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It’s not necessary that all redox reactions are part of energy change. You can notice many reactions around you which involve a negligible amount of energy or occurs without a change in energy. For example, Silver metal is oxidized when it comes in contact in the atmosphere oxygen or hydrogen disulphide.

$4 Ag(s) + 2 H_{2}S(g) + O_{2}(g) \rightarrow 2 Ag_{2}S(s) + 2 H_{2}O(g)$

The film of sulphide which collects on the metal surface acts as a protective layer for further oxidation of metal surfaces. The tarnishing of silver is just one example of a broad class of oxidation-reduction reactions that fall under the general heading of corrosion. The rust of the metals because of corrosion of the surface is an example of the oxidation reaction.

Generally, metals are oxidized in contact with the air and covered by a layer of metallic oxide. In aluminium metal, the cover of aluminium oxide is very hard and thick which protects the metal surface against the later attack of the oxygen of the air. You must have seen a rusty iron hammer, which corrode by oxygen, but at a much slower rate than the burning wood. Do you know why it is so?  Let’s discuss some other points related to corrosion.


What is Corrosion?

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Corrosion is defined as "the degradation of materials by chemical reaction with the environment in which the material resides." This is because of metal oxidation. As metals have a tendency to return to their natural state, it is a natural process which produces either salt or oxides. It requires four elements - anode, cathode, an electrolyte, and a metallic path.

Some Points about Corrosion Materials has Given Below:-
  • They are the materials which are the cause of corrosion.
  • They are toxic in nature.
  • They have very harmful effects as they have a tendency to attack metals and destroy their strength.
  • They also affect the human body, especially tissues. Some acids and bases are included in that.
  • For example, HCl, nitric and sulfuric acid and bases like sodium hydroxide and ammonia.

Corrosion of Metals

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Metal corrosion is the main cause of metal destruction, like steel rusts due to immersion in seawater. Similarly iron reacts with oxygen to form rust by exposure to moist air.

Iron rust is iron oxide Fe2O3. XH2O where X is the amount of complexed water with ferric oxide, which can vary. It shows the color of rust (black to yellow to orange).

It is a very complex process which is completed in the following steps.

  • Oxidation of iron- First the iron gets oxidized into ferrous ions [Fe (II)] with the loss of two electrons.
Fe $\to$ Fe+2 + 2 e-
  • The ferrous ions again get oxidized into ferric ions [Fe(III)] in the presence of water and oxygen.
Fe+2 $\to$ Fe+3 + e-
  • These electrons from the above reactions are used to reduce oxygen.
O2(g) + 2 H2O + 4e- $\to$ 4 OH-

  • The ferric ions combine with oxygen and form ferric oxide [iron (III) oxide]. This ferric oxide gets hydrated with water.

The complete chemical reaction for rust formation is shown below. The mechanism for the rusting process is similar to the electrochemical cell. The electrons formed during the oxidation of iron is conducted through the metal. Thus, the iron ions diffuse from the water layer to the metal surface where oxygen is present. 

This is an electrochemical cell where iron acts as the anode and oxygen gas as the cathode. The aqueous solution of ions behaves like a "salt bridge" as shown in the figure.

Corrosin of Metals

Rusting happens faster in the presence of moisture rather than in a dry environment. This process is also affected by some other factors like the presence of other salts, which increases the rate of rusting, because the presence of salt enhances the conductivity of the aqueous solution formed at the surface of the metal. 

So the rusting of iron and steel is completed rapidly near the ocean (salty) or with salt.

Types of Corrosion

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There are different types of corrosion which depend on the environment surrounding the material, type of material, chemical reaction etc. Some general types of corrosion are described below.

1. Uniform Corrosion

This is also called General corrosion. It is a very common method of corrosion. It deteriorates the whole surface of the metal and makes the surface thin. The damage is done at a constant rate on the entire surface. It can be easily detected by it's appearance. It can be controlled but if it is not, it then destroys the whole metal.

2. Galvanic Corrosion

This type of corrosion occurs with an electrolyte like seawater. Metals have different values of electrical potentials. When they become electrically connected and put in an electrolyte, the more active metal which has a high negative potential becomes the anode. Due to it's high negative potential, it corrodes fast. But the less active metal becomes the cathode. 

The flow of electric current continues till the potentials are equal between both electrodes. So at the joint where the two non similar metals meet, the galvanic corrosion appears. The Galvanic Series shows the list of metals from the most active to the least active (most noble). Thus galvanic corrosion can be controlled by selecting the two metals which are close in series. As platinum is the least active, it is also less active for corrosion.

3. Pitting Corrosion

This occurs because of random attacks on particular parts of the metal's surface. This makes holes which are large in depth. These holes are called "pits". The pit acts as the anode while the undamaged part of the metal is the cathode. It begins with a chemical breakdown in the form of a scratch or spot. The pitting process makes the metal thinner and increases fatigue. For example, it can be very harmful in gas lines.

4. Stress Corrosion Cracking (SCC)

It is a complex form of corrosion which arises due to stress and corrosive environment. This generates brittle and dry cracks in the material. The brittle cracks can inter or Trans granular morphology. The stress is developed in the material due to bending or stretching of the material. It also affects only at a particular section of material. 

The main reasons for stress corrosion are welding, heating treatments, deformation etc. It is very difficult to detect the cracks or detect stress corrosion because they combine with active path corrosion. The active path corrosion occurs generally along grain or crystallographic boundaries. Stress corrosion is strongly affected by alloy

5. Corrosion fatigue

This occurs in the presence of a corrosive environment like saltwater. It is a combination of cyclic stress and corrosion. Corrosion fatigue is produced when a metal breaks at a stress level which is lower than its tensile strength. It is strongly affected by the environment in which the metal resides which affects the initiation and growth rate of the cracks. These cracks are too fine to detect easily. So the stress coupons (metal sample) are used to detect the corrosion.

It can be produced by the influence of various types of stress like stresses applied, thermal expansion, thermal contraction, welding, soldering, cleaning, heating treatment, construction process, casting etc. To prevent corrosion fatigue, the designing and construction process of the materials should be done properly, by eliminating any stress and environmental factors and by eliminating crevices. 

6. Intergranular Corrosion

In the granular composition of metals and alloys, grains (small crystals) are present and their surfaces join with each other. This forms the grain boundaries. Thus the grains are separated by grain boundaries. Intergranular corrosion is also known as inter crystalline corrosion. The Intergranular corrosion is developed on or near the grain boundaries of a metal. This can be due to welding, stress, heat treating or improper service etc. The metal can loose its strength due to the Intergranular corrosion.

7. Crevice Corrosion

It is also known as concentration cell corrosion. This is due to the trapping of liquid corrosive between the gaps of the metal. As the electrolyte has aggressive ions like chlorides, the corrosion reaction is started after settling of liquid in gaps. Oxygen is consumed during the reaction. 

Thus an anodic area is developed near the oxygen-depleted zone while the external part of the material acts as a cathode. Crevice corrosion is similar to pitting corrosion. It’ very difficult to detect crevice corrosion. It can be initiated by materials like gaskets, fasteners, surface deposits, washers, threads, clamp etc.

8. Filiform corrosion

It is a type of concentration cell corrosion. This develops on coated metallic surfaces with a thin organic film. The corrosion generates the defect on the protective coating of metallic surface. The filaments of corrosion product is the cause of degradation of the coating. The filaments look like thin threads. They exist as long branching paths. 

The actively growing filaments do not intersect the inactive filaments. The reflection process takes place when filaments collide with each other. Filiform corrosion is a very specific process because it only affects the surface’s appearance, not the metallic material. 

9. Erosion Corrosion

It is also called flow-assisted corrosion. This is due to the movement of corrosive liquids on metal surface which damages the material. It can be seen in ship propellers which are constantly exposed to sea water or in soft alloys. The damage can be seen as waves or rounded holes etc. It shows the flow of the corrosive liquid. It can be controlled by the use of hard alloys, managing the velocity and flow pattern of the fluid.

10. Fretting Corrosion

It is a form of erosion-corrosion. It shows the combined effect of corrosion and fretting of metal. Due to this corrosion, the material surface starts to disappear. Fretting corrosion exists in the form of dislocations of the surface and deep pits. Oxidation is the main cause of fretting corrosion. It can be controlled by using lubricates, controlling movement etc.

Corrosion Theory

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1. Water on the metal surface dissolves CO2 and O2 from the air.


What is Corrosion?

Corrosion Protection

Corrosion Theory

2. Fe in contact with dissolved CO2 and O2 undergoes oxidation.

Fe $\rightarrow$ Fe2+ + 2e- - Anode

3. Electrons lost by Fe are taken by H+

H+ + e- $\rightarrow$ H

4H + O2 $\rightarrow$ 2H2O

On multiplying the first equation by 4 and adding to the second,

Corrosion Theory Equation

The dissolved O2 can take electrons directly also.

Examples of Corrosion

Corrosion Control Products

4. Fe2+ reacts with dissolved O2 and water

Formation of Rust

Anti Corrosion Coatings

Rust (Hydrated ferric oxide)

Corrosion Protection

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Given below are some of the factors that cause corrosion.
  • Reactivity of metal
  • Presence of impurities
  • Presence of air, moisture, gases like SO2 and CO2
  • Presence of electrolytes
Two methods are used for the protection of materials from corrosion. 
  1. Cathodic protection
  2. Corrosion inhibitors.
Both methods are based on charge control of the metal surface by measuring the potential of the metal. 

1. Cathodic protection

The principle of this method is to alter the electrode potential of the metallic structure so that they can lie in the immunity region. This is the region where the metal is in the stable state of the element and corrosion reactions are not possible. It is mostly used in steel structures in marine and under ground regions.

Two methods are used to apply the cathodic protection to a metal structure. 
  • Impressed Current - This method is used for the protection of pipelines and the hulls of ships in sea water. In this method, an electric current is applied to the metal surface by use of DC electrical circuit. The negative and positive terminal of the current source is connected to the metal requiring protection and an auxiliary anode respectively. The flow of electric current charges the structure with electrons and changes the electrode potential in the negative direction. This process continues till it reaches the immunity region. The current flows from anode to cathode. Thus it protects the metal surface from corrosion.
  • Sacrificial Anode - This is especially used for ships, offshore oil and gas production platform etc. In this technique, the more reactive metal is used to alter the electrode potential and get the immunity region. Zinc is generally used as sacrificial anode. It generates the anodic dissolution current with more negative potential. The cathodic curve intersection is now at a more negative potential which is the immunity region. At this region, the corrosion rate of steel is negligible.

Sacrificial Anode

2. Corrosion Inhibitors

  • According to surface chemistry, the presence of foreign molecules affect the surface reactions.
  • Corrosion processes are also a type of surface reactions. These can be controlled by foreign compounds which are known as inhibitors.
  • The inhibitors get adsorbed on the reacting metal surface. It attaches directly to the surface or adsorbs up to one molecular layer of the metal surface. This is a well known method for controlling the corrosion.
  • The inhibitors can work in different ways; it may block the active sites of corrosion and restrict the rate of anodic or cathodic process, or it may increase the electrode potential etc.
  • Hexylamine or sodium benzoate are used as inhibitors for anodic reactions.
  • Similarly, oxidising agents like nitrite, chromate, red lead, amines, thio-urea etc are also used as corrosion inhibitors.

It is the intrinsic property of a metal or a material that they have resistance to corrosion attack in some specific environmental conditions like pressure, temperature and velocity of fluid etc. These materials are thermodynamically unfavorable for corrosion reaction as in the case of graphite, zinc, cadmium. In these metals, corrosion continues but at an extremely slow rate.

Corrosion Rate

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Corrosion Testing
  1. Corrosion testing is used to measure corrosion. This is done in corrosion testing laboratories.
  2. These are laboratories where experimental testing of materials is done for their verification about corrosion according to various industry standards.
  3. Some standards are used for this purpose like ASTM, ISO, NACE, or custom corrosion testing.
  4. Corrosion testing also includes DOT test, electrochemical and immersion test and heat transfer.
  5. Some standardized methods are also used for testing of glass in different media like acidic, basic or neutral.
  6. Testing methods are done in specific conditions of environment. In the ISO method, glass is put in the ionized water up to 60 minutes.
  7. This solution is then titrated with HCl solution. Thus the amount of HCl for neutralization is measured.
It is also known as corrosion ratio. It is calculated by taking uniform corrosion over the whole surface. Corrosion rate is measured in terms of mpy (mils per year penetration). 
mpy =$\frac{(weight\ loss\ due\ to\ corrosion\ in\ grams) \times (22,300)} { (A) (dt)}$

A = area of corrosion in square inch.
d = metal density in g/cm3
t = time of exposure for corrosion in environment in days.
A complete report and good evaluation is required to measure the type of corrosion and control methods. It is also expressed in terms of metric unit which is mm / y (millimeter per year).

mm {y} = 87.6 x ($\frac{W }{ DAT}$)

W = loss in weight (milligrams).
D = density of metal (g /cm3 ).
A = sample area (cm square).
T = time of exposure for corrosion in environment in hours.
More topics in Corrosion
Effects of Corrosion Voltaic Cell
Causes of Corrosion Corrosive Chemicals
Galvanic Corrosion Electrochemical Corrosion
Corrosion Resistance Corrosion Inhibitor
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