Corrosion fatigue stress

All metals will corrode under certain conditions. Internal corrosion is caused by galvanic corrosion, pitting, corrosion fatigue, stress corrosion cracking, stray currents, etc. 

Corrosion, stress Corrosion fatigue, stress-corrosion cracking... 

Structural materials that are direcdy exposed, such as in cabinets, housings, and heat sinks, are fabricated fix>m materials such as steels, stainless steels, brass, zinc, aluminum, and other metals and alloys with appropriate prof>erties. The types of corrosion encountered in these structures depend on the environment and material as would be exp>ected. Both uniform and localized corrosion can be important when cosmetic corrosion is of concern. In structural applications, crevice corrosion, corrosion fatigue, stress corrosion cracking (SCC), galvanic corrosion, and intergranular corrosion causing reduction in mechanical properties are important. 

Which is the most common cause of corrosion damage, corrosion fatigue, stress corrosion cracking or pitting corrosion ... 

Several tests are not related to any particular part of the corrosion process, but involve only a specific test specimen that responds to corrosion by complete failure. These tests are used in the measurement of certain forms of corrosion involving factors such as stress. Examples are corrosion fatigue, stress corrosion cracking, and hydrogen embrittlement In designing such corrosion tests, the variety of test specimens parallels the number of apphcations. 

Corrosion fatigue is a type of failure (cracking) which occurs when a metal component is subjected to cyclic stress in a corrosive medium. In many cases, relatively mild environments (e.g., atmospheric moisture) can greatly enhance fatigue cracking without producing visible corrosion. 

McEvily A J (ed) 1990 Atlas of Stress-Corrosion and Corrosion Fatigue Curves (Materials Park, OH ASM)... 

Speidel M O, Denk J and Scarlin B 1991 Stress Corrosion Craoking and Corrosion Fatigue of Steam-Turbine Rotor and Blade Materials (Luxembourg Commission of the European Communities)... 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

Corrosion Fatigue Corrosion fatigue is a reduction by corrosion of the abihty of a metal to withstand cyclic or repeatea stresses. 

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. 

Most cracking problems in cooling water systems result from one of two distinct cracking mechanisms stress-corrosion cracking (SCC) or corrosion fatigue. 

SCC and corrosion fatigue differ in the nature of the stresses and corrodents that cause them. SCC occurs under static... 

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. 

No common industrial metal is immune to corrosion fatigue since some reduction of the metal s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which the stressing occurs. Corrosion fatigue produces fine-to-broad cracks with little or no branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families of parallel cracks (Fig. 10.2). They are frequently associated with pits, grooves, or other forms of stress concentrators. Like other forms of... 

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

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

Alter the environment to render it less eorrosive. This approach may be as simple as maintaining clean metal surfaces. It is well known that the chemistry of the environment beneath deposits can become substantially different than that of the bulk environment. This difference can lead to localized, underdeposit corrosion (see Chap. 4, Underdeposit Corrosion ). The pit sites produced may then induce corrosion fatigue when cyclic stresses are present. The specific steps taken to reduce corrosivity vary with the metal under consideration. In general, appropriate adjustments to pH and reduction or elimination of aggressive ions should be considered. 

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. 

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