Page 316 - Chemical engineering design

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MATERIALS OF CONSTRUCTION
of attack in which cracks are produced that grow rapidly, and can cause premature, brittle
failure, of the metal. The conditions necessary for stress corrosion cracking to occur are:
1. Simultaneous stress and corrosion.

2. A specific corrosive substance; in particular the presence of Cl ,OH ,NO ,or
3
C
NH ions.
4
Mild stress can cause cracking; the residual stresses from fabrication and welding are
sufficient.
For a general discussion of the mechanism of stress corrosion cracking see
Fontana (1986).
Some classic examples of stress corrosion cracking are:
The season cracking of brass cartridge cases.
Caustic embrittlement of steel boilers.
The stress corrosion cracking of stainless steels in the presence of chloride ions.
Stress corrosion cracking can be avoided by selecting materials that are not susceptible
in the specific corrosion environment; or, less certainly, by stress relieving by annealing
after fabrication and welding.
Comprehensive tables of materials susceptible to stress corrosion cracking in specific
chemicals are given by Moore (1979). Moore’s tables are taken from the corrosion data
survey published by NACE (1974).
The term corrosion fatigue is used to describe the premature failure of materials
in corrosive environments caused by cyclic stresses. Even mildly corrosive conditions
can markedly reduce the fatigue life of a component. Unlike stress corrosion cracking,
corrosion fatigue can occur in any corrosive environment and does not depend on a
specific combination of corrosive substance and metal. Materials with a high resistance
to corrosion must be specified for critical components subjected to cyclic stresses.

7.4.6. Erosion-corrosion
The term erosion-corrosion is used to describe the increased rate of attack caused by
a combination of erosion and corrosion. If a fluid stream contains suspended particles,
or where there is high velocity or turbulence, erosion will tend to remove the products
of corrosion and any protective film, and the rate of attack will be markedly increased.
If erosion is likely to occur, more resistant materials must be specified, or the material
surface protected in some way. For example, plastics inserts are used to prevent erosion-
corrosion at the inlet to heat-exchanger tubes.

7.4.7. High-temperature oxidation
Corrosion is normally associated with aqueous solutions but oxidation can occur in dry
conditions. Carbon and low alloy steels will oxidise rapidly at high temperatures and their
Ž
use is limited to temperatures below 500 C.
Chromium is the most effective alloying element to give resistance to oxidation, forming
a tenacious oxide film. Chromium alloys should be specified for equipment subject to
Ž
temperatures above 500 C in oxidising atmospheres.