BruceK
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Should a stainless steel thru hull be electrically bonded, the same as a bronze one?
Should a stainless steel thru hull be electrically bonded, the same as a bronze one?
Many theories here to be sure, how ever one or two things to note,
1) If you connect (electrically bonded) your SS thru hull to other non SS (such as BRONZE then you have connected two different metals with differing values on the Galvanic scale and with the Bronze being less noble it could behave like a sacrificial anode (in the right circumstance's) meaning the Bronze will react prior to the SS (that is if the SS is a marine grade such as 316 or 316L and has been "passivized"(which means usually dipped in a mild nitric acid solution to remove any surface free iron and or other impurities that can interfere with SS ability to resist corrosion, So IMHO you are best not to connect to other thru hulls if a different metal.
2) Where you have these two materials together(and electrically bonded) such as your SS prop shaft and Bronze prop you ALWAYS use a zinc anode (Right!) Dont think I will get to many arguments there? so using that very same theory bonding the thru hulls in different materials with out zinc protection will be a possible issue sooner or later,
Cheers Steve
Tom, I see that too, but if we read 2, it suggests that without bonding, where there are dissimilar metals there will be a problem.CS
Your number 1 is nicely dealt with by having a zinc plate in the water with the boats bonding system tied to it. Without the zinc plate (Al too in some cases) it is not a bonding system in the realm of our small boats.
So your answer to the OP should be no problem maybe?
Tom, I see that too, but if we read 2, it suggests that without bonding, where there are dissimilar metals there will be a problem.
I`m fairly confident the ss fittings will be 316, but passivated? No idea and no way of finding out 7 years on.
My current shipwright frankly admits it`s not his area and suggests getting the local sparkie(Aussie for electrician)to take a look.
Not hard to bond them if we decide to, the unconnected strip is right there.
Are there downsides to bonding? Using that timeworn phrase "What could possibly go wrong?"
Steve, I considered starboard as a pad material for its non degradable non compressible properties, but as I doubt anything adheres to it, it becomes difficult securing the thru hulls using it.
Thanks Angus, I searched finding this:https://www.groco.net/products/valves-seacocks/flanged-valves/backing-blocks.
It looks like a better solution than a piece of square half inch teak.
Bruce,
I dont see any connection with wood backing plates and being bonded or not, galvanic corrosion is caused by two (or more) dissimilar metals,
Cheers Steve
Thanks Boatpoker. That accords with comments by the hull surveyor.
The article suggests wood degradation occurs in the presence of corrosion. That being so, should I expect to see corrosion when we pull the s/s thru hulls? And if so, replace with bronze?
This is not quite correct. The acids and alkalinity created around a galvanic reaction do cause wood decay. See attached
Just to note here it's not really comparing Apples with Apples how ever as the article uploaded is basically comparing wood with STEEL and IRON (NOT 316 SS or 316L). Both Steel and Iron and even Bronze have decades of recorded reactions with damp wood and yes wet wood will provide the catalyst/fluid to complete the cycle required for Galvanic issues to take place but again for a TRUE Galvanic reaction you require the THREE elements (two dissimilar metalsand the electrolyte where as in this specific case being discussed does not apply, as you have only one metal being SS,can damp or wet wood cause all sorts of misunderstood issues YES, thats why we now go with G9/G10 or similar material (and no wood/not even epoxy soaked wood).
As earlier stated corrosion/erosion/Galvanic or other wise is a highly controversial issue,
Finally the change to Bronze (check if Silicon Bronze or Manganese Bronze) could be worse in this case(if not changing the wood backing plates) as Both Bronzes are less noble than (316-316 L -Passivized)meaning in a Galvanic series situation Bronze will react sooner than (316/316 L passivized.
I do agree with the use of synthetic thru hulls and NZ has a fairly new range called TRUDESIGN which have impressive testing and even fire ratings and varying government approvals including the well respected Bureau Veritas Marine Division Approval.along with 9093-2 classisications
I have fitted numerous TRUDESIGN thru hulls both on commercial type craft and pleasure vessels in the past 3-4 years in our yard and can applaud there design both in strength and easy to use condition, found Here:Seacocks And Through Hull Fittings | TRUDESIGN.
Also to note today there are many more shafting materials available better than SS 316/316 L both in corrosion and strength,
Just Saying!
Cheers Steve
Manganese bronze was not even mentioned as it is actually in the "brass" class due to it's high zinc content.
There is no confusion or controversy regarding galvanic corrosion among the educated. It is the most straight forward of all the corrosion processes.
Like steel and iron, "stainless steel" is a ferrous metal and suffers from many of the same maladies to differing degrees.
A galvanic cell can develop within within in single piece of metal due to impurities as has been shown by the well known issues of metals imported from a certain country.e.g. Groco recall.
Yes, there are many other materials for shafts however 98% (my guess) of the pleasure craft market uses ss.