Page 321 - Chemical engineering design

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CHEMICAL ENGINEERING
The high silicon irons (14 to 15 per cent Si) have a high resistance to mineral acids,
except hydrofluoric acid. They are particularly suitable for use with sulphuric acid at all
concentrations and temperatures. They are, however, very brittle.
7.8.2. Stainless steel
The stainless steels are the most frequently used corrosion resistant materials in the
chemical industry.
To impart corrosion resistance the chromium content must be above 12 per cent,
and the higher the chromium content, the more resistant is the alloy to corrosion in
oxidising conditions. Nickel is added to improve the corrosion resistance in non-oxidising
environments.

Types
A wide range of stainless steels is available, with compositions tailored to give the
properties required for specific applications. They can be divided into three broad classes
according to their microstructure:

1. Ferritic: 13 20 per cent Cr, < 0.1 per cent C, with no nickel
2. Austenitic: 18 20 per cent Cr, > 7 per cent Ni
3. Martensitic: 12 10 per cent Cr, 0.2 to 0.4 per cent C, up to 2 per cent Ni

The uniform structure of Austenite (fcc, with the carbides in solution) is the structure
desired for corrosion resistance, and it is these grades that are widely used in the chemical
industry. The composition of the main grades of austenitic steels, and the US, and equiv-
alent UK designations are shown in Table 7.7. Their properties are discussed below.
Type 304 (the so-called 18/8 stainless steels): the most generally used stainless steel.
It contains the minimum Cr and Ni that give a stable austenitic structure. The carbon
content is low enough for heat treatment not to be normally needed with thin sections to
prevent weld decay (see Section 7.4.4).
Type 304L: low carbon version of type 304 < 0.03 per cent C) used for thicker welded
sections, where carbide precipitation would occur with type 304.
Type 321: a stabilised version of 304, stabilised with titanium to prevent carbide precip-
itation during welding. It has a slightly higher strength than 304L, and is more suitable
for high-temperature use.
Type 347: stabilised with niobium.
Type 316: in this alloy, molybdenum is added to improve the corrosion resistance
in reducing conditions, such as in dilute sulphuric acid, and, in particular, to solutions
containing chlorides.
Type 316L: a low carbon version of type 316, which should be specified if welding or
heat treatment is liable to cause carbide precipitation in type 316.
Types 309/310: alloys with a high chromium content, to give greater resistance to
oxidation at high temperatures. Alloys with greater than 25 per cent Cr are susceptible to
Ž
embrittlement due to sigma phase formation at temperatures above 500 C. Sigma phase is
an intermetallic compound, FeCr. The formation of the sigma phase in austenitic stainless
steels is discussed by Hills and Harries (1960).