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Diversion Guides > Sailing > Engine Shop > Marine Corrosion > Stray Current Corrosion Basics
Submitted by: BoatUS
We’ve discussed what galvanic corrosion can do, using just the electrical potential in metals. Imagine what happens if you add more electricity. That’s exactly the basis for stray current corrosion.
Stray current corrosion occurs when metal with an electrical current flowing into it is immersed in water that is grounded (such as in any lake, river, or ocean). The current can leave the metal and flow through the water to ground. This will cause rapid corrosion of the metal at the point where the current leaves.
Stray direct current (or battery current) is particularly destructive. Stray current corrosion can cause rapid deterioration of the metal. If the metal in question happens to be an aluminum part like your drive unit, it can be destroyed in a matter of days.
Stray current corrosion is different from galvanic corrosion in that galvanic corrosion is caused by connections between dissimilar metals of your boat’s drive components, and utilizes the electrical potential of those dissimilar metals.
Electrons flow from one dissimilar metal (the anode) to another dissimilar metal (the cathode). In stray current corrosion, electricity from an outside source flows into your boat’s metal components and out through the water for a ground.
For example, your boat may be sitting between a boat leaking DC current and the best ground for that current. Rather than the DC current moving through the water to ground, your boat could provide a path of lower resistance.
The DC current could enter a throughhull fitting, travel through the bonding system, and leave via your drive to the ground. Remember that corrosion occurs at the locations where DC current leaves metal and enters water.
Stray current 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 almost always related to shorepower connections. A boat with internal stray current problems can cause accelerated corrosion to other boats plugged into the same shorepower line if they provide better ground. The stray current would be transmitted to other boats through the common ground wire, but can and should be blocked by installing a galvanic isolator.
A much more subtle, but potentially more damaging cause of stray current corrosion can occur without any electrical problems. Supposed you cruise back to your marina after a weekend on the water, and plug into shorepower to recharge batteries using your automatic trickle charger. Then you go to work for the week.
On Monday, a large steel hulled boat (with scratched and scraped paint) ties up next to your boat. This boat is also plugged into shorepower and goes visiting onshore for a few days. A battery has just been formed -- the large steel hull and your small aluminum drive connected by the shorepower and ground wire.
Depending on the proximity, relative sizes, and how long your neighbor is ashore, when you go out the next weekend you may find your drive highly deteriorated. This unfortunate scenario can also be prevented by the installation of a galvanic isolator.
There is greater danger for boats that connect to AC shorepower: destructive, low-voltage galvanic currents (DC) passing through the shorepower ground wire. Normally, AC is not a corrosion problem, but because the boat, pier, and wire are all connected, or due to a leakage, there can be direct current (DC) also present. This is potentially very damaging and requires additional protection.
Safety regulations require a three-wire cable for carrying shorepower aboard any boat, and that one of these leads grounds all electrical and propulsion equipment to shore. This safety procedure reduces the danger of shock, but also connects the underwater metal components on your boat with metal on neighboring boats using shorepower, steel piers, and metal objects on shore that extend into the water.
This interconnecting of dissimilar metals allows destructive galvanic currents to flow between them. If these currents are allowed to continue, your drive unit will experience severe corrosion damage in a very short time -- as little as a few days.
There is a common misconception that you can overprotect your drive by using too many zinc or sacrificial aluminum anodes. This is not true. The corrosion potential of any metal is a voltage that can be measured by a reference electrode. Such measurements in water commonly are made with a silver/silver chloride reference electrode.
The corrosion potential of a sacrificial anode is a characteristic value for that metal, and it does matter if you have one piece of the metal or 100 pieces. The corrosion potential stays the same. Of course, 100 anodes would be expensive, heavy, and a considerable drag under water.
Only by increasing the corrosion potential by using a different anode material (such as magnesium in seawater) can you overprotect your drive.
Courtesy of Quicksilver Marine.
