I follow battleship New Jersey on YouTube. One of the recent topics is how the hull around the propellers can corrode more than other areas of the hull. Because of this, the navy installed sacrificial anodes.
Why would a large ship corrode around the propellers more. How to anodes prevent this?
In: Chemistry
Almost all boats have sacrificial zincs
When trying to understand corrosion that occurs in salt water, you have to think about salt water as containing electrolytes that create an electrically conducive solution. When metals are introduced into this solution, such as iron, bronze or aluminum that is constructed into boat hulls, ship propellers, outboard engines, storage tanks, and fuel pipelines, each metal will have a type of electrochemical potential or active voltage.
As the electrolytes dissolve, parts of it are drawn to the metal that has an abundant number of electrons, while the other parts are drawn to another metal that has a deficit in the number of electrons it possesses. This back and forth movement of the dissolved electrolytes creates a current, as the salt water breaks apart the metal parts by making them give up its electrons to the saltwater. This process is called galvanic corrosion.
A sacrificial anode consists of a metal alloy, such as zinc, that has a more active voltage when it is introduced into the electrolyte current. The zinc has a greater negative electrochemical potential than other metals when it is placed into salt water. The purpose of the zinc is to have it “sacrifice” its electrons faster than the metal it is mounted to.
As the zinc anode is pulled apart during the electrolyte process, the other metal is protected as the electrolytes are more attracted to the active voltage that the zinc anode possesses. The aluminum, bronze and iron parts in the saltwater undergo less corrosion.
Zinc anodes are the preferred choice in metal alloys for saltwater applications that need a sacrificial anode, because the alloy is less resistant to the saltwater’s electrolytes. The zinc, in essence, stops the oxidation happening to the other metal part as the zinc dissolves away.
The amount of zinc anodes that are needed to protect the other metal surface will be based on several factors, such as how much of the other metal part will be in constant contact with the saltwater, what type of metal is the part made out of, and what type of shape must the zinc anode be made into.
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