With the discovery of electricity in the early 1800s came the awareness of the effects of electrolysis as a known hazard to the submerged hulls of ships. The early use of sacrificial anodes made from iron conflicting with the non-fouling nature of copper hulls ceased their use. It would be another 100 years before the application of aluminium and zinc sacrificial anodes were used chiefly on steel gas pipelines in the United States.
The Science of Galvanic Anodes
It was Michael Faraday’s discovery in 1834 evaluating the relationship between the corrosive weight loss of a surface and electric current that led to the practical use of cathodic protection. Metals are electrochemically active. The physical or electrical contact between two metals submerged in seawater in effect produces a battery with current flowing between them. In the process of galvanic corrosion, the metal possessing the more active voltage loses ions first. Today, these are commonly aluminium sacrificial anodes or zinc sacrificial anodes (see: http://www.cathodicme.com/sacp.html).
An Ashes to Ashes Proposition
Since metals are derived from the natural resources on the planet, they are susceptible to the natural processes of how they are reclaimed by virtue of exposure to such conductive influences as seawater or salty air. Some common structures in a variety of environments facing these driving forces are:
- Home hot water heaters
- Ship and boat hulls
- Pier pilings
- Storage tanks
- Offshore oil rigs
- Onshore oil wells
The Specificity of Metals and Environments to Which They are Best Suited
Possibly those people most directly aware of the effects of galvanic corrosion are boat operators and owners. Zinc sacrificial anodes have been the typical choice of metal to use, although the generalized reference may also include other alloys such as aluminium and magnesium. They are best suited to the particular environments in which they are used such as:
- Freshwater – use magnesium
- Salt water – use aluminium
- Brackish water – use zinc
Busting the Myths
Whether using zinc or aluminium sacrificial anodes, their importance for your boat cannot be overemphasized. Metal is going to dissolve one way or another. Keep a check on the signs of deterioration on both your boat and the anodes. It is possible to achieve the right weight of anode to last a year so that you can perform annual maintenance. If your anode does not last at least that long, you can be sure you need more weight.
Follow these helpful guidelines:
- Maintain low-resistance direct contact whether metal-to-metal or with the use of wire connection.
- Only attach directly to a surface that is bare and bright
- Anodes must be entirely exposed and must not be painted over
- Anodes may be fashioned as hull plates requiring careful monitoring