Anaerobic crevice corrosion and stainless steel keel bolts?

JonnyBoats · Post by **JonnyBoats** » Sun Feb 19, 2006 7:03 pm

Yesterday I was talking with a friend who had looked at a used Beneteau 38 with an external led keel. He told me that when he looked at the boat it was sitting on the hard, and had been out of the water for about three weeks after 12 months in the water.

He explained he saw water weeping out around the joint between the hull and the keel. When he looked in the bilges he saw fresh water lying in the bilge and covering the keel bolts. His opinion was that if fresh water could weep out through the joint, then salt water could enter.

His calculations were that the keel bolts were 80 times stronger than they needed to be, but he was concerned that the stainless steel could be weakened by anaerobic crevice corrosion. Further he stated it was impossible to determine the condition of the bolts inside of the lead keel since one can not x-ray through lead.

My question is has anyone ever heard of stainless steel keel bolts failing because of this type of corrosion, and how likely is it to occur?

Figment · Post by **Figment** » Sun Feb 19, 2006 7:24 pm

I wouldn't automatically assume that the water seeping from the keel/hull joint is coming from the puddle in the bilge. 3 weeks isn't really that much drying time. My boat has been on the hard since November, and I'm still getting a bit of weep from the foam.
I've never heard of keelbolts failing within the ballast. Always within the deadwood.

Short of a brutally hard grounding, I'd think the likelihood of failure to be very low. Those bolts can't be more than 20 years old, right?
Still, the only way to know for sure is to pull one of the bolts and inspect.

Academic: Keelbolts are always overengineered, but 80x seems a bit over-the-top. They may be 80x the deadweight of the ballast, but a transverse force at the end of the foil (a hard grounding in surf) is a whole other calculation.

Post by **Tim** » Sun Feb 19, 2006 9:20 pm

No keel/hull joint is perfect. Water inevitably gets into the seam behind the caulking or whatever is in there, and when the boat is hauled, this water will tend to seep out--often over a very long period of time. See, the water has the benefit of hydraulic pressure to help it enter the crack when the boat is in the water, but out of the water, and lacking this pressure (slight though it may be), the water cannot seep out very quickly, which leads to this sort of long-term seepage. It can take weeks or months.

It's highly unlikely that the water seen inside the bilge is actually what is leaking out. If indeed it is, then that is surely a problem, but I think your friend is overreacting in a natural way. Many people get worked up over keel bolts, or the seams in keels, or the water seepage that is very common. But keel bolt failures are extremely rare indeed.

Crevice corrosion can occur in stainless steel anytime the proper mix of conditions exists. I believe it requires stagnant moisture and air. This mix is unlikely for keelbolts, but not impossible by any means. But I doubt that this is the case with your friend's boat.

As an aside: I thought most Beneteaus had cast iron keels. I'm no expert on those boats, but this is the information I have. That doesn't mean some don't have lead keels, though.

Figment · Post by **Figment** » Sun Feb 19, 2006 9:41 pm

Aside of the aside.... If the keel is iron, the bolts shouldn't be stainless steel, right?

catamount · Post by **catamount** » Sun Feb 19, 2006 9:59 pm

JonnyBoats wrote: His calculations were that the keel bolts were 80 times stronger than they needed to be, but he was concerned that the stainless steel could be weakened by anaerobic crevice corrosion. Further he stated it was impossible to determine the condition of the bolts inside of the lead keel since one can not x-ray through lead.

My question is has anyone ever heard of stainless steel keel bolts failing because of this type of corrosion, and how likely is it to occur?

There are some rules of thumbs (scantlings) out there for calculating how much bolt cross-section you need for a given mass of keel. I don't recall what they are, but know that my boat has plenty (thirteen 3/4" bolts for 5000 lbs of lead, which I think was more than twice what the scantling rule called for).

If the Beneteau has a cast iron keel, you probably can just unscrew the bolts one by one to inspect them, as they will likely be threaded into the metal. If it is lead, chances are that the bolts have a J-hook on the bottom cast into the lead, so you won't be able to unscrew them from the keel for inspection.

The thing to do in that case is to "drop the keel" (on the hard) by undoing all the nuts on the keel bolts and then jacking the boat up off the keel so you can get a look (this is apparently typically done at a boatyard with a travel-lift, particulary yards that are used to handling racing boats, where this is supposedly a pretty standard operation). When you do get the boat lifted off the keel, take a dremel tool and grind out some of the lead around the bolts to get a look at where they go into the metal. You can't really replace them if they look suspect, but you can "sister" them by adding additional bolts. Use oodles of 5200 when you put it all back together as hopefully you won't ever have to do this again!

Is any of this warranted in the case of this Beneteau? Besides wetness is there any other clue that might indicate a problem with the keelbolts, such as rust stains? Of course if the keel is cast iron how would you distinguish rusting bolts from rusting keel? For that matter, if there were any iron contamination when a lead keel was cast, the iron would likely rise to the top of the liquid lead, and so you might still have rust stains at the top of the keel, even if the bolts are fine. Also of importance in the decision making about whether to bother dropping the keel is knowing how the boat has been used in the past -- has it been raced hard? Had a hard grounding? How old is the boat anyway? And how do you plan to use the boat in the future? If you want to cross an ocean, it's probably worth re-assuring yourself about the status of the keel bolts if you have any mis-givings

(No, I don't have any experience with this .... yet!

Dropping my keel is on the schedule for next winter....)

Regards,

Guest · Post by **Guest** » Sun Feb 19, 2006 10:59 pm

Tim wrote: Crevice corrosion can occur in stainless steel anytime the proper mix of conditions exists. I believe it requires stagnant moisture and air.

I probably know just enough to be dangerous, but I thought that stainless used oxygen to help it create a protective oxide layer, and that as a result anytime stainless was deprived of oxygen it could be prone to crevice corrosion in that spot. (For example, stainless rigging under a heavy tape layer - especially in a warm climate.)

I'll admit I just tried to find the answer to this on the web, but became hopelesly mired in technical journals. Perhaps I don't deserve to be posting in the Boat Nerdery section after all....

Oh wait, duh, I bet you meant that the air should also be stagnant, i.e. no air, so we're saying the same thing. Perhaps I *can* be a Boat Nerd :-)

--- Rachel

Rachel · Post by **Rachel** » Sun Feb 19, 2006 11:03 pm

Ooops, that was me just above. I forgot that I wasn't logged in, and for some reason it let me post as a Guest.

--- Rachel

JonnyBoats · Post by **JonnyBoats** » Mon Feb 20, 2006 12:41 am

As an aside: I thought most Beneteaus had cast iron keels.

Actually this particular boat had many "upgrades" during it's life, including a new lead keel so I don't know what the original one from the factory was made of. The reason for the new keel was for racing, it was a deeper keel for performance.

JonnyBoats · Post by **JonnyBoats** » Mon Feb 20, 2006 12:44 am

I thought that stainless used oxygen to help it create a protective oxide layer, and that as a result anytime stainless was deprived of oxygen it could be prone to crevice corrosion in that spot.

That is what I have heard. Also it can happen in water that is not free-flowing, that is why it can happen to shafts inside of shaft tubes.

Rachel · Post by **Rachel** » Mon Feb 20, 2006 1:29 am

I believe the free-flowing water carries oxygen to the stainless.

--- Rachel

(waiting for the metallurgy nerds to chime in)

Post by **Tim** » Mon Feb 20, 2006 7:18 am

Yes, you folks are right about the conditions for crevice corrosion. Apparently I was in "reverso-land" last night when I wrote that air was needed for corrosion, when in fact it's the opposite.

catamount · Post by **catamount** » Mon Feb 20, 2006 10:14 am

Indeed, if the keel to hull joint has been compromised such that sea water can reach the keel bolts, conditions are absolutely ripe for crevice corrosion of Stainless Steel bolts to occur. If the bolts are Monel, Everdur, or some other metal, it might be a different issue.

FWIW, here's why I plan to drop my keel for bolt inspection as soon as it is feasible:

Regards,

krissteyn · Post by **krissteyn** » Fri Jul 07, 2006 11:22 am

My 2c worth - If I had the choice, I would put in silicon bronze keel bolts and laugh at all this "stainless stuff"...

oh and BTW yes 5200 is OK but would rather use SIKA-FLEX...

kris

Post by **Tim** » Fri Jul 07, 2006 11:44 am

krissteyn wrote: oh and BTW yes 5200 is OK but would rather use SIKA-FLEX...

Do you have specific reasons for preferring Sika-Flex? Or is your statement just a matter of personal preference, like Coke over Pepsi?

Ceasar Choppy · Post by **Ceasar Choppy** » Fri Jul 07, 2006 12:37 pm

krissteyn wrote:My 2c worth - If I had the choice, I would put in silicon bronze keel bolts and laugh at all this "stainless stuff"...

oh and BTW yes 5200 is OK but would rather use SIKA-FLEX...

kris

Kris,
I think the stainless is a lot stronger than bronze for this application.

And to the whole oxygen question... just to be a smartass... I think the word "anerobic" in the subject title says it all (anerobic = absence of air or oxygen). ;)

krissteyn · Post by **krissteyn** » Fri Jul 07, 2006 2:21 pm

Where I am now - 5200 is more expensive than Sika - but thats not my reason - I just got used to seeing it work well - yes personal preference.
AND Sika has some fancy yachting award for excellency that 5200 hasnt - probably cause 5200 didnt bother to worry about it - maybe ???

And re bronze - If I had to replace a 3/8 ss bolt with a half inch bronze - SO WHAT ! or even use 5 instead of 4 bolts ...

hey - I promised you 2c - whadayawant - a phd ?

Plastic Classic Forum

Anaerobic crevice corrosion and stainless steel keel bolts?

Anaerobic crevice corrosion and stainless steel keel bolts?

Re: Anaerobic crevice corrosion and stainless steel keel bol