The first thing that hits people whose underwater fittings have suffered from stray current corrosion is the rapid rate of metal deterioration, a common refrain being, "It looked fine last week!" Unlike galvanic corrosion, which works relatively slowly, stray current can dissolve metal fittings in a matter of days. This greatly reduces the likelihood that someone will discover the corrosion before a more serious incident occurs, such as sinking, loss of steering from a failed rudder post, or a broken strut or prop shaft-any metal underwater fitting is vulnerable. Fortunately, the chances that your boat will be affected by stray current corrosion can be greatly reduced if proper wiring and bonding practices are observed. That's good, because corrosion coverage is excluded under all marine yacht and boat insurance policies.
By far, the most detailed marine standards developed by the American Boat and Yacht Council (ABYC) pertain to electrical wiring. The standards specify requirements for wire installation, type and size, insulation, terminal connections, grounding and bonding, and a host of other details that protect the boating public and give boatbuilders infinite headaches. Many electrical requirements are based on one underlying objective: keeping the electrical current contained in the electrical system. Whenever current escapes from the electrical system, it's called leakage current, or stray current.
If a wiring circuit is properly sized and in good condition, there is no reason for the current to stray from its proper path to the appliance and back to the battery; electrical current will always seek the path of least resistance back to its source. For current to stray, there must be 1) faulty or deficient wiring that allows the current to escape, and 2) an alternative, parallel path through another wire or water for the current to flow through. Given two paths. the amount of current flowing through each path depends on how much resistance is in each path. If there is low resistance in the proper path, less current will escape. Suppose two wires chafe together in a wet area; there will be three parallel paths (two wires and the water), each with its own resistance and corresponding amount of current flow.
Stray current that flows through other wires can return to the battery with no corrosive effects (although it can be a fire hazard). Stray current that flows through bilge water or seawater along any part of its alternative path, however, can cause severe corrosion. More specifically, the metal that feeds the positive stray current into the water will likely be driven "anodic" and will corrode; conversely, the submerged metal that receives the stray current becomes "cathodic" and won't corrode. Stray current corrosion is identical to galvanic corrosion, but the corrosive potential of the stray current is increased many times by the output of a 12-volt battery, which can easily overwhelm the natural anodic/ cathodic polarity between any two dissimilar metals immersed in seawater. Put another way, a battery is nothing more than a grouping, or "battery," of galvanic cells linked together to produce 12 volts; therefore, the potential of stray current from a battery of galvanic cells can far exceed the output of a single galvanic cell, depending on the severity of the leak.
In real time, this means that galvanic corrosion may take weeks and months to dissolve a through-hull, whereas stray current can dissolve the same fitting in hours and days if the stray current is high and uninterrupted. Like super-charged galvanic activity, the submerged metal that feeds the stray DC current into the water will lose electrons-the glue that binds metal molecules together-at an accelerated rate, resulting in rapid corrosion. (Note: AC current does not cause corrosion. See "Stray AC versus DC In The Bilge")
Considering the two conditions needed for stray current corrosion to thrive-deficient wiring and an alternative path through the water-the following measures will help prevent its occurrence.
Keep wires high and dry: Since dampness and water can provide a path for stray current, all wiring should be routed as high as practical above the bilge water level. Exposed wire connections in a damp or wet bilge are an open door for stray current. If wire connections must be made near the bilge (e.g., mast wires left short at the base, short appliance wires), they should be enclosed in a weatherproof junction box to seal out moisture, or individually sealed in shrink ,rap and secured as high as possible. Liquid electrical tape or corrosion inhibitor sprays can also help seal terminals from dampness that may infiltrate enclosed junction boxes.
Provide a proper path to ground: Always provide a dedicated, negative wire (the same size as the positive wire) to each appliance, which returns all the way to the negative bus at the electrical panel (or directly to the battery for bilge pumps). Remember that the negative side of the circuit - not just the positive side-is a currentcarrying wire that can also leak destructive stray current.
Protect wire insulation: Wire insulation needs to be protected from damage caused by chafing against sharp comers and moving parts, staples, and errant fasteners. Damaged insulation is also a common source of boat fires.
Ground DC appliances: Appliances that develop an internal current leak to their metal case can be a main source of stray current. Just as the AC green grounding wire is designed to carry stray AC current from the metal appliance case back to shore to protect people, the DC grounding wire is designed to carry stray DC current from the metal appliance case back to the battery to protect the boat. The DC grounding wire is a third wire, which is completely separate from the current-carrying positive and negative wires. It connects to the metal case of any DC appliance, such as a pump (bilge, sump, fish well, wash-down, and freshwater), deck winch, or electric toilet. (Plastic cases do not conduct current, so there is usually no grounding wire.) From there, the grounding wire should run to the grounding bus, which is connected to the engine and battery (see figure 2). DC grounding wires should be no more than one size smaller than the current-carrying wires to the appliance.
Avoid faulty wiring: Switches should always be wired into the positive side of the appliance circuit. The classic example, which is surprisingly common, is the bilge pump float switch that is wired to the negative side of the pump circuit. Every time a bilge pump starts up, it throws tiny particles of carbon inside the housing ("carbon tracking") that can eventually conduct current to the pump shaft sitting in bilge water. If current is constantly running through the pump on its way to the switch, it will still work as expected, but the pump shaft can continuously feed stray current into the bilge water, whether the pump is actively running or not. When the switch is wired properly, carbon tracking is still a threat, but the stray current output is at least limited to when the pump is actively running.
Even after attending to all the likely sources of stray current, the prevalence of stray current and the speed at which it can cause severe damage make additional safeguards worthwhile; carbon tracking in the bilge pump is a case in point. The primary purpose of a bonding system is to provide a low-resistance path for escaped stray current to flow to the battery. If the bonding system is in good condition, the boat's underwater hull fittings will never suffer stray current corrosion.
To control corrosion, the bonding system should connect to every metal fitting exposed to internal (bilge) or external water, including through-hulls, struts, and the rudder post. The prop shaft is already connected to the battery through the engine. Fittings that are completely isolated from any possible exposure to stray current, such as a forward through-hull separated from the main bilge, can be excluded. Remember, bonding dissimilar metals together requires a connection to a zinc anode to avoid galvanic corrosion.
The most extensive cases of corrosion occur when a series of underwater fittings bonded together become isolated by a broken bonding wire or disconnected bonding system. Then all the interconnected fittings upstream of the break point are isolated from both the battery and the sacrificial zinc, and therefore vulnerable to both stray current corrosion and galvanic corrosion.
Special attention should therefore be given to how bonding wires are secured to fittings that are constantly awash in bilge chemicals and seawater. Most bronze throughhulls have a dedicated bonding screw that hold a closed-ring terminal and a lock washer. Crimped-on marine terminals with heat-shrink sleeves work well.. Some people coat the entire terminal connection with clear epoxy (so you can see through it) to seal out bilge water. Avoid using a stainless steel hose clamp to hold a copper wire against a bronze through-hull, which sets up galvanic corrosion. The best solution, however, is to keep the bilge clean and dry.
Most of the time, the wanderings of stray DC current are confined onboard the boat with the problem, but stray DC current can also pass from the source boat to neighboring boats via the AC green grounding wire and surrounding water. This happens because 1) there is inappropriate or deficient wiring somewhere on the boat that allows stray DC current to leak onto the AC green wire, and 2) the AC green wire is not cross-connected to the boat's DC ground system as required by the American Boat and Yacht Council. Adhering to the latter would assure that there is a path for the stray DC current to pass directly to the DC ground system and battery without leaving the boat to do so.
Without the AC/DC cross-connection, the stray DC current on the AC green wire can flow out of the source boat through the shore-power cord and into the marina's grounding network. From there it can pass through the shore-power cord of neighboring boats that are wired with the proper AC/DC ground cross-connection. Once onboard the neighboring boat, the stray DC current flows down the AC green wire to the AC/DC ground cross-connection, onto the bonding system and underwater metals, through the surrounding water to the source boat's prop shaft or bonded fittings, and back to the battery (or battery charger, if that is the source).
Stray DC current can also flow in the opposite direction - from the source boat's underwater fittings, through the water to neighboring boats, through the marina AC green wire, and back to the source boat battery-if the stray DC current exiting the source boat's underwater fitting does not have a path back aboard through another of its underwater fittings. This can occur if the source boat is un-bonded, or its bonding system is disconnected or faulty. A galvanic isolator (see last issue) can stop up to about 1.4 volts of stray current, but more current will easily overwhelm the isolator.
Whichever direction the stray DC current is flowing, the rules remain the same: The metal fittings on the boat(s) feeding the positive stray current into the water are driven anodic and corrode; the metal fittings on the boat(s) receiving the stray current are negative, or cathodic, and do not corrode. This cause lots of hard feelings toward the owner of the offending boat if the neighboring boats wired correctly suffer! At a minimum, make sure the onboard AC green wire is well maintained and cross-connected to the DC ground system so your stray DC currents cannot flow out into the marina to damage other boats.
More importantly, the AC/DC cross-connection also assures that any life-threatening AC current that leaks onto DC wiring will pass directly off the boat to shore ground; otherwise, the AC current can flow into the surrounding water, endangering swimmers.
From Seaworthy, BoatUS Marine Insurance Report, July 2001, "Back to Basics".
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