Page 325 - Chemical engineering design

300
CHEMICAL ENGINEERING
alloys (typical composition 4 per cent Cu, with 0.5 per cent Mg) which have a tensile strength
equivalent to that of mild steel. The pure metal can be used as a cladding on Dural plates,
to combine the corrosion resistance of the pure metal with the strength of the alloy. The
corrosion resistance of aluminium is due to the formation of a thin oxide film (as with
the stainless steels). It is therefore most suitable for use in strong oxidising conditions. It
is attacked by mineral acids, and by alkalies; but is suitable for concentrated nitric acid,
greater than 80 per cent. It is widely used in the textile and food industries, where the use
of mild steel would cause contamination. It is also used for the storage and distribution of
demineralised water.

7.8.9. Lead
Lead was one of the traditional materials of construction for chemical plant but has now,
due to its price, been largely replaced by other materials, particularly plastics. It is a soft,
ductile material, and is mainly used in the form of sheets (as linings) or pipe. It has a
good resistance to acids, particularly sulphuric.

7.8.10. Titanium
Titanium is now used quite widely in the chemical industry, mainly for its resistance
to chloride solutions, including sea water and wet chlorine. It is rapidly attacked by
dry chlorine, but the presence of as low a concentration of moisture as 0.01 per cent
will prevent attack. Like the stainless steels, titanium depends for its resistance on the
formation of an oxide film.
Alloying with palladium (0.15 per cent) significantly improves the corrosion resistance,
particularly to HCl. Titanium is being increasingly used for heat exchangers, for both shell
and tube, and plate exchangers; replacing cupro-nickel for use with sea water.
The use of titanium for corrosion resistance is discussed by Deily (1997).

7.8.11. Tantalum
The corrosion resistance of tantalum is similar to that of glass, and it has been called
a metallic glass. It is expensive, about five times that of stainless steel, and is used for
special applications, where glass or a glass lining would not be suitable. Tantalum plugs
are used to repair glass-lined equipment.
The use of tantalum as a material of construction in the chemical industry is discussed
by Fensom and Clark (1984) and Rowe (1994) (1999).

7.8.12. Zirconium
Zirconium and zirconium alloys are used in the nuclear industry, because of their low
neutron absorption cross-section and resistance to hot water at high pressures.
In the chemical industry zirconium is finding use where resistance to hot and
boiling acids is required: nitric, sulphuric, and particularly hydrochloric. Its resistance
is equivalent to that of tantalum but zirconium is less expensive, similar in price to high
nickel steel. Rowe (1999) gives a brief review of the properties and use of zirconium for
chemical plant.