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**GordMay** · 04-12-2007, 10:23

Quote:

Originally Posted by seaclusion

... There are three types of corrosion underwater metals are subjected to---Electrolytic, Galvanic, and Stray current...
Richard

I believe Stray Current & Electrolytic Corrosion are the same thing.

seaclusion · 04-12-2007, 10:31

Gord,

I will clarify. By stray current I mean voltage/current from a source outside the boat (such as from a nearby boat or poorly insulated/isolated shore power)and electrolytic corrosion from a voltage/current source within the boat (battery grounding/isolation problem).

Richard

Kanani · 04-12-2007, 11:18

Quote:

Originally Posted by seaclusion

Gord,

I will clarify. By stray current I mean voltage/current from a source outside the boat (such as from a nearby boat or poorly insulated/isolated shore power)and electrolytic corrosion from a voltage/current source within the boat (battery grounding/isolation problem).

Richard

You are right seaclusion. That is a huge problem. Especially in marinas. At anchor, not so much.

Pura Vida · 04-12-2007, 11:31

http://www.cruisersforum.com/forums/...the+hot+marina

Eng Nate covered this topic pretty well...

**GordMay** · 04-12-2007, 11:45

I tend to get caught up in semantics, so please pardon my pedantry.

Galvanic & Electrolytic (Stray Current) corrosion are two similar, yet different ,types of corrosion.

Galvanic corrosion results when two dissimilar metals, of sufficiently different potentials (about 0.1 V on the galvanic series) (a galvanic couple can also occur within regions of a single alloy), are electrically connected (bonded; deliberately or not), and immersed in a common electrolyte (water). The same electro-chemical reaction creates the current flow in a storage battery.

Stray Current (or Electrolytic) corrosion results when an applied current flows through immersed conductive materials, other than an intended electrical circuit.
Stray currents can come from an outside source either internal or external to your boat. Internal sources involve a short in your boat’s wiring system, such as a poorly insulated wire in the bilge, an electrical accessory that may be improperly wired, or a wire with a weak or broken insulation that is intermittently wet.
External sources are most often related to shore power connections, or other boats on the dock .

To generate electrolytic corrosion, the stray current must flow onto the metal at one location and then flow off the metal at another location.
When an applied current passes in and out of a metal part, an electrolytic cell is set up, resulting in a localized anodic site, where the positive current exits the metal part. This causes the local oxidation (corrosion reaction) of the metal piece, which may lead to a very rapid metal loss, which is limited only by the amount of current available.

cburger · 04-12-2007, 12:30

Seacocks in picture appear to the Spartan brand, very good quality.

Zach · 07-12-2007, 14:48

So it sounds like all metal fittings should be bonded together. My rudder shaft is currently not bonded to anything. The rudder tube comes up to the cockpit sole.

Should the rudder shaft get its own zinc, or should I use the rudder head as the ground point? (Figuring a long strap to a through bolt in the side of the cockpit.) I've got a bronze tiller head, and a stainless shaft. Would this cause corrosion between the two?

At this point I am thinking to run wires from each through hull, to a bus bar with a wire running to the batteries negative post.

Found this article interesting too. Though the pictures make me ponder plastic fittings... http://www.marinelosssurveyors.com/Article/2T-H.pdf

Pura Vida · 07-12-2007, 15:08

Zach,

My 27 needs only one zinc and it is on the propshaft. Everything else is bonded to that via a groung on the engine. I would suggest that you bond to the engine block and not the battery. Grounding the rudder is a bit unusual on boats as small as ours ( at least I have not seen it in thirty odd year of sailing.) I wouldn't sweat the rudder.

Quality plastic through hulls work fine. The issue is that on some of the older boats there is still life in the original high quality bronze throughhulls, if they have been maintained.

good luck

pv