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It is thanks to elements such as vanadium and chromium that are
responsible for giving stainless steel that special property and
consequently, its name. These elements, when added to the iron and
carbon that make ordinary steel, react with oxygen to form an
airtight coating. The airtight coating in turn protects the steel
from getting rusty. It also ensures that the surface remains
distinctly shiny. We now know why the
dishes in the dish bar advertisements shine so.
Let’s now see why ordinary steel doesn’t have such a luster and why
it rusts. In the case of ordinary steel, the molecules of iron oxide
do not pack neatly together to form a coating. Rust occurs readily
on iron because atomic iron is much smaller than its oxide. Instead
of getting packed tightly together with the atomic iron, the oxide
remains loose on the surface of the metal. If you’ve noticed, rust
falls off in flakes and does not form a thick layer.
In the case of stainless steel, chromium (one of the two elements
added to steel) is a metal that reacts very quickly with the oxygen
in the air. Since the sizes of atomic chromium and its oxide are
similar, they sit comfortably next to each other and the oxide forms
a stable layer. This layer is a few atoms thick and is formed over
the whole surface. If the oxide layer is broken or the metal is cut,
more oxide rapidly forms and covers the exposed surface, preventing
any further corrosion. This is the reason why chromium retains its
shine.
However, stainless steel is not totally stainless. If the proportion
of chromium, added to steel to make stainless steel, is very high,
the same effect is more or less produced. However, the oxide layer
prevents only oxidizing reactions. This means that it is still
vulnerable to attacks from other forms of corrosives, although to a
small extent only.
Stainless steel can corrode. For the airtight coating on the steel
to remain so after it has been damaged, it needs oxygen to reach its
surface to repair the coating. This is not possible when it happens
in water. For example, a yacht keel that is held in place by
stainless steel bolts will drop off the hull in a few months since
the corrosion can continue unabated when the yacht keel is immersed
in the sea. |
