corrosion 101
There are three types of corrosion underwater metals are subjected to---Electrolytic, Galvanic, and Stray current. Bonding underwater metals (and installing a zinc) will cure/prevent the first two types of corrosion but will cause or speed up the last. It is best to find out what type of corrosion your boat is experiencing before deciding on the cure and if you are not experiencing a problem, leave well enough alone. Galvanic & Electrolytic (Stray Current) corrosion are two similar, yet different types of corrosion. Galvanic corrosion results when two dissimilar metals, of sufficiently different potentials (about 0.1 V on the galvanic series) (a galvanic couple can also occur within regions of a single alloy), are electrically connected (bonded; deliberately or not), and immersed in a common electrolyte (water). The same electro-chemical reaction creates the current flow in a storage battery. Stray Current (or Electrolytic) corrosion results when an applied current flows through immersed conductive materials, other than an intended electrical circuit. Stray currents 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 most often related to shore power connections, or other boats on the dock . To generate electrolytic corrosion, the stray current must flow onto the metal at one location and then flow off the metal at another location. When an applied current passes in and out of a metal part, an electrolytic cell is set up, resulting in a localized anodic site, where the positive current exits the metal part. This causes the local oxidation (corrosion reaction) of the metal piece, which may lead to a very rapid metal loss, which is limited only by the amount of current available. TESTING: An ohm meter is a poor way to check bonding connections. I use a 12 volt lamp and make a probe using a screwdriver. Hook a long enough wire to the positive terminal of the starting battery and then poke the screwdriver into each of the through hull fittings and make sure the lamp lights. An ohm meter is only testing the circuit at micro-amps. The following applies mainly to boats left in the water. Trailered boats rarely spend enough time in the water for electrolysis to be a problem. BONDING STRAPS Although copper straps look impressive they are totally unnecessary. At most, the wire only carries a few milliamps and there is no concern for voltage drop so the copper only needs to be heavy enough to resist deterioration from oxidation. A 10 gauge copper wire is more than adequate. I use non stranded 10 gauge bare copper wire for my installations, being careful to secure it so it is not subject to flexing from boat motion. Non stranded wire has much less surface area and avoids the cavities between the strands where moisture and corrosion/oxidation can progress. THEORY Electrolysis only happens when two dissimilar metals are immersed in an electrolyte and connected together. The dissimilar metals have different electrolysis voltages so if you connect them together current flows through the connecting wire one direction and through the water the other. As the current leaves one metal to travel to the other, it causes metal to come off one surface and be deposited on the other like battery plates so the higher voltage metal suffers electrolysis. Electrolysis can also occur when an adjacent structure or boat is injecting DC current into the water and that current goes in one end of your boat and out the other on its way to the destination. This can cause electrolysis even though your boat is not an offender. PROTECTION There are basically 2 ways to reduce electrolysis. The PREFERABLE one is to DISCONNECT the electrical circuit. If this can't be done, the second method is to provide a sacrificial anode (Zinc) so it deteriorates rather than your expensive equipment. a) DISCONNECTING Electrolysis cannot occur on an isolated piece of metal in salt water. It is all at the same voltage but if it is isolated no current can flow so there is no electrolysis. When it is connected to another piece of metal, ESPECIALLY if the other piece is a different metal, you just created a shorted battery and electrolysis will start. By following the wrong advise and bonding everything in the boat you are creating batteries where it is unnecessary and making electrolysis problems worse. Only bond underwater items that are showing symptoms of electrolysis. If it ain't broke, DON'T fix it. Once you bond it unnecessarily you have CREATED the circuit rather than DISCONNECTING it. You are now stuck with providing Zincs since you have removed the first line of protection. Although some through hulls appear to be isolated due to sitting in fiberglass and using non-metallic tubing, they quite often are not and WILL need bonding. For example the raw water cooling inlet for an inboard engine is in fact connected to the engine block by the salt water in the tubing and may need bonding so the current flows though copper rather than the water and in/out of the through hull. b) SACRIFICIAL ZINC There are situations where different metals cannot be disconnected and you are stuck with putting zincs on them to provid a target for the electrolysis. Zincs have a higher electrolytic voltage than marine metals so it is the first to deteriorate. Keep in mind that the zinc will CREATE electrolysis (favorable electrolysis since the zinc is being eaten up). Putting zincs where they were not needed will still cause the zincs to erode away since they create the battery situation where none may have existed. The range of protection a zinc can supply to other metals bonded to it is limited by the conductivity of the water. In salt water you get coverage for a radius of about 4 to 6 feet. A zinc on the stern of a 14 ft boat where everything is bonded is only protecting half the boat. This range gets even less in fresh water and may reduce to only inches. Often magnesium is substituted for zinc in fresh water to provide an even higher voltage to push through the fresh water. Generally, electrolysis problems are created by the shore power connection. You could theoretically disconnect the ground connection in the shore power and avoid electrolysis (and in some cases this is a solution) but ABYC regulations require the AC ground be connected to the DC ground so an electrical fault on the boat won't electrocute swimmers in the vicinity. When you connect your underwater metal to the shore power ground you have "bonded" with every other boat on the docks who have the same connection. Now electrolysis currents are free to flow anywhere in the marina and it only takes one boat with a 12 volt DC leakage to eat up every boat within a wide radius even though the offender has zero electrolysis evidence. Your protection is a Galvanic Isolator in the ground connection of the shore power lead.. Any Galvanic Isolator that meets AYBC specifications will do - they are not rocket science and extremely reliable. On most boats you cannot isolate the DC ground from the underwater metal because the starter motor is bolted to the engine block which in turn connects to the propeller shaft and the water. So without the ground, an internal failure of the battery charger could put 120 or 230 volts AC on your DC ground and into the water.
