What the hell caused this??

[Wightdiver]

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

[Nos4r2]

It didn't have coppaslip on it did it?

[alystra]

Originally Posted by Wightdiver

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.

[Wightdiver]

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.

[Timw44]

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.

[Wightdiver]

Originally Posted by Timw44

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

[Ribochet]

Originally Posted by Timw44

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

[Wightdiver]

Originally Posted by Ribochet

100%

Topic Category: Category: Crevice Corrosion

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.

[PeterM]

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 ?

[Nos4r2]

Originally Posted by PeterM

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.

[jyasaki]

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

jky

[PeterM]

Originally Posted by Wightdiver

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...

[Will75]

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.

[Wightdiver]

Originally Posted by PeterM

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

[martini]

Originally Posted by Will75

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!