I have not had any experience with this type of instance, but my supervisor used to do service and I guess they make units where the coils are made out of copper rather than aluminum. I know they are not cheap but probably cheaper then replacing a unit every few years. The attached picture is a picture of a coil of a condenser unit at a house we rented in Aruba. I asked the owner about it and he said that due to the fact that is was in the area that gets direct winds from the ocean and was mounted high on the side of the house he has to replace every few years. All the other ones were fine. Good luck with the unit.