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.

This prop and shaft were heavily damaged by stray current corrosion.  The fault was due to a short between a corroded starter solenoid terminal and the case of the solenoid.  The engine block was completely isolated which allowed the stray currents to damage the shaft and prop.  Estimated time is 2-4 weeks.

This is a close up view of the shaft in the picture to the left.  Notice the smooth "scooped out" appearance which we have seen before in stray current damage cases.  The necked-down area is where the shaft anode was located.

The above shaft was as shiny as the spot where the zinc was before getting underway for an eight-hour fishing trip. A combination of an alternator fault and an isolated engine block allowed this aggressive stray current corrosion to occur. Below is a close-up of this damage.

Close-up of the damage caused by stray currents over an eight hour period.

This battery terminal was submerged when the engine compartment filled with water from a hurricane.  Note the stray current corrosion on the positive terminal (which was the anode in this stray current cell).  It provides evidence that this battery terminal was covered with water at one time.

This was the shaft of a trawler with stray current damage. The smooth scalloping and tiny pores are typical of stray current corrosion in stainless steel.