Page 223 - Corrosion Engineering Principles and Practice

198 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 199

K , Stress intensity
l
Fast
fracture
K , Critical stress intensity for failure
I
C

Slow
growth

K Threshold range
l
(environment dependent)

Initiation Initiation processes:
• Pitting
• Intergranular cracking
• Stress corrosion cracking
• Corrosion fatigue
• Surface defects
• Erosion
• Wear
(a)
Pit SCC Initiation

Initiation Penetration
K I SCC Exceeded depth, a

s m Slow s m
growth

K I Exceeded
C
Fast
fracture
(b)
FIGURE 6.47 (a) Schematic view of relative magnitudes of K shown with
I
transitions and principal dependencies. The K parameter for threshold range
I
is shown to be variable, (b) Morphological regimes, (c) The ∆a/∆t versus K
for SCC and similar environmentally dependent slow growth (e.g., hydrogen),
(d) ∆a/∆n versus ∆K for fatigue crack growth showing possible effect of SCC
in increasing the slow growth [37].