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Old 11 April 2013, 06:01   #1
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What the hell caused this??

Hi Guys

Just removed the engine mounting bolts from the transom preparing to re flow coat everything and one of the bolts looked like this. Looks like someone has been chewing on it.

The other three are in perfect condition and the transom is totally dry, which is good news.

Some kind of electrolysis?

Anyone seen anything like this before?

Andy
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Old 11 April 2013, 06:20   #2
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It didn't have coppaslip on it did it?
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Old 11 April 2013, 06:42   #3
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Quote:
Originally Posted by Wightdiver View Post
Hi Guys

Just removed the engine mounting bolts from the transom preparing to re flow coat everything and one of the bolts looked like this. Looks like someone has been chewing on it.

The other three are in perfect condition and the transom is totally dry, which is good news.

Some kind of electrolysis?

Anyone seen anything like this before?

Andy
It will be electrolysis, because I think that's how all corrosion works. Stainless steel relies on an impervious layer of oxide to counter corrosion. The oxide layer is only impervious if it forms on a very smooth surface, which is why stainless steel fittings are highly polished. Maybe that bolt was not polished enough, or was scratched or filed, or something?

If the oxide layer fails, or cannot be replaced, then stainless steel will rust very quickly because of electrolysis between the various alloys that make it up. Stainless steel propeller shafts enclosed in a shaft tube suffer this, and it's one reason why boats are not made of SS.

Though the transom holes are dry, if any dampness got between the sealant and the bolt, the oxide coat could be breached and with no air to renew the oxide layer, corrosion will set in and accelerate.

I think that may be a possible cause, but I'm dredging the depths of my head for old knowledge.
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Old 11 April 2013, 06:58   #4
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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.
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Old 11 April 2013, 07:04   #5
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Yes, it looks like crevice corrosion. The oxygen is depleted so the passive layer that makes it stainless isn't renewed, resulting in the pitting. It's even worse if there is any chlorine around from bleach etc.
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Old 11 April 2013, 10:01   #6
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Yes, it looks like crevice corrosion. The oxygen is depleted so the passive layer that makes it stainless isn't renewed, resulting in the pitting. It's even worse if there is any chlorine around from bleach etc.
Well you learn something new every day!!

Thanks Tim
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Old 11 April 2013, 10:35   #7
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Yes, it looks like crevice corrosion. The oxygen is depleted so the passive layer that makes it stainless isn't renewed, resulting in the pitting. It's even worse if there is any chlorine around from bleach etc.
100%

Topic Category: Category: Crevice Corrosion
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Old 11 April 2013, 11:14   #8
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Interesting reading.

Is this something I/we should be worried about?

Only one of the four bolts were effected and as this is a new project to me I have no idea how long they had been in there. If all 4 were in the same condition I could imagine the engine taking a dive off the back in rough conditions.

Is there some form of advised inspection regime? My other RIB has had the engine on the back for at least 6 years, I am getting a bit worried.
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Old 11 April 2013, 11:36   #9
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Surely the surface not renewing would only be an issue if the surface needs to be renewed.......meaning in this case the bolt may have been been 'moving' and waring away the original surface ?

If the bolt doesn't move (at all) the surface wont be eroded, meaning it wont need to renew itself (if there is oxygen or not wont matter) ....

Make sense ?
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Old 11 April 2013, 11:49   #10
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Quote:
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Surely the surface not renewing would only be an issue if the surface needs to be renewed.......meaning in this case the bolt may have been been 'moving' and waring away the original surface ?

If the bolt doesn't move (at all) the surface wont be eroded, meaning it wont need to renew itself (if there is oxygen or not wont matter) ....

Make sense ?
Outboard mounting bolts always move fractionally. There's no way to stop it happening as drill holes aren't that accurate and they're a dissimilar material to what they're bolted to. If it was a stainless transom,outboard saddle and an interference fit all the way through both you might stand a chance but that situation's never going to happen.

Hence using Sika 291 to seal them-it flexes as well as adheres and keeps the seal.

A tiny amount of grease on the bolt (EG, not degreasing it before fitting) could be enough to make the sealant part from it over time.
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