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Reducing galvanic corrosion or electrolysis throughout the vessel

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What is the issue?
When different metals are in contact with each other, and are either submerged 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 large 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 as below.

Bonding metals for galvanic corrosion protection
Photo: Tony Gibson

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.

Used sacrificial anode
Photo: Lamiot via CC BY 3.0
One would expect that this well known pernicious danger with a simple means of protection has been catered for as a matter of course in each and every boat. Well to my surprise you simply cannot make that assumption. I bought a large new boat from one of Europe's premier production houses and to my amazement, I found it had a trivial amount of sacrificial anodes that came with the sail drive and they were not at all connected up as presented above. So it is worth checking.

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, the bottommost noble.

  • • Magnesium alloy

  • • Zinc

  • • Galvanised iron

  • • Aluminium

  • • Mild steel

  • • Cast iron

  • • Lead

  • • Brass*

  • • Magnese bronze

  • • Copper-Nickel

  • • Silicon bronze

  • • Monel *

  • • Stainless Steel*

Below water fittings should not, however, be made of stainless steel, use bronze fittings, fastened of course with bronze fastenings.
* Please note that the actual nobility depends upon the alloy composition of the particular metal.

With thanks to:
Michael Harpur, Yacht Obsession.

How to fit a hull anode

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