Galvanic Corrosion Problems And Questions

[delatbabel]

Hi all,

I have some galvanic corrosion appearing on the keel of my steel-hulled boat and I'm looking at ways to resolve this. So I have some ideas and questions for the electrically minded.

As far as I can tell there is no bonding system on this boat, so in addition to going over the various circuits on board to try to isolate any ground leaks, I'm considering installing a bonding circuit. Nigel Calder talks about one extensively in his "Boatowner's Mechanical and Electrical Manual" but I have a few questions.

Firstly, there is a switch on the negative side of the battery leads, should I remove this and switch only the positive side? Wiring the negative side to ground?

I have a fair bit of electrical equipment at each end of the boat (nav lights and anchor winch on foredeck, lighting and other electronics in aft cabin, as well as a hydraulic pump for the autopilot). I'm considering running a long ground cable the length of the boat under the cabin sole, using reasonably heavy (6AWG or so) tinned cable, tied to a few distribution points along the way. Does this sound reasonable?

If so, should I connect the ground cable to the hull of the boat at more than one point along the way, or at one point only, or not at all? Nigel Calder talks about bonding the ground to the zinc anodes around the boat but of course being a steel hull my anodes are connected directly to the hull on the outside and don't have connections or protrusions into the interior of the hull of the boat (and in any case the two anodes on the rudder would be inaccessible). So I'm thinking I should wire the ground to the hull and let the hull do the conducting.

Del

[redbopeep]

Originally Posted by delatbabel

Hi all,

I have some galvanic corrosion appearing on the keel of my steel-hulled boat and I'm looking at ways to resolve this.

Del

Are you in a marina or cruising? If in a marina, you might wish to get a half-cell and use it to see if there's a stray current that's causing your problems. Within a few short weeks you can have problems if another boat is leaking current or if the shore power is faulty. Further, with a steel hull, you probably should have a half-cell to check anytime you get into a new marina.

If you're on the hook all the time and all you're worried about is your own boat, that's a different issue. Bonding a boat is risky business. I won't comment on it further than that.

Best of luck to ya

P.S.

Hubby is looking over my shoulder on this post, and says he thinks every circuit you wire should have a positive and negative carrying the current to and from your battery and those two lines should be next to each other the entire route to minimize induced electromagnetic fields. You do not want a common ground. EM fields induce currents which can cause problems.

[delatbabel]

Hi redbopeep,

Thanks for answering.

I am in a marina and I suspect that this is making the problem worse but it commenced while on the hook, so I'm sure that it's not the problem. I don't have a shore power connection and don't run AC on board -- pretty much all of my stuff runs off 12V and for the few things that I need an AC charger for (drill, one laptop) I either run a temporary cable to the shore power outlet for the duration or charge them at my office.

I'm investigating the half cell option however and I've found one local supplier after some searching.

Yes I'm sure your hubby is right about the circuits, I'm more concerned about the run from the battery to the main distribution board, which has a switch on both the positive and the negative sides of the circuit. I'm sure it should not have a switch between the negative bus on the distribution board and the negative pole of the battery.

[Istioploos]

Hi Del,

I agree with edbopeep's advice.

I do not think that the switch on the negative side is a problem. It is a safety feature for completely turning off power in case of an emergency (fire) by isolating the batteries from the rest of the boat. I cannot see how it will contribute to galvanic activity. Most of which is caused by either dissimilar metals or by stray induced currents. These you will be able to detect with the half cell.

Good luck

[delatbabel]

I'm having trouble sourcing a half cell. The two Australian suppliers I've tried have both reported that the ones they stock aren't being made any more, and they can't get them in. Anyone else found a supplier?

[seaescape]

Hi guys,

Having a aluminium boat I think I should learn something about this problem. Firstly can someone tell me what is a half cell, what does it look like and how is it used.

Regards

Kevin

[MMNETSEA]

Hi Del,

The problem of galvanic corrosion on Corten steel is rarely occasioned - If areas of the steel are unprotected from salt water normal iron oxidation takes its normal course.

If the boat is out of the water, have the area sand blasted - then treat affected area with a good epoxy primer followed by an epoxy hard coat then your normal anti foul. Check all zincs, replace if showing a white and powdery skin.

Richard

[redbopeep]

The half-cell is made from silver/silver chloride (a silver wire coated with silver chloride). It is immersed about 6" deep in the water around the hull. The half-cell can be called a reference cell or reference electrode in discussion, btw.

In use, it's connected to a multimeter positive terminal. The negative terminal is connected to the boat ground (e.g. the battery negative ground point). Then, the multimeter is set to DC volts. The value you see (in Volts) is the hull potential (lets say you're concerned about your steel hull). You can also just check to see if a particular element, say the propeller, is adequately protected with a zinc, etc, on a fiberglass or wood boat. In that case, you'll measure both with and without the zinc in place. You'd be measuring with the half-cell in the water near the propeller and you'd have to attach the negative terminal to something inside the boat connected to the propeller--say the propeller shaft. There's a range of voltages that are "ok" for good zinc protection. I believe Nigel Caulder talks about this in one of his books. Further, you can get Everett Collier's "Boatowner's Guide To Corrosion" which, as I recall, has some reference info in it regarding half-cell measurements for placement and sizing of zincs.

If you have a metal boat, these measurements are something that you want to make in any marina environment you go into. Even just having a concern for the metal "parts" of your fiberglass or wood boat--thru hulls, propeller, rudder stock, etc, point to the need to measure. There's really no way for you to know if the marina is inducing a current and you're not adequately protected with zincs w/o such a measurement.

Half-cells are hard to come by. Caulder, in one of his books, rather nonchalantly just says get one at a chandler. But, its not possible here in the US to get one that easily.

Google search will show a lot used in the Civil Engineering field. It used to be that a good chandler would have them for marine use and less expensive than the ones intended for professional engineering use. Here links to a couple companies that make or sell them: link 1 link 2.

You might check with a local marine surveyor familiar with steel hulls and ask him/her where they have purchased a half-cell for their use.

Best of luck

[MMNETSEA]

Returning to Steel Hulls and RUST - Many hulls will be seen with serious rusting occurring NOT on the outside but on the inside of the hull at places difficult to examine. Some builders left out 'Limber' holes - which allowed water to collect (salty). Some encapsulated ballast in a heavy skin of cement - the cement eventually allowed water to permeate and get to the hull.

Planned and scheduled 'Self- Survey' of all the difficult places on steel boats can pay dividends.

[redbopeep]

Originally Posted by MMNETSEA

Check all zincs, replace if showing a white and powdery skin.

Richard

Richard, I take it this would indicate a "protective" oxidized coating on the zincs and thus we assume they will not adequately protect the other metal on the boat? If so, why not simply abrade/sand away the "skin to reveal fresh zinc if there is adequate mass left to he zinc anyway?

[MMNETSEA]

Brenda, I should have added the temporary solution of grinding off this skin If there was sufficient mass left - however a problem often seen is where this residue is concentrated at the zinc's bolt and nuts. I still would take the opportunity when the boat is out of the water to replace zincs - especially when the boat is kept in a marina, the 'repaired' ones stored as back ups or hung of the side of the boat on a length of wire.

A quick addition :- the anode/s hanging over the side are useful in providing what is going on in the depths

[osirissailing]

corrision on metal boats - steel. Are you sure your boat is made with Corten steel? If not, I have seen normal steel dissappear in less than a year. Also what kind of bottom paint are you using? If it is copper, then you must have a really good barrier primer on the steel hull before the copper bottom paint is applied. any pits or scars from collision with debris or docks, etc. can expose the raw steel to seawater. Then you will have a wonderful battery formed between the copper bottom paint and the steel hull. Also your zincs should be attached to the hull steel - but - not to any electrical grounds. You install grounding plates for electrical grounds. The idea is to isolate the steel hull from any active electrical circuits.

[Yttrill]

Originally Posted by osirissailing

Also your zincs should be attached to the hull steel - but - not to any electrical grounds. You install grounding plates for electrical grounds. The idea is to isolate the steel hull from any active electrical circuits.

This is impossible on my boat. I have an auto engine which uses the block as the ground. The engine has an alternator so the battery ground must be the engine block too. And then since I have a metal propeller shaft with a steel bearing connected to the hull, the hull is connected to the engine start battery ground already. Since the connection is problematic it is essential to add a secure bond from the engine block to the hull.

My house batteries are also grounded at the same point, this is a good safety feature, otherwise the two grounds would float with respect to each other allowing static discharge: alternator belts come to mind as a source of static electricity.