Corrosion, tie down U-bolt backing plates

F

Falco

New member
Just wanted to pass on the latest suggested quality improvement for the factory and a safety alert.

While doing some work in my 2004 22's port lazarette, I noticed the backing plate for the U-bolt (which most folks use for their trailer tie downs) was essentially dissolved with corrosion. I reached in and flaked off (by hand, with 1 finger) about a third of the plate.

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The sawn off bolts are a nice touch. Hope I can back out the nuts and reuse the U-bolts.

I have no idea what the backing plate was made of - aluminum perhaps.

All anodes on my boat are in #1 condition. I replaced the trim tab and power trim anodes when they showed the slightest pitting. Engine anodes are A#1. All electricals are bonded to a common ground, including the two battery banks. Don't believe this is a stray current problem or unusual galvanic action. (The anodes have shown no unusual wear over 400 hours) It's the material used which acted as a super efficient sacrificial anode.

Would welcome any other suggestions/observations.

These U-bolts are never used. (I use an over the gunnel tie down strap.)

Good thing I never had to tow anyone.

I will replace these "plates" with 3/8 stainless, 4 x 6", 5/16" dia. holes, 1-3/8" OC, oriented vertically, not horizontally, as factory installed.

Suggest you all check these plates on your boats. Wonder what the bow eye backing plate looks like...
 
It looks like those "Blood Sucking Sea Creatures" are starting to spread throughout this site. This is the third or fourth thread they have infected. Bill or Mike may have to spray for them if it continues.
 
Blood suckers indeed - more like wallet and/or time suckers.

FYI - a great source for small quantity but high quality metal stock (eg, 316 SS plate for this application) is:

http://www.onlinemetals.com

I have used this service 4-5 times now. If you live in or are in the Seattle area, you can also drop by their place (in Ballard by Fisherman's Terminal) to: a) pick up your order and/or b) sort through their bone yard bins. I got 2 each 3/8 316 SS plates there today - sized just right, for $0. Great people.
 
Holy Shimolee - first time in years I've agreed with DoD. You've got a galv. prob. for sure and otta get to the root. Next, a case of vanishing motors or something worse.

My dentures looked like that until I changed to ..... (nameless brand).

Something new? Ohmigawd! Love it.

Dusty
 
Although I agree that it would be prudent to look for an electrical connection to other parts of the boat or motor, I'm not so sure you have evidence of a problem which necessarily extends beyond the backing plates.

Per Calder, there are three prerequisite conditions for galvanic corrosion:

"1. There must be two dissimilar metals, or dissimilarities within one piece of metal.

2. The dissimilar metals must be in contact with an electrolyte (an electrically conductive fluid).

3. There must be an electrical connection between the metals (other than the path provided by the electrolyte)."

We seem to have dissimilar metals, one known to be stainless steel, and the other presumably aluminum. If not aluminum, then perhaps stainless with significantly different alloys from those of the u-bolt and nut). And, since one was an apparently uninsulated backing plate, there would have been an electrical connection between the two.

But, what about the electrolyte? I think that was present, too. The forward side of the transom where the u-bolts are mounted is, in essence, a shower stall. Although sometimes dry, it's frequently wet with condensation, and there is no shortage of contaminants which would make the moisture electrically conductive.

So, there could have been two separate, isolated mini galvanic cells between each of the u-bolts and nuts, and their respective backing plates.

Replacement with stainless plates of the same general type (i.e., with the same alloys) as the u-bolts and nuts might well solve the problem. If not, and the aluminum was serving as an unintended sacrificial for something other than the u-bolts (i.e., if there was an electrical pathway between the backing plates/u-bolts and some other part of the boat or motor), the problem should show up in the form of increased corrosion elsewere, hopefully in the form of an increase in the rate of loss of the other sacrificials.

Let us know what happens.
 
Dogone Dory:

-does the boat stay in the water or on a trailer? Trailer. The boat is in the water monthly, on average 1 day at a time.

-do you have a battery charger hooked up all of the time? No.

-were there any ground wires connected to the fittings? No (ie, not bonded).

-was it the same on both sides of the boat? Yes - exactly the same.

I removed the backing plates last night. They are/were definitely aluminum or an Al alloy.

I agree this incident may be the sign of "unusual" galvanic corrosion. I think we need more data. We need more C-Dory owners to check the backing plates on their boats - material? condition? time since install?

I also agree with Almas Only - I too am a big fan of Calder. Corrosion via dissimilar metals is a fact. See http://en.wikipedia.org/wiki/Galvanic_cell. At a minimum the factory, upon choosing dissimilar materials, should have inserted an inert insulative layer (washer) between the two materials. Here's a quote from The Complete Rigger’s Apprentice – Tools and Techniques for Modern and Traditional Rigging 1998: ""Corrosion is always a problem in a saltwater environment, particularly when you mix antagonistic materials such as aluminum and stainless or stainless and carbon fiber; these materials are on different points on the galvanic scale, and when joined by the conductive medium of salt water they set up an electrical potential. The resulting activity corrodes whichever of the two materials is least noble (lower on the galvanic scale) [which aluminum is]. While not as serious with rigging as with permanently immersed items such as hull fastenings, galvanic corrosion can over time weaken aluminum and carbon fiber spars and clog such machinery as winches and blocks. The first thing to do is to take note of places where dissimilar materials are in contact. Then go about isolating them with some form of nonconductive bedding."

This web page: http://www.ocean.udel.edu/seagrant/publications/corrosion.html from NOAA contains good factual information too. While the surface area of the SS bolt vs. that of the aluminum is smaller (results in less corrosion), corrosion nonetheless occurs. See the last picture here: http://corrosion.ksc.nasa.gov/galcorr.htm

I agree using a similar material to the U-bolt (SS plate) may shift the "problem" elsewhere. I'll have to keep an eye on the sacrificial anodes.

I am really interested in learning whether others have or have seen this problem. If most folks have SS/Al combinations at this site and are experiencing no galvanic corrosion, the boat has a problem.

Also, I wonder whether the introduction of a voltage into the water via use of downriggers (about 0.8 volts) might have provided the electrical potential. If others have this problem, it would be interesting to know whether they are also using downriggers on a regular basis.
 
Longtime chemist here.

If those backing plates are truly aluminum, then my guess is that one of two things is at work:

1. some sort of current-driven process in which the aluminum has become sacrificial. Note that in a salt water environment, a salt water track along an otherwise dry surface will remain moist in humid air, and provide a conducting path. Were those U-bolts in use on the trailer (can't recall what you said)?

2. another metal, in trace amounts, has contaminated the aluminum. One which will do this is mercury, in the form of one of its salts. All it takes is a tiny amount of the salt, spread on the surface. This has the effect of forming a galvanic couple on the surface, which assists by continually destroying the tough oxide which normally protects the internal aluminum.

Number 2 can be quite dramatic. When I taught introductory chemistry I used to smear a dilute solution of mercuric chloride onto a couple square feet of aluminum foil as a demonstration. Within two minutes, the foil disintegrates, leaving a powdery residue somewhat similar in texture to those plates. Should not be any mercury anywhere near those U-bolts, but perhaps cadmium, from Cd-plated bolts elsewhere? I'm reaching on this!!
 
No the U-bolts have never been used - for anything, including trailer tie down attachment. I strongly suspect, as suggested, that the conductor is sea water and sea water humidity. Most of my use is in salt water.

As to the mercury and/or cadmium impurities ... Great, I had that crap all over my hands last night and raised lots of dust, inhaled as I crammed my hand and arm along the tops of the gas tanks to remove the nuts/frazzled plates. (Advice: get someone with long skinny arms to do this.) Maybe I should have what's left of the plates tested? Would these compounds still be present?

Maybe the increased mercury/cadmium levels in my body will make me resistant to the Blood Sucking Sea Creatures.
 
dogon's positive experience with aluminum backing plates in contact with SS fasteners jives with mine: after a couple years, I see a little corrosion of the aluminum, at the contact, nothing to worry about, even with no isolation.

Falco's U-bolt problem is exceptional.

As to testing the powder ... yeah, you might get someone to do a qualitative test on it, but because it only takes trace amounts of mercury to hasten breakdown of the protective oxide coating, it probably would not show up.

I would not worry about the dust -- probably not enough in there to matter, even if there is mercury in it, which is far from certain. If it were on my boat, I'd just replace it with herky aluminum, isolated, look around for other places where any aluminum lives, and check for similar corrosion. I'm betting you won't find any, and that there was something peculiar about those plates.

Check the replacements now and then, and that will tell you if the problem is gone.

I bet this is just something weird.
 
All:

Great feedback and info.

But my question still remains: what condition are your trailer tie down U bolt backing plates in? Do they appear to be made of Al? How long have they been in place? Do you use downriggers?

I'd really appreciate hearing from anyone willing to look at this on their boats.
 
Falco

Report:

My 1987 Cruiser has-

1. Stainless backing plates, BUT...

2. Regular, rusty steel nuts!!!

NUTS!!!

Not sure if I can change the old ones out easily, either!

Joe.
 
I checked the starboard backing plate last night. It is aluminum and appears to be in good shape. No rust on the bolts. The bolts were not cut off. I had them put on at the Factory about 3 years ago. The port one will be a little harder to get at as I have the updated glass covers and fuel tank attachment boards. Hope this helps, Big Mac.
 
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