Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

your valuable metal components. It’s time

to replace them.

2 If the anodes were replaced recently, then

the rapid loss of material means that you

may never have had enough anode mate-

rial to begin with, or that your anodes are

protecting other boats on the dock due to

a lack of a galvanic isolator, or that AC or

DC stray currents may be at work.

If the readings are more negative than rec-

ommended, there are two possibilities:

1 It is possible someone has inadvertently

installed magnesium anodes. These are

used in fresh water and should never be

used in a saltwater environment as their

negative potential is high enough to create

an overprotection situation that can be

very damaging to aluminum hulls and

outdrives. (Aluminum is unique among

the common metals used in marine con-

struction in that it can be damaged by

either under- or overprotection from a

cathodic protection system.)

2 There is a DC current leak, either through a

connected fitting on the boat or from an

adjacent boat—via the water—that has a

DC stray-current problem. (We’ll look at

how to check for this shortly.) If the current

leak is coming from someone else’s boat,

call a corrosion specialist as it’s going to

take some serious tracking down to isolate.

a basic corrosion survey 111

engine

negative

terminal

and meter

lead

Touch the meter’s negative (“com”) lead to the battery
negative return, or engine negative terminal, as shown.

The recommended range of cathodic protection from ABYC Standard E-2. (Courtesy ABYC)

A typical meter reading, —0.955 mV, for a fiberglass boat
with bronze running gear and a typical array of bronze
through-hull fittings.