Corrosion-fatigue cracking

Doyle, J. L., Wood, G. R., and Bondurant, P. D. Using Laser-Based Profilometry to Locate and Measure Corrosion Fatigue Cracking in Boiler Tubes, Materials Evaluation, D The American Society of Nondestructive Testing, Inc., Vol. 51, No. 5, pp. 556-560 (1993). 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Several theories have been proposed to explain the corrosion-fatigue phenomena. One is that cyclic stressing causes repeated rupture of protective coatings. Corrosion-fatigue cracks propagate as the coating is successively reformed and ruptured along a plane. 

Numerous factors can have a potentially significant effect on corrosion-fatigue cracking. Most of these relate to stress and the corrosiveness of... 

Figure 10.2 A family of short, transverse corrosion-fatigue cracks originating on the external surface.

Figure 10.3 Blunt fracture edges typical of corrosion-fatigue cracks.

Corrosion-fatigue cracks can be detected by nondestructive testing techniques such as magnetic particle inspection, radiography, ultrasonics, and dye penetrant. Corrosion-fatigue cracks may occur in numerous tubes simultaneously. Nondestructive testing of tubes at locations similar to those in which cracks are observed can be useftil. 

Mitigation or elimination of corrosion-fatigue cracks involves gaining control of the critical factors that govern the mechanism. 

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

Figure 10.5 Circumferential corrosion-fatigue crack adjacent to location of tube sheet.

Figure 10.7 Short, tight, circumferential corrosion-fatigue cracks on the internal surface made apparent by discoloration from weeping. ...

Microscopic examinations revealed that the cracks were unbranched and transgranular, t3q)ical of corrosion-fatigue cracks. These examinations also... 

Intergranular corrosion-fatigue cracks in copper may he difficult to differentiate from stress-corrosion cracking. The longitudinal orientation of the cracks revealed that the cyclic stresses were induced by fluctuations in internal pressure. 

Slides Corroded automobiles, fences, roofs stress-corrosion cracks, corrosion-fatigue cracks, pitting corrosion. 

Fig. 8.50 Corrosion fatigue crack velocity as a function of AX for HY130 (after Vosikovsky )...

Fig. 8.59 Features of a corrosion fatigue crack growth curve...

The complexity of these chemical and mechanical interactions is such that each metal-environment system must be examined on an individual basis to determine the important processes influencing corrosion fatigue crack nucleation and growth rates. Thus, in the ensuing sections, examples are quoted to illustrate commonly occurring phenomena or establish more general principles with reasonably wide applicability for particular classes of metal/environment combinations. It should be noted, however, that when... 

Fig. 8.65 Corrosion fatigue crack growth data for structural steel in seawater at 0.1 Hz, / = -I to 0.85 and -1.10 V (Ag/AgCI) (after Scott...

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