Page 323 - Chemical engineering design

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CHEMICAL ENGINEERING
increasing corrosion resistance, taking type 304 as 1, is given below:
304 304L 321 316 316L 310
1.0 1.1 1.1 1.25 1.3 1.6
Intergranular corrosion (weld decay) and stress corrosion cracking are problems
associated with the use of stainless steels, and must be considered when selecting types
suitable for use in a particular environment. Stress corrosion cracking in stainless steels
can be caused by a few ppm of chloride ions (see Section 7.4.5).
In general, stainless steels are used for corrosion resistance when oxidising conditions
exist. Special types, or other high nickel alloys, should be specified if reducing conditions
are likely to occur. The properties, corrosion resistance, and uses of the various grades
of stainless steel are discussed fully by Peckner and Bernstein (1977). A comprehensive
discussion of the corrosion resistance of stainless steels is given in Sedriks (1979).
Stress corrosion cracking in stainless steels is discussed by Turner (1989).

High alloy content stainless steels
Super austenitic, high nickel, stainless steels, containing between 29 to 30 per cent
nickel and 20 per cent chromium, have a good resistance to acids and acid chlorides.
They are more expensive than the lower alloy content, 300 series, of austenitic stainless
steels.
Duplex, and super-duplex stainless steels, contain high percentages of chromium. They
are called duplex because their structure is a mixture of the austenitic and ferritic phases.
They have a better corrosion resistance than the austenitic stainless steels and are less
susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is
around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex
steels where developed for use in aggressive off-shore environments.
The duplex range of stainless steels can be readily cast, wrought and machined.
Problems can occur in welding, due to the need to keep the correct balance of ferrite and
austenite in the weld area, but this can be overcome using the correct welding materials
and procedures.
The cost of the duplex grades is comparable with the 316 steels. Super-duplex is around
fifty per cent higher than the cost of duplex.
The selection and properties of duplex stainless steels are discussed by Bendall and
Guha (1990), and Warde (1991).

7.8.3. Nickel
Nickel has good mechanical properties and is easily worked. The pure metal (>99 per
cent) is not generally used for chemical plant, its alloys being preferred for most appli-
cations. The main use is for equipment handling caustic alkalies at temperatures above
Ž
that at which carbon steel could be used; above 70 C. Nickel is not subject to corrosion
cracking like stainless steel.