Cathodic Protection and Hull potential testing

Owning a boat is usually expensive. Protecting the hull from corrosion and reducing the time a boat spends out of the water is therefore important to all owners of Aluminium and Steel hulled craft. A metal hull in a salt solution will inevitably have the potential to become corroded. Add a little stray direct current (DC) and this can become devastating.

Careful owners will take the necessary precaution of employing some cathodic protection by attaching Sacrificial Anodes, usually Zinc. 

The good news is that the condition of the hull in terms of the state of oxidation (Corrosion) is measurable. All you need to do is connect a Multimeter to the boat hull (terminal 1) and to an EDT directION reference electrode (terminal 2) and read the potential in mV.  This reading is the potential difference between these two electrodes. (You can consider the hull to be the sensor electrode in this instance). For an Aluminium hull with good protection the potential difference will be between -900 to -1100 mV. As corrosion occurs the values will drift towards -500 which indicates that the hull is being sacrificed.  If the value is over -1100 then the boat is over protected.