If you have ever left a steel item outside, you have probably already witnessed the speed of rust, add saltwater to the equation, and you have an acceleration of corrosion. How does it happen, and what is the cause? Let’s find out why steel rust in saltwater.
Saltwater corrodes steel and other iron objects that are submerged in saltwater fast. Saltwater accelerates rusting because it behaves as an electrolyte allowing the steel to lose its electrons. Dissolved ions let electrons move more quickly on the steel, causing rust to form.
The reddish-brown substance known as rust or iron oxide happens when specific chemical reactions take place. Rust forms when steel and oxygen react in water. This article looks at the process and what causes steel to rust when in contact with saltwater.
What is Rust?
Iron oxide or rust is a reddish-brown flaky coating that is formed on steel or iron objects when oxidation occurs in the presence of water molecules. The rust layer is generally many times wider than the original steel and typically leads to flaking of the surface, reducing its thickness. The buildup of rust can cause failure by breaking adjacent parts apart; this phenomenon is commonly known as rust packing.
The chemical reaction that creates rust happens when electrons and atoms interact. Some chemicals can speed up this process, like acids and saltwater. Rust occurs when the electrical activity between oxygen and iron increases. Saltwater increases the conductivity of water around steel, and rust occurs.
How Does Rust Form?
When water and acidic substances encounter steel or iron, rust builds. Rust is the result of corroding steel after the iron particles have been exposed to moisture and oxygen. The iron reacts and forms iron oxide, which is rust. Both oxygen and water are needed for rust to form.
- Iron + Water + Oxygen = Hydrated Iron (III) Oxide / Rust
Salt – Sodium Chloride
Salt is among the ten most abundant elements on the Planet and a principal component of a healthy functioning body affecting fluid balance, muscle function, and cellular reactions.
Salt, in its chemical form, is also one of the most destructive elements on the Planet. Sodium chloride, or salt as we know it, reveals itself as corrosion on steel, metals, stainless steel, ceramics, and many other materials. Saltwater content in the air can also cause steel to rust.
Saltwater Rusts Steel
Steel is often mischaracterized as metal; however, it’s an alloy of iron and several other carbons. Steel can and does rust in saltwater. Salts and other chemicals increase the conductivity of water around steel. Since objects underwater are continuously exposed to moisture and oxygen, rust formation becomes unavoidable if made of any kind of metal.
Saltwater will corrode steel much faster than freshwater. Saltwater is an electrolyte that conducts ions, lowering the electrical resistance of water and the quicker electrons flow from iron to oxygen, the faster the steel will rust.
When salt sticks to steel surfaces, it strips off electrons that protect the steel. The movement of electrons is called oxidation and is the primary cause of corrosion. If there is no oxidation, corrosion will not happen.
The combination of water, oxygen, and salt, particularly sodium chloride, will damage steel rapidly. This trio eats away at steel, weakening it and causing it to break apart.
Saltwater corrodes iron five times faster than freshwater, and salty humid air cause iron to rust ten times quicker than normal air would. Saltwater, salt air, and road salts will corrode and ruin steel.
Steel doesn’t have to be completely submerged underwater to start rusting; salty air can rust steel just as fast as moisture is always present in the air, which will cause corrosion.
How Fast Does Steel Rust in Saltwater?
Corrosion will start almost immediately, and signs of rust can be seen in 4 to 5 days. However, there are several factors that affect the speed of rust formation, various grades of steel rust at varying rates. The steel type with more significant amounts of iron will rust quicker than those without.
Corrosion studies have shown that steel constantly exposed to saltwater has only three to eight years of service.
What Type of Steel is The Best for Saltwater?
The kind of steel best for saltwater conditions is typically stainless steel. Duplex stainless steel is the best and is used on most offshore oil derricks. The best corrosion-resistant stainless-steel grades are:
- Duplex Stainless-steel.
- 316L Stainless-steel.
- 316 Stainless-steel.
- 304 Stainless-steel.
Stainless-steel are perfect for saltwater conditions; however, there is nothing in nature that rusts steel as successfully as salt. Duplex stainless steel has a more significant chromium content than standard grade stainless steel.
Regardless of thickness or kind of steel, the time will come when cleaning and protection are necessary to maintain the integrity of steel submerged in saltwater.
Stainless steel alloys will experience severe pitting corrosion when exposed to water environments rich in chlorides. When 304-grade stainless steel is used in the naval industry, it can experience pitting corrosion because of seawater contact or salty sea air.
To prevent pitting corrosion, it is crucial to use a stainless-steel grade that is specifically resistant to saltwater like the 316 stainless steel grades. A specialized coating is sprayed onto the steel to stop direct contact with salt water.
At some time, all corrosion-resistant alloys and duplex stainless steels can experience some form of corrosion when exposed to constant saltwater.
The 316-grade stainless steel is the best steel for harsh saltwater environments. The marine-grade stainless-steel nickname is earned for a reason.
The 316-grade consists of 18% more chromium and has more nickel than 304-grade and a 2-3% molybdenum content. These extra additives make 316-grade highly resistant to saltwater.
Titanium is entirely corrosion-resistant and the other preferred metal for use in saltwater conditions; however, it is incredibly costly but worth it for critical applications in marine environments.
Materials that Do Not Rust in Saltwater
Metals that do not rust in saltwater environments are Aluminum, Duplex stainless steel, 316-grade stainless steel, Platinum, Galvanized steel, Copper, Brass, and Bronze.
Platinum does not rust because it is a pure metal that does not contain iron.
Brass, Bronze, and Copper contain no or little iron and do not rust; however, they do react to oxygen. Over time copper oxidizes and forms a green patina, which actually protects the metal from corrosion.
Cobalt chrome is extraordinary because it feels and looks like a precious metal but is so durable it is used in the aerospace industry. Cobalt chrome is long-lasting, extremely hard, and never wears thin; cobalt chrome will last forever.
Why Is Stainless Steel Corrosion- Resistant?
Stainless steel contains a minimum of 10.5% chromium that quickly reacts with oxygen forming an oxide layer on the surface.
The layer of chromium oxide sticks to the stainless steel and becomes a protective barrier.
It is also essential to know that stainless steel is not entirely free from corrosion. Under certain conditions, specific grades of stainless-steel alloys can experience corrosion and show signs of rust or damage.
What Type of Steel is Best for Oil Rigs?
Oil rigs are one of the biggest industries impacted by saltwater corrosion and rust. The oil derricks standing permanently in the sea constantly struggle against corrosion. Saltwater always attacks the exposed material on oil rigs and platforms without end.
Stainless steel is the material most used on oil rigs. The 316L grade austenitic stainless steel is the preferred material used on oil rigs to protect against corrosion. Engineers use stainless-steel structural material above sea level on oil rigs with square hollow or rectangular sections. For oil rigs, protection against corrosion is vital.
Oil rigs consider costs and lifetime and use stainless steel in most forms; pipes, plates, supporting structural areas, and grating. For most oil rigs, the reduced maintenance and excessive price of stainless steel are worth the expense if it offers lifetime use.
Stainless steel is known to be heavy and adds extra weight to any large structure. Keeping corrosion under control is vital, but weight reduction is crucial, particularly in stormy seas. The objective of reducing weight generated the manufacturing of grades of stainless steel like duplex steels, which are excellent for oil rigs on the topside.
There are two stainless-steel grades, Lean duplex, and duplex, that feature heightened rust-resistant properties. The mechanical performance of these two grades gives engineering teams the ability to plan lighter areas onboard.
The two-phase microstructure of duplex stainless steel comprises of austenitic and ferritic stainless-steel molecules that are even stronger than standard stainless steel, which is why they are anti-corrosive and the preferred steel for saltwater conditions.
How To Protect Steel from Rust That Was Submerged in Saltwater
To protect steel that was submerged in saltwater from rust, remove the steel from the water, rinse with fresh water and dry thoroughly. When the steel is dry, paint it with an oil sealant or marine paint.
Painting steel or coating it in special oils is the standard way to protect the metal. Constant maintenance is necessary to one degree or another to keep corrosion at bay.
To prevent rust, it can be controlled with special coatings, like marine paint, varnish, lacquer, or wax that isolate the iron from the elements.
Immense structures that have enclosed box sections, like vehicles and boats, typically use a wax-based product called a “slushing oil” sprayed into those sections.
These type of anti-corrosion treatments typically includes rust inhibitors. Concrete can also be used to cover steel to provide some protection for the steel against corrosion. However, steel can still rust, and expanding rust can fracture concrete, causing problems.
How to Remove Rust by Electrolysis
Simple materials like a plastic bucket filled with an electrolyte solution of washing soda can be used to remove rust from small steel and iron or steel objects by electrolysis.
- Dissolve washing soda in tap water inside the plastic bucket.
- Place a short piece of rebar vertically in the washing soda solution.
- This acts as a conductor.
- Place another short piece of rebar across the top of the bucket to support the suspending piece of rebar.
- Use plastic baling wire to tie to the rusty object and hang the object in the solution from the horizontal rebar.
- Use a battery charger as a power source.
- Clamp the positive terminal to the rebar conductor
- Clamp the negative terminal to the rusty object, which becomes the cathode.
- Watch the rust fall away.
Rust can also be treated with commercial products like a rust converter. These products contain phosphoric acid and tannic acid, which combine with rust and chemically convert rust into black ferric tannate, a more stable material.
Rust can also be removed with organic acids like vinegar, citric acid, and strong hydrochloric acid. Rust can also be removed with chelating agents. Chelating products chemically bind tightly to metal ions making removal of rust easy.
Conclusion
Rusting is caused due to the movement of electrons; saltwater is not the primary cause of rust but does accelerate the process because electrons move more quickly in saltwater than they do in freshwater.
This happens because salt water is an electrolyte solution that contains more dissolved ions than freshwater, allowing electrons to move more quickly over the steel.
Steel rust in certain conditions when iron atoms react with their environment. The trio of oxygen, salt, and moisture, corrodes steel rapidly. With its built-in corrosion resistance, stainless steel is the most preferred steel for saltwater environments and offers long life.
However, stainless steel can rust in specific harsh saltwater environments. Thankfully it does not happen as rapidly or severely as standard steel does. Stainless steel can deteriorate when exposed to harmful chemicals, like sodium chloride, acids, water, or excessive heat for prolonged periods but is still the best steel for saltwater environments.
A special coating can be applied to steel parts to prevent rust from forming. Saltwater will corrode almost all metals, making them brittle and weak.
Sources
- https://sciencing.com/effects-saltwater-metals-8632636.html
- https://sciencing.com/salt-water-rust-metals-5150093.html
- https://www.corrosionpedia.com/2/1432/corrosion-101/an-intro-to-pipeline-corrosion-in-seawater
- https://azrust.com/how-long-for-rust-to-form/
- https://www.electrochem.org/corrosion-science/
- https://en.wikipedia.org/wiki/Rust
- https://www.bluetrailengineering.com/post/choosing-materials-for-use-in-the-ocean
