What makes stainless steel non-magnetic whereas ordinary steel and iron are magnetic?
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We have to first understand how magnetic fields are generated around magnetic metals to answer this question. We know that within each atom, electrons spin on their axis that, in turn, causes magnetic field around them. Some electrons spin clockwise, some counter-clockwise. Generally they are paired so that the magnetic fields are cancelled. Iron which is a magnetic substance has three unpaired electrons. Each electron generates a magnetic field of its own.
If all the fields pull in the same direction then you have a magnet. In other words, the magnetic fields are aligned in a magnet. In the case of stainless steel, there are several types of them. In general they are made of iron (Fe), carbon (C), and about 10 per cent chromium (Cr). Some contain Nickel (Ni).
But other metals are added to obtain different properties. As stainless steel contains iron, a magnetic metal, one it would seem that it would be magnetic. However, when nickel (Ni) is added to stainless steel the result is a non-magnetic form of stainless steel, called austenitic stainless steel. At the atomic level, all the iron atoms act as mini magnets that are aligned in the same direction.
The net effect of this is that collectively the magnetic properties of all the iron atoms add up to produce the overall magnetisation of the material. This is known as ferromagnetism. But the addition of other elements to iron changes the properties. For instance, when chromium and nickel are added, the arrangement of atoms changes completely and this, in turn, affects the magnetic properties of iron. The nickel and chromium that are added to iron tend to cancel the magnetic fields and the net outcome is that stainless steel becomes a non-magnetic substance.