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Protecting Boats Against Galvanic Corrosion
by Leo Block
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Galvanic corrosion -- sometimes incorrectly called electrolysis -- is the best-known form of marine corrosion. It occurs when electrically dissimilar metals (those on opposite ends of the noble scale) are joined together and are exposed to an electrolyte.
The noble scale (Fig. 1) lists metals in the order of their tendency to give off or absorb electrons. Metals with the greatest tendency to give off electrons are called "least noble," and "most noble" metals have the greatest tendency to absorb electrons.
When two electrically dissimilar metals are wired together and exposed to an electrolyte -- such as sea water -- the least noble metal becomes the anode and the most noble one becomes the cathode.
The anode gives off electrons that flow to the cathode, resulting in wastage of metal: corrosion (Fig. 2). This arrangement is called a galvanic cell.
Every storage battery is made up of several galvanic cells. The word "galvanic" pertains to electricity and battery means "two or more of." A conventional 12v battery consists of 6 galvanic cells.
++Spacer Protection==
Whenever possible, two electrically dissimilar metals should not be joined together in the presence of an electrolyte. For
instance, copper and steel pipe fittings should never be connected to each other in a saltwater piping system.
The steel (least noble) metal will rapidly disintegrate due to galvanic corrosion. If it is necessary to use both copper and steel in a saltwater system, a dielectric plastic fitting should be inserted between the two metals to break up the electrical contact between them.
At one time, it was a common practice to mount bronze winches on aluminum masts. When spray or moisture from the air penetrated the gap between the mast and the winch to provide the electrolyte, a galvanic cell was established, resulting in corrosion of the aluminum.
The solution to the galvanic corrosion problem was to insert a non-metallic spacer between the mast and winch. That prevented exposure of both metals to the electrolyte.
++Zinc Protection==
When it is necessary to attach two electrically dissimilar metals and adding a dielectric spacer is not feasible, the anode metal can be protected from corrosion by attaching it to a metal that is higher in the galvanic series -- usually zinc -- to create a new anode.
The original anode is then converted into a cathode and is protected from corrosion (Fig. 3). The new anode, or zinc, is sacrificed to protect the original anode metal.
A typical example of cathodic protection by means of a sacrificial anode is the zinc collar that is attached to the propeller shaft to protect a bronze propeller mounted on a stainless steel shaft.
The sacrificial zinc collar should be inspected every two to three months and replaced if it is significantly decomposed or is no longer tightly attached. This inspection is usually conducted by a diver during periodic bottom cleaning.
Zincs can also be used to protect against electrochemical corrosion, a process that can occur even when electrically dissimilar metals are not involved.
An anode and cathode can be formed between two objects of similar metal construction when the objects have been produced by different manufacturing processes -- particularly if the metal of one of the objects includes more impurities than the metal of the other object.
A local anode and a cathode also can be formed on a single piece of metal due to difference in the electrolyte temperature,
salinity and dissolved oxygen content that may vary from one location to another. The local anode will give off electrons and corrode.
Fortunately, the local anode area can be converted into a cathode by attachment of a sacrificial zinc.
Sacrificial zincs are commonly installed in heat exchangers, oil coolers and other components exposed to the flow of sea water. To facilitate inspection and replacement, pencil zincs (threaded into a pipe plug) are usually inserted in heat exchangers, coolers and other equipment exposed to raw water.
++Shore Power Cables==
Galvanic corrosion can also occur when a boat is hooked up to a dockside shore power system.
As the green ground wire of a shore power cable is connected to the boat's ground, a galvanic cell condition may be established between the boat's underwater fittings and the underwater fittings of another boat or boats -- wired in the same manner to the same dockside power system. The least noble fitting on another boat will become the cathode and the anode fitting will corrode.
Deleting the connection between the green wire and the boat's ground is not an acceptable solution to this problem, as the connection is required to protect against electric shock.
One solution to this problem is the installation of a galvanic isolator in the green ground wire. This is a capacitor device that permits the flow of AC current but does not permit the flow of DC current.
In this manner, the underwater fittings of the boat equipped with the galvanic isolator are isolated from the underwater fittings of other boats connected to the same AC dockside source (Fig. 4).
Fig. 1
The Galvanic Series of Metals
(The Noble Scale)
Metal Volts
_______________________________________
Magnesium-1.60
Zinc-1.10
Cadmium -0.80
Aluminum -0.75
Mild Steel -0.70
Lead-0.55
††††††††††††††???
Tin -0.45
Stainless Steel (Active)-0.26
Brass (Yellow, Red or Admiralty) -0.26
Copper -0.25
Monel -0.20
Silicone Bronze -0.18
Stainless Steel (Passive)0
Silver 0
Gold+0.15
Platinum +0.27
Graphite +0.30
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This article first appeared in the July 1, 1994 issue of Sea Magazine. All or parts of the information contained in this article might be outdated. |
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