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 and slowly loses its shiny brown surface and gains a green coat. This green substance is basic copper carbonate.
Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust. Let us find out the conditions under which iron rusts.
The rusting of iron can be prevented by
painting:-The painting of metal substrates with protective paint systems is called corrosion protection painting. 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 with each other, oiling forms a protective coating for the metal and hence prevent 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 so 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. Zinc, being more reactive can give up electrons easier than iron can. 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, provide corrosion resistance, 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. Anodizing is accomplished by immersing the aluminium into an acid electrolyte bath and passing an electric current through the medium.
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 a very good 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.
If one of the metals is mercury, then the alloy is known as an amalgam. The electrical conductivity and melting point of an alloy is less than that of 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.