Why does my tire leak air?

I have a 2006 Lincoln Zephyr and my front driver side tire keeps going flat. When I bought the car the tire was flat and I thought no big deal and filled it up. It slowly started leaking after that so I had a free flat tire repair and that didn't work so I just recently got a new tire on my car....but it still goes flat? Is it my rim? My rim kind of looks like it's corroding (flaking) but I don't know if it's that. Any ideas what it could be?