Alkaline batteries are subject to leaking, if you are leave your device without use it from some time, and cause irreversible damage to electronic circuit boards. Here is a way to clean up your devise from batter leakage.
My camera had this problem and even after cleaned the contacts, the camera still had battery problems.
So i had to remove the board and check it for any problems. As you can see, one of the battery's contacts had crystallized residue. To clean this residue, dip a cotton swab into lemon juice or vinegar, as the acid can neutralize the leakage from alkaline batteries. Clean everything having liquid or crystallized residue.
If you haven't used your device for some time, like a did, the problem may be deeper and a simple clean can't solve it. I also used rubbing alcohol to clean the board, because the camera still had problems.
So i de-soldered the battery contact, removed the solder and i soldered it again. This solved the problem camera had.
This is the best way to be sure that the soldering is good. The contact and the board must be cleaned very well and check for any soldering that doesn't seems good. With the same way, you can repair devices where liquid has been spilled inside.