There is no argument. Here is the information to support the numbers. There lots of threads that go into the science in depth.
It all deals with the Noble Scale of metals.
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:
Hull Type Recommended Range
Fiberglass w/ Inboard Engine(s) -750 to -1000 mV
Fiberglass w/ Aluminum Outdrive(s) -900 to -1050 mV
Aluminum -900 to -1100 mV Steel -800 to -1050 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, 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.
Source: BoatZincs.com
Hope this helps,
dave
None
