Galvanic%20Action%20(01-25-2K)
 

Question:

Does anyone know why lobster fishermen attach zinc plates to their traps? Also, I have learned that these plates are sometimes attached to the hulls of seagoing vessels. Why?

jk316@pacbell.net

Response No. 1:

Being a commercial waterman from Maryland, I installed zinc bars in my crab pots to stop them from rusting out. A crab pot without a zinc bar only lasts one to maybe one and one-half seasons. With a zinc bar added annually the life of a pot extends four to five years. The same theory applies to the hulls of wooden boats, saving the nails! They are also found on outboard motors, which stops electrolysis from eating the lower units. Yearly replacing the zinc bars would cost less than replacing crab pots, lower units or run the risk of planks coming loose on wooden-hulled boats. Thank God for zinc!

Conrad and Cindy Mercure

Response No. 2:

Metals are ranked relative to one another based on what happens when they are in close proximity in the presence of electric current (which is frequently present in water from a variety of sources including dock electric leaks and improper grounding on boats themselves). This is called electrogalvanic action. More "noble" metals tend to attract material from less "noble" metals, with bronze, for example, being more noble than mild steel. Less noble metals become pitted, and eventually rotted through. It has been found that placing samples of a metal that is extremely low on the scale (that is, one that will lose material to just about any other metal) in the general area, like near bronze propellers and shafts, causes the current to waste its energy taking away the "sacrificial" metal (and in fact coating the other metals with it). Zinc is the common choice for such sacrificial "anodes", and lobstermen might place zinc on their traps if the traps are composed of dissimilar metals like bronze and steel to deter deterioration of the steel.

Norm Petterson, npetterson@mailhost.cle.ameritech.net

Response No. 3:

He answered his own question. Zinc attracts the charged particles rather than attacks the hull of the ship or the motor. These plates are used (sacrificed if you would) since they are cheap to replace.

Jeff Eichman, jse@fast.net

Response No. 4:

Seawater contains many dissolved solids, which are corrosive to metals normally subject to oxidation (rusting). In order to protect these metals, it is common to employ the use of galvanic couples composed of a more active metal (anode) electrically connected and placed in close proximity to the metal to be protected (cathode). A current is generated in this aqueous environment between the two metals, causing the anode to slowly dissolve, while the less active metal is protected (passivated) by the negative charge it receives. The huge oil drilling platforms at sea, U.S. Submarine Fleet, all Navy ships, destroyers, aircraft carriers, etc. (among other vessels and structures) are protected in this manner.

R. C. Mullin, rcmullin@aol.com

Response No. 6:

Another term for the metal plates is "sacrificial metal." This is done to prevent the galvanic reaction of the seawater from corroding the metal hull of the ship. I don't know exactly why lobster boats attach zinc plate to their traps.

Mike Wash, mike_wash@atce.com

Response No. 7:

Marine galvanic zinc anodes are used for cathodic protection of more noble metals and alloys. The first recorded use of sacrificial anodes was recorded when Sir Humphrey Davey had zinc plates installed on the copper-sheathed hulls of British naval vessels in 1824. From this inauspicious beginning, the science of cathodic protection evolved not only in seawater, but also in a multitude of electrolytes including soils. Refer to: http://www.pzm.com/anodes.html, http://bluecrab.richmond.edu/zinc_anode.htm, or http://edie.cprost.sfu.ca/~rhlogan/electrol.html. Since saltwater is very corrosive, sacrificial zinc anodes are attached to a pot which effectively doubles its life span (commercial pots without zinc anodes usually last only one or two seasons). http://bluecrab.richmond.edu/zinc_anode.htm

When I lived in Lake Worth, FL, I used round clamp-on anodes on my propeller shaft (inboard motor), disc-type anodes on my rudder and the little twist-on anodes on my blue crab traps. The anodes are, indeed, the sacrificial lambs.

Don Couch, dcouch@jazz.cybermedia.net

Response No. 8:

Zinc is a sacrificial material that will give itself up before the steel trap will start to rust. This method is also employed on ship hulls as well as other steel objects in contact with highly corrosive environments. Galvanized conduit and tubing is galvanized to deter rust from small scratches. The galvanized coating will sacrifice itself. Check the complete theory and details for corrosion protection in my raceway book.

Dick Loyd, DICKONEL@aol.com

Response No. 9:

As an ex-submariner, the superstructure of my submarine had zinc mounted all over the place to prevent rust. They look like silver ingots.

Tim Shields, Timothy_Shields@cytec.com

Response No. 10:

They are called  "sacrificial anodes." They are "more active" than steel and other exposed metals, and so they corrode in preference to other parts of a vessel. They must be replaced periodically. That's why conduit is zinc plated, to protect against corrosion.

Jess Kray, JessKray@aol.com

Response No. 11:

These plates are called sacrificial anodes. The theory is that the corrosion will attack the zinc and leave the more valuable trap or hull undamaged. Or, to quote the Institute of Marine Engineers "Cathodic protection - Electrolytic system to protect metal from corrosion by providing an electric current neutralizing the current naturally produced in the corrosion process. Current may be provided by sacrificial anodes or through incorrodible anodes in an impressed current system. The electric circuit is completed through the corroding medium to the cathode, the metal to be protected." If you need more information on the recommended number of zincs per square foot, I would recommend contacting a marine engineer. They know the equations.

Lael D. Williams, lael.d.williams@intel.com

Response No. 12:

The zincs are sacrificial elements to help prevent loss of metal due to galvanic action. This galvanic action can be as a result of normal interaction between dissimilar metals or stray electric currents. The boat owners typically use them to prevent their props (brass or bronze) and drive shafts or seals from corroding (as well as other expensive metal parts).

The areas around marinas can have a tremendous amount of stray currents as a result of faulty wiring. A few years back, I was called in to find out why a boat owner (that fell in the water next to his boat), was electrocuted when he climbed up on the dock in Norfolk, VA. In addition to this, many of the boats in the immediate area were losing props and shafts at a record pace. Some of the owners were hanging huge multiple zincs on the transoms to compensate for the problem. The marina owners thought it was the U.S. Navy base across the way.

Upon inspection, I found that the edge of the floating wood dock had a series of diamond plate covers over the electrical trough. Although most of the plate-grounding bonds were broken or missing, there was enough incidental contact to provide a ground reference. There were power pedestals placed periodically along the dock to provide power to the boat slips. When I attached one lead of my digital meter to the diamond plate and dropped the other lead into the water at the point where this unfortunate gentleman fell, I was reading close to 200-volts. To isolate the cause, I began disconnecting the service cords to each boat until the potential disappeared. The boat causing the problem was in the slip right behind the one where the guy was electrocuted. It seems that the owner was too cheap to buy a commercially available "Y" adapter and built one himself, reversing the phase and ground. Essentially, the water around his boat was "live" and the poor guy that fell in the water completed the path when he touched the diamond plate. In addition, the hull of his boat became an electrode and while everyone else's boat was losing metal, his was gaining it.

I had recommended to the marina manager that no boat owner should be permitted to utilize "home" built electrical devices and that each boat should be thoroughly checked before being permitted to connect to the service pedestal (I even offered to build a simple test box for them). To this day, I am not aware of them doing anything about it at all.

L. Bartsch, lbartsch@hns.com

Response No. 13:

Deter? No. Promote? Yes, because of the potential difference between the zinc and the metal hull of the ship the zinc is electroplated to the hull of the ship. As for the fisherman traps, it might either be for electroplating the chain on the tops of the traps (the lobsters like the zinc and are attracted to the field created by the electroplating process), or it is just a cheap and safe weight that won’t readily dissolve in sea water.

Zinc plates are generally attached to boats that are in contact with seawater, which have metal parts also in contact with the seawater. The zinc tabs reduce the galvanic action of seawater on metal parts by concentrating the corrosion on the zinc plates themselves. The plates are not limited to sea-going vessels. My father’s 18-foot wooden boat, driven by an outboard motor, had these zinc plates attached below the transom, directly below the motor. After several months, you could begin to see the corrosion. I believe the plates are used only if long term continuous exposure to the salt water is anticipated.

I don't know if these plates are used on large vessels of metallic construction. I believe they have a separate cathodic protection system. The last time I did any design work involving a loading dock at a marine terminal, special precautions had to be taken to prevent the terminals grounding system from coming into contact with the ship.

Steve Measell, stephen.measell@halliburton.com

Response No. 14:

Lobster fishermen do attach zinc plates to their traps to prevent galvanic corrosion. Not only lobster fishermen, almost every marine vessel now has some sort of zinc plate to prevent galvanic corrosion. The zinc plates acts as a sacrificial metal so that it is attacked first and not other metals used on drive equipment and the like. Take a look at any out-drives of an inboard/outboard motor and you will see a small zinc plate or disk.

Scott Alford, salford@niagarablower.com

Response No. 15:

These zinc objects attached to boats and objects immersed in salt water are called "sacrificial anodes." They are "less noble" (I believe that is the proper term) than the other metal parts that are also immersed in the water and so they corrode, saving the other metal parts in their vicinity. The scientific explanation is very complex, involving dissimilar metals being immersed in an electrolyte (salt water) forming a battery and the "less noble" of the two metals losing molecule, but the zinc anodes really work. Outboard motors have come with them attached for years.

Donald M. Haskin, Haskindm@navair.navy.mil

Response No. 16:

It keeps the mussels from attaching to them.

John Landry

Response No. 17:

The reason for the plates is to control "electrolysis". An electric charge is generated on every object as it moves through the water. Metal is "eaten away" if a plate of zinc or copper or brass isn't attached to the object, which will attract and dissipate the charge. The readily conductive metal is "sacrificial," it is eaten away by electrolysis rather than the metal hull of the ship or the steel wire attached to the nets or traps.

Bill, wct1945@accessky.net

Response No. 18:

The technique is called sacrificial anode. We use it in the huge water pipes of power plants as well. I've installed large blocks of magnesium or zinc (I can't remember which) to protect the walls of the large pipes feeding the cooling towers. I don't know if it works, because I've never been back into one of the water tunnels after it was used for a while, but it is often specified on large projects.

Mike Wood

Response No. 19:

Sacrificial Cathode. Take a look at what a wet cell battery is. Two dissimilar metals in a solution that has a high enough free hydrogen content, or is acidic. Salt water is one of those solutions as well as water with high bromine or chlorine levels. So, you take two different metals and place them in this solution and you have a battery, and as the battery works, one of the two metals will go to solution, unless you return the electrical energy, in which case the metal will return to the cathode that was dissolved. This is why you don't have to replace the lead in your car battery.

In the case of the lobster traps or for that matter any unbonded metal in or around a swimming pool a battery will be set. The zinc plates become the sacrificial cathodes, or go into solution, and can be easily replaced when needed, thus keeping most other metals intact. Zinc is one of the better sacrificial metals because of the cost and how easily it will be dissolved by the acid solution. In our case, we have a 1.5-million gallon pool with epoxy and cement covered mild steel of various varieties, 316 stainless and bronze that stays intact in the high chlorine and bromine solution. 304 stainless can't stay in the pool for long periods and aluminum dissolves in under a week. This also happens by the same means. To keep the different metals alive we bond over 600 zinc ingots to the steel and it dissolves rather than the metals that we want to keep. This is the same thing that ship builders do with their zinc ingots bolted to the hulls of ships. I believe that electrical bonding has a great deal to do with dissimilar metals in pools (stainless, bronze, mild steel mesh and re-bar) to prevent dissolving of these metals, rather than electrical safety. Imagine if the conduit to the pool dissolved...

Michael Wescoatt

Response No. 20:

The zinc is used as a sacrificial anode. The zinc corrodes first and the other metal is protected. The zinc is discarded and replaced when it is excessively corroded. Zinc plates are used in the heat exchangers of ships. I remember being glad I was an electrician’s mate when my mechanic friends onboard the USS George had to the replace those smelly things.

Deannitz@aol.com

Response No. 21:

The anode is designed to be eaten by any electrical current in the water or boat. This prevents the vital engine parts from corroding instead.

JnHn@aol.com

Response No. 22:

I'm an electrician by trade so I couldn't explain the particulars to you but I used to work in a boatyard and I've replaced a ton of zincs. These are designed, as far as I know, to deter galvanic action, and they are usually installed around the shafts, and around the out-drives, (the lower unit portion of the engine). From what I've seen of the bottom of boats, they also work quite well.

INELUKTBL@aol.com

Response No. 23:

Yes. The zinc plates are to prevent galvanic corrosion. The plates corrode instead of the material they are mounted to. We used them all the time in the Navy.

Brad Scheidecker, bscheidecker@genphysics.com

Response No. 24:

The attachment of zinc to ships hulls, etc., is to deter the galvanic action caused by the charge built up on the hull, and metal parts by the movement of the water on the hull. The zinc is expendable, and is used up, "electrode" so to speak.

Ken Teverbaugh

Response No. 1:

HERE'S THE REASON WHY (Lots of Theory) –

GALVANIC POTENTIAL. The galvanic potential of a corrosion cell refers to driving voltage caused by the connection of two differing metals immersed in an electrolyte. The galvanic potential of the cell is calculated as the difference between the open-circuit potentials of the two metals. If the potential difference of the cell is large enough to overcome the resistivity of the metal conductor and the resistivity of the electrolyte, current will begin to flow in the cell. The more anodic (electronegative) material will corrode and the rate of corrosion will be directly related to the magnitude of the potential difference of the galvanic couple. A galvanic couple of metals in which the potential difference is zero will not experience galvanic corrosion.

The half-cell potential of steel freely rusting in seawater is approximately 0.60V. The corrosion action in the galvanic cell stops when the polarization potentials of the metals equalize. It has been determined that corrosion on the steel surface is stopped when the steel has been catholically polarized (charged) to -0.85V (Ag/AgCl/Seawater).

GALVANIC OR DISSIMILAR METAL CORROSION ATTACK. When two dissimilar metals, such as aluminum and steel, are coupled together and subjected to a corrosive environment (such as air, water, salt spray, or cleaning solutions), the more active metal (aluminum) becomes the anode and corrodes through exfoliation or pitting.

CURRENT FLOW. The direction and rate of current flow depends on the composition of the two metals and also factors such as the hardness and cleanliness of the metal surfaces. If several different metals are involved, current will flow from the metal with the higher electron potential (more active) to that with the lower potential (less active). The rate of current flow (and thus the rate of galvanic corrosion) depends on the electron potential difference between the two metals. If a single metal is immersed in an electrolyte and one part of the metal surface is harder or cleaner than another part, current will flow between these parts and galvanic corrosion will occur. The anodic metals such as zinc and aluminum are considered to have a more electronegative potential than the relatively cathodic (less corrosion prone) metals, such as gold and platinum.

SEAWATER RESISTIVITY. When two differing metals (such as steel and zinc) are electrically connected in seawater, the seawater acts as the electrolyte in the electrochemical corrosion cell. In all direct current electrical circuits, Ohm's Law governs the rate of current flow. In the electrochemical corrosion cell, the rate of current flow between the cathode and the anode in the metallic path is determined by the potential difference of the metals (driving voltage) and the fixed resistance of the metal.

CATHODIC PROTECTION AND SACRIFICIAL ANODES. Most seagoing vessels are equipped with sacrificial anodes to offset adverse effects of dissimilar metal connections, stray electrical currents, improper grounding (see ungrounded power systems below), and other sources of electrolytic corrosion. These anodes provide a continuous flow of protective current to corrosion susceptible areas but are sacrificed in doing so. In addition to external hull anode protection, some installations contain anodes, which provide protection in bilge areas, sea chest, condensers, and overboard discharges.

JUST A NOTE FOR OUR BOAT OWNERS –

UNGROUNDED POWER SYSTEMS. Most electric power systems on ships and boats are ungrounded. This means that there SHOULD BE NO PERMANENT, low-resistance connections between the power system and the vessels’ structure. The reason for using ungrounded power systems is to improve the reliability of power to all power using equipment and systems. A secondary reason is to prevent galvanic corrosion of the hull, and engine or motor bearings caused by current flowing through either welded joints or joints of dissimilar metals. An ungrounded power system is never to be considered safe to touch or work on while hot. If a power system is permanently grounded at one point, any accidental ground that may occur on a different leg of the system will immediately cause a fault current to flow. This can be a source of fire and electric shock. On the other hand, if a ground occurs on any leg of a truly ungrounded power system, no current will flow until a second ground occurs on another leg.

Stan Price, stan.price@juno.com

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