Metallurgists, chemists, engineers please. - PeachParts Mercedes-Benz Forum

elchivito · #1 10-09-2012, 02:26 PM

I'm looking for the real answer to the question; can a weak citric acid solution etch or weaken brass?

Cleaning once-fired brass cartridges, dirty with firing gunk mostly but also slightly tarnished from sitting around.

When I put a thousand rounds into 3 quarts of hot (extremely hard) water and add roughly 2 tablespoons of 98% citric acid powder and a squirt of Dawn I get great cleaning results. After about a 2 hour soak the water turns greyish blue. I am having a discussion with some reloading friends. A couple of them tell me I am leaching copper out of the brass body of the cartridge and will weaken the metal. It's my contention that the blueish color is copper that is coming from the copper oxide tarnish being cleaned off and not from the metal body itself. It's been my understanding that citric acid won't etch, or remove, brass from the item being polished. So far there's no difference in the failure rate of brass cleaned this way vs. brass cleaned with media in a tumbler, and a lot less (none) lead dust and mess to deal with.
Anyone know for sure?
Note that I'm not asking how to clean reloading brass. I know how and yes, I have several tumblers and don't like them for a variety of off topic reasons.

Care to confuse the issue some more? Thanks!

beevly · #2 10-09-2012, 04:14 PM

Copper metal certainly can be attacked by organic acids, and copper citrate is indeed blue. The color could also result from the tarnish, which is either copper oxide or copper carbonate, both of which would have some solubility in acid. You're pretty dilute though (about 1%) and not exposed very long. If you don't see any pitting I wouldn't worry about it.

jplinville · #3 10-09-2012, 04:27 PM

The less time in the solution, the better off it will be.

Darktech · #4 10-09-2012, 04:43 PM

I'm showing a production of hydrogen off that reaction, albeit a slow reaction since copper is a less active metal than, say zinc.

2 C6H8O7 + 3 Cu = Cu3(C6H5O7)2 + 3 H2

pj67coll · #5 10-09-2012, 05:15 PM

Hey, you could power your car from your reloading! Now there's a "sustainable" fuel

- Peter.

elchivito · #6 10-09-2012, 07:49 PM

Quote:

Originally Posted by jplinville

The less time in the solution, the better off it will be.

Yeah, actually I think I left it in way too long. The brass turned bright in only a few minutes after some vigorous stirring.
I don't see any pitting, they look great, shiny bright but not jewelry bright the way they come out of a tumbler. Unimportant.

elchivito · #7 10-09-2012, 07:51 PM

Quote:

Originally Posted by Darktech

I'm showing a production of hydrogen off that reaction, albeit a slow reaction since copper is a less active metal than, say zinc.

2 C6H8O7 + 3 Cu = Cu3(C6H5O7)2 + 3 H2

Isn't Zn the other metal in brass? Guess I better not do it in the kitchen next time if it's giving off hydrogen.

layback40 · #8 10-09-2012, 08:30 PM

Quote:

Originally Posted by elchivito

Isn't Zn the other metal in brass? Guess I better not do it in the kitchen next time if it's giving off hydrogen.

Do you see bubbles forming on the surface? If so, you are removing metal.
I would have thought the zinc would be removed first as it is lower down the electrochemical series.
You wont see pitting unless you use a good magnifier.
Dilute caustic soda may be a better cleaner as it wont oxidize the metal.
Once cleaned, rinsed & dry, they should be rubbed with oil to stop corrosion.

Botnst · #9 10-09-2012, 09:28 PM

In addition to producing hydrogen you are in danger of making a salt of an organic acid. These can be poisonous. Be careful. Use a lot of ventilation and nitrile gloves. You are creating (admittedly low concentration) hazardous material and flushing into the sewer. Bad form.

Also I suspect the organic salt might react with the nitrogenous portions of the explosive in a reloaded cartridge.

Why do you need them shiny? Just get an untrasonic cleaner and get the residue out. Forget the oxide.

I just did a cursory search for the chemistry of smokeless powder. Whoa. Start here: C6H7(NO2)3O5

My mind reels at what residual materials might be present and how they might react with a cupric salt (or is it cuprous?). Hell, it's over my head, I dunno.

cullennewsom · #10 10-09-2012, 09:38 PM

Folks over at the gold refiner's forum can tell you everything you want to know.
Gold Refining Forum.com • Index page

The "CM Hoke" book would be a good place to look as well.

Refining Precious Metal Wastes C M Hoke

As an experiment you might be able to drop the Cu, if there is any, out of the solution and weigh it, to see if it is enough to even care about.

elchivito · #11 10-10-2012, 10:01 AM

Quote:

Originally Posted by Botnst

In addition to producing hydrogen you are in danger of making a salt of an organic acid. These can be poisonous. Be careful. Use a lot of ventilation and nitrile gloves. You are creating (admittedly low concentration) hazardous material and flushing into the sewer. Bad form.

Also I suspect the organic salt might react with the nitrogenous portions of the explosive in a reloaded cartridge.

Why do you need them shiny? Just get an untrasonic cleaner and get the residue out. Forget the oxide.

I just did a cursory search for the chemistry of smokeless powder. Whoa. Start here: C6H7(NO2)3O5

My mind reels at what residual materials might be present and how they might react with a cupric salt (or is it cuprous?). Hell, it's over my head, I dunno.

I think I'm gonna wash them again. Maybe put em in a mesh bag and run them through the dishwasher or washing machine. yikes

mytimeyet · #12 10-10-2012, 10:33 AM

once upon a time I worked in a chemical cleaning place. For brass and copper item we did the primary cleaning in caustic soda, rinsed & wiped, then a dip in Oakite 'brightener'. The parts looked awesome and the brightener left a protective layer that did not corrode.
This is the stuff I think: http://www.superkleendirect.com/OAKITE_TDS/Bright_Dip_CB.pdf though the stuff we used was a single dip... Well similar stuff anyway...