I've been searching the internet but haven't found an answer. Are the pins in an ATX connector brass? If not, what type of metal are they?

Most likely nickel plated brass.

If you're just scrapping it you could pull one of the pins and check. If it sticks to a magnet it may be steel or, because nickel is magnetic, nickel plated. Then you can take a file to it or bend it until it breaks from the stress. If there's yellow under the silver color then plated brass. If it's more pink under the silver then it's plated copper.

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Here to learn more so I can recycle more
My grades are my own opinion and not an official grade from Boardsort

Can they be added to the connector category on boardsort?

No, they must have gold plated pins. Somewhere I have a post about connector ends, I will try to find it later.

I just leave ends like that on and take to my local scrap yard. Unless your local yard has a separate category for connector ends/plugs just leave it on.

_________________
Here to learn more so I can recycle more
My grades are my own opinion and not an official grade from Boardsort

Here is my topic on connector ends.

viewtopic.php?f=3&t=18855

Depending on the yard, you may have to remove the connector ends. Mine requires the ends to be removed from the wire, otherwise it is downgraded to a lower category.

Once the connectors are removed, the gold pin ones go to boardsort, and all other non gold connectors go in to the sheet iron pile. But I keep most of the computer power supply connectors for later experimentation for removing the brass out of them.