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Old 11 April 2013, 11:57   #11
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The pitting is likely caused by anaerobic bacteria. Process follows:

Bolt was not sealed adequately. Sea water gets in, but is trapped. The aerobic bacteria in the water consumes the oxygen, and dies (as there is no more oxygen to support it.) Anaerobic bacteria begins feeding on the dead aerobic stuff, producing sulfuric acid as a by-product. Acid eats the steel.

At least that's how I understood the crevice corrosion risk to submerged (or frequently wetted stainless fasteners in marine environments.

jky

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Old 11 April 2013, 12:59   #12
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Guys

Thanks for your replys. As far as I know and can see from the bolts and holes there was no copperslip on them but as I did not do the origional install I cannot be totally sure.

The explenation sounds right to me, looks like I will have to buy another bolt which was not in the budget!! Damn.
How long is it?...I have a new unused one that looks about the same length you can have.

Just looked..the one I have is a 6cm shoulder and just over 8cm thread...
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Old 11 April 2013, 14:14   #13
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Galvanic action.....stainless bolts, aluminium casting, sea water.


"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. The same galvanic reaction is exploited in primary batteries to generate a voltage."

The best solution would be to electrically connect the SS bolts and the engine using a SS strap between the head of the bolt and some high up part of the OBM....but its a bit of an overkill....just replace the bolts.
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Old 11 April 2013, 14:19   #14
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How long is it?...I have a new unused one that looks about the same length you can have.

Just looked..the one I have is a 6cm shoulder and just over 8cm thread...
Peter

Thanks for the kind offer, I think I will buy 4 new bolts when I get around to mounting the engine.

Andy
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Old 11 April 2013, 16:18   #15
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Galvanic action.....stainless bolts, aluminium casting, sea water.

"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. The same galvanic reaction is exploited in primary batteries to generate a voltage."

The best solution would be to electrically connect the SS bolts and the engine using a SS strap between the head of the bolt and some high up part of the OBM....but its a bit of an overkill....just replace the bolts.
If it was galvanic corrosion then the aluminium would erode, not the stainless. Stainless is noble to aluminium.

This is typical crevice corrosion in a situation where all the classic ingredients are present. The oversize holes in the saddle with a washer over the top provide the perfect spot for seawater to get in and sit there stagnating.

All you can do, as has been said, pump plenty of 291 in there to keep the water out. A good bit on the transom round the bolt before the outboard goes up against the transom, then fill the hole in the saddle round the bolt.

Most outboards are drilled for 1/2" bolts and us Brits usually end up using 12mm so there's a bit of space round the bolt which I've never liked much. But that's a whole different subject that's already been done on here!
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