Understanding Galvanic Corrosion - SailNet Community
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Understanding Galvanic Corrosion

Wherever dissimilar metals come into contact in a marine environment, like the stainless-steel track and the aluminum mast in this photo, galvanic corrosion lurks.
Last month, I began an overview of bonding and grounding issues by defining the terms used when dealing with the connection of conducting materials that are not part of the electrical utilization system. Once again: the bonding (the intentional interconnection of separate metallic components) and grounding (the intentional connection of a metal or system of metal components to a specific reference point) of the non-current carrying metal parts of a boat may accomplish three specific and separate objectives:
  1. Reduce the corrosive effects of dissimilar metal galvanic corrosion on expensive and critical boat parts.
  2. Eliminate electrolysis because of being the "ground" for another boat or an entire marina because of wiring defects.
  3. 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.)

This bronze winch is mounted on a chromed base that is also made of bronze, minimizing the amount of galvanic activity that would otherwise occur if the base were steel or aluminum.
One way to understand "difference of potential" is to know that it can be measured in volts. In the case of the dissimilar metals that are in contact, the difference of potential is generated chemically. So what we are dealing with is a battery of sorts. Electrical current will flow if a connection or circuit is complete between the metals having a difference of potential.

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.)

Made of noble bronze, these propeller blades are protected by the less-noble zincs that surround the vessel's stainless-steel prop shaft.
Sacrificing the Cheap Stuff
    Most sailors understand that the least noble anodic metal is sacrificed when a difference of potential is present between the electrically connected metals—that's what zincs are essentially all about. But fewer sailors know that the cathodic metal is actually protected from corrosion even though it is submerged in corrosive seawater. So, utilizing this principle, you can keep your boat from becoming a rustbucket in three steps:
  1. 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.
  2. 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?
  3. 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.

Kevin Hughes is offline  
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