Year unknown. Dimes can rust? Or is this chemical damage? 1 1

[DonnaJo]

Maybe 1993 or 1983. What causes this in coins? 

[Woods020]

Over time as coins are exposed to gases in the air, they develop tarnish (often referred to as toning). It is generally caused by oxygen and sulfur causing a chemical reaction on the coin. Different metals tone in various ways, so clad coins tone slightly different than silver coins for example. 

Edited March 7, 2021 by Woods020

[Woods020]

As I look at this more the obverse has a green tone it appears. Green is usually a sign of PVC contamination. PVC causes a different reaction to the metal leaving the green residue. While toning is natural and seen as appealing to many collectors, PVC damage is universally a bad thing. 

[Henri Charriere]

1 hour ago, Lancek said:

Yup, green is bad.  And keep it away from other coins.  It can spread to any it comes in direct contact with.  Lot of decades old holder were PVC.  Collectors liked then because you could see both sides to the coin.  But today, thanks to lot of those coins turning green, we know PVC plastics are a bad thing.  A little green on the surface starts first.  I call it green slime.  At that stage it can usually be removed with a short acetone bath.

Saw a collection of Morgans at auction.  Every date, every MM.  All in a book with PVC pages and pockets.  Bought a '78 8 tail feather.  Just starting to get some slime.  Soaked in acetone.  Green stuff came off.  Looked great, sent it in for grading, came back an MS 63 and is one of my favorite coins.

If the green stuff gets hard, like it looks to be on your dime.  Then collectable value is likely shot.  That's referred to as verdigris. It has quite often eaten into the coin and removing it could leave big holes.  Making the coin look even worse.

You can buy clear flips today that are safe.  Look for "unplastisized" vinyl.  Or anything that say archival, or museum quality.

With all due respect... verdigris is but one form of patina.  I would like to suggest the OP, whether a coin collector or not, investigate these further on a search engine such as Google to get a full grasp of what each is.  There are many causes of such environmental damage and familiarizing yourself with each will help you understand how and why these phenomena occur and the best course of action to take.

[RWB]

Nickel and copper used in the OPs coin are more reactive than silver. The coin is corroded, nothing more.