What is Cathodic Over Protection?

It is very important to protect metal structures in water. The most common are made of Steel, including: Boats, Wind Turbines and Pipes. The most common method is by using Sacrificial Anodes such as Magnesium, Zinc or Aluminium.

Aluminium Sacrificial Anodes Example

Steel itself has a potential when measured against a Silver/Silver Chloride Electrode of around -600mV. The addition of an Aluminium Sacrificial Anode can increase the negativity to around -650 to -700mV. This would be considered a well-protected structure. The Anodes will need replacing once the Hull or Metal potential gets close to the original pre-protected value.  If a set of Zinc Anodes were added the potential becomes even more negative and could reach around -1000mV. As the difference between the Metal (-600mV) and the Zinc Anodes (-1200mV) is large it is easy to over protect and cause the Anodes to lose electrons rapidly.

 What Happens with Over Protection?

The addition of too many (or too large) Sacrificial Anodes will excessively lower the potential of the steel in the water. When this happens the electrons flow too quickly and the current density becomes too large at the Cathode. In such instances Hydrogen can be formed which, over time, will embrittle certain types of steel.

Coating Damage Exposing Steel to Corrosion

The more common effect however is that the excess electrons coming from the Sacrificial Anode will increase the pH of the water at the metal surface. Alkaline water will often damage coatings and expose the metal to more rapid corrosion.

The combination of these two factors can cause catastrophic surface damage. The loss of electrons at the anode can often be visible in extreme cases as the anode can appear to fizz.  Areas of coating around the anode can be affected by higher pH and gas generation beneath the coating surface.

Hull Testing

The Boat Meter kit contains everything you need to test and monitor the Hull and the effectiveness of Anodes. The REDOX probe will also allow the testing for local factors that may cause corrosion in the marina