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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #11  
Old 10-06-2006
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stainless or stain-less

I aggree, that stainless requires oxygen to form the chromium oxide that is the surface protection that keeps stainless from disapearing like carbon steel. Chlorine and chlorides are among the hardest things for stainless to deal with, and pitting corrosion on surfaces which are closely mated to gaskets or other sufaces is a common problem when chlorides are present.
I work in an industry that provides equipment and vessels to the pulp and paper industry and chlorine is our enemy. As a general rule (with inumerable exceptions) we use 304/L for acidic environment, 316/L for caustic and duplex stainless such as 2205 or 2304 for chloride contact. Where heat and chlorine combine we will often use 254 SMO or Inconel.
There is another situation that stainless is subject to called "stress-corrosion cracking" or pitting which is loosely the errosion or hairline cracking of the surface of the metal in areas that suffer constant flexing or stressing in a corrosive environment. I have seen situations where a tiny pinhole was the entrance to a major cavity which showed up upon ultra sonic testing.
I don't know what alloy my keel bolts are but I might suggest that "rockter" specify a duplex such as 2205 for his replacement shaft. It will be roughly twice as expensive for the material cost and a little more for machining, but it will last longer than him.
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Old 10-06-2006
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Exclamation Like I said Do a little research

You probably got a great quality stainless from China. Do a little research, It's not hard. I thought you would take up on my hints.. Look at duplex stainless. Look at 304 etc. Your shaft probably got some surface cracks from a improperly aligned motor, hitting something, Hell it could of been 40 years old for all I know. I do know if it was the right type (duplex) and you didn't do something to mess it up, it will outlast you.

Being an EE in the marine industry for over 30 years I am not an expert on metalurgy. However in that length of time I have been under over in around boats of all types. I am capable of absorbing other things then just wires and stuff. Do yourself a favor open a book, your eyes and think about things before making blanket statements.

Fair Winds
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Old 10-08-2006
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Gosh Captain Dave, you really must have your eyes open much wider than mine.

As it is, I can see a big corrosion pit on the propshaft of my boat.

You see cracks where the motor is out of line or having run into something.

I remember lining it up with a feeler guage a while back, and I don't remember running into anything.

Hell, maybe it is all bent and haven't noticed and maybe that big corrosion pit really is a crack.
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Old 10-08-2006
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Smile bolts

I am new but if you use 303 stainless for bolts would this work?
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bolts

One other possibility Titanium bolts?
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Old 10-08-2006
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AFAIK the correct grade of stainless for keel bolts is called "marine bronze". Like the stainless kitchen knives with horrible perpetually dual edges that were oh-the-rage after WW2...stainless just isn't always the right answer even if it sells well to the public. MARS Metals, the keel folks, probably could tell you where they obtain keel bolts from.

For prop shafts I think the trade name "Aquamet" is one of the known and proven shaft materials. Prop shafts are probably easier to obtain than similar size alloy rods, is there really any savings to trying to DIY on this?
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SS+Lead=Battery, No?

Based on my experience, and my understanding of the galvanic table, lead and stainless (or bronze, titanium, etc) creates a battery. In this situation your lead will be anodic, and will slowly eat away. I suppose an upside is that your SS will be safe . I would be very careful to have a well zinc'd underbody to prevent the degredation of your keel. Note: this will occur whether or not your SS and lead are bonded.

Lead has a galvanic potential of ~-0.2 and your other metals will be slightly above 0. This gives you a nominal potential of >0.2V.

See this link for more info:
http://www.corrosionsource.com/handbook/galv_series.htm

On a recent underwater experiment that I participated in, a series of lead pigs, secured with SS bolts were left in the marine environment for a month. When everything came out of the water, the lead pigs had about 10% of thier volume eaten away.

Anyone else have thoughts on this?
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Old 10-09-2006
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Marine bronze is a bronze. I don't think it's a stainless steel.

It's probably not a bad material for the keel bolts, but I am sure it will not be as stiff.... it will have a lower elastic constant.
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Old 10-09-2006
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Quote:
Originally Posted by andrewburcham
Anyone else have thoughts on this?
Are SS keelbolts used in conjunction with a lead keel an unusual situation? I cannot remember ever seeing a lead keel suffering significant galvanic corrosion.
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Old 10-09-2006
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Rockter-
"Marine bronze is a bronze. I don't think it's a stainless steel."
My point exactly. (I have a good poker face online, don't I?)

Andrew-
Bear in mind that the lead used in "lead" keels is not 100% pure lead, it has substantial amounts of other materials, some by intent, some by accident. As I understand it, keels aren't considered a critical alloy and that means recycled lead, tire weights with antimony and other content, etc., may all be in there. I'd want to look into exactly what alloy was being tested/used before taking any test as "substantially" correct. (Perhaps you'd know?)

I know lead isn't totally inert, but who runs around with a bare lead keel anyhow? There's usually a heavy layer of old paint and more copper-bearing bottom paint over the lead, which essentially jackets it in sacrificial copper versus any stray current to other parts of the boat. And only a fool (or a prop saft salesman?) would be running a metal shaft and dissimilar metal prop without some zincs up there...wouldn't that make the lead keel a "distant" issue? Literally?

Keelbolts are theoretically embedded dry, solid, "bonded" by the lead cooling around them as they are cast in place. Galvanic corrosion in there shouldn't be a problem unless you like to keep a wet bilge. Heck, the classic way to build wrought iron fences was to pour molten lead into the ground (or masonry) and then stand the wrought iron in the lead, cast in place. When fencework is done that way it lasts hundreds of years, versus rusting out in a decade when the same ironwork is simply embedded in concrete or cement (the modern way). Lead and iron manage to live together very nicely.
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