Q & A — November 2004
By Capt. Ken Kreisler
There is a Difference
| Electrolysis vs. galvanic corrosion, how far an inverter should be from the battery, and more.
What is the difference between electrolysis and galvanic corrosion? K.G., via e-mail
Galvanic corrosion involves two dissimilar metals, where the major change occurs in the metals and not the electrolyte. Here the best example is immersing your boat’s bronze props and stainless steel shafts in sea water, which acts as an excellent electrolyte. With the two metals in direct contact with each other, and the salt water able to conduct electricity, an electrochemical reaction causes the flow of ions to attack and dissolve one of the metals. And as all metals have potential energy in the form of trapped electrons, these electrons migrate from metal to metal. The corrosion takes place because the metals have different electrical potentials. (The Galvanic Series of Metals in Sea Water chart, also known as the Galvanic Scale, lists the most-active to least-active metals.) In our example, the current will flow from the bronze, which has a relatively high electrical potential and is therefore the anode, to the stainless steel, possessing a lower potential and thus taking on the role of cathode. The result is corrosion of the props.
A number of environmental factors contribute to the rate and amount of galvanic corrosion, among them the temperature and salinity of the sea water and how close the anode and cathode are to each other. Warm, highly salinic water is most conducive to galvanic corrosion.
The most common form of protection is the use of sacrificial anodes, or zincs, since they are at the top of the most active metals of the Galvanic Scale and will corrode more easily than the other metals. The illustration here shows a zinc collar, positioned not too close to the strut so as to disrupt the flow of cooling water to the cutlass bearing. It will protect the props against the effects of galvanic corrosion.
This article originally appeared in the October 2004 issue of Power & Motoryacht magazine.