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Tuesday, 2 February 2016

ALLOYS OF NICKEL

The important nickel alloys are : German silver, Monel metal, Nichrome, inconel.


(i) German silver:
          It is a cupro nickel to which zinc has been added. A typical composition is 60% copper, 30% nickel and 10% zinc. Addition of zinc brings down the cost. Its color is silvery with as light pale tinge. It is very ductile and malleable and corrosion resistant. It is used for making electrical contacts, costume jewellery and high quality taps etc., Before the advent of stainless steel, it was also used for household utensils and coinage.

(ii) Monel metal:

          Its composition is 68% nickel, 30% copper, with small percentage of the elements 1% iron, silver remainder manganese etc., 
          Monel metal is superior to brass or bronze in resisting corrosion and in retaining its strength at high temperatures. Its mechanical properties are improved by cold working. Addition of elements like aluminium and beryllium makes the alloy amenable to heat treatment. It is only slightly attacked by acetic, citric and tartaric acids.
          Because of its excellent corrosion resistance properties, it is widely used for parts of water-pumps, propellers, domestic water storage tanks. it is also used for making turbine blades and chemical food handling plants.

(iii) Nichrome:

It is an alloy of nickel and chromium, its composition is 60% of nickel, 15% of chromium and 20% iron.
  1. It is practically non-corrosive.
  2. It can with stand high temperatures without oxidation, its melting point is 14000°C . 
  3. Alloyed with cast iron, it increases resistance of the latter to corrosion and heat, as well as to wear. 
  4. Electrical Resistivity at room temperature: 1.0 x 10-6 to 1.5 x 10-6 ohm-m,
  5. Thermal Conductivity: 11.3 W/moC, 
  6. Magnetic Attraction: None,
  7. Specific Gravity: 8.4,
  8. Density: 8400 kg/m3.
          It is used in the explosives, fireworks, and ceramic making industry, as a heating element. The alloy is very useful in the process of ceramic making, as it can withstand the high temperatures that are generated when a clay object is fired in a kiln. It is also used in motorcycle silencers and in certain microbiological lab apparatus. The chromium present in the wire oxidizes on being heated, and forms a protective layer of chromium oxide. which is used as heat resistant electrical wire in furnaces, and electrical heating devices like geysers, electric iron etc., the alloy is expensive because of its high nickel content.

(iv) Inconel and incoloy:

Alloys principally containing, nickel, chromium and iron. its composition is 75% nickel, 15% chromium, and 9 % iron.
  1. It has high corrosion resistance at ordinary and high temperatures.
  2. Its melting point is 1395C 
  3. Specific gravity = 8.55
  4. Brinell hardness is about 160
  5. It can be welded by the oxyacetylene and metallic arc methods without difficulty.
          Used in electrical industry. It is used for making springs which have to stand high temperature and are exposed to high temperatures. It is also used in exhaust manifold (manifold is the main pipe which carries the explosive mixture from the carburattor to the cylinders of an internal combustion engine) of aircraft engines.
  

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