Aluminum hulls and quicker electrolysis??

[Steve DAntonio]

I was captain/engineer (yeah I was a 'deal' for the owners back then) of a large aluminum motorsailer some years ago. She wasn't a new boat, so I'm surprised I found the problem, but any oil leaking out of the 4 Jimmy diesels (2, 471 gensets and 2, 671 mains) was corroding wherever it dripped on the hull. I immediately had drip pans made and installed.
So, check the engine room carefully.

Oil causing corrosion? That's a first, oil is an insulator, and not cathodic to aluminum. Diesel fuel is an oil of sorts, and we store it in aluminum tanks, many of the trawlers I work with have new and used oil tanks, made of aluminum, many engine heads and blocks are aluminum and they are filled with oil...

I believe you experienced this but there has to be another explanation, oil doesn't cause aluminum to corrode. Did anyone ever definitively identify why this was occurring?

The only theory I can come up with is the engine's bearings, which use copper, were so worn that the oil was heavily laden with copper, which is cathodic to aluminum. You'd still need an electrolyte, i.e. water at the interface, so I suppose oily bilge water... Just kidding, sort of.

 

[dannc]

Oil causing corrosion?
...
The only theory I can come up with is the engine's bearings, which use copper, were so worn that the oil was heavily laden with copper, which is cathodic to aluminum. You'd still need an electrolyte, i.e. water at the interface, so I suppose oily bilge water... Just kidding, sort of.

Copper in the oil is what I was thinking.

There is an article in Professional Boat Builder about a rebuild/refit of an aluminum boat built 50ish years ago. There was corrosion on the aluminum, if I remember correctly, on top of the tanks. They figured that the corrosion was from water leaking from pipes above the tanks. The problem was that the boat was built with copper water supply pipes and they guessed that enough copper was leaching into the water which then slowly dripped onto the aluminum and caused corrosion over the decades. Would would have thunk it?

Later,
Dan

 

[Steve DAntonio]

Copper in the oil is what I was thinking.

There is an article in Professional Boat Builder about a rebuild/refit of an aluminum boat built 50ish years ago. There was corrosion on the aluminum, if I remember correctly, on top of the tanks. They figured that the corrosion was from water leaking from pipes above the tanks. The problem was that the boat was built with copper water supply pipes and they guessed that enough copper was leaching into the water which then slowly dripped onto the aluminum and caused corrosion over the decades. Would would have thunk it?

Later,
Dan

I recall that article well, I wrote it, about the Abeking and Rasmussen yacht built for IBM heir Tom Watson (I think it was built in the early 80s). Inexplicably the builder laced the bilges with copper plumbing, water dripping from a copper pipe, over the course of years, did crater the top of a tank. Copper ions plus water and aluminum equals corrosion. The oil is the part I'm struggling with.