Page 86 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

64 INTRODUCTION AND FORMS OF CORROSION

Silver

Copper

10 μm

5
Figure 1.19 Extrusions and intrusions in copper after 6 × 10 cycles in air. (Reproduced by
permission, John Wiley and Sons (8).)

2. Crack initiation and stage one growth that deepens the intrusions within the
plane of high shear stress.
3. Stage two crack propagation of well-defined cracks on the planes of high tensile
stress in the direction normal to the maximum tensile stress.
4. Ductile fracture propagation.

1.7.37 Crack Initiation

CF cracks are always initiated at the surface, unless there are near-surface defects
that act as stress concentration sites and facilitate subsurface crack initiation. Crack
initiation takes place independently of fatigue limit in air as it can be decreased or
eliminated through the increase of dissolution rates at anodic sites. Localized corro-
sion such as pitting favors fatigue crack initiation through stress concentration and a
local acidic environment. The two main mechanisms of CF are anodic slip dissolution
and HE (80).