galvanic corrosion

[alantani]

from a friend of mine!

Galvanic Corrosion
The following is offered as a primer reflecting the basic information, and as an overview of the topic. Consult a trained technician or consultant for specific issues, applications, and in depth analysis of galvanic corrosion and stray current. When we say a marina is 'Hot', do we mean that the shore power is carrying stray current, the water is carrying stray current, or galvanic corrosion is occurring near the boat? Understanding this stuff is not as murky as it might appear to be. One can feel dumb as mud trying to understand boat electrical current problems, because we are dealing with three different currents that are interconnected -- DC, AC, and the Galvanic potential between different metals in sea water. Think of it like 3 different systems that are connected to each other at one common location. The engine block is the most common connection; although not the best location since the engine cooling water floats at its own potential from the seawater. ABYC recommends that the DC battery negative, the AC ground (the green colored wire), and the bonding system be connected at one point. Also, when trying to understand the standards and recommendations don't confuse what is a safety issue for the vessel with safety issues for people. For example, DC galvanic corrosion can destroy a propeller, and an AC current can electrocute a swimmer if in the direct path back to the source. Keep in mind the difference between bonding for galvanic corrosion, stray current in the boat, stray current outside the boat, and galvanic corrosion surrounding the boat. Also, the term 'electrolysis' is often misused. Electrolysis means the decomposition of an electrolyte by electric current and is not a useful term for this discussion, as the electrolyte in this case is the saltwater.

1- Galvanic corrosion on a boat -- this is the result of two or more metals with more than 200 millivolts potential difference on the galvanic chart that are immersed in the electrolyte seawater (1,000 millivolts = one volt). This is what sacrificial zincs and bonding are for.

2- Stray current in the boat -- like a hot DC bilge pump wire in the bilge. This is a common problem that can be easily tested for by a technician. An old bilge pump can produce enough carbon dust to carry current to the bilge water.

3- Stray current from outside the boat -- stray current corrosion is typically DC, not AC - the electrons need directional flow. In most applications, AC does not cause stray current corrosion, as AC current takes the most direct path back to its origin. What happens here is the DC current is carried through the AC ground (the green wire) to and from shore. Having the vessel properly grounded and adding a properly wired galvanic isolator will eliminate this problem. Larger boats often use an isolation transformer to isolate the shore AC from the ship AC. Properly wiring an isolation transformer will isolate shore current from the boat.

4- Galvanic corrosion can be caused by DC current from improperly grounded boats close to steel pilings, other boats within about 10 feet and jettisoned noble metals on the bottom. This is commonly referred to as a 'hot' marina. You can protect your boat from this by dropping a zinc over the side with a wire attached to the boat's bonding system or propeller shaft(s). Other than the zincs, the propellers are most likely the least noble metal under water. One can buy a zinc 'fish', with a wire and clamp attached, from a chandlery - or make your own -- this is a must have for aluminum boats and inboard / outboards dockside. However, a vessel can also have too much zinc protection.

There are portable and built in corrosion meters available to monitor proper zinc protection. Or, learning how to use a good multi-meter with a half-cell (zinc or silver/silver chloride are most common in the USA) is the key to solving current problems. The above applies to vessels in saltwater - since fresh water is not a good electrolyte, the corrosion issues are usually less. However, there is a much greater likelihood of electric shock from AC current in freshwater.
__________________
Tom Mattusch
Huli Cat
650 726 2926
Safari Club International
First for Hunters

[Bill B]

Good to know.....thank you for sharing....

[Dominick]

I did a crab and rockfish trip on the Huli Cat.  Tom is a good captain.  I never knew about a hot marina.  Good info.  Dominick

[oc1]

I wonder what happens if you have too much zinc protection?  Never heard of that either.
-steve