MarkCO said:
Also, it does not work effectively when used for wet tumbling brass.
Here is the result of conducting the experiment yesterday. I made a solution of 0.75% citric acid, which is the working solution strength of Hornady's ultrasonic cleaning solution at its MSDS maximum percentages. Incidentally, a paper I found online at one point used 40 millimoles/kg (about 0.78%) citric acid to chelate lead battery waste to good effect, but I don't seem to have bookmarked it to put up a link. My point is that we are somewhere in the useful range for chelation, albeit not a high-speed point. But then, we're not talking about high concentrations or thick deposits; just powdered combustion products.
0.75% is far less than the Frankford Arsenal recommendation. From the American Rifleman Staff in the 1960s and reprinted in the NRA book, Handloading, in 1981, Page 77, it states:
"The Arsenal remarked that another method which cleans quite satisfactorily is to soak the cartridge case 5 to 10 minutes in a 5% citric acid solution (a higher percentage in hard water), then thoroughly rinds in water…
…A RIFLEMAN trial of this method gave very good results, and the cleaned case did not tarnish unduly. This method leaves the cases not conspicuously bright, but obviously clean and in good condition."
And that is how these look. They don't have the bright shine I would expect from including a detergent in the mix or a car cleaner with wax. For one thing, those lubricate the surface so pins sliding off are going to burnish it smoother than you get without that lubrication. On the other side, as I've mentioned before, I don't like the pH of the citric acid to be brought down by alkaline detergent products. But I have nothing against stopping the machine and adding it for the last hour, well after cupric oxide and verdigris and white zinc blooming have been removed if bright shine is your objective. From my perspective, though, what I got is clean enough.
MarkCO said:
And...too much citric acid (in a concentration high enough to bind the lead) absolutely will dezincify brass making a very dangerous condition. The slight brown to pink coloration is a sign it has progressed too far.
Are you talking about what you see on the right, here?
The first time I saw that, I made the mistake of assuming the pink was copper left by zinc being etched out of the brass. It's not. That brass is fine underneath it. The pink doesn't change color or tarnish over time as copper does because it isn't pure copper. It is cupric oxide, the red form of copper oxide, and it was already there as part of the case oxidation before the citric acid went to work. The citric acid solution just didn't attack cupric oxide effectively (and that 45 Auto case got the FA 5% treatment). But if you drop that same case into a vibratory tumbler, the red oxide is soon gone and gleaming brass is all that remains.
For fun, I've started another experiment with 30% citric acid (near-saturated solution) and a sacrificial case and I'm taking photos. I'll hold off on what I'm expecting to see and just let it play out and will share the result.