Corrosion may be defined as the deterioration of a metal due to a reaction with its environment. Corrosion on your boat is an electrochemical process. For corrosion to occur areas with different electrical potentials must exist on the metal surface. These areas must be electrical connected and in contact with an electrolyte (sea water- or even so-called fresh water). An electrical current will flows between water and fittings made of different metals, causing some of them to corrode away. On wooden & GRP boats the propeller, shaft, p-bracket and skin fitting are at risk, while on a steel hulled vessel the whole underwater area is vulnerable to costly and potentially dangerous damage.
There are four components in a corrosion cell the Anode, Cathode, a metallic path connecting the anode and cathode, and an Electrolyte (Seawater). The role of each component in the corrosion process is as follows:
At the anode, the base metal goes into solution (corrodes) by releasing electrons and forming positive metal ions. Steel anodic reaction is Fe -> Fe+² + 2e¯.
At the cathode, chemical reactions take place-using electrons released at the cathode. A common Cathodic reaction
is O2 + 2H2 + 4e- -> 4OH¯.
The metallic path provides a way for electrons released at the anode to flow to the cathode.
The electrolyte contains both ions and conducts positive current from the anode to the cathode by ionic movement. The electrolyte contains both negative charged ions called anions and positively charged ions called cations that are attracted to the anode and cathode respectively.
How do Anodes work?
The Anode System exploits this same electrical process to protect your valuable underwater fittings. Sacrificial anodes (Zinc, Aluminium or Magnesium Sacrificial anode) become part of the electrical "circuit" and waste away instead of objects made of more precious metals.
The potential difference between a metal surface and the electrolyte (seawater or freshwater) is normally used as the criterion that cathodic protection has been satisfactorily achieved.
A negative (Cathodic) Steel-to-water potential measured with respect to a Silver/Silver Chloride reference electrode contacting the water should be of at least - 800 mV with the application of cathodic protection.