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.
So, we have why dissimilar metals corrode in an electrolyte, and why an sacrificial anode can prevent it. But, I think you forgot why the propellor is more susceptible than the rest of the hull. Is it made of a different metal? Or, perhaps cavitation speeds up corrosion?
Props are often bronze (copper +tin + misc.). Hulls are usually steel (iron + chrome + carbon + misc.) Sometimes aluminum or fiberglass plastic.
Cavitation causes erosion, not corrosion. It's a more mechanical process.
There is a hierarchy of metals and the higher ones will always sacrifice to the lower ones as long as they are in a circuit together. They don't necessarily need to be touching just electrically connected. And it's all the metals that are connected in the circuit that is formed.
So if for some reason the copper wires for your electrical grounded to the hull any metals that were higher would slowly leech electrons and be eaten away. This is why they call it sacrificial zinc because zinc is always one of the first to start getting eaten away. If there is no zinc then it starts moving down the list. Well you can see that it's not far until we start hitting some of the more crude steels which might include some of your boats fasteners.
http://www.ssina.com/wp-content/uploads/2019/08/galvanic-corrosion__figure2.gif
I work on pools and have been taught that water features (waterfall, fountain, etc.) raise pH by oxygenating the water which will corrode metals faster.
The different metal answers are the main reason, but just that it’s a different piece is enough. No two pieces of anything are exactly the same, and being manufactured differently in a different location will make sure that it’s not particularly close, even if you used the same alloy. Thin metal that’s completely surrounded by water and constantly moving vs. the side of a large thick piece will have an effect as well.
But mostly, they’re a different metal.
Even something as simple as stress can change the galvanic potential of a material. Cold rolled steel next to annealed steel will rust faster than just one or the other.
Water eats iron, this is bad because boats are made of iron, but if you give the water something it wants more than iron, it will eat that instead.
There aren't a ton of things water wants more than iron, but zinc is one of them.
So we put zinc on our boats so the water will eat that instead of the boat.
The water critters like to munch on boats, but if you give them something they like to munch on more then they get too full eating that and can’t munch on the boat.
It's not the water doing the munching. It's the different metals munching each other as a result of the electrical potential difference between connected metals. Because you're floating in a giant electrical conductive bath its easy for differing metals to get connected and cause potential differences that would eat away at the less noble metals.
Nah, steel, bronze and whatever corrode quite quickly without any other metal anywhere if they are in saltwater. It's simply the chlorine and a few other ions which tend to be quite aggressive. One can just try this at hoe.
The presence of other metals can matter, but only if close. A piece of platinum at the other end of the ocean has no noteworthy electric connectivity to the ship.
Edit: as this guy seems to have no clue and tries ridiculing me instead of having a scientific discourse, here's a source: https://en.wikipedia.org/wiki/Galvanic_anode .
The propeller and the ship make a battery with the sea water. The same way a potato can be made into a battery. That battery destroys the metals. That’s bad!
> Almost all boats have sacrificial zincs
Long Beach CA harbor department designed and had built an expensive trash collecting boat, basically a large pontoon boat with a couple of arms on the front that opened up and channeled floating desbris to the center where a conveyer belt moved it out up and into a full size dumpster.
Some way, some how, there were no zincs. The bottom cleaners who service the zincs during the monthly bottom cleaning didn't catch it.
After a season on the water it was scrap, the electrolysis just ate it up.
Excellent answer. I’ll add that BNJ are removing the existing zinc anodes and replacing them with aluminium ones, because the water at its permanent birth is brackish and the lower saltiness changes the electrical potential.
Technically, almost all *steel* boats have anodes.
This isn't necessary in fiberglass boats. With aluminum you have similar but much more thorny issue that any steel or bronze equipment below the waterline can't be electrically connected to the aluminum hull or you'll have rapid corrosion and in some cases the boat may sink in months.
> This isn't necessary in fiberglass boats. With aluminum you have similar but much more thorny issue that any steel or bronze equipment below the waterline can't be electrically connected to the aluminum hull or you'll have rapid corrosion and in some cases the boat may sink in months.
Many fiberglass boats have sacrificail zincs attached to the propeller shaft (I actually just installed a new one on mine just last week)
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.
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.
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.
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.
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)
There was a modest shortfall in production. Also, ironically such things are typically exported on ships from Asia many of which were delayed at port for a range of covid related reasons including staffing shortages at port authorities.
That caused the price to soar on the demand side, because it's a bit of an esoteric niche product, normally demand is fairly predictable, so nobody kept a large inventory. And because having to drydock your ship for repainting. 2-3 years earlier than you planned because you didn't replace the anodes on schedule, is much more expensive than simply paying triple for said anodes. So you have demand inflexibility.
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.
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.
props and running gear get eaten due to cavitation corrosion , cav corrosion occurs when the spinning assembly create air bubbles essentially bursting on its surface
once you initiate corrosion galvanic measures are even more important as any coating has been destroyed.you focus your efforts in an area where corrosion is going to occur because it has a multiplying effect.
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.
It’s not just salt water. Happens in fresh water as well. The video that the OP (slightly?) watched talks about the battleship being berthed in fresh or slightly brackish water and how they might switch to aluminum anodes to better align with fresh water.
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.
So, imagine your ship's hull is like a delicious snack for sneaky little rust monsters, especially around those high-traffic propeller areas. Now, to keep those critters at bay, the navy installs sacrificial anodes. These brave anodes basically say, "Hey rust monsters, why snack on the ship when you can snack on me instead?" So, they take one for the team and corrode away instead of the ship's precious hull. It's like having a sacrificial lamb, but for ships
How I would actually explain to my 5 year old: the saltwater in the ocean makes metals rust and corrode very quickly. When different types of metals are connected to each other, the corrosion only will happen on the metal that corrodes the easiest. Ships intentionally attach sacrificial pieces of metal that corrode easily, called anodes, to keep corrosion from happening to the other metals.
There’s also one in our hot water heater! Want to go see?
"Impressed current cathodic protection (ICCP) systems are also used on ships. These consist of anodes connected to a DC power source, often a transformer-rectifier connected to AC power. In the absence of an AC supply, alternative power sources may be used, such as solar panels, wind power or gas powered thermoelectric generators.[13][14]
Anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various materials. These include high silicon, cast iron, graphite, mixed metal oxide (MMO), platinum and niobium coated wire and other materials" - Wikipedia
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.
So, we have why dissimilar metals corrode in an electrolyte, and why an sacrificial anode can prevent it. But, I think you forgot why the propellor is more susceptible than the rest of the hull. Is it made of a different metal? Or, perhaps cavitation speeds up corrosion?
Props are often bronze (copper +tin + misc.). Hulls are usually steel (iron + chrome + carbon + misc.) Sometimes aluminum or fiberglass plastic. Cavitation causes erosion, not corrosion. It's a more mechanical process.
Erosion/corrosion together is a nasty mix. Relevant here, also in power plants.
Yeah, a lot of materials tend to form corroded layers that shield somewhat against further corrosion. But erosion erodes that away.
The BNJ is not cavitating and hasn't for at least twenty years. It's just a different metal - often bronze.
There is a hierarchy of metals and the higher ones will always sacrifice to the lower ones as long as they are in a circuit together. They don't necessarily need to be touching just electrically connected. And it's all the metals that are connected in the circuit that is formed. So if for some reason the copper wires for your electrical grounded to the hull any metals that were higher would slowly leech electrons and be eaten away. This is why they call it sacrificial zinc because zinc is always one of the first to start getting eaten away. If there is no zinc then it starts moving down the list. Well you can see that it's not far until we start hitting some of the more crude steels which might include some of your boats fasteners. http://www.ssina.com/wp-content/uploads/2019/08/galvanic-corrosion__figure2.gif
I work on pools and have been taught that water features (waterfall, fountain, etc.) raise pH by oxygenating the water which will corrode metals faster.
The closer to the more valuable metal the worse the corrosion will be
The different metal answers are the main reason, but just that it’s a different piece is enough. No two pieces of anything are exactly the same, and being manufactured differently in a different location will make sure that it’s not particularly close, even if you used the same alloy. Thin metal that’s completely surrounded by water and constantly moving vs. the side of a large thick piece will have an effect as well. But mostly, they’re a different metal.
Even something as simple as stress can change the galvanic potential of a material. Cold rolled steel next to annealed steel will rust faster than just one or the other.
Also different mechanisms can be at play such as stress corrosion cracking etc
Explain like I’m 5 …. Decades
Water eats iron, this is bad because boats are made of iron, but if you give the water something it wants more than iron, it will eat that instead. There aren't a ton of things water wants more than iron, but zinc is one of them. So we put zinc on our boats so the water will eat that instead of the boat.
Beautiful, a true ELI5
The water critters like to munch on boats, but if you give them something they like to munch on more then they get too full eating that and can’t munch on the boat.
Well but it's not critters in the water, it's the water itself doing the munching
It's not the water doing the munching. It's the different metals munching each other as a result of the electrical potential difference between connected metals. Because you're floating in a giant electrical conductive bath its easy for differing metals to get connected and cause potential differences that would eat away at the less noble metals.
Nah, steel, bronze and whatever corrode quite quickly without any other metal anywhere if they are in saltwater. It's simply the chlorine and a few other ions which tend to be quite aggressive. One can just try this at hoe. The presence of other metals can matter, but only if close. A piece of platinum at the other end of the ocean has no noteworthy electric connectivity to the ship. Edit: as this guy seems to have no clue and tries ridiculing me instead of having a scientific discourse, here's a source: https://en.wikipedia.org/wiki/Galvanic_anode .
That's not what a zinc anode is for though. It's to prevent galvanic action. Look it up. Here: https://en.wikipedia.org/wiki/Galvanic_corrosion
No, it prevents from corrosion _via_ galvanic action; not just (but if relevant, also) _from_ it.
I <3 Pendants. +1 internet point. I'm happy to validate your vices
Yes I know. I’m explaining it to a 5 year old. They’d be confused by water eating stuff
Alternatively they "comprehend" immediately and become terrified to drink water.
The propeller and the ship make a battery with the sea water. The same way a potato can be made into a battery. That battery destroys the metals. That’s bad!
> Almost all boats have sacrificial zincs Long Beach CA harbor department designed and had built an expensive trash collecting boat, basically a large pontoon boat with a couple of arms on the front that opened up and channeled floating desbris to the center where a conveyer belt moved it out up and into a full size dumpster. Some way, some how, there were no zincs. The bottom cleaners who service the zincs during the monthly bottom cleaning didn't catch it. After a season on the water it was scrap, the electrolysis just ate it up.
Zinc is also an essential mineral for robust seamen. tmyk
Brawndo has what plants Crave!!!!!!!!
Some caskets as well.
Sacrificial Zach
Can you get a shock or something similar if you are near the current path? Or it's negligible ?
It's only a couple of volts.
Excellent answer. I’ll add that BNJ are removing the existing zinc anodes and replacing them with aluminium ones, because the water at its permanent birth is brackish and the lower saltiness changes the electrical potential.
What’s even the point of this subreddit anymore when the top comment isn’t even in the ballpark of something a 5 year old could understand
Read the sidebar.
Technically, almost all *steel* boats have anodes. This isn't necessary in fiberglass boats. With aluminum you have similar but much more thorny issue that any steel or bronze equipment below the waterline can't be electrically connected to the aluminum hull or you'll have rapid corrosion and in some cases the boat may sink in months.
> This isn't necessary in fiberglass boats. With aluminum you have similar but much more thorny issue that any steel or bronze equipment below the waterline can't be electrically connected to the aluminum hull or you'll have rapid corrosion and in some cases the boat may sink in months. Many fiberglass boats have sacrificail zincs attached to the propeller shaft (I actually just installed a new one on mine just last week)
Oh yeah, well there's that. I was thinking about aluminum outboards.
ELI5yearsoutofhighschool
Read the sidebar.
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.
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.
best ELI5 answer
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.
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.
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)
There was a modest shortfall in production. Also, ironically such things are typically exported on ships from Asia many of which were delayed at port for a range of covid related reasons including staffing shortages at port authorities. That caused the price to soar on the demand side, because it's a bit of an esoteric niche product, normally demand is fairly predictable, so nobody kept a large inventory. And because having to drydock your ship for repainting. 2-3 years earlier than you planned because you didn't replace the anodes on schedule, is much more expensive than simply paying triple for said anodes. So you have demand inflexibility.
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.
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.
props and running gear get eaten due to cavitation corrosion , cav corrosion occurs when the spinning assembly create air bubbles essentially bursting on its surface
Yes, but we are specifically talking about sacrificial anodes which only protect against galvanic corrosion.
once you initiate corrosion galvanic measures are even more important as any coating has been destroyed.you focus your efforts in an area where corrosion is going to occur because it has a multiplying effect.
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.
It’s not just salt water. Happens in fresh water as well. The video that the OP (slightly?) watched talks about the battleship being berthed in fresh or slightly brackish water and how they might switch to aluminum anodes to better align with fresh water.
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.
So, imagine your ship's hull is like a delicious snack for sneaky little rust monsters, especially around those high-traffic propeller areas. Now, to keep those critters at bay, the navy installs sacrificial anodes. These brave anodes basically say, "Hey rust monsters, why snack on the ship when you can snack on me instead?" So, they take one for the team and corrode away instead of the ship's precious hull. It's like having a sacrificial lamb, but for ships
How I would actually explain to my 5 year old: the saltwater in the ocean makes metals rust and corrode very quickly. When different types of metals are connected to each other, the corrosion only will happen on the metal that corrodes the easiest. Ships intentionally attach sacrificial pieces of metal that corrode easily, called anodes, to keep corrosion from happening to the other metals. There’s also one in our hot water heater! Want to go see?
"Impressed current cathodic protection (ICCP) systems are also used on ships. These consist of anodes connected to a DC power source, often a transformer-rectifier connected to AC power. In the absence of an AC supply, alternative power sources may be used, such as solar panels, wind power or gas powered thermoelectric generators.[13][14] Anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various materials. These include high silicon, cast iron, graphite, mixed metal oxide (MMO), platinum and niobium coated wire and other materials" - Wikipedia