I suspect you may have a problem with Stray Current corrosion
, the result of a ground-fault on your own boat, or perhaps a hot marina.
Stray current corrosion
involves the same electrochemical reactions as simple and galvanic corrosion
except the reactions are driven by higher DC voltages from sources such as batteries, battery
chargers and alternators. Stray currents (or interference
currents) are defined as those currents that follow paths other than their intended circuit. The conditions causing this type of corrosion require an electrical
fault, along with a problem somewhere in the bonding system of the vessel (the system designed to keep all underwater metals at the same potential to prevent this form of corrosion). The externally induced electrical current
, that attempts to flow between the objects, causes one object to give up ions to the other object, and corrode. Stray current corrosion can strike hard and fast
, often resulting in damage in weeks, days, or even hours
occurs when dissimilar metals are placed in the same electrolyte (like water) and are connected electrically. The transfer of current from one metal to the other, in the completed circuit, results in the corrosion of the more anodic metal and the protection of the most noble of the two. This form of corrosion takes on the order of many weeks to months to manifest
itself in a visible manner.
A common example of galvanic corrosion can be seen when a stainless steel
fitting is attached directly to an aluminum
component, such as a spar. Over time, the aluminum
will corrode in the area around the fitting.
Check out some of our earier discussions:
The Galvanic Series and Corrosion
When to replace the Anodes ?
Alternator and Starter Isolation
Zincs and the 'Hot' Marina