Ride the Lightening
Updated: Oct 4, 2022
Metallic components in contact with each other via water (especially saltwater) potentially face a slow, silent death.
Per Corrosionpedia.com: Electrolytic corrosion is a process of accelerated corrosion. In this process, a metallic surface is continuously corroded by other metal it is in contact with, due to an electrolyte and the flow of an electrical current between the two metals, caused from an external source of electromotive force (EMF).
Here's some Boat Guy science: Harken back to high school chemistry, remember learning about how some metals were more 'noble' than others? In Boat Guy terms, the more noble a metal, the less likely it will get beat up by the other less noble metals on the way home from the bus stop. Metals tend to want to drop the gloves in the presence of an electrolyte, like sea water, and an electrical current. When the metals duke it out, the more noble metal generally walks away in good shape while the less noble metal takes it on the chin.
Why does all of this matter? There many different types of metals in a boat, aluminum, bronze, stainless steel, etc. and many of those are in constant contact with each other via water. Further, many docks, slips, other boats, etc. can emit some electrical current into the surrounding water (hence the phrase 'hot' harbors, slips etc.). Ergo, while all the metals within your boat should be working together to help you have a good time, more likely than not, they are constantly at war with each other. Bottom paints, typically containing heavy metals, are not excluded from this battle either.
This is why all underwater metallic components should be 'zinced' or fitted with sacrificial anodes made from less noble metals. Sacrificial anodes are typically made from zinc, aluminum or magnesium, depending on the application, and will fall on their sword so that the more important metals onboard will be preserved. Anodes degrade over time (that's their job) and need to be replaced on a regular basis, how regular depends on your boat's individual needs. The general rule-of-thumb says that anodes should be replaced at or before they are 50% wasted.
This is also why most boats should have a bonding system. Some metallic components are mounted inboard but are in constant contact with outboard water, through-hull fitting and valves, packing glands, etc. These components need an electrical connection with a sacrificial anode. This is typically arranged through a bonding system, a system of electrical connections between those inboard metallic components and an outboard anode (Diver's Dream) usually made up with wire.
Blistering in bottom paints, greening-up/salting-up and pitting or wasting are all signs of electrolytic battles lost.