Bonded Through Hull Corrosion

[Peregrine1983]

Hi All,

Curious to get the forum's take on this. I bonded my through hulls a few years ago, but now have noticed extensive corrosion around the bonding nut on only one of them.

This through hull also seems to be aging faster than the others and I may even have a slight leak from it, but have not confirmed.

The first photo is the through hull in question and the second is of two through hulls less than 30" away that seem OK. All through hulls are bronze Perkos and the same wire and terminals were used on all of them.

Any ideas on what might be going on?

[goboatingnow]

On a dry grp boat do not bond fittings and certainly don’t ground the bonding. US bonding practices are dated and ABYC suggested practices actually encourage impressed corrosion.

Keep all you fittings isolated and on big metals underwater fit a dedicated anode.

[deblen]

Originally Posted by goboatingnow

On a dry grp boat do not bond fittings and certainly don’t ground the bonding. US bonding practices are dated and ABYC suggested practices actually encourage impressed corrosion.

Keep all you fittings isolated and on big metals underwater fit a dedicated anode.

I agree-no bonding.
From your pics,it appears an iron nut was used on the bonding stud.Suggest you change to a bronze,brass,or SS nut. The existing nut is all that I see that is corroding. The natural green(verdigris) coating on the bronze seacock is likely caused by normal condensation dampness caused by cold seawater travelling in thru the seacock,cooling it's exterior below the dew point of the hot humid interior(engine room?) space.This causes sweating & is normal & harmless.
Cheers/Len

[Peregrine1983]

Thank you Goboatingnow and deblen for your posts.

I read a lot a conflicting info out there about bonding/no bonding - and in 2016 (when I bonded everything) I decided it was worth bonding everything because it made sense to me to keep it all at the same electrical potential.

Why is it that bonding is bad and what has changed? Is it that our boats are more electrically “active” now?

Deblen, I think you’re right about that nut. I’ll just swap it out for a good SS one.

Thanks again.

[ItDepends]

Bonding is a somewhat more complex subject... that a simple YES/NO answer.

People who say NEVER and ALWAYS on this subject are both wrong! It is a fact that there are old boats that are bonded that are in excellent shape, and there are old boats that are not bonded that are in excellent shape.

The more complex the boat, the more bonding is likely to be helpful, and anybody who categorically says NEVER BOND without having ANY information about your boat, you can pretty much ignore.

When you added a bonding circuit you added routine maintenance and testing to be sure it stays as it should be.

This is a complex topic, let's start with your bonding system. The following information is needed (and likely more too) to diagnose your problem. As you might guess from the list of questions, you are asking a question that is pushing what can be done without hands on testing...

What kind of boat?

The thru-hull that has the issue, where is it? What does it supply? How many thru-hulls are connected to the bonding system?

What is connected to the bonding system OTHER than thru-hulls?

Is the bonding circuit connected to a zinc? Size of the zinc, and its location? Is the zinc in good shape and is the connection KNOWN to be good? How long do your zincs last?

Is your bonding system connected to battery negative? Is that connection at ONE place? Is it possible there are other connections to battery negative you do not know about? Have you looked for them?

Do you have any ground leaks? If you say "no" how did you test?

Do you have an AC system on board? Are you plugged into shore power? Do you have a galvanic isolator or isolation transformer? Is your AC Safety ground connected to your bonding system?

[Rumrace]

Copy that brass bronze or zinc nut on top.
Not sure bonding is the best way to isolate dangling things unless hooked up to a mercathode. Salt water being briskly run against metal with a welcoming ground?
Sounds like the Tesla water generator.

[Cheechako]

Bonding can be bad or good. Hard to figure.
I usually go the "if it ain't broke dont fix it" route.

If building new I would NOT bond personally. You are just inviting corrosion or flow of electrons from shore power etc if you ask me. "how do you get electricity to flow? you wire things together!"

The boat in my avatar was bonded at the factory and never an issue though. Passed survey fine 30 years after new. All solid bronze tapered seacocks.

[goboatingnow]

Originally Posted by Cheechako

Bonding can be bad or good. Hard to figure.
I usually go the "if it ain't broke dont fix it" route.

If building new I would NOT bond personally. You are just inviting corrosion or flow of electrons from shore power etc if you ask me. "how do you get electricity to flow? you wire things together!"

The boat in my avatar was bonded at the factory and never an issue though. Passed survey fine 30 years after new. All solid bronze tapered seacocks.

Correct today our boats are very electrical active. Bonding that includes dc negative , ac earth or seawater earthing points is just a recipe for impressed corrosion.

In the past with damp wooden boats the practice developed to centrally bond fittings to an anode . Then our boats got more and more dc systems then ac systems and shore side deficiencies resulted in on board safety compensations and ac, dc & fittings being bonded together resulting in leakage currents all over the place and into the fittings.

With a dry grp hull it’s far better to simply isolate the fitting. With no circuit there is no ( less ) opportunity for corrosion currents

Your fittings will thank you.