Page 324 - Chemical engineering design

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MATERIALS OF CONSTRUCTION
7.8.4. Monel
Monel, the classic nickel-copper alloy with the metals in the ratio 2 : 1, is probably, after
the stainless steels, the most commonly used alloy for chemical plant. It is easily worked
Ž
and has good mechanical properties up to 500 C. It is more expensive than stainless
steel but is not susceptible to stress-corrosion cracking in chloride solutions. Monel has
good resistance to dilute mineral acids and can be used in reducing conditions, where
the stainless steels would be unsuitable. It may be used for equipment handling, alkalies,
organic acids and salts, and sea water.

7.8.5. Inconel
Inconel (typically 76 per cent Ni, 7 per cent Fe, 15 per cent Cr) is used primarily for
acid resistance at high temperatures. It maintains its strength at elevated temperature and
is resistant to furnace gases, if sulphur free.

7.8.6. The Hastelloys

The trade name Hastelloy covers a range of nickel, chromium, molybdenum, iron alloys
that were developed for corrosion resistance to strong mineral acids, particularly HCl.
The corrosion resistance, and use, of the two main grades, Hastelloy B (65 per cent Ni,
28 per cent Mo, 6 per cent Fe) and Hastelloy C (54 per cent Ni, 17 per cent Mo, 15 per
cent Cr, 5 per cent Fe), are discussed in papers by Weisert (1952a,b).

7.8.7. Copper and copper alloys

Pure copper is not widely used for chemical equipment. It has been used traditionally in
the food industry, particularly in brewing. Copper is a relatively soft, very easily worked
metal, and is used extensively for small-bore pipes and tubes.
The main alloys of copper are the brasses, alloyed with zinc, and the bronzes, alloyed
with tin. Other, so-called bronzes are the aluminium bronzes and the silicon bronzes.
Copper is attacked by mineral acids, except cold, dilute, unaerated sulphuric acid. It is
resistant to caustic alkalies, except ammonia, and to many organic acids and salts. The
brasses and bronzes have a similar corrosion resistance to the pure metal. Their main use
in the chemical industry is for valves and other small fittings, and for heat-exchanger tubes
and tube sheets. If brass is used, a grade must be selected that is resistant to dezincification.
The cupro-nickel alloys (70 per cent Cu) have a good resistance to corrosion-erosion
and are used for heat-exchanger tubes, particularly where sea water is used as a coolant.

7.8.8. Aluminium and its alloys
Pure aluminium lacks mechanical strength but has higher resistance to corrosion than its
alloys. The main structural alloys used are the Duralumin (Dural) range of aluminium-copper