Galvanic corrosion occurs when negative ions (electrons) leave a metal and enter its environment, leaving behind positive ions (molecules of the metal that lack one electron), which then combine with non-metallic negative ions such as oxide and chloride.
to work with. Boat Lore Bronze 159 The next least corrosive metal, after monel, is bronze, an alloy
Galvanic
seriesThe galvanic series lists all metals in sequence from most noble, or protected from corrosion, to least noble, or quick to corrode. Many of these metals, such as gold and silver, are not found in fittings on boats. Among those you are likely to encounter, the most noble is monel, a trademarked alloy of mainly nickel with copper and traces of other metals. Although slow to corrode, it is expensive and difficult to work with. bronze, an alloy of copper with anything except zinc (which forms brass) or nickel. Special kinds of bronze are named for their main alloy, such as silicon bronze (Everdur) and aluminum bronze. In common bronze, the alloy is tin. Brass resists corrosion as well as bronze except when its zinc component leeches out, a process called dezincification, caused by metals within the alloy itself forming a galvanic couple. Some brass alloys resist this. Bronze has better properties than brass for most marine applications except decorative items. Bronze that has begun to corrode will have a pinkish color where the zinc has leeched out. Like any alloy of copper, bronze and brass exposed to sea air will form a green coating of copper chloride called verdigris.
Stainless
steelAfter bronze, the next most noble metal on boats is stainless steel. When steel is alloyed with at least 12% chromium, the chrome forms an inert oxide on the surface of the steel that protects the steel from corrosion as long as it is exposed to enough oxygen. There are many grades of stainless steel. The stainless steel fasteners and fittings at a home supply store are probably not what you want on a boat.
Marine grade stainless steel usually contains nickel, which makes it non-magnetic. Stainless steel that sticks to a magnet generally will not last in a marine environment. The most common grade of stainless steel for marine use is type 304 (or A2, containing 18% Cr and 10% Ni). The best grade is type 316 (or A4, containing 18% Cr and 11% Ni and 2% Mo), which adds molybdenum. worthwhile. It is not as strong as type 304, however, and it is more susceptible to work hardening, making it less suitable for some wire rope applications.
Even type 316 stainless steel will not resist corrosion if cut off from oxygen. Sea water usually has enough dissolved oxygen to protect the metal, but it also has chloride ions that attack the protective chromium oxide coating faster than it can be replaced. Stainless steel is therefore rarely used below the waterline except when protected by a sacrificial anode. Stainless steel fittings that are embedded in the deck or installed in poorly aerated places should be removed from time to time and inspected for corrosion or cracking.
or cracking. Crevasse and stress corrosion Water entering a tiny crack in stainless steel can cause crevasse corrosion if the crack is too small to admit sufficient air. Stainless steel
Crevasse
and stress
corrosionWater entering a tiny crack in stainless steel can cause crevasse and stress corrosion if the crack is too small to corrosion admit sufficient air. Stainless steel parts that are always under strain can suffer from stress corrosion that causes brittle cracks. Highly-stressed parts, such as fittings at the ends of cables and stays, should be inspected with a magnifying glass and replaced if they show any signs of cracking or brittleness. To inspect, brush a mix of blue food coloring with WD-40 onto the fitting, let it dry for five minutes, and wipe off using solvent if necessary. Color will remain in any cracks or crevasses.
to flaking Steel and aluminum off and therefore does not protect the underlying metal. The hard and
Least noble of all boat metals is zinc. This metal is most useful in sacrificial anodes, called zincs, which protect other nearby metals from corrosion by supplying negative ions more freely than they do. Zincs can disappear quickly and should be replaced before they are as much as two-thirds depleted.
Fasteners must be more noble than any of the materials they connect. Aluminum rivets are useful only for connecting two aluminum parts, for example, and brass screws should not be used in bronze fittings.
Ordinary steel is suitable for marine use if galvanized by covering it with a layer of zinc at least 100 microns thick. Hot dip or spun galvanizing is thick enough for marine fasteners. Electroplated galvanizing is not and will fail.
Stray
currentAnother kind of electrochemical corrosion, distinct from galvanic corrosion, is stray current corrosion, corrosion sometimes called electrolysis. It occurs, as the name implies, when current leaks from power supply lines into the water or to any part of ground connected to the water. It differs from galvanic corrosion by affecting single metals, all alone, and by the current being supplied externally rather than created by the process. Because of its potentially higher currents, it is generally more severe and can destroy a metal part in a matter of days. Stray current corrosion occurs at the point where the most common where wiring contacts bilge water, which is always problematic. A galvanic isolator that blocks up to about 200 milliamps of DC current in the shore power ground (green) wire protects against stray current from shore power but not, of course, from the batteries.
Test for
corrosionTo test for susceptibility to corrosion, first build a measuring device by connecting a length of 10- or 12-gauge wire with an alligator clip to the negative input of a high-impedance digital voltmeter and connecting another, longer length of wire with a silver/silver chloride reference cell attached to the positive input. Toss the Ag/AgCl half cell over the side, connect the alligator clip to the metal fitting you want to test, and note the voltage there. Do this initially while anchored well away from any other boat, testing each thru- hull fitting and the prop shaft to establish a baseline. If these parts that get exposed to seawater are bonded together, as many—but not all—believe that they ought to be, the voltage should be everywhere the same: -500 to -700 mV at bronze parts, -750 to -850 mV for steel or stainless steel, and -800 to -1,050 mV for aluminum. Any variation indicates a breach in the bonding system. Any number below the protected range indicates that corrosion is occurring. Disregarding the minus sign (it's there because the reference cell connects to the + terminal), the lower the absolute value of the number, the more that part or fitting has been corroded. You cannot test the engine, itself, in this way because its zinc is not immersed in the heat exchanger.
Galvanic
isolatorIf your zincs wear out quickly for no apparent reason, a shore power AC hookup could be the culprit. On most boats, DC ground is connected to AC ground to prevent AC current from entering the water, which could be deadly to a swimmer. Underwater metal on your boat connects your grounded metal through the AC ground to the grounded metal on other boats. If your metal is less noble than theirs, this forms a galvanic couple underwater that will corrode your zincs. A galvanic isolator installed across a cut in the green AC ground wire prevents this. It usually contains a capacitor, which blocks DC, and always has two diodes that block AC below about 1.2 volts.
Modern galvanic isolators usually have a self-test function. To test an older unit, unplug the boat from shore power (important!), then disconnect one green wire and use a voltmeter set to the diode test function. If the voltage across the galvanic isolator rises much above 1 volt, a diode is blown and you should remove the voltmeter immediately to avoid damaging the capacitor. If the voltage is less than half a volt, a diode is shorted. Normally the voltage rises to just under 1 volt. It rises slowly if there is a working capacitor or instantly if not. After performing this test, reverse the voltmeter leads and perform it again to test the other diode.