Bronze nut and aluminium frame corrosion

[kamikazi929]

Hi all. I'm currently overseeing th refurbishment of a small vessel which has two stern drives. The main nut holding the propeller on is completely seized and made of bronze, whereas the frame is aluminium, any attempt to forcefully remove the nut will potentially cause extreme damage to the gearbox.

When in seawater I would expect galvanic corrosion to occurx but this vessel in particular is only in operation for around 40 days at year and stored in a shed the rest of the time.

Is the corrosion of two dissimilar metals faster in the presence of an electrolyte (seawater) or in the presence of oxygen when stored in a shed?

Both sterns drives have this design issue and was wondering why the designers - chose to have a bronze nut that is so susepitable to corrosion with an aluminium frame.

If we were to have 2 nuts manufactured from an alternative material what would be a more resistant material?

 

[EdStainless]

The real problem is likely that the boat is stored wet, so the salt in threads can continue to absorb moisture from the air and continue to corrode while parked.
What metal is the bronze nut screwed onto?
What material is the prop?

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P.E. Metallurgy, Plymouth Tube

 

[DAVIDSTECKER]

I'd think that the Prop shaft is stainless steel.
I was once told the straight Winter Green oil was a very good rust buster.
I have had success with commercial rust busters too.
Unless the Prop is aluminum then it might be helpful to use heat to expand the nut.

 

[WKTaylor]

An illustration showing the parts [such as a break-apart drawing from the parts manual]... with materials-annotation would be helpful.

Regards, Wil Taylor

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