What is Rust?
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.
Given sufficient time, any iron mass, in the presence of water and oxygen, could eventually convert entirely to rust. Surface rust is commonly flaky and friable and provides no passivation protection to the underlying iron, unlike the formation of patina on copper surfaces.
Rusting is the common term for corrosion of elemental iron and its alloys such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called “rust”
What is Rusting?: A Closer Look
Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron(III) oxide, which we see as rust.
Here is the word equation for the reaction: iron + water + oxygen → hydrated iron(III) oxide
Iron and steel rust when they come into contact with water and oxygen both are needed for rusting to occur. In the experiment below, the nail does not rust when air (containing oxygen) or water is not present:
Boiling the water removes the oxygen and the layer of oil prevents it from re-entering. Anhydrous calcium chloride removes water vapour from the air.
Salt dissolved in water does not cause rusting, but it does speed it up – as does acid rain.
Aluminum does not rust or corrode, because its surface is protected by a protective layer of aluminum oxide. This prevents the metal below from coming into contact with air (containing oxygen). Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminum oxide does not flake off.
What is Rusting Of Iron?
Rusting of iron refers to the formation of rust, a mixture of iron oxides, on the surface of iron objects or structures. This rust is formed from a redox reaction between oxygen and iron in an environment containing water (such as air containing high levels of moisture).
The rusting of iron is characterized by the formation of a layer of a red, flaky substance that easily crumbles into a powder.
This phenomenon is a great example of the corrosion of metals, where the surfaces of metals are degraded into more chemically stable oxides. However, the term ‘rusting’ is generally used to refer to the corrosion of objects made of iron or iron alloys.
What is the Chemistry Behind the Rusting of Iron?
The exposure of iron (or an alloy of iron) to oxygen in the presence of moisture leads to the formation of rust. This reaction is not instantaneous, it generally proceeds over a considerably large time frame. The oxygen atoms bond with iron atoms, resulting in the formation of iron oxides. This weakens the bonds between the iron atoms in the object/structure.
The reaction of the rusting of iron involves an increase in the oxidation state of iron, accompanied by a loss of electrons. Rust is mostly made up of two different oxides of iron that vary in the oxidation state of the iron atom. These oxides are:
Iron(II) oxide or ferrous oxide. The oxidation state of iron in this compound is +2 and its chemical formula is FeO.
Iron(III) oxide or ferric oxide, where the iron atom exhibits an oxidation state of +3. The chemical formula of this compound is Fe2O3.
Oxygen is a very good oxidizing agent whereas iron is a reducing agent. Therefore, the iron atom readily gives up electrons when exposed to oxygen. The chemical reaction is given by:
- Fe → Fe2+ + 2e–
The oxidation state of iron is further increased by the oxygen atom when water is present.
- 4Fe2+ + O2 → 4Fe3+ + 2O2-
Now, the following acid-base reactions occur between the iron cations and the water molecules.
- Fe2+ + 2H2O ⇌ Fe(OH)2 + 2H+
- Fe3+ + 3H2O ⇌ Fe(OH)3 + 3H+
The hydroxides of iron are also formed from the direct reaction between the iron cations and hydroxide ions.
- O2 + H2O + 4e– → 4OH–
- Fe2+ + 2OH– → Fe(OH)2
- Fe3+ + 3OH– → Fe(OH)3
The resulting hydroxides of iron now undergo dehydration to yield the iron oxides that constitute rust. This process involves many chemical reactions, some of which are listed below.
- Fe(OH)2 ⇌ FeO + H2O
- 4Fe(OH)2 + O2 + xH2O → 2Fe2O3.(x+4)H2O
- Fe(OH)3 ⇌ FeO(OH) + H2O
- FeO(OH) ⇌ Fe2O3 + H2O
One similarity between all the chemical reactions listed above is that all of them are dependent on the presence of water and oxygen. Therefore, the rusting of iron can be controlled by limiting the amount of oxygen and water surrounding the metal.
Why is Rusting an Undesirable Phenomenon?
Rusting causes iron to become flaky and weak, degrading its strength, appearance, and permeability. Rusted iron does not hold the desirable properties of iron. The rusting of iron can lead to damage to automobiles, railings, grills, and many other iron structures.
The collapse of the Silver Bridge in 1967 and the Mianus River bridge in 1983 is attributed to the corrosion of the steel/iron components of the bridge. Many buildings made up of reinforced concrete also undergo structural failures over long periods of time due to rusting.
Rusted iron can be a breeding ground for bacteria that cause tetanus. Cuts from these objects that pierce the skin can be dangerous.
Since rusting occurs at an accelerated rate in humid conditions, the insides of water pipes and tanks are susceptible to it. This causes the pipes to carry brown or black water containing an unsafe amount of iron oxides.
Factors that Affect the Rusting of Iron
Many factors speed up the rusting of iron, such as the moisture content in the environment and the pH of the surrounding area. Some of these factors are listed below.
- Moisture: The corrosion of iron is limited to the availability of water in the environment. Exposure to rain is the most common reason for rusting.
- Acid: if the pH of the environment surrounding the metal is low, the rusting process is quickened. The rusting of iron speeds up when it is exposed to acid rains. Higher pH inhibits the corrosion of iron.
- Salt: Iron tends to rust faster in the sea, due to the presence of various salts. Saltwater contains many ions that speed up the rusting process via electrochemical reactions.
- Impurity: Pure iron tends to rust more slowly when compared to iron-containing a mixture of metals.
The size of the iron object can also affect the speed of the rusting process. For example, a large iron object is likely to have small deficiencies as a result of the smelting process. These deficiencies are a platform for attacks on the metal from the environment.
How To Prevent Rust: 9 Ways For Any Situation
Rust can quickly become a big problem. It ruins the functionality and stability of important machinery and it can cost your business thousands. Knowing how to prevent rust effectively can save you money and prevent serious problems.
As with many things, some small preventative measures upfront can save you lots of money, time, and frustration later on. We’ve collected the best ways to prevent rust, so you can find a strategy that works best for your equipment or parts.
How to Prevent Rust in Any Situation:
In short, the best way to prevent rust is to prevent moisture from reaching the metal, or by using a material that corrodes more slowly. The following are the best ways to prevent rust. We’ll discuss how to prevent rust using each strategy in more detail later in the post.
- Use an Alloy: The use of alloys, like stainless steel, is one of the most common ways to prevent rust or slow it down. Stainless steel isn’t suitable or economical for all applications, but it will work for many.
- Apply Oil: A coating of oil will help to prevent rust or slow it down since it inhibits moisture from reaching the iron in the metal. However, an oily surface might be problematic for some tools or machines and poses environmental and human health concerns.
- Apply a Dry Coating: Special rust preventative products dry with no residue and form a protective barrier over metal parts and equipment. These are effective for products in use, in shipping, storage, and more.
- Paint the Metal: A good quality paint will slow down rusting by preventing moisture from reaching the metal.
- Store Properly: Store metal parts or products in a low-moisture area, or inside a temperature and humidity-controlled environment to significantly slow down rust. The use of desiccant drying agents in this storage is also helpful.
- Galvanize: Galvanizing coats iron or steel in zinc to protect from rust. Zinc corrodes at a much slower rate than iron or steel, so it’s highly effective for slowing rust.
- Blueing: This process creates a layer of magnetite over the metal to prevent rust. The metal must be regularly oiled to maintain rust resistance, and it will turn blue or black in the process.
- Powder Coating: A layer of acrylic, vinyl, epoxy, or other substances will prevent moisture from reaching the metal, thereby preventing rust.
- VCI Packaging: Vapor Corrosion Inhibitors (VCI) is a type of chemical compound that when infused into various packaging materials, protects metals by emitting rust eradicating vapors into enclosed air space to prevent corrosion on a metal surface.
9 Ways to Prevent Rust
1. Use an Alloy
While all metals corrode, they each corrode at different rates. This is why alloys, which are made from two or more different metals, are resistant to rusting. Technically, all types of steel are already alloys, since they are made from iron and carbon. However, adding other metals, such as chromium, nickel, manganese, and others, will create different types of steel alloys.
Some of these, such as stainless steel, are made to prevent rust completely. Though they are certainly not completely corrosion-resistant, they will rust much more slowly. Other alloys, such as COR-TEN steel, will acquire a layer of rust, but will then stop rusting, under the right conditions.
Changing the composition of the steel also changes its toughness, conductivity, appearance, and many other properties. It is important to consider how to prevent rust, but also make sure that the steel alloy is suitable for the application. Furthermore, consider the welding techniques used and the surrounding environment, as these will all affect the rate of corrosion.
2. Apply Oil
Most gun owners know the importance of keeping firearms well-oiled, even when the weapons are not in use. Oil not only lubricates metal parts and allows them to move with less friction, but oil also forms a protective barrier against rust. The principle here is pretty simple; with a coating of oil, moisture can’t react with the iron in the metal and cause rust.
While a coating of oil can be a simple and effective way to prevent rust, it’s certainly not perfect. Oil also makes it hard to get a grip on an object, and it can cause parts to slip or come unbalanced. It can also be dirty and unpleasant to work with. Finally, oiling must be done repeatedly, which takes time and energy.
3. Apply a Dry Coating
Some products are specifically made to prevent rust. These products work on the same principle as oil—creating a protective barrier against rust—but they don’t leave residue behind. For metal parts or components that need to stay clean or provide a solid grip, a rust preventative dry coating is ideal.
Dry coating rust prevention products such as ARMOR’s Dry Coat Rust Preventative can be applied via spray, dip, or wash. Once they dry, the protective barrier is in place. The metal won’t look or feel any different, so its applications remain the same. Dry coatings can also be used in combination with other ways to prevent rust. For example, you might use a dry coating over a painted or powder-coated object to increase the level of protection.
4. Paint the Metal
Paints will also create a protective layer over metal objects and prevent moisture from reaching them. Of course, no barrier can completely stop moisture from getting through, but painting can be a simple and easy way to slow down rust. If you already want to paint the object a different color or get a different finish, this is an ideal solution.
It is important to use the right paint to prevent rust. The paint must be able to adhere to the metal, so be mindful of what type of paint you’re using as well as any finishes already placed on the metal. You’ll also need oil-based paint, not water-soluble paint if you expect the piece to see excessive moisture or contaminants. Finally, be careful of welded joints or bolts. If there are any weak spots in the painted layer or any crevices not filled, these areas will start to rust.
5. Store Properly
The best way to prevent rust may also be the most obvious—keep the object away from moisture. Water reacts with iron to form rust, so an environment with no moisture will not create rust. However, keep in mind that even regular air contains some moisture in the form of humidity. To completely prevent rust, you’d need air- and water-tight seal. This, of course, would make the object difficult to use, so it makes more sense to prevent rust during storage or shipping.
Galvanizing applies a protective coating of zinc over iron or steel. Since zinc corrodes about 30 times slower than iron, galvanizing can be a cheap and effective way to prevent rust.
Like all of the ways to prevent rust, galvanizing has limitations. The coating of zinc won’t stand up to harsh environmental forces like acid rain or salt. Galvanizing also changes the outward appearance of the metal, and the extra layer can cover up parts of the component, such as the threads on a screw.
Check out our Detail article: What is Galvanizing?
The process of blueing steel actually creates a new layer that is similar to rust, but much less damaging. Blueing creates a layer of magnetite, also called black iron oxide, and gives metals a black or namesake blue appearance.
Blueing is usually accomplished by applying high temperatures and a salt solution. This process is commonly used to economically protect firearms from rust. Blueing works best when the steel is also regularly oiled.
8. Powder Coating
Powder coating is often used to quickly “paint” an object in an assembly line. First, static electricity binds a powdery substance made from acrylic, polyester, epoxy, polyurethane or something else to a metal object. Then, the powder melts in a furnace into a uniform, solid layer. Since there’s no liquid involved, powder coating is ideal for certain finishes or parts.
Powder coating, like painting, covers a metal component in a protective layer. This layer will prevent moisture from reaching the metal and therefore prevent rust. For powder coating to effectively prevent rust, the coating must be intact. Any weak areas will expose the metal and create an entry for rust.
9. VCI Packaging
VCI Packaging is an easy-to-use, clean, and dry packaging option for preventing rust from metal and metal parts. Vapor corrosion inhibitors (VCI) are a type of chemical compound used to protect ferrous and non-ferrous metals from rust and corrosion that are infused into packaging materials including poly films, paper, emitters, chipboards, desiccants, and many other components.
When metal parts are properly stored with VCI Packaging products, VCIs activate and fill up the vapor space inside the packaging. The VCI ions form a shield of protection on the surface of the metal that displaces moisture and eradicates rust. VCI Packaging safely prevents corrosion on protected metals without the need for messy grease, oils, protective coatings, or other time-consuming methods.