7MM Rem cases corrosion

I decided to reload some 7MM Rem cases that I had tumbled (Dillon tumbler with Dillon polish) and then stored in 50 round plastic storage cases neck down about a year ago. When I opened them I found about 80% had this corrosion on the neck. I tried using 400 steel wool but it did not remove it.
Thoughts?
I expect I will be tossing them. Right?

Is it just on the outer surface or clean thru to the inside of the neck?

If just on the outside, a file would tell me how deep it goes. For hunting, i would be tempted to try to neck turn it off. As long as the necks wall thickness doesnt go below .010" while retaining good neck tension.

If the neck cracks or breaks off on firing, you will not know till you eject the shell & try to chamber the next round.

Safety first- scrap all the brass and buy new.

Just for the intrest in understanding if this is damage or a coating. I would take a sharp object and scrap the worst case, even use a pair of metal shears and split the neck to see how thick this discoloration is. I think by scraping and spliting the neck you can get an idea of the strength of the neck. Then decide to toss or load a light load say 3 grans below your normal load, for a test.

Take some Flitz on a rag and see if it will wipe off with Flitz. I have used Flitz on occasion to remove some weird corrosion and it worked.

Don't have any Flitz but I tried some Maas Metal Polish (an aggressive ammonia based, I think, polish). Washed the brass after using the polish to remove any ammonia. It did remove. after some rubbing. the black corrosion. As oneeyejohn proposed, I will start with a lower load (and progressively increase) to see if there is any neck failure. Let you know next week after my trip to the range,
Thanks for the comments.

Rifle brass is a consumable product I would not want the headaches or damage to my chamber or barrel so I would just scrap it and get more

wet tumble and see what happens with stainless steel pins also what brand

If 000 or 0000 steel wool didn't remove it, I would just can the brass. I remove tarnish all the time using steel wool on brass spinning on a drill. Works great.

Wonder if it is remaining seepage from inside the brass....
Maybe shouldn't have bagged it...

Just use steel wool to remove it, it is oxidisation, nothing more.
Vinegar does the same thing, some washing detergents will also oxidise brass black.

Cheers.

Yeah, my guess would be that there was moisture in the brass. When stored neck side down in the box, the brass sat in the residual moisture, containing whatever you used in the tumbler.

It's just tarnish, although heavy tarnish from being sealed up with moisture - clean it off and reload... like others have said when you leave moisture on the brass it will do that. Especially when sealed up. I make sure when I clean and leave them to have something in the container to absorb moisture...

Brasso brass cleaner.

When I am done with my brass, whatever it is that I'm doing, I tumble in dry media treated with car wax. You will never have this problem again and the wax will prevent a chemical bond between the bullet and the case neck