Yacht & Marine Surveyors

What is Cathodic Protection?

If you have two different metals physically or electrically connected and then you immerse them in seawater, they become a battery. Some current flows between the two metals and the electrons that make up that current are supplied by one of the metals giving up bits of itself-in the form of metal ions-to the seawater. This is called galvanic corrosion and can destroy underwater metals.

The obvious casualty is the bronze or aluminum propeller on a stainless steel shaft.

In order to counteract galvanic corrosion you need to add a third metal into the circuit, one that gives up its electrons first.
This piece of metal we call a sacrificial anode, and usually it is zinc.

It cannot be emphasised enough the importance of maintaining the zinc anodes on your boat. When an anode is dead, the metal component it was installed to protect begins to die.

Factors to bear in mind
The larger the surface area of a zinc anode the more protection, although this does depend on the metal it is protecting.
Replace an anode when half gone do not nickel and dime.

Check your anodes at least annually and replace those that are half depleted.

For protection it is CRUCIAL that an anode must be in electrical contact with the metal being protected. Low-resistance, metal-to-metal contact is required so you can

• Either by mounting the zinc directly to the metal being protected
• Or connect the two with a wire.

Ensure that when an anode is mounted directly to the protected metal, e.g bolted to the side of a metal rudder, the surface that the anode is connected to must be bare and bright. Do not paint anodes.

What can go wrong?
This Westerly Pageant had a nice shiny new anode but someone had removed the connection to the stern gear. The result? Dezincification of the propeller and the need for a replacement. Note the pink appearance? When sounded with a hammer the propeller did not ring like a bell, and when struck on it's edge with a file a large piece of propeller blade came away.