Cleaning electrical plugs with brown spots/markings

Have multiple electrical contacts with at least one brown spot or area on the silver area. I don't know if it is corrosion or burn markings. I guess the question ultimately is how to remove or clean them properly?
Updated image to reflect brown spots.

 

Steel wool.

 

Looks like deep scratches.

 

From the sides though the plugs are smooth and flat. Neither does anything catch on the area. Which is why I wonder now if maybe a burn.
I'll need to try steel wool, since brass did nothing.

 

Maybe I'm wrong, but aren't there metal balls in the US sockets that latch onto those holes? They're there to prevent the plug from being accidentally unplugged.

 

Looks like the nickel plating is scrubbed off the prongs. You will see the same thing with cheap tools.

 

Hm. It seems possible (in reply to Stevo9389's comment). Although I just haven't grabbed any steel wool to find out the details (yeah, months later. I'm lazy).
The plug is a PSP charger, by the way.
I think I'll maybe try electrical contact cleaner at some point, unless anyone has any other suggestions?