Page 742 - Corrosion Engineering Principles and Practice

692 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 693

a following section on salt and ash deposit corrosion. Gaseous envi-
ronments associated with sulfidation have divided into the follow-
ing categories [12]:

• Hydrogen/hydrogen sulfide mixtures or sulfur vapor of
a highly reducing nature
• Moderately reducing mixed gas environments that contain
mixtures of hydrogen, water, carbon dioxide, carbon monoxide,
and hydrogen sulfide
• Sulfur dioxide-containing atmospheres
In the first category, sulfides rather than protective chromia are
thermodynamically stable. Hydrogen/hydrogen sulfide mixtures are
typically found in catalytic reformers in oil refining operations.
Organic sulfur compounds such as mercaptans, polysulfides, and
thiophenes, as well as elemental sulfur, contaminate practically all
crude oils in various concentrations and are partially converted to
hydrogen sulfide in refining operations. Hydrogen sulfide in the
presence of hydrogen becomes extremely corrosive above 260 to
288°C. Sulfidation problems may also be encountered at lower
temperatures. Increased temperatures and higher hydrogen sulfide
contents generally lead to higher degradation rates.
For catalytic reforming, the 18Cr-8Ni austenitic stainless steels
grades are considered to be adequately resistant to sulfidation.
However, the use of stabilized grades is advisable. Some sensitization
is unavoidable if exposure in the sensitizing temperature range is
continuous or long term. Stainless steel equipment subjected to such
exposure and to sulfidation corrosion should be treated with a two
percent soda-ash solution or an ammonia solution immediately upon
shutdown to avoid the formation of polythionic acid, which can cause
severe intergranular corrosion and stress cracking [8].
Vessels for high-pressure hydrotreating and other heavy crude
fraction upgrading processes (e.g., hydrocracking) are usually
constructed of one of the Cr-Mo alloys. To control sulfidation, they
are internally clad with one of the 300 series austenitic stainless
steels. In contrast, piping, heat exchangers, valves, and other
components exposed to high-temperature hydrogen-hydrogen
sulfide environments are usually entirely constructed with these
austenitic stainless steels. Figure 15.20 illustrates the corrosion
behavior of austenitic steels as a function of hydrogen concentration
and temperature [3]. In some designs alloy 800H has been used for
piping and headers.
The effects of temperature and H S concentration upon
2
sulfidation of alloys often used in oil refining services are shown in
Figs. 15.21 to 15.25, which represent the metal losses expected after