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In simple terms, a battery is created by connecting two dissimilar metals with wire, and submerging the whole thing in an electrolyte. In the case of a 6 volt battery, the electrolyte is a mixture of sulfuric acid and water. This activates magnetic fields and starts and electrochemical process, changing the chemical form of electricity to the electricity we need to run our appliances. In a sense, a boat is like a battery because you have different metals on your boat, connected with grounding wires, sitting in salt water. The 'noble' metal becomes the cathode and the lesser metal becomes the anode, just like battery terminals. The less noble metal gives up its electrons to the more noble metal, weakening the lesser metal to the point where you can see the corrosion, as the metal gets 'eaten away.' Of course, it's not actually being eaten away, rather, the flow of electrons, away from the weaker metal, takes particles of metal with it. Eventually there would be nothing left.
This kind of corrosion, specifically, occurs when two dissimilar metals are submerged in water. The water becomes the pathway (electrolyte) between the two metals, similar to the 'water' in a battery. The metals have to be different because one must be more chemically active/less stable than the other, in order for a reaction to take place. If we remember back to high school chemistry, we know that all atoms have electrons, which make metals electrically charged, which produces some kind of measurable current. So, the lesser metal corrodes faster as its electrons flow out and away from it. Let's take a bronze propeller, for example, and attach it to a stainless steel shaft- the bronze is the more chemically active/less stable, called the anode, and the stainless steel (steel-chromium alloy, specifically engineered to be corrosion-resistant), the cathode, less chemically active (stronger, more 'noble'), so the bronze will corrode, depositing particles of itself onto the stainless steel. Eventually, the bronze will dissolve away completely. Electrons flow from the bronze, through the salt water, and the bronze atoms become ions and break away. So, for each ion that is released by the bronze, electrons in the stainless steel react to form a negative ion.
Another cause of galvanic corrosion is the shore power hook-up. When you plug in, you're basically tying your boat's electrical system to the other boats around you, through the shore power green grounding lead. Your boat is now part of a large galvanic cell, interconnected with onshore metal that is in the water- as well as other boats- and corrosion may be greatly accelerated. Salt water is very conductive, especially in hot climates where the water temperature is higher. Zinc, aluminum and magnesium anodes corrode first, so these are 'sacrificed' by being fastened in various places on your boat, below the waterline. Most boats need protection against galvanic corrosion, since most have at least some dissimilar metal fittings or equipment. That's why we make sure we don't paint the anodes because that will render them useless.
Minimizing Galvanic Corrosion:
-use similar metals wherever possible.
-make the smaller, more expensive parts (such as propellers, rudders, and sea cocks) from a more noble metal (graphite, platinum, titanium, stainless steel, etc.) than the larger, less expensive items.
-insulate dissimilar metals with a gasket or flexible compound to avoid contact (or 'electrical conductivity') between them.
-bond similar metals to a common ground.
-avoid the use of graphite grease. Instead, use a lithium or moly based grease, which are advertised as being 'non-conductive.'
-install anodes. Since it's almost impossible to prevent all galvanic activity, a zinc anode is the most common solution. The zinc is placed in strategic locations where it can be monitored.
These are solid-state devices that are part of a series, connected in line to the boat's green safety ground lead, ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents coming aboard you boat on the shore power ground wire, but still maintaining the integrity of the safety grounding circuit. These currents could cause corrosion to your underwater metals- through hulls, propeller, shaft, etc.
The galvanic isolator has two pairs of diodes, set up so that a voltage of about 1.2 volts is required to cause them to conduct. As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup. Normally, no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault. Therefore, it is important to have a good quality unit that will not overheat when required to carry the rated load. Also, some heat will be generated by the voltage drop and the unit must be able to withstand this.
The purpose of the galvanic isolator is to disconnect your wet metal parts from the dock supply in order to prevent electrolysis. The trouble is, you need them connected so that if there is an electrical short on the boat, it doesn't make the boat alive at 120 volts, or worse, which can give you quite a kick when you step off an aluminum dock. The galvanic isolator relies on the fact that electrolysis voltages are quite low- usually less than one volt- whereas electrical failure voltages are quite high. Silicon diodes, which are used to conduct electricity in one direction but block it in the reverse direction, have a built-in forward voltage drop of about 0.6 volts. It is not like a resistor voltage drop- no current has to flow to create the drop- so below 0.6 volts, it is disconnected. Above this, it conducts with very little resistance to current flow.
Stray Current Corrosion:
Galvanic activity usually progresses slowly, sometimes taking months or years before serious corrosion is apparent. The voltage difference between the two metals may be only millivolts (1/1000ths of a volt). Stray currents, on the other hand, can be thousands of times greater, and can destroy expensive components in hours.
-poor insulation, especially in damp areas of the boat, such as the bilge.
-undersized wiring, which causes excessive voltage drops. The electricity then tries to find a better flow path.
-cheap appliances that leak electricity.
-radio grounds with different voltages than the battery ground.
-lack of common ground point.
-tying the AC systems neutral to the boat's ground system without an isolation transformer.
-defective shore power wiring can cause problems between two boats electrically tied together at a marina.
-wire the boat like your house, not your car. Modern homes have three wires to every outlet. One from the electricity source, a return line to the electricity source and a ground wire (your car has only one wire that goes from the battery to the appliance. The car's chassis is used as the return line. There is no path to ground because of the rubber tires).
-use two-wire marine appliances, not single wire automotive appliances (e.g. engine alternators and starters, bilge pumps, etc.). Make sure both the electrical supply wire and the return wire are large enough. Devices that work well on land may be unsuitable in a marine environment.
-use common ground for all systems. Use keel bolt to an external ground plate, not the engine block.
-use type-B isolation transformer to tie the neutral side of the AC system to the boat's common ground.
-install an isolation switch to disconnect your battery when not in use.
-check your boat with a voltmeter. Look for voltage readings where there should be none.
Stray current corrosion is similar in concept to that of galvanic corrosion, however the electric current generated isn't due to just having dissimilar metals in contact. With a stray current, there is a wire that is touching something it shouldn't, like a faulty bilge pump float or degraded wiring lying in the bilge, which sends current into the water, away from its intended path, and the stray current uses the boat, or its underwater fittings as the new path. In most cases, it's the boat's battery bank or battery charger system which is supplying the power.
With stray current corrosion, we are dealing with relatively high currents, and damage normally occurs in hours to days, vs months, as in the case of galvanic corrosion. The stray current can come from your boat, or from a neighboring boat, bit it will usually be your boat that is causing the problem. A boat can lost its shaft, rudder, and propeller in less than 2 weeks. It is also possible for a vessel to be sunk by as stray current, in a matter of just days, if the current flow is high enough. Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring, through the common ground on board, or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shore power grounds, there is potential for widespread damage. Aside from concerns of corrosion, there is also potential for electrocution if shore power chords are allowed to lie in water, let alone the unwise people who leave their shore power chord plugged in at the dock while they go for an afternoon cruise.
Metal boats can be more prone to it because the hull material itself is conductive, and any stray wire or connections using the hull as a 'ground,' will result in electrolysis. But all hull materials, including GRP or composite boats, will suffer too, as they have metal parts below the waterline- propellers, prop shafts, through hull fittings, radio ground plates, etc.