Galvanic Corrosion.
Galvanic action is an electro-chemical reaction that occurs between two different metals when placed in close proximity in an electrolyte. i.e. Seawater. When immersed in an electrolyte, the metal with the highest negative potential acts as an anode, whilst that with the lowest negative potential acts as a cathode. This causes a electrical current to flow from the anode to the cathode, at the same time corroding the anode. The closer the proximity the greater the reaction. Also the warmer and saltier the water the faster the corrosion.
This can present a considerable corrosion problem on a metal hulled vessel especially where hull fittings are made of a different material.
| Galvanic Table of Nobility | |
| Material. | Notes. |
|
Magnesium Zinc Aluminium Cadmium Mild steel Wrought Iron Stainless steel (Low Chromium) Lead Tin Brass Copper Bronze Copper-nickel alloy Nickel-copper alloy Stainless steel (High Chromium) |
The table on the left shows the order of nobility
of the materials commonly used in boat building. Metals placed at the top of this table have a higher negative potential will corrode faster than those with lower negative potential. Metals at the lower end of the table are known as passive, and are less likley to corrode. The greater the difference between two metals on the table, the greater will be the galvanic reaction between them. |
The main preventative measures that can be taken to minimize this corrosion are as follows:-
1. Every effort should be made to ensure that as many of the metallic components used during construction are compatible.
2. Contact with the water should be restricted as far as possible using an effective paint system.
3. The insulation of all hull fittings from the hull.
4.The fitting of sacrificial anodes, normally made from zinc to the relevant areas of the hull, rudder and shaft etc. Care must be taken to fit anodes of the correct size.
5. The anodes should be correctly bonded to the relevant fittings.
6. Electrical equipment on board must also be correctly bonded.