Corrosion

Corrosion: definition

Corrosion in metals

In metals, Corrosion is the natural conversion of refined metal into a more stable state such as metal oxide, sulphide, or hydroxide.

So, corrosion is a chemical reaction, in which a material reacts with elements found in the atmosphere or water, and gets oxidized.

Causes of Corrosion

You may ask, how can corrosion happen naturally? The causes of corrosion can be many:

• Corrosion of metal may occur due to exposure to humidity or condensation of water on metal surfaces. A metal can react with Oxygen and Hydrogen present in water to convert into metal oxides. You will learn more about this when you read about how Iron gets corroded due to water.
• Exposure to hydrogen and oxygen, or corrosive gasses such as Chlorine, Ammonia, Sulphur Oxides, or Hydrogen Peroxide. In the process of corrosion, metals get oxidized i.e. they lose electrons. So, naturally, corrosive substances are good at oxidizing metals i.e. making them lose electrons and gaining electrons themselves.
• Exposure of metal to electric currents for a long time can also cause them to corrode.
• Metals such as steel can undergo stress corrosion when they are subjected to a corrosive environment and high stresses. High stresses can cause the metal to develop cracks in its micro-structure, which can then get corroded. This is extremely dangerous as it cripples buildings or bridges.

Corrosion Prevention

A metal gets corroded when it is exposed to the environment. An exposed piece of metal is susceptible to corrosive agents and is dangerous for the entire structure which is not exposed. Corrosion can be prevented by placing a barrier between the material and the environment. There are many ways we can do this, like:

• Painting.
• Greasing.
• Electroplating.
• Galvanization.

Galvanization is a process in which a layer of Zinc is deposited on the surface of the metal object. Zinc is more reactive than Iron and therefore gets corroded to prevent corrosion of Iron. For this reason, Galvanization of Iron is also called sacrificial protection of Iron. The corroded Zinc can easily be replaced with another coat of Zinc to ensure the Iron underneath does not rust. Zinc can be coated on a metal by painting with zinc-rich paints, or electroplating.

Corrosion reaction

Corrosion is an electrochemical reaction between the corroding agent (which can be oxygen and water, or any corrosive material), and the material being corroded.

Let us look at what happens when Iron gets corroded. Corrosion of iron is called rusting. Iron gets rusted when exposed to air and water, or steam. The chemical formula for rust is ${\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{H}}_2\mathrm{O}$. By studying the detailed process of rusting, you will see how the flow of charges in this electrochemical reaction takes place.

Fig. 1: Rusting Process of Iron | CorrosionPedia

Let us see which chemical reactions lead to the formation of rust.

Rusting is a complex process, but can be summarized in 1 chemical reaction -

$$4Fe+3O_{2}+6H_{2}O\rightarrow 4Fe(OH)_{3}$$

Ferric Hydroxide (Fe(OH)₃) dehydrates to form Fe₂O₃.H₂O.

1. In the first step, Iron undergoes oxidation when it comes in contact with water, and Iron (II) ions are formed.

$$Fe_{(s)}\rightarrow Fe^{2+}_{(aq)}+2e^{-}$$

Iron loses electrons to become iron (II) ion.

The electrons released by Fe in this reaction react with the hydrogen ions in water and the dissolved oxygen to produce water.

$$4e^{-}+4H^{+}_{(aq)}+O_{2(aq)}\rightarrow 2H_{2}O_{(l)}$$

Hold on, rust has not formed yet. These 2 reactions produce water and Iron (II) ions. Now the second step takes place.

2. In the second step, Hydrated Iron Oxide (rust) is formed. The consumption of H⁺ in the first step leaves Hydroxide ions (OH^-) in the solution. Fe²⁺ ions react with these hydroxide ions to form Ferrous Hydroxide, or Hydrated Iron Oxide.

$$Fe^{2+}_{(aq)}+2OH^{-}_{(aq)}\rightarrow Fe(OH)_{2(s)}$$

Fe(OH)₂ is also called green rust. Simultaneous to this reaction , Fe²⁺ also reacts with more H⁺ and oxygen to get oxidized further to Fe³⁺ .

$$4Fe^{2+}_{(aq)}+4H^{+}_{(aq)}+O_{2(aq)}\rightarrow 4Fe^{3+}_{(aq)}+2H_{2}O_{(l)}$$

These Fe³⁺ ions combine with OH^- ions to form Ferric Hydroxide.

$$Fe^{3+}_{(aq)}+3OH^{-}_{(aq)}\rightarrow Fe(OH)_{3}$$

This Ferric Hydroxide dehydrates to form Fe₂O₃.H₂O, which is what makes the reddish-brown rust we see.

Types of Corrosion

Corrosion can be broadly categorized into three types:

• General attack corrosion.
• Localized Corrosion.
• Galvanic corrosion.

General Attack Corrosion

General attack corrosion is also called Uniform Attack Corrosion. It occurs when the entire exposed surface of a metal is corroded evenly and at the same rate. It is the most common type of corrosion.

Localized Corrosion

In localized corrosion, a small area of an exposed metal surface is corroded. Localized corrosion is of three types:

Pitting

Pitting is when a small hole or cavity is formed on the metal surface. This can penetrate the metal and ultimately deteriorate it to failure.

Crevice Corrosion

Happens due to stagnant environment conditions like under a gaskets, washers, or clamps. Liquids or oxygen may get trapped in such crevices for a long time, and give way to corrosion for the exposed metal.

Filiform Corrosion

Filiform corrosion occurs when water, acids or other corroding substances enter cracks/gaps in the paint coating or metal coating. Filiform corrosion can spread inside the structure and cause structural weakness.

Galvanic Corrosion

This is interesting. Galvanic corrosion is referred to the corrosion that occurs when two dissimilar metals are in electrical contact with each other, and are exposed to a corrosive electrolyte. When such a condition is present, the two metals corrode at different rates. One of the metal corrodes faster than it would have corroded alone, and the other corrodes slower than it would have alone. The metal which corrodes faster is called the sacrificial metal.