remove sheared screw from electrical outlet box?
Corrosion isn't your problem. The screw was too long and it bottomed out on what looks like a cable clamp in the box.   The screw is in compression.  If you get in there with a Dremel with the tiny cutoff wheel, you should be able to saw through it.  Once you do the compression will be gone and you should be able to finger-spin it out.  

It looks like his mistake was using extra long screws because of the depth of the plaster there.   And he went extra extra long, and was too long and bottomed out.   Choose length more carefully, or use the feature on your wire stripper to cut bolts.