Coated aluminum corrosion in seawater pump application

[trygw]

I know the basics about electrolysis and how it corrodes aluminum in salt water. Heres my question though.

In a sea water pump application were no electrical current is present, if a coated aluminum housing is used, will the aluminum still corrode due to electrolysis. I'm told that any contact of aluminum with dissimilar metals, even in the non wetted part of the pump, will cause electrolysis. Is this true?

 

[MikeHalloran]

Yes.

The dissimilar metals plus electrolyte comprise a battery cell, and will produce a current of their own with no external assistance.



Mike Halloran
Pembroke Pines, FL, USA

 

[strider6]

my question is:

the aluminium coated surface is in contact with seawater?
where do the galvanic corrosion came from, i mean what is the other material?

S

Corrosion Prevention & Corrosion Control

 

[MikeHalloran]

The aluminum will corrode wherever it's in contact with the seawater, e.g. through pinholes in the coating. There _will_ be pinholes in the coating.

But that wasn't your question. You asked about dissimilar metals in contact with the aluminum, on the dry side of the pump.

Where seawater is involved, there _is_ no dry side of the pump. The pump will eventually develop a leak, and the seawater will find its way to the metal-metal interface.






Mike Halloran
Pembroke Pines, FL, USA

 

[trygw]

Yes, pumps do tend to leak over time. Ignoring that fact, my question is really specific to the dissimilar metal contact in areas that are not intended to come into contact with salt water. For example, the bolts that hold the cover plate on. They never see any salt water. Would there still be electrolysis of the aluminum even though the dissimilar metal is not in contact with the saltwater?

Thanks,

Tryg Waterhouse

 

[MikeHalloran]

Theory would say that no electrolyte = no electrolysis.

Old salts would say the electrolyte will be there.

The pump doesn't understand the theory.

Don't assume the aluminum will be the metal attacked, either.



Mike Halloran
Pembroke Pines, FL, USA

 

[trygw]

Hey Mike,

Thanks a lot for the information. It sounds like its worth at least prototyping and testing.

Tryg Waterhouse

 

[chief]

I have seen a bronze ship side valve with steel stud bolts holding the bonnet, totally eaten away. Nearly lost the ship because of it. The studs just wasted away. The valve was nicely painted so the dissimilar metal was not apparent. The studs had no direct contact with sea water either.
Aluminum in sea water is not a good idea and has a short lifespan,especially if you use steel bolts.
The old DC3(Dakota) aero engine had an aluminum engine block. Old hands will tell that the steel bolts kept stripping out of the block, and these engines were air cooled.



Offshore Engineering&Design