Page 82 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

60 INTRODUCTION AND FORMS OF CORROSION

days is an example of CF failure. Steel pumping tubes used in oil wells have a limited
life span because of the CF experienced in oil well brines. Pipes carrying steam or
hot fluids of variable temperature may fail because of thermal cycling (8).

1.7.31 Morphology of CF Ruptures
CF results in fine-to-broad cracks with little or no branching, unlike SCC that exhibits
branching cracks. The cracks appear as families or parallel cracks and are filled with
dense corrosion product. The sample may have pits, grooves, or some other forms of
stress concentrator. Transgranular fracture paths, often ramified or branched are more
common than intergranular fracture with the exception of lead and zinc (Fig. 1.18).
Some systems show a combination of transgranular and intergranular forms of
fracture (8, 9).
The surface of fatigue ruptured material on examination shows two zones; namely,
(i) a smooth, silken zone not having undergone any plastic distortion and the second
zone resulting from the propagation of fatigue crack through the metal. The cyclic
constraint tends to smooth the surface of the rupture by wearing. A final rupture zone,
rough and damaged because of plastic deformation, is noted. The crack propagates

Figure 1.18 Corrosion fatigue crack through mild steel sheet. (Reproduced by permission,
John Wiley and Sons (8).)