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 mV
The correct answer is D.
The recommended range of cathodic protection for a vessel depends on hull material and underwater metals:
| Hull Type | Recommended Range |
|---|---|
| Fiberglass w/ Inboard Engine(s) |
-750 to -1100 mV
|
| Fiberglass w/ Aluminum Outdrive(s) |
-950 to -1100 mV
|
| Aluminum |
-950 to -1100 mV
|
| Steel |
-850 to -1100 mV
|
| Wood |
-550 to -600 mV
|
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, saildrives, hulls) • Minimum corrosion protection voltage is -950mV
Mild Steel (e.g., hulls, rudders, tanks)
• Minimum corrosion protection voltage is -850 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.