Corrosion of Metals and Its Prevention

CORROSION

Corrosion is a process where the metal corrodes. Corrosion is a natural process, and chemically active metals get corroded in a moist atmosphere. Contrary to popular belief, rusting and corrosion are not the same.  

Rusting is the process where iron corrodes due to exposure to the atmosphere. Corrosion is a process where the water or the moisture on the surface of the metal oxidizes with atmospheric oxygen; it is an oxidation reaction.

The main circumstance of corrosion occurs with iron because it is a structural material in construction, bridges, buildings, rail transport, ships, etc., with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. Some other metals also get tarnished in this manner.

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Covering the surface of the metal with enamel and lacquers helps to protect the metal against corrosion. Parts of machines that move can be protected by coating layers of water-repellent oil or grease. Another way of protecting iron and steel is by painting on them, as it creates a barrier between the surface of the metal and moist air or water. The other methods of protecting a metal are alloying, galvanizing, electroplating, etc.

Alloying

An alloy is a mixture of two or more metals. Alloying is a process where metals like iron or steel are mixed with a less reactive metal like chromium, magnesium, etc., to protect against corrosion and create non-rusting alloys, E.g. Brass is an alloy which consists of copper and is a cheap and non–reactive alloy. Another example of a non-rusting alloy is stainless steel, a mixture of iron and carbon.

Galvanizing

Coating iron or steel with Zinc to prevent corrosion is known as galvanizing. The most common method is hot-dip galvanizing, in which the parts are immersed in a bath of molten hot zinc. Coating iron or steel with zinc involves immersing it in a liquid containing zinc and using it as the negative cathode. Through electrolytic deposition, a layer of zinc is formed on the surface of the metal. Zinc will either oxide or corrodes creating a zinc oxide layer that does not flake off like iron oxide rust.

Electroplating

Electroplating is a process where a metal is coated by electrolytic deposition with chromium, silver, or another metal. This process is generally held at room temperature from aqueous electrolytes. It is one of the most popular and common methods to prevent corrosion.

Silver articles become black after some time when exposed to air. This is because it reacts with sulphur in the air to form a coating of silver sulphide.

Copper reacts with moist carbon dioxide in the air, slowly losing its shiny brown surface and gaining a green coat. This green substance is basic copper carbonate.

When exposed to moist air for a long time, iron acquires a coating of a brown, flaky substance called rust. Let us find out the conditions under which iron rusts.

Painting:- Painting metal substrates with protective paint systems is called corrosion protection paint. The purpose of corrosion protection painting is to protect the metal substrate against atmospheric corrosivity, i.e.rust

Oiling:- As oil and water do not mix, oiling forms a protective coating for the metal, preventing it from rusting.

Greasing:- Similar to oiling, greasing also forms a protective layer on the surface.

Galvanising:- The method of protecting steel and iron from rusting by coating them with a thin layer of zinc.

The galvanised article is protected against rusting even if the zinc coating is broken. Can you reason this out?

→Zinc is more reactive than iron and will always oxidise before iron. Even if the zinc is just in a large block and iron is fully exposed, it will still oxidise first; this is how sacrificial protection of ships works.
To be oxidised, a metal must lose electrons. Being more reactive, Zinc can give up electrons easier than iron. If an iron atom lost an electron, another would be conducted from the block of zinc to replace it. This will continue until all the zinc has oxidised or unless it is no longer in direct contact.

Chrome plating:- Chrome is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, resist corrosion, ease cleaning procedures, or increase surface hardness.

Anodising:-Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. Anodising is accomplished by immersing the aluminium in an acid electrolyte bath and passing an electric current through the medium.

Making alloys:-

An Alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. It is prepared by first melting the primary metal and then dissolving the other elements in it in definite proportions. After that, it is cooled at room temperature.

Alloying is an excellent method of improving the properties of a metal. We can get the desired properties by this method. For example, iron is the most widely used metal. But it is never used in its pure state. This is because pure iron is very soft and stretches easily when hot. But, if it is mixed with a small amount of carbon (about 0.05 %), it becomes hard and strong. When iron is mixed with nickel and chromium, we get stainless steel, which is hard and does not rust.

The alloy is known as an amalgam if one of the metals is mercury. An alloy's electrical conductivity and melting point are less than pure metals.

For example, brass, an alloy of copper and zinc (Cu and Zn), and bronze, an alloy of copper and tin (Cu and Sn), are not good conductors of electricity, whereas copper is used for making electrical circuits.

Solder, an alloy of lead and tin (Pb and Sn), has a low melting point and is used for welding electrical wires together.