Electrolytic Corrosion

Few things can scare the wits out of a boater like stray current in the water. With good reason. When you take the recipe for corrosion – an anode, a cathode, a metallic path, an electrolyte – and add an outside electrical current to the mix, there are sure to be consequences. Likely severe. Potentially catastrophic. This form of corrosion – known as electrolytic corrosion or stray current corrosion – represents a different kind of problem than what we’ve discussed here so far.

The introduction of current (from your own boat, a neighbor, or a shoreside outlet) changes things dramatically and immediately. Anodes are obliterated quickly, leaving your boat defenseless. From there, it can be truly astonishing how quickly your boat’s underwater metals can be damaged (or destroyed).

How can you tell when you’re dealing with electrolytic corrosion? Well, the best way is to test for it with a reference cell. A silver/silver chloride cell will provide accurate, repeatable results, which will let you act on the problem. That may mean sussing out the problem on your own boat, or else finding its source in the water around you. In any case, it’s a serious concern that should never be ignored.

Without a reference cell’s readings to go on, stray current corrosion is still going to present a few clues. For example, electrolytic corrosion is almost always directional. Say the neighbor on your port side is, unbeknownst to them, throwing current into the water. You can expect to see the damage on your boat confined to that same side. A twin-screw boat in this scenario may have no issues at all with their starboard side. Anodes intact, perfect prop shaft and blades. But the side facing the power source would tell a very different story. Remember this: asymmetric damage is a telltale sign of electrolytic corrosion.

Of all the topics we’ve covered so far, electrolytic corrosion is perhaps the trickiest to tackle in a single post. We hope to have provided a useful, broad strokes overview of the topic. For a deeper look, as always, we can recommend turning to Everett Collier’s The Boatowner’s Guide To Corrosion.