Do you know the reason behind the collapse of the Silver Bridge and the Mianus River Bridge in the United States?
Both the bridges collapsed because of the corrosion of the steel/iron components of bridges.
Corrosion is a deterioration process in which a substance gets converted into its respective oxide, hydroxide, sulfide, or carbonate. Some common examples of corrosion are –
- Rusting of Iron
- Tarnishing of Silver
- Discoloration of the Taj Mahal
Iron is one of the most abundant metals in the earth’s crust. It belongs to the d-block of the periodic table with the atomic number 26. It plays an important role in sustaining life on this planet.
The bulk is used in making bridges, alloys, pipes, utensils, appliances, medical equipment, vehicles, and other structural steelwork. However, rusting of iron degrades its desirable properties like strength, permeability, and appearance.
In this article, we will discuss the most searched questions related to rusting of iron like
- Is rusting of iron a chemical or physical change?
- The chemical reaction of rusting of iron
- What causes iron to rust?
- How to prevent rusting of iron
So, Is rusting a chemical change? Yes, Rusting is a chemical change. Rusting of Iron is the formation of a reddish-brown coating on the surface of the metal and its alloys due to the action of moisture and air over a long period. This phenomenon is a chemical change as Iron combines with Oxygen in presence of water to form a new compound, Iron oxide. This weakens the bonds between the atoms of Iron in the structure and as a result, rusted iron loses its desirable properties.
Let’s get started checking it in more detail.
Why is Rusting of Iron a Chemical Change?
A chemical change is a change that brings alteration in the composition of the material and causes it to form a new substance with distinct properties.
In the case of rusting of iron, the oxygen atoms form a bond with iron atoms in an environment containing water.
This chemical reaction does not occur instantly. However, it takes an extended time to form iron oxide, commonly known as rust and there is no physical process to get the iron back from the rust.
In short, the formation of a new substance (rust) and the occurrence of permanent changes in the composition of species during the rusting of iron satisfy all the characteristics of a chemical change.
Hence, rusting of iron is known as a chemical change.
Chemical Reaction of Rusting of Iron
Rusting of iron is a redox reaction in which iron acts as a reducing agent and oxygen acts as an oxidizing agent.
In a redox reaction, one species undergoes oxidation (loss of electrons) while another species undergoes reduction (gain of electrons).
Rusting of Iron begins with the transfer of electrons from a reducing agent, Iron to an oxidizing agent, Oxygen.
Rust composes of two types of iron oxide – Ferrous oxide and Ferric oxide that differ in the iron’s atom oxidation state (+2 and +3).
Ferrous oxide is also called Iron (II) oxide with the chemical formula of FeO in which the oxidation state of Iron is +2. Whereas, Ferric oxide is also called Iron (III) oxide with the chemical formula of Fe203 in which the oxidation state of Iron is +3.
Being a reducing agent, Iron donates its electrons
Fe → Fe2+ + 2e–
Being an oxidizing agent, the oxygen atom accepts the electrons from iron and increases its oxidation state from +2 to +3.
4Fe2+ + O2 → 4Fe3+ + 2O2-
In the presence of water, the following acid-base reactions occur between Iron cations and water molecules to form iron hydroxides.
Fe2+ + 2H20 ⇌ Fe(OH)2 + 2H+
Fe3+ + 3H20 ⇌ Fe(OH)3 + 3H+
Iron cations and hydroxide ions also form Iron hydroxides through the following direct reaction
O2 + H20 +4e– → 4OH–
Fe2+ + 2OH– → Fe(OH)2
Fe3+ + 3OH– → Fe(OH)3
Further, the resulting iron hydroxides follow the dehydration equilibria and produce the iron oxides that account for rusting of Iron.
Fe(OH)2 ⇌ FeO + H2O
4Fe(OH)2 + O2 + xH2O → 2Fe2O3.(x+4)H2O
Fe(OH)3 ⇌ FeO(OH) + H2O
FeO(OH) ⇌ Fe2O3 + H2O
Is Rusting of Iron a Physical Change?
No, rusting of iron is not a physical change. However, it is a chemical change as the composition of iron changes and a new substance, iron oxide (rust) is formed.
Rusting of Iron would be a physical change if the composition of iron remains the same throughout the process. Let us understand it by taking a few examples,
Bending of iron rod, Drawing a wire of iron metal, Melting of Iron are physical changes because iron changes its form only, not the chemical composition in all these processes.
However, during the process of rusting, iron atom and oxygen atom react together to make a new substance, Iron oxide in the presence of an electrolyte, water.
Must Read: Does Copper Rust
Chemical Change versus Physical Change
|No chemical reaction occurs between species thus the chemical composition of species remains the same throughout the process
|A chemical reaction occurs between species, thus the chemical composition of species gets altered
|No new substance is formed
|The new substance is formed with distinct properties
|Temporary and Reversible in nature
|Permanent and Irreversible in nature
|Changes occur only in the appearance of species
|Changes occur in both physical and chemical properties of species
Freezing of water
Melting of ice to water
Breaking of a glass
Chopping of vegetables or fruits
Sublimation of dry ice
Cracking of an egg
Melting of candle wax
Burning of firewood
Ripening of fruits
Digestion of food
Souring of milk
Rusting of iron
How Long Does It Take Iron to Rust?
The rate of rusting of Iron depends on multiple factors like Moisture, Oxygen, pH, impurities, the thickness of iron material, and more.
Availability of water is one of the most important factors in allowing rusting of iron.
Iron cations react with water molecules to form Iron Hydroxides during the process of rusting. Therefore, an iron stand placed in a wet area is more likely to rust faster than an iron stand placed inside a dry area.
Moreover, the nature of water – saltwater of pure water can also decide the rate of rusting of Iron. The presence of multiple ions in saltwater speed up the rusting process than pure water.
Rusting of Iron cannot occur without Oxygen as it acts as the electron acceptor in the formation of rust.
Therefore, iron materials placed in an open space are more likely to rust faster than materials placed inside a closed space like a house, office.
Lower pH of the environment surrounding accelerates the rusting process while higher pH slows down the rusting process.
Therefore, acid rain is more likely to speed up the rusting of iron bridges than normal rainwater.
Impurities accelerate the rusting of Iron. Therefore, Iron-containing a mixture of metals tends to rust faster than pure iron.
The Thickness of the Material
A thick iron rod tends to rust slower than a thin iron rod as thicker surfaces seem to be stronger which slows down the rusting of layers of iron.
Moreover, the volume percentage of iron in a material is also an important factor in altering the rate of rusting. The more the iron concentration, the faster the rate of rusting and vice versa.
Is Rusting of Iron a Reversible Change?
A change is called reversible only if the original material can be recovered.
For example, the ice gets converted into liquid during melting, however, after lowering down the temperature to 00 C, liquid water gets converted to its former state, ice again. Therefore, melting ice is a reversible change.
However, in the case of rust, the iron atom gets converted to iron oxide (rust) due to the action of water and oxygen, and the oxide layer of iron once formed cannot be converted back into iron.
Therefore, rusting of iron is not a reversible change.
What Causes Iron to Rust?
When the iron is exposed to wet air, it rusts because of the chemical reaction that occurs between iron and oxygen. Iron oxidizes to iron cations (Fe2+ and Fe3+) and then gets converted into Iron oxide during the reaction.
The iron oxide or rust appears reddish-brown flaky layer which gets deposited on the outer surface of iron and then progressively become worse over time.
If rust is left unaddressed in the initial stages, it causes lower layers of material to rust which eventually jeopardize the integrity of the iron
How to Prevent Iron from Rusting?
Some of the ways to prevent rusting of Iron are as follows:
Zinc has a lower reduction potential than Iron so it can oxidize more easily than iron. So in the process of Galvanization, iron is covered with a protective layer of zinc to prevent its oxidation or rusting
Use of Alloy
Alloy is a mixture of two or more metals or metals with non-metal. The rusting can be slowed down by using iron alloys like stainless steel.
Good quality paint or oil coating is used to form a protective layer on the surface of iron material which inhibits moisture or water from reaching the iron and chemical reaction between them.
Hence, painting or oil coating prevents or slows down rust.
Cathodic protection is useful for not only preventing rusting of iron but corrosion of many other metals.
It uses a sacrificial anode of more reactive metals like magnesium, zinc, aluminum with more negative electrode potential than that of iron. As a result, iron does not get oxidized at the cathode.
You must also read about Nickel plating in the article on does nickel rust.
Rusting is a type of corrosion in which a redox reaction occurs between oxygen and iron in presence of moisture which leads to the formation of the layer of red-brown flaky iron oxide.
Rusting is a chemical change that includes the formation of a new substance, rust. It can be either ferrous oxide (+2 oxidation state) or ferric oxide (+3 oxidation state). It would be a physical change if there is no change in the composition of the iron.
The rate of rusting of iron can vary from a few minutes to days, months depending on factors like environmental conditions (moisture and oxygen), pH, impurities, thickness or volume percentage of iron, and more.
Rusting is a reversible change as the formation of rust cannot be reversed or undone. So, it is better to prevent iron materials from rusting by coating, Galvanization, cathodic protection, and more.