rapid corrosion of aluminum hull in fresh water
rapid corrosion of aluminum hull in fresh water
(OP)
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?





RE: rapid corrosion of aluminum hull in fresh water
Mike Halloran
Pembroke Pines, FL, USA
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdoweb/
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
2) Dc is not generally run ungrounded in watercraft. Even if there is no bonding wire, if there is an engine present the engine provides a bond through its mounting. Bonding allows the use of a single overcurrent device per circuit rather than two.
3) Is the sea wall bonded to the secondary neutral of a distribution transformer, which is in turn bonded to a multigrounded primary neutral? If so, DC potential may be present due to the various interconnected metals involved, and AC may find your sea wall makes a good low impedance path back to the source. I would suggest providing an isolation transformer.
RE: rapid corrosion of aluminum hull in fresh water
Also, is the hull painted, and if yes, how does the distributoin of corrosion relate to areas of paint damage or other unpainted hull areas?
One more question: can you confirm positively that the boat is not in electrical contact with the sea wall?
There is a possibility of "stray current corrosion", but I'll wait for your responses to the questions above.
RE: rapid corrosion of aluminum hull in fresh water
the corrosion is all over the one side of the boat.
The only electrical contact between the hull and the wall is through the water.
the hull is only painted above the prop and the corrosion is under the paint also causing it to bubble.
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
If this is the case you should have an earth leakage detection system. This would ensure that your elv system is electrically sound.
By having an isolation transfomer in your shore power connection you should have no galvanic couple set up between the vessel hull and the wharf. Before going too much further could you varify these points.
RE: rapid corrosion of aluminum hull in fresh water
Whatever the cause of the DC flow, you need to reverse it. Connect a DC source to the seawall and the boat and crank it up until your corrosion stops.
An alternative would be to try and give the stray current a better path - say a copper ground wire from the boat to the seawall.
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
It will rot unless you coat it.
Haul your boat and paint your entire hull with a good Anticorrosive followed by an antifoulant below the waterline. Replace the anodes.
I suspect the existing coating failure is likely due to pinholing from a poorly applied "one coat" coating system, poor surface prep, contamination prior to painting, or any combination of the same.
RE: rapid corrosion of aluminum hull in fresh water
Grounding the vessel will provide a low resistance electrical path and hence reduce the current that may leave the hull and enter the water.
RE: rapid corrosion of aluminum hull in fresh water
After it leaves the hull and enters the ground and the grounded neutral of the electical distribution system where does it go? The return path for any battery is through the electrolyte. The battery I'm speaking of is not the onboard one, but the one formed by immersing dissimilar metals in water.
Ground is not some magical sink for coulombs as some believe, but is only part of the loop.
RE: rapid corrosion of aluminum hull in fresh water
second - yes - you should coat underwater part of the vessel with coating thickness up to 350 microns of anticorosives;
third - change anodes - the one you use are not zinc one - otherwise you would see them corroding as well as the side of the vessel.
RE: rapid corrosion of aluminum hull in fresh water
RE: rapid corrosion of aluminum hull in fresh water
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.
RE: rapid corrosion of aluminum hull in fresh water
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.."