Easy to be confused, as we are really talking about 3 inter-related topics:
* Electrolysis
* Bonding
* Galvanic Corrosion
Let's take
ELECTROLYSIS first. All metals have a base electric potential. The lower the number of it, like zinc, then the faster and more likely it will be attacked. This occurs naturally but is exacerbated when a boat and the metal is immersed in saltwater, as it makes an excellent current between the cathode and anode, which the zincs are also called or referred to.
The higher the e-potential, then the more "noble" the material is, like zincs are low, aluminum a bit higher, and the high quality marine bronzes and stainless steels being very high. Note, magnesium has an even lower potential than zinc and this is typically used for the anodes in freshwater.
Stray current can be a BIG problem in marinas, that's why metal objects need to be bonded (see below). One can check the level of electrical contact in your boat's area by putting a silver/silver chlroride cell off a digital multi-meter and reading the value of the current in the water. Most marinas with boat repair facilities have the means to do this. Note that it might not be the fault of the marina's dock wiring, but a poorly wired neighboring boat could be the culprit. Whenever you see zincs lasting but weeks on only a handful of boats, the wiring on those piers are suspect as as the boats on them.
BONDING is simply a means to put all metal objects in contact with each other, so that when in the water, they all have the same electric potential. The intent is to unify them and "raise them all up" to a level, or value, that is HIGHER then the potential of the zincs installed on the hull, OB, bracket, or other running gear.
From my example above, if you needed your "protection level" to be above 650, one item was below it. But by bonding them together, you put them all to > 700. Do not confuse bonding here with adding an earth ground.
GALVANIC CORROSION also naturally occurs in nature and this is due to the effect of using dis-similar metals, but there are 3 conditions that must be met for this to happen.
* The metal juncture must be wet with a conductive liquid
* There must be metal to metal contact
* The metals must have sufficiently different potentials
You can see this can be bad in saltwater as it is not only conductive, but the salts can make it work even faster ... me thinks the salt makes it a better "battery", as that in essence is what's happening.
Example: If you screw a SS bolt into an aluminum skiff, the area around the bolt head will or can bubble off the paint, show a white powder, or even get pitted, and eventually fail. This is because the SS is more noble than the aluminum and thus there is a current flowing between the 2 and the aluminum gets eaten away.
You can prevent this from occuring by using "barriers" to isolate the one metal from the other. I do this by putting heatshrink onto the body and/or threads of the bolt, using grease like TefGel (made to isolate metal alloys) or some waterproof synthetic grease, or nylon washers under the SS fasteners, so the nylon sits on the aluminum hull.
On some spinning reels, you can see white powder corrosion and paint flaking where a SS screw was put into the aluminum reel housing .. that is the effect of dis-similar metals at work in a saltwater environment.
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FYI - this info is NOT meant nor intended to be the defacto treatise on these conditions ... so if your boat has signs of some of these or needs to be protected ... you advised to do further on-line research.
... back to work now
