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.
Galvanic corrosion is the term you’re after. When you have dissimilar metals submerged in conductive liquid (water in this case), the more reactive metal of the 2 (or more) will have its ions stripped out and cause accelerated corrosion. To combat that ships have an even more reactive metal attached to the ship as an anode, commonly zinc, so that the ions from the zinc get stripped as a sacrificial part instead of the ship itself.
Water corrodes steel because of atomic forces that make iron and oxygen want to bond together. These forces are magnified in salt water. A sacrificial annode works by pulling those forces from the metal you want to save to the small block of zinc metal. This works because zinc wants to form zinc oxide more than steel wants to form iron oxide.
However the alloy that makes up ship propeller create forces closer to the ones zinc experiences when attached to normal steel. So you need more zinc to create a greater attraction for oxygen to bond with the zinc rather than the propellers.
I’ve worked w boats for years and this has become a real pain in the ass since Covid, bc Zincs have tripled in price for no good reason (thanks corporate greed assholes)
The topic is called cathodic protection.
Basically, metals are good at being conductive because they have extra electrons. These extra electrons would much rather be paired off. If an atom with too few electrons like oxygen comes close, the metal might bond with the oxygen. This can happen even if the oxygen is already in another molecule if the energy state is favorable. Oxidized metals are often called “rust”.
So you create conditions where it is energetically favorable for those atoms with too few electrons to bond to something else. Easiest way is to electrically connect a more reactive sacrificial anode to the metal. Zinc is most often used.
If you have a bunch of metal in salt water you make a junky battery. The battery runs down by ruining the metal. If the metal is something important like a ship you hate that, so you intentionally make a better battery by adding some zinc so that can run down and get ruined instead.
Different metals protect each other from corrosion depending on where they fall on the [galvanic scale](https://en.m.wikipedia.org/wiki/File:Galvanic_series_noble_metals.jpg). Propellers are a copper alloy like bronze and the hull is steel. Steel protects bronze because it’s higher on the scale. Zinc protects steel because it’s higher again on the scale again. The zinc annode is sacrificial. It protects the other metals from corrosion by itself corroding. It is simply replaced periodically to maintain protection.
Propellers are likely made of a different material than the hull. When you put dissimilar metals in a conductive solution like ocean water, they tend to undergo galvanic corrosion where the 2 metals start swapping atoms. The anode will be something that is easier to corrode galvanically so that it will be dissolved instead of the props.
Early Batteries were basically just two different metals put into salt water. They are empty once one of the metals has been eaten away completely.
Ships‘ propellers are usually a different metal than the hull.
To avoid your ship’s important parts becoming a giant battery and getting eaten away, you have to put a little bit of a different metal on it so it becomes eaten away instead.
Top comment is great, but not really in the spirit of ELI5..
Salt water likes to eat metal, and it likes to eat some metals more than others. If you give the salt water the choice to eat steel (what really big boats are made out of) and zinc (a nice shiny metal), the salt water will almost always choose to eat the zinc. The people who fix the boat can then go and replace the zinc (anode) much easier than they can replace the main parts of the boat.
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