Video Transcript
In the picture, a corroded nut and
bolt can be seen, which have protected the rest of the metal from corrosion. What name is given to the corrosion
prevention technique that works in this manner?
When metals are in contact with
each other, corrosion will occur according to the order of their reactivity. More reactive metals corrode first
or more easily. Most corrosive processes involve
the oxidation of a metal. For example, when iron rusts, it is
converted from neutral iron into a form of hydrated iron(III) oxide. Some metals remain relatively
strong when corroded. But when iron corrodes, or rusts,
it loses its strength and becomes weak and flaky. However, if a more reactive metal
species is present with iron and in contact with it, the more reactive species will
be oxidized instead of the iron, preventing iron from corroding.
For example, suppose a block of
zinc was attached to a sheet of iron. Since zinc is more reactive than
iron, it loses electrons more readily than iron. In other words, zinc is oxidized in
preference to iron. Zinc donates its lost electrons to
iron and thereby helps prevent iron from oxidizing. So, in the presence of an oxidizing
species, such as oxygen in the air, zinc will lose electrons and be oxidized, but
iron will not.
The picture shows an example of
this process. We are not told what metal the nut
and bolt are made from. But we presume it is iron or steel,
since the nut is red brown in color. And so the metal is probably a
ferrous metal, in other words, an iron-containing metal. But we are told that it has
corroded, preventing the rest of the system from corroding. So we know that the nut and bolt
must be made of a different and more reactive metal than that in the rest of the
system. If the nut and bolt do contain
iron, this must mean that the rest of the system is made from a metal or metals
below iron in the reactivity series.
Now that the nut and bolt have
corroded, they no longer offer protection to the other metal, and so the nut and
bolt should be replaced. When a more reactive metal protects
a less reactive metal from corrosion, the corrosion prevention technique is called
sacrificial protection. Sacrificial protection is a
corrosion prevention technique in which a more reactive metal is oxidized and
donates electrons to a less reactive metal to which it is attached, thereby
preventing the oxidation of the less reactive metal.
Finally, what name is given to this
corrosion prevention technique? The answer is sacrificial
protection.
