Alloys

The solid substance obtained from mixing any molten metal or nonmetal with another molten metal and cooling the mixture is called alloy.

The time between 5000 BC to 3000 BC, the humans used to make ornaments, weapons, and machinery with copper. It is called the Copper age in history. But copper is a soft metal and things made of copper did not last long. That is why people at that period used to mix molten copper with molten tin to get bronze. Basically, bronze is an alloy. The 3000 BC to 1000 BC is called the Bronze age. The invention of the alloy by those ancient people was an epoch-making invention. At this period, people used to make weapons and machinery with bronze.

We know, alloys are more effective than metals in making different types of machinery. Iron and carbon are mixed to make steel which is another alloy.

Different alloys and its uses

Gold is another popular alloy. its composition and uses are also given below:

You have already learned alloys are compositions of various metals where they do not contain the equal amount of the metals to mix. In alloy, one metal is the representative metal while the other metals are non-representative metals. For example, in bronze, the representative metal copper is 65% while there is 35% zinc. The alloy is named after the representative metal. So, bronze is copper’s alloy.

Symptoms, Causes, and Prevention of Corrosion of Certain Metals and Alloys

Generally, exposure of pure metals and their alloys to open air for a long time creates a different coloured substance which is known as corrosion of metal.

Example: if things made of pure copper, brass or bronze are exposed to air for a long time, a black/brown/green coloured layer is created. This layer is called copper slag and copper decays by producing copper slag. Different coloured copper slag has different elements in them like cuprous oxide (Cu₂O), cuprous sulfide or celcoisite (Cu₂S) etc. That is why copper slag does not have a specific formula.

Similarly, if things made of iron or iron alloy are left in the open for a long time, their upper part produces a reddish-brown substance. This substance is called rust and iron decays by producing rust. The substance created due to corrosion of iron is the hydrated ferric oxide (Fe₂O₃.nH₂O).

Some metals usually donate electron (oxidation reaction) to turn into positive ion when they are exposed to open air. Some ingredient of the atmosphere accepts that electron to turn into negative ion in a reduction reaction. Then, the positive and negative ion combines into a new compound. This new compound undergoes reaction with the metal and corrodes the metal.

The rusting reaction on iron occurs very slowly and in stepwise.  It requires oxygen (O₂) and water (H₂O ) from the atmosphere. The water of the atmosphere converts into H⁺ and OH^– as follows, When iron comes in contact with H⁺ of open air it donates the electron and turns into Fe²⁺. It is an oxidation reaction.  The electron donated by Fe is accepted by O₂ and H⁺ to produce H₂O which is a reduction reaction. At this stage, Fe²⁺, H⁺, and O₂ undergo reaction and produce Fe³⁺ and water. Then, Fe³⁺ undergoes reaction with OH^– and produces Fe(OH)₃. At last, this ferric hydroxide turns into hydrated ferric oxide or rust Fe₂O₃.3H₂O. 

Prevention of Metal Corrosion

We know the less reactive metals usually do not undergo reaction with oxygen but the more reactive metals rapidly undergo reactions with the oxygen of air and water. Metals and their alloys do not corrode if they are not in direct contact with air. The iron made doors and windows of our homes are painted so that they do not come in direct contact with air.

Thus, a coating of less reactive metal is laid on more reactive metals then the more reactive metals are saved from the corrosion. This can be done in various ways like painting, electroplating, galvanizing etc.

Electroplating

Generally, electroplating is the method of laying a coating of less reactive metal on more reactive metal by electrolysis. In this method, the metal that will be the coating is connected to the positive charge of the battery (anode) and the metal that will get the coating is connected to the negative charge (cathode). Then the electroplating is done through electrolysis.

For example, to add a coating of copper on iron, a solution of CuSO₄ is taken firstly.  The copper rod is connected to the positive charge while the iron rod is connected to the negative charge of the battery.

 

When electricity is passed through the cell, Cu donates two electrons which go into the solution as Cu²⁺. 

Now, this Cu²⁺ accepts two electrons from Fe rod to become Cu and attaches to Fe rod. 

Galvanizing

Galvanizing is the method of laying zinc coating on any metal and it does not require electrolysis. Zinc is coated anyway on any metal.

Recycling of Metals

The amount of every element is definite in the world. After using metallic products, instead of discarding them, they are sent to the factories. Usable metal is made from that trashed metal and this method is called Recycling of metal.

For example, trashed iron is sent to steel mills for recycling. Trashed aluminium utensils are sent to aluminium factories to recycle new aluminium. 60% of aluminium used in Europe is recycled aluminium and 21% of the total copper used in America is recycled copper.

*Reference- NCTB Books