Page 207 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Carbon steel has less than 1.5 wt% carbon, can be given varying amounts of hardness or ductility, is easy
to weld, and is cheap. It is still the material of choice in the CPI when corrosion is not a concern.
• Low-alloy steels are produced in the same way as carbon steel except that amounts of chromium
and molybdenum are added (chromium between 4 and 9 wt%). The molybdenum increases the
strength of the steel at high temperatures, and the addition of chromium makes the steel resistant
to mildly acidic and oxidizing atmospheres and to sulfur-containing streams.
• Stainless steels are so-called high-alloy steels containing greater than 12 wt% chromium and
possessing a corrosion-resistant surface coating, also known as a passive coating. At these
chromium levels, the corrosion of steel to rusting is reduced by more than a factor of 10.
Chemical resistance is also increased dramatically.

• Nonferrous alloys are characterized by higher cost and difficulty in machining. Nevertheless, they
possess improved corrosion resistance.
Aluminum and its alloys have a high strength-to-weight ratio and are easy to machine and cast, but in
some cases are difficult to weld. The addition of small amounts of other metals—for example,
magnesium, zinc, silicon, and copper—can improve the weldability of aluminum. Generally,
corrosion resistance is very good due to the formation of a passive oxide layer, and aluminum has
been used extensively in cryogenic (low-temperature) operations.
Copper and its alloys are often used when high thermal conductivity is required. Resistance to
seawater and nonoxidizing acids such as acetic acid is very good, but copper alloys should not be
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used for services that contact ammonium ions (NH ) or oxidizing acids. Common alloys of copper
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include brasses (containing 5–45 wt% zinc) and bronzes (containing tin, aluminum, and/or silicon).
• Nickel and its alloys are alloys in which nickel is the major component.
Nickel-copper alloys are known by the name Monel, a trademark of the International Nickel Corp.
These alloys have excellent resistance to sulfuric and hydrochloric acids, salt water, and some
caustic environments.
Nickel-chromium alloys are known by the name Inconel, a trademark of the International Nickel
Corp. These alloys have excellent chemical resistance at high temperatures. They are also capable of
withstanding attack from hot concentrated aqueous solutions containing chloride ions.
Nickel-chromium-iron alloys are known by the name Incoloy, a trademark of the International
Nickel Corp. These alloys have characteristics similar to Inconel but with slightly less resistance to
oxidizing agents.

Nickel-molybdenum alloys are known by the name Hastelloy, a trademark of the Cabot Corp. These
alloys have very good resistance to concentrated oxidizing agents.
• Titanium and its alloys have good strength-to-weight ratios and very good corrosion resistance to
oxidizing agents. However, it is attacked by reducing agents, it is relatively expensive, and it is
difficult to weld.

As previously shown, the combination of operating temperature and operating pressure will also affect
the choice of MOC. From Table 7.9, it is evident that the number of MOCs available is very large and
that the correct choice of materials requires input from a trained metallurgist.

Moreover, information about the cost of materials presented in this text is limited to a few different
MOCs. The approximate relative cost of some common metals is given in Table 7.10. As a very
approximate rule, if the metal of interest does not appear in Appendix A, then Table 7.10 can be used to
find a metal that has approximately the same cost. As the metallurgy becomes more “exotic,” the margin
for error becomes larger, and the data provided in this text will lead to larger errors in estimating the
plant cost than for a plant constructed of carbon steel or stainless steel.