Although ranking far behind steel, the second most important metal in
which manganese plays an important alloying role is aluminium. Some 23 million
tons of aluminium are produced annually. Small amounts of manganese are found
in many of these Al alloys, enhancing corrosion resistance. The explanation for
this beneficial effect is simple. Intermetallic compounds formed with iron and
silicon have an electrolytic potential which is far less negative than that of
aluminium. This means that the aluminium surrounding such particles will
corrode under corrosive conditions, with disintegration further spreading as a
series of deep pits are formed which spread the process to other particles.
Manganese replaces iron-silicon compounds with manganese-iron-silicon compounds
which have an electrolytic potential very close to that of aluminium. As a
result there is no potential difference and therefore no corrosion.
Aluminium cans for soft drinks
Manganese is used as an alloying element up to its solubility limit of
about 1.5%. Aluminium-manganese alloys and aluminium-manganese-magnesium
alloys, which have been sold under different trade names, have found
applications in such diversified areas as kitchenware, roofing, car radiators
and transportation. By far the most important use of aluminium-manganese alloys
is for beverage cans, of which some 100 billion units are produced each year.
The market for aluminium-manganese cans has grown steadily, thanks to the fact
that such cans can be recycled.
Aluminium alloys containing up to 9% Mn have promising properties, but
they cannot as yet be economically produced. Technologies to produce these
commercial amorphous metals through very fast cooling are of
potential interest but the processes used are still very expensive and can only
be applied to high value materials used in the aerospace