Stray Current Corrosion
This form of corrosion is similar in concept to that of galvanic corrosion, however the electric current generated is not due to just having dissimilar metals in contact. In this case there is a power source generating the current. In most cases it’s the boat’s battery bank or battery charger system which is supplying the power. We are dealing with relatively high currents in stray current corrosion, and damage normally occurs in hours to days, vice months to years as in the galvanic case.
Simply stated, if there are two pieces of metal underwater that are at different voltage potentials, and they are connected to a power supply, a stray current will flow, and underwater metal damage will occur. In order for this to happen, there must be some form of short circuit from the DC positive system to the DC negative system (a ground fault in a battery charger might be an example). There also must be a fault in the ground system which is isolating these two pieces of metal from each other (a perfect example would be a break in that green wire system, or bonding system running through your boat).
A fault like this will cause underwater metal damage in most cases, not only to the boat with the problem, but to neighboring boats as well. And these neighboring boats don’t even have to be plugged into shore power to be affected!
We have the technology to detect these types of faults in boats and marinas. Below are some pictures of the damage that can be done by stray DC currents in a short period of time.