It's been a bit humid and rainy down where I live, so why not throw some ammo on my grass and see how it does. Results? Brass > Lacquer > Polymer. Brass corroding or getting tarnished doesn't matter as much to me when the steel ammo rusts. The brass cased ammo looks like the m193 I get from Federal Ammo cans on stripper clips...except a few spots of green and black oxidizing. The lacquered steel ammo has less rust than the lacquer. There tends to be sweet spots for rust on both casings; the rim and neck. The lacquer has small amounts all over the spot where the extractor would rest, like a ring, but minimal and looks like just surface rust. The polymer ammo has over 5x the amount of rust in random spots, and the rust looks like it is starting to change dimensions of the casing to the visible eye. The primer socket is also rusted on the polymer cased ammo, but to be fair the lacquered cased one has a primer sealant. While the lacquer ammo looks like surface rust, polymer looks like it is built up.

Ammo:
Federal 223
Brown Bear HP
Wolf FMJ

Sorry I don't want to upload like 10 pictures...

EDIT: here ya go
https://i.imgur.com/6Y8p9cu.jpg
https://i.imgur.com/DPpjyzi.jpg
https://i.imgur.com/LsmuKJ6.jpg

Some how this thread looses it's appeal without pics.

Aloha, Mark

Can't we get just one pic of the 3 casings side by side?

C'mon, man.

DAmnit fine I will xD

In for the disappointment.

A picture is worth a thousand words....

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they still look ok to shoot

Thanks, glock-coma

I have had similar results. I could not get lacquer coated steel cased ammo to rust in just a few days in the rain elements etc.

For my test, I also chambered the rounds a few times thinking that the usual scratches on the case from chambering will facilitate rust. But it did not appear to.

But I'm sure steel cases will rust eventually while brass will only tarnish and be OK after years outside.

I noticed the poor corrosion resistance of polymer coated ammo some time ago, and avoid the stuff when I'm stacking it deep for a rainy decade (no pun intended).  Lacquer coated stuff is way betterer.

The zinc plated stuff is even worse, IMHO, because the corrosion feels very abrasive.

I was impressed at how much it didn't rust the first few days. The last 2-3 days it rained during the day and at night the fog limited visibility to maybe 100 yards max, so humidity was pretty high. That's when the polymer ammo started to rust a lot faster. I think if the ammo were touching each other it would rust faster, so if you have steel ammo in a magazine in the same conditions it would cause more problems. I came to this conclusion when I put 1 wolf 1 brown bear in a cup with water and salt, previously. As usual, it rusted at the rim and neck first, but it also left some streaks where they touched each other.

One note is on the first day, none of the steel ammo had any rust, but the brass ammo immediately had tarnishing and green/black corrosion spots.

Exact same experience. If I were to buy steel cased ammo it would be lacquered.