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CHEMICAL ENGINEERING
7.4.8. Hydrogen embrittlement
Hydrogen embrittlement is the name given to the loss of ductility caused by the absorption
(and reaction) of hydrogen in a metal. It is of particular importance when specifying steels
for use in hydrogen reforming plant. Alloy steels have a greater resistance to hydrogen
embrittlement than the plain carbon steels. A chart showing the suitability of various
alloy steels for use in hydrogen atmospheres, as a function of hydrogen partial pressure
Ž
and temperature, is given in the NACE (1974) corrosion data survey. Below 500 C plain
carbon steel can be used.

7.5. SELECTION FOR CORROSION RESISTANCE
In order to select the correct material of construction, the process environment to which
the material will be exposed must be clearly defined. Additional to the main corrosive
chemicals present, the following factors must be considered:

1. Temperature affects corrosion rate and mechanical properties.
2. Pressure.
3. pH.
4. Presence of trace impurities stress corrosion.
5. The amount of aeration differential oxidation cells.
6. Stream velocity and agitation erosion-corrosion.
7. Heat-transfer rates differential temperatures.
The conditions that may arise during abnormal operation, such as at start-up and shutdown,
must be considered, in addition to normal, steady state, operation.

Corrosion charts
The resistance of some commonly used materials to a range of chemicals is shown in
Appendix C. More comprehensive corrosion data, covering most of the materials used
in the construction of process plant, in a wide range of corrosive media, are given by,
Rabald (1968), NACE (1974), Hamner (1974), Perry et al. (1997) and Schweitzer (1976)
(1989) (1998).
The twelve volume Dechema Corrosion Handbook is an extensive guide to the inter-
action of corrosive media with materials, Dechema (1987).
These corrosion guides can be used for the preliminary screening of materials that are
likely to be suitable, but the fact that published data indicate that a material is suitable
cannot be taken as a guarantee that it will be suitable for the process environment being
considered. Slight changes in the process conditions, or the presence of unsuspected
trace impurities, can markedly change the rate of attack or the nature of the corrosion.
The guides will, however, show clearly those materials that are manifestly unsuitable.
Judgement, based on experience with the materials in similar processes environments,
must be used when assessing published corrosion data.
Pilot plant tests, and laboratory corrosion tests under simulated plant conditions, will
help in the selection of suitable materials if actual plant experience is not available. Care
is needed in the interpretation of laboratory tests.