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?

 

[MikeHalloran]

How is the hull moored? Conductive?



Mike Halloran
Pembroke Pines, FL, USA

 

[shawn04]

the hull is not moored conductive. This has been checked and the electrical system has been checked for grounding problems to ensure not connection to the shore power cable ground. As far as we can tell there is no connection to the hull except thru the water

 

[SJones]

Is there impressed current cathodic protection on the sea wall or nearby?

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[shawn04]

none that I am aware of the wall is attached to the utility system ground near by.

 

[stevenal]

1) I believe magnesium is generally favored over zinc for protecting aluminum 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.

 

[mshimko]

Please describe the corrosion that you've observed on the one side. Is it spread out over the entire side, or is it primarily at or near "edges"?

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.

 

[shawn04]

there is an isolation transformer on the boat.

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.

 

[stevenal]

I meant to use an isolation transformer for whatever electrical equipment is on the sea wall. I assume this equipment is why and how the sea wall is bonded to the utility. Not sure why else it would be bonded.

 

[shawn04]

I'm sorry. the sea wall is bonded to the utility all over the city per the city code I haven't figured that one out, but the city requires it so it would require alot of isolation transformers.

 

[Compositepro]

It seems pretty clear that there is a curent flow though the hull toward the sea wall. The source maybe on the boat or it could be that your boat is in the path of the current through the water. Fresh water has a high resistance and aluminum is an easier path. Check for a voltage gradient in the water from near the sea wall to the other side of your boat. The solution would be an impressed current protection for your boat. Magnesium anodes could posibly provide enough additional voltage over zinc to provide protection also.

 

[stevenal]

City codes do not generally apply to electic utilities, since it would be difficult for a utility to comply with all the various resulting codes within its coverage area. In the USA, the code in question is the NESC which is then adopted with or without modification by the various states. My copy says nothing of sea walls.

 

[TheBard]

Am I to understand that your elv system is a two wire insulated return and your voltage is floating above earth.
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.

 

[jzeeff]

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.

 

[mshimko]

Something totally different to consider - install a ground between your vessel and the pier. If they are at the same ground potential, AND your electrical system is properly set up, there should be noher stray current corrosion (if indeed that is what was happening)

 

[stevenal]

Joining dissimilar metals that are immersed in an electrolyte will not stop current from flowing. This would be like shorting the terminals of a battery to stop current. The affect is to increase current.

 

[MASE81]

Its an aluminum boat.

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.

 

[mshimko]

For Stevenal - flowing current by itself does not result in corrosion. Its only when current LEAVES the metal and enters the electrolyte that you get corrosion, and that's the phenomenum that seemed to be described above.

Grounding the vessel will provide a low resistance electrical path and hence reduce the current that may leave the hull and enter the water.

 

[stevenal]

mshimko,

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.

 

[medlerici]

I would say you should change mooring plase first;

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.