Page 88 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

66 INTRODUCTION AND FORMS OF CORROSION

effect on fracture behavior below a static intensity factor K the threshold intensity
ISCC,
stress factor for the growth of stress corrosion cracks in tensile operating mode. Above
K , the crack velocity increases with increasing stress intensity factor K (region I).
ISCC
In region II, da/dt is independent of K. In region III, there is a steep increase in crack
velocity as the maximum intensity factor approaches the fracture toughness of the
material (K IC region III).
The figure illustrates type A true CF growth pattern in which the synergistic
interaction between cyclic plastic deformation and environment produces cycle- and
time-dependent crack growth rates. True CF influences cyclic fracture, even when
the maximum stress intensity factor K max in fatigue is less than K ISCC .
The cyclic load form is important. The crack growth rate increases, and the fatigue
threshold decreases as compared with that in air. The crack growth rate obeys a Paris
Law with increase in crack growth rate and a decreased fatigue threshold compared
with the behavior in air (73).
The figure shows the stress CF process, type B, purely time-dependent CF prop-
agation, which is essentially a simple superposition of mechanical fatigue and SCC.
Stress CF occurs only when K max > K ISCC . In this case, the cyclic character of loading
is not important. The combination of true CF and stress CF results in type C, the most
general form of CF crack propagation behavior (79).
This behavior is generally characterized by a plateau region, which prevails over
a definite threshold K . It is usually referred to as stress CF as SCC systems usually
th
exhibit this behavior, and the most common theory assumes that the crack growth rate
is because of the addition of SCC and pure fatigue crack advance. This type of syn-
ergistic effect is observed in systems not sensitive to SCC such as ferritic stainless in
seawater under cathode polarization. It is often associated with HE. It is possible that
the plateau behavior is because of the control of crack growth rate by nonmechanical
processes such as transport processes (73).
The figure shows the cyclic time-dependent acceleration in da/dN below K
ISCC
combined with time-dependent cracking (SCC) above the threshold (79). This is a
combination of environmental effects.
Examples of CF types and a combination of two types (A and C) have been
observed in similar steels under different potential. Type C behavior is clearly
associated with cathodic potential and thus with HE (73). Thus this is a mix of true
fatigue and stress CF where one can dominate the other in its influence on crack
growth, depending on the properties of the interface (83).

1.7.39 Prevention of CF
One or a combination of the following procedures is recommended to prevent CF:

1. Redesigning to reduce or eliminate both temporary and permanent cyclic
stresses. It is recommended to reduce the magnitude of stress fluctuation (79).
2. Selecting a material or heat treatment with higher CF strengths.
3. Use of inhibitors, reduction of oxidizers, or pH increase, depending on the sys-
tem and the environment, can delay the initiation of CF cracks. For example,