Expert: Mike Fulcher - 1/1/2008

Question
I have paint bubbling on my aluminum boat. It has been suggested that it is may be caused by stray current leakage from the cottage (Great Lakes - fresh water) submersible pump which is approximately 50 feet from the wooden dock where the boat is tied. Can AC leakage cause corrosion of this type, or is it DC only? The zinc sacrificial on the motor is in excellent shape as is the paint on the motor shaft and near the stainless steel prop.

Answer
Given that the anode for your motor is OK and the pump is a good distance away, I'm guessing that the pump is not a problem.  If it's a newer boat, I'd say it just had a spotty paint job.  If it's an older boat, the bubbling paint may just be a results of years of weathering.  If the paint provides a good seal, the pump shouldn't matter since there's no access for water to contact the metal hull.  If there are pinholes in the paint, then any water contact will cause the underlying metal to oxidize eventually, regardless of what the level of leakage current is.