Page 226 - Corrosion Engineering Principles and Practice

200 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 201

However, if products from general corrosion are trapped so as to exert
stress in a structure, they can cause SCC.
The idea, once prevalent, that only alloys and not pure metals
are susceptible to SCC is quite possibly correct. The question is,
“How pure is pure?” Copper containing 0.004 percent phosphorous
or 0.01 percent antimony is reported to be susceptible to SCC in
environments containing ammonia-based ions. Cracking has been
produced in a decarburized steel containing less than 0.01 percent
carbon, but still containing small amounts of manganese, sulfur,
and silicon in a boiling ammonium nitrate solution. SCC has been
produced in commercial titanium containing, among other
constituents, 600 ppm of oxygen and 100 ppm of hydrogen. Hence,
the idea that a given material cannot fail by SCC because it is
commercially pure is not correct [21].
Transgranular SCC progresses by localized subsurface attack in
which a narrow path is corroded randomly across grains without any
apparent effect of the grain boundary on the crack direction.
Transgranular SCC may occur during the SCC of austenitic stainless
steels and less commonly during the SCC of low-alloy steels. It can
also occur in the SCC of copper alloys in certain media (e.g., ammonia).
It seldom occurs in aluminum alloys.

6.5.2 Corrosion Fatigue
Fatigue is the failure of a metal by cracking when it is subjected to
cyclic stress. The usual case involves rapidly fluctuating stresses that
may be well below the tensile strength. As stress is increased, the
number of cycles required to cause fracture decreases. For steels, there
is usually a stress level below which no failure will occur, even with an
infinite number of cycles, and this is called the endurance limit. In
practice, the endurance limit is defined as the stress level below which
no failure occurs in one million cycles. A typical S-N curve fatigue
curve, commonly known as an S-N curve, is obtained by plotting the
number of cycles required to cause failure against the maximum
applied cyclic stress.
When a metal is subjected to cyclic stress in a corrosive
environment, the number of cycles required to cause failure at a
given stress may be reduced well below the dotted line obtained for
the same metal in air shown in Fig. 6.48. This acceleration of fatigue
called “corrosion fatigue”, is revealed by comparing the solid line in
Fig. 6.48 with the dotted line reference. The solid curve indicates that
metal life under such conditions can be much lower than the reference
curve established in air. The S-N curve with corrosion tends to keep
dropping, even at low stresses, and thus does not level off, as will the
ordinary fatigue curve.
A marked drop in or elimination of the endurance limit may occur
even in a mildly corrosive environment, especially in the case of a
film-protected alloy. For example, deionized water, which ordinarily