rapid corrosion of aluminum hull in fresh water

[shawn04]

I have an aluminum hell with Zn anodes that is corroding on one side and the keel. the side that is corroding is the side toward the sea wall. I have verified there to be no electrical issue with the DC system (ungrounded) on board. There is also no electrical connection between the grounding system for the shore power cable and the hull or the hull and the steel sea wall. The wall is bonded to the system ground for the utility and a DC potential was detected in the area. I have been reading about electrolysis corrosion due to the DC current. Any thoughts?

 

[fdac]

Corrosion only on the one side, nearest the seawall, certainly sounds like an electrolysis problem to me. Get a CP engineer to do a hull potential survey. This will tell you what the anodes are doing and whether the hull is cathodically protected, as well as identify what might be causing your problems. This type of stray problem is often caused by remote anodes emitting current which goes onto the hull and then jumps off. It's the 'off' bit that does the damage (in conventional current terms!)

 

[jzeeff]

stevenal - this is a case of current flow from a piece of metal, through a solution, then to another piece of metal. So a ground wire will help by giving the electrons an easier path. Applying a DC flow in the reverse direction will help even more (you could zinc electroplate/galvanize the hull).

Where does the current go after going to the seawall and then into the ground? Back to the source of course.

 

[stevenal]

Immerse two differing metals in an electrolyte with no external connections, and you have an open circuit. Like a battery on the shelf, no current flows in the electrolyte or the metal. Connect one of the metals to ground, and the situation has not changed. Bond the two metals together, and you will now have the maximum flow of current through the electrolyte and the now present external circuit. If the external circuit cannot be avoided (the two metals directly contact each other and cannot be isolated), change the metal (galvanize, or add anodes), or apply another source (impressed current system.)

Source of course? Reminds me of a song. "...go right to the source and ask the horse, he'll give you the answer that you'll endorse.."