Since water is a great conductor, and rust is an electrochemical reaction, rusting is possible underwater. Keep in mind that steel and iron are both susceptible to humid, damp conditions, so simply being submerged underwater may not be enough for rusting unless the temperature is relatively warm. However, does metal rust underwater?
Yes. Metals, especially ferrous iron, rust underwater and in moist air, and they rust faster if the medium of reaction is acid rain or saltwater. When water touches metals like iron, the water reacts with carbon dioxide (CO2) in the atmosphere to form weak carbonic acid.
This phenomenon occurs because salt water is an electrolyte (electrons move freely within the solution), and the number of dissolved ions present in the solution is higher than the ions present in freshwater.
In this article, you will learn how metal rusts underwater, the duration of the rusting process, how to prevent metal from rusting underwater, and what happens to metal under saltwater
How Do Metal Rust Under Water
The process through which metal rusts under water is known as oxidation. This oxidation occurs when iron reacts with oxygen and water to form an oxide which is commonly known as rust. For rusting to occur in iron or steel, water and oxygen must be present.
The first reaction that takes place in the early stage of rusting is the breaking down of oxygen molecules by water, which helps the metal’s reaction with oxygen. After this process, iron will lose its electrons while oxygen absorbs them.
Then, water will react with the ferrous and ferric ions contained in the iron to give ferric hydroxide, hydrogen, and ferrous hydroxide. The formation of more iron compounds will depend on how the hydroxides lose their water content.
When these chemical processes happen, it will help to form the rust particles that will later drop off the surface of the metal, making the inner part susceptible to more corrosion because it will get acted on as well.
What’s the Duration of Rusting of Metals Under Water?
Once a metal is in an environment where water and oxygen are present or in abundance, the iron may start to show signs of rust within a minimum of four to five days. Under water, green rust will first appear after a reaction between iron and chloride, and this rust is a result of the corrosion of iron or its alloys like steel.
If the iron is in an outdoor environment with relatively low humidity (low amount of water vapor in the air), the iron will start to show signs of rusting from two to four days, while if it is in an outdoor environment with relatively high humidity (high amount of water vapor in the air), the sign of rusting will start to occur from the first day the iron was kept.
Note: Different types of metals exist and these differences play a significant role in the rate at which they rust. Metals with a high level of iron composition will rust faster under water than iron with a low level of iron composition, or metals that are made of steel.
Do Salt Water Rust Metal Faster?
When a metal rusts, it shows that a chemical reaction took place in the metal, and hydrated iron oxide is one of the outcomes of exposing iron to water or moist air.
Under salt water, iron rusts rapidly because the presence of salt in the water triggers an electrochemical reaction which inhibits the formation of a protective film that would have stopped the metal from rusting, hence, the iron continues to rust without a hindrance.
Note: Pure water or dry oxygen does not affect iron. Rust will not occur when the iron is kept under water that contains dissolved salt, rather, the presence of dissolved salt will speed up the rate of the chemical reaction the way acid rain does.
How to Prevent a Metal From Rusting
To prevent metal from rusting after removal from salt water, wash the metal thoroughly in fresh water and dry it well, especially the small openings where there might be some residues of saltwater.
Metals that are always kept in saltwater should be immersed completely in kerosene, oil, or antifreeze when storing them. This method of prevention is what keeps boat engines and other metals that are always inside the water when a boat is ferrying from rusting.
Also, metals that are always inside water, especially saltwater can be galvanized. Galvanization is a process through which metals especially steel can be prevented from corroding or rusting. It is done by dipping the metal in a zinc coating that makes it not partake in the oxidation reaction where iron reacts with oxygen and water.
But scratching or scraping a metal that has been galvanized can make the exposed part prone to reacting with oxygen and water which will eventually make the metal start rusting. Also, there are specially produced paints that can be used to coat metals to prevent them from rusting.
Frequently Asked Questions
Does Carbon Steel Rust Under Water?
Yes, mild steel which is commonly known as carbon steel will rust with time because the small quantity of carbon present in mild steel cannot prevent it from rusting, except if the metal has a protective coating that is capable of preventing it from corroding.
When elements like titanium and chromium are added to steel components, they will only help to enhance the corrosion resistance strength of the mild steel, but cannot make the steel rust-proof.
Do All Metals Rust Under Water?
No, not all metals rust under water. Aluminum, for example, will not rust under water because its surface is coated with a protective layer of aluminum oxide. This protective layer acts as a barrier that stops the metal from having direct contact with the water and oxygen.
Metals can rust under water once the conditions responsible for the reaction to take place are present; conditions like the water being saltwater, oxygen being present or the environment having either a low or high humidity.