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· Sea Slacker
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ah..so they did not advise me of that when I called them last year.

so help me understand if electrolysis is an issue - how come the stainless shaft does not appear to have any electrolysis issue?
Shaft is big, and it is also towards the boat from the prop. So the electricity flow is from shaft to prop to water. This prevents much corrosion occuring at shaft. Pins are small and they are between prop and water, so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
 

· Sea Slacker
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1.- The size of the shaft relative to the pins and its position relative to the prop and boat do not affect whether or not, or to what extent, electrolytic corrosion will occur.

2.-

The first part of this sentence (besides being completely irrelevant) has absolutely no relation to the second part.

3.- The pins are made of monel because monel is galvanically closer to bronze than stainless is (as you correctly pointed out). However, this is to prevent galvanic corrosion, not electrolytic corrosion.
The two are intimately related, in particular when we are talking about prop shaft and anything attached to it. Practically all diesel engines are electrical ground for their alternators and as such, shaft/prop etc act as a ground path to the water (unless specifically disconnected through a coupler).

Boat ground is higher in potential than water and it does create current. Galvanic and electrolytic corrosion have essentially same mechanism - difference in potential created in one case by external source of electric potential and in another by different pontential between dissimilar metals in electrolyte.
Generally speaking simple difference of potential between stainless and bronze is not sufficient to create significant corrosion, but the ground current from the boat helps this process quite a bit.
 
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