Slocombe, 23, will provide cover for first-choice keeper Josh Lillis. The deal is for 12 months, with the club having an option to extend it for a further year.
Slocombe joined the Iron from local non-league side Bottesford Town in 2008, and has made five competitive appearances.
Four of those were off the bench, with his only start coming in last year's Carling Cup first round win over Oldham Athletic.