Rusting of Iron: Characteristics, Reactions and Factors

Rusting of iron is a chemical change. Objects made of iron or iron alloys over a course of time deposit a thin reddish brown coloured flaky layer especially in the presence of a humid environment. 

• This layer is known as Rust and the phenomenon is known as rusting of iron. 
• It is a very common phenomenon in our day to day life. 
• The layer of rust has a mixture of several iron oxide, iron hydroxides and iron hydrates, all of these are chemically stable and have properties different from iron.
• Generally rusting degrades the strength and durability of iron. 
• Rusting if left unchecked can eventually lead to the collapse of iron maid structures or render the iron made objects as useless. 
• The prevention of rusting is of immense importance.

Rusting is actually corrosion, but the term 'rusting' is used specifically for the corrosion of iron and iron alloys. Iron is a very important metal which has a lot of applications and uses across all Industries. Use of methods to prevent rusting is very essential to increase the durability of iron products.

Key Terms: Iron, Ferrous, Ferric, Oxidation, Rust, Rusting Of Iron, Air, Moisture, Corrosion, Molecule

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What is Rusting?

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Rusting of iron is the deposition of Rust (a reddish brown coloured flaky layer) on the surface of iron. This occurs because iron is placed in a corrosive environment. 

• More technically, rusting of iron can be defined as the process of slow oxidation.
• Oxidation of iron with water (as humidity) and oxygen (from air) leading to the formation of various iron oxides and iron hydroxides.

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Characteristics of Rust

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The rust so formed is:-

 (i) Reddish Orange in colour.

(ii) Brittle and flaky in texture. 

(iv) Fragile in nature 

(v) It gradually diminishes the efficiency, strength and durability of iron. 

A humid environment and presence of air are the two most prominent causes of rusting of iron.

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Chemistry Behind Rusting of Iron

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Rusting of Iron happens when the iron atoms slowly react with oxygen molecules in the presence of moisture. This process takes quite a long time but once it is initiated, it cannot be reversed. 

• Rusting of iron is an oxidative process. 
• This implies that iron is oxidised by reacting with oxygen which is provided by air or water molecules. 
• In this process, the iron atoms also undergo loss of electrons as well.

It generally begins by :

1. Breaking of interatomic bonds in iron. This renders iron atoms free to react with oxygen in the vicinity. 
2. The freed iron atoms undergo an increase in oxidation state accompanied by a loss of electrons. 

The iron metal can be excited to any of the following two oxidation States.

1. The ferrous Oxidation State- The ferrous iron has undergone a loss of two electrons and has a positive charge of 2. It is symbolised as Fe²⁺.
2. The ferric Oxidation state- When the ferrous ions acquire greater energy for excitation, it loses one more electron. Now it is left with a positive charge of 3 3 and is transformed into a ferric ion.. The ferric iron is symbolised as fe³⁺.

The ferrous iron combines with oxygen to give ferrous oxide (FeO). The ferric iron combines with oxygen to give ferric oxide (Fe₂O₃). Another thing that is crucial in rusting is the compatible chemical nature of iron and oxygen. 

• Iron is a good reducing agent while oxygen is a good oxidising agent. 
• When in contact the iron readily gives the electrons and oxygen really except electrons to form chemical bonds.

Read More: Molecular Orbital Theory

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Reactions Involved in Rusting

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The reaction involved in the process of rusting is depicted in the following section in a step wise manners:-

Step 1

Loss of electrons by iron- Iron gives two of its electrons to change into ferrous iron. The reaction is:-

Fe → Fe²⁺ + 2e^-

Step 2

Further oxidation of iron- Now this ferrous iron so formed reacts with a molecule of oxygen to give a higher oxidation state of iron- the ferric iron. The chemical equation for this is:-

4Fe²⁺ + O₂ → 4Fe³⁺ + 2O₂^-

Step 3

Occurrence of acid base reactions / Formation of iron hydroxide- The acid- base reactions between the iron cations and the hydroxide anions take place in the following manner:-

1. Fe²⁺ + 2H₂O → Fe(OH) + 2H⁺
2. Fe³⁺ + 3H₂O → Fe(OH)₃ + 3H⁺

Other reactions by which the formation of iron hydroxide takes place in the process of rusting are listed below:-

1. O₂ + H₂O + 4e^- → 4OH^-
2. Fe²⁺ + 2OH^- → Fe(OH)₂
3. Fe³⁺ + 3OH^- → Fe(OH)₃

Step 4

Conversion of hydroxides to oxides- The iron Hydroxide so formed under body hydration to yield iron oxide that forms the rust. This conversion is done through various chemical reactions some of those are given below:-

1. Fe(OH)₂ → FeO + H₂O
2. 4Fe(OH)₂ + O₂ + xH₂O → 2Fe₂O₃.(x+4)H₂O
3. Fe(OH)₃ → FeO(OH) + H₂O
4. 2FeO(OH) → Fe₂O₃ + H₂O

From above chemical reactions, it can be concluded that water and oxygen are very essential for rusting of iron. In each chemical step, there is either oxygen or water as one of the reactants. Hence limiting the contact of iron with air and water has the potential to prevent the process of rusting.

Read More: Thermal expansion

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Conditions Necessary for Rusting 

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The conditions necessary for rusting of iron are:-

• Presence of Moisture- A humid environment 
• Presence of Air- It is the oxygen present in the air that helps in rusting of iron.

Read More: Pseudo First Order Reaction

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Experiment to Prove That Air and Moisture are Necessary for Rusting

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The following iron nail experiment can be helpful to demonstrate the rust that rusting requires moisture and air:-

Aim

To prove that air and water are essential for rusting of iron using Iron nails.

Requirements

• Iron nails
• 3 test tubes
• Anhydrous Calcium Chloride
• Boiled distilled Water
• Oil
• 3 Corks
• Tap water

Procedure

1. Take three test tubes and thoroughly clean each test tube.
2. Put tap water in one of the test tubes along with 1-2 iron nails. Then seal the test tube with a cork. Mark this as a test tube as A. 
3. Take the second test tube, put Iron nails in it and boil distilled water. 
4. Add a few drops of oil to form a layer of oil on the surface of water. 
5. Mark the test tube as B and cork it. 
6. Take the last test tube, put some Iron nails in it followed by anhydrous calcium chloride. 
7. Now mark this test tube as C and cork it. 
8. The anhydrous calcium chloride works to remove any trace of moisture present inside the test tube.
9. Leave the test tube for a few days before making any observation.

Observations

Iron nails in the test tube A rust’s. The nails in the test tube B and C do not rust at all. 

• This is because the nails in the test tube A were in contact with both air and water. 
• In the test tube B, the nails were in contact with only water as the layer of oil prevented air from reaching the nail. 
• In the test tube C, the nails were surrounded by only dry air as the anhydrous calcium chloride absorbed the moisture from the test tube.

Read More: Heat transfer

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Factors Affecting the Process Of Rusting

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Apart from the presence of moisture and air, there are certain other factors that have the potential to accelerate the formation of rust on iron. Such factors are listed below:

1. Water: When the surface of iron comes in direct contact with water the rusting is inevitable. 

• Excluding moisture, other ways by which the iron gets exposed to water are rain and presence of water bodies nearby.
• Iron structures located close to water bodies such as bridges over rivers are most prone to rusting.

2. Acidic pH: A lower pH increases the rate of rusting. 

• This happens when the iron is in constant contact with acidic substances such as acid rain, which have been a common occurrence in many places.
• On the other hand a higher pH retards the process of rusting. 

3. Salts: The presence of different salts increases the instances of rusting of iron. 

• This can be mostly seen in iron present in seas eg. 
• Ships are at a risk of getting their parts rusted as the mineral salts of the seas increase the rate of rusting due to electrochemical reactions.

4. Impurity: It is observed that pure iron rusts slowly as compared to iron mixed with various other metals.

Read More: Metals and Non-Metals

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Prevention of Rusting

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Rusting of iron is considered to be an undesirable phenomenon as it degrades the quality of iron. Rusted iron does not have the efficiency, strength and durability of normal iron.

• Rusted iron is weaker, fragile, breaks and sheds into pieces leading to a loss of quantity of iron and exposing the underlying iron layers to rusting.
• Rusting of iron leading to degradation of iron made machinery and structures can have several hazardous effects on human life. 

Some of the associated hazards are listed below

1. Rusting of iron could lead to collapse of iron made structures such as bridges, huge machinery and equipment in the factories and in parts of a ship. 
2. This can result in casualties, injuries which could be either curable or incurable. 
3. Such accidents and delays the normal operation and increased expenses as well.
4. Rusted iron becomes a breeding ground for bacteria like tetanus. 
5. Cuts and wounds exposed to such rusted iron can cause tetany.
6. The insides of water pipes and water tanks are most susceptible to rusting. 
7. This could contaminate water with huge amounts of iron oxides that are supplied as black coloured water.

Methods to Prevent Rusting of Iron

Thus, we can see the detrimental and hazardous effects of iron rusting can prove to be very costly. There are several ways through which iron made structures and objects are secured from the risk of rusting. Several such methods are discussed in brief here:-

1. Alloys: It has been proven that some alloys of iron have a nature to protect themselves from rusting. 

• Examples of such alloys are- stainless steel and weathering steel. 
• Stainless steel is not only safe from rusting but also has a greater tensile strength than iron. 
• Nowadays, many structures, which previously used to be made of iron, are being made of stainless steel to prevent rusting and adding strength.

2. Coatings: Iron can be protected from getting rusted with the help of coatings done by using Paints varnishes and wax tips. 

• The coating works to prevent rusting by acting as a barrier between iron and the surrounding air. 
• The result is that the iron does not come in direct contact with the surrounding air or moisture and is saved from rusting. 
• Care should be taken to regularly coat the iron with suitable material because with time the coatings wear away. 
• It is for the same reason that many industrial machinery is lubricated with grease so that there are reduced instances of rusting.

3. Galvanization: Galvanization is the process of depositing a layer of another metal such as zinc on iron by dipping the iron piece in molten zinc or by the process of electroplating. 

• The zinc layer hence deposited gets rusted in the presence of air and moisture and saves the iron from getting rusted.
• Usually stainless steel is galvanised with a layer of zinc to prevent rusting. 
• The disadvantage of galvanization is that it can work only for a specific amount of time. 
• Hence, this method cannot be used for iron made in highly corrosive areas (where cadmium coating can be used instead).

4. Cathodic Protection: The electrochemical reactions that lead to rusting can be inhibited by providing the metals with an electric charge. 

• This is done by cathodic protection.
• This is done by making the iron/steel a cathode and attaching a sacrificial anode to it.
• This sacrificial anode must have an electrode potential that is more negative than that of iron.
• Metals that are commonly used as sacrificial anodes are zinc, magnesium, and aluminium. 
• Once they are corroded away, they must be replaced in order to protect the iron/steel.

Also Read:

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Things to Remember 

• Rusting is the process of degradation of iron by the gradual conversion of iron into iron oxides 
• In the presence of air and moisture which deposits on the iron surface as a reddish brown flaky substance.
• The presence of air and moisture, both are necessary to cause rusting. 
• There will be no rusting in the absence of any of these two requisites.
• Rusting of iron is detrimental and hazardous as it can lead to collapse of structures, increase the risk of tetanus infections ,etc.
• Rusting of iron can be prevented by- Coating the iron surface with paints, varnishes and wax tapes, galvanization with zinc, cathodic protection and by using rust free alloys of iron.

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