Corrosion fatigue continued

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. 

It should be remembered [39], that when evaluating data about fatigue limits in the pulsating or alternating mode which have been determined with inert fluid environments, that the corrosion fatigue may not only effect remarkably the reduction of the admissible stresses but also depends on the number of cycles (a continuous small slope of the Wohler-curve). 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

The development and use of appropriate standards for SCC response in both research and characterization for application is extremely important if continued improvements in the literature database are to be realized. ASTM has put forth the most extensive effort in this endeavor, dating to the early 1960s, with the first standard published in 1972 (ASTM G 30). Since that time, 17 standards have been published on specimens, environments, alIo3 , and classifications, but only three have been added since the first edition of Manual 20. The development of such standards deaUng with stress corrosion has been the responsibility of ASTM Subcommittee GO 1.06 on Stress Corrosion Cracking and Corrosion Fatigue, under ASTM Committee GOl on Corrosion of Metals. An additional important contribution to the standards development process is the sponsorship of technical symposia, which provide an effective technical forum for specialists to present current work in progress. The subcommittee has sponsored nine such symposia [1,41,58-64 ], which have served as catalysts for many developed standards. 

Corrosion fatigue failure is generally not a common failure in boilers. The failure is observed when the boilers are not used continuously or subjected to frequent startups and shutdown. Proper control of startup and closedown procedures can eliminate corrosion fatigue. 

Fig. 2.39 Transgranular brittle striations by corrosion fatigue on admiralty alloy in water diluted ammonia (reproduced with permission of [41]). Four concentrically circles appear intersecting the continuous array of cleavage steps. Black arrows indicate propagation direction...

With binary copper—lead, the continuous copper phase provides the primary load support while pockets of 20—50% lead supply a continuous lead surface film. Tin content of 3—5% is commonly incorporated with the lead to minimi2e corrosion. Copper—lead alloys, either cast or sintered on a steel back, provide good fatigue resistance for heavy-duty main and connecting rod bearings for auto, tmck, diesel, and aircraft engines. 

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