When is toning corrosion? - Lincoln Cents

picked up some bank rolls. Appears this 1971-S cent roll wasn’t stored properly. Maybe too humid?
All edges are very dull and seem to have gone from red to brown and back to a different red with more of a rusty color.
Most coins have a very nice chocolate color with original luster (am I describing that accurately?). A few have heavier toning though.
Question is - when is toning considered more of corrosion and environment damage? Is this a desirable look?
I think they look cool at least

 

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The third coin has a very neat toning.
I like it.
The First coin has indeed some enviromental problems, especially on the rim.
Well, it depends where the coin was setted in the roll, I believe.

 

All toning is actually environmental damage - it's thin films of metal compounds such as silver sulfide.

If it has a pleasing appearance, it's toning. If it's ugly or too thick, it's damage.

 

Well said.

 

When selling it is toning, when buying it is corrosion.

 

That's a fine line to walk.

 

Thanks everyone.
Luckily the majority of the roll actually looks like the third one. Very milk chocolate.
Think these caught me off guard because it’s the first time I’ve actually owned/looked closely at unc heavily toned coins. Most of the time they’re oiled up to appear as such or all luster is gone. Lots of corrosion from contaminants other than air too.

Pulled a couple of neat proofs. One is very brown.