How to prevent galvanic corrosion - Soundings Online

How to prevent galvanic corrosion

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A galvanic isolator can keep your slip mates from depleting your boat’s underwater metal

A galvanic isolator can keep your slip mates from depleting your boat’s underwater metal

Read the other story in this package: Simple galvanic corrosion test

Properly wired and maintained boats connected to AC shore power at a marina are susceptible to galvanic corrosion caused by neighboring boats with depleted zincs. Galvanic corrosion isn’t caused by the AC current but by a small DC current moving between electrically connected dissimilar metal components submerged in an electrolyte (seawater), which will dissolve the less-noble metal.

The illustration shows how, in a marina, the AC ground wire provides the electrical connection between submerged metal components on all of the boats connected to it. Take a sterndrive powerboat as an example: Since aluminum is less noble than stainless steel or bronze, first the zincs and then the aluminum outdrive can waste away as they protect the stainless steel shaft, bronze propeller and metal through-hulls on a neighboring boat with depleted zincs.

If marina wiring isn’t the problem but rather the depleted zincs on someone else’s boat, what can you do? A galvanic isolator, a simple electronic device that is easy to install and requires no maintenance, will mitigate this problem. Installed in series with the shore power AC grounding wire, the galvanic isolator consists of silicone diodes and a capacitor that block small DC galvanic currents but allow AC fault currents to pass unimpeded.

A well-found boat properly wired for safety — in other words, bonded — is particularly susceptible to galvanic corrosion. A bonded boat’s through-hull fittings at or below the waterline — such as seacocks, shafts and propellers, and steering gear components — are all interconnected on the inside of the hull with a copper strap or wire. Additionally, fuel fills, metal tankage, the windlass, mast and the like are connected to the bonding system.

The copper bonding strap or wire is connected to underwater zinc anodes, and both the AC and DC grounds are connected to the bonding circuit. The AC shore power neutral is never connected to a boat’s grounding and bonding circuits. For safety reasons the AC shore power neutral is completely isolated on the boat and goes to ground ashore.

Suppose an AC “hot” wire chafes through its insulation on a metal tank or the insulation melts through on a hot metal surface. Since a properly wired boat is bonded, metal masses like the fuel and water tanks and the engine are connected to the bonding circuit, which is connected to the grounding circuit causing a short circuit and the shore breaker to trip. The alternative to bonding is for you to find out that a tank is “hot” by becoming part of the grounding circuit when touching both it and a grounded metal mass. Bonding also protects swimmers by providing a path ashore for an AC short instead of through the water, a condition that has electrocuted swimmers.

On a boat with a bonded mast or lightning rod the bonding system should mitigate the effects of a lightning strike by providing a path for the lightning current to ground through the bonded underwater components. The addition of an immersed ground plate connected directly to the mast or lightning rod and to the common ground point on the boat is the gold standard for lightning protection. Also, bonding can help protect against rapid corrosion occasioned by stray DC currents that can be far more destructive than small galvanic generated currents (measured in millivolts). That, however, is another subject.

The downside to bonding is that dissimilar bonded metals submerged in seawater and connected to a ground wire ashore are potentially part of a huge chemical battery. The elements of the battery are the collective underwater parts of every boat (connected by the marina’s AC ground) and the electrolyte in which the bonded underwater parts are submerged (seawater, which conducts electricity). Note that I say “potentially,” because if every boat on this circuit is properly wired and has sufficient zinc anodes, the underwater metal parts of your boat are safe. But what if the guy down the dock hasn’t renewed his sacrificial zincs? Your zincs are now going to be protecting his boat, and when your zincs are depleted your underwater metal is subject to galvanic corrosion.

The good news is that only DC current causes galvanic corrosion and that the current produced by connected dissimilar metals submerged in an electrolyte is less than the voltage needed to bias (open) two silicon diodes wired in series (approximately 1 volt). Diodes permit DC current to flow in one direction only; the photograph of the guts of a basic galvanic isolator shows two sets of two diodes each, wired in parallel to conduct in opposite directions. With this arrangement the isolator blocks only the small DC current that causes galvanic corrosion, and the continuity of the grounding circuit is preserved. The net effect is that you can enjoy the safety aspects of a bonded boat, and protect your sacrificial zincs and submerged metals from rapid corrosion occasioned by the galvanic currents in your marina.

The drawback of the diode galvanic isolator is that AC fault currents will bias the diodes, opening them to permit the flow of DC current. This problem is solved by adding a capacitor to the galvanic isolator, like the Newmar Power GI-50, which has four silicon diodes and a 25,000 micro Farad capacitor (www.newmarpower.com ). The capacitor blocks DC current but provides a path for low-level AC leakage current to exit the vessel’s grounding system. The capacitor also prevents this AC leakage current from contributing to the isolator’s diodes forward conduction point, which preserves the isolator’s galvanic current-blocking capabilities.

The American Boat and Yacht Council currently requires a status monitor to be fitted to the galvanic isolator to test for proper functioning. (Professional Mariner, www.promariner.com, offers such a system.) I tested the Newmar Power GI-50 aboard my Grand Banks 42, Maramor. It is well-made, reasonably priced and rated to provide protection up to 1.4 volts DC but has no status monitor. Instead, I tested the GI-50 with a digital multimeter.

Testing the diodes is done without disassembly and in place by simply connecting the leads of a digital multimeter to the two ground studs protruding from the case of the galvanic isolator. Disconnect the boat from shore power and test the diodes with the meter’s diode test feature — in both directions. Remember, there are two sets of diodes wired in parallel to conduct in opposite directions ensuring the continuity of the grounding circuit. The diode test will measure about 0.9 volts.

You also can test the capacitor. Discharge it by shorting the two ground terminals with a screwdriver, then disconnect the ground wires and measure the resistance across the terminals. The resistance should rise as your digital multimeter charges the capacitor. The GI-50’s 25,000 micro Farad capacitor reached a resistance of about 400K ohms, although that took some time. Now discharge the capacitor and reverse the leads; the resistance readings should be about the same.

The important test is the diode test to ensure continuity of the ground. It is unlikely there would be a problem, since the diodes in the GI-50 are capable of carrying 135 percent of the rated shore power current (67.5 amps) while maintaining a maximum exterior surface temperature on the heat sink of 90 C. Also, the GI-50 is rated for a maximum short circuit current of 5,000 amps. Notwithstanding, I would definitely test the diodes after a lightning strike.

You can find more detailed information on this subject in Nigel Calder’s “Boatowner’s Mechanical and Electrical Manual,” (International Marine/McGraw-Hill).