What is the issue?
When different metal are in contact with each other, either submersed in, or subject to seawater spray, galvanic corrosion or electrolysis occurs. This is an exchange of electrons, atomic particles, ions etc causing an electronic difference of potential between the metals. The less noble or anodic metal can be very quickly corroded away by the seawater conducting its ions to the more noble or cathodic metal. Aluminium, ordinary steels and the more base metals are less noble and highly subject to corrosion. The more noble materials include bronze and stainless steel.
Why address this?
Irrespective of what a fitting is, how much it costs and the critical function it performs aboard, the item made from the least noble metal, or most anodic, will have its structure consumed by accelerated corrosion and turn to dust unless protective measures are taken.
How to address this?
Use sacrificial zinc anode(s) and concentrate the dissimilar metal corrosion to them by connecting and bonding all the crafts metals together and to the anode see figure 1.
All sailors are aware of the need to place zincs on their vessels. Few know that the noble metals are by reverse protected from corrosion by the same process.
Hence if one connects every metal part of the craft into a single bonded metal system, and then connect this to a sacrificial anode(s) in contact with the seawater, the corrosion of the entire bonded metal system is focused upon that anode(s). Thus the process of galvanic corrosion can be used to protect the more noble metals throughout the vessel that are connected to the sacrificial zinc.
The zinc should be watched carefully as it will be eaten away very quickly acting as the single point of galvanic corrosion for the vessel. Be very careful not to cover anodes with antifouling and they must be replaced when erosion or severe pitting has reduced their effectiveness.
Finally opt for more noble metals where you can. Here is a nobility table from the top, with the last noble material, bottom most noble.
* Please note that the actual nobility depends upon the alloy composition of the particular metal.
Below water fittings should not hwever be made of stainless steel, use bronze fittings, fastened of course with bronze fastenings.
Michael Harpur, Yacht Obsession.