The brass in that photo had been in a bag during a basement flood at my dad's place. To check for damage, after the ultrasonic cleaning I subsequently polished it in treated corncob and all the pink turned into polished brass. So the pink wasn't deep enough to cause weakness.
I see your point. I keep looking at pins and I own a Thumbler B, but haven't tried them myself as I am looking to get away from polishing rifle cases altogether at this point. I've seen several experiments and have done an initial one of my own that seem to indicate necks polished on the inside produce less consistent bullet pull and therefore greater MV spread than dull ones. So I have been running my polished necks over a turning case neck brush to roughen the inside lightly and have been leaving the chemically cleaned ones alone. That seems to be working out thus far, but I haven't determined which is best. The interesting thing to look at will be how the loads perform as they age.
Another element to all this is oxidation. Long ago (1920's, I think) Hatcher placed polished and unpolished cases on the roof of the Frankford Arsenal building which was in a location that had fumes from chemical plants and other industries in the air in those pre-EPA days. After a year on the roof, the polished cases had corroded through, while the unpolished ones were OK. So surface oxides offer some protection to the brass which can affect long term storage life.
I find that if you clean brass with vinegar and rinse it and leave them out, you will get fairly heavy surface oxide over time, with purples and greens. It might be good protection, but it's also good camouflage in the grass, so those cases are easy to lose in grass. The citric acid cleaned cases stay yellow. It gradually darkens, but is unmistakably brass, so the surface isn't activated the way vinegar affects it. I assume this oxide also has good corrosion resistance (untested), but it doesn't get lost in the grass. Indeed, I find it easier to spot the dull yellow than polished yellow because it doesn't mirror the grass. (Nickel-plated brass is the pits from this standpoint. It is practically invisible in grass as the mirroring doesn't alter the grass color.)
If someone using pins can run some cases with just soap and some others with the citric acid (or Lemishine or lemon juice as the source) added and set them up side by side for a month or two and report any difference in appearance, I would be interested to hear of it.
Experiments with greater or lesser amounts of citric acid would be interesting to hear of, too. You may have noticed that primer residue will fizz in acid, so there seem to be carbonates or other alkaline substances in it. For that reason it may be that primer pockets don't clean as easily in basic solutions as they do in acidic solutions, so that's another reason for the acid quantity to be experimented with.
Also, if anyone polishing their cases heavily and chronographing their loads would like to add to the shooting world's general body of knowledge, please also try experimenting with an inside neck brush and compare the results to loading cases polished on the inside of the neck to see what you get by way of velocity consistency differences. Of particular interest is how ammo shoots that's been sitting loaded for a year or so, brushed vs. not brushed. If the discrepancy worsens over time or goes away over time, it would be worthwhile knowing. I am aware ammunition is not wine and that aging it isn't be a normal practice, but if you load progressively for a season or two's matches all in advance, or load in advance for storage for the next mass extinction event or other survival scenarios, it would be good to know how your loading method's product ages in your storage conditions.
Magnum Wheel Man,
Citric acid in 10 lb quantity can be had for about $27 postpaid, here
. I bought mine from him and service was prompt. That's enough for about 23 gallons of the 5% arsenal solution, which is already reusable for a long time. Probably a lifetime supply. Enough to make a group buy with friends worthwhile.