Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737




The zink anode:

Galvanic corrosion.
When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.
Even a single type of metal may corrode galvanically if the electrolyte varies in composition, forming a concentration cell.

Galvanic corrosion is of major interest to the marine industry. Galvanic series tables for seawater are commonplace due to the extensive use of metal in shipbuilding. It is possible that corrosion of silver brazing in a salt water pipe might have caused a failure that lead to the USS Thresher sinking with all men lost.

There are several ways of reducing and preventing this form of corrosion.

  • One way is to electrically insulate the two metals from each other. Unless they are in electrical contact, there can be no galvanic couple set up. This can be done using plastic or another insulator to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminium and steel parts. Use of absorbent washers that may retain fluid is often counter-productive. Piping can be isolated with a spool of pipe made on plastic materials or made of metal material internally coated or lined. It's important that the spool should have a mimimum length of approx 500mm to be effective.
  • Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy, and allowing them to dry.
  • Coating the two materials or if is not possible to coat both, the coating shall be applied to the more noble, the material with higher potential. This is necessary because if the coating is applied only on the more active material, in case of damage of the coating there will be a large cathode area and a very small anode area, and for the area effect the corrosion rate will be very high.
  • It is also possible to choose metals that have similar potentials. The more closely matched the individual potentials, the lesser the potential difference and hence the lesser the galvanic current. Using the same metal for all construction is the most precise way of matching potentials.


Sacrificial anode
.
A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components.

In laymen's terms, it is a piece of more readily-corrodible metal attached by a conductive solid to a less readily-corrodible metal, with both metals immersed in a conductive liquid, typically fresh or salt water. The more active metal corrodes first and generally must oxidize nearly completely (hence the term "sacrificial") before the less active metal will corrode, thus acting as a barrier against corrosion for the protected metal.

More scientifically, a sacrificial anode can be defined as a metal that is more easily oxidized than the protected metal. Electrons are stripped from the anode and conducted to the protected metal, which becomes the cathode. The cathode is protected from corroding, i.e., oxidizing, because reduction rather than oxidation takes place on its surface.

For example when zinc and iron are put together (in contact) in the presence of oxygen and water, the zinc will lose electrons and go into solution as zinc cations. Electrons released from the zinc atoms flow through metallic conduction to the iron where, on the surface, dissolved oxygen is reduced, by gaining the electrons released by the zinc, to hydroxide anions. Were the zinc not present, the same reduction of oxygen to hydroxide would occur on the iron surface, however in that case the electrons for reduction would be furnished by the iron thus oxidizing the iron. Therefore, the zinc, when present,is "sacrificed" by being oxidized instead of the iron. The iron is "safe" until all of the zinc has corroded. As zinc is more costly than iron, this method of protecting iron, or steel, would not be cost effective were it not for secondary chemical reactions that form coatings on the iron surface thus shutting down the electrochemical reaction to a trickle and thus greatly prolonging the life of the zinc anode.

Other examples of protection by use of sacrificial anodes include protection of voids in the glass lining of mild steel water heater tanks via use of magnesium or aluminum alloy anodes, protection of off-shore oil rigs via special alloy anodes for use in salt water, protection of lock gates in water ways, etc.

It is important to understand that for this mode of corrosion protection to function there must be simultaneously present an electron pathway between the anode and the metal to be protected (e.g.,a wire or direct contact) and an ion pathway between the anode and the metal to be protected (e.g., water or moist soil) to form a closed circuit; thus simply bolting a piece of active metal such as zinc to a less- active metal, such as mild steel, in air will not furnish any protection.

Source of the above: Wikipedia

The making:

Here's how my sub got it's zink anode.

 
   I got the zink from the general hardware store. I had to buy a large sheet, normally used for roofs on houses.
I guess that explains the look in the clerk's eyes when he learned that it was for a submarine.. hehe.

The zink is connected to my ground connection, which again has electrical connection to the shaft, push rods, well... all exposed metal.

The white wire that passes behind it, has got nothing to do with the zink setup, it's the wires for the upper aft rudder top light.





Webmaster: Robert Holsting