Which voltage reading comes from a vessel's bonding system that's protecting underwater metals from seawater corrosion?

A)  0 mV
B)  -405 mV
C)  -536 mV
D)  -908 mV
E)  -1420 m

The correct answer is D.

The recommended range of cathodic protection for a vessel depends on hull material and underwater metals:

Voltage readings below (more positive) indicate that underwater metals
connected to the bonding system are under-protected and may be corroding.

Voltage readings above (more negative) indicate that underwater metals
connected to the bonding system are over-protected and causing vessel
damage.

Voltages Below Recommended

Each underwater metal is protected from corrosion when its cathodic
protection system is able to induce and maintain a negative shift of at least
200 mV. For the four most common marine metals this means:

Aluminum (e.g., outdrives, hulls)
 • Minimum corrosion protection voltage is -900mV

Mild Steel (e.g., hulls, rudders, tanks)
 • Minimum corrosion protection voltage is -800 mV

Stainless Steel (e.g., propeller shafts, rudder posts, trim tabs, propellers)
 • Minimum protection voltage is -750 mV

Bronze (e.g., propellers, thru hull fittings, seacocks, struts)
 • Minimum corrosion protection is -550 mV

If the voltage of one of your underwater metals is less (more positive) than
these, you need to check your cathodic protection system -- usually done by
simply replacing depleted sacrificial anodes.

Voltages Exceeding Recommended

Overprotection can create conditions that damage underwater hull coatings, aluminum alloy metals and wooden hulls.

 • Steel and fiberglass hulls -- decreased effectiveness of anti-fouling
paints and barrier coatings when made more negative than -1100 mV.

 • Aluminum hulls and outdrives -- highly susceptible to alkali corrosion of
its metal, and hydrogen blistering of its paint coatings, when made more
negative than -1200 mV.

 • Wooden hulls -- destruction of wood fibers (alkali delignification) occurs
around metal fittings made more negative than -650 mV.

Overprotection usually occurs from one the following conditions:
Using the wrong type of anodes (e.g., magnesium anodes in saltwater);

 • A defective impressed-current corrosion controller;

 • Stray DC currents originating from defective wiring or equipment within
your vessel.

The most common source of stray currents is a defective bilge pump, float
switch or wire insulation -- each of which carry +12 VDC submerged in bilge
water. Make sure these electrical devices (and wire splices!) are inspected
and tested for electrical leakage.