2001 Saturn SL2 Tires leaking

Tires and Wheels problem
2001 Saturn SL2 90000 miles

My husband took his car in to get new cars in September of 2009. Since then all 4 tires have been leaking slowly. He has to fill the tires once every week or two. I had the tires installed at Firestone and when I called them to state the problem I was told that since the tires were put on around winter and the wheels are aluminun salt has gotten into the beading and caused it to basically corrode so the tire is not sitting properly. This is what is causing the leak. I am not buying it. This is the 3rd set of tires we have had on this car and we have never had this problem. I talked with my father and he is guessing that they did not properly clean the rim before installing the new tires. They want to charge me 25 a tire to clean and reinstall. Any input or adive would be appreciated.