Stainless steel screws under epoxy !?

No pissing match going on.

I posted this thread because I was surprised someone professional would epoxy over ss-screws below the waterline, because of crevice corrosion. Then, of course, some people found it necessary to deny that there even was such a thing as crevice corrossion/oxygen starvation, hence the discussion got heated.

When I began this thread, I thought everyone knew about oxygen starvation, hence my first post.

With regards to Par's statement, that this was about being "armchair" and "dictating" what is "correct" and what's not, it wasn't. I just simply couldn't pretend that oxygen starvation wasn't real just because some thought it would be swell to deny it's existens - no matter how many old crap-boats they pulled apart.

Btw, if you guys read the article, they're even talking about giving the owner the choice of having it lacquered (as in "you get to see the wood") or painted.

Longliner,
I have no idea how heavy that keel of yours is, but let's say the keelbolts aren't holding the keel in place years down the road, thus making the glass fibre take the weight, wouldn't that stress other parts of the boats, and not the original set-up (floors, frames stringers etc)? I'd think that could become a problem in the long run.

 

Yep we all deny that "there even was such a thing as crevice corrossion/oxygen starvation" and why don't you throw in the holocaust as well.

How you got that into your head?

Anyway time to let that go...no?

 

By the way that's sarcasm...

 

ballast is 3300lbs,,it is bolted to and through the keel and keelson,,per plans,plans are old 1967,,boat has been in a barn for the entire time ,untill 3 yrs ago,,but coverd and kept dry,,plan calls to fill the shaft holes of the bolts with something like 5200,,I wish to seal it from outside to avoid a lack of oxegen,,not coating the bolts with anything but,,, I could just swap them out with silicon bronze

 

Ah, yes, the holocaust-thingy. I wouldn't know if people would deny that as well, and I have no intention of finding out.

Well, duh!

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Or, you could perhaps get someone to do a survey and to do some calculations? I wouldn't know, but since it's a rather big keel in my world I'd propably do the silicon bronze-thing as I would be able to get it of the boat if it (the keel) was damaged. Plus, it wouldn't be _that_ expensive all things considered.

Edit/add: Btw, out of curiosity, I'd love to see some pictures of it, if you have any on hand (not the technical ones, but just to see how it looks).

 

Longliner,

From what I have read you can use cathodic protection for things such as keel bolts. A zinc anode in strong electrical contact with the stainless steel should offer protection. I think that salt water qualifies as strong electrical contact. Phil can probably answer that one.

Interestingly that may mean prop-shaft anodes may actually be giving some boat owners more protection than is realised.

 

Phil here is one for a chemistry teacher. Is it possible that the copper in a lot of anti-fouls is providing some sort of cathodic protection for any stainless present?

 

I understand ,,anodes,,,but I wonder if you cut air off from ss,,are you asking for trouble?,,and do you technically cut air off ,,if you dont encapsulate?

 

Longliner,

Even a small crevice (it could even be from a weld, a painted area or the backside of the gudgeon) could lead to crevice corrossion. Let's say that 60 centimeters/2 feet of the bolts are painted, or put into the keel (same thing, in this case), the 60 cms would be prone to oxygen starvation. It's not a matter of having a "breather hole" (for want of a better word), because the surface where it's at needs to get oxygen. I guess you could simply swap them ever so often. Pull one of them once a year or every other year to see if it's okay?

The crap part of this sort of corrossion is that it's happening where you cannot see it, so ...

Btw, just below this thread there are some good threads about this. Especially the one about the "stainless steel boat".

 

I'm not sure but from what I read the important point is the sound electrical connection. So I would understand from that if the head of the bolts are exposed to the anode then the whole bolt is protected regardless of the conditions at say the middle or the end of the bolt. Is that what you are getting at or is it some other cause.

If you cut oxygen off from the ss then it appears the ss is defenseless against corrosion attack if the existing oxide layer is breached. Some things I have read suggest that some mechanical breach of the existing oxide is required while others suggest that the chloride in salt water is enough to cause it. In the absence of either mechanical abrasion or stagnant salt water, I'm not sure what dose it but its seems logical that for corrosion to occur you need some agent to breach the existing oxide layer be it mechanical or chemical. Flowing water protects because of the oxygen content, the question you are left with is how much flowing water is needed? and will keel bolts get enough?

One article suggested that it can be very random they gave an example of setting up a dozen or so screws in a sealed salt water environment only to see one of them break down and corrode. I don't know if its possible but in that instance it sounds like the corroding screw has in effect become the anode and protected the rest, I suspect that once that screw had corroded sufficiently the next would break down. I might just set up an experiment for the heck of it.

Apologies for any repetition.

 

"Stainless owes its corrosion resistance to its Chromium content,
which forms an oxide film which is resistant to attack (material is
then referred to as passive). Nickel improves welding properties.
18/8 (% Chromium and Nickel) is the minumum grade (304 grade).
Better is 316 grade which has Molybdenum, which improves
corrosion resistance.
If stainless is starved of oxygen (e.g. under seals or barnacles) it
loses its protective oxide film and becomes active. It will then
corrode readily. This can also occur in microscopic crevises resulting
in almost invisible corrosion which can cause sudden failure.
Good for deck fittings it is not recommended for use underwater
(except when galvanically protected as, for example a fastener in an
aluminum outdrive)."

 

I have pulled heel bolts on sailing boats only 8 years old, totallt waisted (they were eaten away), and had to obviously be replaced.
There is pitting corrosion also that eats the metal when in a briny solution and gets worse as the temperature increases.

Forget stainless steel underwater unless it is an economic situation, then be prepared to remove and replace as required.

the best underwater metal is monel, followed by silicon bronze, 316L (low carbon) stainless is a later unattractive alternative.

 

The question remains could they have been protected with successfully with an anode. The last line says it all "(except when galvanic ally protected as, for example a fastener in an aluminum outdrive)." mind you what of the outdrive? I will assume that some zinc is bolted to it and is the final layer of protection.

Yeah I take what you say and have seen similar but there are many boats of much greater age carrying SS keel bolts. Take for example SS bolts in cast iron keels as is done on quite a few production keel boats, iron is further up the galvanic table so one would assume that it is providing protection to the stainless steel. In that instance it works, you don't see old Northshores (LOL, well I have not, anyway ) with keels dropping off. I think its a matter of understanding exactly what it is you are doing and where the protection is or is not coming from.

As you probably know Monel is a stainless steel, its just a brand name for a certain alloy (range) that is designed for highly corrosive environments. Point being we probably should refer to the alloy's because if you choose the stainless steel alloy that is Monel it makes lie of a lot of what has been said. Great stuff though even if a bit expensive and hard to find down here, I have to go to town to source it normally. I asked at the Wooden Boatshop once and they looked at me blank.... never heard of it ????! You have to wonder.

 

Good to see some technical discussion here in addition to the unfortunate argument, apparently the result simply of a bit of confusion over what each was talking about.
Just in case that comes back though... Wikipedia calls it horsemeat, http://en.wikipedia.org/wiki/Wikipedia:STICK and I think all of us know when to move on.
Keep the technical data coming, and maybe we should look at getting some of this in a page on the BDnet wiki?

 

"As you probably know Monel is a stainless steel, its just a brand name for a certain alloy "

Sorry mate, quite incorrect......Monel or Alloy 400, is a nickel alloy containing 65-70 percent nickel, 20-29 percent copper, and small amounts of iron, manganese, silicon and carbon ....hardly really a "stainless steel"......it certainly is regarded as a stainless alloy as such.