrogen being released from the muriatic acid (HCl) as zinc chloride is being formed.
Also not a chemist, Bob
Just to add a question to the question, what is the difference between inhibited muratic and non-inhibited muratic?
I know we have some chemists on the list.
Wes
Its not plating.....hint..what is brass made of?
Which of the two metals is being removed by the acid?
Gunner
Political Correctness is a doctrine fostered by a delusional, illogical liberal minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
I guess I admit to being a chemist but I'm really rusty on this kind of stuff. First, pure hydrochloric acid will not dissolve copper metal, but hcl with oxygen dissolved in it will, so that is one difference between fresh acid and "used" stuff. The used stuff most likely does have copper ions dissolved in it. Then, since zinc is more electrochemically active than copper, when you put the copper ions in contact with the zinc metal the zinc is oxidized and the copper will plate out. Now, at the same time the hcl is chewing up the zinc as fast as it can so it comes down to how "used up" the acid is. If the hcl is mostly consumed so it doesn't react with the zinc very fast, and if the solution has picked up lots of dissolved air (oxygen) in use and then used that to dissolve lots of copper along the way, that copper will have time to plate out on the zinc before the remaining hcl eats away the zinc. If you took fresh hcl and just added pure zinc until the acid was mostly used up, it should not plate copper out if you then put it on brass. This is all my take on this without doing any research to back it up, ymmv, it's worth what you paid for it, etc ;-).
----- Regards, Carl Ijames
I think inhibited muratic acid is saturated by dissolving all the zinc possible in it. I think it is used to remove mineral deposits from galvanized pipe and for soldering flux. I believe it is also called "killed" acid.
Don Young
Apparently much of the "Aztec Gold" that the Spanish went chasing after was naturally occurring copper-silver-gold alloy that they made their objects out of, then soaked in a suitable solvent (one such "suitable solvent" being, IIRC, piss that had been left in the bowl until it was smelly -- I don't know if I'm _that_ dedicated of a metalworker).
The solvent would dissolve the copper and silver, leaving porous gold behind. Then the craftsman would beat on the thing to crush the gold into a nice solid-looking layer.
This is all recollection of an article read a looooong time ago, so if someone has a more accurate reference share it, by all means.
Carl, Thanks for this answer, I may not be nuts! : )
LLB
I'd be more concerned about annealing the tube during the melt-out, countering whatever work hardening happened during the bending. Unless an annealed condition of that particular part has been recommended by Sheldon the Cat, its dent susceptibility makes it somewhat less desirable.
But what about just buffing the scale off in the first place?
No it's leaching the zinc and leaving the copper. ...lew...
The annealed condition is ok. I tried buffing and it can be done, it is just a question of time. 10 seconds in the bath takes the scale off and then I just scrub it with a plastic scratch pad to take the worst of the copper off, its very thin. The whole project is buffed when finished. Using the acid really saves time and effort.
LLB
snip---
That's what is happening. Zinc on the surface is displacing the copper in solution, so it is, indeed, plating the copper. This process is commonly used in refining where recovery of values is accomplished with base metals. Zinc is amongst the best possible choices.
Harold
Now you'll need a chemist, which I definitely am
Thanks, Harold, and thanks to Carl (who owned up to being a chemist). I sort of smelled like that to me, but doing chemistry by smell is not to be advised. d8-)
So it sounds like there is some degree of dezincification and some degree of copper plating going on, the relationship depending upon the state of the hydrochloric acid solution. Interesting. It could almost get me interested in chemistry.
-- Ed Huntress
Heh! Were it not for my years of refining experience, I wouldn't have had a clue. I have never studied chemistry in my life, aside from taking one quarter at the community college in Utah, just before I started using cyanide for extracting values from gold ore. That was about 30 long years ago, and I've forgotten everything I learned.
It's nothing short of amazing how one can learn processes without having a fundamental understanding of why they happen.
Harold
Would a paint stripping gun get hot enough to remove your bending medium without producing scale? --Glenn Lyford
Would a paint stripping gun get hot enough to remove your bending medium without producing scale? --Glenn Lyford I bet it would. My heat gun works great for mass de-soldering of old circuit boards. I can just melt the solder, whack the board against something solid, and all the components fall off.
Don Young
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