- Reduce the corrosive effects of dissimilar metal galvanic corrosion on expensive and critical boat parts.
- Eliminate electrolysis because of being the "ground" for another boat or an entire marina because of wiring defects.
- Protect persons and equipment from damage because of lightning.
When two different types of metal are in contact and subject to a corrosive environment, the least noble metal will be sacrificed. This is known as galvanic corrosion. (John Vigor's Practical Mariner's Book of Knowledge is an excellent source to identify the galvanic series of metals in seawater and most other essential sailing data.) A corrosive environment would include submersion in saltwater (and even freshwater), or the incidence of seawater spray.
The process of accelerated corrosion begins because of an exchange of ions, electrons and other atomic and subatomic particles at the point where these metals touch. This exchange of particles at the junction of these metals causes an electrical difference of potential between the metals. (This term is actually the root of all evils discussed in this article, and thus it needs to be fully understood.)
A common lead-acid battery ceases having a difference of potential across its terminals, or is dead when the lead oxide plates are chemically converted to lead sulfate through use and discharge of the battery. A brass plumbing fitting threaded into a Monel bilge tank will cease to have a difference of potential with each other when all the zinc in the brass corrodes away and the remainder of the fitting crumples into dust.
In the case if submerged dissimilar metals, the more anodic (or less noble) metal corrodes and gives up part of its structure (in the form of metal ions conducted through the seawater) to the more cathodic (or more noble) metal. The anodic metal is essentially consumed. If you read my article last month ("Understanding Grounding and Bonding"), you'll recognize that this information repeats a portion of that article, but understanding this concept is extremely vital to solving both the induced voltage problems of galvanic action and the imposed voltage problems of both lightning and electrolysis. (We'll cover the latter two in a future article.)
- Use only very noble metals for critical parts and parts exposed to the most corrosive environment. Underwater parts should be made of silicone bronze or better. Stainless steel should not be embedded or otherwise sealed off from the atmosphere, as oxygen-starved stainless steel is much less noble than stainless steel that is exposed to air or water. And do not use common brass or mild steel for any permanent parts making up the structure of the vessel or its systems.
- Connect every metal part of a vessel to every other metal part of the vessel. This may seem like asking for trouble; after all, isn't one of the conditions of galvanic corrosion between dissimilar metals that they be in contact? How then can linking them together be a solution?
- Place sacrificial anode(s), connected to the bonded metal system, in contact with the seawater. These are usually made of zinc, a pretty non-noble metal and one that's relatively inexpensive.
Now, with everything important on board made of noble metal, and all of it connected together, the zinc anodes in the water are doomed! They will succumb to the very pox we are attacking—dissimilar metal corrosion. But the nature of this galvanic corrosion protects the more noble metals in the water that are connected to the sacrificial zinc. Once you realize how this works, you'll begin asking yourself, ‘how many other problems in life end up having the solution already in place?'
Next month I'll tackle electrolysis—not the kind that gets rid of unwanted body hair—the kind that preys upon your sailboat.
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